Special & General Relativity Questions and Answers If a photon
travels at the speed of light, why isn't its mass infinite?
Because the photon is one of those handful of particles (
photon, graviton, gluon) which has 'zero rest mass'. The special
relativistic formula that shows mass increasing with speed only
applies to particles with non-zero rest mass such as neutrinos,
electrons, quarks and so on. Return to the Special & General
Relativity Questions and Answers page. All answers are provided
by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe,
part of the NASA Education and Public Outreach program.