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.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.