Constraining the Photon Lifetime

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.