Positronium
= Positronium
<Positronium> is the electromagnetic bound state of an <electron> and a <Positron>. Their equal masses give <reduced mass> $m_e/2$, half the hydrogenic Rydberg binding scale and approximately twice the electron-proton separation of hydrogen. Its <spin> singlet and triplet differ in hyperfine <energy>; <annihilation> also affects their physical lifetimes and splitting.