Pairla

Meet Pairla, a gamma-ray photon who turns light into matter.

Einstein's E = mc² works in both directions. A photon with enough energy, passing close to an atomic nucleus, can vanish and leave behind an electron and its antiparticle, a positron. The nucleus is needed to take up a little momentum, because a lone photon cannot turn into a pair and conserve energy and momentum at the same time. Carl Anderson discovered the positron in cosmic rays in 1932 and shared the 1936 Nobel Prize in Physics for it.

The photon must supply at least the rest energy of both particles. The electron's rest energy is 0.511 MeV, so the threshold is 2 × 0.511 = 1.022 MeV. Any energy above that becomes kinetic energy of the pair.

Pair production is one of the main ways high-energy gamma rays lose energy in matter, so it matters for radiation shielding and for the detectors used in particle physics and gamma-ray astronomy.

It is the reverse of annihilation, where an electron and positron meet and become gamma rays. Below 1.022 MeV it cannot happen at all, however intense the light.