While most people seem to be obsessed with losing weight, physicists have the opposite problem: they want to know how fundamental particles such as electrons put on weight. According to the very basic laws of nature, these particles should not weigh anything at all. So where does their weight - or mass, as physicists prefer to call it - come from? And why do they all have different weights?
Most physicists believe that the answers to these questions can be found in another particle called the Higgs boson. But no one has ever seen a Higgs boson, even though it was first predicted by the British physicist Peter Higgs in the summer of 1964. This is why governments across Europe and beyond are spending some £3bn on a massive underground particle accelerator called the Large Hadron Collider (LHC) at the Cern laboratory in Geneva.
The Higgs boson is so important to particle physics that it has been called the "God Particle" by one Nobel Prize-winner, Leon Lederman. "That's a bit embarrassing," says Higgs, sitting in the living room of his flat in Edinburgh's new town. "Lederman's story is that he wanted to call his book The Goddamn Particle, rather than The God Particle, but his publishers wouldn't agree to it. I'm not a believer in God, but I thought his rather flippant use of the term might be offensive to some people."
Higgs does not mind upsetting people over matters of principle. Earlier this year he was awarded the Wolf Prize - which is often considered to be the second most important prize in physics after the Nobel - but he refused to fly to Jerusalem to receive the award, because it was a state occasion attended by the President of Israel, Moshe Katsav, and Higgs is opposed to Israel's actions in Palestine.
Born in Newcastle in 1929, Higgs went to school in Birmingham, Bristol and London - his father was an engineer with the BBC, so the family moved around - and then studied physics at King's College in London. In 1960 he landed a permanent position at Edinburgh University, and four years later he did the work that has made him famous in the world of physics and resulted in his portrait hanging in the Scottish National Portrait Gallery.
Yet no one realised his research was important at the time. Indeed, the second paper he wrote about the Higgs boson was rejected by Europe's leading journal for particle physics, and Higgs had to send it to a rival journal in the US. "I don't know why I separated the work into two pieces," he recalls. "Maybe if I had written it as one continuous paper, they would have rejected the lot." It should have helped that two other physicists - François Englert and Robert Brout of the Free University of Brussels - had independently arrived at much the same conclusions as Higgs at around the same time, but their work was ignored as well. That may explain why Higgs is reported to have sent a note that year to one of his PhD students, saying: "This summer I have found something that is totally useless."
Higgs's work involved one of the most important concepts in physics - symmetry. Just as we say that a circle is symmetric because it looks exactly the same after it has been rotated, physicists say that a theory is symmetric if it does not change when we change something else. In a nutshell, the problem in the early 1960s was that the basic theories of particles were too symmetric. Physicists had to find something that could "break" this symmetry and allow the particles to put on weight.
Today this something is known as the Higgs mechanism, although Higgs himself is quick to point out that at least half a dozen other physicists deserve a share of the credit. "Most of what has been attached to my name shouldn't have been," he says, "but probably the Higgs boson is correctly attached to me, because I was the person who drew most attention to it."
Although Higgs is little known outside the physics community, his name hit the headlines in 2002 when comments he made about Stephen Hawking led to a story headlined "Clash of the atom-smashing academics" in The Scotsman. According to Higgs, some remarks he had made about a lack of communication between Hawking, who is primarily a cosmologist, and the particle-physics community were taken out of context. The two men have since resolved their differences, although Hawking still believes that the Higgs boson won't be found.
It goes without saying that the basic idea of the Higgs mechanism is not easy to explain. That is why William Waldegrave, when he was the government's science minister in 1993, offered a bottle of vintage champagne to anyone who could explain on a single sheet of A4 paper what the Higgs boson was.
The winning entries in the competition used a variety of analogies to explain the Higgs. David Miller of University College London, for instance, described a cocktail party organised by a political party with guests scattered evenly across the floor. A former prime minister arrives at the party and is surrounded by a cluster of people as she moves about the room. In this analogy, the people at the party represent something called the Higgs field, and Mrs Thatcher gains weight as she tries to move through this field. Someone less important would attract fewer acolytes and hence would gain less weight. Of course, Peter Higgs didn't explain it like this in his influential papers in 1964.
The story of the Higgs boson started in 1961, when Yoichiro Nambu, a physicist at the University of Chicago, showed that a process called "spontaneous symmetry breaking" might be able to explain where mass comes from. To visualise what is meant by this, imagine a marble sitting on top of a Mexican hat. This scenario is symmetric, because it looks the same from all directions. However, the marble is not stable and the symmetry is spontaneously broken when it falls into the rim of the hat. This might sound like another populist analogy, but the Higgs field actually is shaped like a Mexican hat.
A year later Jeffrey Goldstone, then at Cambridge University, and two future Nobel Prize-winners, Abdus Salam and Steven Weinberg, showed that there was a flaw in this approach. Higgs's first contribution to the problem - made 40 years ago- was to show that Goldstone and his colleagues had also made a mistake. This breakthrough was published in Physics Letters.
Higgs then went on to show that spontaneous symmetry breaking could explain how particles that were already known at the time could have mass. Moreover, Higgs's theory predicted the existence of what he describes as a "left-over particle". This new particle was the Higgs boson.
Higgs sent this second result to Physics Letters as well, but he was told that it was not suitable for rapid publication and was advised to send it to another journal. However, he later heard through a colleague that the paper had been rejected because the editors felt that "it was of no obvious relevance to physics". At first Higgs was "indignant", but he later realised that the first draft of the paper "had been short on sales talk". He added two paragraphs and sent the paper to a US journal called Physical Review Letters, where it was accepted.
However, it took longer for the particle-physics community at large to realise the significance of what the Edinburgh physicist had done. Higgs remembers giving talks on his work to sceptical audiences at Harvard and Princeton. "I was facing audiences who at first thought I was a crackpot," he recalls. "At the end of the day they accepted that I wasn't, but they didn't realise that you could do something useful with the work."
In the end it was Weinberg and Salam - whose earlier result Higgs had shown to be wrong - who used the Higgs mechanism to make one of the biggest breakthroughs in the history of physics, when they combined the weak and electromagnetic forces into a single "electroweak" force. Weinberg, Salam and a US physicist, Sheldon Glashow, shared the 1979 Nobel Prize for Physics for this work, and the Higgs boson became part of the furniture in particle physics.
The rest is not quite history, because no one has actually found a Higgs boson yet. Higgs and thousands of other physicists are confident that it will show up at the LHC. If it does, Higgs - and possibly Englert and Brout - will be packing their suitcases for the Nobel Prize ceremony in Stockholm. And if the Higgs boson cannot be found at the LHC, physicists will still have a massive problem with their weight.
Peter Rodgers is the editor of 'Physics World' magazine