Peter Higgs locks the door of his top-floor flat in Edinburgh’s New Town and hurries down the 84 steps—one for each year of his life. At ground level, he pokes his balding head out of the front door of the Georgian building and checks that the coast is clear on Heriot Row. He then pulls up the collar of his coat and heads for Princes Street, where he boards a bus to Leith. In an anonymous café by the Firth of Forth, he eats an early lunch while, 1,300km away in Stockholm, the Swedish Academy announces Higgs as the joint winner of the 2013 Nobel prize in physics and all hell breaks loose—as the world’s media try to track down Scotland’s fugitive physicist.
Thus begins Frank Close’s engaging book about Higgs, a man every bit as hard to find and as enigmatic as the Higgs boson, whose existence he predicted on a July afternoon in 1964. At one point Higgs tells Close that predicting the particle “ruined my life”. A shy and retiring adult, whose childhood ill health had made him comfortable with isolation, all he wanted was to go unnoticed. The Higgs boson, however, has thrust upon him a level of fame that makes that impossible.
Close’s aptly named Elusive is a semi-technical account of Higgs’s life and his contribution to fundamental physics. It is a book that requires from its author the skills of a theoretical physicist, a populariser and a biographer—and, almost inevitably, Close sometimes fumbles one of these roles. But we should forgive him that. This topic is the hardest physics known to humanity and his human subject is not exactly the easiest character, either. Though Higgs has explored the far frontier of modern physics, he famously has no mobile phone, television or computer. Kudos to Close for pinning him down and interviewing him for this book.
According to Close—himself a particle physicist—Higgs’s interest in physics was sparked at Cotham School in Bristol, when he noticed that the name of an ex-pupil was mentioned several times on the honours board. In 1928 Paul Dirac had plucked from thin air an equation that describes an electron travelling near the speed of light. The “Dirac equation”, which predicted the existence of a previously unsuspected realm of “antimatter”, is one of only two equations inscribed on flagstones on the floor of Westminster Abbey (the other is Stephen Hawking’s formula for the temperature of a black hole).
Dirac’s triumph was quantum field theory, which unifies into a seamless framework two apparently incompatible characteristics of the ultimate building blocks of matter, such as electrons: their ability to behave as both localised particles and spread-out waves. In Dirac’s picture, an electromagnetic field fills all of space. Imagine that at every point in space is a bead that can oscillate on a vertical spring. All the springs are connected, so the field is like a spring mattress. Disturbing the mattress—that is, injecting energy into the field—causes a ripple to propagate across it. This is a subatomic particle—in the case of the electromagnetic field, a photon. Thus the theory incorporates, at a fundamental level, the particle and wave aspects of reality.
A remarkable feature of the quantum field theory of the electron is that it is possible to deduce from it, almost trivially, the existence of the electromagnetic force—that is, all the bewildering variety of electric and magnetic phenomena discovered by scientists over the past 200 years. The key phenomenon that makes this possible is symmetry, which concerns properties of an object that remain unchanged when the object is transformed in some way. A square, for instance, looks the same when rotated by a quarter-turn and so is said to be symmetric under rotations of a quarter of a turn.
In 1918, the German mathematician Emmy Noether revealed a remarkable discovery about symmetry. According to Noether’s theorem, the great conservation laws of physics are merely manifestations of deep symmetries. For instance, the conservation of energy, a foundation stone of physics, maintains that energy can neither be created nor destroyed, merely morphed from one form to another. This turns out to be nothing more than a consequence of “time translation symmetry”—the fact that, all things being equal, the outcome of any given experiment is the same, whether it is done today, tomorrow or in 10 years’ time.
As Close points out, physicists have a technical name for the something that changes without altering any physics. They call it a gauge. If a billiard table is raised a metre in the air, or two metres, or any other height imaginable, Ronnie O’Sullivan would still be able to play snooker on it because the balls would continue to travel along paths and carom off each other in the way decreed by Newtonian physics. Here the gauge is the height of the billiard table, and raising the table everywhere at once is known as a “global gauge transformation”.
Changing a gauge everywhere at once, however, turns out to be impossible in a universe in which no influence can travel faster than a cosmic speed limit set by Einstein: the speed of light. Imagine a billiard table that is 10 light years across: if the near side is raised, it will take 10 years before a signal travelling at the speed of light carries that information to the far end. Think of the change as a bump in the billiard table that moves across the table at the speed of light.
Despite the gauge being different at every point of space and time, however, we can reasonably expect the laws of physics to remain the same everywhere. The only way this can happen is if, at every point on the billiard table, there is a force that compensates for the bump in the table and keeps the balls running in the way that we would expect them to. This is the key point. As Close tells us: the preservation of “local gauge symmetry” requires the existence of a compensating force.
It was subsequently discovered, by the American physicist Julian Schwinger in the 1950s, that if these sorts of changes are wrought on electrons, then the compensating force that keeps the physics the same is none other than the well-known electromagnetic force. Remarkably, all electric and magnetic phenomena are nothing more than a trivial consequence of local gauge symmetry. This discovery left a huge impression on Higgs.
So striking was the result that it was natural to wonder whether nature’s other fundamental forces—such as the weak and strong forces that are at play inside atomic nuclei—were also consequences of local gauge symmetry. But there was a problem. Quantum field theory could only go so far at this point; it was plagued by quantities that blew up the equations, making them nonsensical. These could in theory be excised, but only if the force carriers—the gauge bosons—were massless.
The photon—the gauge boson of the electromagnetic field—is indeed massless. But this is not true for the “weak” nuclear force that is responsible for radioactivity; it has a very short range. In quantum theory, this is synonymous with having force carriers with mass.
For a while, it seemed as though there was a way around this. Suppose the gauge bosons of the weak force are intrinsically massless but somehow acquired their masses extrinsically. How could that happen? Well, a system in which every fundamental particle has zero mass is perfectly symmetric, since changing one particle for another makes no difference. This symmetry could, however, be “spontaneously broken”—resulting in the gauge bosons acquiring mass.
Sorted, right? Not quite. In 1962, the British physicist Jeffrey Goldstone found that such symmetry-breaking by necessity conjures into existence new massless particles—massless particles that, crucially, had never been seen in any experiment. Another mystery had been created.
This was the state of play in 1964. Most physicists had lost interest in quantum field theory because of the undetectable “Goldstone bosons”. But then the 35-year-old Higgs at the University of Edinburgh had, in his words, “the only really original idea I’ve ever had”. He found out that if empty space is not empty at all, but instead filled with a new field whose symmetry is being broken in the presence of gauge bosons, then miraculously the Goldstone bosons are effectively “eaten”. There was now a theory for why they were undetectable—and interest returned.
Higgs, in one fell swoop, had not only rescued quantum field theory but also found the mechanism that endows matter with mass. In the physics equivalent of a “buy one, get one free”, the Higgs field endows the carriers of the weak force with masses. In turn, the massless building blocks of matter—the quarks and leptons that compose your body—gain mass by interacting with the Higgs field.
Actually, five other physicists hit on the same mechanism at the same time. But what distinguished Higgs from the rest of the “Gang of Six” was that he alone realised that, of the four Goldstone bosons, only three would be eaten. One would be left and, unusually, it would have a mass. This was the particle that would acquire the name “Higgs boson”.
Over time, as nature’s other building blocks were discovered, the Higgs boson was left as the missing piece. It became the Holy Grail, the particle whose discovery would prove not only the existence of the enigmatic Higgs field but also the otherwise undetectable mechanism that endows matter with mass.
And so at a cost of €5bn, the biggest machine in history was constructed beneath fields grazed by cows on the French-Swiss border. With its subterranean detectors the size of cathedrals, the Large Hadron Collider at Cern, near Geneva, slammed together protons at unimaginable speeds while physicists sifted the subatomic shrapnel for the tell-tale signatures of the Higgs boson, a particle that lives for less than a billionth of a billionth of a second.
Close was with Higgs at a summer school in the Sicilian hilltop town of Erice when the call came to go to Cern. A few days later, on 4th July 2012, Higgs was in its auditorium with François Engelert (another of the Gang of Six and the joint Nobel winner with Higgs) as the discovery of the boson was announced. Tears filled his eyes as wild applause broke out and people crowded round to offer congratulations and shake his hand. After 48 years, Higgs’s prediction was vindicated. (Ironically, Higgs had left the field of fundamental physics decades before, finding it too complicated.) “The portion of my life for which I am known is rather small—three weeks in the summer of 1964,” he once said. “The amount of labour was rather small, and I am staggered by the consequences.”
The Higgs field is something entirely new under the sun. Whereas the source of the gravitational field is mass and the source of the electromagnetic field is electric charge, the Higgs field exists in empty space without a source. From the moment we are born to the moment we die, we are immersed in it. But, like fish living in the sea, we are unaware of the universal medium in which we swim.
The Higgs field is responsible for two incredibly important things. First, if it were not for its existence, fundamental particles would have no mass and the atoms of which you, the stars and galaxies are made would not exist.
The second important consequence is that it both gives the three bosons of the weak force the heft they have and the force itself the incredible weakness it has. It is this weakness that makes the first step of the nuclear reactions that heat the sun so ridiculously slow. It is why the sun takes 10bn years to burn all its hydrogen fuel, rather than a split-second, resulting in enough time for the evolution of complex life like you.
All this matters because, incredibly, we have found the simple principle from which everything arises. Facebook, you and me, crisp packets, snails, soap operas, toddlers, giraffes, the sun and the moon—all exist to enforce local gauge symmetry! Now why should that be so? Nobody knows. As with so many discoveries in science, the discovery of the Higgs inspires a host of new questions. What is the origin of the Higgs field? What is it made of? Where do dark matter and dark energy, which together account for 95 per cent of the universe’s mass-energy, fit in?
The last word, however, should go to Higgs. Asked whether he expected that the Higgs boson would be found, he replied: “No. Not in my lifetime.” Little wonder why—on that day in 2013 when he was awarded the Nobel—fear got the better of him, and he bolted for the shadows.
