is generally considered to have zero rest mass, and is a stable particle. The experimental upper limit on the photon mass is very small, on the order of 10^-53 g; its lifetime would be more than 10¹⁸ years. For comparison, the age of the universe is about 13.8e9 m years.

Single photons have been shown to travel at the speed of light in vacuum. The measurement uses a heralded single photon source.