Although the simple model of a massless photon may turn out to be correct, the prospect of one with mass raises some intriguing possibilities. For instance, if photons do decay, what do they decay into? "That's, of course, the key question," Heeck says. One possibility are particles called neutrinos, which come in several varieties, or flavors: "If the lightest neutrino were massless, then that would be the most obvious final state" for the photon, Heeck says. A photon "could also in principle decay into other unknown particles."*
Heeck's paper, published July 11 in Physical Review Letters, represents the first calculation of the minimum lifetime of a photon, but it relies on some simplifying assumptions, he says. Most important, it neglects interactions of photons and matter after the cosmic microwave background formed, which could have a strong effect if photons have very small masses. For example, photons may be absorbed by interstellar matter and reemitted, changing their properties. Berti agrees that the assumptions complicate things. "The calculation in principle is very interesting," he says. "But then the devil is in the details."
If these matter interactions can be better understood and taken into account, Heeck says he could calculate tighter limits on the photon’s lifetime. More precise data about the cosmic microwave background would also help, although the reason Heeck didn't use measurements from more recent spacecraft than COBE, such as the Wilkinson Microwave Anisotropy Probe, is because those newer observations lack the necessary temperature data to understand the light's blackbody spectrum.
*Correction (7/30/2013): An earlier version of this story stated that gluons represent a potential end state for photon decay. But this is not possible because a property of gluons called color charge prevents them from being found on their own.