"[C]reation and annihilation concepts antedate quantum mechanics. The concept of annihilation of pairs of oppositely charged, elementary particles... dates from the turn of the twentieth century. It became important in astrophysics about 1924... The annihilating pairs were first positive and negative electrons, later protons and electrons, and finally, starting in 1931, electrons and anti-electrons. ...In Dirac's "hole" theory of 1930... pair annihilation was neither novel nor central. Dirac's object was to deal with a difficulty... that the theory allowed electrons to make transitions to . ...interpreting electrons in states of negative energy as unobservable, and empty negative-energy states, or "holes" as protons. As a by-product, when an electron jumped into a vacant negative-energy state, an electron and a proton disappeared together into radiation. Since pair annihilation was already an accepted concept, this... was admissible."