Is the Principle of Relativity Valid for Faster-than-Light Propagation?

[nikeadidas]

Michelson Morley experiment was sought to find out Earth's motion through ether, which allegedly produced ether wind. Thus the time required for two rays of light would be different, thereby producing interference pattern. Since Earth revolves arouns sun, and sun revolves around something else, and our galaxy is rotating, revolving. So is it not that if there was any ether wind, it would be the resultant of all these motion, and not just Earth's motion around sun. The equations in the textbooks take into account only the Earth's motion. I agree it is not possible to take into account all these motions while calculating ether wind. But I think it would render the calculations made for these experiments invalid. Had M&M factored this in their experiment..I am confused

 

[Matterwave]

They detected NO ether wind, period. So, either the Earth with all these combined motions is somehow "absolutely stationary" wrt to this ether, or there is no such thing as ether.

 

[ghwellsjr]

Isn't it possible that the combination of all the motions you described resulted in such a low value for the ether wind that the accuracy of the MMX instrument couldn't detect it?

Actually, you left out the most important motion which is the surface of the Earth caused by its rotation. So even if all the motions added to zero at some point in time, twelve hours later, it should have been large enough for the instrument to measure.

Michelson concluded that since he couldn't measure any ether wind, the Earth must be dragging the ether along with it. He proposed repeating the experiment at the top of a high mountain to minimize the drag.

But before he could carry out his proposal, some other scientists explained the null result of the experiment, not by discarding the idea of an absolute stationary ether, but by concluding that lengths must contract along the direction of motion through the ether. And that is exactly the same "explanation" that you get when you use Special Relativity to define any inertial Frame of Reference and see what happens when the surface of the Earth and thus the MMX instrument are moving in that FoR.

So if Special Relativity comports with reality, it also comports with the idea of an absolute stationary ether as it only affirms the fact that its state cannot be determined. It is discarded on philosophical preference, not on the result of any measurement.

 

[jtbell]

Actually, you left out the most important motion which is the surface of the Earth caused by its rotation. So even if all the motions added to zero at some point in time, twelve hours later, it should have been large enough for the instrument to measure.

They also tried it at different times of year, when the Earth is at different points in its orbit, so the Earth's orbital velocity was in different directions with respect to the supposed "ether wind" and the two would add differently.

 

[Dale]

If the aether breaks wind in the forest and there is nobody around to hear, is the speed of light still c?

 

[ghwellsjr]

If the aether breaks wind in the forest and there is nobody around to hear, is the speed of light still c?

Don't you mean "smell"?

 

[Dale]

Hehe, I was debating which to say.

 

[ghwellsjr]

It's a tough philosophical question but I recommend using Occam's razor to cut the wind.

 

[nikeadidas]

Yes totally correct. Guess I was confused about what I was exactly confused about. The thing that keeps nagging me is how M&M experiment is described in textbooks. I mean if the ether wind can arise anyhow, then isn't it naive to assume that the light ray traveling in the direction of motion would be traveling slowly compared that with the light ray traveling perpendicular to it. Wasn't it possible that due to uneven ether wind, the speed of light traveling perpendicular to motion would be faster?

I know that this notion about ether has been abandoned now. But I am looking at it just for sake of that experiment. Can anybody elucidate me on this (I hope I am being clear, coz this is as articulate as I can be)

 

[ghwellsjr]

Remember that a round trip of light is always involved so even if the light ray traveling in the direction of motion is slowed down, the light ray that is reflected back will be sped up, making the round trip take the same time as the rays involved in the perpendicular direction. But this requires that the mirrors be closer together along the direction of motion compared to the mirrors along the perpendicular.

 

[kill_SR]

It's funny how relativist like to talk about "null results" in M&M experiment but "forget" about Dayton Miller's "Ether-Drift Experiments at Mount Wilson"

 

[Doc Al]

It's funny how relativist like to talk about "null results" in M&M experiment but "forget" about Dayton Miller's "Ether-Drift Experiments at Mount Wilson"

Not as funny as those still desperately clinging to the Ether.

As far as Miller's results go, see: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#Miller

 

[ZapperZ]

And just in case people missed it, Doc Al's reference clearly falsify the notion that we "forget" about Miller's result. People who make such claim clearly haven't done their homework.

Zz.

 

[zonde]

I mean if the ether wind can arise anyhow, then isn't it naive to assume that the light ray traveling in the direction of motion would be traveling slowly compared that with the light ray traveling perpendicular to it. Wasn't it possible that due to uneven ether wind, the speed of light traveling perpendicular to motion would be faster?

Speed of light in direction of motion is c-v for forward trip and c+v for backward trip. So the round trip speed is c(1-v^2/c^2).
In perpendicular direction speed for forward trip and backward trip is the same - \sqrt{c^2-v^2}
or c\sqrt{1-v^2/c^2} and it is faster.

So it is correct that the speed of light traveling perpendicular to motion would be faster.

I know that this notion about ether has been abandoned now. But I am looking at it just for sake of that experiment. Can anybody elucidate me on this (I hope I am being clear, coz this is as articulate as I can be)

This actually has nothing to do with aether. It's about Galilean transformation.

 

[Tracer]

They detected NO ether wind, period. So, either the Earth with all these combined motions is somehow "absolutely stationary" wrt to this ether, or there is no such thing as ether.

This interpretation is based on the concept that the ether is stationary and unmoving. Other interpretarions are possible if the ether is considered to exist and have the ability to move. Why are these other interpretations taboo?

 

[ghwellsjr]

They detected NO ether wind, period. So, either the Earth with all these combined motions is somehow "absolutely stationary" wrt to this ether, or there is no such thing as ether.

This interpretation is based on the concept that the ether is stationary and unmoving. Other interpretarions are possible if the ether is considered to exist and have the ability to move. Why are these other interpretations taboo?

The whole idea of an ether is that it is what undulates or what waves. Just like waves on the surface of the water medium, the light wave travels at some speed, but the medium itself stays put. Having a medium that can move around would create more problems than it would solve.

By the way, prior to Einstein, most if not all scientists still believed in an absolute stationary ether. They explained the fact that no ether wind could be detected because their apparatus shrunk along the direction of motion through the ether and clocks would run slower when moving through the ether. This idea, called Lorentz Ether Theory or LET explains everything just as well as Special Relativity but it affirms the existence of the ether. Only problem is, no one knows where its rest state is so it has been deemed of no consequence, whether or not it actually exists.

 

[Tracer]

The whole idea of an ether is that it is what undulates or what waves. Just like waves on the surface of the water medium, the light wave travels at some speed, but the medium itself stays put. Having a medium that can move around would create more problems than it would solve.

By the way, prior to Einstein, most if not all scientists still believed in an absolute stationary ether. They explained the fact that no ether wind could be detected because their apparatus shrunk along the direction of motion through the ether and clocks would run slower when moving through the ether. This idea, called Lorentz Ether Theory or LET explains everything just as well as Special Relativity but it affirms the existence of the ether. Only problem is, no one knows where its rest state is so it has been deemed of no consequence, whether or not it actually exists.

Lorentz Ether Theory also assumes an absolute stationary ether. The CMBR is a good starting point for consideration as the rest frame of the ether. If it is not the correct rest frame it is the very next best thing for consideration.

If a quanta of ether contained energy, why would not that quanta be accelerated by a gravitational field exactly the same as any other object having mass or energy equivalent mass? It should be since the acceleration caused by a gravitational field is independent of the mass or energy equivalent mass of the body being accelerated.

In short, other than not being the mainstream consensus, why is the concept of moving space wrong?

 

[Dale]

In short, other than not being the mainstream consensus, why is the concept of moving space wrong?

Because it has failed every experimental prediction that it has made.

Also, if the idea is that light is a vibration in the aether medium then the aether needs to be solid since light is a transverse wave and fluids don't support transverse waves, and it needs to be both very rigid and low-density to get waves propagating at c.

 

[ghwellsjr]

LET, like SR, does not include and, in fact, ignores gravity, which means they don't apply to the whole universe or anything as big as CMBR.

In any case, both LET and SR do not consider a frame (or ether) to contain anything that could be quantized and as I said before, having a moving frame (or ether) in which parts of it can move around would create problems that the simple construct of a frame couldn't handle.

If something isn't mainstream consensus, there's usually a good reason.

 

[Histspec]

If a quanta of ether contained energy, why would not that quanta be accelerated by a gravitational field exactly the same as any other object having mass or energy equivalent mass? It should be since the acceleration caused by a gravitational field is independent of the mass or energy equivalent mass of the body being accelerated.

In short, other than not being the mainstream consensus, why is the concept of moving space wrong?

Because the (old) theory of "complete aether drag" is incompatible with
a) Aberration of light
b) Sagnac effect due to Earth's rotation (included in GPS)

This was the reason, why the Fresnel-Lorentz idea of a stationary aether was preferred - until it was refuted by Michelson-Morley, so that the only remaining choice was special relativity.

http://en.wikipedia.org/wiki/Aether_drag_hypothesis
http://en.wikipedia.org/wiki/Michelson–Gale–Pearson_experiment

Regards,

 

[zonde]

This interpretation is based on the concept that the ether is stationary and unmoving. Other interpretarions are possible if the ether is considered to exist and have the ability to move. Why are these other interpretations taboo?

This interpretation is based on the concept that matter is completely independent of aether. Aether is viewed as something like air - where there is matter there is no aether.

But this interpretation is very primitive considering modern concepts about chemical structure of material objects. Basically chemical bonds are of electromagnetic nature therefore chemical structure should be affected by changes in speed of light (not mentioning that aether as medium for electromagnetic field should be even inside atom).

And if you consider things that way difference between Einstein's spacetime and luminuferous aether is not obvious.

 

[Chronos]

Google on 'gravity probe B' for a more compelling discussion.

 

[Saw]

the Fresnel-Lorentz idea of a stationary aether was preferred - until it was refuted by Michelson-Morley, so that the only remaining choice was special relativity.

http://en.wikipedia.org/wiki/Aether_drag_hypothesis
http://en.wikipedia.org/wiki/Michelson–Gale–Pearson_experiment

The MM experiment did not refute LET. Since LET and SR use the same math, that means they are experimentally indistinguishable. If so, LET has not been refuted by any experiment, just as SR has not.

The Wikipedia link you mention does not contradict what I have just said. See this other Wikipedia link for a more detailed explanation.

The difference between LET and SR is only that the former ventures to put on the table an explanation for the math (there is a stationary and non-ponderable aether), albeit admitting it is unprovable (because such aether is undetectable), whilst the latter does not go into that complication and remains at an abstract level (that is the way the universe works: principle of relativity plus the constancy of the speed of light, full stop).

Somehow paradoxically SR is preferred for that very same reason, for its prudence: as Lorentz himself acknowledged, if you admit the hypothesis of the aether, you start wondering about its attributes and you waste your time in useless discussions.

 

[Dale]

The difference between LET and SR is only that the former ventures to put on the table an explanation for the math (there is a stationary and non-ponderable aether), albeit admitting it is unprovable (because such aether is undetectable), whilst the latter does not go into that complication and remains at an abstract level (that is the way the universe works: principle of relativity plus the constancy of the speed of light, full stop).

The latter also puts an explanation for the math, as you yourself have indicated.

 

[Saw]

The latter also puts an explanation for the math, as you yourself have indicated.

Yes, but of a different level. Am I wrong if I assume that LET also endorses the two postulates? If so, on top of that, it puts forward a deeper level explanation, meaning one that could be the reason for the fact that the two postulates are true. Couldn't we say that the postulates are kinematics and the aether is an attempt at grasping the dynamics? (In any case, to avoid any doubt: I am not defending the idea of an aether...!)

 

[Dale]

Am I wrong if I assume that LET also endorses the two postulates?

Yes, that is incorrect. LET assumes that Maxwell's equations are only valid in the aether frame, so it rejects the first postulate. The strange properties of the aether are proposed in order to make it so that even though Maxwell's equations are only valid in the aether frame our clocks and rulers are distorted in such a manner as to make it impossible to detect the deviations from Maxwell's equations in different frames.

Neither theory is any "deeper" than the other. Both use the same equations, one derives them from two postulates, the other postulates an aether which has those properties. Neither explains why the postulates are the way they are nor why the aether has the properties it has.

 

[ghwellsjr]

LET assumes that Maxwell's equations are only valid in the aether frame, so it rejects the first postulate. The strange properties of the aether are proposed in order to make it so that even though Maxwell's equations are only valid in the aether frame our clocks and rulers are distorted in such a manner as to make it impossible to detect the deviations from Maxwell's equations in different frames.

I've been saying that LET assumes the principle of relativity, the same as Einstein's first postulate, which is what you ended up saying (it's impossible to detect deviations in different frames), but rather it's Einstein's second postulate (the propagation of light is c in all frames) that LET rejects and instead affirms a different second postulate (the propagation of light is c only in the aether frame). It's only the choice between these two second postulates that makes the two theories different.

Neither theory is any "deeper" than the other. Both use the same equations, one derives them from two postulates, the other postulates an aether which has those properties. Neither explains why the postulates are the way they are nor why the aether has the properties it has.

I would add that the net effect of Einstein's second postulate is that any inertial frame you want to pick has all the properties of the one elusive aether frame. And this is why Einstein pointed out in his 1905 paper introducing Special Relativity that this second postulate is "only apparently irreconcilable with the former". He's saying it seems impossible that any frame could be a candidate for the frame which LET claims is unique but it turns out that it is possible.

 

[Dale]

I've been saying that LET assumes the principle of relativity, the same as Einstein's first postulate, which is what you ended up saying (it's impossible to detect deviations in different frames)

I would say that for LET the impossibility of detecting deviations is a derived result, not an assumption. It is not uncommon that a set of axioms and theorems can be re-axiomatized such that some of the old axioms become theorems.

However, I must admit that I have never bothered to look for an "official" axiomatization of LET.

 

[DrGreg]

I was under the impression that LET postulates the existence of an aether such that:

• light speed is isotropic relative to the aether
• length contracts by the Lorentz factor relative to the aether
• time dilates by the Lorentz factor relative to the aether

(this is an incomplete list). The Principle of Relativity therefore becomes a derived result rather than a postulate. One ugly aspect is that if you want to extend LET to cover various branches of physics, you have to keep adding more postulates, e.g. that "mass dilates by the Lorentz factor relative to the aether" (equivalent to relativistic mass), etc, whereas Einstein's first postulate automatically encompasses all branches of physics.

I may be wrong, but I suspect there may never have been a rigorous statement of LET's postulates because the theory was abandoned before it gained much momentum.

 

[Dale]

That is in line with my impressions also.

 

[Histspec]

The MM experiment did not refute LET. Since LET and SR use the same math, that means they are experimentally indistinguishable.

I wasn't talking about LET. I was talking about the Fresnel-Lorentz idea of a stationary aether, which was considered an observable entity when it was developed in 1818 by Fresnel and 1892 by Lorentz. Of course, it was later (between 1892-1904) modified by including the Lorentz transformation and all of its effects, in order to explain Michelson-Morley, Trouton-Noble, etc.. Though the resultant theory, now called LET, was never considered a viable alternative to special relativity.

Regards,

 

[Saw]

I wasn't talking about LET. I was talking about the Fresnel-Lorentz idea of a stationary aether, which was considered an observable entity when it was developed in 1818 by Fresnel and 1892 by Lorentz. Of course, it was later (between 1892-1904) modified by including the Lorentz transformation and all of its effects, in order to explain Michelson-Morley, Trouton-Noble, etc..

Ah, sorry, I misinterpreted you.

Yes, that is incorrect. LET assumes that Maxwell's equations are only valid in the aether frame, so it rejects the first postulate. The strange properties of the aether are proposed in order to make it so that even though Maxwell's equations are only valid in the aether frame our clocks and rulers are distorted in such a manner as to make it impossible to detect the deviations from Maxwell's equations in different frames.

That may be a good historic account of how Lorentz's ideas developed: first, he said that the PoR (Maxwell's equations are valid in all frames) did not apply (the equations are only valid in the aether frame), later he changed his mind (the PoR applies in all frames, because -as you say- " it is impossible to detect the deviations from Maxwell's equations in different frames"). But in the end LET accepts the PoR, doesn't it? Really, if person A accepts a principle without giving a reason and person B gives a reason (good or bad) but also accepts the principle, one has to admit that both A and B accept the principle. I may agree with you that that the reason given by B (LET) is "contrived, artificial, wrong or whatever you want to call it", but still it is a reason, which is lacking in what we call today SR.

rather it's Einstein's second postulate (the propagation of light is c in all frames) that LET rejects and instead affirms a different second postulate (the propagation of light is c only in the aether frame). It's only the choice between these two second postulates that makes the two theories different.

I do not see that. Again, it may be historically true (is it? Poincaré did affirm the principle of the constancy of the speed of light in connection with the PoR). But in any case, I do not see how one could today defend the existence of an aether, defend the first postulate, defend the LT and not defend the constancy of the speed of light in all frames.

I would add that the net effect of Einstein's second postulate is that any inertial frame you want to pick has all the properties of the one elusive aether frame. And this is why Einstein pointed out in his 1905 paper introducing Special Relativity that this second postulate is "only apparently irreconcilable with the former". He's saying it seems impossible that any frame could be a candidate for the frame which LET claims is unique but it turns out that it is possible.

I fully agree with this idea, which you often repeat. The gist of Einstein's idea is that all frames measure as if they were at rest wrt an aether, no matter if the latter exists or not. That is why the they all measure c as the speed of light and they find fault in all other frames (lack of sync, TD and LC). But that is fully compatible with LET, isn't it? LET simply adds to that that, in its opinion, an aether does exist.

To sum up: I admit I am splitting a hair…

 

[Saw]

The Principle of Relativity therefore becomes a derived result rather than a postulate.

Yes, that is it.

One ugly aspect is that if you want to extend LET to cover various branches of physics, you have to keep adding more postulates, e.g. that "mass dilates by the Lorentz factor relative to the aether" (equivalent to relativistic mass), etc, whereas Einstein's first postulate automatically encompasses all branches of physics.

Well, yes, when you give reasons, the problem is that your conclusions are limited by the nature and robustness of your reasons. But you also have advantages: you do not admit absurd conclusions like time travel...

I may be wrong, but I suspect there may never have been a rigorous statement of LET's postulates because the theory was abandoned before it gained much momentum.

You are right. In fact, it may be not worthwhile to develop LET any further. As I said before, it is more pragmatic assuming, as we do, that the 2 postulates work and that is it. As long as one does not think that he is giving an explanation for the postulates when he is not.

 

[Dale]

But in the end LET accepts the PoR, doesn't it?

LET views the principle of relativity in the same way that NASA conspiracy theorists view the moon landings. They recognize that there is a lot of evidence for it, but they believe that all of the evidence is faked, and it irritates them that they can't prove it. I wouldn't call that "accepting".

As long as you have an aether you are rejecting the principle of relativity, at least philosophically. However, if you throw out the philosophical baggage of the aether and just concentrate on the math then LET takes time dilation and length contraction as postulates and derives the principle of relativity and the invariance of c. SR takes the principle of relativity and the invariance of c as postulates and derives time dilation and length contraction.

Neither is deeper than the other as far as that goes nor does either provide more "reasons" than the other, they simply swap the assumptions and the conclusions.

Woot! 10k posts!

 

[lugita15]

As long as you have an aether you are rejecting the principle of relativity, at least philosophically.

Not necessarily. Sound waves have a preferred frame, the rest frame of air, yet that doesn't invalid the principle of relativity, because the wave equation for sound waves is not a law of physics and thus does not need to be true in all frames. We could have had an analogous situation where Maxwell's equations needed to be modified in order for them to be true in all frames.

Neither is deeper than the other as far as that goes nor does either provide more "reasons" than the other, they simply swap the assumptions and the conclusions.

Lorentz's historical theory was "deeper" than SR in at least one sense: it tried to provide physical underpinnings for relativistic effects. For instance, Lorentz believed that a moving electron was compressed by its own electromagnetic field.

 

[ghwellsjr]

I was under the impression that LET postulates the existence of an aether such that:

• light speed is isotropic relative to the aether

The Principle of Relativity therefore becomes a derived result rather than a postulate.
...
I may be wrong, but I suspect there may never have been a rigorous statement of LET's postulates because the theory was abandoned before it gained much momentum.

One thing is for sure, LET does not accept Einstein's second postulate and whether or not any of its promoters prior to Einstein ever even considered it, they probably wouldn't have given it a second thought because they believed it was irreconcilable with the principle of relativity, whether or not that principle was formulated as a postulate or derived. But the LET that we talk about today does affirm the principle of relativity which Einstein said he raised to the status of a postulate (his first).

So the only significant difference between LET and SR is that although they both affirm the principle of relativity, LET postulates the existence of an aether such that light speed is isotropic relative to the aether (and only to the aether), whereas SR postulates that light speed is isotropic in any inertial frame.

 

[Saw]

Woot! 10k posts!

Beware, you are becoming wordy!

LET views the principle of relativity in the same way that NASA conspiracy theorists view the moon landings. They recognize that there is a lot of evidence for it, but they believe that all of the evidence is faked, and it irritates them that they can't prove it. I wouldn't call that "accepting".

I do not know why you think so. Conspiracy theorists argue that NASA did NOT land on the Moon. LET affirms that the PoR DOES apply. A different thing is that LET thinks the PoR applies for a physical reason, which generates a fortunate compensation of effects.

Lorentz's historical theory was "deeper" than SR in at least one sense: it tried to provide physical underpinnings for relativistic effects. For instance, Lorentz believed that a moving electron was compressed by its own electromagnetic field.

Yes, with that I agree. As you imply, "deeper" here does not mean "better" since the underpinning may be wrong. An example to reinforce your point: you may model how hot a body is by measuring temperature and mass; for most purposes, that is good enough, but to gain a better understanding of the phenomenon, you may try to find a deeper-level explanation; in this sense, you may hypothesize that the physical reason is (a) a fluid that goes into the body or (b) the internal motion of particles. Both (a) and (b) are "deeper-level" explanations, although (a) is wrong.

So the only significant difference between LET and SR is that although they both affirm the principle of relativity, LET postulates the existence of an aether such that light speed is isotropic relative to the aether (and only to the aether), whereas SR postulates that light speed is isotropic in any inertial frame.

As DrGreg pointed out, the problem with LET is that nobody knows very well what it means since it has never been fully developed. But for the purpose of this discussion, can we make a convention? Let us define LET as SR (= PoR + invariant c) plus the hypothesis that the reason for all this is an aether. Are the two things logically incompatible? I do not think so.

When you say that for LET "light speed is isotropic only relative to the aether", we should clarify what we mean by that. If those concepts ("isotropic", "speed") are fed by absolute measurements of time and distances, then not only LET also I do affirm that light CANNOT be isotropic in any frame. But LET admits that absolute measurements are not in practice possible. We must content ourselves, in real life, with relative measurements. And if you feed the concept of speed with those relative measurements, you forcefully get (also for LET!) isotropic light speed.

 

[harrylin]

This interpretation is based on the concept that the ether is stationary and unmoving. Other interpretarions are possible if the ether is considered to exist and have the ability to move. Why are these other interpretations taboo?

Those other interpretations (presumably all of them, but it's hard to be sure about an undefined group) are incompatible with special and general relativity as well as some key experiments that give support for those theories. Special relativity emerged from Lorentz's electron theory which assumed a stationary ether model and succefully explained such things as the Fizeau "drag".

 

[harrylin]

Yes, but of a different level. Am I wrong if I assume that LET also endorses the two postulates?

There was no "LET". Einstein based the second postulate on Maxwell's theory which also was part of Lorentz's theories. Thus Lorentz didn't disagree with Einstein's 1905 paper, and Einstein clarified that the second postulate is made plausible by Lorentz's preceding theory of electrons. Moreover, Lorentz had next derived the Lorentz transformation equations (although not in their symmetrical form) in order to conform to the first postulate.

Let us define LET as SR (= PoR + invariant c) plus the hypothesis that the reason for all this is an aether. Are the two things logically incompatible? I do not think so.

Neither did Einstein think so, as he explained in a very elaborate way in his Leiden inauguration speech.

 

[harrylin]

[..] So the only significant difference between LET and SR is that although they both affirm the principle of relativity, LET postulates the existence of an aether such that light speed is isotropic relative to the aether (and only to the aether), whereas SR postulates that light speed is isotropic in any inertial frame.

No, the isotropy of light in any inertial frame is a derived result, although most textbooks for students skip that. As Einstein put it in 1905:
"We now have to prove that any ray of light, measured in the moving system, is propagated with the velocity c, if, as we have assumed, this is the case in the stationary system; for we have not as yet furnished the proof that the principle of the constancy of the velocity of light is compatible with the principle of relativity."

 

[zonde]

I think that problem about LET is not whether or not speed of light is isotropic in all inertial frames (we just construct coordinate systems so that it is so).
I think that problem is about c being universal speed limit. Because Lorentz transform works only for one speed. Any other speed will be different after Lorentz transform. That universality is the unreasonable part. And that universality ensures that principle of relativity holds.

On the other hand we can view principle of relativity as external constraint. Matter that does not follow this principle can not change it's state of motion i.e. it will be fragile and break down when accelerated. If we assume that matter can replicate itself (say pair prodaction can happen only in presence of matter) then more flexible matter will be evolutionary preferable. At least that is my reasoning why principle of relativity holds.

 

[ghwellsjr]

[..]So the only significant difference between LET and SR is that although they both affirm the principle of relativity, LET postulates the existence of an aether such that light speed is isotropic relative to the aether (and only to the aether), whereas SR postulates that light speed is isotropic in any inertial frame.

No, the isotropy of light in any inertial frame is a derived result, although most textbooks for students skip that. As Einstein put it in 1905:
"We now have to prove that any ray of light, measured in the moving system, is propagated with the velocity c, if, as we have assumed, this is the case in the stationary system; for we have not as yet furnished the proof that the principle of the constancy of the velocity of light is compatible with the principle of relativity."

So you're saying that Einstein's so-called second postulate is also a derived result, just like others on this thread have pointed out that the principle of relativity is a derived result for LET and not a first postulate?

 

[lugita15]

No, the isotropy of light in any inertial frame is a derived result, although most textbooks for students skip that. As Einstein put it in 1905:
"We now have to prove that any ray of light, measured in the moving system, is propagated with the velocity c, if, as we have assumed, this is the case in the stationary system; for we have not as yet furnished the proof that the principle of the constancy of the velocity of light is compatible with the principle of relativity."

I think here Einstein is proving the compatibility of the first and second postulates, not the truth of the second postulate, which is of course an assumption of his theory.

 

[lugita15]

On the other hand we can view principle of relativity as external constraint. Matter that does not follow this principle can not change it's state of motion i.e. it will be fragile and break down when accelerated.

What exactly do you mean by this? Why can't an object which does not "follow" the PoR change its state of motion?

 

[zonde]

What exactly do you mean by this? Why can't an object which does not "follow" the PoR change its state of motion?

Object that does not follow PoR would have to have different structure for different states of motion. Well, I am assuming that object can't have many different stable configurations and smoothly slip from one structure to other without breaking down. Something like that.

 

[lugita15]

Object that does not follow PoR would have to have different structure for different states of motion.

I still don't understand what you're talking about. And in any case, it doesn't really make sense to talk about objects "following" the PoR; you can only talk about laws of physics which follow it or not. For example, in Aristotelian physics the law was "An object at rest stats at rest, and an object in motion comes to rest", which is obviously incompatible with the PoR. How could a law of this kind lead to structural instability of physical objects?

 

[Dale]

But LET admits that absolute measurements are not in practice possible.

This is true, but the problem with the rest of your comments is that LET nevertheless asserts the existence and reality of unmeasurable things. Thus, what is real is considered different from what is measured. Conversely, what is measured is considered to be an inaccurate reflection of reality.

Conspiracy theorists argue that NASA did NOT land on the Moon. LET affirms that the PoR DOES apply.

I disagree here. LET argues that the PoR does not apply in reality, but that reality is unmeasurable.

But for the purpose of this discussion, can we make a convention? Let us define LET as SR (= PoR + invariant c) plus the hypothesis that the reason for all this is an aether.

So we should make a convention that we agree you are right? I prefer the convention where we all agree with me.

 

[zonde]

I still don't understand what you're talking about.

Ok, let me try one more time.
Object has different interactions that determine it's structure - different molecular bonds, interactions determining structure of atom and interactions inside nucleons.
Let's say that interactions all happen at different speeds. They have at some distance potential minimum. When we change speed of object this minimum potential distance changes by different amounts for different interactions (because of different interaction speed - you can imagine that we use Lorentz transforms for different interactions with different values of "c"). So if internal distances determining structure of object do not change in unison it should eventually break.

 

[Saw]

I disagree here. LET argues that the PoR does not apply in reality, but that reality is unmeasurable.

Again it depends on what we mean by PoR. If by PoR we mean knowing absolute simultaneity, then, yes, LET argues that we do not have that knowledge. And so do I... Don't you? I would think anybody would agree on that, maybe I am wrong, but it looks quite obvious... (By absolute simultaneity I mean the one that would be established in the eather frame, if the aether existed, or by an instantaneous means of communication, if such thing were possible.)

Instead if by PoR we mean that, even if we do not have such ambitious knowledge (absolute simultaneity), we live quite well without it (we can solve all known practical problems), then LET should agree with that.

 

[ghwellsjr]

So the only significant difference between LET and SR is that although they both affirm the principle of relativity, LET postulates the existence of an aether such that light speed is isotropic relative to the aether (and only to the aether), whereas SR postulates that light speed is isotropic in any inertial frame.

As DrGreg pointed out, the problem with LET is that nobody knows very well what it means since it has never been fully developed. But for the purpose of this discussion, can we make a convention? Let us define LET as SR (= PoR + invariant c) plus the hypothesis that the reason for all this is an aether. Are the two things logically incompatible? I do not think so.

The only attribute of the aether of LET is that it is a preferred frame, the one nature operates according to. It does not offer any other explanatory mechanisms regarding how nature interacts with light to cause length contraction or time dilation. So we don't want to conceive of LET as being SR plus something. It's the principle of relativity (which is not SR) plus something just like SR is the principle of relativity plus something else.

When you say that for LET "light speed is isotropic only relative to the aether", we should clarify what we mean by that.

I was using DrGreg's terminology in my quote. I usually use Einstein's terminology such as "light always propagates at c" or "any ray of light moves at c" but they mean the same thing. In LET, light propagates at c only in the aether.

If those concepts ("isotropic", "speed") are fed by absolute measurements of time and distances, then not only LET also I do affirm that light CANNOT be isotropic in any frame. But LET admits that absolute measurements are not in practice possible. We must content ourselves, in real life, with relative measurements. And if you feed the concept of speed with those relative measurements, you forcefully get (also for LET!) isotropic light speed.

There is no measurement that can determine how light propagates. If there were, we wouldn't have the principle of relativity which means we wouldn't have either LET or SR because they are both based on that principle (plus a postulate about how light propagates).

Prior to Maxwell, the principle of relativity was well established. As a result of Maxwell's discovery that his equations describing an electromagnet field predicted waves that happen to travel at c, he (incorrectly) concluded that this would provide a means to experimentally measure the rest state of that field and do away with the principle of relativity. When MMX could not experimentally measure the rest state of that field, the principle of relativity was upheld and LET was formulated to explain what was happening still based on a field at absolute rest. The principle of relativity (again, not to be confused with Special Relativity) was what forced Lorentz, et al, to formulate LET, not the other way around. This resulted in time and space being absolute even though they were aware of the concepts of time dilation and length contraction for objects moving with respect to the aether but it never occurred to them that space itself or time itself, in other word's, nature, could ever be relative. In this sense, absolute time and absolute space are derived results from the experimental evidence of the principle of relative plus the postulate that light travels at c only in one rest state (the one nature operates on).

Einstein's two postulates, along with his definition of a Frame of Reference incorporating the amalgamation spacetime (instead of leaving space and time as independent coordinates) had a derived result of time and space being relative to the defined Frame of Reference rather than an absolute of nature.