This latest paper examines and calculates the lower limit of a photon's lifetime based on our best upper limit of the photon's mass (additional review of this work can be found at PhysicsWorld).
For a photon to decay, it must have a mass—otherwise there’d be nothing lighter for it to decay into. A photon with nonzero mass is not ruled out by theory, but experiments with electric and magnetic fields constrain the mass to less thanIn other words, it is horribly longer than the age of our universe by many orders of magnitude! I'd say the odds of observing such a decay is zero.
10−54kilograms. Heeck assumed this upper limit and worked through a generic model in which photons decay into even lighter particles, which could potentially be neutrinos or some more exotic particles.
As a constraint, Heeck considered the CMB, the relic emission from the hot, opaque plasma that persisted for several hundred thousand years after the big bang. The CMB spectrum matches very closely a perfect blackbody, which implies very few, if any, of the CMB photons decayed on their
13billion year journey. Heeck calculated that the minimum lifetime is 3years in the photon’s rest frame. This might seem ridiculously small, but the photons are extremely relativistic. When time dilation is taken into account, a visible wavelength photon in our reference frame would be stable for 1018years or more.