A question about the speed of light and electromagnetism

[PeroK]

I'm not convinced that the oversimplification that time is not passing in the reference frame of a particle with no rest mass has no use.

The reference to particle decay is interesting, but we were trying to establish the kinematics of light in relativity. Light is studied using an affine parameterisation (so we don't need proper time along a light-like trajectory). Ironically, many of the tests of GR are experiments involving light: the Pound-Rebka experiment; Eddington's original experiment using a solar eclipse; the Shapiro delay. So, far from being problematic, the kinematics of light are well understood and modelled within relativity, and provide many of the key tests of the theory.

 

[PeterDonis]

the reference frame of a particle with no rest mass

There is no such thing. It is impossible to have a reference frame in which a zero rest mass object is at rest.

You can have a coordinate chart in which the worldline of a zero rest mass object is one of the coordinate axes. But that chart will not meet the requirements for a reference frame, because it is impossible to construct an orthonormal frame field using that coordinate chart's basis vectors.

 

[PeterDonis]

This oversimplification could be useful in understanding why individual photons, gluons, and gravitons do not decay, no matter how much energy they carry.

No, it isn't, because even though the spacetime arc length along the worldlines of such particles is zero, those worldlines still consist of distinct events, and a decay could still happen at any of those events, if all we are looking at is the spacetime geometry. To explain why decays do not happen at any of those events, you have to show that the amplitude for such decays is zero at all of those events; and just showing that the arc length along the worldline is zero is not sufficient to do that.

none of the forces carried by zero rest mass bosons do experience CP violation.

I don't think this is true. Above the electroweak symmetry breaking threshold, the electroweak gauge bosons are massless, but AFAIK they can still induce CP violating interactions.

 

[Mister T]

I'm not convinced that the oversimplification that time is not passing in the reference frame of a particle with no rest mass has no use.

It may have some use, but it's still an oversimplification and is the source of a lot of misunderstanding.