How come photons have no mass if they have energy and $E=mc^2$?

Question

If photons have no mass, and yet have energy, how is that compatible with $E=mc^2$? Anything having mass should have energy (and the opposite should be true as well no)?

Related, possible duplicates: https://physics.stackexchange.com/q/2229/123208 & https://physics.stackexchange.com/q/62744/123208 — PM 2Ring, Apr 21 '20 at 08:29

score 2 · Accepted Answer · answered Apr 21 '20 at 08:40

2

$E=mc^2$ is a statement about energy in the rest frame. The photon rest frame does not exist but one can approach it in principle by moving along with a photon at near light speed. The photon will be red shifted and its energy will approach zero. In this limit $E=mc^2=0$.

answered Apr 21 '20 at 08:40

my2cts

24,097

2

Very helpful, thanks a lot! – GaelF Apr 21 '20 at 09:36

How come photons have no mass if they have energy and $E=mc^2$?

1 Answers1