Twenty-one Centimeter Emission

Radio wavelength emission that originates with a neutral 1H atom. The proton and the electron each have a quantum "spin," which points either "up" or "down." Spins can be parallel (both of them "up" or "down"), or antiparallel
(opposite states). The antiparallel state has slightly less energy than the parallel state, so if an atom in the parallel state changes to antiparallel, a
21-cm radio photon is emitted.