Relativistic/rest mass of photon in non-vacuum?

Question

A photon has zero rest mass yet positive relativistic mass. This is possible only if v=c. Yet this only holds in vacuum. I can imagine that the energy budget holds if light is seen as waves, but still there should be a solution to this paradox. Is the rest mass positive in non-vacuum?

Answer 1

Hi helene_thygesen

The rest mass of a photon is always zero. In relativistic mechanics, the rest mass is identified with the length of the energy momentum 4 vector [E/c,px,py,pz]. For a particle with a lightlike worldline (such as a photon) the length of this vector is always zero, it's a property of minkowski (or lorentz based-) space-time. This means the photon always has zero rest mass.

Because this is a flat space property it's easily generalised to other coupled fields to describe non-vacuum situations: the result still holds.


Hope this helps!
The Chicken

Answer 2

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