One Way Speed of Light: Is it Constant?

[CompSci]

TL;DR

Is the one way speed of light constant?

Is there any reason to believe that the one way speed of light is constant? Not isotropic, but constant in the sense of being independent of position.

 

[Nugatory]

Is there any reason to believe that the one way speed of light is constant? Not isotropic, but constant in the sense of being independent of position.

It is a postulate of special relativity that the one way speed of light is constant (although it takes a bit of care to state the postulate precisely). Theories based on other assumptions can be constructed, but all that are consistent with observations are more complicated, require other assumptions that are less palatable, and make no testable predictions that special relativity does not. Thus there are no good reasons not to proceed as if the one-way speed of light is constant.

 

[Dale]

Summary:: Is the one way speed of light constant?

Not isotropic, but constant in the sense of being independent of position.

The term for that is homogeneous. Usually “constant” refers to being independent of time.

The one way speed of light in vacuum is a convention. The two way speed of light in vacuum is homogenous, isotropic, and constant. Such symmetries are useful, so it would be rather silly to adopt a convention which does not respect them. In principle, you could do it, but I have never seen an application for doing so except as an incidental byproduct of something else.

 

[Ibix]

Summary:: Is the one way speed of light constant?

Is there any reason to believe that the one way speed of light is constant? Not isotropic, but constant in the sense of being independent of position.

Einstein essentially defined it to be constant, homogeneous, isotropic, and frame invariant. You can define it other ways if you like - it turns out to be just a matter of choosing a non-trivial coordinate system. An obvious example is a rotating coordinate system, where the coordinate speed of light varies with radius.

 

[Herman Trivilino]

Is there any reason to believe that the one way speed of light is constant?

As others have pointed out, it's an assumption. But it's an assumption that has consequences. And when those consequences have been verified by observation and experiment (as they have) it gives us reason to believe the assumption.

 

[jbriggs444]

But it's an assumption that has consequences.

The only consequences are to coordinate systems. Not to observation or experiment.

 

[Vanadium 50]

How do you measure the one-way-speed of light?
If you try to design an experiment, you will discover that this is tied up with your synchronization convention.

 

[Herman Trivilino]

The only consequences are to coordinate systems. Not to observation or experiment.

Ah, yes. Of course. Because it depends on a simultaneity convention.

 

[Nugatory]

Ah, yes. Of course. Because it depends on a simultaneity convention.

Nonetheless, it’s an an important consequence - relativity is by far the cleanest and most elegant formulation of the physics.