Positronium and Quantum Hide-and-Seek: How to 'Push' a Photon to Uncover the Mystery
An electron and its antiparticle — the positron, predicted by Дираком, — upon meeting, momentarily form позитроний: a tiny pair circling each other, like in an atom but without a nucleus. Almost instantly it decays, emitting two photons that turn out to be mysteriously linked.