Understanding the Higgs 8: Physics is worth a mass

People remember 1967 for different reasons.

It was the year of the Six-Day War in the Middle East and the military coup in Greece. It was the year when North Sea gas came ashore in Britain, when the Beatles issued Sergeant Pepper, and when Celtic won the European Cup. Nicole Kidman was born, Elvis Presley got married, and Woody Guthrie died.

Dr Christiaan Barnard carried out the first heart transplant and Jocelyn Bell discovered pulsars.

And two physicists, working independently, applied the Higgs mechanism to unite electromagnetism and the weak interaction into a single overall structure. Twelve years later, they would share the Nobel Prize.

The two men were very different in background. Steven Weinberg, born in New York City, is an atheist and a strong supporter of the state of Israel. He takes a view of nature which can sometimes seem quite bleak.

Abdus Salam, born in Jhang, Punjab, was a Muslim who quoted the Quran in support of the scientific quest for knowledge. He had a particular interest in symmetry.

‘That may come from my Islamic heritage for that is the way we consider the universe created by Allah with ideas of beauty and symmetry and harmony, with regularity and without chaos.’

But the two also had much in common, including a deep concern to act on global issues. Weinberg has written, spoken and campaigned strongly on the dangers of nuclear weapons, and continues to do so. For a time in the early 1970s he was a consultant to the U.S. Arms Control and Disarmament Agency, ACDA, providing them with technical background for the SALT arms reduction talks with the Soviet Union. Salam established the International Centre for Theoretical Physics in Trieste as a place where physicists in developing countries could go for periods of creative stimulation, enabling them to keep at the forefront of their subject without joining a brain-drain to America and Europe.

And in physics, both were clear about the need to pursue the task of seeking structure and using the techniques of symmetry and of quantum field theory – and if necessary to opt out of the tide of fashion and choose their own direction.

Salam had studied mathematics at the University of the Punjab and then won a scholarship to Cambridge, where he was a research student of Nicholas Kemmer. When Kemmer moved to Edinburgh, Salam was appointed in his stead, and then in 1957 became professor at Imperial College London where he built up a brilliantly talented team, among them Tom Kibble who told him about the Higgs mechanism.

Weinberg had been in the same class at high school as Sheldon Glashow, and had gone on to study at Cornell and Princeton, and to become professor of physics at the University of California in Berkeley. in 1966 he took leave from his post at Berkeley to go to Massachusetts, so that his wife could study at Harvard Law School; today she is Professor of Law at the University of Texas at Austin. In Massachusetts Weinberg spent time at Harvard with Schwinger, who he would later succeed there, and at MIT. In 1967 he was working on the strong interaction, trying to apply the Higgs mechanism to it and getting conflicting results – and then realised that his model was in fact telling him about the weak interaction instead.

Turning the key

‘When I started doing research in early 1950s,’ Weinberg observed recently, ‘physics seemed to be in a dismal state.’ The problem was the proliferation of all kinds of particles and forces.

‘Nature, like an enemy, seemed intent on concealing from us its master plan.

‘At the same time, we did have a valuable key to nature’s secrets. The laws of nature evidently obeyed certain principles of symmetry, whose consequences we could work out and compare with observation, even without a detailed theory of particles and forces… It was like having a spy in the enemy’s high command.’

Both Weinberg and Salam realised that the Higgs mechanism provided a means of keeping the symmetry of the electroweak interaction and at the same time breaking it sufficiently for the pulses of the weak force to be concentrated in particles with a non-zero mass.

Their papers predicted three such particles – and in 1983 all three were found by the Super Proton Synchrotron at CERN. The team leaders, Carlo Rubbia and Simon van der Meer, were awarded the Nobel prize the following year for the discovery.

The new particles were the W+ and the W, and the neutral Z0. They are very massive, with the W about 80 times the mass of a proton, and the Z just over 90 times. By comparison, the mass of an iron atom is around 55 times the proton’s mass.

With such a large mass, their lifetime is very short indeed, and they only travel an incredible short distance before vanishing.

The Weinberg and Salam papers also predicted a fourth particle – one that was a kind of leftover after all the necessary weak particles had been put in place. This additional particle was the Higgs, and so its discovery puts one large additional piece of support for Weinberg and Salam’s work.

In addition to the experimental support, a strong theoretical underpinning came from two Dutch physicists, Martinus Veltman and his student Gerardus t’Hooft. They showed that mathematically the theory held together so well that it had none of the infinities that had plagued so much of field theory for so long. That work earned them the Nobel Prize for Physics in 1999.

The Standard Model

The momentum from the success with the electroweak interaction carried forward to the strong interaction, using the idea of SU(3) symmetry and quarks, with various physicists playing a part in developing what is now called the Standard Model.

But the aim of science is always to press on to get closer to the truth, and it is clear that the Standard Model is not the final stage. The big unresolved challenge is to reconcile it, the theory of the very small, with general relativity, the theory of the very large, and that seems a long way from completion. The Standard Model encompasses the strong, weak and electromagnetic forces, but not gravitation.

And indeed although the Higgs particle consolidates the Standard Model, it also provides deep and perhaps unsettling philosophical challenges.

First of all, it finally removes any hope of a material base to physics. That hope was really lost in 1926 when Schrodinger produced his wave theory of matter, picturing an unknown substance in motion – its unknown nature was why he gave it the mathematical symbol psi – out of which matter somehow formed as the waves on the ocean. In the Schrodinger picture we see the wave and can model it and calculate its shape, but we know nothing of the deep sea beneath.

The Schrodinger picture was countered by Heisenberg, who insisted that we could still have particles, while not needing to look at their deeper nature. We could not zoom too closely in on their direct moments of contact, he said, but instead we much stand back at a distance and simply correlate the measurements of what went in and what comes out.

Heisenberg’s approach meant that physicists could avoid the issue of what matter is and simply talk about how it behaves. So they could continue to talk about particles in fairly familiar – and fairly material – language.

The concept of the Higgs field means that this approach is no longer good enough. It tells us that what makes matter the way it is – is something quite specific but also quite non-material, a field. We start with something fundamental, whose essence we do not know, namely light. We add in something even more non-material, the concept of symmetry. And we add in something else non-material, the Higgs field. And the outcome of these various insubstantial and non-material entities is – the stuff that builds up through atoms and molecules into lumps of iron and concrete.

2500 years of argument

The old debate amongst the Greeks was between Aristotle and Plato, who argued about what was the primary factor of existence. Plato said that it was form and Aristotle said that it was substance. Plato said that the fundamental essence was a set of patterns – universal templates or archetypes – which created the real world when they were somehow stamped onto a formless kind of clay.

Aristotle said that the clay was the essence, and that forms were secondary, being the patterns which we notice. He gave the example of the concept of a snub nose. Could we say that snubness somehow existence as a form and was imposed on the various noses of the world? He asked. No, was his reply, snubness is a concept that we create to enable us to better catalogue the noses around us.

The debate was one by Aristotle and he and the Greeks set us off down a trail of substance. This was why they developed the concept of an atom as something fundamental – it was part of a picture in which substance was primary.

But now we have a theory in which substance is not primary, but created out of light, mathematics and a field. A field is simply a form – a kind of abstract structure in space – indeed if an atom is an example of pure substance, then a field might be seen as a case of pure form.

That in turn leads to the question of whether or not we should be talking about particles, or whether we should use the language of waves.

And deep under the surface is the question of order. Symmetry is now invoked to impose order on nature – but where does symmetry come from? And how does it ‘impose’ itself?

So in one way the Higgs discovery reinforces a remarkable theory that took its present shape in the early 1970s. In another way it undermines a worldview that formed 2,500 years ago – and thus points the way forward to the next development in physics, whatever it might be, with as many questions as answers. These are good reasons for physicists to be cheerful – and to burst into song.

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