On this day in 1932, Caltech physicist Carl Anderson discovered the positron, the first particle of antimatter to be identified. Anderson had not set out to hunt antimatter. He had built a cloud chamber to determine the composition of cosmic rays, high-energy particles that rain down from space. The instrument included a magnet, which allowed Anderson to determine whether particles passing through were positively or negatively charged, and a lead plate to slow the particles down. Anderson took hundreds of photographs of tracks taken by cosmic ray particles, but he was stymied by tracks like the one in this post. The curve of the trajectory suggested the particle was positively charged yet far less massive than a proton. An editor at the journal Physical Review suggested the name positron, since the particle was resembled a positively charged electron. Only later did Anderson realize that the positron was identical to the electron except for the opposite charge—Paul Dirac had predicted the existence of such an antiparticle in 1931. Anderson was awarded the 1936 Nobel Prize in Physics for his discovery.
