Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
P.A.M. Dirac (1902-1984), the British mathematical physicist, shared the 1933 Nobel prize with Schroedinger. His theory of quantum mechanics was broad enough to encompass both wave and matrix mechanics. From purely mathematical considerations, he concluded that the electron could exist in two energy states, positive or negative, and hence that there should exist its antiparticle with a positive charge. Such a particle, the positron, was discovered in 1932 from cloud chamber photographs, where in a magnetic field the tracks of particles of opposite charges curve in opposite directions, as on this Swedish stamp. Since then other particles of antimatter have been observed, each having its corresponding mirror particle. By combining quantum mechanics with the special theory of relativity, Dirac was also able to explain the spin of the electron.