Relativity vs competing theories

[mangaroosh]

hey guys,

I'm just wondering about the existing competing theories; it's been mentioned in a number of threads that there are a couple of theories which equally explain the results of relativity experiments, but which are based on different underlying principles. I've also read that relativity is the only one that involves relativity of simultaneity; is that true? Do the other theories involve absolute simultaneity?

I'm also wondering what those other theories are?

 

[Dale]

The only one I am aware of is Lorentz aether theory (LET). There is some disagreement on another thread about what LET is and questions like the one you raised. LET was essentially abandoned early on, so it doesn't have a real clear definitive interpretation. So the arguments tend to be difficult to resolve as far as what LET itself means.

 

[bcrowell]

There are probably quite a few theories of classical gravity that are consistent with all the empirical evidence. Whether these are particularly well motivated is a matter of taste.

Brans-Dicke gravity is viable, but only for values of the omega parameter that make it very similar to GR.

Østvang’s quasi-metric relativity: arxiv.org/abs/gr-qc/0112025v6

Jacobson Einstein-Aether gravity: Ted Jacobson, David Mattingly, "Gravity with a dynamical preferred frame," 2000, http://arxiv.org/abs/gr-qc/0007031 ; Ted Jacobson, "Einstein-aether gravity: a status report," 2008, http://arxiv.org/abs/0801.1547 ; http://relativity.livingreviews.org/Articles/lrr-2006-3/ (has experimental tests)

Of these three, the only one I really know anything significant about is BD gravity, which incorporates relativity of simultaneity in the same way as GR. I can't imagine how any viable theory of gravity could fail to incorporate relativity of simultaneity, since that's a feature of SR, and SR has been verified to extremely high precision.

 

[Dale]

Oh, good point. I was just talking about SR, not gravity.

 

[tom.stoer]

I would like to add Einstein-Cartan-theory which is (as an enhancement of GR) indistinguishable from GR due to the tininess of the spin-current and torsion effects, but which is favoured by theoretical reasons.

 

[harrylin]

hey guys,

I'm just wondering about the existing competing theories; it's been mentioned in a number of threads that there are a couple of theories which equally explain the results of relativity experiments, but which are based on different underlying principles. I've also read that relativity is the only one that involves relativity of simultaneity; is that true? Do the other theories involve absolute simultaneity?

I'm also wondering what those other theories are?

Probably you refer here to Special Relativity (SR) and physical models for it ("metaphysical" explanations). As a matter of fact, SR is based on postulates about phenomena and not directly on physical models. However, the second postulate was based on the success of the Maxwell-Lorentz model of light propagation, and that theory involved the concept of absolute simultaneity. Since SR emerged from those foundations, it is not really a "competing" theory. Einstein phrased it as follows:

"We [...] assume that the clocks can be adjusted in such a way that
the propagation velocity of every light ray in vacuum - measured by
means of these clocks - becomes everywhere equal to a universal
constant c, provided that the coordinate system is not accelerated.
[..this] "principle of the constancy of the velocity of light," is at
least for a coordinate system in a certain state of motion [..] made
plausible by the confirmation through experiment of the Lorentz theory
[1895], which is based on the assumption of an ether that is
absolutely at rest".
- http://www.soso.ch/wissen/hist/SRT/E-1907.pdf

The essential point of SR is that "absolute rest" cannot be detected; and consequently also no "absolute simultaneity" can be detected either. SR only relates to things that can be detected.

Addendum: there are competing explanations for SR, just as there are also many competing explanations for quantum mechanics. Some main explanations of SR:
- the existence of a (3D) "ether" (also called physical space or vacuum)
- the existence of a 4D physical "spacetime" (also called block universe or 4D ether)
- shut up and calculate (a non-explanation, but possibly most popular)

 

[ghwellsjr]

hey guys,

I'm just wondering about the existing competing theories; it's been mentioned in a number of threads that there are a couple of theories which equally explain the results of relativity experiments, but which are based on different underlying principles. I've also read that relativity is the only one that involves relativity of simultaneity; is that true? Do the other theories involve absolute simultaneity?

I'm also wondering what those other theories are?

Just in case you are asking about competing theories to Special Relativity (and not General Relativity), the wikipedia article on the One-way speed of light provides two, Lorentz ether theory and [STRIKE]Roger's[/STRIKE] Edwards' theory. The only difference between these three theories is the differing "second" postulate that they are based on. The "second" postulate has to do with simultaneity, that is, how they arbitrarily decide to divide the time of the round-trip measurement of the speed of light into its two parts, one for the outbound part of the trip and one for the inbound part of the trip. Special Relativity makes them equal. Lorentz ether theory makes them unequal except in one presumed state of rest. [STRIKE]Roger's[/STRIKE] Edwards' theory emphasizes that the division is truly arbitrary and let's you pick and choose anyone you want in any particular situation.

Please note that all three theories share the same "first" postulate, the principle of relativity. This adds to the confusion when you ask about "relativity" without specifying what you mean. I'm assuming that you meant "Special Relativity" which is one of the three relativity theories all based on the principle of relativity.

 

[harrylin]

Just in case you are asking about competing theories to Special Relativity (and not General Relativity), the wikipedia article on the One-way speed of light [..].

If Einstein were still alive, he would probably call that Wikipedia article extremely misleading as it turns a fly (different ways of formulating or interpreting the same theory) into an elephant (different theories)...

 

[ghwellsjr]

Just in case you are asking about competing theories to Special Relativity (and not General Relativity), the wikipedia article on the One-way speed of light[..].

If Einstein were still alive, he would probably call that Wikipedia article extremely misleading as it turns a fly (different ways of formulating or interpreting the same theory) into an elephant (different theories)...

You just quoted Einstein in which he referred to Lorentz theory so I think the facts do not support your speculation.

 

[ghwellsjr]

The essential point of SR is that "absolute rest" cannot be detected; and consequently also no "absolute simultaneity" can be detected either. SR only relates to things that can be detected.

It is also an essential point of LET and Edwards' theory that "absolute rest" cannot be detected--that's the whole point of the "principle of relativity" that Einstein discussed in the 1907 paper that you referenced. It is true that "absolute simultaneity" cannot be detected but that doesn't mean that a "relative simultaneity" can be detected. Einstein's whole point, if you read what he says, is that no simultaneity can be detected, so for you to claim that SR relates to things that can be detected is wrong. That's why these three theories are indeed three separate theories, because they are each built on a different "second" postulate that defines the meaning of simultaneity in three different ways. Simultaneity of any type cannot be detected. All three of these theories equally comport with all experimental evidence. There is no way to detect which one of these three theories is preferred.

Once you understand that Edwards' theory let's you decide how to define simultaneity (which was really Einstein's idea all along), then you can see that Einstein's arbitrary choice is the preferred one because it is simpler, not because it is based on something that can be detected.

 

[Naty1]

Here is a suvery of the experimental basis of special relativity...supporting experiments.

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

In brief:

as of this writing there are no reproducible and generally accepted experiments that are inconsistent with SR, within its domain of applicability.

But this article does not discuss, that I could see in a quick scan, if any other theories also meet these all these test criteria.

 

[ghwellsjr]

But this article does not discuss, that I could see in a quick scan, if any other theories also meet these all these test criteria.

Sure it does, right here:

3.2 One-Way Tests of Light-Speed Isotropy

Note that while these experiments clearly use a one-way light path and find isotropy, they are inherently unable to rule out a large class of theories in which the one-way speed of light is anisotropic. These theories share the property that the round-trip speed of light is isotropic in any inertial frame, but the one-way speed is isotropic only in an aether frame. In all of these theories the effects of slow clock transport exactly offset the effects of the anisotropic one-way speed of light (in any inertial frame), and all are experimentally indistinguishable from SR. All of these theories predict null results for these experiments. See Test Theories above, especially Zhang (in which these theories are called “Edwards frames”).

 

[mangaroosh]

It is also an essential point of LET and Edwards' theory that "absolute rest" cannot be detected--that's the whole point of the "principle of relativity" that Einstein discussed in the 1907 paper that you referenced.

That point about "absolute rest" is an interesting one, because the effect of Einstein's relativity (and perhaps others) appears to be the same as treating inertial reference frames as being absolute rest frames, from their own perspective.

It is true that "absolute simultaneity" cannot be detected but that doesn't mean that a "relative simultaneity" can be detected. Einstein's whole point, if you read what he says, is that no simultaneity can be detected, so for you to claim that SR relates to things that can be detected is wrong. That's why these three theories are indeed three separate theories, because they are each built on a different "second" postulate that defines the meaning of simultaneity in three different ways. Simultaneity of any type cannot be detected. All three of these theories equally comport with all experimental evidence. There is no way to detect which one of these three theories is preferred.

Once you understand that Edwards' theory let's you decide how to define simultaneity (which was really Einstein's idea all along), then you can see that Einstein's arbitrary choice is the preferred one because it is simpler, not because it is based on something that can be detected.

Einstein's theory involves relative simultaneity; do the other theories involve absolute simultaneity?

 

[ghwellsjr]

That point about "absolute rest" is an interesting one, because the effect of Einstein's relativity (and perhaps others) appears to be the same as treating inertial reference frames as being absolute rest frames, from their own perspective.

This is true for Einstein's Theory of Special Relativity but not for the others.

Einstein's theory involves relative simultaneity; do the other theories involve absolute simultaneity?

The expression "relative simultaneity" only makes sense when the idea of "relative time" is significant. Prior to Einstein's ideas, it just didn't occur to anyone that time could be actually relative. Even though LET uses all the same equations, the terminology is that what we now call "Proper Time" (the legitimate time displayed on any clock) was called by them "local time", meaning that it was not really the time that nature operates on, but rather a "modified" or "corrupted" time. They viewed the time on their clocks as suffering from an unknown amount of time dilation caused by unavoidable motion through the ether.

I looked up Edwards' paper and I would say he wasn't promoting a new theory but merely elaborating on Einstein's observation that the two parts of the round-trip speed of light measurement can be assigned arbitrarily in any proportion, including making the speed in one direction c/2 and ∞ in the other direction.

He has a section on using the slow transport of clocks "for the absolute synchronization of clocks in a coordinate system" and he concludes "a clock that is moved to all positions in a coordinate system to set the other clocks does not establish an absolute synchronization" [emphasis his].

But he concludes his paper saying, "For most problems the most convenient assumption to make is still that of isotropic space", in other words, Einstein's synchronization convention.

 

[mangaroosh]

The expression "relative simultaneity" only makes sense when the idea of "relative time" is significant. Prior to Einstein's ideas, it just didn't occur to anyone that time could be actually relative. Even though LET uses all the same equations, the terminology is that what we now call "Proper Time" (the legitimate time displayed on any clock) was called by them "local time", meaning that it was not really the time that nature operates on, but rather a "modified" or "corrupted" time. They viewed the time on their clocks as suffering from an unknown amount of time dilation caused by unavoidable motion through the ether.

Under LET, how is "the time that nature operates on" determined?

 

[ghwellsjr]

Under LET, how is "the time that nature operates on" determined?

It isn't. It's just presumed that it exists. Put yourself back before Einstein's time. Would you ever imagine that time runs differently under different circumstances? Not just that your clocks might be affected by those circumstances but that time itself is relative? Or that each observer determines that the other ones time is running slower? It's one thing to believe in Galilean relativity where time and spatial distances are absolute where all you have to do is have the obvious sort of offsets to get from one frame to the other but what Einstein came up with is a giant leap and not at all obvious.

 

[Dale]

Please note that all three theories share the same "first" postulate, the principle of relativity.

I still disagree with this. I think that all aether theories, including LET, violate the principle of relativity. That is, IMO, the point of the aether.

 

[mangaroosh]

It isn't. It's just presumed that it exists. Put yourself back before Einstein's time. Would you ever imagine that time runs differently under different circumstances? Not just that your clocks might be affected by those circumstances but that time itself is relative? Or that each observer determines that the other ones time is running slower? It's one thing to believe in Galilean relativity where time and spatial distances are absolute where all you have to do is have the obvious sort of offsets to get from one frame to the other but what Einstein came up with is a giant leap and not at all obvious.

That's what I was thinking, I just wanted to make doubly sure, cheers.

Just to try and get a better understanding of the differences, would you by any chance know how the light clock on the train experiment would be interpreted under LET?

Would the clock on the train slow down in that reference frame?

 

[ghwellsjr]

Just to try and get a better understanding of the differences, would you by any chance know how the light clock on the train experiment would be interpreted under LET?

Exactly the same way as under SR.

Would the clock on the train slow down in that reference frame?

Yes, but nobody knows by what amount.

The difference is that LET would never consider the ground frame because it represents the stationary ether and we don't know where that is. We're all doomed to ride around on the train having our clocks running slow (by an unknown amount) and having our lengths contracted along the direction of the train (again, by an unknown amount).

Einstein said, "You don't need the ground--just assume that you are stopped and anyone moving with respect to you is on a different train. All you care about is the relative difference in your respective trains. And everybody agreed.

 

[harrylin]

It is also an essential point of LET and Edwards' theory that "absolute rest" cannot be detected--that's the whole point of the "principle of relativity" that Einstein discussed in the 1907 paper that you referenced. It is true that "absolute simultaneity" cannot be detected but that doesn't mean that a "relative simultaneity" can be detected. Einstein's whole point, if you read what he says, is that no simultaneity can be detected, so for you to claim that SR relates to things that can be detected is wrong. [..]

That's not what I meant; probably I should have written that SR only relates to things can be experimentally verified. My point was that while the predictions of a theory of physics relate to the setting of measurement instruments, no theory depends on those settings - also not SR. No theory postulates how experimenters will act. On a side note, relative simultaneity is detected continuously with GPS receivers in the sense that corrections must be made for the motion of the geoid.

 

[harrylin]

I still disagree with this. I think that all aether theories, including LET, violate the principle of relativity. That is, IMO, the point of the aether.

The historical facts are that Lorentz and Langevin were among the most prominent early relativity teachers and Einstein came back to the ether without abandoning the PoR.

 

[harrylin]

You just quoted Einstein in which he referred to Lorentz theory so I think the facts do not support your speculation.

You are confused (perhaps due to a mistranslation): Lorentz's theory of 1895 to which Einstein refers there was not relativistic. In contrast, Einstein discusses there Lorentz's paper of 1904 and his own paper of 1905 as the same theory. Therefore also the bundle "The Principle of Relativity" starts with Lorentz-1904.

 

[harrylin]

That point about "absolute rest" is an interesting one, because the effect of Einstein's relativity (and perhaps others) appears to be the same as treating inertial reference frames as being absolute rest frames, from their own perspective.

Einstein's theory involves relative simultaneity; do the other theories involve absolute simultaneity?

The ether interpretation involves absolute simultaneity as a concept of something that exists in nature but cannot be detected; only relative simultaneity can be detected. In contrast, the physical spacetime interpretation regards relative simultaneity as slices through spacetime, without the existence of absolute simultaneity.

 

[ghwellsjr]

Please note that all three theories share the same "first" postulate, the principle of relativity.

I still disagree with this. I think that all aether theories, including LET, violate the principle of relativity. That is, IMO, the point of the aether.

I'm only repeating what Einstein repeatedly says in all his papers, books, and talks.

For example, look at his 1907 paper that harrylin quoted in post #6:

He starts off describing how Newton's laws of physics retained their form under Galilean transformation which he specifically called "the principle of relativity". But then the Lorentz's laws of electrodynamics based on "a stationary immobile ether" put that principle in jeopardy because they did not transform intact. And so the search went out to locate the ether culminating in MMX which affirmed the principle of relativity but it had to be based on a different transformation which Lorentz came up with. The only difference between Lorentz's approach and Einstein's is that Lorentz derived the transformation after an experiment forced him to, whereas Einstein starts with the generalized principle of relativity and derives the transformation from a necessary consequence of applying the second postulate. And he specifically states that it is this second postulate that makes the difference between Lorentz's theory and his own theory of Special Relativity when he says:

It required only the recognition that the auxiliary quantity introduced by H. A. Lorentz, and called by him "local time", can be defined as simply "time."

This then leads to the statement that the ether "does not fit in" with his theory.

Poincaré, in his 1904 paper, also recounts under the heading of The Principle of Relativity, how Lorentz "valiantly defended" the principle of relativity by coming up with a new transformation.

Now I'm not saying that anyone but Einstein ever formally presented two postulates as the basis for a theory or even that Einstein was claiming that Lorentz had his own two postulates but the net result was the same as if he had and I think it is helpful to point this out.

 

[harrylin]

[..] he specifically states that it is this second postulate that makes the difference between Lorentz's theory and his own theory of Special Relativity when he says:

It required only the recognition that the auxiliary quantity introduced by H. A. Lorentz, and called by him "local time", can be defined as simply "time."

[..] Now I'm not saying that anyone but Einstein ever formally presented two postulates as the basis for a theory or even that Einstein was claiming that Lorentz had his own two postulates but the net result was the same as if he had and I think it is helpful to point this out.

Yes I think so too - thanks.
Note that everyone was forced to come up with a theory that conforms to the PoR, Einstein also refers to those experiments in his 1905 paper; however Lorentz's approach was a bit messy while Einstein derived the transformation equations in a cleaner way.

But please clarify what you mean with "he specifically states", as in that sentence he doesn't mention the second postulate and the way he phrases the second postulate in that 1907 paper is in agreement with Lorentz's 1904 paper.

 

[ghwellsjr]

You are confused (perhaps due to a mistranslation): Lorentz's theory of 1895 to which Einstein refers there was not relativistic. In contrast, Einstein discusses there Lorentz's paper of 1904 and his own paper of 1905 as the same theory. Therefore also the bundle "The Principle of Relativity" starts with Lorentz-1904.

Yes, Einstein traces that development in Lorentz's ether theory but he never stops calling it a theory different than his own or pointing out that it is his second postulate which make the difference as a starting point and the lack of an ether as an ending point. I count at least seven times in the first column of page 513 where Einstein refers specifically to Lorentz's 1904 theory and two of those times are in contrast to his own theory of relativity. He never claims that they are merely two interpretations of the same theory. In this and other papers, he always shows a contrast to Lorentz's ether theory as a result of his second postulate.

 

[harrylin]

Yes, Einstein traces that development in Lorentz's ether theory but he never stops calling it a theory different than his own or pointing out that it is his second postulate which make the difference as a starting point and the lack of an ether as an ending point. I count at least seven times in the first column of page 513 where Einstein refers specifically to Lorentz's 1904 theory and two of those times are in contrast to his own theory of relativity. He never claims that they are merely two interpretations of the same theory. In this and other papers, he always shows a contrast to Lorentz's ether theory as a result of his second postulate.

First of all, no "Lorentz ether theory" existed in those days. Perhaps you mean Lorentz's 1895 theory of electrons, to which both Einstein and Lorentz referred in their respective papers? For the rest, I can find little to nothing of what you say in the paper itself, perhaps because I base myself on the German original and not on a poor translation (I think that I already made you attentive on that subtle but important detail in an earlier discussion).

 

[ghwellsjr]

But please clarify what you mean with "he specifically states", as in that sentence he doesn't mention the second postulate and the way he phrases the second postulate in that 1907 paper is in agreement with Lorentz's 1904 paper.

He doesn't use the words, "second postulate" in the first column of page 513 but he does talk about Lorentz's "local time" in contrast to Einstein's "new time concept" which come directly out of his second postulate.

Then ending on page 513 and continuing on to the first column of page 514, he develops this "new time concept" describing his synchronization convention for remote clocks and calling it the "principle of the constancy of the velocity of light". He then affirms "the experimental confirmations of Lorentz's theory, which is based on the assumption of an absolutely stationary ether". In other words, his synchronization convention is identical to the absolutely stationary ether. What "is not at all self-evident" is that this same convention, reserved only for the ether frame in LET can be arbitrarily applied to any reference frame in SR.

So LET claims that there is only one frame that has the ether-like characteristic in which light propagates at c in all directions, SR claims that any frame has that ether-like characteristic. They are two different theories based on two different "second" postulates.

 

[ghwellsjr]

First of all, no "Lorentz ether theory" existed in those days. Perhaps you mean Lorentz's 1895 theory of electrons, to which both Einstein and Lorentz referred in their respective papers? For the rest, I can find little to nothing of what you say in the paper itself, perhaps because I base myself on the German original and not on a poor translation (I think that I already made you attentive on that subtle but important detail in an earlier discussion).

The first rule of PF is that "All posts must be in English". I would think that would apply to all links too, so when you link to Einstein's original paper in German and claim that you can translate it better than the link I provided in English, it doesn't appear to me to be playing by the rules. If you want to provide another link to his paper in English that you believe is better than the one I provided, I'll be happy to consider the differences but for now, I'm going to have to point you to quotes in the paper I linked to.

Now you are claiming that when Einstein originally referenced Lorentz theory, it was not in regard to ether, just about electrons. Well, I started this train of posts at #24 quoting the beginning of Einstein's paper with regard to "Lorentz's laws of electrodynamics based on 'a stationary immobile ether'" but I changed Einstein's wording. He called it "the electrodynamic theory of H. A. Lorentz" and then he said, "That theory is founded, namely, on the assumption of a stationary immobile ether." He doesn't provide a year for this theory so I don't know if it is the same one you are talking about, but he traces the progress of Lorentz's theory from that point on, never failing to call it a theory.

I don't know if at that time it is was specifically called "Lorentz ether theory" or LET but I also don't know if Einstein's theory was called "Special Relativity" so why should we care about precise terminology?

 

[harrylin]

He doesn't use the words, "second postulate" in the first column of page 513 but he does talk about Lorentz's "local time" in contrast to Einstein's "new time concept" which come directly out of his second postulate. [..]

Hmm ... Einstein doesn't claim or suggest there that that is his new time concept, nor does he suggest that that concept comes directly out of his second postulate. However, we know that it corresponds to Poincare's new time concept which was based on Lorentz's "local time"; and Poincare had suggested to his readers that it was fully Lorentz's idea.

What "is not at all self-evident" is that this same convention, reserved only for the ether frame in LET can be arbitrarily applied to any reference frame in SR. [..]

As you probably know, no "Lorentz Ether Theory" was known by Lorentz or Einstein... Perhaps we should start a thread on that 1907 paper, as it's not exactly the topic of this thread.

 

[harrylin]

The first rule of PF is that "All posts must be in English". I would think that would apply to all links too, so when you link to Einstein's original paper in German and claim that you can translate it better than the link I provided in English, it doesn't appear to me to be playing by the rules.

If you want to provide another link to his paper in English that you believe is better than the one I provided, I'll be happy to consider the differences but for now, I'm going to have to point you to quotes in the paper I linked to. [..]

Sorry, I don't think that your extension of rules is what was intended. Many people in this forum can read and understand German, and those who can't should rely on those who do (the translation that you refer to even contains pertinent misleading mistranslation, so big that even non-German speaking people can easily verify it).

Anyway in an earlier discussion I did refer to a better English translation which however is not online; and I also gave a link to the German original. And once more: if you like to discuss that paper in detail, I propose to start it as a topic, so that we not further hijack this thread and avoid diluting the same information in different threads.

[..] I don't know if at that time it is was specifically called "Lorentz ether theory" or LET but I also don't know if Einstein's theory was called "Special Relativity" so why should we care about precise terminology?

The error is in making a distinction that Einstein did not make and then projecting that view on his words.

Harald

 

[Dale]

The historical facts are that Lorentz and Langevin were among the most prominent early relativity teachers and Einstein came back to the ether without abandoning the PoR.

I disagree with both of these claims too. Particularly the comment about Einstein. I know the speech you are referring to, but he clearly mentions that the "aether" of GR did not have a rest frame, making it unrelated to the luminiferous aether of earlier theories.

 

[Dale]

The ether interpretation involves absolute simultaneity as a concept of something that exists in nature but cannot be detected; only relative simultaneity can be detected.

I agree. This is why my understanding is that LET regards the PoR as being something that is violated in nature but cannot be detected.

 

[ghwellsjr]

The ether interpretation involves absolute simultaneity as a concept of something that exists in nature but cannot be detected; only relative simultaneity can be detected.

I agree. This is why my understanding is that LET regards the PoR as being something that is violated in nature but cannot be detected.

We all agree with the first part of harrylin's quote, but the question is can relative simultaneity be detected? Einstein says no, it has to be defined.

 

[harrylin]

I disagree with both of these claims too. Particularly the comment about Einstein. I know the speech you are referring to, but he clearly mentions that the "aether" of GR did not have a rest frame, making it unrelated to the luminiferous aether of earlier theories.

I did not refer to "a speech" but to a number of articles, including a speech; and you seem to contradict Einstein's precision that the ether of the general theory of relativity is transmuted conceptually into the ether of Lorentz if we substitute constants for its functions of space. But we seem to agree that the label "ether" doesn't identify competing theories, so I won't elaborate more here as that would again deviate from the topic.

 

[harrylin]

We all agree with the first part of harrylin's quote, but the question is can relative simultaneity be detected? Einstein says no, it has to be defined.

That's also correct; as I elaborated, it depends on what you mean with that. To elaborate more: with GPS time and local "Einstein synchronization" we should be able to detect the difference in standard clock synchronization (which is commonly called "relativity of simultaneity") between the ECI frame and a lab frame.

 

[harrylin]

I agree. This is why my understanding is that LET regards the PoR as being something that is violated in nature but cannot be detected.

I'm fed up with the strawman called "LET"; I now conclude that it is less than useless - thus I will from now on ban it completely.

 

[mangaroosh]

Exactly the same way as under SR.
Yes, but nobody knows by what amount.

Apologies, I was probably a bit vague in the question. I thought I had read on here (may have been elsewhere) that LET would treat the slowing of the ticking clock as being a result of the mechanics of the clock being affected by the motion i.e. that the moving clock ticks slower because the photon has a longer distance to travel.

The difference is that LET would never consider the ground frame because it represents the stationary ether and we don't know where that is. We're all doomed to ride around on the train having our clocks running slow (by an unknown amount) and having our lengths contracted along the direction of the train (again, by an unknown amount).

Einstein said, "You don't need the ground--just assume that you are stopped and anyone moving with respect to you is on a different train. All you care about is the relative difference in your respective trains. And everybody agreed.

Could LET be formulated without the superfluous rest frame? For example, if we were to say that if an absolute rest frame were to exist, then the Earth would be moving relative to it with a velocity v and if an absolute rest frame were to exist, a train moving relative to the Earth would be moving with a velocity u relative to that absolute rest frame.

Instead of assuming that we are stopped in the absolute rest frame, and considering everything else in motion with respect to us, could we assume that if an absolute rest frame were to exist, then we would be in motion with respect to it, as would everything else?

Again, we don't need the rest frame, we can just hypothesise that if one were to exist, that we would be in motion relative to it.

 

[ghwellsjr]

Apologies, I was probably a bit vague in the question. I thought I had read on here (may have been elsewhere) that LET would treat the slowing of the ticking clock as being a result of the mechanics of the clock being affected by the motion i.e. that the moving clock ticks slower because the photon has a longer distance to travel.

Prior to Einstein, Lorentz and others spent a lot of time trying to figure out the mechanics of length contraction and time dilation but nowadays, when we speak of LET, we have striped it of everything except the idea of a preferred rest frame.

Could LET be formulated without the superfluous rest frame?

Well the idea of LET is that nature needs it in some unknown way to make things happen so there wouldn't be much point in formulating LET without that rest frame. It's not superfluous to LET.

For example, if we were to say that if an absolute rest frame were to exist, then the Earth would be moving relative to it with a velocity v and if an absolute rest frame were to exist, a train moving relative to the Earth would be moving with a velocity u relative to that absolute rest frame.

Instead of assuming that we are stopped in the absolute rest frame, and considering everything else in motion with respect to us, could we assume that if an absolute rest frame were to exist, then we would be in motion with respect to it, as would everything else?

Again, we don't need the rest frame, we can just hypothesise that if one were to exist, that we would be in motion relative to it.

Sure, even in SR you don't have to pick a Frame of Reference in which you are at rest or the Earth is at rest or the solar system is at rest or the galaxy is at rest. You can pick one in which you are traveling at any speed in any direction (as long as it is less than the speed of light).

 

[mangaroosh]

Prior to Einstein, Lorentz and others spent a lot of time trying to figure out the mechanics of length contraction and time dilation but nowadays, when we speak of LET, we have striped it of everything except the idea of a preferred rest frame.

Sorry, I'm not sure I get what you mean by "stripped it of everything".

Well the idea of LET is that nature needs it in some unknown way to make things happen so there wouldn't be much point in formulating LET without that rest frame. It's not superfluous to LET.

I'm just wondering, does it actually require an absolute rest frame? Again, we could hypothesise that if one were to exist, then we would be moving relative to it, but we don't really need to assert that it actually exists, simply that we [the earth] are in motion and not at absolute rest.

Sure, even in SR you don't have to pick a Frame of Reference in which you are at rest or the Earth is at rest or the solar system is at rest or the galaxy is at rest. You can pick one in which you are traveling at any speed in any direction (as long as it is less than the speed of light).

Does that carry with it an implicit assumption that some reference frame is at rest?

 

[ghwellsjr]

Sorry, I'm not sure I get what you mean by "stripped it of everything".

Stripped of any characteristics of an ether to explain why lengths contract and clocks run slower. I thought I explained that. And I didn't say "everything", I said "everything except the idea of a preferred rest frame." Think of it exactly like any rest frame in Special Relativity except that there is only one in which the speed of light is actually c.

I'm just wondering, does it actually require an absolute rest frame? Again, we could hypothesise that if one were to exist, then we would be moving relative to it, but we don't really need to assert that it actually exists, simply that we [the earth] are in motion and not at absolute rest.

LET postulates that there exists a single rest frame in which light travels at c. There is no proof or evidence for this but neither is there any proof or evidence that denies that it can't happen.

Does that carry with it an implicit assumption that some reference frame is at rest?

Yes.

It's no different than Einstein's second postulate that light travels at c in any reference frame. That also has no proof or evidence but neither is there any proof or evidence that denies that it can't happen.

It sure seems to me like you're understanding this. I don't see why you keep asking the same questions and getting the same answers. Maybe you could explain why you're struggling with this.

 

[harrylin]

[..] Could [..] be formulated without the superfluous rest frame?

That is just what Einstein did in 1905, and the result was SR. For that purpose he rephrased Maxwell's assumption about light propagation in vacuum by the same assumption relative to an unidentified but presumably arbitrary Newtonian system of reference (which he identified with "empty space") and made it a postulate. I find that physically a bit messy (discarding a physical model while keeping the phenomenon that is based on it), but it did the trick.
- http://www.fourmilab.ch/etexts/einstein/specrel/www/
- http://www.bartleby.com/173/7.html

For example, if we were to say that if an absolute rest frame were to exist, then the Earth would be moving relative to it with a velocity v and if an absolute rest frame were to exist, a train moving relative to the Earth would be moving with a velocity u relative to that absolute rest frame.

That is correct of course. See also the discussion here which highlights the philosophical differences between interpretations:
- http://www.bartleby.com/173/7.html

Instead of assuming that we are stopped in the absolute rest frame, and considering everything else in motion with respect to us, could we assume that if an absolute rest frame were to exist, then we would be in motion with respect to it, as would everything else? [..]

Obviously.

 

[harrylin]

[..] in an earlier discussion I did refer to a better English translation which however is not online; and I also gave a link to the German original. And once more: if you like to discuss that paper in detail, I propose to start it as a topic, so that we not further hijack this thread and avoid diluting the same information in different threads. [..]

As a correct understanding of that paper may be of general interest, I now started a discussion of it as a new topic.

Harald

 

[mangaroosh]

The first rule of PF is that "All posts must be in English".

I thought the first rule of PF was "don't talk about fight club"

 

[mangaroosh]

It sure seems to me like you're understanding this. I don't see why you keep asking the same questions and getting the same answers. Maybe you could explain why you're struggling with this.

I think I am getting an understanding of it, but I'm not yet certain I understand the nuances of both; so my apologies if the questions are repetitive, but I'm not sure I that I have fully understood the implications; also, the statement that LET and SR would treat the light clock [on the train] thought experiment exactly the same, kind of threw me; because I thought both offered different explanations for what happens.

Stripped of any characteristics of an ether to explain why lengths contract and clocks run slower. I thought I explained that. And I didn't say "everything", I said "everything except the idea of a preferred rest frame." Think of it exactly like any rest frame in Special Relativity except that there is only one in which the speed of light is actually c.

I'm not sure if you expressed it exactly so, but the understanding I've garnered, from discussing it with you, would definitely be in accord with that; as mentioned though, the statement above kind of threw me. With regard to the "stripping of everything"; I probably should have been more explicit in my reference, because it was the "everything [else]" that I wasn't 100% on.


Please forgive my continual recourse to the light clock thought experiment, if it seems either naiive or monotonous; it's just that I [personally] find it quite helpful as a visual aid to addressing my understanding, or lack thereof. Essentially, the differences [as I understood them] are:

• the reciprocity of the contractions i.e. there is reciprocity under SR but not LET
  Essentially, under SR each observers clock ticks normally, from their perspective, and it's the moving clock that ticks slower; but under LET, both clocks tick slower than the rest frame clock, but the trains clock ticks slowest (assuming a higher velocity relative to the rest frame)

• -the attributed cause of the contractions i.e. under SR time slows down, under LET it's the mechanics of the clock.
  My understanding is that, under SR, the relative motion of the reference frame causes spacetime to be affected, such that lengths contract and time slows down; but under LET a slower ticking clock is ascribed to the mechanics of the clock arising from the motion relative to the rest frame - i.e. the photon has a longer distance to travel between mirrors in a moving clock.

My understanding of LET is that, if the observer on the train were in a windowless carriage - like Galileo's observer on the ship - they would not be able to determine if they were at absolute rest, so they could not tell if their clock was ticking at the "normal" rate, or if it was actually ticking slower, because they would have nothing to compare it to. If, however, they were able to see the earthbound clock, they would presumably be able to calculate that it was ticking faster (assuming the Earth is traveling with a lower velocity relative to the rest frame)

I had a more detailed explanation of what I mean typed out, but I wanted to try and make it more concise. I can post that if the above isn't very clear.

LET postulates that there exists a single rest frame in which light travels at c. There is no proof or evidence for this but neither is there any proof or evidence that denies that it can't happen.

Yes.

It's no different than Einstein's second postulate that light travels at c in any reference frame. That also has no proof or evidence but neither is there any proof or evidence that denies that it can't happen.

I'm still a bit unclear as to the status of the second postulate; initially I thought it was absolutely set in stone, but I'm a lot less certain of that now. I fully trust what you say about it, but when others reference experiments which purportedly verify the second postulate, I wonder if there is some nuance that I haven't picked up on. To me it seems as though there is [at least limited] scope in the formulation of this postulate.

Again, just to return to the light clock thought experiment, to try and clarify my [lack of] understanding: the tick-tock of a light clock is determined by the distance the photon has to travel in the light clock; under SR this is represented by a line perpendicular to the midpoint of both mirrors. This would be the same as if the clock were at absolute rest.

I've been told that this representation of the path of the photon is not based on an assumption, but is something that is borne out by experiment; or from the verification of the second postulate; that it is because the speed of light is constant, regardless of motion relative to the source, that it has to be so, otherwise the observer on the train would measure a slower speed of light.

To me it appears as though this is not the case under LET, where, as you mention, we can just consider the train's clock from the absolute rest frame - which would be the same as the observer on the platform's view, in the Einsteinian version. In this case the tick-tock of the trains clock is not simply the perpendicular line between the midpoints of the two mirrors, rather the line as represented by the hypotenuse of a right angled triangle; and this would be as true for the observer on the train as for the observer in the absolute rest frame - even if the observer on the train could not determine that it was so.


Hopefully the above makes sense, and if I haven't entirely exhausted your patience, if you could point out anywhere I might be going wrong, I would be very greatful.

 

[harrylin]

[..] I thought both offered different explanations for what happens. [..]

Well yes, but the explanations are at different levels, just as Newton's explanations differ from what nowadays is called classical mechanics. As Lorentz later put it: “Einstein simply postulates what we have deduced [...] from the fundamental equations of the electromagnetic field”. Note that that remark refers to the second postulate: the PoR was not deduced from the field equations but appeared to be at odds with it.

[..] if the observer on the train were in a windowless carriage - like Galileo's observer on the ship - they would not be able to determine if they were at absolute rest, so they could not tell if their clock was ticking at the "normal" rate, or if it was actually ticking slower, because they would have nothing to compare it to. If, however, they were able to see the earthbound clock, they would presumably be able to calculate that it was ticking faster (assuming the Earth is traveling with a lower velocity relative to the rest frame)

I can see no reason for such an assumption in any interpretation of SR.

I'm still a bit unclear as to the status of the second postulate; initially I thought it was absolutely set in stone, but I'm a lot less certain of that now. [..]

There are several formulations of it; its purpose was to provide a necessary condition based on Maxwell's electrodynamics for the derivation of the new transformation equations. A postulate based on an alternative electrodynamics theory such as by Stokes or Ritz leads to other transformations.

Again, just to return to the light clock thought experiment, to try and clarify my [lack of] understanding: the tick-tock of a light clock is determined by the distance the photon has to travel in the light clock; under SR this is represented by a line perpendicular to the midpoint of both mirrors. This would be the same as if the clock were at absolute rest.

I've been told that this representation of the path of the photon is not based on an assumption, but is something that is borne out by experiment; or from the verification of the second postulate; that it is because the speed of light is constant, regardless of motion relative to the source, that it has to be so, otherwise the observer on the train would measure a slower speed of light.

That is slightly inaccurate; as I think was highlighted in this thread or in another recent thread, experimental results of that time discarded all light models except that of Maxwell-Lorentz. However, in view of several other experiments such as MMX also that one was in problems, and a fix was needed to make it PoR-compatible.

[..] we can just consider the train's clock from the absolute rest frame - which would be the same as the observer on the platform's view, in the Einsteinian version. In this case the tick-tock of the trains clock is not simply the perpendicular line between the midpoints of the two mirrors, rather the line as represented by the hypotenuse of a right angled triangle; and this would be as true for the observer on the train as for the observer in the absolute rest frame - even if the observer on the train could not determine that it was so. [..]

Your line of thinking seems to be a preparation for MMX, as well as, potentially the later KTX:
http://en.wikipedia.org/wiki/Kennedy–Thorndike_experiment

With such known and future experimental results was dealt with here:
http://en.wikisource.org/wiki/Electromagnetic_phenomena

Harald

 

[mangaroosh]

Hey Harry, thanks for replying. Firstly, can I assume, from the lack of comment, that what I wrote about the reciprocity of contractions and the attributed cause of contractions is accurate?

Well yes, but the explanations are at different levels, just as Newton's explanations differ from what nowadays is called classical mechanics. As Lorentz later put it: “Einstein simply postulates what we have deduced [...] from the fundamental equations of the electromagnetic field”. Note that that remark refers to the second postulate: the PoR was not deduced from the field equations but appeared to be at odds with it.

I would potentially question what is deduced from the fundamental equations, particularly because Lorentz's ether theory appears to be based on a slightly different deduction; am I right in that?

I can see no reason for such an assumption in any interpretation of SR.

It wasn't an assumption for SR, it was the application of the principle of relativity to LET; at least that was the intention.

There are several formulations of it; its purpose was to provide a necessary condition based on Maxwell's electrodynamics for the derivation of the new transformation equations. A postulate based on an alternative electrodynamics theory such as by Stokes or Ritz leads to other transformations.

Would it be equally possible to formulate it as, the speed of light is constant with respect to the point of emission, but independent of the motion of the source? Would that conflict with any experimental evidence?

That is slightly inaccurate; as I think was highlighted in this thread or in another recent thread, experimental results of that time discarded all light models except that of Maxwell-Lorentz. However, in view of several other experiments such as MMX also that one was in problems, and a fix was needed to make it PoR-compatible.

Apologies, I keep having to refer back to the light clock on the train thought experiment; does that mean that both the treatment of the path of the photon in SR and LET are possible?

When I say the treatment of the photon under LET, I mean my understanding as I've outlined above i.e. that the path of the photon is not necessarily the perpendicular line between the midpoints of the two mirrors, but rather the path represented by the hypotenuse of a right angled triangle.

Your line of thinking seems to be a preparation for MMX, as well as, potentially the later KTX:
http://en.wikipedia.org/wiki/Kennedy–Thorndike_experiment

With such known and future experimental results was dealt with here:
http://en.wikisource.org/wiki/Electromagnetic_phenomena

Harald

To a certain extent it is a prep. for MMX; again, I'm wondering if the representation of the path of a photon [in a light clock] under LET were a representation of a photon in the MMX, would the null result still arise; I presume it would, but I'm not sure.

 

[ghwellsjr]

• the reciprocity of the contractions i.e. there is reciprocity under SR but not LET
  Essentially, under SR each observers clock ticks normally, from their perspective, and it's the moving clock that ticks slower; but under LET, both clocks tick slower than the rest frame clock, but the trains clock ticks slowest (assuming a higher velocity relative to the rest frame)

There is no difference in what we know is actually happening under SR or under LET. It is only a difference in what you are assigning to things that you can't know are happening. In SR once you assign a Frame of Reference, which defines times and distances, then the reciprocity disappears. If you select a FoR in which you are not at rest, or if you start moving after you select a FoR in which you started out at rest, then time for you is dilated and your lengths are contracted, although you can't tell that. It's only when you then transform all the distances and times (events) into a new definition (a different FoR) that what was assigned a shorter distance or a longer time in one FoR takes on a longer distance or a shorter time in the other FoR.

The easiest way I know to communicate this idea is to think just about relative motion. If A and B are in relative motion, in one FoR, A is assigned a speed of zero and B is assigned a non-zero speed. In another FoR, B is assigned a speed of zero and A is assigned non-zero speed. Do you think that using a different FoR caused A to suddenly start moving? I don't think so, I think with regard to motion, different frames make perfect sense. The same thing applies to length contraction and time dilation. The ones that have motion according to the FoR that you are using are the ones for which lengths and times take on a different meaning.

• -the attributed cause of the contractions i.e. under SR time slows down, under LET it's the mechanics of the clock.
  My understanding is that, under SR, the relative motion of the reference frame causes spacetime to be affected, such that lengths contract and time slows down; but under LET a slower ticking clock is ascribed to the mechanics of the clock arising from the motion relative to the rest frame - i.e. the photon has a longer distance to travel between mirrors in a moving clock.

Do you think that just because in one FoR B is stationary and in another one B is moving that spacetime is affected? Nothing is happening in space or time or spacetime just because you choose to use a different FoR to assign distances and times differently such that A is stationary under one of those definitions and moving under a different definition.

In the early days, Lorentz and others were trying to determine how the mechanics of a clock would cause it to slow down as it traveled through the ether but now we don't worry about that. What we do instead is verify that the laws of physics transform intact under the Lorentz transformation. If they don't, we know they are not valid and we change them until they do. If they do, then we don't have to be concerned about the specific mechanics, it will happen automatically.

 

[harrylin]

Hey Harry, thanks for replying. Firstly, can I assume, from the lack of comment, that what I wrote about the reciprocity of contractions and the attributed cause of contractions is accurate?

Not really, it only means that when skimming over that part I didn't see anything shocking.
I wasn't sure about what you meant and decided to leave it. [Addendum: I agree with George's clarifications.]
However, with my remark in post #38 I meant that I decided from now on to abstain from discussions about a hypothetical competing theory to "relativity theory" as I find it too artificial - surely you notice that "ban" in my latest posts.
Historically there are different interpretations of what started out as the "Lorentz-Einstein theory of relativity" and the idea that Lorentz and Einstein proposed competing theories emerged later. In the early years they promoted the new theory together and their interpretations were not distinguished in presentations and papers. That should hardly be surprising, as physicists tend to focus on the predictions of a theory and not on "metaphysics".

I would potentially question what is deduced from the fundamental equations [..]

Lorentz already answered that: deduced was what Einstein made a postulate. The fundamental equations suggested to Maxwell and followers that light is a wave with propagation constant c in vacuum.

Would it be equally possible to formulate it as, the speed of light is constant with respect to the point of emission, but independent of the motion of the source? Would that conflict with any experimental evidence?

Not clear what distinction you make: the speed with respect to one point is equal to the speed with respect to all points - that makes no difference.

[..] I mean my understanding as I've outlined above i.e. that the path of the photon is not necessarily the perpendicular line between the midpoints of the two mirrors, but rather the path represented by the hypotenuse of a right angled triangle. [..]

I'm not sure that I understood your set-up, if it wasn't a bit like MMX; and as I already indicated, basic (x,t) problems like MMX and the later KTX were perfectly dealt with in Lorentz's 1904 paper.

 

[mangaroosh]

There is no difference in what we know is actually happening under SR or under LET. It is only a difference in what you are assigning to things that you can't know are happening. In SR once you assign a Frame of Reference, which defines times and distances, then the reciprocity disappears. If you select a FoR in which you are not at rest, or if you start moving after you select a FoR in which you started out at rest, then time for you is dilated and your lengths are contracted, although you can't tell that. It's only when you then transform all the distances and times (events) into a new definition (a different FoR) that what was assigned a shorter distance or a longer time in one FoR takes on a longer distance or a shorter time in the other FoR.

The easiest way I know to communicate this idea is to think just about relative motion. If A and B are in relative motion, in one FoR, A is assigned a speed of zero and B is assigned a non-zero speed. In another FoR, B is assigned a speed of zero and A is assigned non-zero speed. Do you think that using a different FoR caused A to suddenly start moving? I don't think so, I think with regard to motion, different frames make perfect sense. The same thing applies to length contraction and time dilation. The ones that have motion according to the FoR that you are using are the ones for which lengths and times take on a different meaning.

Thanks gh; I think I understand the scenario above.

If we consider both A and B as "objective" observers, which I think we can do because it is a thought "experiment". From A's perspective, he can consider himself at rest and so the instruments in B's reference frame undergo contraction; while B can also consider himself at rest and it is A's instruments that contract, form B's perspective. The instruments will contract by the same amount.That is what I meant by reciprocity.

As I understand it, under LET the contractions aren't reciprocal, in this way. If for example we were to have 3 observers: A is in the absolute rest frame; B is at rest on earth; C is on the train; where the train is moving with a higher relative velocity to the rest frame, than the earth. Then A will see the instruments in the other reference frames contract; B will see C's instruments contract but A's will "expand" [just to contrast with "contract"]; while C will see the other instruments [in both reference frames] expand.

Do you think that just because in one FoR B is stationary and in another one B is moving that spacetime is affected? Nothing is happening in space or time or spacetime just because you choose to use a different FoR to assign distances and times differently such that A is stationary under one of those definitions and moving under a different definition.

In the early days, Lorentz and others were trying to determine how the mechanics of a clock would cause it to slow down as it traveled through the ether but now we don't worry about that. What we do instead is verify that the laws of physics transform intact under the Lorentz transformation. If they don't, we know they are not valid and we change them until they do. If they do, then we don't have to be concerned about the specific mechanics, it will happen automatically.

I suppose I'm just trying to see what the logical conclusion is. If there is absolute simultaneity under LET, but there are clocks which tick at different times, then it seems like it has to be the mechanics which causes the clocks to slow down and not "time" slowing down. Under SR, it appears as though the slowing of the clocks is attributed to the slowing of time.

Does that make sense, or is it way off?