Relativity vs competing theories

[mangaroosh]

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".

It wasn't immediately clear, probably because I was reading them through the lens of "relativity vs competing theories"; though, I was wondering how you managed to quote a part of my post and leave out the important qualifier referencing LET I presumed it was intentional but I wasn't clear as to what the intention was.

I know physicists tend not to focus on the "metaphysics" but scientific theories have "metaphysical" implications, and insofar as physicists are concerned with accurately describing reality they are concerned with metaphysics; of course, where they are not concerned with accurately describing reality, they are not concerned with metaphysics. Given that the differences between [what has been described in this thread as] LET and Einsteinian relativity are effectively metaphysical, it is probably unavoidable to discuss them without discussing the metaphysics. Who knows a certain level of metaphysical consideration could actually facilitate the unification of QM and SR.

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.[/quote
The speed of light being a constant (c) in vacuum, and the speed of light being c regardless of the motion relative to the source, appear to be somewhat different.

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.

This is an area where I am not clear. The speed of light being constant with respect to one point, doesn't necessarily mean that it is constant with respect to all points - which I know is not what you were suggesting.

Has the latter been borne out by experimental evidence, or has it just not been contradicted by experimental evidence?

Part of my confusion arises because it seems as though LET has a different "second postulate" than Einsteinian relativity, which also fits the evidence. This leads me to believe there is limited scope for deducing a postulate with respect to the speed of light - would that be correct?

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.

If we consider the two light clock examples, form SR and LET (as I understand it).

Under SR the path of the photon traces a line perpendicular to the two mirrors; while under LET [as I understand it] the photon traces a line as represented by the hypotenuse of a right angled triangle.

Under SR, the photon has a strictly vertical velocity component of c, and so the observer would measure the speed of light to be c, relative to the carriage.

Under LET, the photon has a velocity of c in the horizontal (or diagonal) direction - due to the motion of the train and the clock - but not a velocity of c in the vertical direction. If the observer can only measure the speed relative to his reference frame he would not measure the speed of light to be c, but some lower value.

If both theories equally explain the evidence, then it suggests that the speed of light is not necessarily constant in all directions, regardless of the motion relative to the source.


It has been suggested [by others, elsewhere] that the MMX demonstrates that the speed of light is constant in all directions, regardless of the motion relative to the source; however, if the null result of the MMX is equally explained under LET, then this seems to suggest that this isn't the case.

Again, hopefully the above makes sense.

 

[ghwellsjr]

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.

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?

You have to make a distinction between what an observer actually sees or perceives or measures and what is defined by a Frame of Reference which includes things that observers cannot see or perceive or measure. A theory such as SR or LET cannot have any bearing on what an observer sees or measures or perceives, can it? If a theory said that an observer sees or measures or perceives something different than what the observer is experiencing, then that theory does not comport with reality.

No observer will ever see, perceive or measure another object to be expanded (or time to be contracted), they always see things moving with respect to themselves as contracted and clocks running slower.

A Frame of Reference is a scheme to allow us to assign dimensions of space and time. When we say that an object is contracted in a given FoR and other objects are normal, we don't mean that the contracted object (or observer) will see the normal one as expanded, they are still going to see it as contracted.

Under the idea of LET, there is only one Frame of Reference that was considered to be real. All objects moving in that frame will be contracted and time will be running slower depending on their speeds. These were considered to be real contractions and clocks slowing down but the observer cannot be aware of this. When they view other objects and clocks, even ones that are at rest in the ether, they still see them as contracted and time dilated, but it is not considered reality, it is considered in LET to be an artifact.

So under LET, the contractions are perceived to be reciprocal but they are not believed to be actually reciprocal. Under SR, under any particular FoR, the contractions are also not reciprocal although they will be perceived to be reciprocal. It's just that under SR, we don't regard anyone FoR to be preferred so we don't claim that one FoR is the only one for which the contractions are actual and other contractions are artifacts.

When we are talking about reference frames, we don't say that A sees B to be contracted, everybody that is using the assigned dimensions for the frame of reference says that B is contracted while A is not if B is the one that is moving and A is stationary.

 

[mangaroosh]

You have to make a distinction between what an observer actually sees or perceives or measures and what is defined by a Frame of Reference which includes things that observers cannot see or perceive or measure. A theory such as SR or LET cannot have any bearing on what an observer sees or measures or perceives, can it? If a theory said that an observer sees or measures or perceives something different than what the observer is experiencing, then that theory does not comport with reality.

Apologies, this is somewhat like learning a new language for me; as with learning a new language it is usually easier to understand what is being said, than it is to express oneself. Similarly, I think my understanding is somewhat betrayed by my inability to express that understanding clearly and using the correct terminology. Hopefully it isn't as frustrating for you [and others] as it is for me.

I understand that theories, such as SR and LET, cannot affect what an observer measures or perceives; am I right in saying that a theory is an explanation of what an observer sees and/or perceives, which also makes predictions about what an observer should see under certain conditions; the veracity of these predictions makes or breaks the theory. In the case of both LET and SR, both have stood up to the test - LET indirectly at least.

No observer will ever see, perceive or measure another object to be expanded (or time to be contracted), they always see things moving with respect to themselves as contracted and clocks running slower.

A Frame of Reference is a scheme to allow us to assign dimensions of space and time. When we say that an object is contracted in a given FoR and other objects are normal, we don't mean that the contracted object (or observer) will see the normal one as expanded, they are still going to see it as contracted.

Under the idea of LET, there is only one Frame of Reference that was considered to be real. All objects moving in that frame will be contracted and time will be running slower depending on their speeds. These were considered to be real contractions and clocks slowing down but the observer cannot be aware of this. When they view other objects and clocks, even ones that are at rest in the ether, they still see them as contracted and time dilated, but it is not considered reality, it is considered in LET to be an artifact.

So under LET, the contractions are perceived to be reciprocal but they are not believed to be actually reciprocal. Under SR, under any particular FoR, the contractions are also not reciprocal although they will be perceived to be reciprocal. It's just that under SR, we don't regard anyone FoR to be preferred so we don't claim that one FoR is the only one for which the contractions are actual and other contractions are artifacts.

When we are talking about reference frames, we don't say that A sees B to be contracted, everybody that is using the assigned dimensions for the frame of reference says that B is contracted while A is not if B is the one that is moving and A is stationary.

I'm not sure I fully understand, so forgive me if I return to the comfort blanket of the light clock thought experiment.

Reciprocity in SR
According to SR, the observer on the platform will see the clock on the train ticking slower, because, from their perspective, the photon travels a longer distance between the mirrors. Simultaneously [in the relative sense], the observer on the train will see the exact same for the clock on the platform. In this sense the observed contraction (or time dilation) is reciprocal. Why does the perception differ from the actual; does time dilation actually occur in any of the reference frames?

I know that A will perceive it to happen in B's reference frame; while simultaneously [again, in the relative sense] B will perceive it to happen in A's reference frame; but both are free to label themselves as being at rest, so it should be reciprocal. But if it is not actually reciprocal in which reference frame does it occur, if at all?


Reciprocity in LET
I also have trouble seeing why, according to LET, an observer will perceive reciprocal contractions, just the same as in the SR interpretation.

As I understand it, there is a key difference between SR and LET [in terms of the light clock thought experiment]; according to SR both observers clocks will tick at the "normal" rate in their own reference frames, and this is partly what leads to them observing the other clock running slow; because it is by comparison. If their clock were running just as slow, then they would presumably see both clocks ticking equally.

This is slightly different under LET, as I understand it; if we, again, just stick with the observer on the platform as being in the absolute rest frame, and the train moving relative to it, then, according to LET, the trains clock will actually be running slower in the observer on the train's reference frame as well. They, however , will not be able to tell by how much it is running slower because they cannot detect the absolute rest frame - in this case, they are in a windowless carriage and can't see the clock on the platform.

However, if they could see the clock on the platform, they would presumably be able to calculate that it is ticking faster; if they knew the distance from themselves to the platform clock at any given moment. In terms of what they would perceive, I find it difficult to imagine, perhaps the further away they are the slower the clock would appear to tick, but it would probably appear to speed up as they approach it, peaking as they are in line with it and then slowing down again because the light from the clock has further to travel to meet their retina. Again, though, I presume they would be able to calculate that it was ticking faster.

Remove absolute rest
Of course, they cannot detect the absolute rest frame, they can only observe other objects that are moving relative to this fantasy FoR. If we imagine that the train is traveling relative to the earth, such that the train has the higher velocity relative to the rest frame, then the Earth's clock would be ticking faster than the trains clock, and just as they would do with the absolute rest frame clock [if they could detect it], they could presumably calculate that the Earth's clock was ticking faster - by a smaller amount than the rest frame clock (if they were able to detect it).


That would be my understanding of it, but I'm not sure if there is something I've overlooked, or that I'm not aware of.

 

[harrylin]

[..]
The speed of light being a constant (c) in vacuum, and the speed of light being c regardless of the motion relative to the source, appear to be somewhat different.

Not if you understand the physical concept behind it: it was (and still is) taken for granted that the speed of a wave is completely independent of that of the source. In the introduction of his 1905 paper Einstein simply phrased it as follows:

"light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body"

This is an area where I am not clear. The speed of light being constant with respect to one point, doesn't necessarily mean that it is constant with respect to all points - which I know is not what you were suggesting.

According to SR (but not GR) it is everywhere the same constant relative to all points in space as defined with a Newtonian reference system ("inertial frame"). Thus, probably I did not understand what you meant (and I still don't!).

Has the latter been borne out by experimental evidence, or has it just not been contradicted by experimental evidence?

That is the same thing: the best that can be borne out by experimental evidence is a persistent lack of contradiction. In practice that ideal can't even be reached as no experiment is perfect.

Part of my confusion arises because it seems as though [Lorentz] has a different "second postulate" than Einstein[..], which also fits the evidence. This leads me to believe there is limited scope for deducing a postulate with respect to the speed of light - would that be correct? [..]

I don't know what you mean with that; Lorentz did not really work with "postulates" but developed theories based on physical models and the results of experiments.

However, as Lorentz came to prefer Einstein's derivation over his own (which indeed was much more complicated), perhaps it's useful to highlight how they they fit together, although Lorentz might phrase a few sentences a little different from Einstein. For the subtlety of the difference is, if I see it correctly, what you are missing.

So, please take Einstein's 1905 derivation and I'll show you how easy it is to switch between Lorentz and Einstein. The following translation is quite OK:
http://www.fourmilab.ch/etexts/einstein/specrel/www/

To make his derivation perfectly "Lorentz compatible", only a few sentences need slight modification (in italics) as follows:

Examples of this sort, together with the unsuccessful attempts to discover any motion of the Earth relatively to the “light medium,” suggest that the phenomena of electrodynamics as well as of mechanics possesses no properties that permit the detection of absolute velocity. They suggest rather that, as has already been shown to the first order of small quantities, the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good. We will raise this conjecture (the purport of which will hereafter be called the “Principle of Relativity”) to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell's theory for stationary bodies. The concept of a “luminiferous ether” will be helpful to explain the second postulate; however the theory will not provide a “preferred stationary space” with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.

That's about it; if I see it correctly, there isn't much else that really needs to be modified although Lorentz would add here and there some more qualifiers as "true", "apparent", "local" etc.

However, Einstein makes a subtle switch from "empty space" to "reference system" without a clear motivation. To make the logic as well as the derivation easier to follow for the readers, Lorentz could instead stick with the original, physical meaning of the second postulate as formulated in the introduction - let's call it Maxwell's light postulate.

Next, Lorentz could explain that if we combine Maxwell's light postulate with the PoR, it follows that this postulate should also appear to hold in an inertial reference system that is in motion with respect to the ether, so that it could appear to be a "stationary" system just as in Newton's mechanics. He could refer to his 1895 and 1899 papers that explain how this works.

If we next operationally define all terms such as "speed", "time" etc. as described in section 1 (free from metaphysical meaning), then we obtain the following result:

Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.

I hope that this sufficiently clarifies the perceptual differences as well as the complete lack of difference in practice.

Harald

ADDENDUM: I forgot to point out that from Lorentz perspective one may proceed the derivation directly based on Maxwell's version of the light postulate, it is not necessary to make the intermediate step from "empty space" to an arbitrary Newtonian reference system.
Einstein wouldn't like that but it's much more straightforward (and of course, Newton would like that).

The Lorentz transformations that then result describe primarily a transformation between a system that is at rest in the ether and one in inertial motion. Thanks to the form of those transformation equations (they form a group), the same transformation equations are valid between inertial reference systems - just as is the case with the Galilean transformations.

 

[ghwellsjr]

You have to make a distinction between what an observer actually sees or perceives or measures and what is defined by a Frame of Reference which includes things that observers cannot see or perceive or measure. A theory such as SR or LET cannot have any bearing on what an observer sees or measures or perceives, can it? If a theory said that an observer sees or measures or perceives something different than what the observer is experiencing, then that theory does not comport with reality.

Apologies, this is somewhat like learning a new language for me; as with learning a new language it is usually easier to understand what is being said, than it is to express oneself. Similarly, I think my understanding is somewhat betrayed by my inability to express that understanding clearly and using the correct terminology. Hopefully it isn't as frustrating for you [and others] as it is for me.

I understand that theories, such as SR and LET, cannot affect what an observer measures or perceives; am I right in saying that a theory is an explanation of what an observer sees and/or perceives, which also makes predictions about what an observer should see under certain conditions; the veracity of these predictions makes or breaks the theory. In the case of both LET and SR, both have stood up to the test - LET indirectly at least.

Yes, you are right, but that was not my point. My point was that both SR and LET have different explanations for what an observer will never see under any circumstance, specifically, the propagation of light.

And both LET and SR have stood the test in exactly the same way, LET is no more indirect than SR. They are different because they have a different second postulate about what cannot ever be perceived under any circumstance by any observer, the propagation of light.

No observer will ever see, perceive or measure another object to be expanded (or time to be contracted), they always see things moving with respect to themselves as contracted and clocks running slower.

A Frame of Reference is a scheme to allow us to assign dimensions of space and time. When we say that an object is contracted in a given FoR and other objects are normal, we don't mean that the contracted object (or observer) will see the normal one as expanded, they are still going to see it as contracted.

Under the idea of LET, there is only one Frame of Reference that was considered to be real. All objects moving in that frame will be contracted and time will be running slower depending on their speeds. These were considered to be real contractions and clocks slowing down but the observer cannot be aware of this. When they view other objects and clocks, even ones that are at rest in the ether, they still see them as contracted and time dilated, but it is not considered reality, it is considered in LET to be an artifact.

So under LET, the contractions are perceived to be reciprocal but they are not believed to be actually reciprocal. Under SR, under any particular FoR, the contractions are also not reciprocal although they will be perceived to be reciprocal. It's just that under SR, we don't regard anyone FoR to be preferred so we don't claim that one FoR is the only one for which the contractions are actual and other contractions are artifacts.

When we are talking about reference frames, we don't say that A sees B to be contracted, everybody that is using the assigned dimensions for the frame of reference says that B is contracted while A is not if B is the one that is moving and A is stationary.

I'm not sure I fully understand, so forgive me if I return to the comfort blanket of the light clock thought experiment.

Reciprocity in SR
According to SR, the observer on the platform will see the clock on the train ticking slower, because, from their perspective, the photon travels a longer distance between the mirrors. Simultaneously [in the relative sense], the observer on the train will see the exact same for the clock on the platform. In this sense the observed contraction (or time dilation) is reciprocal. Why does the perception differ from the actual; does time dilation actually occur in any of the reference frames?

Everything you stated in the preceding paragraph is what each observer can see, perceive and measure and has nothing to do with either theory or with any concept of a reference frame. But let me ask you a question before I answer yours:
Since each observer perceives the other one as moving but in the platform's reference frame only the train is moving and in the train's reference frame only the platform is moving, does the perception of motion differ from the actual; does motion actually occur in any of the reference frames?

Now I would hope that you will recognize that when we pick a frame to describe and analyze what is happening to both the train and the platform, we assign all the motion to one or the other but not both. So in each frame, all the motion is assigned to the other one.

In exactly the same way, when we choose the platform frame, all the time dilation is assigned to the train and none of it to the platform. When we choose the train frame, all the time dilation is assigned to the platform and none of it to the train.

So to answer your question in a more general sense, yes, time dilation actually occurs in any reference frame for any object that is in motion, the faster it moves, the more the time dilation.

I know that A will perceive it to happen in B's reference frame; while simultaneously [again, in the relative sense] B will perceive it to happen in A's reference frame; but both are free to label themselves as being at rest, so it should be reciprocal. But if it is not actually reciprocal in which reference frame does it occur, if at all?

The "it" that you are talking about is "time dilation" but change it to "motion" and see how silly the statement sounds:

I know that A will perceive motion to happen in B's reference frame; while simultaneously [again, in the relative sense] B will perceive motion to happen in A's reference frame; but both are free to label themselves as being at rest, so it should be reciprocal.

What you should be saying is: I know that both A and B will assign all the motion to A in B's reference frame; while A and B will assign all the motion to B in A's reference frame but both are free to choose any reference frame, including one which assigns an equal but lesser motion to both of them but there will never be a reference frame in which both of them have no motion.

Now just change "motion" to "time dilation" and say it again: I know that both A and B will assign all the time dilation to A in B's reference frame; while A and B will assign all the time dilation to B in A's reference frame but both are free to choose any reference frame, including one which assigns an equal but lesser time dilation to both of them but there will never be a reference frame in which both of them have no time dilation.

Reciprocity in LET
I also have trouble seeing why, according to LET, an observer will perceive reciprocal contractions, just the same as in the SR interpretation.

If you are having this trouble then you don't really believe what you said earlier in your post:

I understand that theories, such as SR and LET, cannot affect what an observer measures or perceives

As I understand it, there is a key difference between SR and LET [in terms of the light clock thought experiment]; according to SR both observers clocks will tick at the "normal" rate in their own reference frames, and this is partly what leads to them observing the other clock running slow; because it is by comparison. If their clock were running just as slow, then they would presumably see both clocks ticking equally.

There is NO difference between what SR and LET claim about the rates that clocks tick at. The ONLY difference is in what they say about those rates. SR affirms that they are real and legitimate. LET denies this and claims that they are artifacts, unreal, illegitimate, and not actual. But they don't calculate the values of those rates to be any different than SR.

This is slightly different under LET, as I understand it; if we, again, just stick with the observer on the platform as being in the absolute rest frame, and the train moving relative to it, then, according to LET, the trains clock will actually be running slower in the observer on the train's reference frame as well. They, however , will not be able to tell by how much it is running slower because they cannot detect the absolute rest frame - in this case, they are in a windowless carriage and can't see the clock on the platform.

However, if they could see the clock on the platform, they would presumably be able to calculate that it is ticking faster; if they knew the distance from themselves to the platform clock at any given moment. In terms of what they would perceive, I find it difficult to imagine, perhaps the further away they are the slower the clock would appear to tick, but it would probably appear to speed up as they approach it, peaking as they are in line with it and then slowing down again because the light from the clock has further to travel to meet their retina. Again, though, I presume they would be able to calculate that it was ticking faster.

You really should quit trying to make these artificial distinctions between LET and SR. The only distinction is that LET claims the absolute rest frame is in some unknowable state, they would NEVER assign it to the platform or to the earth. That would defeat the whole concept of LET which is that no actual clock is running at the absolute rate that the universe is running at.

Remove absolute rest
Of course, they cannot detect the absolute rest frame, they can only observe other objects that are moving relative to this fantasy FoR. If we imagine that the train is traveling relative to the earth, such that the train has the higher velocity relative to the rest frame, then the Earth's clock would be ticking faster than the trains clock, and just as they would do with the absolute rest frame clock [if they could detect it], they could presumably calculate that the Earth's clock was ticking faster - by a smaller amount than the rest frame clock (if they were able to detect it).

That would be my understanding of it, but I'm not sure if there is something I've overlooked, or that I'm not aware of.

I think it is not going to be productive to defend LET or to keep bringing up these examples. You need to learn SR and once you do you will understand why scientists were struggling so much prior to Einstein showing them the way out of their confusion.

 

[harrylin]

[..] I would like te have a question, i hope it isn't improper.
There are some people, who claims they detected that "aether wind", motion to light medium, something similar to the measurements of COBE, what do you think about them?
I think, gravity waves for example, can cause some weak effects, of course this effects are not so definite.

It would be a proper question under this topic if any of those claims corresponds to a competing theory that is discussed in a serious physics journal, and to which you can refer us.

 

[GTOM]

I deleted it.
( I asked it in the first place, because i thought it can correspond to the Lorentzian theory, that there is a kind of absolute frame, and maybe it isn't impossible to detect, although you have to watch very small effects.
But again I am sorry if i was improper.)

 

[Agerhell]

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?

I would say that you can in principle get three different relativity theories.

1. Special Relativity - which essentially says that no inertial frame is preferred. Thus the relativity of simultaneity. Also you cannot really know who's clock is running faster out of two clocks both moving inertially.

2. LET, - Which essentially says that there is one preferred frame, that of "the aether". You still get the same equations of time dilation and relativistic mass as in special relativity but there is no relativity of simultaneity and the clock that moves faster relative to the eather ticks slower.

3. Locally preferred frame relativity - Could in principle be formulated in many ways, but the most natural way is to treat the local gravity field as a preferred frame. Under this formulation the clock onboard a vehicle traveling close to the Earth ticks slower the faster the clock goes in relation to the centre of the Earth. There is no relativity of simultaneity. When Nasa wants to determine how fast the clocks onboard their vehicles are ticking this is basically the approach taken. Close to earth, such as for GPS satellites, the Earth is what matters. Out in the solar system, velocity in relation to the Sun is dominant and close to, for instance, Jupiter, Jupiter is what counts.

Note that SR is the only one of the above that incorporates relativiy of simultaneity. I do not know if GR is more like the third of the above, someone else has to help you on that one.

 

[Dale]

the most natural way is to treat the local gravity field as a preferred frame.

The gravity field does not have a rest frame. If you wish to discuss this further you need to provide a scientific reference.

 

[mangaroosh]

Yes, you are right, but that was not my point. My point was that both SR and LET have different explanations for what an observer will never see under any circumstance, specifically, the propagation of light.

And both LET and SR have stood the test in exactly the same way, LET is no more indirect than SR. They are different because they have a different second postulate about what cannot ever be perceived under any circumstance by any observer, the propagation of light.

Ah OK, apologies, I read it differently.

Would that mean then, that the theory that makes the fewest assumptions is the one that would be preferred?

Everything you stated in the preceding paragraph is what each observer can see, perceive and measure and has nothing to do with either theory or with any concept of a reference frame.

I'm not sure I fully understand that point. My intention is to use the light clock thought experiment in the same way it is used to clarify such things as time dilation and length contraction. For that reason I don't understand how the light clock thought experiment would have nothing to do with Einsteinian relativity. I see it as more of an explanatory aid to discuss the differences and potential consequences of the theories.

But let me ask you a question before I answer yours:
Since each observer perceives the other one as moving but in the platform's reference frame only the train is moving and in the train's reference frame only the platform is moving, does the perception of motion differ from the actual; does motion actually occur in any of the reference frames?

If either observer perceives themselves as being at [absolute] rest relative to the ether, as you have mentioned, and their counterpart in motion, when the opposite is in fact true, then the perception of motion differs from the actual. The principle of relativity effectively says that an observer can't tell if they are in motion or at rest, but if they perceive themselves as being at rest when they are in motion then then the perception would differ from the actual.

There is an alternative, more intuitive, interpretation, however; both observers could perceive the Earth to be in motion (orbiting and rotating) and that the train starts off at rest relative to the Earth and that the train then actually starts moving, such that both observers ascribe the motion to the train (and the observer on the train); this is presumably what would be categorised by the term "absolute relativity"

Now I would hope that you will recognize that when we pick a frame to describe and analyze what is happening to both the train and the platform, we assign all the motion to one or the other but not both. So in each frame, all the motion is assigned to the other one.

In exactly the same way, when we choose the platform frame, all the time dilation is assigned to the train and none of it to the platform. When we choose the train frame, all the time dilation is assigned to the platform and none of it to the train.

So to answer your question in a more general sense, yes, time dilation actually occurs in any reference frame for any object that is in motion, the faster it moves, the more the time dilation.

I do recognise that when we choose a frame we ascribe all motion to one or the other, but to try and clarify the point of reciprocity, if we put ourselves in the stead of the two observers; let's say that I am on the train and you are on the platform.

From your perspective my instruments are contracted, but simultaneously [in the relative sense] from my perspective your instruments are contracted, such that contraction is supposed to actually be reciprocal.My understanding of LET is that this would not necessarily be the case; if we say that the train I am on is traveling with a higher velocity relative to the rest frame, then my instruments would be more contracted than yours, such that if we were to measure them - in the manner employed by the Einsteinian light clock thought experiement - then we could probably deduce that my clock was ticking more slowly.

The "it" that you are talking about is "time dilation" but change it to "motion" and see how silly the statement sounds:

I know that A will perceive motion to happen in B's reference frame; while simultaneously [again, in the relative sense] B will perceive motion to happen in A's reference frame; but both are free to label themselves as being at rest, so it should be reciprocal.

But if we exchange "motion" for "a slower ticking clock" it doesn't sound quite so silly:

I know that A will perceive a slower ticking clock in B's reference frame; while simultaneously [again, in the relative sense] B will perceive a slower ticking clock in A's reference frame; but both are free to label themselves as being at rest, so it should be reciprocal.

This suggests that substituting "motion" for "time dilation" isn't necessarily the same thing.

What you should be saying is: I know that both A and B will assign all the motion to A in B's reference frame; while A and B will assign all the motion to B in A's reference frame but both are free to choose any reference frame, including one which assigns an equal but lesser motion to both of them but there will never be a reference frame in which both of them have no motion.

Now just change "motion" to "time dilation" and say it again: I know that both A and B will assign all the time dilation to A in B's reference frame; while A and B will assign all the time dilation to B in A's reference frame but both are free to choose any reference frame, including one which assigns an equal but lesser time dilation to both of them but there will never be a reference frame in which both of them have no time dilation.

I'm not sure that we can mix reference frames like that can we, by saying that B will ascribe all motion to B in A's reference frame; is it no more correct to say that A will ascribe all motion to B in A's reference frame?

Again, if we substitute ourselves for both A and B, then we can take a more concrete look at it: If you are A and I am B, then it isn't quite as easy to say that I will ascribe all motion to myself in your reference frame, when in my reference frame I will ascribe all motion to you.

Similarly, we can't really say that you will ascribe all motion to yourself in my reference frame, while you will ascribe all motion to me in your reference frame. I think it is more accurate to say that I would ascribe all motion to you in my reference frame, while you would ascribe it to me in your reference frame; and so reciprocity should arise.

If you are having this trouble then you don't really believe what you said earlier in your post:

Reciprocity in LET
I also have trouble seeing why, according to LET, an observer will perceive reciprocal contractions, just the same as in the SR interpretation.

I understand that theories, such as SR and LET, cannot affect what an observer measures or perceives

Hopefully the clarification above outlines my understanding of how the LET interpretation of relativity suggests that there would not be reciprocal contractions, while the Einsteinian interpretation suggests there would.

There is NO difference between what SR and LET claim about the rates that clocks tick at. The ONLY difference is in what they say about those rates. SR affirms that they are real and legitimate. LET denies this and claims that they are artifacts, unreal, illegitimate, and not actual. But they don't calculate the values of those rates to be any different than SR.

You really should quit trying to make these artificial distinctions between LET and SR. The only distinction is that LET claims the absolute rest frame is in some unknowable state, they would NEVER assign it to the platform or to the earth. That would defeat the whole concept of LET which is that no actual clock is running at the absolute rate that the universe is running at.

I might have been better advised to qualify some of the preceding statements, because my intention wasn't to suggest that LET would treat the platform as the absolute rest frame. The intention was to demonstrate, using the light clock thought experiment, how observers might deduce that their clock was ticking slower than that of another observer. I intended to use the platform as the absolute rest frame for explanatory purposes only, on the basis that the under the Lorentzian interpretation, the absolute rest frame would, in effect, be the same as the platform frame of reference from the Einsteinian interpretation. The train reference frame could equally be used as the absolute rest frame, for explanatory purposes, but I think it is more intuitive to use the platform - or at least, we are more conditioned to consider it intuitive (that is not to say that it is actually the absolute rest frame.

From there we can examine the situation by first considering how an observer moving relative to the rest frame might deduce that their clock was ticking slower, then considering how an observer on the Earth might deduce that their clock was ticking at a different rate. We can then remove the idea of the absolute rest frame and consider how observers moving relative to each other might deduce which clock is ticking faster or slower.

I think it is not going to be productive to defend LET or to keep bringing up these examples. You need to learn SR and once you do you will understand why scientists were struggling so much prior to Einstein showing them the way out of their confusion.

It's not that I am trying to defend Lorentzian relativity, I'm trying to understand the practical differences between it and Einsteinian relativity; I have an understanding of both and I think the light clock thought experiment is useful for discussing the differences, because it is quite an easy example to understand.

I know that my understanding of both "theories" of relativity is not nil, and my representation of that understanding above is, I believe, fairly logical, but I'm not sure if there are things I have missed which might affect the conclusions drawn.

 

[mangaroosh]

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

But would it not be possible to discard the physical model and the phenomenon that is was based on?

Based on what I have encountered with regard to Einsteinian relativity to date, and re-inforced by George has said, it seems as though Einsteinian relativity, in many cases, treats reference frames as though they are at absolute rest in the ether, or that they are just at absolute rest. I would think that even this is unnecessary, and is a superfluous assumption.

I've heard that this treatment is supposed to be based on the constancy of the speed of light, but because Lorentzian relativity says something different about the propagation of light it suggests that the Einsteinian treatment of reference frames [as being at absolute rest] isn't definitively a necessity. I get the impression that it would be possible to drop the idea of an absolute rest frame, without the Einsteinian retention of the phenomenon it was based on.

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

I had a read of the first few pages, but couldn't really see anything that addresses the philosophical differences; the pages I read are concepts I am familiar with. Do you know if there is a specific section that does that?

Obviously.

But Einsteinian relativity appears to treat reference frames as though they are at absolute rest, or at rest in the ether, as George mentioned.

 

[Agerhell]

The gravity field does not have a rest frame. If you wish to discuss this further you need to provide a scientific reference.

? What are you saying?

I am just saying that you could in principle think of three types or relativity...

1. LET - There is one preferred frame, that of the eather and time dilation and relativistic mass increase depends on the velocity relative to this universal preferred frame.

2. SR - There are no preferred frames. All inertial frames are identical. This leads to relativity of simultaneity etc. People in different inertial frames disagree on each others clock rates etc.

3. Some kind of system with locally preferred frames. This could in principle be done in an infinite number of ways, but some kind of system relating to the locally dominant gravitational field to me is the most natural. For instance, when Nasa wants to compute the clock-rate onboard their space vehicles they take into account their speed and postion relative to the Earth for near Earth vehicles. Out in the solar system is is velocity and position relative to the Sun is what matter the most and close to, for instance Jupiter, the impact of Jupiter is the dominant factor.

They have a thick manual online, I do not have the link right now, that discusses such matters.

 

[Dale]

? What are you saying?

I am saying two things:
1) A gravity field does not have a rest frame
2) If you wish to discuss your third type of relativity then you need to provide a mainstream scientific reference for it.

For instance, when Nasa wants to compute the clock-rate onboard their space vehicles they take into account their speed and postion relative to the Earth for near Earth vehicles. Out in the solar system is is velocity and position relative to the Sun is what matter the most and close to, for instance Jupiter, the impact of Jupiter is the dominant factor.

Sure, you can make a reference frame relative to a star or a planet or some other object. Objects have rest frames, the gravitational field does not have a rest frame.

 

[harrylin]

[..]3. Some kind of system with locally preferred frames. This could in principle be done in an infinite number of ways, but some kind of system relating to the locally dominant gravitational field to me is the most natural. For instance, when Nasa wants to compute the clock-rate onboard their space vehicles they take into account their speed and postion relative to the Earth for near Earth vehicles. Out in the solar system is is velocity and position relative to the Sun is what matter the most and close to, for instance Jupiter, the impact of Jupiter is the dominant factor. [..]

It's similar to a centre of mass system of mechanics: that kind of system isn't preferred for the laws of nature, but only for simplicity of calculations.

 

[mangaroosh]

Here is the original reply, without the invalid reference.

Not if you understand the physical concept behind it: it was (and still is) taken for granted that the speed of a wave is completely independent of that of the source. In the introduction of his 1905 paper Einstein simply phrased it as follows:

"light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body"

An article I linked to, in the "constancy of c" thread, offers an interpretation of what the italicised could potentially mean, which seems to make sense. I posted the article only because I have a better understanding of the sentiments that I would be able to express myself.

Essentially, the point was, that the motion of the source of a light wave does not affect the propagation of that wave, once the wave is released because the source will not travel fast enough to affect the rate of propagation of the wavefront. It think the point was also that physical.

There was more to it, but I wouldn't do it justice if I tried to outline it - I've PM'd you the link and can PM the link to anyone that wants to read for the sake of this discussion.

According to SR (but not GR) it is everywhere the same constant relative to all points in space as defined with a Newtonian reference system ("inertial frame"). Thus, probably I did not understand what you meant (and I still don't!).

I don't know what you mean with that; Lorentz did not really work with "postulates" but developed theories based on physical models and the results of experiments.

However, as Lorentz came to prefer Einstein's derivation over his own (which indeed was much more complicated), perhaps it's useful to highlight how they they fit together, although Lorentz might phrase a few sentences a little different from Einstein. For the subtlety of the difference is, if I see it correctly, what you are missing.

So, please take Einstein's 1905 derivation and I'll show you how easy it is to switch between Lorentz and Einstein. The following translation is quite OK:
http://www.fourmilab.ch/etexts/einstein/specrel/www/

To make his derivation perfectly "Lorentz compatible", only a few sentences need slight modification (in italics) as follows:

Examples of this sort, together with the unsuccessful attempts to discover any motion of the Earth relatively to the “light medium,” suggest that the phenomena of electrodynamics as well as of mechanics possesses no properties that permit the detection of absolute velocity. They suggest rather that, as has already been shown to the first order of small quantities, the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good. We will raise this conjecture (the purport of which will hereafter be called the “Principle of Relativity”) to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell's theory for stationary bodies. The concept of a “luminiferous ether” will be helpful to explain the second postulate; however the theory will not provide a “preferred stationary space” with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.

That's about it; if I see it correctly, there isn't much else that really needs to be modified although Lorentz would add here and there some more qualifiers as "true", "apparent", "local" etc.

However, Einstein makes a subtle switch from "empty space" to "reference system" without a clear motivation. To make the logic as well as the derivation easier to follow for the readers, Lorentz could instead stick with the original, physical meaning of the second postulate as formulated in the introduction - let's call it Maxwell's light postulate.

Next, Lorentz could explain that if we combine Maxwell's light postulate with the PoR, it follows that this postulate should also appear to hold in an inertial reference system that is in motion with respect to the ether, so that it could appear to be a "stationary" system just as in Newton's mechanics. He could refer to his 1895 and 1899 papers that explain how this works.

If we next operationally define all terms such as "speed", "time" etc. as described in section 1 (free from metaphysical meaning), then we obtain the following result:

Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.

I hope that this sufficiently clarifies the perceptual differences as well as the complete lack of difference in practice.

Harald

ADDENDUM: I forgot to point out that from Lorentz perspective one may proceed the derivation directly based on Maxwell's version of the light postulate, it is not necessary to make the intermediate step from "empty space" to an arbitrary Newtonian reference system.
Einstein wouldn't like that but it's much more straightforward (and of course, Newton would like that).

The Lorentz transformations that then result describe primarily a transformation between a system that is at rest in the ether and one in inertial motion. Thanks to the form of those transformation equations (they form a group), the same transformation equations are valid between inertial reference systems - just as is the case with the Galilean transformations.

Just on the general point of the differences between Lorentzian relativity and Einsteinian, I understand that they are quite similar, and that the maths is essentially the same, but there are some key differences; one striking difference pertains to relativity; it is relative according to Einsteinian but absolute according to Lorentzian.

While I understand that Lorentzian relativity isn't necessarily based on "postulates" per se, how it appears to me is that if we were to try and formulate a postulate [about the propagation of light] for the Lorentzian interpretation, then it would be different to the Einsteinian; that is, both say somewhat different things about the propagation of light, neither of which has been contradicted by experiment.

Note: I see that George refers to a "postulate" when speaking about Lorentzian relativity; while I understand that Lorentzian relativity isn't based on postulates per se, I presume George is aware of this and uses the term for brevity rather than specificity - I'm using the term in a similar manner.

[Not being falsified by experiment] is the same thing [as being borne out by experiment]: the best that can be borne out by experimental evidence is a persistent lack of contradiction. In practice that ideal can't even be reached as no experiment is perfect.

While I understand that point, if my understanding of the difference in "postulates" is accurate, then there is a distinction between the two bracketed qualifiers above, because neither the Lorentzian nor the Einsteinian postulates have been falsified, but the evidence hasn't distinguished between the two i.e. neither one has been borne out by the evidence.

 

[harrylin]

But would it not be possible to discard the physical model and the phenomenon that is was based on?

Based on what I have encountered with regard to Einsteinian relativity to date, and re-inforced by George has said, it seems as though Einsteinian relativity, in many cases, treats reference frames as though they are at absolute rest in the ether, or that they are just at absolute rest. I would think that even this is unnecessary, and is a superfluous assumption.

Such theories as you seem to suggest, or at least the ones that are known, were discarded because of the experimental evidence.
According to SR (more according to Lorentz than Einstein who might have not liked to formulate it that way), you can treat any inertial frame as if it is at rest in the ether.

I've heard that this treatment is supposed to be based on the constancy of the speed of light, but because Lorentzian relativity says something different about the propagation of light it suggests that the Einsteinian treatment of reference frames [as being at absolute rest] isn't definitively a necessity. I get the impression that it would be possible to drop the idea of an absolute rest frame, without the Einsteinian retention of the phenomenon it was based on.

That sounds like a misinterpretation of both Lorentz and Einstein. It's difficult for me to imagine all possible ways to misinterpret what looks to me like straightforward explanations by them...

The simple answer is that the Lorentz transformations relate to inertial frames, else you mess up like many students with the twin paradox.

I had a read of the first few pages, but couldn't really see anything that addresses the philosophical differences; the pages I read are concepts I am familiar with. Do you know if there is a specific section that does that?

I think that I made a copy-paste error - sorry! Here is the chapter that I had in mind and that highlights the philosophical difference between interpretations:
http://www.bartleby.com/173/16.html
Especially halfway the section starting with " In one of the most notable of these attempts Michelson devised a method".

But Einsteinian relativity appears to treat reference frames as though they are at absolute rest, or at rest in the ether, as George mentioned.

One could say that, yes. That is what the PoR demands; it's done exactly the same in classical (Newtonian) mechanics.

 

[Dale]

While I understand that Lorentzian relativity isn't necessarily based on "postulates" per se

All theories are based on postulates (you cannot avoid it). One big problem with these discussion is that LET was never developed so it does not have a definitive set of postulates published anywhere. It has postulates, but they are kind of unspoken and not entirely agreed upon.

 

[Agerhell]

All theories are based on postulates (you cannot avoid it). One big problem with these discussion is that LET was never developed so it does not have a definitive set of postulates published anywhere. It has postulates, but they are kind of unspoken and not entirely agreed upon.

I would expect the definition of LET to be that.

1. There is one prefereed frame, that of "the universal eather".
2. Length contraction, time dilation and relatvistic mass increase occurs according to the same formulas as in Special Relativity but it is the velocity relative to "the universal eather" that counts and not the relative velocity between intertial reference frames as in SR.

But if other people have other definitions, then for sure it is hard to have a fruitful discussion.

 

[Dale]

But if other people have other definitions, then for sure it is hard to have a fruitful discussion.

Agreed!

 

[harrylin]

[..] An article I linked to, in the "constancy of c" thread, offers an interpretation of what the italicised could potentially mean, which seems to make sense. [..] Essentially, t[..] the motion of the source of a light wave does not affect the propagation of that wave, once the wave is released because the source will not travel fast enough to affect the rate of propagation of the wavefront. It think the point was also that physical.

There is no need for speculations, Einstein clearly explained what it means in several papers, talks and interviews: Maxwell's model of wave propagation. And this was well understood at the time. But apparently that is also what you mean with your suggestion; thus I don't know why you bring it up.

Just on the general point of the differences between Lorentzian relativity and Einsteinian, I understand that they are quite similar, and that the maths is essentially the same,

Again (this was the very point of my elaboration): Lorentz preferred Einstein's method of derivation. The resulting math is necessarily not "essentially the same", but identical.

but there are some key differences; one striking difference pertains to relativity; it is relative according to Einsteinian but absolute according to Lorentzian.

That is a matter of words and interpretation, and regretfully few textbooks discuss it. Although I haven't read it yet, that amazing fact is apparently explained in the textbook by Mermin (Space and Time in Special Relativity, regretfully ).
Anyway, it's exactly the same with interpretations of classical (Galilean) relativity. If you understand that, then it's easy to understand the same for SR.

While I understand that Lorentzian relativity isn't necessarily based on "postulates" per se, how it appears to me is that if we were to try and formulate a postulate [about the propagation of light] for the Lorentzian interpretation, then it would be different to the Einsteinian; that is, both say somewhat different things about the propagation of light, neither of which has been contradicted by experiment.

I think that I gave a clear presentation about the way Lorentz agreed with the second postulate while not following Einstein's suggestion of a change of interpretation.
[ADDENDUM:] Note however that certain predictions of QM are thought to be incompatible with Einstein's interpretation. This is more commonly discussed in the QM group (search for "Bell's theorem".

Note: I see that George refers to a "postulate" when speaking about Lorentzian relativity; while I understand that Lorentzian relativity isn't based on postulates per se, I presume George is aware of this and uses the term for brevity rather than specificity - I'm using the term in a similar manner.

I was a bit sloppy in one of my replies, sorry. As Einstein correctly summarizes in his 1907 paper, Lorentz's paper of 1904 (as well as his own paper of 1905) combines the earlier Lorentz theory of electrodynamics with the relativity principle. While Lorentz's 1904 paper doesn't call it a "postulate", it certainly uses it that way.

While I understand that point, if my understanding of the difference in "postulates" is accurate, then there is a distinction between the two bracketed qualifiers above, because neither the Lorentzian nor the Einsteinian postulates have been falsified, but the evidence hasn't distinguished between the two i.e. neither one has been borne out by the evidence.

Experimental evidence can support or disagree with predictions; it cannot tell us about interpretations of those predictions. Just as a picture cannot tell you how you should interpret it.

 

[harrylin]

[..] But if other people have other definitions, then for sure it is hard to have a fruitful discussion.

A "definition" (of characterizing aspects?) of a theory isn't identical - except by pure chance - to postulates on which it is based...