## Unification the long hard road

In 1979, Sheldon Glashow (1932-), Abdus Salaam (1926-1996) and Steven Weinberg (1933-) shared the Nobel Prize for 'contributions to the theory of the unified weak and electromagnetic interaction between elementary particles...'. The theory had taken a long time to mature. From the late 1950s, the Nobel laureates had been working sometimes together and sometimes independently, seeking a single theory which would encompass both weak and electromagnetic forces. The goal was to describe the weak and electromagnetic interactions in terms of a mathematical symmetry. As Salaam later pointed out, 'The electroweak picture was being pieced together with almost exasperating slowness. At first we were trying to gauge the wrong symmetry.'

As one technical problem was overcome, a new one appeared. These excerpts from the lectures at the Nobel ceremony give an indication of the difficulties in formulating a coherent new theory:

Weinberg: 'At that point we had reached the right solution, but to the wrong problem.'

Glashow: 'I had "solved" the problem of strangeness changing neutral currents by suppressing all neutral currents; the baby was lost with the bath water.'

There were other problems, one of them being that the formalism was 'not renormalizable,' which meant that the calculations encountered infinities. This latter problem was solved in 1971 by the Dutch physicist Gerard 't Hooft (1946-). In the words of theorist Sidney Coleman (1937-2007): 't Hooft's work turned the Weinberg-Salaam frog into an enchanted prince.'

### The end of the road

Despite the difficulties in formulating the electroweak theory, experimental evidence was emerging that the theory was on the right track. The theory had predicted that there should exist Weak neutral currents' — weak interactions in which the particles involved do not exchange electric charge. No such interactions had ever been seen until 1973, when a neutral current neutrino interaction was observed at CERN. In 1978, an experiment at the Stanford linear accelerator showed some delicate interference effects between electromagnetic and weak forces, as predicted by the theory. The time was right to look for the jewel in the crown — the carrier of the weak force.

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