The photon – the quantum of light or other electromagnetic radiation – is normally considered to have zero mass. But some theories allow photons to have a small rest mass and one consequence of that would be that photons could then decay into lighter elementary particles. So if such a decay were possible, what are the limits on the lifetime of a photon? That is the question asked by a physicist in Germany, who has calculated the lower limit for the lifetime of the photon to be three years in the photon's frame of reference. This translates to about one billion billion (1018) years in our frame of reference.

An issue of mass

The idea that photons have a finite lifespan, and therefore mass, is difficult to imagine. Indeed, astronomers looking at distant cosmic objects regularly detect photons that are billions of years old. But some theories suggest that photons could have a non-zero rest mass, albeit a small one – the upper limit for the mass of the photon is constrained to 10–18 eV or 10–54 kg thanks to experiments with electric and magnetic fields. And with this small mass, a photon could decay into other lighter elementary particles, such as a pair of the lightest neutrino and an antineutrino, or even particles that are currently unknown and beyond the Standard Model of particle physics.

Now, Julian Heeck of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, has turned to cosmological observations for signs of this photon decay (Phys. Rev. Lett. 111 021801). He looked at the cosmic microwave background (CMB), a remnant of the Big Bang that came into being when the universe was very young – only about 380,000 years old.

Background glow

Before that time, matter and radiation were intrinsically linked. But as the universe underwent a period of extreme growth known as "inflation" and expanded, the hot plasma of electrons and light nuclei cooled enough to allow neutral atoms to form. This "decoupling" of matter and radiation suddenly allowed photons to travel freely across the universe. Over time, their wavelengths were stretched by the expansion of the universe to leave a faint glow of radiation in the microwave region of the spectrum – an emission of uniform, black-body thermal energy – in every direction that we can detect today.

More than 100 experiments have studied the CMB since it was first discovered, including NASA's Cosmic Background Explorer (COBE) satellite, its Wilkinson Microwave Anisotropy Probe (WMAP) and more recently the European Space Agency's Planck mission, all of which have made increasingly precise measurements of this radiation. In fact, the CMB spectrum is the most precisely measured black-body spectrum in nature.

A long lifetime

It is this spectrum that Heeck used as a constraint for his calculations – he used extremely accurate data from the COBE mission and compared it to his calculated spectrum, which included the photon decay.

If the photon has mass and is decaying into lighter particles, then the number density of photons in the CMB should decrease as the photons travel. But this in turn would mean that the CMB spectrum would no longer fit the near-perfect thermal curve that is observed. Heeck reasons that as the CMB is an almost a perfect black body, very few photons, if any, will have decayed during the 13.8-billion-year existence of the universe and so the CMB measurements can constrain the photon's lifetime.

Using a combination of the mass and CMB constraints, Heeck calculates the photon's lifetime within its own rest frame to be three years. But as these photons with tiny mass travel at nearly the speed of light, time dilatation must be accounted for to obtain their lifetime in our frame of reference, for visible light – and this was calculated to be 1018 or a billion billion years. Improving this limit might be difficult until new studies can probe the early universe further.

The research is published in Physical Review Letters.