What We Don't Know in Physics

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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.