# The invariance of the speed of light is not only a hypothesis?

• I
• zoltrix
So no, the invariance of the speed of light cannot be proven, but there is a test that can be done to determine whether or not relativity is valid.f
If I would define the one-way speed as non-isotropic, then the transformation formula would be come more complicated, whithout added value:
https://en.wikipedia.org/wiki/One-w...ansformations_with_anisotropic_one-way_speeds

From what I understand the synchronizations of Reichenbach and Grünbaum are only other conventions of synchronizations. But I don't want a synchronization convention at all. It may have a practical interest, but it's nothing physical.
If Einstein's theory is based on a convention and not on a physical hypothesis it seems to me it has no physical value.

The real is that which has practical consequences and which does not depend on a convention
I do not agree with that definition, but as you use it then indeed length contraction is not “real”. Nor is electrical charge, the magnetic field, the electrical field, gravity, kinetic energy, potential energy, momentum, and I am sure other things. So I am perfectly content with length contraction being not “real” with your meaning. It is in good company with lots of other important but not-“real” things.

I'm not making any assumptions
Nonsense. You cannot avoid making assumptions. You can only be clear on what you have assumed. By believing that you have not made any assumptions you are tricking yourself.

It is particularly amusing to hear a LET proponent disparage making assumptions

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PeterDonis
If Einstein's theory is based on a convention and not on a physical hypothesis it seems to me it has no physical value.
The two SR postulates are physical hypotheses.

I'm not making any assumptions, it's the reality of the closing speed, which is something that can be verified experimentally. Why make other assumptions?
Closing rate between you and light does vary with your speed relative to whoever is measuring. This is true in Einsteinian relativity as well as Galilean relativity. Assuming that closing rate and relative speed are the same thing, though, is assuming Galilean relativity.

As Dale says, you cannot avoid assumptions, but you need to be clear about what they are.

The two SR postulates are physical hypotheses.
I thought so too but Dale and others say that isotropic speed of light is a convention.

It is particularly amusing to hear a LET proponent disparage making assumptions
As Dale says, you cannot avoid assumptions, but you need to be clear about what they are.
The assumption in LET is existence of the eather. In BU it is isotropic speed of light.

I thought so too but Dale and others say that isotropic speed of light is a convention.
You seem to think that those are mutually exclusive. The list of important physical quantities that are defined by some convention is quite substantial

PeterDonis
You seem to think that those are mutually exclusive. The list of important physical quantities that are defined by some convention is quite substantial
We can choose by convention that the speed of light is 1, but that will have no physical consequence, only a mathematical consequence.

We can choose by convention that the speed of light is 1, but that will have no physical consequence, only a mathematical consequence.
Correct. As long as the 2-way speed is the experimentally confirmed invariant speed, the one way speed is a matter of convention with no physical consequence (the synchronization convention). So we may as well postulate the simplest convention consistent with the observations.

PeterDonis
Correct. As long as the 2-way speed is the experimentally confirmed invariant speed, the one way speed is a matter of convention with no physical consequence. So we may as well postulate the simplest convention consistent with the observations.
This is where I disagree, I think the invariance of the one-way speed has physical consequences compared to non-invariance. So it can't be used as a convention. It can only be used as a physical hypothesis.

This is where I disagree, I think the invariance of the one-way speed has physical consequences compared to non-invariance.
Then you are wrong. Consider doing some maths to determine what the effect on a measurement would be if you chose a different simultaneity convention.

Dale
This is where I disagree, I think the invariance of the one-way speed has physical consequences compared to non-invariance.
Can you show how and where?
So it can't be used as a convention.
You need a clock at the emission and clock at the detection to measure the speed. If those clocks are not at the same place in space (are spacelike separated) the clocks need to be synchronized -> need to have a simultaneity convention -> one way speed of light is a convention.

dextercioby
I think the invariance of the one-way speed has physical consequences compared to non-invariance.
OK. So please identify and calculate any measurement that could detect any such physical consequence. I will then show that such a measurement is also consistent with a different one-way speed.

PeterDonis and Ibix
Then you are wrong. Consider doing some maths to determine what the effect on a measurement would be if you chose a different simultaneity convention.
I do no want to choose any simultaneity convention. Why do you want a simultaneity convention? How do you know there is such a thing ?
Can you show how and where?
Time dilation, length contraction for example. Thess are physical consequence.
You need a clock at the emission and clock at the detection to measure the speed. If those clocks are not at the same place in space (are spacelike separated) the clocks need to be synchronized -> need to have a simultaneity convention -> one way speed of light is a convention.
But if it's a convention, you can't derive from it the origin of physical phenomena like time dilation. When you measure the world line that represents proper time on a Minkowski diagram, is this world line physical or is it just a mathematical representation in your opinion? If this is just a mathematical representation, then what is the true origin of time dilation?
The need to be synchronized is a practical need, it has nothing to do with physical reality.
OK. So please identify and calculate any measurement that could detect any such consequence. I will then show that such a measurement is also consistent with a different one-way speed.
If you don't assume isotropy (or other similar convention like Reichenbach), how do you derive time dilation, contraction, etc ?

If you don't assume isotropy (or other similar convention like Reichenbach), how do you derive time dilation, contraction, etc ?
Reichenbach is non-isotropic. That is exactly what I would assume (although I would use Anderson’s approach). However, you have avoided answering the challenge. You claimed that “the invariance of the one-way speed has physical consequences compared to non-invariance”.

So again: please identify and calculate any measurement that could detect any such physical consequence. I will then show that such a measurement is also consistent with a different one-way speed.

You have made the claim. Now back it up or rescind the claim

I do no want to choose any simultaneity convention.
How do you propose to avoid it?

But if it's a convention, you can't derive from it the origin of physical phenomena like time dilation.
Einstein disagrees with you.

PeterDonis
I thought so too but Dale and others say that isotropic speed of light is a convention.
In SR postulate 2, Einstein speaks only of the invariant "speed of light in vacuum".

He does not need to distinguish there between round-trip- and one-way speed. Both have there the same value because of his clock-synchronization scheme for defining the time coordinates of the inertial reference coordinate systems.

Einstein disagrees with you.
I think Einstein thought it was a physical reality.
Reichenbach is non-isotropic. That is exactly what I would assume (although I would use Anderson’s approach).
It's a convention, it's not a natural isotropy that depends on relative velocities.

How do you propose to avoid it?

Simply assuming the clocks are out of sync. If two clocks are synchronized and then they accelerate to high speed, they are out of sync, why artificially resynchronize them?
In SR postulate 2, Einstein speaks only of the invariant "speed of light in vacuum".

He does not need to distinguish there between round-trip- and one-way speed. Both have there the same value because of his clock-synchronization scheme for defining the time coordinates of the inertial reference coordinate systems.
So isotropic speed of light is only a convention.
Thus acording to you there is no Einstein's special theory of relativity, Einstein only made mathematical abstractions about relativity.
Here is my point of view : Either BU is a physical theory and it opposes LET, or it is only a convention and in this case LET is the only physical theory.

I do no want to choose any simultaneity convention. Why do you want a simultaneity convention? How do you know there is such a thing ?
How do you tell if two clocks show the same time or not? The answer to that is a synchronisation convention. You can't not have one unless you simply refuse to look at more than one clock.

why artificially resynchronize them?
So that you can measure one way speeds with them.

Still dodging the challenge, I see. So are you going to rescind your claim or just hope I ignore your avoidance? It isn’t a bad bet, I will probably forget at some point, but by then I think the message will be clear enough anyway.

PeterDonis and Ibix
So that you can measure one way speeds with them.
Indeed. If you don't, you could have one clock at one end of your experiment and two at the other end, all three showing different times. You could get two one way speeds out of one experiment!

Dale
How do you tell if two clocks show the same time or not? The answer to that is a synchronisation convention. You can't not have one unless you simply refuse to look at more than one clock.
In fact we cannot tell if two clocks show the same time. I am quite willing to establish a convention of simultaneity, but you must not give this convention physical consequences that it does not possess.

I am quite willing to establish a convention of simultaneity, but you must not give this convention physical consequences that it does not possess.
It has no physical consequences; it only changes your interpretation of your measurements.

So isotropic speed of light is only a convention.
No, that is only correct for the one-way-speed of light. The isotropy of the two-way speed of light in vacuum is not only a convention. It was tested in the MM experiment.

Thus acording to you there is no Einstein's special theory of relativity, Einstein only made mathematical abstractions about relativity.
That' not correct, see above.

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Dale
So isotropic speed of light is only a convention.
Thus acording to you there is no Einstein's special theory of relativity, Einstein only made mathematical abstractions about relativity.
Here is my point of view : Either BU is a physical theory and it opposes LET, or it is only a convention and in this case LET is the only physical theory
This is an absolutely silly objection. You object to the use of a synchronization convention in BU, and assert that the use of such a convention makes the theory only a mathematical abstraction. But your absurd solution is to push LET which not only uses the same synchronization convention but also adds an undetectable aether and a convention about which frame it is at rest. If you want to use LET that is fine, but the idea that it is somehow superior on the grounds of physicality is ridiculous. It makes all of the same assumptions as BU, plus some more.

In any case, making testable assumptions and arbitrary conventions is part of all physical theories. The idea that the existence of a convention precludes a theory from being physical displays a profound ignorance of science. But, I guess it doesn’t matter much any more.

vanhees71 and Sagittarius A-Star