22 - Grand unification

Summary

When in 1931 Paul Dirac concluded from his equation for the electron that there should exist an antiparticle with opposite electric charge, he felt very embarrassed. Such a particle had not yet been discovered, and he really did not want to disturb the scientific community with such a revolutionary proposition. ‘Maybe this strange positively charged particle is simply the proton,’ he suggested. When shortly after that the real antiparticle of the electron (the positron) was identified, he was so surprised that he exclaimed: ‘My equation is smarter than its inventor!’

Nowadays, physicists no longer suffer from such modesty. High energy physicists are now accused of arrogance, and not fully without reason: when the new gauge theory for the weak force became popular, Weinberg and Salam had no trouble at all advertising it as a ‘unification’ of the weak force with the electromagnetic force; and to avoid any misunderstanding: ‘the most important unification since Sir James Clerk Maxwell unified electricity with magnetism’.

I have always maintained that the Weinberg-Salam model does not fully deserve such laudable comment. Did the new theory not start off with two different gauge fields, which we indicate by the mathematical terms ‘SU(2)’ and ‘U(l)’? That is not unification. On the other hand, however, you could emphasize that both force systems are based on exactly the same mathematical principles, and furthermore they are intimately mixed to produce the phenomena that are now explained. And, as in any meaningful theory, numerous new predictions emerged that could be vindicated by experiment. It was much less the case with the previous formalisms, so in this sense one could talk of unification.