Big Bang Light Reveals Minimum Lifetime of Photons
The notion of the speed of light as the cosmic speed limit is based on the assumption that particles of light, called photons, have no mass. But astrophysical observations cannot rule out the slim chance that photons do have a tiny bit of mass—a prospect with wide ramifications in physics. For instance, if photons weigh nothing at all, they would be completely stable and could theoretically last forever. But if they do have a little mass, they could eventually decay into lighter particles. Now, by studying ancient light radiated shortly after the big bang, a physicist has calculated the minimum lifetime of photons, showing that they must live for at least one billion billion years, if not forever.