How does light slide sideways?

[harrylin]

[..]
The rules (i.e., postulates of SR) that I have to believe in are not themselves explained physically, and I guess this is what hangs me up. The postulates are a distillation of our understanding, in the form of presumptions (axioms?) or rules, about the behavior of light and physical systems in different reference frames (and of course the constant speed of light common to all inertial frames) that imply equations (e.g., lorentz transform) that in turn lead to accurate analytical predictions of scenarios and thought experiments. The recipes for analysis, however, are not themselves an explanation of the physics of systems, only an extension of the presumptions used in making specific case by case predictions and for formulating rules of thumb.

Our understanding is phenomenological, though it has been refined by nuanced experimentation and empirical evidence over the years. I see SR as more of a mathematical theory than a physical one, because it doesn't really explain, it only describes what is. I was hoping there would be something deeper we might stumble onto so I could see SR as a physical theory that explains at a deeper level (not necessarily all the way down to the deepest secrets of nature, but deeper than just an accurate prediction of the physical measurements) what the mechanisms that lie beneath the physical behavior of the systems in our scenarios and thought experiments. I cannot help but think there is something deeper going on implied by the scenarios we have discussed here that we are not recognizing. [..].

Actually many of us do recognize that but it has been the subject of unresolvable debate and so that topic is not anymore appreciated on this forum, just as it has been secretly banned from many physics journals. Not because it's overlooked / unimportant / censored, but because more discussion about it seems useless and counterproductive for this forum.

In a nutshell, historically the following three interpretations have been given (at risk of oversimplification):

1. stationary ether (Lorentz)
2. all motion is truly relative in a Machian sense (Einstein)
3. block universe (Minkowski)

Einstein's idea that all motion is truly relative led to GR, but in-depth analysis of his 1918 defense of that idea showed that it leads to inconsistencies - and nowadays very few physicists adhere to it. As Einstein's thinking and the issues with it are hardly known, I wrote a short overview here: https://www.physicsforums.com/threa...paradox-as-paradox.780185/page-7#post-4921049

You can read up about the two main other interpretations by searching this forum for "block universe".
And a short summary of those discussions with elaboration of the reason to ban debates on that topic can be found in this forum's relativity FAQ, here: https://www.physicsforums.com/threads/what-is-the-pfs-policy-on-lorentz-ether-theory-and-block-universe.772224/#post-4859428

 

[PeterDonis]

2. all motion is truly relative in a Machian sense (Einstein)

I'm not sure I understand what this interpretation is supposed to mean.

in-depth analysis of his 1918 defense of that idea showed that it leads to inconsistencies

By "his 1918 defense", I assume you mean this:

https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity

Is there a particular point in this where Einstein gives a short description of what you are calling the "all motion is truly relative" interpretation?

 

[harrylin]

Yes I mucked that bit up.

Not really! See this old discussion over words here: https://www.physicsforums.com/threads/is-acceleration-absolute.59283/
The expression "acceleration is absolute" commonly means what Langevin meant with it: acceleration leads to phenomena that are universally observed, independent of reference frame (one of his examples became known as the "twin paradox").

 

[harrylin]

[..] https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity

Is there a particular point in this where Einstein gives a short description of what you are calling the "all motion is truly relative" interpretation?

It would be going too much off topic to elaborate in this thread (I elaborated in the other thread), but I'll expand here on my summary in the other thread with some links to Einstein's interpretation - and I'll leave it at that!

- Einstein refers in his 1918 paper to criticism on his publications about GR. We can infer his own interpretation of that time from many bits and pieces in his Dialog (e.g. "A gravitational field appears" "one can neither say, that the gravitational field in a certain place is something "real', nor that it is "merely fictitious" "all the stars that are in the universe, can be conceived as taking part in bringing forth the gravitational field; because during the accelerated phases of the coordinate system K' they are accelerated relative to the latter and thereby can induce a gravitational field").

It's at least as clearly expressed in his earlier publications that led to that criticism:

- https://en.wikisource.org/wiki/The_...ain_the_extension_of_the_relativity-postulate.
(extracts: "epistemological defect, which was perhaps first clearly pointed out by E. Mach" "The cause must thus lie outside the system." "essentially conditioned by the distant masses," "Can any observer, at rest relative to [PLAIN]https://upload.wikimedia.org/math/4/f/4/4f45bf1507f5ace45ff25334e53fece4.png, then conclude that he is in an actually accelerated reference-system?" "The reference-system https://upload.wikimedia.org/math/4/f/4/4f45bf1507f5ace45ff25334e53fece4.png has no acceleration. In the space-time region considered there is a gravitation-field which generates the accelerated motion".)

- https://en.wikisource.org/wiki/Rela...ument_for_the_General_Postulate_of_Relativity
(extracts: "he consequently comes to the conclusion that the chest is suspended at rest in the gravitational field. Ought we to smile at the man and say that he errs in his conclusion? I do not believe we ought to if we wish to remain consistent" "Even though it is being accelerated with respect to the "Galileian space" first considered, we can nevertheless regard the chest as being at rest." "A gravitational field exists for the man in the chest, despite the fact that there was no such field for the co-ordinate system first chosen." "the observer in the railway carriage experiences a jerk forwards as a result of the application of the brake, [..] He might also interpret his experience thus: "My body of reference (the carriage) remains permanently at rest.[..] however, there exists (during the period of application of the brakes) a gravitational field which is directed forwards and which is variable with respect to time."

One might say that Einstein tried to make acceleration "relative" in the sense of Langevin by creating equally "relative" gravitational fields.
And for good understanding: these fields are supposedly induced by motion relative to the distant stars.

It is also instructive to compare http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gr.html (and notice the stark contrast between Einstein's remarks above and the remark that 'General Relativity is the study of real gravitational fields').

 

[PeterDonis]

One might say that Einstein tried to make acceleration "relative" in the sense of Langevin by creating equally "relative" gravitational fields.

I would put it that Einstein made coordinate acceleration relative, by creating equally relative gravitational fields. But Einstein did not try to make proper acceleration relative.

these fields are supposedly induced by motion relative to the distant stars.

Yes, but not in any "instantaneous action at a distance" way. The influence of the distant stars had to propagate to our local region of spacetime; the metric in our local region of spacetime is not due to the influence of the distant stars "right now", but to the influence of the distant stars in our past light cone. Many "criticisms" of this aspect of GR simply ignore this fact.

notice the stark contrast between Einstein's remarks above and the remark that 'General Relativity is the study of real gravitational fields'

Yes, but that's because Baez defines "real" fields as fields associated with nonzero spacetime curvature. This is a different usage of the term than Einstein's--Einstein's usage equated a "gravitational field" with nonzero connection coefficients. Even in curved spacetime, you can find local inertial frames in which the connection coefficients are zero, and in these frames there is no "gravitational field" in Einstein's sense. So Einstein and Baez are talking about two different concepts. It's unfortunate that the term "gravitational field" has been overloaded in this way, but it has, and we just have to deal with it.

 

[Dale]

Either way I feel SR is clearly more fundamental, let's not go through the exercise of defining "Fundamental". Though note to your point GR is more encompassing, which is inclusive of SR.

I would say GR is more fundamental. You can recover SR as a limiting case of GR.

 

[harrylin]

I would put it that Einstein made coordinate acceleration relative, by creating equally relative gravitational fields. But Einstein did not try to make proper acceleration relative.

I already clarified my understanding of what Einstein meant. Obviously he cannot have meant what nowadays is called proper acceleration": if an accelerometer beam is deflected in one reference system and sends out a related signal, this is necessarily the case in all reference systems. And as A.T. pointed out, coordinate acceleration is already relative in SR; consequently he cannot have meant coordinate acceleration either. See also next.

Yes, but not in any "instantaneous action at a distance" way. The influence of the distant stars had to propagate to our local region of spacetime; the metric in our local region of spacetime is not due to the influence of the distant stars "right now", but to the influence of the distant stars in our past light cone. Many "criticisms" of this aspect of GR simply ignore this fact.

I argued that Einstein 's 1918 paper hides even worse than "instantaneous action at a distance" (I provided the link). If you disagree, please start a continuation thread on the twin paradox; it will be interesting! (I will not participate in further hijacking the topic of this thread).

Yes, but that's because Baez defines "real" fields as fields associated with nonzero spacetime curvature. This is a different usage of the term than Einstein's--Einstein's usage equated a "gravitational field" with nonzero connection coefficients. [..].

Indeed, and it's similar with "acceleration": both meanings as used by Einstein are obscured by modern usage. It's very unfortunate that even in physics words acquired altered meanings. Altered meanings can be due to misunderstanding or lack of understanding (my favourite example: "billion"!), but it can also be the result of philosophical disagreement.

 

[Dale]

It's very unfortunate that even in physics words acquired altered meanings.

As our understanding evolves, our words and their meanings evolve also. Using words the same way and with the same meaning from a century ago only hinders progress. All it helps is learning historical trivia.

Personally, I think it is far preferable to read modern treatments of a topic using modern terminology and concepts than it is to read the original works with their outdated terminology and concepts.

 

[harrylin]

As our understanding evolves, our words and their meanings evolve also. Using words the same way and with the same meaning from a century ago only hinders progress. All it helps is learning historical trivia.[..]

From experience and in agreement with George Orwell, I came to a rather different conclusion.

Personally, I think it is far preferable to read modern treatments of a topic using modern terminology and concepts than it is to read the original works with their outdated terminology and concepts.

Personally I think that the study of both modern treatments and the original works is necessary for a correct understanding of a topic. And happily, thanks to the Web, everyone is increasingly enabled to observe and understand the modifications that took place.

 

[PeterDonis]

Obviously he cannot have meant what nowadays is called proper acceleration":

And as A.T. pointed out, coordinate acceleration is already relative in SR; consequently he cannot have meant coordinate acceleration either

Since these are the only two kinds of acceleration there are, I don't know what you think he meant.

I argued that Einstein 's 1918 paper hides even worse than "instantaneous action at a distance" (I provided the link). If you disagree, please start a continuation thread on the twin paradox

Yes, I will; the previous thread you posted in is dormant so I'll open a new one and reference it. I'll post a link here when I do.

 

[Dale]

in agreement with George Orwell

Probably not the best source for a physics discussion.

 

[harrylin]

Probably not the best source for a physics discussion.

Our discussion had drifted to how "words and their meanings evolve": Orwell was an expert on that topic! Sure, it''s better to stick to physics here.

 

[PeterDonis]

I'll post a link here when I do

Here's the link to the new thread:

https://www.physicsforums.com/threads/using-gr-to-analyze-the-twin-paradox.806102/

 

[harrylin]

Here's the link to the new thread:

https://www.physicsforums.com/threads/using-gr-to-analyze-the-twin-paradox.806102/

Regretfully the title suggests to me that you intend to analyze the twin paradox with modern GR. If so, it cannot address the issues with Einstein's interpretation that I mentioned in the other thread. But I'll have a look at it, and reply if and insofar as it has anything to do with that.

 

[PeterDonis]

the title suggests to me that you intend to analyze the twin paradox with modern GR

The intent is not to analyze the paradox from first principles, but to look at Einstein's analysis in the dialogue you linked to, and the criticism you quoted in the other thread, and consider whether the criticism invalidates Einstein's analysis.

 

[Dale]

it''s better to stick to physics here.

That would be a welcome change.