4 ms·

Photos by definition don't have a rest mass because they are never at rest. They must always travel at the speed of light, which means they don't experience tim

by brabel 6y ago

Photos by definition don't have a rest mass because they are never at rest. They must always travel at the speed of light, which means they don't experience time, so rest does not even make sense for a photon.

jlokier 6y ago
That makes sense in English, however "rest mess" is also a technical and mathematical term, and in that terminology it has the value zero for photons.