Like all scientists at the highest level, Dirac was not afraid to descend from the pinnacle and discuss more down-to-earth matters. Here are two examples. Much of our knowledge comes from light scattered by matter; in particular, that is how we see. In a clever stroke of lateral thinking, Dirac realized that the quantum symmetry between waves of light and waves of matter implied that it is also possible for material particles to be scattered by light, a ghostly possibility that could be observed, as he showed in 1933 in a paper with Peter Kapitza. This was observed for the first time about ten years ago and the manipulation of atoms by laser beams is now a thriving area of applied quantum mechanics – a fact recognized with a Nobel prize last year (Physics World November 1997 p51, print version).
It is not my intention to write about what sort of person Dirac was. But I must mention the genre of “Dirac stories”. He was so unusual in the logic and precision of his interaction with the world, both in and out of physics, that tales have become attached to him and have acquired a life of their own. I suppose it matters to a historian whether they are true or apocryphal (or as Norman Mailer says, “factoids”), but to us they have a deeper resonance that transcends fact. Resisting temptation, I retell just two less well known ones.
Like many scientists, Dirac was known to sleep during (other people’s) lectures, and then wake and suddenly make a penetrating remark. Once, a speaker stopped, scratched his head and declared: “Here is a minus where there should be a plus. I seem to have made an error of sign.” Dirac opened one eye and said: “Or an odd number of them.” Another time, Dirac was at a meeting in a castle, when another guest remarked that a certain room was haunted: at midnight, a ghost appeared. In his only reported utterance on matters paranormal, Dirac asked: “Is that midnight Greenwich time, or daylight saving time?”