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What We Don't Know in Physics

MIT physicists seek the origins of mass, the nature of dark matter, and beautiful equations to describe the universe.
December 22, 2008

Standing on the fire escape of his office in Geneva, Switzerland, assistant professor of physics Steven Nahn, PhD ‘98, enjoys the evening air as he enumerates some of the universe’s greatest mysteries. The question he’s working on, he says, is so simple a child could ask it: “Where does mass come from?”

Cosmic Detective: Professor Gabriella Sciolla has built an apparatus to detect dark matter.

Answering it requires explaining why the fundamental par­ticles that make up all the matter in the universe have mass. So Nahn and about 30 other MIT researchers and students are working on experiments to run on the Large Hadron Collider, the new particle accelerator based at the European Organization for Nuclear Research. The LHC is an exercise in extremes: at temperatures near absolute zero, it will accelerate particles to the highest energies ever achieved experimentally, its thousands of powerful magnets guiding protons along a circular path 27 kilometers in circumference until they collide. The instrument is slated to be fully operational later this year, and Nahn figures he’ll spend half of the year working there; his students and other researchers are spending most of their time at the LHC. Each wants to be part of the team investigating one the biggest questions in physics.

“I find it ironic,” muses Nahn. “It seems like a very simple question, but it takes an enormous accelerator and thousands of physicists working on it to try to find out the right answer.”

Many of the other questions driving MIT physicists are just as basic. Another big one that LHC experiments could resolve concerns the nature of about 23 percent of the universe–the so-called dark matter, whose existence is inferred from gravitational effects on visible objects. Physicists simply don’t know what it is. “Where is all the stuff?” Nahn says, half joking. “You’d think we’d know.”

Additional Information: "What Else We Don't Know"

To be a physicist is to puzzle over what the rest of us take for granted–that objects have mass, that the universe consists of matter instead of antimatter, that gravity works.

Physicists around the world are now working to expand and revise the parts list of the universe–what’s known as the standard model, a compact distillation of about 100 years of research, which attempts to describe the particles and forces that account for all physical phenomena. The standard model includes 12 fundamental particles that constitute matter as we know it, plus their equal but opposite antiparticles. It includes the four fundamental forces governing interactions between particles: gravity, electromagnetism (which is responsible for light, magnetism, and electricity), and the strong and weak forces (which mediate the interactions within atomic nuclei). And it includes particles that carry the four forces–although the one carrying gravity remains hypothetical.

This framework ties together everything that particle physicists know to be true. It tells us that atomic nuclei, once thought to be indivisible, are made up of protons and neutrons; protons and neutrons are further divisible into particles called quarks, which are held together by the strong force, whose carrier is the gluon.

Professor Frank Wilczek, who won the Nobel Prize in physics in 2004 for his work on the strong force, says that the standard model is a good working description of how the world works. But it doesn’t all fit together as nicely as he and others think it should. The lack of experimental evidence for gravity’s carrier, the graviton, is one source of frustration–although MIT physicists have played a pioneering role in trying to detect it and are currently upgrading machinery that may be the first to succeed (see “Catching ­Einstein’s Waves,” May/June 1008). And that’s just one of several major loose ends that MIT physicists are trying to tie up.

To that end, they’re building dark-matter detectors; searching for fundamental particles that complement those we know; and eagerly awaiting the results of particle collisions at the LHC, which will at last allow physicists to test decades of theoretical work on these stark mathematical descriptions of our universe.

Why Do Things Have Mass?

For Nahn, the most intriguing missing piece of the puzzle is mass. “If you just take the barest theory, it would tell you that all [the ­particles] are massless,” he says. Whether you’re a layperson or a physicist armed with ­sophisticated particle detectors, this prospect seems absurd. Electrons, which make up a negligible fraction of the mass in individual atoms, have a mass of about .0005 giga-electron-volts (GeV); the heaviest fundamental particle, the top quark, has a mass of about 175 GeV. “Somehow, you have to incorporate into the theory a way to generate this diversity of mass,” says Nahn. The simplest way to do this is to posit another particle, which has come to be called the Higgs boson. What photons are to an electromagnetic field, Higgs bosons are to the putative Higgs field, a medium that surrounds everything in the universe and interacts with elementary particles in a way that gives them mass.

Wilczek says that without the Higgs boson, we’re like a race of intelligent fish that don’t know they’re immersed in water. These fish would have a better chance of understanding the laws of their universe “if they realized the environment they took for granted was a material that modified the way they moved,” ­Wilczek says. “Similarly, if we assume that what appears to us as empty space is a medium … we have nicer equations than otherwise. But we don’t know what [the Higgs boson] looks like–as if we hadn’t seen molecules of water.”

Physicists will readily admit that to the uninitiated, invoking hypothetical, never-seen particles to resolve problems with your theories may seem contrived or even, in Nahn’s words, “a little bit crazy.” But this approach has proved sound before. In the late 19th century, Dmitri Mendeleev developed the periodic table and predicted several chemical elements that were subsequently observed, including gallium and germanium. In 1931, Paul Dirac postulated the existence of antimatter in order to explain a puzzling consequence of an equation he’d derived to reconcile our understanding of electrons with relativity. And MIT’s Wilczek predicted the gluon, which was directly detected in 1979.

Since Higgs bosons are highly unstable, the only way to observe one is to create it in a high-energy collision. And no previous particle accelerators were powerful enough to produce a reliably detectable number of Higgs bosons, which are predicted to have a mass between 114 GeV and 184 GeV. The LHC, however, will smash protons together at energies seven times as high as those achieved by the most powerful accelerator now in operation. “We have to find this Higgs particle, or something like it, in this energy scale,” says Nahn. Physicists hope that they find the Higgs because if they don’t, they’ll be forced to conclude that the standard model’s mass problem has a more complex solution. But for many of them–including Nahn–it’s exciting enough just to be able to finally test the Higgs theory experimentally. The new collider, which was shut down for repairs shortly after it opened in the fall, is scheduled to go back online in spring 2009; until then, Nahn and his students are working on software that will monitor the operations of one of the LHC’s detectors and eventually analyze the data it generates (see “The Making of a New Collider,” May/June 2008).

Are the four forces unified?

Theorists like Wilczek are also attempting to make the standard model itself more mathematically beautiful and experimentally viable. Each of the four forces has its own set of governing equations. But “the equations are lopsided,” says Wilczek. He and others believe, however, that the forces are like four “sides” of a mathematical die. They’re discrete, but each is also part of a whole. ­Wilczek points out that although the forces generally have different strengths, for particles very close to one another, they have the same strength. This suggests that the mathematical impulse to bring the forces together into a whole governed by a grand unification theory is on the right track. Electromagnetism and the weak force fit together well enough mathematically that already they are often referred to as one force, the “electroweak.” The equations for the strong force are similar to those for electromagnetism and the weak force. The one that’s difficult to fit in, says Wilczek, is gravity.

It may seem strange that physicists put so much faith in the predictions of mathematics. However, Wilczek says, “I don’t trust my own opinions unless nature gives us some encouragement.” It’s probably no accident that the equations are so similar, he observes. “The forces didn’t have to come together,” he says. “The equations didn’t have to look like different faces of the same die.”

Wilczek hasn’t yet had the satisfaction of seeing unification borne out experimentally: physicists simply haven’t had the means. There is a way to test the theory, though. Adding another batch of particles to the standard model makes the math for unification work. Each of these theoretical “supersymmetric” particles would interact with other particles in the same way that one of the known particles does but would be much more massive. Wilczek hopes that the LHC’s high-energy collisions will produce at least one supersymmetric particle. Theorists like him have been working on questions for decades without being able to test them; now, he says, “the experimentalists are catching up.”

What Is Dark Matter?

Gabriella Sciolla, an associate professor of physics at MIT, hopes to validate supersymmetry through experiments on dark matter. Physicists know, from observing the gravitational interactions of galaxies and other celestial objects, that there is much more mass in the universe than they can account for by looking for the kinds identified by the standard model. This missing mass is called “dark matter” because it doesn’t interact with photons. It cannot be seen with optical or x-ray telescopes. “For sure, I am a little biased, but for me, the most interesting open question in physics is, What is dark matter?” says Sciolla. One simple explanation is that it is made up of one or more of the supersymmetric particles.

In the bowels of Building NW13, in a windowless cinder-block room that her research group calls “the dungeon,” Sciolla is testing a new apparatus called the Dark Matter Time Projection Chamber–essentially a large stainless-steel tank of gas flanked by two digital cameras. The principle behind the detector is simple. When a particle of dark matter strikes a gas atom, the atom will recoil, knocking loose electrons that will be detected by the cameras. By tracing the paths of these electrons, Sciolla will be able to see not only that a particle struck, but from which direction. That will be important in establishing that the detector is actually seeing dark matter, not something else. If, as many physicists believe, our galaxy is rotating through a stationary region of dark matter, then the dark matter should strike the atoms in Sciolla’s detector like rain hitting the windshield of a moving car. The direction of this “rain” should vary by about 90º every 12 hours, because the axis of Earth’s rotation is about 45º with respect to the dark matter.

Sciolla and her research group will station their detector in an underground lab to isolate it from cosmic rays, a major source of noise, and they’ll spend 2009 gathering preliminary data to prove that the concept works. In a year, Sciolla hopes to have a one-cubic-meter detector that will be 50 times as sensitive; in five years, she hopes to have a detector as large as a few hundred cubic meters.

Finding dark-matter particles would be the physicist’s equivalent of hitting the jackpot. “All these big questions in physics are somehow connected,” Sciolla says. “Dark matter is the one answer that would satisfy so many different unanswered questions in different fields of physics.” Detecting it would provide strong evidence for supersymmetry.

If dark matter turns out to consist not of supersymmetric particles but of axions, hypothetical particles that Wilczek did important work to describe, that finding could get at another huge question. Axions figure prominently in an esoteric theory that explains why matter–as opposed to antimatter–prevails in the universe, even though the Big Bang produced all the particles and their antiparticles in equal numbers.

What Is Dark Energy?

Even if dark matter is detected and its nature revealed, another curious phenome­non that physicists have called dark energy raises plenty of other questions. ­”Galaxies are being pushed apart by some repulsive force,” explains Edmund ­Bertschinger, head of the physics department. “The measurements of the last decade tell us that something very much like gravitational repulsion has taken over the universe.” That is, the universe is expanding at an accelerating rate, but physicists don’t know why. Is it because of dark energy? Or is dark energy just a concept that’s patching over a misunderstanding of the laws of physics?

Despite the similarity in their names, dark energy is probably completely unrelated to dark matter–and is a much greater mystery. “There are plausible explanations of dark matter,” says Bertschinger. “We do not have plausible models of dark energy that make sense in the context of high-energy physics.” Work done by Bertschinger and many others has shown that tests to distinguish between dark energy and a modified form of gravity will be very difficult to develop. However, Bertschinger is doing theoretical work that he hopes will lead to such tests over the next decade.

“This is a big time in physics,” says Sciolla. “Everything will get an answer in the next few years, we hope.” And then what? Well, then she and her colleagues might be out of a job, Sciolla jokes. “But,” she adds, “I’m sure there will be plenty of new questions that will be unanswered.”

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