## Dirac's equation predicts antiparticles

In 1928, British physicist Paul Dirac wrote down an equation that combined quantum theory and special relativity to describe the behaviour of an electron moving at a relativistic speed. The equation would allow whole atoms to be treated in a manner consistent with Einstein's relativity theory. Dirac's equation appeared in his paper *The quantum theory of the electron,* received by the journal *Proceedings of the Royal Society A * on 2 January 1928. It won Dirac the Nobel prize in physics in 1933.

But the equation posed a problem: just as the equation x^{2}=4 can have two possible solutions (x=2 or x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. But classical physics (and common sense) dictated that the energy of a particle must always be a positive number.

Dirac interpreted the equation to mean that for every particle there exists a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron there should be an "antielectron" identical in every way but with a positive electric charge. In his 1933 Nobel lecture, Dirac explained how he arrived at this conclusion and speculated on the existence of a completely new universe made out of antimatter:

If we accept the view of complete symmetry between positive and negative electric charge so far as concerns the fundamental laws of Nature, we must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about, these stars being built up mainly of positrons and negative protons. In fact, there may be half the stars of each kind. The two kinds of stars would both show exactly the same spectra, and there would be no way of distinguishing them by present astronomical methods.