pair production and annihilation

Two of the most vivid demonstrations that energy and matter are interchangeable happen routinely in quantum electrodynamics. In pair production, a sufficiently energetic photon vanishes and in its place two particles of matter appear: an electron and its antimatter twin, a positron. In annihilation, the reverse occurs: an electron meets a positron and the two destroy each other, their mass converting entirely into energy that flies off as photons. Pure light becomes matter; matter becomes pure light.

Both processes obey strict conservation laws, which is why a photon cannot simply turn into matter anywhere it likes. To create an electron-positron pair, the photon must carry at least enough energy to supply both particles' rest mass — a threshold of about 1.022 million electronvolts, since each particle's rest energy is about 0.511 MeV. It also needs a nearby nucleus or another particle to absorb recoil, because a lone photon cannot conserve both energy and momentum while making a pair. Annihilation in turn typically yields two photons flying off in opposite directions, sharing the released energy so that momentum balances.

These are everyday workhorses, not exotic curiosities. Electron-positron annihilation is the basis of the PET scan, where positrons emitted inside the body annihilate and the resulting back-to-back gamma rays are detected to build a medical image. Pair production is how high-energy gamma rays shower into cascades of particles in detectors and in the atmosphere when cosmic rays strike. Together they are the cleanest demonstrations of Einstein's mass-energy equivalence playing out one particle at a time.