Triumph out of difficulty

Just as the problem appeared to be solved, a difficulty appeared. The solutions to the equations were such that energy could have both positive and negative values. How could a particle be in a state of negative energy? Dirac felt that either the negative solutions were 'non-physical' and could be ignored, or perhaps the equations were 'trying to tell him something!'.

Having agonised over this for some time, Dirac felt that the 'difficulty' could not be ignored. His own equations were 'more clever than he was'! They were telling him about some law of nature that had not yet been discovered.

Let us quote from Dirac's own account of his conclusions in a lecture he gave at a conference in Budapest in July 1977, to mark the 50th anniversary of his discovery: '... the difficulty was making the rather bold assumption that negative energy states exist, but normally in a vacuum they are all filled up. If such a negative energy state is not filled, there is a hole which appears as a physical particle. It would be a particle similar to the electron, and it would have positive instead of negative charge, and it would have positive energy.'

Dirac's discovery was a 'discovery of the mind', made on the basis of logic and mathematics. It predicted the existence of a new particle which nobody had yet observed experimentally, an 'antiparticle' to the electron (later named the 'positron'). Within a few years came confirmation by experiment, when, in 1932, C.D. Anderson observed the track of a positively charged particle of the same mass as the electron when studying cosmic ray interactions. Antimatter really existed!

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