Higgs 7: Three roads converge
By 1964 the challenge was clear. The familiar phenomena of electricity and magnetism looked as if they could be part of something even bigger. This ‘something’ was the electroweak interaction, with the electromagnetic field one of its aspects and the weak interaction another.
The reasoning came from symmetry. Looked at from one perspective, the electroweak field appeared as electromagnetism. Vewed from another perspective, it appeared as the weak interaction which causes radioactivity.
But because these two interactions, weak and electromagnetic, are so different, the symmetry could not be complete. So somehow the symmetry had to be broken.
Is it possible to keep symmetry and yet break it? No, if we have in mind something like a vase which loses all symmetry when we let it shatter on the floor.
But we don’t have to go that far in the concept of breaking things. We could think of the symmetry of a circle or a five-pointed star and allow it to break a little – and get a rose. The original symmetry is still there, but with edges that are somehow ‘softer’ – and which indeed can look beautiful.
The problem was that Goldstone’s theorem seemed to force the choice to be between two extremes – either a perfect vase or a smashed one. But Philip Anderson’s 1963 paper had suggested that in superconductivity a similar problem was in fact resolved by a process of condensation. Particles in a crystal have their behaviour shaped by the lattice of the crystal.
That suggestion from Anderson was enough to encourage physicists at three centres to take up the challenge anew – and each independently to come up with a solution.
Breaking it gently
The way to resolve the problem is to invoke a new field which fills all of space – today called the Higgs field. This field obeys the rules of symmetry, just as does the existing field of the electroweak theory. But it somehow condenses into a single state. It does this in the same kind of way as happens with bar magnets, when the many jumbled-up magnetic domains drop down into place in a single direction which locks in. Or in the way in which the electrons in a superconductor lock in to a single coordinated structure.
So the field itself is symmetric – but when it condenses into a single fixed state the symmetry is broken.
And it is this field that provides mass to particles that would otherwise be as massless as light. The Higgs field links with the Yang-Mills field (the field which has to exist to maintain gauge symmetry) – and the particles of the Yang-Mills field acquire mass as a result.
The outcome is not only the photon for electromagnetism but no less than four other particles. Three of these are concentrations of the weak interaction, given mass by the condensing of the Higgs field. And there is a fourth, a kind of by-product of the process, something new – the particle which has become known as the Higgs.
The first to publish were at the Free University of Brussels – Francois Englert and Robert Brout.
Englert’s first degree had been in engineering, but he switched to physics and went to work at Cornell as Brout’s research assistant. They found their approaches to physics so similar that when the time came for Englert to go back to Belgium, Brout decided to move there with his family, and went on to acquire Belgian citizenship and play a big part in the further development of physics there, up till his death last year.
One of the features Englert and Brout shared was a readiness to look afresh at any problem, and that, says Englert today, is how they made their breakthrough with what has come to be known as the Higgs particle.
“The generality of our results is largely attributable to the use of quantum field theory, which at the time was largely ignored in elementary-particle physics. Its use in deriving the mechanism was no accident. Driven by his unusual faculty to translate abstract concepts into tangible intuitive images, Robert always conspicuously disregarded academic knowledge and favored entering any subject from scratch. For him, the fact that he was no expert on particle physics was an advantage: He could easily free himself from fashionable trends in the quest for a consistent theory of short-range fundamental forces.”
Imperial College London was the base of Tom Kibble, who had taken his physics degree and doctorate at Edinburgh. He was of Scots descent and born in Madras in India, where his grandfather had been a medical officer and his grandmother, Helen Bannerman, a well-known writer of children’s books. Working with him at Imperial were two visiting Americans, Gerry Guralnik and Dick Hagen, who had both attended lecture courses by Julian Schwinger during their earlier studies.
The paper written by the three was just being put in the post for publication when the papers by Englert and Brout and by Peter Higgs arrived.
Meanwhile at Edinburgh Peter Higgs had been following the arguments about Anderson’s work on superconductivity. In particularly, he noted a criticism made by Wally Gilbert, a remarkable man who started out as a physicist but switched to biology after meeting James Watson (of double helix fame) and went on to win the Nobel Prize for his work in sequencing DNA. Today in retirement, he is an artist-photographer whose work is exhibited in galleries across the US, with the images including an Orkney series.
In 1964 Gilbert had argued that there was a fault in the argument that symmetry-breaking could lead to mass. At first Peter Higgs thought that this was indeed the end of any hope of a way forward. But he then realised that a method developed by Julian Schwinger could be applied to show Gilbert was wrong. He sent a letter to the journal Physics Letters which is published at CERN, the particle accelerator centre near Geneva.
He then worked on a second paper, to put together the actual way in which the symmetry-breaking occurs, using the Higgs field. He sent off this paper to Physics Letters as well – only to have it returned as not suitable for publication. It turned out afterwards that the editor of the journal had thought the paper ‘not to have any relevance to particle physics’.
“I was rather shocked,” he says. “I did not see why they would accept a paper that said this is a possible way to evade the Goldstone theorem, and then reject a paper that showed how you actually do it.”
So during August he revised the paper and with the encouragement of one of his colleagues, Euan Squires, added on a paragraph that drew attention to the fact that the theory had practical experimental consequences – that it predicted a new particle which could be looked for. This emphasis was one of the factors that associated Peter Higgs’s name with the search for the particle.
He sent the paper to the editor of another journal, Physical Review Letters, who confirmed that he would accept it and mentioned the paper by Englert and Brout which he was also about to publish.
The Edinburgh connection