Relativity vs competing theories

[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...