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.
