Nature’s simplest atom does not appear in the periodic table. Positronium (Ps), the short-lived bound system of an electron and its antiparticle, the positron, was independently predicted by Arthur Ruark and John Wheeler after the positron was discovered in 1932. Four years before positronium itself was discovered in 1951, Egil Hylleraas and Aadne Ore had performed a variational calculation 1 and concluded that Ps atoms could combine to form the diatomic molecule Ps2. Two Ps atoms can also interact by swapping the spins of their two electrons or positrons.
Interactions of Ps atoms with each other are much more difficult to observe than to envision. To create Ps, researchers shoot positrons at a suitable target. If the Ps atoms are to be close enough to react, a large number of positrons must be accumulated and then quickly dumped onto a small-diameter spot. That’s a challenge, because the...
