Twenty One Centimeter Emission is a radio wavelength emission that originates with a neutral hydrogen atom, i.e., a single proton, or hydrogen nucleus, with its accompanying electron. The proton and the electron each have a quantum "spin", which points either "up" or "down." They can both be "up" or "down" (Parallel spins), or one can be "up" and the other "down" (antiparallel spins). 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. Thus, cold neutral hydrogen in space emits this radiation which can be detected using a radio telescope.