On the Fourth of July, physicists at the biggest particle accelerator ever built, the Large Hadron Collider, announced that they had found a new subatomic particle. Many believe it to be the long-sought Higgs boson, which accounts for the basic mystery of why certain other elementary particles have mass. Finding the Higgs boson will surely result in a well-deserved Nobel prize, perhaps as soon as this year.
But amid the applause, the popping of champagne corks, and the rare phenomenon of breathless news coverage for a physics experiment, some scientists are beginning to get nervous. For many particle physicists, lurking beneath the celebration is a fear—a fear that finding the Higgs doesn’t mark a beginning for science, but an end.
This possibility is what some physicists call the “nightmare scenario.” The existence of the Higgs boson confirms the Standard Model of particle physics, a theory that has correctly predicted the results of essentially every particle experiment ever done, with accuracy unparalleled in science. But for all its importance, scientists already know the Standard Model is flawed, and are hoping for clues to what lies beyond it. And if the largest and most powerful particle accelerator humans have ever managed to build can’t show us anything beyond the Higgs boson, it may be a very unwelcome sign that we’ve hit the limits of what we can learn.
This might sound strange. After all, a discovery, by definition, teaches us something new about the world. But at this level, physics isn’t just a matter of discovering new particles: It’s also a process of learning what the limits of discovery will turn out to be.
In a sense, the Higgs boson is not a “discovery” at all. Physicists first theorized the particle 50 years ago. Soon after, it was recognized that the Higgs—or something like it—was a necessary part of the Standard Model, our best description of the forces that shape the subatomic world. As evidence steadily accumulated for the rest of the theory, physicists came to take the Higgs boson for granted.
Nonetheless, it had not been directly observed before this most recent series of experiments. Indeed, the Large Hadron Collider was built at a cost of about $9 billion precisely for the purpose of smashing together protons with enough energy to produce a Higgs boson. But even as physicists built a Higgs hunter, many hoped the collider would do more.
“Discovery of a Standard Model-like Higgs is as exciting as if Columbus had actually set sail and reached India, rather than bumping into a new continent,” said Daniel Whiteson, an experimental particle physicist at the University of California, Irvine. “A technical feat, but one that teaches us very little we don’t already know.”
The real hope, for physics, was that the Higgs experiments would inch open the door to something new, offering insights into questions the Standard Model can’t yet answer. As far as we know, there are four fundamental forces of nature. The Standard Model describes three of them astoundingly well. But it has nothing to say about the fourth force, gravitation. Gravitation is mysteriously different from the other three forces. For instance, it is much weaker—10^32 times weaker than the weakest Standard Model force. The unexplained weakness of gravity relative to the other forces is known as the Hierarchy Problem.
“The main reason to be disappointed if it turns out there is nothing at LHC scales but the standard model Higgs is that the Hierarchy Problem remains unexplained,” says Lee Smolin, a researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, and an expert on attempts to develop a unified theory of gravity and the Standard Model forces.
The Standard Model leaves other problems unresolved, too. Astronomical observations reveal that the universe is not only expanding, but accelerating—a process that requires a huge, unexplained source of energy. Physicists call it “dark energy” because they have no idea what it could be. Meanwhile, measurements of galaxies’ masses reveal that they are much heavier than one would expect from merely adding up their visible matter. The rest of the mass comes from so-called dark matter, which many physicists think will be explained by a new particle that is not described in the Standard Model.
Together, dark energy and dark matter make up 96 percent of the known mass-energy of the universe. The Standard Model accounts only for the other 4 percent. Many physicists hoped the Large Hadron Collider would provide a hint of what the rest of the universe is made of. But so far, no luck.
If the Large Hadron Collider uncovers other types of particles in the neighborhood of the Higgs boson—theoretically postulated particles known as “sparticles,” for instance, or alternative “impostor” versions of the Higgs—that could lend support to big theories that try to push physics beyond the Standard Model. But if the Higgs particle exists, as it now seems it does, and if it behaves exactly as the Standard Model predicts, then that could be telling us that any glimpse past what we already know lies outside our experimental reach.
Some worry that to find something really new, we would need to go much, much deeper than we have so far—all the way to what’s called the Planck scale. To explore matter at that level requires energies 10 quadrillion times larger than what the Large Hadron Collider can produce. The Large Hadron Collider is 17 miles in circumference and took a decade to build. A collider to probe the Planck scale would need to be, literally, the size of an entire galaxy.
Still, it is too soon to give up hope. Though the Large Hadron Collider has revealed a Higgs-like particle, the jury is still out on whether it is the Standard Model boson—and what to make of it if it is.
In the days since the original announcement, several physicists have shown that the Large Hadron Collider data is compatible with both the Standard Model Higgs and several more interesting alternatives. “My gut is telling me it’s a nonstandard model Higgs boson,” says Ian Low, a theorist at Northwestern University and the Argonne National Laboratory.
Other physicists point out that even if this new particle is just the standard Higgs, things could have been worse. The real nightmare, according to theorist John Ellis of King’s College London, would have been finding the Higgs at a slightly larger energy, “in which case there would have been no need for new physics before the Planck scale.” As it is, the energy of the observed Higgs implies that there are instabilities in the Standard Model that could lead to new physics under less extreme conditions.
And the Large Hadron Collider may yet tell us more. Next year, the collider will be revamped. When it opens again, it will operate at twice the energies it has managed so far. It is always possible that something new will be found.
Even so, these worries are likely to arise again. Fundamental physics has reached a stage where major experiments cost billions of dollars and require the efforts of thousands of physicists. Each time we push the boundaries of what we know forward, we get closer to the line at which it will no longer be feasible to push any further. We are faced with a struggle between the questions we want to answer and the limitations of our abilities—and at some point, perhaps soon, our limitations will win the day.