Post by: varsigma on 26/04/2023 02:00:21
A couple of examples are 1) how we can make a large, complicated object like a planet, into a less-complicated 'blob' of matter; since the mass of the planet is the important detail we don't need to know about how the matter is distributed or anything else about it. We can replace the planet with an equivalent mass, theoretically.
Then 2) there's the particle in a rectangular well, a box with infinite potential.
--In electronics you study circuits with ideal elements even though real electronic devices aren't ideal.
And at the really simple end are time and distance. The first, time, is still the subject of debate as to its conceptual existence, or actual physical existence. Distance on the other hand isn't a thing we question very much, although there are theories that suggest the universe is a hologram they are quite complicated, not easy to understand the way we understand distance innately.
If we had a simple explanation for why time and distance appear to be so different, maybe it would all be simple and easy to understand.
But usually you start at the simple end, right? I don't know, perhaps the way momentum starts with a mass and a velocity, but depending on whether you have a relativistic or a quantum context you then have a more complex physical thing. Or do you just have a more complex idea?
p.s. I was asked more than once, in the discussion I mentioned, to define what I thought "physical" means. So I said anything with physical units is physical. I also would say a distance is about as physical as you can get.
You don't erase a distance by measuring it; time on the other hand doesn't seem to work that way.
(?)
Post by: Eternal Student on 26/04/2023 10:40:22
I'm not really sure what you wanted to discuss (here in this forum).
It doesn't matter too much but the general policy is to try and construct the title of the forum post as a question. I'm not a moderator and I don't really mind, it just can help to identify what the other users need to do, for example answer a question, provide opinions or engage in some other sort of interaction.
But usually you start at the simple end, right? I don't know, perhaps the way momentum starts with a mass and a velocity, but depending on whether you have a relativistic or a quantum context you then have a more complex physical thing. Or do you just have a more complex idea?Did you want a broad and general discussion about the history of scientific development or how new theories in physics are usually developed? Are you asking me what I do? (I don't usually develop new theories).
- - - - - - - -
You have plenty of good points that could be discussed, I just don't know which bits you did actually want to discuss.
Best Wishes.
Post by: alancalverd on 26/04/2023 11:09:37
We generally start with the simplest model and add sophistications to account for observed anomalies or discovered additional or limiting factors, hence for instance simple aerodynamics says that the bigger you make the wings of your wind turbine, the more efficient it gets, but the discovery of compressibility effects limits the tip speed (supersonic speeds are easily achievable but will reduce effficiency and/or break the blade) and therefore the range of wind speeds over which the machine can operate.
As for time, you can define it as the dimension that separates effect from cause. Then you apply the concept of entropy and discover that you can't decrease the entropy of the universe, so time is essentially irreversible.
As for momentum, a lot of physics is about conserved quantities and dimensions. Experimentally we observe that mass (up to a point), energy (likewise) and momentum (always) are conserved, which makes our modelling very robust and also tells us where to look to resolve some anomalies, e.g. in particle decay and planetary astronomy.
*neatly expressed by my engineering colleagues: mechanical engineers build things that move, civil engineers build things that don't move.
Post by: Bored chemist on 26/04/2023 12:45:57
neatly expressed by my engineering colleagues: mechanical engineers build things that move, civil engineers build things that don't move.Who builds swing bridges?
Post by: alancalverd on 26/04/2023 13:25:26
My daughter is a surveyor whose job is to resolve conflicts in the construction industry!
Post by: Origin on 26/04/2023 16:12:29
I was asked more than once, in the discussion I mentioned, to define what I thought "physical" means. So I said anything with physical units is physical.Are there nonphysical units or do you just mean units? The reason I ask is because energy has units and energy itself is not what I would call a physical thing.
Post by: Bored chemist on 26/04/2023 17:13:27
energy itself is not what I would call a physical thing.Would you call it a religious thing, or a grammatical thing or an agricultural thing?
Or are you just saying it's not a thing.
Post by: alancalverd on 26/04/2023 19:07:58
An entity is a distinct object - electron, motor car, whatever
A quantity is a dimension or combination of dimensions,such as mass M, length L, time T, energy ML2T-2, etc.
A unit is that by which we express the amount of a quantity: gram, meter, second, joule.....
So you can talk about the mass of a motor car (1500 kg) and the energy density of its fuel (40 MJ/kg) and so forth.
Energy is a physical quantity which, in classical mechanics, is conserved: that is, the amount of energy in the universe is the same before and after an event. The forensic excitement arises from trying to work out where some of it went, and in some cases we end up considering the relativistic possibility that it might have converted into mass, or vice versa.
Post by: varsigma on 27/04/2023 01:36:34
My opinion is that physics is the business of building mathematical models of things that happen (or don't happen - see below*) in order to predict what will happen next or if we alter something.Yes, fair enough. I would add that the models arise because Physics is also the science of measurements; it's about "how are we measuring", not so much what the thing being measured is except that "it's physical".
Try to put that into the very simple context of measuring a distance. How do we measure a distance? We use a fixed unit of . . . distance. Algebraically speaking, we tile a one dimensional space or make a pattern appear. Patterns always contain some kind of information.
Post by: varsigma on 27/04/2023 01:46:34
Are there nonphysical units or do you just mean units? The reason I ask is because energy has units and energy itself is not what I would call a physical thing.Ok. Richard Feynman said nobody knows what energy is, I'm guessing that still holds. So, can you or anyone say it's not a physical thing?
In the lecture where Feynman says that, he also describes forms of energy, like heat, electricity, electromagnetic radiation etc. We know that energy has different forms and these can be converted; we know how to make heat engines that do work.
We know a lot about this thing called energy, but according to Feynman we can't say what it is, although it's ok to think of it as like a number that doesn't change.
Post by: Bored chemist on 27/04/2023 07:58:57
Ok. Richard Feynman said nobody knows what energy is, I'm guessing that still holds. So, can you or anyone say it's not a physical thing?Yes.
Post by: alancalverd on 27/04/2023 18:03:25
I always found Feynman uncharacteristically mystical on this subject, but no worse that the writers of the National Curriculum who talk about "the 'go' of things".
Post by: Zer0 on 27/04/2023 19:55:08
' Energy ' .
What i learnt from it was same as it's Defined.
Energy is the Ability to do Work.
But that OP went deeper, saying the Difference in levels of Energy states, is Crucial for it to Flow.
@OP
Hopefully you aren't thinking in Metaphysical terms, are you?
Post by: varsigma on 27/04/2023 21:43:33
Hopefully you aren't thinking in Metaphysical terms, are you?No. I think I might be thinking in terms of: If Feynman is correct, nobody can say what energy is.
If someone else says energy isn't physical they need to explain how they aren't saying they know what energy is.
It's just logic, really. I don't think metaphysics comes into it.
Or perhaps there's an idea that if you can't say what it is, because nobody knows, you can still say what it isn't.
Like, you can say energy isn't time, or distance. Can you say energy isn't physical? What does that mean?
Post by: Eternal Student on 28/04/2023 00:15:28
How do we measure a distance? We use a fixed unit of . . . distance. Algebraically speaking, we tile a one dimensional space or make a pattern appear.Actually that is a debateable point. It's not really how the modern idea of a physical distance is thought about.
To keep it simple.... Historically there was a physical stick or pole of some length, that length was agreed to be 1 unit of distance (lets say 1 yard). The stick was used as a measuring stick and many other sticks were cut down to the same length as it. These were distributed to various places and organisations for use as the standard unit of 1 yard of distance. Anyway, that sort of thing does describe or define a "distance" much in the way that you have suggested. To measure a distance you would be checking to see how many of these special sticks you can fit between point A and point B.
The more modern definition of distance is NOT like this. It's actually a measurement of something much more like time. One way to think about the modern unit of 1 metre is that it is how far light would travel in a vaccum after a small fraction ( 1 / 299 792 458 ) of a second. To measure a distance you MUST actually measure THE TIME it would take for light to travel travel there, you cannot do it just by putting some sticks between the two points.
Conceptually this is very important, I'll just give one example of where it matters: Suppose that light is generally slowing down as the universe ages (with respect to old fashioned conventional notions of distance or yard sticks). If light was slowing down like this, then two fixed points on the ground that are 1 metre of distance apart today will not be 1 metre of distance apart tomorrow. However, you would still fit the same number of yard sticks between the two points today or tomorrow. The modern definition of distance would not agree with the old fashioned definition of distance.
The way you define distance matters, it matters a lot. The modern understanding of distance is NOT based on fitting many sticks between two points or "tiling" a one dimensional space as you described. Under the modern understanding of "distance" we cannot meaningfully say that light has slowed down compared to yesterday. Instead we can only conclude that all sticks and all distances between two points have increased. To make a relevant connection with some Astrophysics that is popularly known: We say that the universe is expanding, in that distances between two co-moving points in space will increase over time BUT using a different understanding of distance we could instead conclude that distances are not changing, all that is happening is that light is slowing down as the universe ages.
I've spent too long on this minor issue - I was just trying to illustrate how important it can be.
Best Wishes.
Post by: Eternal Student on 28/04/2023 00:44:17
Let's take the next point:
Ok. Richard Feynman said nobody knows what energy is, I'm guessing that still holds.@Origin actually did very well bringing this up as a point of discussion.
@alancalverd has his own opinion and is very good at keeping things simple and pragmatic.
@Zer0 has stated that many other forum threads do exist on the topic already. You didn't give the link to your own question @Origin. I think it was this: https://www.thenakedscientists.com/forum/index.php?topic=85721.0
If you ( @varsigma ) have time to spare you could glance through that discussion - it's about 4 pages so skim read as required, or just put more questions or discussion requirements in here.
Best Wishes.
Post by: varsigma on 28/04/2023 01:28:33
An entity is a distinct object - electron, motor car, whateverOk. I got into this a bit with other people who seemed keen to point put that once you define an entity, it has attributes or properties. But are the properties the things that are identified and in what way is an entity separate from its attributes?
For example the electron is a distinct particle, an entity, it "has" mass, charge, and spin. It seems to me the electron is, in fact, mass, charge, and spin, in a kind of superposition. That is, the electron is its attributes, not something separate from them, or in addition to them.
Further, is the universe a distinct object? What are its attributes? Is an entity just a thing with attributes or are entities the sum of their attributes? Can an entity be an attribute? Does an entity/attribute classification always work?
Post by: varsigma on 28/04/2023 01:54:51
The way you define distance matters, it matters a lot. The modern understanding of distance is NOT based on fitting many sticks between two points or "tiling" a one dimensional space as you described. Under the modern understanding of "distance" we cannot meaningfully say that light has slowed down compared to yesterday.Isn't the modern understanding about having a more precise way to measure distances? I can still use my 'tiling algorithm' and it's quite serviceable.
Post by: Eternal Student on 28/04/2023 04:48:04
Isn't the modern understanding about having a more precise way to measure distances?That is only one half of it. Yes we have a more precise or accurate way of specifying a distance.
However, there is a conceptual shift in understanding what a distance is. It's not an accidental consequence of the way some official decided to define a metre. The decision wasn't made just to improve accuracy of measurements. They were well advised by scientists.
With special and general relativity there was a widespread use of ideas about a "metric" and a "space-time interval". For the physics to work well, it is important that what we think of as a "distance" between point A and point B is by definition set equal to the "space-time interval" between A and B (which is really just a "space interval" since A and B must have had the same time co-ordinate for us to sensibly think about measuring the distance between them). To say that another way, we would want our understanding of distance to be precisely how much time it takes for light to travel from A to B. We would not want it to be based on how many sticks we can fit between A and B because if it was then we need to make a lot of assumptions about how sticks behave over different times and in different regions of space, some of which may have very unusual spatial curvature (gravity). We do not need to make any of those assumptions, it's far better just to adjust our understanding of what a distance is supposed to be.
* I've taken c=1 to be able to say "equal" in most places instead of merely "proportional to".
I can still use my 'tiling algorithm' and it's quite serviceable.For practical purposes, yes. Your model might have c, the speed of light, vary over time or space and it might be unsuitable for predicting real world phenomena in regions of space with unusual curvature (gravity). For planet earth and over short timescales (like the lifetime of planet earth) it should be OK.
Best Wishes.
Post by: alancalverd on 28/04/2023 11:47:13
For example the electron is a distinct particle, an entity, it "has" mass, charge, and spin. It seems to me the electron is, in fact, mass, charge, and spin, in a kind of superposition. That is, the electron is its attributes, not something separate from them, or in addition to them.I've walked across a nylon carpet and am sitting in my rotating office chair. I have mass, charge and spin. I am not an electron. Nor is an electron a proton.
OK, let's suppose the quantities are identical. I have £50 and The Boss has £50. I am not a woman, and she is not Alan.
More identities: John and Tom are identical twins, down to the last atom of their DNA. They are still distinct entities.
But if you want to be mystical, conduction electrons really are indistinguishable!
Post by: Eternal Student on 28/04/2023 17:11:58
An entity is a distinct object - electron, motor car, whateverDo physicists really use the term "entity" that much? Sounds a bit metaphysical.
According to Wikipedia we have this definition:
An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It does not need to be of material existence.
I can't find any specialist scientific definition of the term "entity" on a quick search of the internet - except for computer science where it is considered to be one object about which you can store data in a database. It has a meaning in the English Language and you could obviously chose to use it in a piece of scientific text just as that.
OK. I got into this a bit with other people who seemed keen to point put that once you define an entity, it has attributes or properties. But are the properties the things that are identified and in what way is an entity separate from its attributes?It depends how much science you want to discuss. To keep it simple, then yes - real objects have properties and it makes some sense to ask further questions along the lines you have presented. However, if you wanted to consider Quantum Mechanics (QM) then it is no longer so obvious that objects must have properties.
In QM the notion of "local realism" cannot be sustained. As such one answer to your question "in what way is an entity separate from its attributes?" is that an object and its attributes could be separated by many thousands of miles. There are many ways in which an object can be separated from its properties or attributes. Moreover, an object does not need to have any definite value of its attributes until those are measured. Furthermore, it cannot be assumed to have properties much like hidden variables that existed but were just unknown to you. In some situations it is just not possible to explain the behaviour of some objects if their attributes were fixed and established by nature prior to the measurement being made.
If you're interested look up the EPR paradox and Bells inequality. (Glance at these Wikipedia entries for example: https://en.wikipedia.org/wiki/Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox and also https://en.wikipedia.org/wiki/Bell%27s_theorem ). However they aren't easy going or an ideal introduction. This video: "Bell's Theorem: The Quantum Venn Diagram Paradox" by minute physics is available on YouTube and it's a much lighter introduction to the topic (it will still take about 15 minutes, even "minute physics" didn't seem able to keep it short).
Best Wishes.
Post by: alancalverd on 28/04/2023 22:35:01
Do physicists really use the term "entity" that much?The high priests of pedantry, the International Standards Organisation, use the term exactly as I did. It's been a while since I was involved in such matters but I don't think energy is an entity in ISO - it is a quantity. Feynman was a good musician but I think he got this one wrong.
Post by: varsigma on 28/04/2023 22:41:29
I've walked across a nylon carpet and am sitting in my rotating office chair. I have mass, charge and spin. I am not an electron. Nor is an electron a proton.Ok. Sort of.
OK, let's suppose the quantities are identical. I have £50 and The Boss has £50. I am not a woman, and she is not Alan.
More identities: John and Tom are identical twins, down to the last atom of their DNA. They are still distinct entities.
But if you want to be mystical, conduction electrons really are indistinguishable!
You can't really compare classical objects with quantum particles, can you?
Conduction electrons are indistinguishable because all electrons are identical. Identical in a very exact way, whereas classical copies are only identical in an approximate way.
Print two copies of a document; close examination of the characters will show some differences, never mind how different the sheets of paper might be.
Copying information classically never gives you identical copies--even in a digital computer with very tight manufacturing tolerances. Classically we have to deal with approximate copies.
Quote
It depends how much science you want to discuss. To keep it simple, then yes - real objects have properties and it makes some sense to ask further questions along the lines you have presented. However, if you wanted to consider Quantum Mechanics (QM) then it is no longer so obvious that objects must have properties.The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments. But those are what we might term the local properties or attributes; the position and momentum are also "properties" because--spacetime . . .
It's difficult to put nonlocality into the frame when it comes to the location of an electron, then the location of its charge, its mass, and its spin. Most electron experiments I've looked at appear to assume these are all localised. For instance they propagate together in a beam of electrons.
The only real evidence I've seen of de-localised properties is the appearance of quasiparticles on the surface of certain materials at low temperatures. Here you see electrons interacting with the material and each other, and appearing to separate into localised quanta of charge (chargons), spin (spinons), and mass (magnons). This result confirms my suspicions that electrons are a superposition of field quanta. Since the result also yields some information about geometry and dimensions, that has to be a measurement.
Post by: alancalverd on 28/04/2023 22:52:28
Now Pauli says you can't have two fermions with identical quantum numbers, so electrons are distinguishable.
Except when they aren't.
Post by: Bored chemist on 29/04/2023 00:24:07
nobody can say what energy is.A dictionary editor can.
Post by: Bored chemist on 29/04/2023 00:29:24
To measure a distance you MUST actually measure THE TIME it would take for light to travel travel there, you cannot do it just by putting some sticks between the two points.No
Nobody does that.
Apart from anything else, how do you measure time?
Also there's a reason for the weird number in
how far light would travel in a vaccum after a small fraction ( 1 / 299 792 458 ) of a second.And it's arbitrary.
Post by: geordief on 29/04/2023 01:20:08
Apart from anything else, how do you measure time?Is a time unit arrived at by counting the spontaneous and random emissions in the radioactive decay of an atom?
Post by: Eternal Student on 29/04/2023 03:34:46
The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments.The spin of an electron is exactly the sort of property that could become entangled with the spin of another particle and illustrate behaviour similar to photons with entangled polarisation that were described in Bells inequality.
You can't prevent it just by declaring that an experiment was "classical", all you can do is choose to do an experiment where it wouldn't be important. However, you can certainly choose not to include it in your discussion, all physics is just a model and a decision must be made about what is sufficient.
ES said: To measure a distance you MUST actually measure THE TIME it would take for light to travel travel there,.....1. Nobody eats 5 portions of vegetables every day but they should. Distance is defined a certain way, that's not my fault, that's just how it is in the SI system.
BC replied: ...Nobody does that.
2. Some people do measure distances just by measuring the time for light to travel. Estate agents have radar guns to measure the dimensions of a room quickly. As you know, estate agents are paragons of truth.
Also there's a reason for the weird number in ( 1 / 299 792 458 ) of a second.Yes. Setting it to 1 would have been easier but, I suppose, there were limits on what number they could chose without putting the modern definition too far out of tolerance with the older standards already in use. It's an integer, so... glass half full.
And it's arbitrary.
Apart from anything else, how do you measure time?Well, as I expect you already know there is a standard established for the second. It looks like @geordief has already asked about this. I was tempted not to even make a start discussing time, it's too late in the day. For practical purposes I use the elephant method ( just count 1 elephant, 2 elephants, 3 elephants...etc. ).
Best Wishes.
Post by: Bored chemist on 29/04/2023 11:24:26
No, it's defined in terms of microwaves emitted by caesium atoms.Apart from anything else, how do you measure time?Is a time unit arrived at by counting the spontaneous and random emissions in the radioactive decay of an atom?
But the tricky bit is that, in principle, anything perturbs that emission- even if it's only by gravitational shifts.
So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your ruler.
Post by: Bored chemist on 29/04/2023 11:26:58
ome people do measure distances just by measuring the time for light to travel. Estate agents have radar guns to measure the dimensions of a room quickly. As you know, estate agents are paragons of truth.Those are interesting gadgets, but they don't work by time of flight of photons.
On the other hand, if you use them to measure the size of your swimming pool, you get the answer wrong by a factor of the refractive index of water.
Post by: alancalverd on 29/04/2023 11:48:34
And I have, at least twice in this thread.nobody can say what energy is.A dictionary editor can.
Post by: alancalverd on 29/04/2023 11:50:47
The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments.The sentence is meaningless, so you can't "safely assume" its validity.
Post by: geordief on 29/04/2023 11:54:17
So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your rulerWhy is that important.?We can never attain absolute accuracy in measurements ,can we?
If the clock was right but the spatial distance was wrong that really would be a problem,surely.
Post by: alancalverd on 29/04/2023 11:54:57
But the tricky bit is that, in principle, anything perturbs that emission- even if it's only by gravitational shifts.Not true, my friend! Both space and time are distorted by the same phenomenon so your measurement is correct!
So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your ruler.
Post by: alancalverd on 29/04/2023 11:57:42
2. Some people do measure distances just by measuring the time for light to travel. Estate agents have radar guns to measure the dimensions of a room quickly. As you know, estate agents are paragons of truth.I rarely deal with estate agents, but I frequently rely on the opinion of radar operators who use exactly this method to tell me about conflicting traffic.
Post by: Eternal Student on 29/04/2023 15:12:53
Those are interesting gadgets, but they don't work by time of flight of photons.I thought most radar systems measure distance this way. I'm almost tempted to ask how estate agents devices do work - but I have only a passing interest so a short answer would be fine.
But the tricky bit is that, in principle, anything perturbs that emission- even if it's only by gravitational shifts.I've got to agree with @alancalverd and @geordief here. It's not "wrong" as such, it just is the local flow of time. It's not the same rate as you might obtain elsewhere but since Einstein's results we were expecting or counting on that. It is exactly how we would want time to be measured or defined.
So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your ruler.
There are a few caveats: 1. We assume you can keep an atomic clock small enough so that all of it is effectively in one location and experiencing the same local conditions. For example, in the vicinity of the event horizon of a small black hole the curvature of space could change significantly even over the width of just a few atoms and not all of your Caesium atoms will be under the same conditions. This is only a problem for practical use of the equipment, theory can have time defined this way without problems.
2. It does assume that no significant forces or fields exist which would influence the electron orbit energy levels but a good clock should be designed to control the E and B fields at the Caesium atoms (along with everything else we think is important and know about).
3. There could be unknown unknowns. If there is some currently unknown physics that could influence these electron orbit energy levels OR shift the frequency of radiation before it's detected by the detectors - then it isn't measuring exactly what we wanted.
Best Wishes.
Post by: Eternal Student on 29/04/2023 16:03:29
Is a time unit arrived at by counting the spontaneous and random emissions in the radioactive decay of an atom?Are you happy with the answers / discussion so far, or did you want a reference to the way that time is measured?
There are some references and discussion in Wikipedia or on the internet. Just use sensible keywords like "second", "time", "caesium", "atomic clock" etc.
It isn't really about the radioactive decay of an atom. It is just about a Caesium atom being allowed to have an electron make a transition from one energy level to another. As you probably know, for any given transition between two energy levels, a photon of precisely one frequency is emitted (or absorbed). We get the equipment to make a specific transition happen and assert that 1 second is however long it took for 9192631770 cycles of that electromagnetic wave to pass a point in space (e.g. a detector). So, as opposed to a radioactive decay process, where your clock substance would be used up and depleted, the Caesium atoms are re-useable, you just excite the electrons back to the high energy state again.
There are other systems and atomic clocks but the general concept allowing us to define a second is as described above. Some of the more reliable and practical methods use some simpler device (like a quartz crystal) to keep time and the Caesium atoms (or whatever substance they have chosen) just become a test. The entire system can be imagined as something which guesses the time and then applies corrections. For example, you can bombard atoms with radiation which is just guessed to be the right frequency (based on the quartz crystal system) and then adjust the frequency upwards or downwards slightly until enough of the Caesium atoms are detected as being moved to the higher energy state. Computer algorithms automatically apply adjustments to the time keeping that has been done in between the bombardment tests.
Best Wishes.
Post by: Zer0 on 29/04/2023 20:44:39
Hopefully you aren't thinking in Metaphysical terms, are you?No. I think I might be thinking in terms of: If Feynman is correct, nobody can say what energy is.
If someone else says energy isn't physical they need to explain how they aren't saying they know what energy is.
It's just logic, really. I don't think metaphysics comes into it.
Or perhaps there's an idea that if you can't say what it is, because nobody knows, you can still say what it isn't.
Like, you can say energy isn't time, or distance. Can you say energy isn't physical? What does that mean?
If it isn't meant to be Metaphysical, but purely Logical...
Then Energy seems to be a Calculable entity.
It's a Real thing.
It can be Measured.
We can Control & Utilize it.
(I do Not feel the need to bring Prof Feynman into this.
& I do Not believe He ever made such an Absolutist statement.
Maybe for QM, but not ' Energy ' )
Apologies i Misunderstood the Topic to be based on Abstractions, hence felt it was a Metaphysical dialogue.
Post by: varsigma on 29/04/2023 22:20:27
The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments.If the sentence is meaningless, how did Milliken do his experiment? What did he assume?
The sentence is meaningless, so you can't "safely assume" its validity.
Post by: geordief on 29/04/2023 23:00:30
decay of an atom?
Are you happy with the answers / discussion so fa
I am not sure I agreed with you regarding yard sticks.
If you create a yard stick for use as a universal unit of measurement and then wait until (let's hypothesise) the speed of light slows appreciably.
As per what I understand you to be saying those yard sticks remain unchanged in length but I am wondering whether their length is dependent on the speed of light insofar as the bonds holding the Atoms together are those of electric attraction.
Since light is em radiation I was wondering if the force of electric attraction was proportional to the speed of em radiation -ie the speed of light.
If so ,the yard sticks would change in length.
Am I right?
Post by: alancalverd on 29/04/2023 23:19:15
f the sentence is meaningless, how did Milliken do his experiment? What did he assume?There are no assumptions in Millikan's experiment. He "simply" discovered that charge is quantised - it's a clever and quite difficult experiment but yielded a remarkably accurate value of the quantum of charge.
Post by: alancalverd on 29/04/2023 23:21:19
Then Energy seems to be a Calculable entity.It is a quantity, not an entity. See reply #7 above.
Post by: Eternal Student on 30/04/2023 01:44:00
I am not sure I agreed with you regarding yard sticks.You've asked a lot of sensible questions about yard sticks and you are recognising the problems. The main point is precisely that real physical sticks do tend to be made of something and they could change dimensions for all sorts of reasons. When distance is defined as how far light travels in a unit of time then we avoid the need to worry about any of this.
As per what I understand you to be saying those yard sticks remain unchanged in length
I think that was from a post I made a while back. In that post the sticks were "idealised sticks" in that we were trying to consider what happens if light slows down and for that we needed our sticks to be immune to whatever it was that might have caused light to slow down. Don't spend too long worrying about that - the key point is that sticks are not good ways to measure a distance with, we do not know exactly how they would or should behave. For example, we wouldn't know exactly which effects we wanted our "idealised sticks" to be immune to.
Let's re-phrase this: Just for that particular situation you need to assume that we had access to a "god-given" stick of length 1 metre which was 1 metre and for evermore shall be 1 metre (and god knew what a "distance" was supposed to be better than we do, so he/she knew precisely what effects that stick had to be made immune to).
I was wondering if the force of electric attraction was proportional to the speed of em radiation -ie the speed of light.The answer is yes, assuming Maxwells equations continue to model the behaviour of light. The speed of light, c, is given by c = 1 / √(με) where ε and μ are the permittivity and permeability of space (things that will influence how strong the electric or magnetic attractions will be). So if c changes over time then at least one of μ or ε must be changing with time. Assuming it's ε then we have: c decreasing => ε must be increasing => the electric attraction between charges is proportional to 1/ε => so that would have been decreasing. That might end up with the atoms in the stick being less strongly pulled together so that the stick might grow longer. (It might also be that the attraction is now so weak the molecules can't even hold together and the stick falls apart). However, the reason or cause for light to be slowing down was deliberately left arbitrary and hypothetical - it may be that it wasn't following Maxwells equations in that future due to some as yet unknown physics.
Anyway, this sort of lengthening of sticks would be precisely the opposite of what you want to happen if you're hoping to measure the same distances with sticks and with light being allowed to travel. Half the speed of light and you should half the distance it travels in 1 second. But since your sticks have grown you can't even fit half the number of sticks into that new length measurement, you'd be lucky to get one-third as many sticks into that length measurement. So it would now be even easier to conclude that measuring distances with sticks or with a travel time of light leads to disagreements (if the speed of light changes over time).
Overall, the important point is that yard sticks are not the best way to measure a distance - there are far too many questions and complications about how sticks might behave.
Best Wishes.
Post by: varsigma on 30/04/2023 02:43:53
There are no assumptions in Millikan's experiment.Sorry, I just can't see how that could be possible.
I still don't understand why it's meaningless to assume that the electron's properties occupy the same place. I just don't get what you mean.
Instead of "in the same place", perhaps I should say the mass, charge, and spin of an electron are all in phase.
p.s. I realised I should qualify that last sentence with "under normal circumstances".
Post by: alancalverd on 30/04/2023 11:42:00
What Millikan (and countless subsequent undergraduates) did was to measure the voltage required to prevent a charged droplet from falling under gravity between two parallel plates. It turned out that the required voltage has discrete values, from which he deduced that charge is quantised. Clever, difficult (I've never known an undergraduate get it to work first time), but no assumptions.
Post by: varsigma on 30/04/2023 22:30:18
Mass and charge are not wave functions and thus do not have a phase.The mass and charge of an electron aren't a part of its wavefunction? Or have I misinterpreted you?
What Millikan (and countless subsequent undergraduates) did was to measure the voltage required to prevent a charged droplet from falling under gravity between two parallel plates. It turned out that the required voltage has discrete values, from which he deduced that charge is quantised. Clever, difficult (I've never known an undergraduate get it to work first time), but no assumptions.
Well, I think any experiment has to make assumptions. One assumption I think Milliken had to make was that oil droplets can be charged, that electrons stick to them and stay stuck in the experiment.
Post by: alancalverd on 30/04/2023 23:28:01
The mass and charge of an electron aren't a part of its wavefunction? Or have I misinterpreted you?You have misinterpreted physics. A wavefunction is the mathematical model we use to describe the probability of finding an object at a point in space. The electron doesn't "have" a wavefunction, but we assign one to it. You are not alone in your misconception, by any means!
If the droplets weren't charged, they wouldn't be prevented from falling by the electric field. If the charge dissipated, the stationary droplet wouldn't remain stationary. Nobody said anything about electrons. The experiment demonstrates that charge is quantised.
Post by: varsigma on 01/05/2023 02:22:10
You have misinterpreted physics. A wavefunction is the mathematical model we use to describe the probability of finding an object at a point in space. The electron doesn't "have" a wavefunction, but we assign one to it. You are not alone in your misconception, by any means!Ok. If there's a Schrodinger equation that describes the probability of finding an electron near a proton, say in a Hydrogen atom, in what sense does it not describe the position of the electron's mass or any other property of that electron?
If the droplets weren't charged, they wouldn't be prevented from falling by the electric field. If the charge dissipated, the stationary droplet wouldn't remain stationary. Nobody said anything about electrons.Nobody said anything about electrons and having measured their charge? I thought that was the whole point of the experiment.
How then did Milliken know what he measured?
I still fail to see how anyone can do an experiment--any experiment--and not make assumptions.
Post by: Eternal Student on 01/05/2023 05:53:10
If there's a Schrodinger equation that describes the probability of finding.......Minor detail: It's not the Schrodinger equation that describes these things, it's the wave function which appears in that equation.
[I've edited this post to remove more details, the post was getting too long].
in what sense does it (the wave function) not describe the position of the electron's mass* or any other property of that electron?Various ways exist. These are some of the easiest ones to explain:
1. The wave function may be in a superposition of states. Until a measurement is made a single unique value of that property cannot be assigned to the object.
2. Measurement of one property will cause a wave function collapse and the wave function is then changed. This can alter the value of other properties of the object. To say this more clearly, you cannot measure all the properties one after the other and hope you'll know everything at the end. As you continue measuring more things you will unavoidably mess up some of the previous things you measured.
I hope that makes some sense.
* You actually mentioned "mass" as one example of a property. Simple QM models will assume the mass of a particle is a fixed unchanging constant. You want the mass as a parameter to generate the Schrodinger equation. The above discussion applies to all the "other properties" of the electron you've ever mentioned (e.g. spin, momentum, position etc.) and the general spirit of it should apply to mass.
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I'm not sure that quantum mechanics needs to be too important for your discussion. For large scale models we can assume objects have properties with definite values, these exist at all times and these properties are located with the object.
No model in physics is perfect and no teacher would try to present the most complete or accurate model of physics to their class. Instead you ( @varsigma ) can make a decision about how much detail or simplification is useful and just pick a model that is sufficient for the purpose.
You didn't say much about the original discussion you were having .....
Hi. I recently had a discussion online about the subject of physics, in which I posted something about simplification, and how that seems to be where physics starts, at least.I assumed you might be wanting to get some new ideas for that other online discussion from this web forum. That's why I have sometimes rambled on about a very small detail or niche area.
You don't have to - but would you be able to tell me a bit more about that other online discussion and/or about what you're hoping to obtain from this discussion here on this forum?
Best Wishes.
Post by: alancalverd on 01/05/2023 11:04:43
Ok. If there's a Schrodinger equation that describes the probability of finding an electron near a proton, say in a Hydrogen atom, in what sense does it not describe the position of the electron's mass or any other property of that electron?You are beginning to get the picture. The wave function does what I said - it maps the probability density of finding the electron. It is not a property of the electron, because it is a function of the environment (it's different for an electron in a hydrogen atom compared with a hydrogen molecule), not just the entity.
Nobody said anything about electrons and having measured their charge? I thought that was the whole point of the experiment.I repeat: Millikan demonstrated that charge is quantised. How did he know what he measured? Whenever an oil drop was stationary he looked at the voltmeter. No assumptions (other than that the voltmeter measured volts). You (and almost everyone else) have made the assumption that the quantum is the charge of an electron. Why not just read the Wikipedia article?
How then did Milliken know what he measured?
I think the best experiments are true null investigations, like dropping a couple of rocks from a tower. No room for any assumptions: what you see is what you get.
Post by: alancalverd on 01/05/2023 14:20:25
Measurement of one property will cause a wave function collapse and the wave function is then changed.I seriously disparage this statement!
You can plot the outcome of dice throws as a wave function. You throw the dice and get a number. You haven't done anything to the hypothetical wave function, just chosen one value of it, which you could not predict. If you roll the dice again, you will get an equally unpredictable number. If you had "collapsed" the wave function, you would have restricted the range of future possibilities - the gambler's fallacy.
It is perfectly true that if you measure any property of a subatomic particle you will have altered its state in some way, e.g. by bouncing a photon off it, and thus biased its future state and wave function because you have changed its energy and momentum, but the notion of "collapse" rather militates against Heisenberg.
Post by: geordief on 01/05/2023 15:01:59
So collapsing the wave function is a bit like swatting a fly ?Measurement of one property will cause a wave function collapse and the wave function is then changed.I seriously disparage this statement!
You can plot the outcome of dice throws as a wave function. You throw the dice and get a number. You haven't done anything to the hypothetical wave function, just chosen one value of it, which you could not predict. If you roll the dice again, you will get an equally unpredictable number. If you had "collapsed" the wave function, you would have restricted the range of future possibilities - the gambler's fallacy.
It is perfectly true that if you measure any property of a subatomic particle you will have altered its state in some way, e.g. by bouncing a photon off it, and thus biased its future state and wave function because you have changed its energy and momentum, but the notion of "collapse" rather militates against Heisenberg.
You kill it but there is always another one.....
Post by: alancalverd on 01/05/2023 18:01:09
Do you collapse the wave function of a pair of dice? No, you make an observation of an event that you can't predict, though you have a very good idea of the likelihood of scoring less than 2 (not possible), 7 (highly probable) or 12. The only difference is that Heisenberg's limit on the precision with which you can simultaneously know the position and momentum of an object is a bit more important for an electron than for macroscopic dice.
Post by: Eternal Student on 01/05/2023 18:34:43
I seriously disparage this statement!OK.
It's still perfectly true unless the wave function was already in an eigenstate of what you were about to measure. Even then you can still say that the measurement would "collapse" the wave function but in this situation that new function will just turn out to be the same wave function.
In most situations two operators representing different observables do not share a common set of eigenfunctions, so that measuring one forces a collapse to a state that cannot be an eigenstate of the other operator.
Rather than write out the mathematics in something like Dirac notation for this, I suspect most of the readers would just prefer a concrete experiment to be discussed.
So the experiment similar to the one in post #20 is one of the clearest demonstrations.
Have some polarised light and start sending it through polarising filters. Have a first filter that will collapse the wave function so that the lights polarisation is entirely in the x-axis direction. Put that through a second filter which permits only light polarised along the y-axis to pass and no light will get through. However, if you add a third filter turned at 45 degrees between the x- and y-axis and insert that in between the two other filters, then you will now get some light to pass the last filter. The measurement of the polarisation along the 45 degree axis has forced a wave function collapse which has now made sure that the wave function is back in a superposition of states for polarisation along the x-axis or y-axis.
This has been discussed in several other threads and here's the YouTube video that tends to be chosen to go with it. ( "Three polarising filters: a simple demo...." , available on YouTube, duration ~ 1 min 30 seconds).
You can plot the outcome of dice throws as a wave function.It's an awful example but if you really want to use it we can.
Consider an observable we can call "the last dice roll result". If it was a 6 then that is now a fact (actually it is still a random variable but a very trivial one - it has only one possible value and a 100% probability of having that value). All of the characteristics we want are still being exhibited. We have a situation comparable to wave function collapse, roll the dice and the value of the last dice roll has to be updated immediately. Throw the dice again. If the dice roll you obtained was a 5 then that replaces the previous dice roll value and that is now the last dice roll result.
About the gamblers fallacy --> They are just betting on the wrong things. Bet on the last dice roll result because there has been a wave function collapse, the wave function is now one where the last dice roll will always be a 6 (or whatever it was). They will win every time (until a new measurement is made).
Best Wishes.
Post by: Eternal Student on 01/05/2023 20:58:15
Sorry, I spent so long writing my reply, you ( @alancalerd ) had written another one in between.
Do you collapse the wave function of a pair of dice?Yes. You haven't really clearly defined what you are considering as a "wave function" but I'll just assume it involves a description of the system that is sufficient to predict the outcome of dice rolling (and nothing much else). Exactly how you're going to get that into the Schrodinger equation or what you are considering as the Hamiltonian is questionable - but hey, whatever, this is 2023 and we'll just go with it. Let's just assume you are conceptualizing the dice as some system that is described by something which is loosely like a wave function.
Prior to making any measurements your wavefunction presumably has these characteristics: Each die can take one of six values {1,2,3,4,5,6} with equal probability = 1/6 for each value. The evolution of your wave function is not all that interesting by whatever version of the time dependant Schrodinger equation it follows. As time progresses, the probability of getting a particular die roll result doesn't change.
Now make a dice roll and measure the result (let's say it's a 1 and a 5 = 6 total). That dice roll result is clearly not following anything like your old model any longer. The wave function is not the same as it used to be, it has been changed, there was something analogous to a wave function collapse. Your wavefunction is now one where the dice roll result would always be a 1 and a 5 making a total of 6. Future results might still follow your random prediction model but the previous results certainly don't.
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On a very minor and tangential topic. The entire notion of Random Variables in pure mathematics is quite an interesting one. With Quantum mechanics it is now reasonable that probability is something that exists in nature instead of being only a theoretical or abstract mathematical concept. Since the number of people likely to be interested in pure mathematics is not high, I'll just leave off that discussion.
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It's just a pointless and confusing expression, implying that a wave function has more significance than a mere model.Well, the premise of QM is that every system can be described by a wave function and that wave function describes everything that is knowable about the system. So, for example, if the mathematics says that you can't determine all the components (x, y and z- Axis components) of angular momentum simultaneously (which they do), then many physicists will assume that this would be true and you can not actually do that in reality.
What I'm saying is that for many physicists, Quantum mechanics is of earth-shaking importance that changes our understanding of what reality might actually be. You could choose to consider that a wave function is the best example of a property that a system can have and the only property you would need to assume it has. All other observable properties that it might have can be determined from that wave function. Some properties will never be knowable simultaneously or just are not properties that the system can have. Change that wave function somehow and you can change the behaviour including various physical properties of the system. If you know the wave function has not changed then you can assert that the system has not changed its properties or behaviour etc... The wave function is the over-arching governor of, or register of properties for, the system.
But it is just a model and not the law of our nation. So you don't HAVE to assume the wave function has any underlying significance if you don't want to.
Best Wishes.
(LATE EDITING to fix some spelling / grammar - which still isn't perfect.).
Post by: alancalverd on 01/05/2023 22:21:32
You haven't really clearly defined what you are considering as a "wave function" but I'll just assume it involves a description of the system that is sufficient to predict the outcome of dice rollingNo! A Schrodinger wave function does not (cannot) predict the position of an object, but the probability density of its distribution in space. You can't predict the outcome of a dice roll but you can write down the probability density of each possible outcome. Same thing.
Post by: alancalverd on 01/05/2023 22:31:20
What I'm saying is that for many physicists, Quantum mechanics is of earth-shaking importance that changes our understanding of what reality might actually be.I think you are confusing physicists with philosophers. There is nothing earthshaking about quantum mechanics - it is a good model of what happens: the everyday currency of physicists.
100 years ago it was a new model, but since it (a) explained a lot of things that didn't make sense in a continuum/billiard ball model and (b) predicted a few things we hadn't yet observed, it was just a better way of doing business. Like relativity.
Post by: alancalverd on 01/05/2023 22:43:03
Have a first filter that will collapse the wave function so that the lights polarisation is entirely in the x-axis direction. Put that through a second filter which permits only light polarised along the y-axis to pass and no light will get through. However, if you add a third filter turned at 45 degrees between the x- and y-axis and insert that in between the two other filters, then you will now get some light to pass the last filter. The measurement of the polarisation along the 45 degree axis has forced a wave function collapse which has now made sure that the wave function is back in a superposition of states for polarisation along the x-axis or y-axis.Er, no. There being no polarisation along the 45 degree axis (because you said the first filter confined the beam to x polarisation only) the intermediate filter cannot have "measured" anything. What it did was to rotate the plane of polarisation between its input and output. If each filter had measured rather than reformatted the incoming beam, there would be very little loss and the superposition would result in close to 100% transmitted intensity overall.
The system determines the wave function, not the other way around. On proton, one electron, spherical distribution. Two protons, two electrons - a dumbell 1H2 molecule Add a neutron, and it's 3He - spherical again! The wave function has a lot of significance - it helps us predict the behavior of a system. But wave function "collapse" doesn't.
Post by: Eternal Student on 02/05/2023 02:45:02
We're at risk of hijacking @varsigma 's post entirely. I'll keep this short.
.....(Stuff removed to keep it really short....)...
If you're not happy with light and polarising filters, then you can do much the same experiments and get the same results using atoms or electrons (instead of light) and their spin (instead of polarisation). For that you replace the filters with Stern-Gerlach apparatus.
Best Wishes.
Post by: alancalverd on 02/05/2023 09:25:12
Quote
Given that the input to the second S-G apparatus consisted only of z+, it can be inferred that a S-G apparatus must be altering the states of the particles that pass through it.(my italics)
which is much more reasonable, and also makes sense if applied to the 45 degree optical polariser.
I may be a bit pedantic in distinguishing between segregating (black sheep to the left, white to the right) and measuring (counting the sheep in each pen after segregation) but that's the residual chemist in me: qualitative and quantitative analysis are not the same thing. The "triple S-G" experiment simply selects white sheep then arbitrarily paints them red or blue.
*the measurement phase of MRI comes later: we listen to the radiofrequency emission as the selected spins relax and realign to the primary field.
Post by: Eternal Student on 03/05/2023 03:49:13
We might have lost the OP. I'm inclined not to continue saying much until they've had a fair time to reply. At any point you ( @varsigma ) can steer the discussion in some direction.
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@alancalverd has made some good points. Thanks for spending some time to look up something about Stern-Gerlach apparatus and bothering to make a reply, it is appreciated. However, measurement and exactly what constitutes a measurement is just too big a topic for me to start discussing at the moment.
Best Wishes.
Post by: alancalverd on 03/05/2023 08:23:06
The S-G experiment was part of the undergraduate syllabus 60 years ago, and, beneath the layer of dust, my notes are all about polarisation and selection, not measurement! Maybe time to grumble to Wikipedia?
I've been thinking about the 45 degree polariser. Quite simply, it comes down to the resolution of a vector. If we represent the E field of a polarised EM wave as a unit vector in the x plane, we can resolve it as the sum of 1/√2 vectors at ±45° to the x plane. So the (idealised) polariser at +45° will transmit about 1/√2 of the incident energy as an EM wave at 45° to the original plane. Then the third polariser does the same, resulting in about half of the original beam being transmitted. Again, this isn't "measurement" but manipulation by absorption and retransmission through the polar medium.
Post by: Zer0 on 03/05/2023 20:41:28
The OP seems to have lost all Energy..
To Charge ahead!
Post by: Eternal Student on 03/05/2023 21:10:09
Well... this was something I had prepared, with a couple of questions to work through, if you're bored, @Zer0 or anyone else. But as usual it started to get too long. It's only going to get a person half-way to a quantum model of light and how filters work. I didn't really have the time to finish it PLUS the common sense to assume no-one would read it. BUT, if you're bored then there are two questions here to work through, like a lunch-time crossword puzzle etc. I've not checked the spelling or grammar or done much editing.
I've been thinking about the 45 degree polariser. Quite simply, it comes down to the resolution of a vector.Yes. The Classical explanation of how a polarising filter works is exactly like this. Well done and keep the dust off those old notes, they're always useful.
Using classical models, an idealised polarising filter could be imagined as something with conduction electrons that can oscillate in only one direction (e.g. some thin wires running in one direction like a multi-wired chesse-slicer for people who want to cut mutiple slices and aren't prepared to do it one slice at a time). For optical wavelengths of e-m radiation, the wires are really small and really closely packed together so that a crystalline substance is the best way to go because no-one would want cheese cut that thinly. Anyway, an idealised polariser is able to absorb all the oscillation that is in the direction of its orientation when an e-m wave passes through. Only the oscillation perpendicular to its orientation is transmitted.
The only minor detail is that the energy in an e-m wave is proportional to the square of its amplitude. So the amplitude was reduced exactly as you claim (1/√2 for a polarising filer at 45 degrees) but the energy is reduced by 1/2. (The remaining calculations you suggested then have a follow-through error).
The next thing that can be interesting is to ask yourself what the frequency of the transmitted light must be. If the light incident on the filter has frequency f and all the filter does is remove a vector component of that oscillation what is the frequency of the transmitted light?
(a) It is still f. (It may have a different polarisation but the greatest deflection along any axis would be precisey when the incident light was at its greatest deflection etc.)
(b) fout ≠ fin .
[Minor aside, which you probably already know: Photographers used to like using a pair of polarising filters and just adjusting their orientation relative to each other as a way of controlling the total brightness of what they are filming or photographing. It reduces the amplitude of everything quite uniformly, e.g. putting the filters at 45 degrees to each other halves the intensity at all frequencies. In particular it does not change the colour balance or favour some frequencies over another. Using some dark coloured glass is cheaper but then the filter does knock out some colours more than others. It doesn't matter so much with modern technology - light editing on photoshop will correct a red saturated image of a bus into a stunning photo of a young man with perfect teeth.]
Now, assume you turn the incident light down in intensity as much as you can. With modern technology and a laser pulse, let's say you could get only a few photons to hit the filter.
The next multiple choice question has to be: What will be transmitted at the other side of the filter? Somehow you must have a lower intensity at the other side of a filter (unless the orientation was precisely inline with the polarisation of the incident light).
(a) Same number of photons (incident and output), with typically lower frequency on the output. E = hf, lower f, lower E.
(b) Same number of photons and they also have the same frequency. A component of the amplitude in some direction was removed, so the photons have a bit less energy.
(c) Light is quantised. So you get a fraction of the photons at the other side rather than fractional-photons or anything that could have satisfied (b).
(d) Something else. Just don't consider photons.
---------
You'll probably take option (d) though - just don't use Qunatum theory. That's ok, it's a choice. QM is just a model.
Best Wishes.
Post by: alancalverd on 04/05/2023 07:02:03
First, I admit to the "amplitude/energy" error. It occurred to me whilst driving home yesterday but you beat me to publication!
The simplest way we can discuss polarisation of EMR is with a classical wave model, which fits neatly with our models of the electronic structure of chiral molecules and all the way down to radiofrequency polarisers and reflectors. It is less obvious why a chiral molecule or wire grid should preferentially transmit a boson with spin up or down, and even less obvious how it can rotate the spin of an incoming photon, nor what "frequency" means to a particle model - surely some energy is lost in transmission so why do the exit photons have the same energy as the incident photons (they do - your sunglasses are grey, not red!)?
However it is clear that at very low intensities we can count individual photons, so we are back to an adage I used in some other threads: our best model of EM transmission and propagation is Maxwellian, but detection (particularly at low intensities) is distinctly a quantum phenomenon.
Your option c is the only one that makes sense and describes the "black box" transfer function of multiple polarisers, but whilst Stern-Gerlach segregation is explicable in terms of spin/field interaction it gets very difficult to explain what happens inside a molecular polariser box unless you use a wave model.
As for the photographic use of polarisers, it seems like a complicated way of reducing intensity, compared with simply reducing the aperture of the lens, which doesn't affect the visual impact of the image. A single polariser, however, is very useful for suppressing the polarised glare from a reflective surface and thus increasing the contrast or conspicuity of the object of interest. Excellent sunglasses for coarse fishing and gliding, but dangerous for driving!
PS - my undergraduate notes on Stern-Gerlach are from the quantum physics course, not classical electromagnetics!
Post by: varsigma on 04/05/2023 09:53:28
Interesting that the Wikipedia article, when discussing sequential S-G systems, talks about "measuring" when a particle passes through a nonhomogeneous magnetic field. When we use magnetic fields to select regions for analysis by spin resonance (e.g. MRI) we talk about polarising or forcing, not measuring*.
What I've seen about measurement of quantum states is that preparing a state, such as by applying an external magnetic field, is the same thing as a measurement. It's really about deciding where the inputs and the outputs are, in terms of the quantum information.
I read a sort-of interesting SciAm article which included a discussion of SG and the way you don't get a Boolean logic if you try to build a circuit, so to speak, using SG 'gates'.
The reason as I recall, is that spin doesn't distribute logically; you can't write down an expression like "spin is up in the x direction and spin is up or down in the z direction", after doing some measurements.
p.s. I've been following the discussion but haven't had a lot of opportunity to post anything. Also there are some problems with my ability to connect. I'm back now though, so, hi.
Post by: alancalverd on 04/05/2023 11:03:47
If you look at the Wikipedia article https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment there's a very neat diagram at the bottom ("sequential experiments") of a triple S-G assembly.
You might argue that the first unit is doing some sort of measurement, though it is what I called "segregation" rather than measurement. But the second one is merely rotating the spin axis of the incident beam: there is no element of counting or segregating anything and the output is identical to the input, just rotated.
I think a lot of confusion arises from an elementary "explanation" of indeterminacy, where people talk about measuring the speed and position of a particle by bouncing a photon off it, and argue that the transfer of momentum is responsible for the "uncertainty" of the result. That wasn't what Heisenberg said at all!
Post by: varsigma on 04/05/2023 13:04:28
It seems to me the application of an entity/attribute model to physics theories or experiments isn't optimal.
Instead of for example, arguing about whether energy is a real physical thing, whether you can put it in a bottle or whether it's an entity or an attribute, just say it's part of physics, it has physical units, it's physical.
The concept of work is completely ingrained into our existence, we have to do work all the time in a gravitational field. Can we put gravity in a bottle? Does that question mean anything?
What does the bottle have to be? An entity? What attributes should it have? Isn't gravity an attribute of the universe, so already "in a bottle"?
Or complex quantum probability amplitudes, what kind of bottle can you put them in?
Post by: alancalverd on 04/05/2023 18:59:20
Gravity is a phenomenon with an attribute - it sucks. You might be tempted to call it an attribute of mass, since it is always associated with mass, but dark matter seems to confound that statement!
Work and energy are quantities.
A quantum probability amplitude is a mathematical model.
Post by: Eternal Student on 04/05/2023 22:30:42
It occurred to me whilst driving home yesterday but you beat me to publication!Since it's a friendly forum, it's not as if it matters a lot. I only made the comment since there may have been 1 other person reading it and getting confused for a day.
Your option c is the only one that makes sense and describes the "black box" transfer function of multiple polarisers,...Yes absolutely. A "black box" approach is all I would have tried for if I had continued developing the argument for how a polarising filter works when a photon approaches it. I wasn't entirely sure exactly how I was going to finish that development and make plausible arguments for polarisation being thought of more as a blend of answers "yes" and "no" for whether a photon would pass a filter of a given orientation etc. I was just fairly sure that going straight for a mathematical representation or any argument that polarisation is spin would not be useful for many readers. (I don't know why I worry about that too much, even on a busy day there will be only 2 readers plus a moderator or two who felt obliged to read it).
It seems to me the application of an entity/attribute model to physics theories or experiments isn't optimal.Which seems like a very reasonable statement to me. It's quite different to what was suggested in some of your earlier posts. In much of QM it is very unclear how attributes behave and should be associated with an entity.
However, I'm not claiming to have a better description of what physics is or how you should think about it. Physics is an attempt to explain or model what we (human beings) observe in our surroundings and it can be done in various ways. If it works and is useful, it's OK and you can call it physics.
Best Wishes.
Post by: Eternal Student on 04/05/2023 22:42:46
As for the photographic use of polarisers, it seems like a complicated way of reducing intensity, compared with simply reducing the aperture of the lens,....Aperture may be limited in adjustment (1,2,3,4, units etc.), while polarisers aren't - you can spin them to any angle.
Setting the aperture as wide as possible will help to make sure only what you have manually focused on will be in focus, stuff in the foreground and background gets some natural blur. This allows some artistry in photography. If you had to cut the brightness down by reducing the aperture then everyting ends up in focus.
Best Wishes.
Post by: varsigma on 05/05/2023 00:16:02
If you can't put it in a bottle, it isn't an entityWell, again, what kind of bottle?
What kind of material is the bottle made of?
And, is the universe an entity? If it is, what kind of bottle does it go in?
Post by: Bored chemist on 05/05/2023 08:44:37
Excellent sunglasses for coarse fishing and gliding, but dangerous for driving!Why?
Post by: alancalverd on 05/05/2023 09:34:46
And, is the universe an entity? If it is, what kind of bottle does it go in?A really big one. Or, if you are of a mathematical turn of mind, a Klein bottle, which resolves the dispute between Big Bang and Continuous Creation.
Post by: alancalverd on 05/05/2023 09:43:01
1. You can see below the reflecting surface of still water.Excellent sunglasses for coarse fishing and gliding, but dangerous for driving!Why?
2. It improves the conspicuity of cloud structures, for the same reason.
3. Car windscreens have polarising stress patterns and some instrument panels have anti-glare polarised surfaces, giving you blind spots. Glider canopies in contrast are annealed Perspex with very few stresses, and analog aircraft instruments have plain glass fronts - but beware of LCD displays on all GPS systems.
Post by: paul cotter on 05/05/2023 09:54:30
Post by: paul cotter on 05/05/2023 10:01:13
Post by: alancalverd on 05/05/2023 12:10:47
Post by: alancalverd on 05/05/2023 12:18:31
Quick question concerning the nature of energy: work can definitely be described as a boundary phenomenon, can energy also be categorised as such? ( WAG question ).The conversion between energy and work involves at least a hypothetical boundary, and in most engineering embodiments there is a material boundary between the energy source and the object we want to change, so some energy is "lost" in heating or moving the boundary. I would term that loss as a boundary phenomenon, but stick to considering work and energy as quantities, not phenomena.
Post by: paul cotter on 05/05/2023 13:47:33
Post by: alancalverd on 05/05/2023 14:46:10
Post by: Eternal Student on 05/05/2023 20:14:37
Quick question concerning the nature of energy: work can definitely be described as a boundary phenomenon, can energy also be categorised as such?I don't know, that's something I had to look up. Thanks for expanding my knowledge of stuff. To be honest I'm still not sure what a WAG is (internet says: a wife or girlfriend of a football player).
Anyway, it seems like @alancalverd's answer is reasonable based on what I could find.
In the theory stuff we do often want to talk about an "energy density" which is an amount of energy in some space or in some material. No boundary between any two things is necessary. By contrast we don't talk about a "work density", work is definitely always done on something (on some boundary between two things etc.) rather than just being spread out over a region of space etc.
I know we've had other discussions in other threads about energy. Basically "work" and especially mechanical work is quite easily defined while "energy" is much more nebulous.
I worked for a short time in the video industry and all the lens assemblies we used had continually variable irises....And also relating to @alancalverd 's comments about photography and polarising filters earlier...
I did mention in that post that I hadn't done a lot of editing or polishing. I found a reference to the use of polarising filters as a method of controlling brightness and went with it. It fitted the theory and tied up with everything in the post nicely, linking the theory to some practice. So it's not "wrong" in that at least some photographers did use it - but a bit of poetic licence was applied. When I said "Photographers used to like using a pair of polarising filters...". I really didn't care if they liked it or how commonly it was done, it was needed for the post.
Best Wishes.
Post by: paul cotter on 06/05/2023 11:42:59
Post by: varsigma on 11/05/2023 08:04:25
What do you think of the use of modern day information science in theoretical physics?
If particles in the Standard Model are fundamental, does that mean they are a form of information; are particles like a Shannon message?
Do you think there is a better understanding of information these days, at least classical information?
Given that we know how to compress images and so, erase 'redundant' information while keeping enough so, you know, it's still recognisable? Just a for instance.
Post by: alancalverd on 11/05/2023 12:14:45
Post by: Eternal Student on 11/05/2023 21:26:55
I haven't brought this up yet, but here goes.Actually, it wasn't hard to guess that you were going to talk about "information" in some profound sense:
post #65:
...in terms of the quantum information...post # 22:
...Copying information classically never gives you identical copies...- - - - - - -
What do you think of the use of modern day information science in theoretical physics?In principle, there's no need to draw boundary lines separating "physics" from "computer science" or "geology" or any other science. If something is useful, then it's useful.
Do you think there is a better understanding of information these days,Yes, most fields of knowledge do develop over time, Information science isn't any different from other areas of human knowledge or endeavour.
If particles in the Standard Model are fundamental, does that mean they are a form of information...As I expect you already know there are some recent theories that tie some concepts in physics to some concepts in information science.
Examples:
1. The possibility that Black holes store information on their boundary along with further developments that what we perceive as 3-Dimensional reality could be a holographic projection of information just stored on a 2-D surface.
2. Thermodynamic entropy being linked with Shannon entropy.
Information science is useful when it explains something or allows something to be modelled, it's almost irrelevant if everything in reality is just information or not. Philosophy might be concerned about what the universe really is, physics is just concerned with how things behave irrespective of any terminology you might apply to describe those things.
Best Wishes.
Post by: varsigma on 12/05/2023 00:10:26
I think you need to distinguish between mapping (where you lose dimensionality that can be inferred), minimisation (losing genuinely redundant information) and lossless compression (coding recognised sequences into shorter sequences that fully identify the original).Ok well, I think the concept most people have of classical information is related to our sense of vision, primarily.
It might be why double-slit experiments are given as examples a lot.
So I think about it like this: dots appear that can be seen--the dots are classical. Up close, real close they look completely different. "it looks like a dot" is a far field effect. So far, one dot, one particle and one bit of information.
But lots more dots make up a pattern. So now the idea is compression of this information such that the pattern is conserved.
How many dots are needed to see the interference pattern from a distance? Is it the same number for any kind of particle?
And so on. We know that up close, each particle that leaves a dot actually leaves a lot more than a pointlike mark, but we ignore it. Because we ignore it, that means we see an approximation--an already compressed image--and its a choice we make.
I think its important, with quantum information, to keep track of this thing we do without really thinking about it. It is pretty hardwired.
Post by: evan_au on 12/05/2023 09:06:44
Quote from: alancalverd
beware of LCD displays on all GPS systems.Some years ago I had a car with a small driver information panel. It used LCD technology, with a linear polariser.
- Unfortunately, they had oriented the linear polariser so that if you wore polarized driving glasses, you couldn't read the information display.
- I sometimes have the same problem if I wear my polarising glasses onto the train station - the LCD panels displaying the route of the next train appear black.
This has been resolved on smartphones and newer cars by using circularly-polarised LCD screens. You can still read them with linearly-polarised sunglasses (but you lose a bit of contrast).
Post by: alancalverd on 12/05/2023 22:47:04
How many dots are needed to see the interference pattern from a distance? Is it the same number for any kind of particle?Er, no. If you are thinking about single-photon two-slit experiments, we know that each receiver event involves a single photon with the same energy as the one that left the transmitter: the photon clearly doesn't split and interfere with itself because that would give you two red dots from each blue photon, but what we observe is a pattern of blue dots.
And so on. We know that up close, each particle that leaves a dot actually leaves a lot more than a pointlike mark, but we ignore it.
So how many events constitute a pattern? That is pretty much the same question as how long is a piece of string. The more you know about the cause, the less you need to know about the effect to calculate the entire pattern - a case of fully encoded lossless compression. If I know you have a blue light source and two slits with a defined geometric relation, I can tell you what the interference pattern of an infinity of photons will look like as soon as I have detected just one.
If I know nothing about the slit geometry I will need enough receiver events to indicate where I might find maxima and minima, and the more information (events) I have, the more confident I can be in describing the slit geometry. But some of the events will be redundant repeats, not contributing any more data to my calculation, so it looks as though the error bounds decrease as 1/√N.
Post by: varsigma on 13/05/2023 04:28:03
Er, no. If you are thinking about single-photon two-slit experiments, we know that each receiver event involves a single photon with the same energy as the one that left the transmitter: the photon clearly doesn't split and interfere with itself because that would give you two red dots from each blue photon, but what we observe is a pattern of blue dots.I'm not sure that you got the gist of the question: how many dots are needed so a pattern is recognisable?
If you use a pattern-recognition algorithm and single-particle events, when does an interference pattern exist?
What would the algorithm need to decide this?
Post by: varsigma on 13/05/2023 04:31:36
So how many events constitute a pattern? That is pretty much the same question as how long is a piece of string. The more you know about the cause, the less you need to know about the effect to calculate the entire pattern - a case of fully encoded lossless compression. If I know you have a blue light source and two slits with a defined geometric relation, I can tell you what the interference pattern of an infinity of photons will look like as soon as I have detected just one.I'm talking about doing the experiment. In that case you wouldn't expect to see the same pattern twice, especially any time before the pattern emerges.
The problem here is recognising the interference pattern, so how many particles will do that? That is, given any pattern of dots from any double-slit experiment, how many can be erased so there is still a pattern?
Post by: alancalverd on 13/05/2023 05:52:04
I'm sure ES has a better grasp of formal statistics than I, but the χ2 "goodness of fit" test is rattling about in the recesses of my memory. Essentially, you partition your expected distribution E into "cells", calculate the fraction of the total area in each cell, and count the fraction of actual events A in each cell. Then the sum of the squares
∑x (Ex/E - Ax/A)2
tells you how close your actual distribution is to the hypothetical continuum.
The point at which you announce the result is up to you! My favorite story concerns the Indian Queens Bypass on the A30 through Cornwall. Not sure how accurate it is but this is my recollection from the local School of Mines about 20 years ago: The engineers had asked for 1000 trial boreholes along the proposed route, to determine the subsoil structure. The first two tests struck granite at a couple of meters below the surface, in the middle of the route. Politicians and accountants announced that the subsoil was granite and no more tests were required so land was purchased and construction commenced immediately on that basis. It turned out that these were the only two granite boulders in 30 miles of peat bog. The project ran umpteen years late and God knows how many times over budget.
Post by: varsigma on 13/05/2023 07:23:09
If the laws of physics are constant (which seems to be the case) you would always expect to see the same pattern because the probability distribution will be the same each time.The only issue I have with the phrase "the same pattern" is if, say you have a single-particle beam and stop after say 20 particles. Now replace the screen and repeat, the next pattern of 20 dots will be different from the first.
Quote
The question is how many dots are required to recognise that a sampled distribution is consistent with your theoretical continuous distribution.Yes, and because it is a sample of a statistical distribution you expect different samples to be statistically different.
Even though two interference patterns are in the same class of objects, say. But more generally, two or more interference patterns contain the same information independently of how large the sample. That is, they convey the same "message" with equal expectation.
Post by: alancalverd on 13/05/2023 11:18:48
Einstein said that repeating an action and hoping for a different result is madness, but two random samples of the same thing is not a repeat - definition of randomness! So the question is what level of confidence you require to assert that they are samples of the same distribution.
The power of the χ2 test is it can tell you not only the extent to which your samples may be said to be representative, but also if the fit is "too good" - evidence of a failure of the mechanism, such as a bit of the primary beam getting through your supposed filter.
Post by: Eternal Student on 13/05/2023 21:56:23
I'm sure ES has a better grasp of formal statistics than I, but the χ2 "goodness of fit" test is rattling about in the recesses of my memory....I don't think exactly which test you use is going to be important for answering the OP, the important point is only that it's all about rejecting some hypothesis with some probabilities. You may need an infinite amount of data from an infinite number of repetitions of the experiment before you can conclude with certainty, that the data does conform to the distribution predicted by the model.
Just for completion we need to mention at least one situation where you wouldn't need an infinite amount of data: There may only be a finite number of photons in the universe and time may be discrete and finite. In that situation it's much easier. Just sample it all. Once your "sample" is the entire population then you're done, you have a complete description of the distribution. (I didn't say it was practical).
Best Wishes.
Post by: alancalverd on 14/05/2023 00:09:34
Note the difference between physicists ("consider a spherical cow in a vacuum.......") and mathematicians ("an instantaneous sample of all the photons in the universe......").
Or as others have put it, "Rocket science is two equations. Rocket engineering is a lot more complicated."
Post by: varsigma on 15/05/2023 09:36:55
One is, that philosophy has a problem it seems, with being able to deal with modern physics. One argument I've seen about why this is has to do with philosophy being largely anthropocentric, objects are distinct (you can see their boundaries, you can pick them up etc), they have properties which again are things we experience in the far field, or at much greater than atomic dimensions.
The anthropocentric frame, if you will, is where we observe particle or wave behaviour. But this is when quantum interactions occur which we can't see; except we get the notion that at those atomic dimensions, the physical world must be quite different. Quantum logic isn't like any other kind of logic, and philosophy needs logic.
So why no consistent quantum philosophy? I think there possibly will never be such a thing, and one thing that explains is why you get so many (apparently) different answers from physicists when you ask them "what is a photon?".
Post by: alancalverd on 15/05/2023 14:23:31
On the other hand I'm embarrassed on behalf of my colleagues if they give you different answers to "what is a photon". It is a quantum of electromagnetic energy, modelled as a particle with zero mass. Anything else would have a different name.
Post by: Eternal Student on 15/05/2023 17:39:56
So why no consistent quantum philosophy?A few philosophers are throwing out papers and discussions involving Quantum theory. See, for example, https://plato.stanford.edu/entries/qt-issues/ The online Stanford Encyclopedia of Philosophy, which has a lengthy discussion and cites many other modern articles. It's still a bit too early to know if there will be a consistent quantum philosophy (in my opinion).
However, a lot of philosophy is human centred and human beings don't typically observe quantum effects or build models in their mind of how things work that are remotely like quantum mechanical models. So there is no reason why quantum mechanics should be important for all of philosophy.
An explanation of the nature of morality that involves pages of complicated mathematical equations would be of no use or benefit to most people. It doesn't help them understand their own nature or the nature of things around them.
Let's back this up with some analysis and opinions taken from the paper "The Influence of Quantum Physics on Philosophy", F.A. Muller, first published 2021, an on-line version is available here: https://link.springer.com/article/10.1007/s10699-020-09725-6 .
Concerning Analytic Philosophy, we can take heed of the results of the Philosophical Papers Survey, conducted by David Chalmers and David Bourget (2014; an update and extension is in the making). They asked opinions about 30 controversial issues in philosophy and obtained 3226 responses:
(a list of 30 issues follows including:)
17. Moral judgment: cognitivism or non-cognitivism?
18. Moral motivation: internalism or externalism?
Quantum physics had no discernible influence on any of these debates, full stop. Should it have influenced these debates? For most issues, I don't see what it could have contributed or how it should contribute.
Muller discusses the burgeoning field of "philosophy of physics" and makes it clear that quantum physics has had considerable impact on this. However, with respect to the wider fields of philosophy the conclusion is as follows:
9. Recapitulation:
Although quantum physics has influenced philosophy in the sense that it has grown a new flourishing and blossoming branch of the tree of philosophy, apart from some recent contact between philosophy of physics and metaphysics, quantum physics has had hardly any influence on philosophy at all, and at best some influence on metaphysics, mostly in recent times. With regard to prominent issues intensely thought about by philosophers, such as those on the Chalmers-Bouget list, we dare conclude that it is difficult to see how quantum physics could bear on those issues. If it cannot, it ought not, for ought implies can.
Best Wishes.
Post by: varsigma on 16/05/2023 02:09:12
On the other hand I'm embarrassed on behalf of my colleagues if they give you different answers to "what is a photon". It is a quantum of electromagnetic energy, modelled as a particle with zero mass. Anything else would have a different name.Ok. I agree that most sources will say that a photon is a discrete particle or quantum of the electromagnetic field.
However, if you go to physorg or stackexchange, things are a bit less clear. It depends, apparently to a large extent, on how close you are to a photon as it were. Again with the near-field/far-field distinction, as to what a thing might appear to be.
And generally we represent photons and other particles in a diagrammatic way, and what the diagram looks like depends on the context. In electronics the electric field of some signal propagates at the speed of light; electronic circuits emit radiation in all directions. Photons don't really make sense at radio frequencies, even if radio signals are a whole lot of photons with long wavelengths.
Add to that, physics is by far a done deal, I guarantee there are quantum photon effects we haven't discovered yet, and we've discovered a lot that weren't predicted by any theory. So I think that's one way to gauge our understanding of quantum particles--how many discoveries have there been which were a "complete" surprise?
Given that such "new" quantum effects have appeared unexpectedly, what does that do to any quantum philosophy?
The philosophical problem might be connected to how the theories we have, don't tell us all that much in terms of what to expect. Unlike Newtonian mechanics which generally does.
Post by: alancalverd on 16/05/2023 11:37:45
Is a man the embodiment of a creative and beautiful soul, or 70 kg of water and a few bones? Depends on whether you are dealing with a live musician or a dead one.
"Complete" surprise is rare. A lot of particles were hypothesised because of an apparent breach of the usual conservation rules, and then discovered when we have worked out where to look.
Post by: Zer0 on 16/05/2023 20:27:21
Given that such "new" quantum effects have appeared unexpectedly, what does that do to any quantum philosophy?
The philosophical problem might be connected to how the theories we have, don't tell us all that much in terms of what to expect. Unlike Newtonian mechanics which generally does.
Maybe the Theories propose the idea that We should accept Probabilities, not similar to expecting the unexpected.
Post by: varsigma on 16/05/2023 20:37:06
"Complete" surprise is rare. A lot of particles were hypothesised because of an apparent breach of the usual conservation rules, and then discovered when we have worked out where to look.Ok. I'd say that the Higgs boson and the top quark qualify as discoveries that at least had an expectation of being detected.
But another example: the fractional quantum Hall effect, was not hypothesised although it was detected.
And there are plenty of other examples of quantum effects, that have been detected experimentally but were not expected.
So that could explain to some extent why philosophers struggle with, you know, ontology or objective reality.
How are they dealing with a nonlocal universe? Even understanding what that means has to be a problem for a lot of people. I admit I'm probably one of those.
Post by: alancalverd on 16/05/2023 21:57:20
So that could explain to some extent why philosophers struggle with, you know, ontology or objective reality.Philosophers invent things to struggle with, because they don't have meaningful lives.
In my world people have real problems and I get paid to understand and solve them with physics, chemistry, maths, brute force and duct tape. Very satisfying.
On reflection, radioactivity was indeed a complete surprise, along with the discovery of life in deep ocean vents, Earth's cyclic climate, and a whole lot of stuff that hadn't been seen until someone looked out of curiosity rather than necessity. In that context, the exploration of the moon and Mars hasn't provided much bang for your buck - so far.
Post by: varsigma on 17/05/2023 19:59:12
In my world people have real problems and I get paid to understand and solve them with physics, chemistry, maths, brute force and duct tape. Very satisfying.I would agree that hands on is more interesting than sitting around wondering what everything is, erm, is.
I personally find it frustrating when people don't seem to be able to question their own understanding or even admit they don't, nor does anyone else really, know that much about it after all.
Sure there is all that mathematics and a bunch of concepts, there are things that can go into bottles, and things that can't, but is that it? I shudder when I consider the possibility.
This other "discussion" (another forum) about what the word physical means, as opposed to what a physical thing "is", has problem right off the bat. The universe is full of information, and we as observers decide what it means. Information isn't "encoded" with any meaning (at all).
Assume (ignore any objections here) that Hydrogen is a form of information. To us as observers the existence of this information leads us to theorise that it means stars, in the future. Hydrogen "by itself" tells us no such thing.
How useful is the "put it in a bottle" approach? What is a bottle, first of all?
Is it an entity, then what are its attributes? Is it an attribute because it's a particular shape, the material it's made of is the entity?
Then what does "in the bottle" mean? The material the bottle is made of is "in" the bottle a priori, and material has mass so mass is "in" the bottle. Now you struggle with what the mass of this bottle means, or with the difference between what the bottle is made of and what you can put in the ah, interior, whether this or that is an entity or an attribute--the bottle "has" mass, so it's an attribute of this thing you're calling a bottle.
What a headache.
Post by: geordief on 17/05/2023 22:40:12
What a headacheOr what fun.
Plus ,what is to say that those abstruse considerations may not give rise to practical outcomes eventually?
What of Einstein's proposal that spacetime was curved? How practical must that have seemed to anyone else at the time?
Post by: varsigma on 18/05/2023 21:44:16
Or what fun.Abstruse considerations being the "can it go in a bottle" rule?
Plus ,what is to say that those abstruse considerations may not give rise to practical outcomes eventually?
I haven't been able to see any advantage in doing this. As I stated, what does a bottle already have in it? What does "in" mean?
This attempt to apply a somewhat ill-defined "model" to the physical world I think fails because, for one, it's anthropocentric. It assumes that anything physical is something humans can see and feel.
I think one question that upsets this model is "is an atom a bottle, what does it contain?"
Further, I think any arbitrary distinction at the macroscopic level (it goes in a bottle or it doesn't) is going to have to leave a lot of physics in some kind of twilight zone. We already know that computing with classical information is a completely different physical context than computing with quantum information (i.e. entanglement between particles).
You can't put entanglement in a bottle; therefore . . . ??
Post by: varsigma on 18/05/2023 21:59:54
I've been having a back and forth with someone about this topic, which centres around something Richard Feynman said in one of his lectures. This was that "in physics today, it's important to understand that nobody knows what energy is".
It's important to note that Feynman distinguishes between knowing what a thing is, and understanding it. We certainly do understand how to "use" energy. We build machines (heat engines) that cycle this thing called energy. We understand energy well enough that we can convert it from one form to another. So, it's interesting that we can do all that with a thing we have no knowledge of, in terms of "what" it is.
To my way of thinking, energy could well be an anthropic invention, a very useful one. It can be seen as a kind of physical boundary, or limit. It is undeniably a very useful and fruitful idea. But does it "really exist"?
Does it matter? What does "really exist" mean, anyway (one for the philosophers, perhaps)?
One more detail: Feynman says that energy is a conserved numerical quantity. What's your opinion of that? Do you think he's saying energy is just a number?
If he is, is he contradicting his "nobody knows" proposition?
Is energy a physical thing, or a numerical quantity, and are those two things different?
Oops, I just realised the answer is, nobody knows.
Post by: Eternal Student on 19/05/2023 02:31:42
I'd like to canvas some responses, about the subject of energy and how this is understood.There was a fairly recent (end of 2022) discussion in this thread:
https://www.thenakedscientists.com/forum/index.php?topic=85721.0
I don't suppose anyone would mind discussing it again but there's already quite a bit of information and opinion in that thread. It's recent enough that many of the regulars will remember a little about it.
@alancalverd said this:
In simple terms, there isn't a simple explanation or even a simple definition. Energy is one of the quantities that is conserved in classical physics, and very few adults have any idea what that means.
@Bogie_smiles said this:
In the simplest terms, I have always understood energy as the ability to do work.
@paul cotter said this:
To return to the original question, I propose the following: energy is the capacity to do work with the limitation that in the case of thermal energy some or all( worst case ) will not be able to do useful work.
@Eternal Student (me) said stuff that took a lot of space and I'll just try to summarise here:
1. School level definitions don't really do it (Energy) any justice.
2. Energy is that which appears in Noether's theorem and as such it can only be identified as a conserved quantity in systems with time translation symmetry. For example, in our expanding universe, there isn't a conserved quantity you can call energy. (To condense that severely: What most people think of as Energy is not conserved).
- - - - - -
One more detail: Feynman says that energy is a conserved numerical quantity. What's your opinion of that? Do you think he's saying energy is just a number?Yes, that is pretty much the gist of what he was saying in that lecture.
There were some other lectures discussing symmetries and conservation laws but they are more specialised. He discusses symmetry and conservation laws mainly in the context of Quantum Mechanics:
You see, therefore, the relation between the conservation laws and the symmetry of the world. Symmetry with respect to displacements in time implies the conservation of energy;...
[Taken from Feynman 17-3 between eq. 17.24 and 17.25. https://www.feynmanlectures.caltech.edu/III_17.html ]
In the time of those lectures the relationship was mainly used just in Quantum mechanics but that particular relationship is much more general and not just limited to QM.Best Wishes.
Post by: varsigma on 19/05/2023 10:40:40
Yes, that is pretty much the gist of what he was saying in that lecture.
There were some other lectures discussing symmetries and conservation laws but they are more specialised. He discusses symmetry and conservation laws mainly in the context of Quantum Mechanics:
You see, therefore, the relation between the conservation laws and the symmetry of the world. Symmetry with respect to displacements in time implies the conservation of energy;...
I think you need to be careful about the phrase "numerical quantity" that Feynman uses. My opinion of it is that he's reminding everyone he also used an analogy of counting up 'abstract' children's toy blocks.
Numerical quantity I think should properly belong in the theoretical ballpark because it either means a measured quantity of say, matter, or it means an abstract quantity that has physical units.
It can't be that nobody knows what energy is, but we know it's a number, because . . . we then know what it is.
Post by: Eternal Student on 19/05/2023 14:26:37
I think you need to be careful about the phrase "numerical quantity" that Feynman uses. My opinion of it is that he's reminding everyone he also used an analogy of counting up 'abstract' children's toy blocks.You are free to make of Feynman's lectures what you please, however there are some conventional understandings of what was said and intended. Feynman wasn't really using language or addressing his lectures to an audience of philosophers, he was aiming at scientists and that particular lecture was an early one for the students (in their progress through undergraduate studies). So he was aiming to break some misconceptions from school and provide a good introduction to undergraduate level physics.
Given the audience, it's fair to say that Feynman was attempting to communicate something different when he said "Energy is a numerical quantity". He meant that it is a quantity AND ALSO it has numerical properties.
Most people would assume the natural division was between numerical quantities and non-numerical quantities. Even the word "quantities" is an issue since in the English language one tends to think of something you can count and number. Overlooking that, a numerical quantity is one where a number is assigned to it. A non-numerical quantity is something that doesn't meaningfully behave like a number.
How many people are in this room? How long is this string (in inches)? Those are numerical quantities.
What is the name of this person? What is the colour of this t-shirt? Those are non-numerical quantities.
Numerical quantities follow all the properties you expect of things that are numerical. For example you can order them, 4 (people in this room) is more than 2 (people in this room). You can also multiply and add them meaningfully, a room with 4 people in really does have 2 times the number of people as a room with 2 people in it. Similarly you can add the people from two rooms together and you will get a number of people that is the sum.
At best, non-numerical quantities can be ordered but they do not have all the properties of numbers. Quite often you can't even order them. Example: "Pink" is not more than "Blue", there is no natural order relation on non-numerical data like colours. If you assessed peoples size as "Small", "Medium" or "Large" then that data actually is partially ordered but it still lacks all the other properties of numbers, for example you cannot add small people together and end up creating a medium person. Meanwhile, if you had actually measured the mass of the person, then that would be a numerical quantity.
What Feynman was saying is that Energy has the properties you expect of a numerical quantity rather than being some qualitative or descriptive data only. So, it's not as if at the start of an experiment all we can say is that the total energy was "medium" and at the end of the experiment the amount of energy was also "medium". Energy isn't just any old sort of descriptive quantity, it is a numerical quantity. So you can meaningfully add two energies together AND write a conservation law as a conventional mathematical expression:
The numerical sum of all energies at the start = the numerical sum of all energies at the end.
It can't be that nobody knows what energy is, but we know it's a number, because . . . we then know what it is.It is commonly understood that what Feynman was saying is that all you (his students) should accept about energy is just it is a numerical quantity which seems to be conserved (does not change with time) in all experiments and observations. Just don't make any further assumptions about it - because you can't.
As discussed in an earlier post, Feynman did have more to say about energy in some later lectures. So these are not the last or final word from him but I am confining my attention to the spirit and meaning of just this lecture.
Best Wishes.
Post by: varsigma on 19/05/2023 22:03:45
You are free to make of Feynman's lectures what you please, however there are some conventional understandings of what was said and intended.I've noticed that plenty of people seem to be able to make what they please of what Feynman said in that lecture. For instance, they maintain doggedly that Feynman means energy is a concept. That's what it is in any theory, but what about the physics?
Feynman wasn't really using language or addressing his lectures to an audience of philosophers, he was aiming at scientists and that particular lecture was an early one for the students (in their progress through undergraduate studies). So he was aiming to break some misconceptions from school and provide a good introduction to undergraduate level physics.Yep. I think he was trying to uncover the big secret about physics; theories don't really tell you what physical things are, mathematics is about relations between sets of numbers. I can't see that therefore taking away the idea that he says "energy is a number", when he actually says "energy is a conserved numerical quantity", follows at all.
If we don't know what energy "really" is, and if mathematics doesn't tell us beyond it being conserved (numerically), that's as far as it goes. What about experiments and measurements?
What is a measurement "really"?
Given the audience, it's fair to say that Feynman was attempting to communicate something different when he said "Energy is a numerical quantity". He meant that it is a quantity AND ALSO it has numerical properties.Yes. I would caution though, that that conclusion needs to stay in the theoretical domain. Recall that properties are things that entities have, except so far we have that energy is numerical. Numbers are entities because they have properties or attributes, right? Numbers most certainly don't have a physical existence, all they have is a value.
Perhaps that's why some people like to believe they know what energy is--it's a number. Except that isn't what Feynman actually said.
So, measurement. A physical quantity is measured directly or indirectly and mapped to a numerical quantity so we can do some math. I maintain that the only physical quantity that can be measured directly, is distance. Believe it or not, I've been trying to have a discussion with someone who is convinced that distance, because you can measure it, is a concept. Although the out here is, space is real.
Yeah, doesn't work for me.
Post by: Eternal Student on 20/05/2023 00:08:08
I've noticed that plenty of people seem to be able to make what they please of what Feynman said in that lecture.Yes, that's what people will do when they read a document that is about 3 pages (or listen to lecture of about 1 hour), they will summarise and/or condense it. Different words are bound to be used by them.
If you read that lecture I don't think there is a compact sentence that is held up high as the perfect definition. There isn't a single sentence that you should try to utilize as a self-contained definition of energy. He takes an entire lecture to make his point and provide examples. He is frequently telling you what energy isn't instead of attempting to tell you what it definitely is. Overall, a scientist should have some appreciation of what energy is and isn't by the end of that lecture.
Split the lecture into thirds:
1st section: Smash pre-conceived ideas. Illustrate that all we know is that energy is some abstract numerical quantity.
2nd section: Provide some examples. Illustrate that what we have understood is enough to do a lot.
3rd section: Explain that it is even more complicated than this lecture suggested and even more things are going on.
Example phrases from the 3rd section: There are many other forms of energy, and of course we cannot describe them in any more detail just now. ; independence of time has to do with the conservation of energy* ; we should note that available energy is another matter?.... The laws which govern how much energy is available are called the laws of thermodynamics and involve a concept called entropy
* - In a previous post I've already taken some extracts out of his discussion of conservation laws and time translation invariance. I haven't taken extracts from lectures on thermodynamics or many other things but they are there.
I think he was trying to uncover the big secret about physics;OK, sure. I wasn't looking that deeply. On the face of it, he was just presenting a lecture to educate his students. So trying to introduce them to some of the big ideas in physics or uncover the big secrets etc. is precisely what he was trying to do. Although, in the wider sense, his own motivation for studying and teaching physics may very well have been trying to discover some of the fundamental questions he has himself (why are we here?, what is the nature of our world? etc.)
I can't see that therefore taking away the idea that he says "energy is a number", when he actually says "energy is a conserved numerical quantity", follows at all.OK. Although you asked about it in post #107, so you got a reply.
Do you think he's saying energy is just a number?There is at least one sentence where he does DIRECTLY talk about energy with the word "number":
...it is just a strange fact that we can calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same...
However, I am not advocating that you take just that sentence as a definition, the whole point is that you can't, you need the whole lecture (and more). However, if you did "take away" the idea that energy is a number then that's OK and you're not wrong (although it is only a small portion of what is in the lecture).
If we don't know what energy "really" is, and if mathematics doesn't tell us beyond it being conserved (numerically), that's as far as it goes.As discussed previously. Feynman is not the only source of information about what energy is or isn't. However, just confining our attention to this one lecture, yes, that is pretty much what he was saying here. Well done.
Numbers are entities because they have properties or attributes, right? Numbers most certainly don't have a physical existence, all they have is a value.I'm not sure I can discuss the nature of numbers in a small amount of space and this post has already taken too much time (to write or to read).
Best Wishes.
Post by: varsigma on 20/05/2023 09:31:36
Overall, a scientist should have some appreciation of what energy is and isn't by the end of that lecture.I think it goes further, in that all physical quantities are abstract numerical quantities.
Split the lecture into thirds:
1st section: Smash pre-conceived ideas. Illustrate that all we know is that energy is some abstract numerical quantity.
Mathematics doesn't say much about what measurements are. On the other hand operator algebras with a momentum operator do appear to.
And I think physicists like Seth Loyd or Lenny Susskind would agree that there is definitely a connection between information and energy.
Post by: Zer0 on 20/05/2023 20:33:53
" Matter " .
But of course, what is Matter?
(Doesn't Really Matter)
Post by: alancalverd on 25/05/2023 09:53:41
School curricula tend to be written by educationalists, not teachers, and therefore serve only to confuse the student and put him off "difficult" subjects like physics. Fact is that physics is really dead easy because it is about what happens (dynamics) or doesn't happen (statics) - stuff you see in everyday life. Quite unlike history (deciding which account of stuff you never experienced is less unreliable) languages (the grunts made by apes who look like us but live somewhere else) literature (many books of bad English written about a few lines of good English) or religion (you'd be prosecuted for selling any other product that doesn't work).
Post by: alancalverd on 25/05/2023 10:03:53
At best, non-numerical quantities can be ordered but they do not have all the properties of numbers.Friendly grumble:
I'd reserve quantity for something that can be associated with a numerical value, and quality for a property that can't. Keeps life simple and explicit.
You could of course analyse the spectrum of a pink dawn, but whilst your numbers would mean something to a blind physicist (even to the extent of estimating the sun altitude and the nature of atmospheric dust) "pink" wouldn't convey anything of value.
Post by: alancalverd on 25/05/2023 10:10:15
What is a bottle, first of all?A bottle is something we make (or imagine) to contain something else. It is a member of the set of containers, which includes boxes, cages, and finite bounded universes.
Post by: Eternal Student on 25/05/2023 10:33:43
Friendly grumble: I'd reserve quantity for something that can be associated with a numerical valueAgreed and it was agreed originally.
Even the word "quantities" is an issue since in the English language one tends to think of something you can count and number. Overlooking that....Since Feynamn said ".... Energy is a numerical quantity...", it was much more natural just to change one word in that and have numerical quantities vs. non-numerical quantities rather than changing everything and talking about numerical data vs. non-numerical data.
Best Wishes.
Post by: alancalverd on 25/05/2023 15:43:54
Post by: varsigma on 27/05/2023 12:01:20
What kind of container is needed for information? Do you agree with the Wikipedia article that says information is an abstract concept?
Here's a question: in the military, information about the enemy is relevant during wartime, actually it's relevant in peacetime too.
So if some officer decides the best way to get a dispatch to another friendly base is to print it, put the printout in a satchel, then order someone to deliver it personally, where is the information?
That's a trick question, information is everywhere; but where is the information which is relevant to the officer or to the enemy?
When I was enrolled in a course on finite-state automata and formal languages, I recall saying something about information to the lecturer, which was, we decide when information is relevant, he agreed. Information can still be meaningful but be irrelevant. That dispatch will be relevant, but not after the war is over. For instance.
Maybe I'm just listing some attributes of information, whatever it is. Or maybe I'm using the notions of relevance and meaning to put a fence around it.
Post by: alancalverd on 27/05/2023 23:24:07
Quote
Abstractly, information can be thought of as the resolution of uncertainty.
When the war is over, (or more simply when the coin has landed) there is no uncertainty of the enemy's position and capability. General Schwarzkopf was asked, at the outset of the first Iraq war, "what shall we tell the Press?" to which he replied "Right now, tell them nothing. When it's over, tell them who won." Meanwhile, the object of military intelligence is to minimise uncertainty, so in the case cited, it is the words in transit.
Shannon's mathematical formalism of information theory deals with the number of bits (however represented - notches on a stick, pulses on a telephone line, whatever...) needed to reduce the receiver's entropy to a level, (a) such that the receiver is confident that his interpretation coincides with the transmitter's intention and (b) in the presence of random noise.
A lot depends on prior mutual understanding. "Turn left heading wun fife zero and descend flight level too tree fife" is absolutely explicit and can be heard through a lot of noise, but it presumes that the pilot has a working magnetic compass and an altimeter set to standard pressure, knows left from right, and indeed how to turn and descend an aircraft. The problem with the Hollywood emergency talkdown scenario is that the recovering alcoholic who has just occupied the right-hand seat after the captain has died, doesn't know which dial is which, or what knob makes the plane go up and down, so the instruction hasn't altered his entropy very much. By Shannon's formalism, information content is not determined solely by the transmitter!
Post by: varsigma on 28/05/2023 00:39:14
In physics you manipulate symbols using mathematical logic, you do experiments which are effectively information-gathering exercises.
So an experiment to determine Newton's constant: you have a context, perhaps weights attached to vertically suspended springs. You measure "displacements", you also need to know some other stuff like the mass of the earth, and its radius to calculate a value for G.
But what is it? It is relevant to the force between two masses separated by a distance, but what does it mean? Does it even qualify as information, or is it something else?
Post by: geordief on 28/05/2023 00:53:17
Suppose a sentient being received some sensory input ,is there a maximum amount of physical sensory inputs it must receive so as to output something like a work of art(maybe extremely primitive)?
Is there a correlation between the physical input and the mental output or can a minimal physical input produce an unrelated large mental (informational?) output?
Post by: varsigma on 28/05/2023 01:31:14
I never really appreciated that information could be a learning discipline in itself.I took a course in communications, and I was surprised that information has entropy. At the time I figured it was something somebody borrowed from "real" physics. But what did I know?
Post by: varsigma on 28/05/2023 02:13:43
Suppose a sentient being received some sensory input ,is there a maximum amount of physical sensory inputs it must receive so as to output something like a work of art(maybe extremely primitive)?I'd try asking ChatGPT about it. What would you need to tell it, in descriptive terms, so it outputs the required work?
Is there a correlation between the physical input and the mental output or can a minimal physical input produce an unrelated large mental (informational?) output?I'd say that depends on what the sentient being knows already.
Post by: geordief on 28/05/2023 03:49:38
I'd say that depends on what the sentient being knows alreadyYes, I thought that too.The brain creates it's own inputs.Any external input goes through a huge amount of processing before anything like an output can be observed.
In fact I don't know if it is possible to correlate an individual input with an individual output.
Post by: alancalverd on 28/05/2023 11:27:35
I'd try asking ChatGPT about it. What would you need to tell it, in descriptive terms, so it outputs the required work?"Required" disqualifies it from being art. The more closely the customer specifies the end product, the more the process becomes engineering rather than art.
Going back to my air traffic example, "left135 FL235" is a precise specification to line up with the runway and avoid other traffic, to be carried out to the letter and not surprise anyone. "Amaze the crowd with some freeform aerobatics" is a request for an artistic display of the same skills, though the sensible controller still requires the performance to be contained within an allocated box of sky.
Post by: varsigma on 28/05/2023 11:58:14
Here it is: can you put information in a bottle by itself, and detect it?
There is no explanation of what information "by itself" is. My answer would be the bottle is information, so it already has information in it, namely that it is a bottle, a particular arrangement of material which--informs you of its functionality. The shape (a pattern) tells you you can put stuff in it. It says what "bottle" means.
Information by itself is an idea of the nonexistence of information, I suppose. What's your take on this somewhat psychiatric ward question? It suggests a serious misunderstanding of what information actually is. A bottle by itself with no humans around to detect it would be what? Undetected information? Is a mind required here?
Post by: alancalverd on 28/05/2023 14:48:19
Slightly more practical, the Dead Sea Scrolls (found in jars) tell us a lot about the siege of Masada that we didn't know previously, but still follow Shannon's requirement of prior mutual understanding, in this case how to read ancient Hebrew.
More abstractly (assuming this is what the questioner thought he was asking), information requires a carrier and a terminator. A bottle serves as both. In the air traffic case, the terminator is implicit because a real pilot (not the Hollywood drunken hero) expects to receive (and acknowledge) two 3-digit numbers in a specific order.
There are classic tales of unterminated data streams such as "send me ?500 or else...." which raises the temperature (entropy) of a relationship if it is not terminated by ".... collect the goods yourself for ?400". So it's a good idea to put a cork in the figurative bottle.
Post by: varsigma on 28/05/2023 19:29:08
More abstractly (assuming this is what the questioner thought he was asking), information requires a carrier and a terminator. A bottle serves as both.Actually I thought the questioner believed their question was a way to show that information is a concept, it's not something you can physically put in a bottle. This person is either a bit crazy or doesn't understand what information is.
Information by itself is a phrase that contradicts the fact that it needs a carrier, as you say. And if I start with a lump of ordinary glass and end with a bottle, the information that it is a bottle is self-evident.
So I have put the information "this is a bottle made of glass" into the bottle. I think this questioner would reject my "hypothesis", given they style themselves as a philosopher,
Can I put information in a bottle by itself and detect it? Can I detect a bottle if the bottle is by itself? Where is a bottle when it's by itself? Yikes.
But I can keep asking questions about "the information" to try to lower my uncertainty about what the questioner is asking. Heh.
Post by: varsigma on 28/05/2023 21:35:26
Shannon explains the question this philosopher posed, and its uncertainty. It's the kind of question a philosopher might believe is an important examination of the nature of information--they presume the answer is no. (Who cares)
But the question contains information and all information has an entropy. The phrase "by itself" can be queried. Is the question "by itself"? no it isn't. Is any part of the question "by itself", evidently not.
Nothing in the universe, apparently, is by itself, everything is connected.
Can I put a label on a glass bottle that says "this is a bottle made of glass", without changing any meaningful information-about-bottles? I'm implicitly communicating with someone or some thing that can decode what's on the label. Without the label, the shape and the glass communicate some information to an observer.
So the question turns into quite a good example of what Shannon was talking about, back when.
Easy peasy. And yes I think philosophy sucks too.
Post by: varsigma on 28/05/2023 22:09:13
A pair of informations go into a bar. The bartender asks, "You're by yourself, you two?"
Post by: alancalverd on 29/05/2023 11:26:12
I think this questioner would reject my "hypothesis", given they style themselves as a philosopher,You have correctly defined a philosopher as a person who makes a living by telling you that you don't understand what you just said. A narcissist with no redeeming features.
Post by: Zer0 on 29/05/2023 19:40:14
Post by: varsigma on 30/05/2023 15:38:39
Leonard Susskind says information is entanglement. Entanglement is another well-misunderstood term.
If I think about what he's saying, that implies electrons are entangled with protons in ordinary atoms: they are "charge-entangled". Where is the information? How do you measure it.
There's another word that you need a good understanding of the meaning of. The protons and the electrons in orbitals are measuring each other, just not classically.
If you want classical measurements you will need some classical equipment; today you can scan your solid state of choice with a scanning-tunneling electron microscope. You recover classical information because the measuring device has a much larger phase space than the sample being scanned.
Post by: alancalverd on 30/05/2023 15:52:05
We use search dogs to solve the inverse problem.
Post by: varsigma on 30/05/2023 16:01:11
The information is the position of an electron. You have no idea where a free electron might be found, but if you know the location of a proton you have significantly reduced your uncertainty.Electron position has more entropy than proton position because of the mass difference (I guess). But classical measurements sort of smear this out, because classical measurements are always a representative sample.
When you say free electron, do you just mean not bound to a proton? As in, flowing in a conductor?
Sorry, I see you were just commenting on the different states, and how a bound electron has less entropy because protons or atomic nuclei, in general, are easier to locate. I'm recovering from a cold so a bit woolly.
Post by: varsigma on 30/05/2023 17:41:29
These patterns might be seen in the activities of enemy communications, even if what is being communicated isn't known. Information with a high degree of uncertainty because it's encrypted is commonly sent and received in military contexts.
Patterns are physical, information in patterns is necessarily physical too. ChatGPT also agrees that the shape of physical objects is one of these physical patterns.
An ordinary glass wine bottle say, "remembers" its shape. It is a store of information even when its empty.
I can't understand why anyone shouldn't see this obvious thing. But it seems some people just can't.
Post by: alancalverd on 30/05/2023 19:12:22
There are classic cases of ENIGMA and TUNNY decodes that depended on human stupidity such as always finishing with HH, sending the same message twice with the same seed, or the interceptor knowing that weather readings are always transmitted on the hour, but I doubt that such patterns occur much these days - everyone has read the book!
Post by: varsigma on 30/05/2023 22:51:16
ChatGPT tells me the information is the meaning; the information content is how much meaning, in that case.
Then I asked about encryption, where the idea is to maximize the entropy of "the information" so its as uncertain as possible.
To have meaning, a message has to be encoded somehow, just as languages like English encode this thing called meaning. So if information is meaning and information content is how much meaning, what are the letters and the words, and the sentences etc, in a natural language? What are the ones and zeros in a binary string with an unknown context (I found it in a computer memory)?
ChatGPT tells me that the meaning is context-dependent. So there is no context-free information. Uh huh.
So what meaning can I assign to a randomly located binary string, other that "it's a string of 1s and 0s". Is that devoid of meaning?
Anihoo, I can see that ChatGPT appears to line up with most of what (I think) I learned in that course. One thing I recall is the surprise factor--a message you don't expect has more information content than a message you do expect.
But unexpected messages are generally less probable, in an ordinary world (most of the information your brain processes is familiar--you expect to see a world that looks like it did the day before, more or less). Hence--entropy.
Post by: alancalverd on 30/05/2023 23:40:34
Strictly, of course, a code is a string that identifies a longer string stored in the receiver, whereas a cipher simply substitutes one symbol at a time so that the decrypt contains the same number of bits or whatever as the encrypted string. Thus "BMBO" is a simple cipher for "ALAN", but "ALAN" is a code for "an old geezer in Cambridge with nothing better to do with his time".
The unexpected message will only contain information if the receiver has some preconception of what the sender means. Thus ALAN or even .- .-.. .- -. will probably denote at least "a British bloke" to many earthlings but conveys nothing at all to a Martian tree frog.
Post by: varsigma on 31/05/2023 21:01:17
ChatGPT seems like a longwinded philosopher rather than someone who actually understands information theory. Try Wikipedia - the entry seems to have been written by folk who know what they are taking about.Thanks. Are you really an old guy living in Cambridge, and is that the US or the UK? I had online dealings with a person who was studying string theory at King's College. Bit of a maths rottweiler.
Had an interesting chat once about superfluids and viscosity.
Yes I'm cautious about ChatGPT, and I've managed already to get it to contradict itself about entropy. Such is AI, it comes with a warning label. Actually I've written a few programs that I should have put a warning label on.
As to philosophy, it seems some people think it's about discussing how uncertain you are, how little you can frame in ordinary language. I've read philosophical tracts that were pretty much impenetrable; they appeared to presume that "philosophical meaning" can be packed into a single word so it contains more meaning somehow, than what you find in a dictionary.
Supervenience, for example. The capacity to, something something.
Whereas, in physics the philosophy is start simple, then treat complexity as a sum of simple things, not as a single concept which is, impenetrable, not known, yada yada. Then the philosophers only have ideas, everything is conceived, so if you can't conceive of it, it fails the test. What test? Who said there's a test?
Post by: varsigma on 31/05/2023 21:15:55
Perhaps it's why I find the Information Theory approach a convenient way to look at complex systems.
Binary logic is simple, you can derive nice, hard facts like, it isn't a universal logic, computationally, because it isn't reversible--the laws of physics are though. You get to see that it's because of an insufficiently large phase space (volume) and how to increase the volume by adding more inputs and outputs, solving the universality "problem".
Which of course, engineers don't bother doing because of the cost, in keeping all that information around,
Post by: alancalverd on 01/06/2023 00:33:46
You see, my friend, it all depends on what you think you mean by Cambridge. Is there a universal meme that can be deconstructed as a set of paradigms sufficiently delocalised in spacetime that your Cambridge and mine are the same but not identical, or identical but not, in whatever sense you think you are talking about, the same? In what sense does the Pythagorean essence of Cambridge heuristically or existentially conflict with the Aristotelian ur-Cambridge such that they cannot coexist?
I could say that in a few minutes I can walk across a bridge over the river Cam, but a philosopher would ask "how do you know that it is really you, and whilst the river was named by the Saxons, since the water that was there at the time is no longer there, is it still the Cam?
Post by: geordief on 01/06/2023 00:57:29
Last time I looked, I was definitely an old bloke, and AFAIK this Cambridge is cold, wet, and near the East Coast, just like the other one.Does the idea of a convention have import when discussing what "information" is?
You see, my friend, it all depends on what you think you mean by Cambridge. Is there a universal meme that can be deconstructed as a set of paradigms sufficiently delocalised in spacetime that your Cambridge and mine are the same but not identical, or identical but not, in whatever sense you think you are talking about, the same? In what sense does the Pythagorean essence of Cambridge heuristically or existentially conflict with the Aristotelian ur-Cambridge such that they cannot coexist?
I could say that in a few minutes I can walk across a bridge over the river Cam, but a philosopher would ask "how do you know that it is really you, and whilst the river was named by the Saxons, since the water that was there at the time is no longer there, is it still the Cam?
Is the idea of a convention the mirror image of solipscism?
In philosophy do we,as conscious beings travel.through time in the same way as a physical object has its worldline in.the "real" world?
Ps ,what on earth is ur-Cambridge when it is at home?
Post by: varsigma on 01/06/2023 06:57:12
Does the idea of a convention have import when discussing what "information" is?Information is what we say it is. But you have to choose a physical basis for it in order that it is "representative".
Why I say the above is because, in the classical domain there are any number of choices for a representation.
Quantum mechanics has fewer choices, perhaps because at that scale nature restricts our choice and so it restricts what we can call it. Maybe.
Conventions, like using bra-ket notation have no real effect on what quantum information "is", since mathematics can't really tell you that. It does tell you about unitarity and the conservation of information though. Whatever it "really" is.
I'll see if I can explain the above with the usual suspect, an interference pattern from a beam of particles.
The pattern is independent of whether you do it one particle at a time, or lots of particles. Either way you see the same pattern and so you see the same information, it's up to you to decide what the context is.
But the reason the pattern is the same, however you sample the distribution in terms of number of particles at a time, is because of unitary logic and conservation laws. So each sample must be representative, even if the sample has one particle in it.
So the question "when does a pattern appear" which is recognizably an interference pattern, is a classical question about a classical pattern, each dot is a classical measurement. We decide when it looks like an interference pattern, or when there is enough information content.
The rule of thumb is that information reduces uncertainty; in this case the information is equivalent to answering the question, how many particles will form a classical pattern? But the number and the pattern aren't the same information, right?
So, context. Seeing a classical interference pattern of dots, each made by a single particle, is in the same philosophical domain as seeing a binary string with a pattern in it. What does the pattern mean?
Post by: alancalverd on 01/06/2023 10:56:30
Ps ,what on earth is ur-Cambridge when it is at home?It is Cambridge, at its original home!
Post by: alancalverd on 01/06/2023 11:06:37
We decide when it looks like an interference pattern, or when there is enough information content.I've just been learning about "hyperuniformity", particularly in the case of the distribution of prime numbers. Humans are very good at discerning patterns but unlike pigeons (who are brilliant at it) we tend to see patterns where by mathematical proof (prime numbers) or statistical sampling (pseudoartefacts on medical images), none exist.
Thus many people think there is a disjuncture between classical and quantum physics because you can't explain the gross interference pattern in terms of particles and you can't explain the distribution of individual events in terms of waves. It's all a matter of vanity: fact is that some folk like to imagine that reality is governed by their preconceived models, but this isn't economics, politics, religion or philosophy - it's science!
Post by: varsigma on 02/06/2023 00:09:05
Thus many people think there is a disjuncture between classical and quantum physics because you can't explain the gross interference pattern in terms of particles and you can't explain the distribution of individual events in terms of waves.Waves and particles are things we inject into the experiment, we attempt to use that context so we can understand the effects we see.
They are observations we make from a distance, about how the pattern is encoded in space and time, by some function. It must be a function because it maps particles to locations, and this appears to be procedural; there is an algorithm in there somewhere.
Another thing about experiments with very small things is, no detail is insignificant. After a particle leaves a mark, it leaves altogether--you can know very little about its state, including where it is. This is connected to our notions of writing information and erasing it. That is, for a particle to write a mark on a detector screen it has to vanish after this event. It's a quantum requirement, say.
In terms of entropy, the time of appearance of a dot reduces classical uncertainty about the position, but at later times the uncertainty is maximal, relative to the frame of the experiment (and its physical basis for the information you decide is there somewhere).
Post by: alancalverd on 02/06/2023 08:25:00
I'll allow "inject" as a colloquialism in this context: I think you mean "assign to the model of" an experiment. When we actually inject energy or stuff, the object is to see what happens next - understanding may come later.
Procedures and algorithms are predetermined processes through which we force data or real stuff. There is nothing predetermined about the fate of a photon in the double-slit experiment, and there can't be: the outcome of any given event is essentially random.
On a grumpy day, I'd even reject "quantum requirement"! Quantum mechanics is how we describe and predict what happens. The fact that we can correctly predict an outcome, is a measure of the validity of the model, not a god-given and preordained demand as to how nature should work.
"Classical uncertainty" is the sum of random and systematic experimental errors that estimates the size of the ballpark that contains the truth. Not to be confused with Heisenberg's indeterminacy, which is an inherent property of the entity, nothing to do with error, and denies the existence of a single truth.
It has been argued that the progress of modern science actually rests on a religious belief that the universe was created and run by a consistent directive, and science is merely a search for the creator's plan. I deplore such arrogance. As I see it, by inventing and polishing mathematical models of what we know, we get better at anticipating the outcome of experiments that we haven't yet done.
Post by: varsigma on 02/06/2023 12:10:12
Oh dear! I think you are still not distinguishing between observations and models.Yes perhaps I should have said that we decide when we are observing wavelike effects and when we aren't.
I'll allow "inject" as a colloquialism in this context: I think you mean "assign to the model of" an experiment. When we actually inject energy or stuff, the object is to see what happens next - understanding may come later.
Procedures and algorithms are predetermined processes through which we force data or real stuff. There is nothing predetermined about the fate of a photon in the double-slit experiment, and there can't be: the outcome of any given event is essentially random.
I'm more trying to go with the basics in any interference experiment; what is known and what isn't.
This should apply beyond the start or end of the experiment itself. Information is meaning, so what do random or regular patterns of dots mean? One thing it means is you see a symmetrical pattern--an interference pattern with a central maximum and less intense fringes each side--so you don't need the whole thing, half of it will do. You can't do that with a low intensity input beam because it isn't, you know, a large enough sample.
I'm not sure about the algorithmic aspects, but I can't explain then what connection a double-slit experiment might have to quantum computation. I'm pretty sure there is a connection.
Post by: alancalverd on 03/06/2023 15:28:37
what do random or regular patterns of dots mean?The dots arrive at random times, and the position of the next dot cannot be predicted from past events, but every time we repeat the experiment for long enough we get the same overall distribution.
Post by: geordief on 03/06/2023 15:48:20
And what is "long enough"?
Two dots? Three?
Is the " overall distribution" always there but we only see it above a certain number?
Post by: alancalverd on 03/06/2023 22:19:04
Pattern recognition is an odd business.Humans tend to see "hyperuniformity" - imposing patterns on data where none exist, because we have a preference for geometry. This has some advantages, say in navigating through a remebnered but now modified landscape, because we can ignore a few anomalies, but it can lead to errors in making very confident predictions that turn out to be completely wrong. Simple example: I say 1,3,5... and you say 7,9.... Wrong! I was enumerating primes, so the amswer is 7,11... and whilst it can be proved that there is no pattern to the occurrence of prime numbers, they do tend to occur in sort-of-periodic clumps!
Post by: varsigma on 03/06/2023 23:23:13
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One of the first observations if not the very first one of the implications of the quantum mechanics to the computational complexity was made by a most famous physicist, Nobel Prize winner Richard P. Feynman, who proposed in his seminal article [8] that a quantum physical system of R particles cannot be simulated by an ordinary computer without an exponential slowdown in the speed of the simulation. On the other hand, the simulation of a system of R particles in classical physics is possible with only a polynomial slowdown.--https://www.utupub.fi/bitstream/handle/10024/162051/Hirvensalo_InterferenceV3.pdf?sequence=1 (https://www.utupub.fi/bitstream/handle/10024/162051/Hirvensalo_InterferenceV3.pdf?sequence=1)
The main reason for this is that the mathematical description size of a particle system is linear in R in classical physics but exponential in R according to quantum physics. As Feynman himself expressed:
. . . But the full description of quantum mechanics for a large system with R particles is given by a function ψ(x1, x2, . . . , xR, t) which we call the amplitude to find the particles x1, . . ., xR, and therefore, because it has too many variables, it cannot be simulated with a normal computer with a number of elements proportional to R or proportional to N . [8]
Number N in the previous citation refers to the accuracy of the simulation: the number of points in the space, as Feynman formulates. In the same article, Feynman considered the problem of negative probabilities, and returned to the same issue a couple of years later [9]. Feynman's approach may be earliest formulations to understand the role of interference in the probabilities induced by quantum mechanics.
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Following Feynman's idea and using quantum mechanical systems for bearing the information and carrying out the computation, it is possible to design algorithms that benefit from the interference: the undesired computational paths may cancel each other, whereas the desired ones may amplify.--ibid.
This phenomenon is generally believed to be the very source of the power of quantum computing.
I myself would classify the appearance of an interference pattern, in single particle or low particle number experiments as a computation. A computation of what, apart from what can be seen, is in the details I guess.
Simulations of quantum interference are around; how do these differ from the real deal?
Post by: varsigma on 04/06/2023 02:42:34
You also encounter the fact that there aren't any fixed ways to describe physical stuff like light.
The photon, the Standard Model asserts, is an exchange particle that carries the electromagnetic force.
That is actually quite a loose definition or description. A photon is a gauge boson; the electromagnetic field is a connection in a fiber bundle over spacetime. A photon is representative of the field in that it has the same dimensions as the field. An electromagnetic force is the analog of a Newtonian mechanical force; there's a symmetry in the equations of motion for an LRC circuit and a simple pendulum.
This only tells you that Newtonian momentum has an electromagnetic equivalent, which is inductance multiplied by the current. There is a correspondence between Newtonian oscillations and Maxwellian ones. Ok, but what does it mean?
I don't know if I can say, or if anyone can. It suggests that mass is a way to store energy though. If inductance represents a way to. Mass curves spacetime around it, magnetic potential does that for electrons.
Right, so the electromagnetic field is the gauge field or U(1) symmetry, of the fermion field or SU(2) symmetry in the first family of the Standard Model. Another detail here is you can't break the gauge symmetry of a theory, or then it's unphysical.
I've studied group theory and I'm still wondering why there are so many, but only a handful in the best model we have of fundamental physics.
Perhaps the answer is the universe needs these ones to make itself classical, with complexity. Entropy being one of the complexities, meaning is another complex thing. Is that only relevant to sentient life, and so on.
Post by: varsigma on 05/06/2023 03:45:17
Is there any other way we can reproduce that effect?I think I can give a tentative answer to your question, which is, it depends.
And what is "long enough"?
Two dots? Three?
In experiments that demonstrate interference, it's nice to see a pattern that we can say is definitely there.
But in quantum computers, the wavefunctions of two particles can be in superposition, such that it's a form of constructive or destructive interference.
We arrange for this to happen that way, and so it must have a nonzero probability of occuring in that case.
Post by: varsigma on 05/06/2023 05:36:47
Newtonian mass is defined in terms of a resistance to being displaced from rest, or from regular motion.
I'd say that since Newton the heuristic has changed some. Nowadays there seems to be a difference in meaning between a heuristic and an ansatz, which can mean in German, an educated guess. Which is what heuristic means.
But scientists like to borrow words and give them a closely defined meaning, so ordinary usage is excluded. A mass ansatz is more a way to employ a technique, mathematically, to solve a certain kind of problem.
For instance, a photon with a large energy can decay spontaneously into an electron and a positron. The gauge transformation conserves charge and mass; the electron and its antiparticle are a (complex) conjugate pair whose product is the required mass ansatz--the right tool.
Post by: alancalverd on 05/06/2023 08:46:49
And beware of "required": it is a technical term in mathematics that constrains our models, but nothing is "required" in physics - it either happens or it doesn't, and if it doesn't, we have to change our model.
Post by: geordief on 05/06/2023 11:20:25
So,is that to say that when we have just two.(or even one?) dots on the screen they can be graphically represented as an interference pattern even though our own optical system (the eyes and the brain) do not process it that way?Is there any other way we can reproduce that effect?I think I can give a tentative answer to your question, which is, it depends.
And what is "long enough"?
Two dots? Three?
In experiments that demonstrate interference, it's nice to see a pattern that we can say is definitely there.
But in quantum computers, the wavefunctions of two particles can be in superposition, such that it's a form of constructive or destructive interference.
We arrange for this to happen that way, and so it must have a nonzero probability of occuring in that case.
Eventually, with enough dots we do see the underlying interference pattern "with our own eyes "?
Would that be a standard interpretation?
Hopefully not to cloud the issue(an ironic analogy,perhaps) ,but if optical measuring instrumentation was sufficiently sensitive might it ,in theory pick up directly the interference pattern from one ,two or three dots?
Post by: Eternal Student on 05/06/2023 14:11:45
I seem to have a bit of time and have tried to read some of the later posts. I thought I'd start with some questions.
the electromagnetic field is a connection in a fiber bundle over spacetime.I'm willing to put my hand up and say that I don't understand that sentence. I'm no expert and not too worried about looking ignorant - it was ambiguous and confusing to me.
The main problem is understanding what is meant by a "connection" and the possibility that "a fibre bundle over spacetime" meant the usual one associated with general relativity. A "connection" would then be understood as an "affine connection". However, you probably didn't mean that but just used the word "connection" to imply some relationship or link between elements of a fibre bundle that just has 4-D spacetime as a base space.
Overall, I'm not really sure what that sentence meant. If the sentence was important then it needs some references or further development.
The next sentence is equally uninformative for me:
A photon is representative of the field in that it has the same dimensions as the field.What dimensions does the field have? Do you mean length, time, mass - those sorts of dimensions OR the dimension of some linear space (the number of vectors in a basis etc.) OR a measurement of just space occupied ( 3 cm x 10 cm x 12 cm ) OR something else? i.d.k.
Best Wishes.
Post by: Eternal Student on 05/06/2023 15:05:30
Your discussion of LRC circuits and analogies is interesting and many textbooks will exhibit that analogy. Indeed, it is commonly said that as a student of physics all you will do is study the harmonic oscillator in ever increasing levels of complexity and detail. They are ubiquitous across most of physics.
So rather than just go along with that I thought it would be worth taking a moment to consider a different viewpoint: Perhaps it is mainly that human beings go looking for such simple relationships.
An electromagnetic force is the analog of a Newtonian mechanical force; there's a symmetry in the equations of motion for an LRC circuit and a simple pendulum.There is some analogy between how a LRC circuit behaves and how a pendulum or harmonic oscillator behaves but you've been very human in finding this analogy. You've ignored the thousands of electrical circuits and situations where Current was not like velocity and/or there wasn't anything like mass or inductance. Instead you've identified a situation where analogies can be made, assumed that would be important and run with it. That's ok in that sometimes these analogies and ideas will lead somewhere and turn out to be very useful in very general situations. However, sometimes they remain just happy co-incidences - an analogy that existed for that one situation only.
Given enough time human beings will identify ways in which any two things are similar or analogous. The way my dog eats spaghetti is going to be analogous to the way the moon orbits the earth, I just need to find some quantities I can calculate or measure which will show a suitable correspondence (and ignore all the things I could measure that don't). For example, the square of the orbital time period is proportional to the cube of the semi-major axis. Given enough time and effort we will find something about the spaghetti eating that will have the same correspondence.
This only tells you that Newtonian momentum has an electromagnetic equivalent, which is inductance multiplied by the current.Does it? It's an analogy you can make for an LRC circuit but not necessarily for other electrical circuits or for all things involving electromagnetism.
For example, an electromagnetic field does contain a perfectly conventional Newtonian momentum, it has to be there and it has nothing to do with a current being observed anywhere. If you considered current as a quantity analogous to a velocity then this means there can be a non-zero momentum where there is zero velocity (i.e. the analogy seems to break down).
When you started this thread you ( @varsigma ) were interested in a Feynman lecture, so you might like section 27-6 of the Feynman lecture documented here: https://www.feynmanlectures.caltech.edu/II_27.html
In that lecture it discusses precisely how much momentum is contained in the E and B fields. In any region with non-zero E and B fields, there is a momentum density g = (1/c2 ) . E x B . That's all we need - just a non-zero E and B field, no current has to flow from somewhere to anywhere for momentum to exist in the electromagnetic field. (Minor note: It's commonly said that the momentum is "in the field" but I would prefer to say only that it is in the space permeated by those fields - i.e. avoid suggesting that the fields could hold it, just that momentum is in the space somehow when the fields are in that space).
It is possible to analyse and think about the LRC circuit in terms of how it changes the momentum of the space around the inductor and between the capacitor plates. As you know, the E and B fields are changing in those components, so the momentum in the space around those components is changing (see the Feynman lecture above). The pendulum had a mechanical component (a mass or bob) which shows a change in momentum as it swings back and forth, the LRC circuit has space (or the fields within that space) as the equivalent component showing a change in momentum. This might take a moment to think about, so I'll say it again: When the E and B fields change, the momentum in the space is changing, so it is exactly like the electrical components are applying a force onto space - pulling and pushing it.
Now, as mentioned earlier, we human beings will eventually identify a similarity or analogy between any two phenomena if we spend enough time and ignore all the things that just didn't show the correspondence we wanted. I wouldn't exclude my own suggestion from that. I think it's interesting to imagine an LRC circuit as being exactly like a pendulum - it just has space as the component swinging back and forth instead of the pendulum bob - but I wouldn't encourage anyone to run too far with that idea. It works nicely for this situation.
Best Wishes.
Post by: alancalverd on 05/06/2023 17:09:03
if optical measuring instrumentation was sufficiently sensitive might it ,in theory pick up directly the interference pattern from one ,two or three dots?Easier with x-rays, but the answer is yes, we can detect individual photons, and they do indeed arrive at random with the spatial probability distribution as calculated from the continuum-wave analysis.
Post by: alancalverd on 05/06/2023 17:16:57
it is commonly said that as a student of physics all you will do is study the harmonic oscillator in ever increasing levels of complexity and detail.Spoken like a mathematician!
Never mind the spherical cow in a vacuum, I once said "physics is a trivial particularisation of mathematics" and then spent 10 years calculating, designing and building just one particularisation of physics, based on a thermal diffusion equation, not a harmonic oscillator!
Post by: varsigma on 05/06/2023 20:10:41
Not complex. They are both real particles with charge and mass.In field theories particles have complex probability amplitudes.
Post by: varsigma on 05/06/2023 20:23:51
When you started this thread you ( @varsigma ) were interested in a Feynman lecture, so you might like section 27-6 of the Feynman lecture documented here: https://www.feynmanlectures.caltech.edu/II_27.htmlYes, there you describe the free field; in an LRC circuit the evolution of the field is constrained. It's an interesting exercise finding the correspondences; although you can say there's an electromagnetic momentum which is "equivalent" to Newtonian momentum in the context of field oscillations, it's far from a unifying principle. But it's still interesting that, in order for oscillations to appear certain constraints are needed. Certain physical things need to be fixed in place.
In that lecture it discusses precisely how much momentum is contained in the E and B fields. In any region with non-zero E and B fields, there is a momentum density g = (1/c2 ) . E x B . That's all we need - just a non-zero E and B field, no current has to flow from somewhere to anywhere for momentum to exist in the electromagnetic field. (Minor note: It's commonly said that the momentum is "in the field" but I would prefer to say only that it is in the space permeated by those fields - i.e. avoid suggesting that the fields could hold it, just that momentum is in the space somehow when the fields are in that space).
Post by: varsigma on 05/06/2023 20:37:59
The main problem is understanding what is meant by a "connection" and the possibility that "a fibre bundle over spacetime" meant the usual one associated with general relativity. A "connection" would then be understood as an "affine connection". However, you probably didn't mean that but just used the word "connection" to imply some relationship or link between elements of a fibre bundle that just has 4-D spacetime as a base space.The connection and its curvature is the field. Sean Carrol explains what that means to some extent in one of his online lectures. (it's number 15 Gauge Theory)
There's a good article in an old issue of SciAm, by Bernstein and Phillips, that discusses the geometry and the topology of a gauge field. The article starts out explaining what a fibre bundle is and how to construct one for the sphere.
Then the geometry of a gauge field is seen in neutron spin precession in a magnetic field, and the topology is seen in the Aharonov-Bohm effect. Both experiments "exploit" interference.
The first experiment is much simpler in terms of the required analysis, as the the path lifting rule is pretty obvious. Which suggests the geometry of the gauge field is less complex than its topology.
Post by: varsigma on 05/06/2023 20:58:22
What dimensions does the field have? Do you mean length, time, mass - those sorts of dimensions OR the dimension of some linear space (the number of vectors in a basis etc.) OR a measurement of just space occupied ( 3 cm x 10 cm x 12 cm ) OR something else? i.d.k.The dimensions are the electric and magnetic fields--photons have two field dimensions; when they propagate in the vacuum they 'occupy' all three spatial dimensions. The dimensions of spacetime aren't the dimensions of the electromagnetic field.
Post by: varsigma on 05/06/2023 21:42:53
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1. Idea--https://ncatlab.org/nlab/show/Aharonov-Bohm+effect (https://ncatlab.org/nlab/show/Aharonov-Bohm+effect)
The Aharonov-Bohm effect is a configuration of the electromagnetic field which has vanishing electric/magnetic field strength (vanishing Faraday tensor F=0) but is nevertheless non-trivial, in that the vector potential A is non-trivial. Since the vector potential affects the quantum mechanical phase on the wavefunction of electrons moving in an electromagnetic field, in such a configuration classical physics sees no effect, but the phase of quantum particles, which may be observed as a interference pattern on some screen, does.
More technically, a configuration of the electromagnetic field is generally given by a circle-principal connection and an Aharonov-Bohm configuration is one coming from a flat connection, whose curvature/field strength hence vanishes, but which is itself globally non-trivial. This is only possible on spaces (spacetimes) which have a non-trivial fundamental group, hence for instance it doesn't happen on Minkowski spacetime.
In practice one imagines an idealized electric current-carrying solenoid in Euclidean space. Away from the solenoid itself the magnetic field produced by it gives such a configuration.
Note how it says "in practice". That means in experiments, one has to build a solenoidal coil magnet. It took a while for the first experiment to be realised because of the dimensions of the required device. Like, really small.
Post by: alancalverd on 05/06/2023 22:46:58
But the probability is A2 so real particles have real distributions.Not complex. They are both real particles with charge and mass.In field theories particles have complex probability amplitudes.
Post by: varsigma on 06/06/2023 00:24:21
But the probability is A2 so real particles have real distributions.Yeah. I don't know that I can explain why the square of a complex amplitude is a mass term (in a Lagrangian). Sean Carroll might be able to, or we might be able to discuss what he's talking about.
Post by: varsigma on 06/06/2023 06:11:10
Intensity is a general kind of thing about waves and wave motion.
The intensity of a beam of particles and the square of an amplitude are meant to mean something.
Interference is heuristically wavefunction overlap, in a sense that some information is thereby encoded 'in' the superposition.
Post by: alancalverd on 06/06/2023 07:07:10
The intensity of a beam of particles is the number of particles passing through unit area in unit time. Spatially, it's going to look like a wave function of some sort if you wait long enough.
And once again you are in danger of confusing model with reality. We can predict an interference pattern by superposing wave functions, but where an individual photon/electron/buckyball goes within that distribution is entirely random. It's no big deal: you can predict the outcome distribution of an infinite number of dice throws or even coin tosses very accurately, but each throw is unpredictable. Nothing is "coded in a superposition" to determine what happens next.
Post by: varsigma on 06/06/2023 19:00:32
And once again you are in danger of confusing model with reality. We can predict an interference pattern by superposing wave functions, but where an individual photon/electron/buckyball goes within that distribution is entirely random. It's no big deal: you can predict the outcome distribution of an infinite number of dice throws or even coin tosses very accurately, but each throw is unpredictable. Nothing is "coded in a superposition" to determine what happens next.I think the best model of an interference pattern is that it's a way to verify that your quantum computer is
"up and running". It tells you that interference is a kind of universal test, it's independent of the type of particle.
Although the dimensions, the size of the two slits and how far apart they are, are engineering requirements.
Unless there is a way to further encode known information into the beam, the interference pattern is the result of a test run, it validates . . . quantum behaviour(?)
Post by: varsigma on 06/06/2023 20:04:43
Feynman's big idea, or one of them, was that there is plenty of room to store and manipulate information, at the quantum field level.
Store and manipulate, as in, copy, transmit, erase, and so on. Bennett and Landauer explored the fundamental limits of computation; that is, what does it mean in terms of the energy required? What does the existence of energy mean, anyway?
My guess is it has to have a physical basis, there are forms of energy. We accept that in physics, there are two fundamental forms in any generalized "system of particles".
We can't really compare the oscillations of a simple pendulum with the free oscillations of an LRC circuit, because the fundamental gauge groups are, well, fundamentally different in ways we are still figuring out. With gauge theories there is a common approach to answering some of the questions. After all that's what physics is really, asking questions and trying to understand the answers. Philosophy tries to decide which questions are meaningful; Physics just does some experiments.
A footnote about one of the "big ideas", namely fibre bundles and quantum fields:
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In short: all global structure in field theory is controled by fiber bundles, and all the more the more the field theory is quantum and gauge. The only reason why this can be ignored to some extent is because field theory is a complex subject and maybe the majority of discussions about it concerns really only a small little perturbative local aspect of it. But this is not the reality. The QCD vacuum that we inhabit is filled with a sea of non-trivial bundles and the whole quantum structure of the laws of nature are bundle-theoretic . . .--https://ncatlab.org/nlab/show/fiber+bundles+in+physics (https://ncatlab.org/nlab/show/fiber+bundles+in+physics)
Post by: alancalverd on 07/06/2023 09:14:47
What does the existence of energy mean, anyway?Physics is the business of constructing mathematical models of what happens (or doesn't happen - the branch of physics known as civil engineering). Within those models, energy is a conserved quantity in classical physics, and remains conserved as mass-energy in relativistic physics. Nothing more or less.
Post by: alancalverd on 07/06/2023 09:23:37
Philosophy tries to decide which questions are meaningful; Physics just does some experiments.No. Philosophers assert which questions are meaningful, then question the meaning of meaningful until they disappear through their own anal sphincters. Physics is about discovering and predicting useful and interesting stuff.
As an experimental physicist I'm quite used to the banter of engineers ("bloody scientist...") and theoretical scientists ("oily-fingered engineer...") but nobody has ever insulted me with the title of philosopher!
Post by: varsigma on 07/06/2023 18:13:40
Unfortunately it isn't a good copy, if you're subscribed to the magazine online you can see a better copy.
Anyway, despite the importance of fiber bundles in gauge theories, the rather long article is a bit impenetrable. There is almost no mathematics, there are diagrams and explanations so it's up to you to map any ideas you have about topology and geometry into the mix.
Total spaces and base spaces are presented early as diagrams. A fiber is just a set of points defined over each point in the base space. They introduce the idea of combing the hair on a sphere, which is mathematically equivalent to defining a basis vector at each point so that all the basis vectors are parallel.
You can do this for a part or patch of the sphere but not the whole of the total space. The patch can be as large as a hemisphere though; the base is then the 2d projection of the hemisphere. The fiber over each point in the base is a line with a point on it identifying the angle of each basis vector. So far the ability to parallel transport along an arbitrary closed curve in the total space is not defined, something extra is needed.
This extra is the map of the gradient of each tangent space at each point on the hemisphere, relative to the flat tangent space at the "north pole". This map has to preserve the holonomy. Movement in the base is mapped to movement in the total space along the 'generalized' tangent planes, so moving from fiber to fiber is a smooth, or flat, path. The path is along the flat connection on the bundle.
Having the gradient map in the fiber bundle means having a way to lift a path from the base space to the total space. Without it all you have is basis vectors in the projection.
On a more lightfooted er, footnote, I tried discussing Bernstein and Phillip's ideas with ChatGPT. It did eventually touch on the fibrations and foliatons of the hemisphere. To foliate the hemisphere, define a set of latitude lines (circles), each leaf of the foliation has a cylinder of fibres. Each cylinder or section of the circle foliation has the same generalised gradient in the fibration. Check out how this gradient is re-configured in the diagrams on page 18 of their article.
Yeah, otherwise the bot was repetitive and not especially helpful, although it might have helped me clear up a couple of things.
Post by: Bored chemist on 07/06/2023 18:46:25
nobody has ever insulted me with the title of philosopher!I had assumed you were a PhD.
Post by: varsigma on 08/06/2023 02:25:49
They actually state that neutron interferometry demonstrates the topology of the gauge field, and the A-B experiment demonstrates the geometry. The gauge field is the magnetic vector potential. This was considered a mathematical nicety, required to complete Maxwell's system of equations that describe the E and B fields.
You can call it a redundant degree of freedom. Bernstein and Phillips say its geometry is a truncated cone, with a hemisphere glued back. The topology (in spin precession in a real magnetic field) is a Mobius strip; the spin vector is transported around the strip so takes two complete rotations over the base space, a circle, to be parallel again to its initial direction. It's really a space of phase shifts in the spin vector. Moreover, the global twist in a Mobius strip is indeterminate; it's there, but not in any localised sense.
This is why the global phase of the field--the neutron matter field--is indeterminate as well.
The A-B experiment takes the notion of parallel transport in a bundle of directions (the tangent vector spaces at each point) to the notion of a bundle of phases, in such a field. Instead of the geometric version of parallel transport, you have the topological version, a path-lifting rule.
One big difference between the two experiments is the mass of the quantized field elements, neutrons vs electrons. The fiber bundles are classical, the equipment is classical.
The gauge field is there because electrons see a phase shift, they interact with the magnetic field (not detectable) determined by the vector potential.
Post by: alancalverd on 08/06/2023 17:27:57
But a gentleman wouldn't draw attention to it, surely?nobody has ever insulted me with the title of philosopher!I had assumed you were a PhD.
Post by: Bored chemist on 08/06/2023 17:59:41
I wasn't aware that PhDs were handed out by gentlemen.But a gentleman wouldn't draw attention to it, surely?nobody has ever insulted me with the title of philosopher!I had assumed you were a PhD.
Post by: varsigma on 08/06/2023 18:07:59
They say that the base space of the experiment, in which the beam and the coil are three dimensional, is also three dimensional and the total space is four dimensional. The extra dimension is the "circle of phases" for quantum fields over each point in the base space.
So consider a two dimensional base space in which the electrons move, and the total space is three dimensional. This is otherwise called a pullback. You can push forward from this to a different four dimensional space, maybe take some laws of physics along.
Post by: varsigma on 14/06/2023 05:31:35
A good example I can think of is when, during an electronics tutorial a student asked why is time mathematically a real number, like in all the formulas? The lecturer more or less said, "because it works that way". Or, doing it another way doesn't work, so that's why.
Wikipedia's article on the A-B effect and its implications, or the physical nature of the EM field as it were, goes over some of the problems philosophy has with it.
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The Aharonov?Bohm effect is important conceptually because it bears on three issues apparent in the recasting of (Maxwell's) classical electromagnetic theory as a gauge theory, which before the advent of quantum mechanics could be argued to be a mathematical reformulation with no physical consequences. The Aharonov?Bohm thought experiments and their experimental realization imply that the issues were not just philosophical.--https://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect#Magnetic_solenoid_effect (https://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect#Magnetic_solenoid_effect)
The three issues are:
1. whether potentials are "physical" or just a convenient tool for calculating force fields;
2. whether action principles are fundamental;
3. the principle of locality.
So there's that word "physical"; also 3. implies that measurement is local, or at least localised.
I think because of the way measurements change (or break) a symmetry, there has to be a gauge field for measurement to make physical sense, and not just in the A-B effect.
p.s. on second thought, that might be a bit controversial. It might not be true for classical measurement.
Post by: alancalverd on 14/06/2023 11:24:54
We all have to deal with philosophy. Physics, "by itself", doesn't really try to cover philosophical questions or answers.Best way to deal with philosophy is to hold your nose and walk away. Philosophical questions do not have answers because if they did, philosophers would be out of work.
Why is time a real number? Because it is the dimension that separates sequential events, and real numbers are those that we invent to delineate sequence and quantity.
Post by: varsigma on 14/06/2023 21:36:09
If you read the SciAm article the authors go over the details but, they don't go into why a silicon 'waveguide' does the job, or why it needs to be a certain shape. One requirement of the experiment is that, after splitting the neutron beam, one half has to go through a real magnetic field, so its path is longer than the other half.
The topology of spin precession in fermionic matter (fields), as the article explains, is the Mobius strip. I did a bit of graph theory so I know you can embed a Mobius strip in a solid torus; actually you just have to embed the edge on the surface so you have a torus knot, with a single twist. If the base space is a circle you have a fiber with two points--two angles 180? apart--over each point in the base.
Bernstein and Phillips use the phrase "path lifting rule", but it seems this is nowadays just "the lift" and there are two components--the horizontal lift and the vertical lift. Since the base space and the total space are 1-dimensional there is no vertical lift in spin precession in fermions. This is a nice heuristic.
p.s. it could also qualify as a cool thing to say at parties.
p.p.s. if you have to explain it, you can say it's because the geometry of the magnetic vector potential is topologically a hemisphere, and all fermion geodesics stay at the same latitude, if they start that way so they are perpendicular to the B field lines. The spin of all fermion fields has no vertical lift in general, whether the phase of their matter fields does and the phase of the field, not the spin, can be lifted vertically.
By now whoever you thought was interested probably isn't.
Post by: varsigma on 15/06/2023 19:49:25
The simplest way to see what it all means is in terms of information, and how to encode quantum effects:=phase shifts, with classical information, and how to process it. So clearly you can't encode the global phase of a matter-wave or its spin vector, you have to have a difference so you need at least two distinct waves (!).
Also, an external magnetic field is a way to encode a phase difference, and, clearly it's not the field itself but the potentials that are "effective". But the external field strength is the control factor.
Post by: alancalverd on 15/06/2023 22:54:00
Post by: varsigma on 16/06/2023 04:43:11
A proton and a neutron go into a magnetic field bar. The neutron heads straight towards the bartender, but the proton tries to and ends up smacking into a wall.
The bartender says to the neutron, "Your mate ok?", The neutron says, "I think he's a bit charged up about something".
Post by: varsigma on 16/06/2023 06:42:50
A pair of topological fiber bundles go into a bar. After finding some ambient space, one of them sees a sign on the wall saying "Edges only Night".
He says to the other bundle, "We might as well come back another time, or we could have a completely pointless evening".
Post by: varsigma on 16/06/2023 14:40:45
Has anyone explained anything to it, so it says "I see", then tells you why?
It's been explaining the development of a surface to me, I've been explaining why Bernstein and Phillips use a truncated cone for the, ah, gauge field geometry seen by charged fermions.
'meh'
Post by: Origin on 16/06/2023 14:48:15
A physics joke.I believe the neutron would also be affected by a magnetic field due to the neutrons magnetic moment. Of course it would be much less than the proton.
A proton and a neutron go into a magnetic field bar. The neutron heads straight towards the bartender, but the proton tries to and ends up smacking into a wall.
The bartender says to the neutron, "Your mate ok?", The neutron says, "I think he's a bit charged up about something".
Post by: alancalverd on 16/06/2023 19:26:42
Post by: varsigma on 16/06/2023 22:08:04
I believe the neutron would also be affected by a magnetic field due to the neutrons magnetic moment. Of course it would be much less than the proton.The neutron scattering experiment was finely tuned. Actually both experiments that Bernstein and Phillips explain are highly engineered to very close tolerances.
The velocities of particles in incoming beams is a known. So the time neutrons spend in a magnetic field is also known, and the field strength is known.
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The total rotation angle induced along the path through the magnetic field is equal to the Larmor precession frequency multiplied by the time the neutrons spend in the field. The angle can therefore be calculated from measurements of the velocity of the beam, the intensity of the field and the distance across the field. In the version of the experiment done by Rauch, Bonse and their colleagues the neutrons travel through a magnetic field 1.5 centimeters wide at a speed of 2,170 meters per second, so that each neutron spends a little less than seven microseconds in the field.
When the electromagnet is operating at maximum current, the strength of the field is 400 gauss, which corresponds to a Larmor frequency of 433 million degrees per second. At this rate, in seven microseconds the spin vector of each neutron rotates about eight full turns. If each 360-degree rotation of the spin vector restored a neutron to its original state, one would expect to observe eight cycles of maximum and minimum counts. The actual result is significantly different. As the magnetic field increases from zero to its maximum the number of neutrons detected at the counter passes through only four cycles.
I would first (shut up and) calculate the deBroglie wavelength and energy of the neutrons. Neutrons have no electric charge (U(1) charge), their magnetic moment is explained by, I think weak hypercharge and that explains why free thermal neutrons decay spontaneously. It's a weak SU(2) sector interaction between field energies. Why is the B field maximum 400 gauss , that has to be an electromagnet that can pack a few field lines into R^3
p.s. I think I managed to get ChatGPT to agree that magnetic field lines are an heuristic device, which in essence "decorate" a magnetic field with closed loops and lead to the notion of magnetic flux. The flux density through a surface is heuristically the density of the field lines through the surface. Also because of the closed loop it must be conserved.
Inside a solenoid the (electric) current is in the same direction as the magnetic flux along the field lines, outside it's inverted and opposes the current.
Post by: alancalverd on 17/06/2023 00:05:01
Inside a solenoid the (electric) current is in the same direction as the magnetic flux along the field lines, outside it's inverted and opposes the current.Not sure we are looking at the same picture here! The field of a solenoid is merely an extension of the field of a single loop. The current direction is circular, with the field lines axial to the solenoid.
Confusion arises because for most practical purposes we make solenoids by winding a wire over a cylinder, and talk about magnetic field in ampere-turns or ampere-turns per meter, so you may be misled into thinking that the current passes from one end of the solenoid to the other. Fact is that "turns" is actually irrelevant! If you had a flat sheet of perfect conductor, passed a current from one edge to its opposite, and rolled it into a cylinder with its axis parallel to those edges, you would get the same field for the same number of amps per meter, with the current obviously circling round the cylinder and not along it. The "turns" business is just a matter of engineering practicality: it's much easier to make a long homogeneous field by driving 1 amp through 100 turns of wire than by driving 100 amps around 1 turn of rolled sheet - though superconductors do allow very large currents to circulate through very few turns.
Post by: varsigma on 17/06/2023 07:04:44
Bernstein and Phillips use a diagram (of course) to illustrate some of the finer details. Imagine an incoming beam of spin-polarized neutrons so all spin up. This is represented by a sequence of alternating red and grey stripes above a line. So for an unpolarized beam, the stripes would span this line and change colour across it. The stripes represent the probability amplitude of spin up or down as a regularly spaced pattern.
So a rotation of this spin amplitude in an external magnetic field rotates the pattern across the line; a 180? rotation pushes a red stripe over to a grey stripe, and this is meant to be interpreted as a flip from spin up to spin down. The phase of the amplitudes doesn't shift.
With a flat surface of pure silicon, neutrons aren't reflected but for a thick enough potential barrier, neutrons reflect internally such that constructive interference generates a diffracted beam at right angles to the transmitted beam. This depends on how flat the external surfaces are and on the amount of silicon between them. The same effect means a second barrier will add 1/2 the partial beams together and a third barrier will generate a single output beam, with 1/2 the intensity of the input beam.
All that sounds a bit like what happens when you have three polarizing filters and some light.
.
Post by: varsigma on 17/06/2023 07:18:42
The "turns" business is just a matter of engineering practicality: it's much easier to make a long homogeneous field by driving 1 amp through 100 turns of wire than by driving 100 amps around 1 turn of rolled sheet - though superconductors do allow very large currents to circulate through very few turns.Yes, I would say that it's because of the convenience of having the same current density in a metal coil as in a metal cylinder.
In both cases you increase the density, but you should bear in mind that electrons behave a bit differently when their motion is restricted by the dimensions of a metal conductor.
As the authors do near the end of that SciAm article, consider a flat ring of metal with magnetic flux through the centre. A current around the ring can go in two directions, and according to B & P, the parallel transport of the phase vector is that of a geodesic on a cone; slice the cone open and it's flat--the geodesic is a straight line and the phase vector is pointing in the same direction everywhere along it.
This geometric cone isn't "really there", unless you use electron phase to determine that it is. It might only look like a truncated cone, this vector potential field, or classical gauge field, if you're a fermion with U(1) charge.
Post by: varsigma on 18/06/2023 22:30:39
The argument is that simplification of complexity in the natural world leads to useful theories, because we can formulate simple relations and operations, then use them to fabricate more complex 'structures'. The close relation between mathematics and physics is fairly obviously a requirement or necessary condition for the fabrication of theories and real devices (telescopes, microscopes, interferometers, . . .) which illustrate or support the ideas we have about "the universe".
So this paper refers to sophistication instead; in what sense is the sophistication of ideas (theories) a better path to understanding than simplification is? Or in other words, how can simplification and sophistication, work together?
An example of what this paper says is a sophistication, is the construction of a topological space, equivalent to a geometry. So wrapping the circle around itself is a sophistication; simply stated any number of windings is a map to the integers (#crossings in the graph) and from the circle to itself.
Now add the condition or restriction that none of the circles can be contracted because real 3-space has a line removed and all the wrappings go around it. So the 3-space, with a line removed is a circle, topologically. Circles or loops that don't go around the removed line are contractible.
Post by: alancalverd on 18/06/2023 22:50:07
how can simplification and sophistication, work together?Said it before, will say it again. Rocket science is just two equations. Rocket engineering is a lot more complicated. The trick is to keep adding bits of science until your model is good enough for practical purposes.
Post by: varsigma on 22/06/2023 23:32:09
I learned that electrons barely move in a typical low frequency circuit; that power supply for your computer is not a thing that "delivers" electrons to the motherboard. The electrons in the motherboard and in all those chips and other discrete devices, are already there. So what happens?
Apparently what happens is all outside of the wires where the fields have free space and all the charges appear on the surface; so in a semiconductor this surface effect and external fields must also be the case. It's all about changes in the fields, some are internal but most of the action is in free space.
Electron flow and induction explain how transformers work, but there is no flow of electrons between two windings. Induction is a field effect.
Post by: alancalverd on 22/06/2023 23:43:07
Quote
Assume a current I = 1 ampere, and a wire of 2 mm diameter (radius = 0.001 m). This wire has a cross sectional area A of π ? (0.001 m)2 = 3.14?10−6 m2 = 3.14 mm2. The charge of one electron is q = −1.6?10−19 C. The drift velocity therefore can be calculated as.....
.....2.3 x 10-5 m/s
If the difference is difficult to comprehend, imagine pushing a stick into soft tar. The far end of the stick moves almost as soon as you start to push (depending on the speed of sound in the stick) , but the rate at which the stick moves through the tar is very slow.
Post by: varsigma on 22/06/2023 23:57:50
There is movement of electrons, transmitted at the speed of light in the medium, but the net flow (drift) is very slow.You mean the movement is transmitted, not the electrons?
Another reason the electrons don't move much is, they don't have to, because electrons pack a lot of energy into a small volume.
Post by: Eternal Student on 23/06/2023 00:50:23
What does anyone here think of the idea, commonly found, that electrons flow along wires and that's what you pay for, when the bill is due?Advice: If you plan on making a significant change in topic from what you started in this thread, then it may be best to start a new thread. I'm not staff and it doesn't bother me, it's just that you might get more replies and/or more relevant replies if people know they don't have to read the last 11 pages to join the current discussion.
Anyway, in answer to that question: It's a good enough idea for school level physics. For higher level physics it's understood that E and B fields cause a flow of energy in the direction of the Poynting vector. So the energy is not really flowing along the wires. As @Alancaverd mentioned, some electrons are slowly moving along the wires but you aren't getting super-energy electrons delivered and low-energy tired old electrons taken away at the other side or anything like that. Energy is flowing out to space from the battery and then in from space to the rest of the wires and the device that is consuming power, the flow of electrons just provides some magnetic field to make the Poynting vector E x B do what is needed in the right places.
None-the-less a model or conceptualisation based on
(LATE EDITING: *postive charge carriers and conventional current may be discussed instead of electrons).
There are several PopSci videos on YouTube explaining the situation. Veritasium did a video about a year ago called "Energy doesn't flow in wires" which most people will quote but I think this video (below) actually beat him to it by 2 years.
"Circuit Energy doesn't flow the way you think", Science Asylum, (duration under 8 minutes).
Best Wishes.
Post by: alancalverd on 23/06/2023 09:14:15
You mean the movement is transmitted, not the electrons?effectively, yes
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Another reason the electrons don't move much is, they don't have to, because electrons pack a lot of energy into a small volume.Er, no. There are just an awful lot of free electrons in a conductor! Current is the quantity of charge passing through a plane per unit time.
In 1 m3 of copper, there are about 8.5x1028 atoms. Copper has one free electron per atom, so n is equal to 8.5x1028 electrons per cubic metre.
Post by: varsigma on 24/06/2023 16:49:01
In 1 m3 of copper, there are about 8.5x1028 atoms. Copper has one free electron per atom, so n is equal to 8.5x1028 electrons per cubic metre.I was trying to make the point that a small fraction of that number is what moves under an applied electric field.
So the energies aren't because a lot of electrons are moving, it's because electrons are charged. You only have to disturb a small number to see a proportionally large current. Or see a spark jump across a gap.
And you have Fermi levels involved in the dynamics. The applied electric field changes the Fermi levels, but around defects in a metal lattice electron flow is . . . different.
Post by: alancalverd on 24/06/2023 17:58:49
I was trying to make the point that a small fraction of that number is what moves under an applied electric field.No, all the conduction electrons move. Same as my "poking tar with a stick" example.
Post by: varsigma on 24/06/2023 19:00:03
No, all the conduction electrons move. Same as my "poking tar with a stick" example.All the electrons move, but only a fraction move in the same direction at the drift velocity. This is much lower than the random velocities, with no net flow.
Post by: alancalverd on 24/06/2023 19:10:56
Post by: varsigma on 24/06/2023 19:54:51
If there is no net flow, why do we measure a current?The "no net flow" condition is when there is no applied field (no voltage or current source). An applied field gives some of the valence electrons a net flow, with a corresponding flow of holes in the opposite direction. The bulk of the electrons remains dynamically thermal though, so I say there is no net flow for those.
I guess at the fundamental level a picture is needed of how electrons "flow" in conductors. Or in semiconductors.
Post by: alancalverd on 24/06/2023 20:24:33
Valence electrons are irrelevant: we are talking about electrons in the conduction band. The picture is simple - just think of the stick model, or if that's too complicated, imagine a crowd leaving a stadium. When the final whistle blows they all move. The ones nearest the gate leave immediately, the number leaving the stadium per unit time (the current) depends on the ratio of the width of the gate to the width of a person, and the drift velocity of those inside the stadium may be very slow indeed.
There is no necessary reverse flow of holes. The Hall effect shows that the moving entities are EITHER electrons (in most metals and all n-type semiconductors) OR holes (some metals, all p-type semiconductors).
Post by: varsigma on 24/06/2023 21:38:19
The picture is simple - just think of the stick model, or if that's too complicated, imagine a crowd leaving a stadium. When the final whistle blows they all move. The ones nearest the gate leave immediately, the number leaving the stadium per unit time (the current) depends on the ratio of the width of the gate to the width of a person, and the drift velocity of those inside the stadium may be very slow indeed.Ok. There's always a problem with analogies but I get the gist of what you say. The big difference is when electrons do this, the changes in the fields move too, at the speed of light, and most of the field is outside the conductor. In free space.
It isn't the flow of electrons or holes, although that's what explains semiconductor v-i characteristics, but the flow of energy in the field around the wire. In semiconductors you have junctions between p and n type crystals in say a diode. There's a region--the space charge layer--which is part of the lattice. There isn't any real benefit in analysing the fields outside the solid state, but you could.
Post by: alancalverd on 25/06/2023 00:18:17
Not sure what you mean by changes in the external field. The external field around a straight wire at distance r is always μi/2πr for a current i, regardless of the drift velocity of the carriers.
Post by: varsigma on 25/06/2023 02:09:46
Not sure what you mean by changes in the external field.I mean in the sense electromagnetic signals travel at c.
Whether you ignore the fields external to the conductor or not, they're still there.
Post by: alancalverd on 25/06/2023 11:10:20
You can't actually measure the speed of propagation of a magnetic or electric field because varying one would generate the other, and produice a selfpropagating electromagnetic pulse.
Post by: Eternal Student on 26/06/2023 05:22:02
While I agree with a fair amount of what you've said @alancalverd , I don't think you are being entirely accurate and consequentially you may be dismissing a few ideas or comments from @varsigma that actually aren't entirely wrong. I'll address one of your points first because you probably won't be too offended.
@varsigma, I don't think you're getting or presenting the arguments based energy transfer with E and B fields in the best way. I think that @alancalverd has correctly found some faults.
The main thing is: I'm no expert or final arbitrator. It's a forum, this is a discussion, that's all.
Now, Alancalverd seems to be denying E and B fields and their importance in delivering power in electrical circuits, that's something I might be able to change with some discussion.
A d.c. flow of electrons produces a static magnetic field around the conductor but no external electric field.That's only a simplified truth. A more complete truth is that there will also be a static electric field around the conductor.
Wires have some small resistance. As such a small electrical potential gradient must be established along them in order to maintain a steady d.c. flow.
How is that done? It is thought that charges re-distribute themselves slightly, such that when you take a cross-section through the wire you would have some surface charges on that conducting wire. At steady state, the surface charges are going to be positive (abbreviated +ve) and fairly dense near the +ve end of the battery, becoming less dense as you move away from the +ve battery terminal toward the power drawing component of the circuit. After reaching approximately 0 surface charge about half-way around the circuit, the surface charge will now start to become -ve as you move toward the -ve end of the battery.
With this arrangement of surface charge, an electrical field is maintained inside the wire or equivalently an electrical potential gradient exists along the wire.
(https://msuperl.org/wikis/pcubed/lib/exe/fetch.php?w=300&h=213&tok=1d27d2&media=184_notes:thinresistorefield_new_.png)
Image taken from "Projects and Practices in Physics", https://msuperl.org/wikis/pcubed/doku.php?id=184_notes:resistors, mainly because they display a "non-commercial share-alike license" on their website so I don't think they'll mind me using the diagram.
A gradient of surface charges is shown in the circuit, with the steepest gradient existing over the resistor (the narrow piece of wire).
Anyway, this distribution of surface charges means that there will inevitably be an electrical field outside the wire in addition to there being an Electrical field inside the wire in the direction of the conventional current.
I'm keeping this post short so won't say much more now. To start with, we just need to establish that electric fields do exist OUTSIDE of a current carrying wire. It's already commonly known we will have magnetic fields outside the wire, so in another post we'd be ready to start looking Poynting vectors E x B both inside and outside of the wire.
1. There is work done by Jefimenko (et.al.) that show the existence of an electrical field in the space outside a current carrying conductor [ there's a copy of a 1961 article here: https://zjui.intl.zju.edu.cn/course/ece329/Secure/LectureNotesforCalendar/optional/Jefimenko62.pdf with pictures on the second page (numbered page 20) for those who prefer pictures ].
2. This paper: The Electric Field Outside a Stationary Resistive Wire
Carrying a Constant Current, by Assis, Rodrigues and Mania, Foundations of Physics, Vol . 29, No. 5, 1999 is also commonly cited. Although it does fall back on some numerical approximations, formulae for the force exerted on a point charge located some distance away from a current carrying wire and purely due to surface charges on the current carrying wire are exhibited (see section 4 of the article).
3. More modern treatments exist. Using the basic ideas of continuity conditions at a boundary between two media for electromagnetic fields we can actually show that there is an E field outside the wire with very little work (see https://en.wikipedia.org/wiki/Interface_conditions_for_electromagnetic_fields ). But until or unless LaTeX support for mathematical symbols is restored I'm not going to do that today.
Best Wishes.
Post by: paul cotter on 26/06/2023 11:53:57
Post by: alancalverd on 26/06/2023 14:27:36
Wires have some small resistance.Interesting. I've just spent the morning measuring the magnetic field outside a closed superconducting ring where this statement is clearly not true!
Post by: paul cotter on 26/06/2023 15:50:51
Post by: Eternal Student on 26/06/2023 17:50:41
For a superconductor, you're (@alancaverd) right. I'm going to take for granted that for a more ordinary wire, you'd agree that it does have some resistivity.
Say you have a superconducting wire delivering power to a load what would the Poynting vector be?
That's actually a very interesting question. You just won't be able to establish a potential gradient across a length of superconductor. It has precisely 0 resistance and will not support an infinite current in steady state (So V=IR always gives V=0 since R=0 etc.). So there is no need for surface charges to accumulate on the surface of the superconducting wire (similar to the diagram a few posts ago). There is zero E field inside the conductor (in steady state), any current that was there will just always keep flowing, it does not need any E field to overcome any resistivity. In practice, if you tried connecting a battery then you'll have a problem, the current will escalate rapidly and the material will get warm fast, so in practice the superconductivity will be lost or something else in the circuit (like the wires to the battery) will break etc.
For one thing, you'll need to be very specific about what the load is and how it was attached without itself being a superconductor: For example, if it's a resistor (or lamp filament or something) which is also supercooled and superconducting then you just can't establish a potential across the resistor (or filament) either, so no work can be done when charge passes through that resistor. Specifically, it's not sufficient to say "you have a superconductor delivering power to..(whatever)....", in some cases you just would not deliver power or have power consumed by what you thought was the load. If you dunk a superconducting circuit complete with a lamp filament that can also become superconducting into a tub of liquid Nitrogen then you may find that when you connect a battery, the lamp filament just would not light up.
To connect a battery and have it all work (hopefully), there must be some regions where you don't have a superconductor. For example the battery and some wires to it are NOT in liquid nitrogen, they are at room temperature, similarly the load (lamp filament or whatever) is not superconducting and it has some wires to/from it which are also not in the liquid nitrogen. Now you're in a position where there are some regions of the circuit where E and B fields can be set up fairly conventionally. So there are some E fields outside the superconducting piece of wire just from the bits of circuit that were not superconducting.
That's my best estimate anyway:
1. There will still be a magnetic field outside the superconducting piece of wire because there were moving charges in it.
2. there will still be E fields outside the superconducting piece of wire but they were sourced from the non-superconducting bits of the circuit rather than directly from surface charges on the superconducting piece.
3. If you prevent (2) and don't have E fields produced in space from anything or anywhere in the circuit (e.g. dunk the whole circuit into liquid nitrogen) then the bulb does not light and NO power is delivered anywhere or consumed by any part of the circuit.
Best Wishes.
Post by: alancalverd on 26/06/2023 17:56:58
Now, Alancalverd seems to be denying E and B fields and their importance in delivering power in electrical circuits, that's something I might be able to change with some discussion.It is entirely reasonable that a static E field can be detected where there is a potential gradient, and obvious that there is a static B field around a conductor carrying a steady current, but varsigma was talking about an electromagnetic field, which I take to mean a time-varying and self-propagating field generated by accelerating charges.
Mea culpa, possibly.
Post by: varsigma on 27/06/2023 06:42:34
It is entirely reasonable that a static E field can be detected where there is a potential gradient, and obvious that there is a static B field around a conductor carrying a steady current, but varsigma was talking about an electromagnetic field, which I take to mean a time-varying and self-propagating field generated by accelerating charges.Actually I was talking about the difference between an open circuit with no current flowing, and a circuit with a DC current. When you apply the current (resp. the voltage), the changes in the circuit (inside and outside) propagate through it, at c.
That has to hold as well for AC. It's the changes in the E and B fields that propagate.
If instead you attach an open wire to a battery terminal, it has the same potential as the terminal for the same reason.
Post by: Eternal Student on 29/06/2023 03:41:36
There's a bit of a problem with your phrasing @varsigma, which could easily leave people muddled or confused.
First line implies some differences exist:
Actually I was talking about the difference between an open circuit with no current flowing, and a circuit with a DC current.
Later line doesn't:
If instead you attach an open wire to a battery terminal, it has the same potential as the terminal for the same reason.
When you apply the current (resp. the voltage), the changes in the circuit (inside and outside) propagate through it, at c.There needs to be a careful use and understanding of the word "changes". It would be helpful to make a distinction between the changing of the potential and the (final) change in potential that is achieved.
Your sentence reads as if the final state of the circuit is reached much like something is transmitted at the speed c. That doesn't happen. If you close a switch near the battery, it can take a while for the region away from the battery to reach its final steady state. Indeed it can take a while for the region immediately next to the battery (and switch) to reach its final steady state. However, the changing of the potential can propagate at the speed c: If the potential of a piece of wire near the battery starts to rise, then the potential of a piece of wire far away from the battery will then start to rise with a delay time between those two which is precisely as if something is travelling at about c.
The total time taken to reach something approximating the steady state is short anyway because the electrons don't have to move far in the wires (or components) to start establishing the final E fields. Specifically, the electrons can move sideways (e.g. from the centre of the wire to the outer surface) which is a distance of a few millimetres (width of the wire) to start establishing the surface charge distributions and this will get you close to the conditions of the final steady state . None-the-less that is some distance that the electrons have to move, so no part of the circuit is going to jump to the final steady state conditions instantly.
You can consider circuits like this:
circuit.jpg (27.95 kB . 1056x345 - viewed 765 times)Once the switch is closed, the bulb starts to glow within 1 (metre) / c seconds, as if power has passed in a straight line through the open space between the switch and the bulb - it has clearly not travelled all along the length of the wires. However, this is only the first or early glow of the bulb. The final steady state will not be reached for a while longer and the bulb will progressively get brighter.
Presenting diagrams and experiments like the above is a slightly softer introduction to the idea that E and B fields are important. Most of the useful ideas you may want to present can be seen or exhibited and many school level notions of how electrical power is delivered can be dismissed (e.g. pushing a train of charges through the wire, which should only start getting power to the bulb at ~ speed of sound in the wire and demands the entire length of the wire is travelled).
Best Wishes.
Post by: paul cotter on 29/06/2023 08:28:08
Post by: alancalverd on 29/06/2023 08:58:29
power has passed in a straight line through the open space between the switch and the bulb - it has clearly not travelled all along the length of the wires.AAAGH! (to quote the bad guy getting his comeuppance in all the best comics).
Power is the rate of transfer of energy. This is a science forum, not a school for bad journalism!
What this thread is all about is the difference between phase velocity (v) and group velocity (u) of electrons (and holes). In the absence of inductive effects, v = c, and u is the drift velocity we calculated a few pages ago.
The observed delay in a filament lamp reaching full brightness is due to thermal inertia. The current flow is virtually instantaneous to maximum and the power actually decreases as the filament reaches white heat. If we replace it with a LED or a spark gap, there is no delay - every electron that moves, generates a photon.
And before the pedantic sharks attack, yes, the power dissipated in a LED or spark gap does indeed increase with time but again this is a thermal effect (tungsten has a positive temperature coefficient of resistance, semiconductors and plasmas generally negative) not a consequence of the finite value of v or u in the wires!
Post by: paul cotter on 29/06/2023 10:45:09
Post by: alancalverd on 29/06/2023 11:36:18
At the other end of the scale, I've used an analog delay line to make a very stable sinusoidal oscillator with no harmonics. We had the material in stock: it looked like a coaxial cable but instead of a braid, the "shield" was a single continuous winding with a ?ferrite core. You just asked the storekeeper for "30 microseconds" or whatever, and he cut the appropriate length off the reel.
Sadly, knowledgeable chainsmoking storekeepers and sweaty analog electronics seem to have been abolished. Or was it the other way around?
Post by: paul cotter on 29/06/2023 13:40:18
Post by: varsigma on 29/06/2023 16:55:00
And before the pedantic sharks attack, yes, the power dissipated in a LED or spark gap does indeed increase with time but again this is a thermal effect (tungsten has a positive temperature coefficient of resistance, semiconductors and plasmas generally negative) not a consequence of the finite value of v or u in the wires!LEDs and transistors have a smooth v-i response, except at the beginning or end of a switch from off to on (or on to off).
In the initial and final parts of a switching (or equilibrium change), the response is chaotic, briefly. It's also more interesting than the smooth response which is just boring old linear stuff.
Post by: Eternal Student on 29/06/2023 17:29:44
A rigorous analysis would require transmission line treatment with the parameters of inductance and resistance of the conductor and interconductor capacitance and admittance( in air 0 ).Yes. I've seen seen transmission lines modelled like this:
(https://upload.wikimedia.org/wikipedia/commons/thumb/1/11/Transmission_line_element.svg/250px-Transmission_line_element.svg.png)
I'm not an electrical engineer but I had always assumed that is just a model. There are no capacitors or inductors between the power lines (or along the lines) but none-the-less the thing acts as if there is. This model is useful for analysing what happens mainly because it allows one to use more conventional ideas or notions in electronics to determine what is happening. However, the capacitance is not found in discrete little capacitor components spaced every few metres, it is much more of a continuous thing spread out over all the length of the wire. Similarly the inductors aren't found as discrete components. The model is just a way of emulating the way E and B fields will be established in each wire and how they would propagating through the space between the wires etc.
I don't think there's a perfect model based on how the E and B fields would spread or a microscopic explanation of what is happening to charges in the wires either. A lot of those models assume simple geometry (like round wires rather then cheese wedge shapes in cross section along with the wires usually being quite straight instead of curved wiggly wires). More generally the first moments of current flow (i.e. at the moment the switch is closed) is often explained with an awkward hybrid of mechanisms - some of it is a bit like a train of charges being pushed along the wire, some of it is more like charges moving to the surface of the wire etc. Additionally a real world circuit never has the behaviour we would want for the E and B fields spreading through space: (1) Air is a mixture of stuff and fields don't move through it exactly like a vacuum, (2) The battery, wires and load are NEVER the only things in the universe. There's always some other ferromagnetic material and/or fields sourced from other things in the region. For very long power transmission lines there's no reason to assume that the power station in Nottingham is the place where all the power being consumed in my light bulb is coming from, even if the wiring was only connected to that Nottingham power station. My light bulb can have power delivered to it from whatever E and B fields are in the local vicinity to it. To say that another way, the conservation of energy only requires the Nottingham power station delivers energy to the fields that exist in space equal to the amount of energy my light bulb can pull out of space. However the path of the energy flow may be from Nottingham to Rodney Smith's light bulb and not my light bulb, meanwhile whatever power station and power transmission lines that Rodney Smith's house is wired to may be where my energy for my light blub is coming from.
{Overlap with @varsigma who finished writing before I did. Yes... (I think) the early moments of current flow is usually explained with an awkward hybrid of mechanisms, i.e. we don't really have an exact model or understanding for that - just some ideas we can patch together well enough. I'm no expert, that's just my opinion.}
Best Wishes.
Post by: alancalverd on 29/06/2023 17:34:20
Post by: paul cotter on 29/06/2023 18:24:45
Post by: alancalverd on 29/06/2023 20:22:23
Post by: varsigma on 07/07/2023 23:51:02
I'd like to canvas an opinion or two about the likeness between two well-understood things, namely energy and information.
I'll list some of the coincidental things:
Energy is conserved, so is information. Both depend on a local context, i.e. both depend on locality.
Energy can be transported from place to place, so can information. Both can be lost along the way, via dissipative processes.
Information can be written or stored. Energy can also be stored.
Information can be erased, but this only changes a pattern, so it's agreed that the pattern is "blank". Energy can be converted which in a sense erases some energy, although conversion of energy is itself a process that uses energy.
Post by: Eternal Student on 08/07/2023 00:31:10
I'd like to canvas an opinion or two about the likeness between two well-understood things, namely energy and information.You could start a new thread and put a Poll on it. It appears right at the top of the thread and is automatically updated. People can easly select a response and a bar chart of results is automatically generated (or hidden until some deadline is reached). Experiment with the options and you'll see for yourself.
See https://www.thenakedscientists.com/forum/index.php?topic=85028.msg681214#msg681214 for an example of a poll and some screenshots showing you exactly which buttons to push to create one.
Best Wishes.
Post by: alancalverd on 08/07/2023 09:54:35
Post by: paul cotter on 08/07/2023 14:59:11
Post by: varsigma on 08/07/2023 23:24:44
With one tap on a keyboard I can eliminate any arbitrarily large amount of information, in principle.Can you explain what you mean by eliminate?
Do you mean something like, make indistinguishable from a background?
Is energy well-understood? Does the meaning of well-understood need a review? It's a thing that is certainly well-understood in a mathematical sense. It is not however, a thing that can be said to be patterned, except in a rather abstract sense. That sense is an increase or decrease in entropy. Entropy is system-dependent.
Post by: paul cotter on 09/07/2023 10:23:07
Post by: alancalverd on 09/07/2023 10:59:43
Any journalist will tell you that power is the ability to make things happen and is synonymous with force, strength and energy, and resilience is the ability to deform and dissipate energy.
Post by: varsigma on 11/07/2023 04:04:41
They should teach you about how all the energy we use is largely due to the sun's radiation.
And about how we use energy at different scales, and the scale of information processing related to the distribution and conversion of energy is at the low end of the scale. It might be a convenient fiction but it seems to be a thing we need, both the idea and the thing itself, whatever that is. Something physical, perhaps. Why I can say that is because physics is both theoretical and practical.
I don't think anyone has managed to show we don't need this idea of energy, as a kind of source of information, or information loss, which arguably still means there is such a thing as thermodynamic or thermal information. This is just something you can postulate as a change in entropy that makes a system distinguishable from its environment, and call it say, heat information.
Here I'm underlining that information is what we say it is, but it has to be recognizable--a pattern that corresponds to a particular configuration of matter. A determinate system of particles.
Such as the ones that form the text in this sentence.
Post by: alancalverd on 11/07/2023 08:17:54
IIRC we were taught 70 years ago that almost all the energy we use comes from the sun. The teaching process is simple enough: sunlight makes the grass grow, and so forth..... You don't have to use the plants immediately: we store firewood, and coal and oil are the oxidisable residue of things that lived a long time ago. And there it is: solar, potential, chemical, kinetic, thermal: what connects them all? A conserved quantity.
It was all pretty obvious in the days of steam engines: coal, heat, useful work. Less so when the conversion of chemical to electrical energy takes place hundreds of miles away, but you can't have fire and boiling water in a modern classroom in case you offend any Zoroastrians or commit the sin of making carbon dioxide in public.
It would be fun to teach entropy at a primary level - something to think about!
Post by: paul cotter on 11/07/2023 09:00:36
Post by: Bored chemist on 11/07/2023 10:49:17
but the National Curriculum used to require primary school teachers to establish that "Energy is the 'go' of things"That phrase is sometimes attributed to J C Maxwell.
Not sure he worked for the National Curriculum.
Post by: alancalverd on 11/07/2023 14:23:37
Post by: geordief on 13/07/2023 21:27:08
Maybe not, but it was quoted in the NC even if they didn't coin it. Maybe that's why so many people still think about aether - the Dept of Education's only physics textbook is 100 years out of date!With all the water vapour out there it can't be long before they track down the aether.
https://roboticsandautomationnews.com/2021/01/20/scientists-have-found-more-water-in-space-than-they-ever-knew-possible/39771/#:~:text=The%20cloud%20of%20water%20in,accounting%20for%20its%20remarkable%20size.&text=The%20gas%20cloud's%20higher%20temperature,in%20space%20is%20constantly%20heated.
Post by: paul cotter on 14/07/2023 13:32:01
Post by: geordief on 14/07/2023 16:23:50
Aether does not exist.'twas said in jest(not joust).
Post by: Zer0 on 20/07/2023 19:40:49
Aether does not exist.'twas said in jest(not joust).
@Gee
I Wish you'd speak your mind a bit more Often.
You seem to Know alot of Stuff.
Others could Benefit from it.
ps - it can't always be take, take n take...sumtims U gotta Give!
Post by: varsigma on 26/08/2023 04:41:02
Thanks to some stuff I learned about image compression I can talk about an interference pattern of dots as an uncompressed image. One way to compress it is by making one dimension, say the vertical, redundant. So all the dots lie on the same horizontal line. Does that change the number of dots needed to see a pattern?
This horizontal line can be anywhere, and in a sense, its position represents another redundancy. All an algorithm needs is the 'x-address' of each dot.
Post by: alancalverd on 26/08/2023 08:24:49
A Laue x-ray diffraction pattern is essentially a 2-dimensional inverse transform of the structure of a single crystal, and both x and y, or more correctly r and θ, of each spot are required to elucidate the structure. Powder diffraction, however, produces essentially circularly symmetric interference so only one dimension is relevant. [Sort of. In fact the intensity of each spot or line is also important to identify its source, so they are actually "n + 1" dimensional.]
Note that there is a difference between seeing a pattern and determining one. The human (and probably most animal) brain is very adept at spotting geometric patterns where none exist. One annoying case is the "ring artefact" that lots of folk report inside a CT scan image of an entirely uniform cylindrical noise source - it seems to derive from the "clue" of the circular boundary. My favorite example is to ask a witness, who claims to have spotted a pattern in someone's behavior, "1,3,5,7, what is the next number?" "Obviously, 9" "Are you sure it's not 11?"
Post by: varsigma on 26/08/2023 20:39:24
Even if there isn't, you need a horizontal scan to see the pattern, which alternates in a regular way. An algorithm could look for this regular alternating pattern and I think, detect interference with a smaller number of dots. Someone needs to do the experiment, or the analysis of existing experimental data.
I maintain that a two-dimensional screen which is the 'measurement space' for double-slit experiments has one redundant degree of freedom in the output, and the significance of this fact speaks to the encoding of information.
Post by: alancalverd on 27/08/2023 10:28:10
The heuristic I'm using is that, given a pattern from a double-slit 'interaction', that is, given that interference is detected, there is a symmetry in that, along any vertical scan line there should be a regular (semi-regular ?) distribution of dots.No.
Assuming the slits are vertical, the horizontal distribution of the interference pattern is the product of the random distribution of photons with the transform of the slits. The vertical distribution of photons in any one line is entirely random.
So you will see something like a bell curve of intensity in either direction, but modulated by a pattern only along the horizontal axis.