Verification of c isotropy vs c in Einstein synchronization

[randyu]

I'm confused by something I read, that said: Ref: http://adsabs.harvard.edu/abs/1988AmJPh..56..811B

It was suggested by Einstein and later greatly elaborated on by others that the methods used to synchronize distant clocks are a matter of convention. The standard method, in which it is assumed that the speed of light is isotropic, obviously yields an isotropic light speed when such clocks are involved in determining the speed of light. Another method, in which clocks travel symmetrically but otherwise arbitrarily in opposite directions, may also be used to synchronize distant clocks. This method establishes whether or not the clocks are synchronized in a physically significant way in the sense that it allows a distinction to be made between a contrived anisotropic light speed and an anisotropic speed that is physically significant or real. Specifically, a contrived anisotropic light speed results in laws of physics that are not symmetric, whereas a true anisotropic light speed does not affect the symmetry of physical laws. Furthermore, when invariance in the speed of light is imposed, the invariant interval may be identified with the lapse of proper time in the case in which anisotropy is contrived. But, in the case of true anisotropy, this identification is not possible. Experiment reveals that, on the basis of symmetry in physical law, any anisotropy in the speed of light is contrived and not physically significant.

What does that last statement (underlined by me) mean, "any anisotropy in the speed of light is contrived and not physically significant". Why is it not physically significant? I thought the very definition of isotropy confirmed physical significance. Seems like it should be physically significant in both experimental verification and application of the Einstein convention. And how is it that it is considered contrived? I mean I know we pick a convention, but that doesn't mean it's contrived, or does it?

 

[WannabeNewton]

Read this and the references therein given as "Further Reading": http://en.wikipedia.org/wiki/One-wa..._appear_to_measure_the_one-way_speed_of_light

The entire page consists of a very detailed overview of the issue that should clarify your confusion.

 

[PeterDonis]

This is a comment I made in the previous thread that spawned this one, about the abstract that the OP quotes from:

One thing in this abstract puzzles me. It says:

[W]hen invariance in the speed of light is imposed, the invariant interval may be identified with the lapse of proper time in the case in which anisotropy is contrived. But, in the case of true anisotropy, this identification is not possible.

This doesn't make sense to me; AFAIK in standard relativity (special or general), the "invariant interval" along a timelike curve can *always* be identified with the lapse of proper time along the curve. So I don't understand what the abstract is talking about here. It's quite possible that this paper was speculative and not "mainstream" physics; not every paper that gets published in a physics journal is correct.

 

[randyu]

Read this and the references therein given as "Further Reading": http://en.wikipedia.org/wiki/One-wa..._appear_to_measure_the_one-way_speed_of_light

The entire page consists of a very detailed overview of the issue that should clarify your confusion.

Thanks for the great reference. I found this part particularly useful

experiments also confirm agreement between clock synchronization by slow transport and Einstein synchronization.[2] Even though some authors argued that this is sufficient to demonstrate the isotropy of the one-way speed of light,[10][11] it has been shown that such experiments cannot, in any meaningful way, measure the (an)isotropy of the one way speed of light unless inertial frames and coordinates are defined from the outset so that space and time coordinates as well as slow clock-transport are described isotropically[2] (see sections inertial frames and dynamics and the one-way speed). Regardless of those different interpretations, the observed agreement between those synchronization schemes is an important prediction of special relativity, because this requires that transported clocks undergo time dilation (which itself is synchronization dependent) when viewed from another frame

So, my conclusion is that isotropy is first assumed and the basis of experiments are established from this assumption. Then, the results confirm the assumptions. However, that's not is say that the whole concept is rock solid since, there could be other assumptions which lead to conferrable results of another theory.

In any event, my take away would be that on the basis of what we know today. Light speed is isotropic and it is physically significant. Therefore, I would disagree with the abstract quoted in the OP. What say you?

 

[WannabeNewton]

Within the framework of SR, whether or not the one-way speed of light is isotropic is merely a matter of synchronization convention and does not affect the values of physical observables. It is just a prescription for constructing coordinate systems. As such it is not physically significant. In SR the only significance is in the isotropy of the two-way speed of light.

 

[randyu]

Thanks for the distinction.
Seems like using the term isotropic or anisotropic in the same sentence as two-way light speed is a bit meaningless or at least confusing. And that the author quoted in the OP must have been talking about one-way light speed. But, in general I would be comfortable with c being constant and physically significant on the basis of symmetry in physical law.