The first particle beams will race through Switzerland's Large Hadron Collider, the biggest particle accelerator ever built, this Wednesday. After 15 years—and something like $8 billion in construction costs—the machine should start producing early results by midfall. According to the usual story, particle physicists everywhere are anxiously awaiting evidence of an as-yet-unseen elementary particle called the Higgs boson.
Last summer, I argued that the discovery of the Higgs could spell disaster for the field. If we find the Higgs, the Standard Model of high-energy physics would provide a theoretical account of all known particles and their interactions. Physicists could use it to predict the results of every particle accelerator experiment ever performed with near-perfect accuracy, given a big enough computer. But the Standard Model isn't intuitive enough to provide insight into why the world happens to be the way it is. Most physicists hope that there's a deeper, more revealing theory waiting to be discovered; if all the LHC finds is the Higgs, they will be sorely disappointed. Fortunately—and this I didn't mention last year—there's no particular reason to expect that the Higgs will show up.
That the Higgs boson appears in the Standard Model at all is more a matter of historical and sociological coincidence than a prediction based upon firm scientific data. Indeed, there is no direct (or indirect) proof that the Higgs boson is real. The hodge-podge of theories that are rolled up in the Standard Model are such that everything we think we know about particle physics may be exactly right—and yet the "God particle" could be a fiction.
The story of the Higgs boson goes back almost 50 years, to when the theories that would ultimately be combined into the Standard Model were first developed. At that time, physicists were aware of three apparently distinct forces that could influence the motion of particles: the electromagnetic force, responsible for familiar phenomena like thunderstorms and televisions, and two other forces that were important in nuclear processes, known as the strong and weak forces. They knew of a fourth force, too—gravity—but it was far too weak to be important in these experiments.
The Higgs saga begins in 1960 with a physicist named Sheldon Glashow, who had just wrapped up his Ph.D. at Harvard and was working in Copenhagen, waiting for a visa to come through so he could begin his postdoctoral work in Russia. Glashow had a hunch that two of the three forces—electromagnetism and the weak force—were actually manifestations of the same thing, an "electroweak" force. In 1961, he published a paper that tried to describe both forces with a single mathematical framework.
But there were problems with Glashow's theory. When two electrons exert a force on each other, that force is "carried" by a different kind of particle that travels between them. Electromagnetism, for instance, is carried by photons, the same stuff that makes up light; when two electrons repel each other, they exchange photons. To make the connection with the weak force, Glashow needed to suppose that there was a kind of analog to the photon that would carry the weak force. But no one had ever seen a particle like this, and if they were really anything like photons, they should have been very easy to observe.
One way around this problem was to suppose that the weak-force carriers were very heavy. The heavier a particle is, the bigger the accelerator you need to produce it; by theorizing large masses for these force carriers, it was possible to put them outside the range of 1960s technology. This would have explained why no one had seen them, but it also threatened to make Glashow's theory untenable: The only mathematically consistent way that anyone knew of to make these new force-carrying particles so heavy involved adding a new, very light particle to the theory called a Goldstone boson. (A boson is one of two types of particle—the others are called "fermions"—distinguished by differences in their internal rotation. Photons are bosons; electrons are fermions.) These Goldstone bosons would have been easy to detect if they were real, again because they're so light. Yet they were conspicuously absent in experiments.
Meanwhile, a young Scottish physicist named Peter Higgs was interested in an entirely different area of particle physics: the strong force. Here, too, no one knew how to create a consistent theory without also predicting pesky, nonexistent Goldstone bosons. At least until 1964, when Higgs proposed a solution.
Higgs' idea, now called the Higgs mechanism, was to expand the playing field: He studied what happens mathematically if you suppose that force carriers all act the same within the scope of the theory you're trying to build but behave differently when they meet something outside the theory. (This outside influence could be one or several new particles, or it could be something you already knew about but which you wouldn't expect to be relevant to your theory.) He found that it's possible for some force-carrying particles to interact so strongly with an outside particle that they become inextricably entwined, such that what experimenters saw in the lab were really mixtures of the two. Other force-carrying particles don't interact with the outside particle at all. If Higgs was right, all of the force-carrying particles would be very light, but some would appear heavy because they mix with this massive particle. And that means there wouldn't be any Goldstone bosons to worry about.
The original work on the Higgs mechanism didn't worry about the details of the outside particle; to make his point, he just made the simplest possible choice mathematically. The important thing was that, whatever was at the heart of the Higgs mechanism, it could be massive enough to explain why no one had seen it at work.
It wasn't until 1967 that Steven Weinberg and Abdus Salam tried using the Higgs mechanism to save Glashow's electroweak theory. But to get the Higgs mechanism to work, they needed to pick a specific particle from outside the theory—a consideration that Higgs didn't have to worry about when he developed his mechanism abstractly. We now know that there were several things that Weinberg and Salam could have proposed that would have fit snugly into their big theory. One set of possibilities, now known as Technicolor models, would have posited a whole bunch of new fermions, rather than a new boson. (Weinberg worked out the specifics of Technicolor models in 1980.) A related set of theories might have accounted for the mass of the force-carrying particles with an exotic interaction—called a condensate—between the top quark (which has since been observed) and its anti-particle.
The ideas behind these alternatives were already well-established in 1967, but there was no evidence to lead Weinberg and Salam toward any one of them. So they chose the most parsimonious possibility as a placeholder until experiments could catch up with a more complete picture of the electroweak force: They published their theory with the single new elementary particle that Higgs originally used as an example and that we now call the Higgs boson. Nothing in the rest of the theory compelled this decision, and though we now have considerable evidence for the Higgs mechanism, we don't have any for the eponymous boson.
Why, then, does the Higgs boson get so much attention, among physicists and in the popular press? Weinberg and Salam's papers, though ignored until the early 1970s, are now immensely influential because of the theory's other predictions. Indeed, Glashow, Weinberg, and Salam shared the 1979 Nobel Prize for their electroweak theory. In 1974, John Iliopoulos cobbled together the Glashow-Weinberg-Salam electroweak theory—Higgs boson and all—with a theory of the strong force to write down the Standard Model as we now understand it. Over the last 35 years, as the Standard Model recorded success after success, it became easy to forget that the Higgs boson is—and has always been—independent of the rest of the theory.