"Classical Physics Is Wrong" Fallacy - Comments

[Val28]

He, he, I was a high school kid when entertained in such an obvious topic. Of course Newton was and is perfectly correct in its sphere of applicability, of low velocities and week gravitational fields. Take a limit of all SR formulas as c goes to infinity and you recover the clasical mechanics. Set Planks constant to zero and same for QM. I have a warning to all future Einsteins: Make sure you explain what we already know (which is now quite a lot) and your new theory recover ours in some limiting case. Good luck!

 

[mfb]

I have a warning to all future Einsteins: Make sure you explain what we already know (which is now quite a lot) and your new theory recover ours in some limiting case.

Something all serious physicists do, but nearly no crackpot even tries. Typically because the crackpots don't even know what is known.

 

[JohnNemo]

That’s because he didn’t know any better. WE now know the limits where it is valid.

Zz.

Exactly, but I’m not sure this point comes out clearly in your article. To say that

1. Newton was wrong (about a lot of things)

is not incompatible with saying that

2. It just so happens that within certain limits (which limits we now are aware of and he wasn’t) parts of his theory are mathematically correct to a very good approximation and very useful in fields of engineering.

 

[Ibix]

It just so happens that within certain limits

It doesn't "just so happen". It's a necessary feature of any theory that it looks like Newton in the classical limit because the real world looks like Newton in the classical limit. If it doesn't look like Newton it doesn't look like the real world.

parts of his theory are mathematically correct to a very good approximation and very useful in fields of engineering.

But that's true of relativity and quantum too. And we will always have to suspect that any future theory may be invalid outside the regime we have tested. So we can describe every theory as wrong and regard it as mere coincidence that it works. Or we can take @Orodruin's point that a theory is better regarded as its testable predictions, not its untestable assumptions, and regard it as right when those predictions are accurate to available precision.

 

[JohnNemo]

It doesn't "just so happen". It's a necessary feature of any theory that it looks like Newton in the classical limit because the real world looks like Newton in the classical limit. If it doesn't look like Newton it doesn't look like the real world.

It is only *necessary* in the weak sense that you are defining “classical limit” as meaning “that part of Newton’s theory which is approximately mathematically correct”. It is necessary only because of your definitions. Ultimately it is a happenstance.

 

[Orodruin]

you are defining “classical limit” as meaning “that part of Newton’s theory which is approximately mathematically correct”

The aim of a theory in empirical science is not to be "mathematically correct" and it is unclear what you would even mean by something like that. What matters is if the theory makes correct predictions or not.

Also, "approximately" and "mathematically correct" would typically not be two words that mathematicians would like to string together in a sentence. That 2 = 2.000123 is not approximately mathematically correct, it is wrong.

 

[JohnNemo]

The aim of a theory in empirical science is not to be "mathematically correct" and it is unclear what you would even mean by something like that. What matters is if the theory makes correct predictions or not.
.

Yes, by mathematically correct I meant approximately correct predictions in terms of *measurement*. Not in the sense of getting your sums right!

Measurement is not all of Physics. There are also concepts. Newton’s predictions for the effects of gravity may be approximately correct in terms of measurement in certain circumstances, but GR regards his idea of gravity being a force as incorrect (in all circumstances).

 

[Dale]

Ultimately it is a happenstance.

No, it is not happenstance. It is a necessary condition for future theories to be considered valid. To be valid, both relativity and QM must unavoidably have a Newtonian limit. Not as a matter of coincidence but precisely because Newtonian physics has a large body of experimental validation.

If they did not have a Newtonian limit then they would have been immediately discarded. They would have been falsified by all of the experiments that validate Newtonian physics.

 

[JohnNemo]

No, it is not happenstance. It is a necessary condition for future theories to be considered valid. To be valid, both relativity and QM must unavoidably have a Newtonian limit. Not as a matter of coincidence but precisely because Newtonian physics has a large body of experimental validation.

If they did not have a Newtonian limit then they would have been immediately discarded. They would have been falsified by all of the experiments that validate Newtonian physics.

I think you are unconsciously using (as many people do) the words “Newtonian limit” to mean the whole of Newton’s theories. But it is only a subset of Newton which approximately corresponds to a subset of relativity. The boundary of the subset of Newton’s theory is a happenstance conditioned by the limits of the experimental opportunities he happened to have available to him.

 

[Dale]

I think you are unconsciously using (as many people do) the words “Newtonian limit” to mean the whole of Newton’s theories. But it is only a subset of Newton which approximately corresponds to a subset of relativity.

I am not sure what you mean. In the limit v<<c and weak gravity we recover Newton’s three laws and his law of gravitation. What more is there?

 

[JohnNemo]

I am not sure what you mean. In the limit v<<c and weak gravity we recover Newton’s three laws and his law of gravitation. What more is there?

Newton did not regard his theory as limited to v<<c and did not regard his theory as limited to weak gravity.

I’m just querying your use of the words “Newtonian limit”. This is a modern concept, not one that Newton recognised.

I think a lot of the controversy about whether Classical physics is “wrong” is caused by different people using the same words with different meanings.

One group means by Classical Physics “that part of (or those circumstances in which) Newton’s theory produces predictions which we now know to be still approximately correct” - this group is outraged by the idea that anyone should seem to write off Classical Physics as simply “wrong”

The other group means by Classical Physics “Newton’s theories taken as a whole” - this group can’t understand why the other group is so outraged by any suggestion that Newton’s theories may not be wholly correct.

Each group can be tempted to exaggerate its position in order to try to redress the balance!

 

[Dale]

’m just querying your use of the words “Newtonian limit”. This is a modern concept, not one that Newton recognised.

You are missing two points. The first and most important is that the Newtonian limit is not a feature of Newtonian mechanics; it is a feature of subsequent theories. The Newtonian limit does not belong to Newtonian mechanics; it belongs to relativity and it belongs to QM. If those two theories did not possesses a Newtonian limit then they would be wrong. The existence of a Newtonian limit for relativity and QM is not a necessary condition for Newtonian physics to be right; it is a necessary condition for QM and relativity to be right. It is certainly not happenstance since the founders of relativity and QM worked hard to include a Newtonian limit in their theories; they recognized the necessity.

The second and much more minor point is that Newton is not the final authority on what Newtonian mechanics is. It is named after him because he started it, but its development continued long after him. He started it, not finished it. Many things that we recognize today as being parts of Newtonian mechanics were from the work of others, not Newton.

 

[JohnNemo]

You are missing two points. The first and most important is that the Newtonian limit is not a feature of Newtonian mechanics, it is a feature of subsequent theories. The Newtonian limit does not belong to Newtonian mechanics, it belongs to relativity and it belongs to QM. If those two theories did not possesses a Newtonian limit then they would be wrong. The existence of a Newtonian limit for relativity and QM is not a necessary condition for Newtonian physics to be right, it is a necessary condition for QM and relativity to be right. It is certainly not happenstance since the founders of relativity and QM worked hard to include a Newtonian limit in their theories since they recognized the necessity.

The second and much more minor point is that Newton is not the final authority on what Newtonian mechanics is. It is named after him because he started it, but its development continued long after him. He started it not finished it. Many things that we recognize today as being part of Newtonian mechanics was from the work of others, not Newton.

I agree with everything you say (except your first sentence!) and that is exactly my point. Most of the controversy arises because people are using the phrases “Newtonian Mechanics” and “Classical Physics” with different understood meanings without declaring what definition they are using.

 

[bhobba]

-"Classical physics is used in an overwhelming majority of situations in our lives." so what? do examples prove that a theory is correct?

No it doesn't - nothing can prove any theory correct and classical mechanics is no exception. All theories are just provisional.

But I think before forming an opinion you need to read Landau - Mechanics.

It is based on the principle of least action which must be true if QM is true providing you are dealing with the regime such that close paths cancel - only stationary paths do not cancel - they reinforce - this is from Feynman's path integral approach which is logically equivalent to usual QM. In Landau, assuming just that and symmetry you basically get classical mechanics. Does that make classical mechanics correct - of course not - but if its wrong then we have a rot in QM and that would be very revolutionary indeed. In fact we have reason to believe QM at levels we can currently reach would be unlikely to be wrong:
https://arxiv.org/pdf/quant-ph/0401062.pdf

Again QM is just provisional, like any theory - but what Weinberg's paper shows is if it is wrong there is something basically wrong with some very fundamental things - so fundamental it should have been picked up by now - but doubt is one of the essential basics of science so one never knows - more likely though if its wrong its at levels we can't currently reach.

Thsnks
Bill

 

[ZapperZ]

Exactly, but I’m not sure this point comes out clearly in your article. To say that

1. Newton was wrong (about a lot of things)

is not incompatible with saying that

2. It just so happens that within certain limits (which limits we now are aware of and he wasn’t) parts of his theory are mathematically correct to a very good approximation and very useful in fields of engineering.

"not incompatible" means "compatible"?

If that is so, then I do NOT see those two as being compatible. To say that Newton's laws are wrong, and then to turn around and use it, is an inconsistent act.

To say that it is accurate over a certain limit, and then to turn around and use it within those limits, is NOT an inconsistent act.

Unfortunately, this is now semantics. But this is not how people who have asked this on this forum think, because they are under the impression that we should NOT be using Newton's laws, under ANY limits, because it has been shown to be "wrong" by QM and SR/GR. THAT is the argument that I'm countering against.

Zz.

 

[JohnNemo]

Unfortunately, this is now semantics. But this is not how people who have asked this on this forum think, because they are under the impression that we should NOT be using Newton's laws, under ANY limits, because it has been shown to be "wrong" by QM and SR/GR. THAT is the argument that I'm countering against.

Zz.

I think you make that point very well here:

"Moral of the Story

Classical physics WORKS for our ordinary situation, so it HAS to be valid at some level.

Classical physics has been shown to be derivable from SR and QM under special conditions that apply to our ordinary situation.

Any theory MUST have the ability to show that it merges to the classical description when applied to ordinary situation."

For each of the three points you are careful to stress the limits ("at some level", "our ordinary situation", "to ordinary situation")

But the start of your article is, I think, more polemical and you are exaggerating to redress the opposite extreme of how you perceive "people who have asked on this forum think"

 

[Dale]

I agree with everything you say (except your first sentence!) and that is exactly my point.

I am glad that we now agree. Although if you agree with my first paragraph then I cannot see how you could claim that it is “happenstance”

 

[ZapperZ]

But the start of your article is, I think, more polemical and you are exaggerating to redress the opposite extreme of how you perceive "people who have asked on this forum think"

And like I said, that is now a matter of semantics and personal preference. I stated that based on many years of observations of members here in this forum when we repeatedly dealt with such topics.
Zz.

 

[JohnNemo]

I am glad that we now agree. Although if you agree with my first paragraph then I cannot see how you could claim that it is “happenstance”

Like a lot of things in this thread it has to be read in context but basically what I was saying was that the fact that Newton's theories (as originally proposed by Newton) make predictions which are approximately accurate only up to a certain limit is a "happenstance". It is not part of the theory (as initially proposed by Newton) that the theory is only intended to apply up to a certain limit.

 

[bhobba]

The second and much more minor point is that Newton is not the final authority on what Newtonian mechanics is. It is named after him because he started it, but its development continued long after him. He started it, not finished it. Many things that we recognize today as being parts of Newtonian mechanics were from the work of others, not Newton.

Indeed - Dirac had an interesting view on this (we don't discuss philosophy here but I can't resist - Kuhn - watch out):
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.485.9188&rep=rep1&type=pdf

Thanks
Bill

 

[Dale]

It is not part of the theory (as initially proposed by Newton) that the theory is only intended to apply up to a certain limit.

Of course not, the Newtonian limit is part of relativity and QM, not part of Newtonian mechanics. I have read the context and your statements repeatedly show this misunderstanding.

 

[JohnNemo]

Of course not, the Newtonian limit is part of relativity and QM, not part of Newtonian mechanics. I have read the context and your statements repeatedly show this misunderstanding.

Anyway. I am glad we are now agreed!

 

[Dale]

To say that Newton's laws are wrong, and then to turn around and use it, is an inconsistent act.

To say that it is accurate over a certain limit, and then to turn around and use it within those limits, is NOT an inconsistent act.

Particularly well said.

 

[bhobba]

But the curvature isn't "real" in some philosophical sense - there is an unobservable flat background.

This is bit off topic but I think it interesting enough to mention. That is Steve Carlip's view (look him up if you don't know him). I have been reacquainting myself with GR recently since I find as a mentor I answer more of these queries. Anyway if you don't know Lovelock's theorem you might find it sheds light on this.

Thanks
Bill

 

[PeterDonis]

That is Steve Carlip's view

Can you give a particular reference? I've read a fair number of Carlip's papers and I'm not sure I've seen him expound the "unobservable flat background" view.

 

[bhobba]

Can you give a particular reference? I've read a fair number of Carlip's papers and I'm not sure I've seen him expound the "unobservable flat background" view.

It was from when I posted a lot on sci.physics.relativity. He posted quite a bit until it became infested with too many cranks, but mentioned a few times it was an essentially undecidable philosophical question whether space-time was curved or was flat and simply acted like it was curved.

I must say that was at least 10 years ago so may have changed his mind. I do know he is a very approachable guy and I don't think he would mind anyone asking him what his current view is. Mine is he is correct - and until he mentioned it, it never even occurred to me - it was sort of a revelation..

Thanks
Bill

 

[PeterDonis]

He posted quite a bit until it became invested with too many cranks, but mentioned a few times it was a an essentially undecidable philosophical question whether space-time was curved or was flat and simply acted like it was curved.

Ah, ok. Yes, from a philosophical point of view, I agree the question is undecidable. The spacetime curvature interpretation of GR is, strictly speaking, an interpretation, not a claim about "how things really are". It just happens to be a very, very useful interpretation, so much so that physicists routinely talk about it as if it were a fact.

 

[Dale]

It just happens to be a very, very useful interpretation, so much so that physicists routinely talk about it as if it were a fact.

That is indeed a problem. In the Newtonian limit of GR, for example, the Christoffel symbols naturally become the force of gravity. So although we speak of gravity in GR as not being a force, in fact there is a mathematical quantity in GR that does serve that role and can be interpreted as a force. Choosing to not interpret that way is a matter of philosophy and can be discarded as needed. But that common (and useful) philosophical interpretation probably serves to exacerbate the “Newtonian physics is wrong” impression.

 

[bhobba]

Ah, ok. Yes, from a philosophical point of view, I agree the question is undecidable. The spacetime curvature interpretation of GR is, strictly speaking, an interpretation, not a claim about "how things really are". It just happens to be a very, very useful interpretation, so much so that physicists routinely talk about it as if it were a fact.

For what it's worth I am with Feynman who thinks the geometrical interpretation is marvelous - it's hard to think its not actually true - but strictly speaking like the difference between SR and LET you can't really prove it. I have read his lectures on gravitation and he starts with flat space-time and spin 2 particles. You still get the EFE's but if space-time actually curved is not so clear.

Later I discovered Lovelock's Theorem that put it more in context for me. Thinking of the metric as a field or it's natural geometrical interpretation doesn't make any difference - the theorem says you get the same result - but strangely only in 4 dimensions which is rather interesting.

Thabks
Bill

 

[PeterDonis]

You can make prediction of the consequences of a particular spacetime geometry and then measure those consequences

No, you can make a prediction of the consequences of particular solutions of particular equations, and then compare the predictions with measurement. But the equations don't have labels on them that say "these equations describe a spacetime geometry". Spacetime geometry, as I said in a previous post, is an interpretation of the equations. You don't need an interpretation to make predictions and compare them with measurements. Interpretation is a conceptual crutch for us humans, because we don't naturally think in equations, so we need some kind of story to tell about what the equations "mean". But the "spacetime geometry" story is not the only story you can tell about the equations of GR.