341

Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 15/05/2023 09:36:55 »

Just to round things out, I'd like to touch on some philosophical implications.

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?".

342

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

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.

343

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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?

344

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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?

345

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

346

Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 11/05/2023 08:04:25 »

I haven't brought this up yet, but here goes.

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.

347

Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 05/05/2023 00:16:02 »

If you can't put it in a bottle, it isn't an entity

Well, 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?

348

Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 04/05/2023 13:04:28 »

About the other discussion

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?

349

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

350

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

351

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

352

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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".

353

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

The sentence is meaningless, so you can't "safely assume" its validity.

If the sentence is meaningless, how did Milliken do his experiment? What did he assume?

354

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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, 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!

Ok. Sort of.
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.

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.

355

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

356

Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 28/04/2023 01:28:33 »

An entity is a distinct object - electron, motor car, whatever

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?

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?

357

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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?

358

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

359

Physics, Astronomy & Cosmology / Re: Talking about Physics

« 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.

360

Physics, Astronomy & Cosmology / Talking about Physics

« on: 26/04/2023 02:00:21 »

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. At the simple end.

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.
(?)