Photons could conceivably decay, but new analysis of the cosmic microwave background shows that a visible wavelength photon is stable for at least ${10}^{18}$ years.

Can a photon decay? It’s hard to imagine, especially considering how long starlight travels to reach us. Still, if photons happen to have a small, imperceptible mass, then they could decay into lighter particles. A search for signs of these decays uses the oldest light in the universe—the cosmic microwave background or CMB. In Physical Review Letters, Julian Heeck of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, shows that the blackbody spectrum of the CMB rules out decays and thus sets a lower limit on the photon lifetime.

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 than ${10}^{-54}$ kilograms. 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 $13$ billion year journey. Heeck calculated that the minimum lifetime is $3$ years 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 ${10}^{18}$ years or more. – Michael Schirber

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