Wearing hiking boots,fleece jacket and jeans, experimental physicist Roger Dixon looks ready to scramble up the hillside behind us into the wilds of Minnesota's North Woods. But on this mild morning in early autumn, we meet at a defunct iron mine to descend into the earth instead. Nearly half a mile beneath our feet, scientists are trying to solve a cosmic mystery: what is the universe made of?
The rusty frame of the mine's head trace looms overhead as we step into a metal box no bigger than a broom closet. It's a bare-bones elevator, built some 70 years ago when the Soudan mine was still active. The door rattles closed, and down we go in total darkness. No one bothers to speak over the roar of the car's wheels. Three minutes later, Dixon and I walk across the concrete floor of a vaulted cavern. The place is roomy enough to park a blimp.
Here, 2,341 feet underground, researchers are setting a trap for an exotic, yet-to-be-detected form of matter. Their quarry is an invisible and furtive character known as a WIMP, or Weakly Interacting Massive Particle (see box, page 53). Many physicists today suspect that this subatomic speck--so far found only in theoretical equations--is the basic component of the mysterious "dark matter" that makes up 80 percent or more of the universe. WIMPS have been frustratingly elusive, however, and will remain hypothetical until researchers catch a few.
Scientists want to identify the particles in dark matter to flesh out a startling picture of the cosmos that has taken shape during the last 30 years. We live in a shadowy universe, it seems, dominated not by glittering stars and gaudy galaxies but by some aloof, unseen stuff, a kind of universal glue whose presence we infer by the gravity it exerts. Dark matter--perhaps 10 times as abundant as visible matter made of ordinary protons, neutrons and electrons--shapes the galaxies, dictates the movements of stars and helped sculpt the structure of the universe itself. While an understanding of dark matter may not promise any practical payoffs (it won't cure cancer or prevent tooth decay), it could help answer the most profound questions in the universe. How did the cosmos form? What is the ultimate fate of the universe? How do we fit into the bigger picture? We may be stardust, but most of what's out there is something else entirely.
Right now, at least 20 scientific teams around the world are racing to snare the first certifiable WIMP--an achievement that would finally yield evidence of dark matter's makeup. One group, an Italian-Chinese collaboration working deep under the Apennine Mountains near Rome, has already declared possible success, but most physicists dispute that claim. Given the magnitude of the discovery, whoever wins the race will almost certainly capture a Nobel, or, as University of Chicago cosmologist Michael Turner puts it, "strike Swedish gold."
In the Soudan mine, the search hasn't started yet, but already some handicappers are giving this contender an edge. Just as a bomb shelter would protect occupants from nuclear fallout, the stone-shielded cavern should block incoming radiation, which triggers misleading signals. The researchers will use newly designed detectors--frozen chunks of crystal the size of hockey pucks--that are ultrasensitive to WIMPS. The Soudan experiment is the centerpiece of the Cryogenic Dark Matter Search, a $26 million, federally funded project that has pulled together some 50 scientists from 10 U.S. institutions. Stanford experimental physicist Blas Cabrera is one of three co-directors. Dixon is the project manager in Minnesota, visiting the mine once a month from his base near Chicago at the Fermi National Accelerator Lab.
Next fall, scientists will switch on the new detectors and wait for a subtle flicker of energy that suggests a WIMP has landed.
The first person to suspect a dark presence in the universe was Fritz Zwicky, a brilliant and belligerent Swiss astronomer who spent his professional life at Caltech. In the dark-matter drama, Zwicky (1898-1974) plays the mad prophet who is scorned in his lifetime, only to win posthumous vindication.
His insight came in 1933, after he trained his telescope on a clump of galaxies known as the Coma cluster. Zwicky calculated the speeds and masses of the individual galaxies and discovered a jarring discrepancy: the cluster should have ripped apart long before. The visible matter couldn't muster enough gravitational pull to prevent the fast-moving galaxies from scattering.
Zwicky's creative mind saw a solution to this apparent paradox. The cluster could stick together if it contained a massive amount of unseen material--which he dubbed dark matter--whose gravity held the galaxies in formation. To exert sufficient force, this "missing mass" would have to outweigh the ordinary matter in the cluster by about 10 times, he calculated.
He had it right, but almost no one followed up on the idea for 40 years. Why? For one thing, Zwicky's eccentric behavior made it easy to dismiss him as "crazy old Fritz." His attack-dog style and explosive temper (he once threatened to kill a colleague) alienated many in the gentlemanly community of astronomy. And he further tarnished his reputation by hatching crackpot ideas, including a scheme to improve telescope viewing by firing cannons into the air. On the dark-matter problem, though, Zwicky was far ahead of his contemporaries, who weren't prepared for the uncomfortable notion that most of the universe lies out of sight. Capturing the prevailing view at the time, Jesse Greenstein, longtime astronomy chair at Caltech, wrote in a 1970 essay: "I hope the missing mass isn't there."
But it was there--and not just in the Coma cluster. Almost everywhere astronomers pointed their telescopes, they saw signs that dark matter was at work behind the scenes. Without it, the behavior of galaxies simply didn't make sense. Take our own Milky Way. Stars on the rim are flying so fast they should shoot off into space--unless the galaxy contains far more matter than meets the eye. Just as in the Coma cluster, the gravitational attraction from our galaxy's visible matter is too weak to rein in these speedy stars. From that and other evidence, astronomers have deduced that the Milky Way is enveloped by an enormous dark-matter halo that extends far into space; no one is sure how far.
The universe teems with dark matter--but what is it?
Blas Cabrera can tell you just how vexing that question has been. When Cabrera decided to pursue wimps in the late 1980s, he was already a veteran dark-matter hunter. His team at Stanford had spent nearly a decade trying to snare an overweight, magnetically charged and entirely hypothetical particle known as a monopole, which was a leading candidate for dark matter back then. Monopoles should have been lumbering through the universe since shortly after the Big Bang--or so the theory went. Cabrera, PhD '75, and his colleagues built three generations of monopole detectors, but like everyone else looking for the same quarry, they were unsuccessful and eventually gave up.
Cabrera, his dark hair now fringed with gray, describes the wild goose chase with equanimity. If he's disappointed about spending such a chunk of his life on a failed search, he doesn't let on. The same fate, of course, could befall the search for wimps. "These things are always high risk," he admits, "because there's never any guarantee of nature having picked that particular avenue."
If Cabrera had listened to his father, who recommended a career in biology, he wouldn't be chasing ghostly particles. But he decided to join the family business: both his father and grandfather were physicists. As a kid, Cabrera loved taking apart electronic gadgets to figure out how they worked. He also saw how much physicists enjoyed their work. "When my parents got together with friends, most of whom were physicists, I noticed that they all talked about what they did with great enthusiasm."
His interest in superconductivity drew him to Stanford for PhD studies, and he's been there ever since--as postdoctoral fellow, professor and, from 1996 to 1999, chair of the Nobel-studded physics department. Dark matter hasn't absorbed all Cabrera's time. Recently, he helped design an optical detector to aid astronomers studying rapidly moving objects. The first of these went into action last winter at McDonald Observatory in West Texas. He also has been running experiments on superconductivity (the phenomenon in which some materials become super-efficient electrical conductors at low temperatures). But for much of the past 20 years, his fascination with the biggest question around -- what is the cosmos made of?--has kept dark matter at the center of his universe.
Cabrera started the Cryogenic Dark Matter Search in the late 1980s with UC-Berkeley physicist Bernard Sadoulet. They co-direct it with David Caldwell, '48, an emeritus physicist from UC-Santa Barbara who's now based at Stanford. What particularly excites Cabrera is the breadth of the overall effort. "There's almost no area of physics," he says, "that isn't included in either the intellectual aims of the project or the detector itself."
In just the last second, trillions of wimps probably zipped through you, never rustling an electron. This is the challenge facing WIMP chasers. How do you trap a stealth particle that flies invisibly through everyday matter, neither reflecting nor emitting any detectable radiation?
Answer: you try to catch it in the unlikely event of a crash.
That, at least, is the plan for the Minnesota mine experiment. Cabrera and colleagues designed the detectors to sense a WIMP in the rare act of colliding head-on with the nucleus of an atom in a crystal. The wimp will bounce off and continue on its way. But the atom that takes the blow will recoil, causing tremors -- called lattice vibrations -- to spread outward through the crystal. Sensitive electronics will pick up those tiny rumblings. Scientists will also measure the charge of electrons dislodged by the collision, which should help them discern WIMPS from most radiation. Using semiconducting crystals--in this case, silicon and germanium--allows them to make that distinction.
Freezing the detectors helps, too, by holding the atoms still. At room temperature, even crystallized atoms would jiggle and bounce like first-graders eager for recess, making a recoil harder to detect. Chilling them to nearly absolute zero (-459 F), the temperature at which all atomic motion ceases, stops this distracting vibration. As a result, says Cabrera, the "lattice vibrations stand out like ripples when you drop a pebble into a still pool."
Beneath the Stanford campus, in a dogleg tunnel lined with white concrete, Cabrera and his colleagues are testing that strategy and fine-tuning the equipment. At the end of the tunnel sits a 6-foot black box holding three of the crystal disks. Lining the rest of the corridor are the tanks and pipes of the chilling system and the electronics that monitor the detectors.
Thirty-five feet overhead, bikes whiz along Panama Street and students sun themselves on the steps of the Terman building. Up there, "we basically live in a bath of radiation," Cabrera notes. Being underground blocks out some of that, but plenty of stray particles still make their way down. So the researchers have gone to great lengths to protect the detectors, encasing the "icebox" in three kinds of shielding: a layer of ordinary lead, a layer of polyethylene and an inner wall of "old" lead liberated from a French ship that sank in the 18th century. Hundreds of years beneath the ocean have allowed much of the lead's natural radioactivity to leak away, so it's less likely to trigger false readings.
But even these precautions don't eliminate distracting radiation, as a quick visit to the chilly tunnel demonstrates. Wearing baby-blue paper booties to keep down dust, Tarek Saab, a grad student of Cabrera's, shows how the detectors work. He flips a switch on an oscilloscope monitoring their activity, and almost immediately, a green peak erupts on the screen, meaning something just set a detector ringing. Then another peak sprouts, and another. Are we witnessing a historic moment?
Not likely. The detectors get about 25,000 such "hits" a day, Cabrera explains, and virtually every one of them comes from an ordinary particle that managed to sneak into the icebox. A few might conceivably be wimps slamming into the crystals, but such exceptions would be difficult to pick out in the constant rain of other particles. According to theorists' calculations, wimp collisions should be so infrequent that the detectors might register only six or seven a year.
The main culprits in the confusion are cosmic rays streaking in from space. This type of radiation is particularly deceiving because it can jar loose a neutron that may have blundered into a detector. Neutrons are hard to distinguish from WIMPs because either could strike an atom's nucleus. Although sophisticated monitors can factor out most of this radioactive "noise," the din makes it harder to "hear" an arriving WIMP.
That's why this is only a testing phase, and the real search will be deep in a mine. Thirty-five feet of choice Palo Alto dirt and rock won't suffice, but a half-mile of sturdy Minnesota stone should do the job. Cabrera says the cavern will shield the detectors from nearly all cosmic rays, boosting their sensitivity 100 times.
The old mine nonetheless seems an odd place to plant a physics experiment, more than 2,000 miles from the California labs of the directors, 250 miles from Minneapolis and probably 100 miles from the nearest Radio Shack. But the location has several pluses aside from its rock-shielded depth. For one, says Cabrera, it allows the wimp search to piggyback on existing infrastructure, since the mine already houses a proton-decay detector run by the University of Minnesota and will soon have an experiment probing yet another exotic particle, the neutrino. The presence of scientists and engineers working on other projects is particularly important, Cabrera notes, when you're asking someone to spend the winter in backwoods Minnesota.
On my visit with project manager Roger Dixon, the place seems pleasant enough. Although it's hard to forget we're 2,300 feet below ground (you can't look out a window or step outside for a stroll), the cavern is warm and dry and has Internet access. A casual, jesting attitude seems to prevail. Four-toed red dinosaur tracks mark the way to the fire exit, and a sign reading "Tornado Shelter" is plastered on one of the doors.
Putting together an experiment down here is like building a ship in a bottle, says Dixon, because everything has to descend through the same narrow shaft we did. They've sent down plenty of unlikely stuff, from a forklift to a cherrypicker. Walking over to a metal building resembling a large garden shed, where the icebox and shielding will sit, Dixon points to some fuzzy brown lumps stuck to the cavern wall. "Those are bats," he says. They plunge down the shaft, possibly attracted by the warmth, but can't find their way out and eventually starve. The staff keeps a net handy in case one bites someone and has to be trapped for a rabies test.
By September, crews had turned the shed-like building into a "clean room," with gleaming white walls that look like porcelain. (Dust is the bane of the project because it can carry radioactive particles.) This winter, spring and summer, they will assemble the chilling system, install the electronics and construct the icebox to house 42 detectors, which are being sent up from Stanford in batches. The first six detectors should be in place and awaiting WIMPs by October 2001; the rest will go live by the end of 2002. The project will collect data through at least 2005.
That schedule, Dixon notes with glee, will require some Californians to spend at least a few frigid weeks in northern Minnesota. "It's about time those wimps see what it's like up here in winter," he says.
Catching a wimp is one thing; proving you've done it is another. With 20 teams competing for a prize of this importance, the scientific community will demand impeccable evidence before proclaiming anyone's finding a success. Confirming a wimp signal is "a little like [verifying] the Loch Ness monster," Cabrera says. "You have to have enormous amounts of data before you can convince everyone."
A preview of such intense scrutiny came last February when a joint Italian-Chinese team calling themselves DAMA (for DArk MAtter) announced on their website a "possible" wimp signal. Shortly afterward, they repeated the assertion at a dark-matter conference in Marina del Rey, Calif.
DAMA's detectors, less sensitive than the Stanford-built version, sit in an underground cavern near Rome. To infer the presence of wimps, the group looked for a hypothesized seasonal fluctuation: the Earth's orbit around the sun might, in theory, make WIMP hits more frequent in summer than in winter. And that's what dama reported at the conference: 1 percent more hits in June than in December. The seasonal difference, they concluded, suggests some of these arrivals may have been WIMPs.
The researchers didn't actually claim to have nabbed a WIMP. Nonetheless, they were grilled at the conference, and the scientific skewering hasn't stopped. "It's a lousy experiment--it doesn't tell you what you're seeing," says Joel Primack, a physicist at UC-Santa Cruz who did much of the pioneering theoretical work on wimps in the early 1980s. Though Primack, PhD '70, isn't a part of any current search, he's keenly following the chase.
One problem was that the Italian-Chinese results didn't jibe with preliminary data from the underground detectors at Stanford. Richard Gaitskell, a UC-Berkeley astrophysicist working with the Stanford team, presented those findings at the same conference. Over the entire year of 1999, he reported, after as many extraneous signals as possible were ruled out, the campus detectors recorded only 13 potential hits. If the DAMA figures were right, then wimps should be much more abundant than theorists have predicted, and Stanford should have seen half again as many signals. "We exclude their results," Cabrera says.
The Stanford data established that the new equipment is sensitive enough to detect a WIMP. Beyond that, however, the findings were ambiguous. Cabrera doubts that any of the 13 signals picked up under Panama Street were actually wimps. Further analysis suggests that at least eight were neutrons. Then again, the other five could not be conclusively identified, which means they might have been the real thing. It's a tantalizing prospect, but according to Cabrera, there just isn't enough evidence to know.
Whoever makes the next wimp claim can expect the same kind of grilling from skeptics. The only way to win scientists over, Cabrera says, is to show that you've ruled out everything that might conceivably mimic a WIMP--the errant neutrons, the electronic glitches--and to record a reproducible signal over time. You also must go through the equivalent of an irs audit: opening up your experiments and data to people whose sole purpose is to find fault with your work. Ultimately, confirmation will not depend on a single experiment. wimps will have to show up in a separate search, preferably one by a different team using a different method, Cabrera says, "so we can't be fooled in the same ways."
The scientific infighting over DAMA's findings generated plenty of media coverage, as a scrap over the basic ingredient of the universe is bound to do. But Michael Turner, a cosmologist at the University of Chicago, has a different take. "Look, what's exciting is that the experiments have reached the sensitivity where they can search for wimps," says Turner, MS '73, PhD '78, an original WIMP theorist who gave the particles their catchy name. If WIMPs do exist, he says, someone will apprehend them soon.
Scientists can hardly wait. For centuries, they've been groping in the dark to fathom the workings of the universe. With dark matter finally unveiled, they'll be able to look farther into the past, see the present more clearly and maybe predict what's ahead. "We're talking about an epochal discovery," says Primack. It seems safe to say that dark matter has a bright future.
Mitchell Leslie is a frequent contributor living in Albuquerque, N.M. He last wrote for Stanford about Lewis Terman.
