[varsigma (OP)]

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.
(?)

[Eternal Student]

Hi.

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.

[alancalverd]

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.

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.

[Bored chemist]

neatly expressed by my engineering colleagues: mechanical engineers build things that move, civil engineers build things that don't move.

Who builds swing bridges?

[alancalverd]

CivEng does the foundations and approaches, MechEng does the bit in the middle. ElecEng complains that you specified the cable lengths but didn't mention sea water.

My daughter is a surveyor whose job is to resolve conflicts in the construction industry!

[Origin]

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.

[Bored chemist]

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.

[alancalverd]

We physicists are a pedantic bunch. We talk about entities, quantities and units.

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.

[varsigma (OP)]

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.

[varsigma (OP)]

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.

[Bored chemist]

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.

[alancalverd]

Energy is a quantity that is conserved. Like mass and momentum. We don't use undefined words like "physical thing".

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".

[Zer0]

Previously, i had created an OP on
 ' 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?

[varsigma (OP)]

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?

[Eternal Student]

Hi.

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.

[Eternal Student]

Hi again.

   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.

[varsigma (OP)]

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?

[varsigma (OP)]

  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.

[Eternal Student]

Hi.

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.

[alancalverd]

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!