Matter from Light: The Physics of Pair Production
Pair production is one of the most dramatic demonstrations of E = mc² in nature. A high-energy gamma ray photon, passing near an atomic nucleus, spontaneously converts into a particle-antiparticle pair — typically an electron and a positron. Pure energy becomes matter, with the photon's energy distributed between the rest mass and kinetic energy of the newly created particles.
The Threshold and Conservation Laws
Pair production requires the photon to have at least 1.022 MeV of energy — twice the electron's rest mass energy (0.511 MeV). Below this threshold, there simply is not enough energy to create the mass. A nearby nucleus is essential: it absorbs the recoil momentum, allowing both energy and momentum to be conserved simultaneously. The probability increases with the nuclear charge squared, making heavy elements like lead excellent pair production targets.
Spiraling Through Magnetic Fields
In a magnetic field, the newly created electron and positron spiral in opposite directions — the electron clockwise and the positron counterclockwise (or vice versa, depending on field orientation). This characteristic V-shaped pattern in cloud chamber photographs was how Carl Anderson first identified the positron in 1932, confirming Dirac's prediction of antimatter.
Annihilation: The Reverse Process
The reverse of pair production is annihilation: when the positron eventually encounters an electron, both particles vanish, producing two 0.511 MeV photons traveling in opposite directions. This process is the basis of PET (Positron Emission Tomography) scanning in medicine, where radioactive tracers emit positrons that annihilate with electrons in the body, and the resulting photon pairs are detected to create 3D images.