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Physics

The waiting game

A few physicists have spent decades searching for the rarest events in the Universe — and seen nothing. But their enthusiasm for the hunt is undimmed. Geoff Brumfiel asks what keeps them going.

Credit: D. NEWTON

Blas Cabrera still remembers Valentine's Day 1982. Entering his lab on a Sunday afternoon the young physicist made a heart-stopping discovery. His custom-built detector had just sensed something nobody had seen before — a particle called a magnetic monopole. For three years, Cabrera had been fine-tuning his experiment at Stanford University, California, and it seemed as if his diligence had paid off. “It certainly looked exciting at the time,” he recalls. But today, more than 20 years later, he has yet to see a second monopole.

Most scientists would go crazy waiting years for a result that might never materialize. But a small number of physicists are doing just that. They devote years and even decades to the search for hypothetical particles and forces.

What keeps them going is a devotion to fundamental physics, an obsession with the strange machines they build to achieve their goals, and an opportunity for independence from the giant accelerator groups that now dominate much of particle physics. “It's like drugs: you get addicted and you can't get out of it,” says Giorgio Gratta, another physicist at Stanford University who will soon begin searching for a kind of nuclear decay that happens roughly once every trillion trillion years. Despite funding setbacks and the occasional false alarm, many of these researchers establish successful careers even when their detectors fail to record a single particle.

Part of the reason they command respect is because their quests address nagging questions left over from the past century's revolutions in physics. Einstein's general theory of relativity explained gravity's inner workings, and quantum mechanics led to a standard model that could predict the interactions of tiny particles, such as quarks — but try combining them and you get mathematical gibberish. A similar mismatch arises between cosmological theories and laboratory observations that fail to detect the matter thought to hold together much of the Universe. These inconsistencies leave physicists feeling they are missing a big part of the picture.

To fill the gaps, theorists have generated a stream of hypothetical particles and forces. But these notions face a problem: if they were easy to find, somebody would have seen them already. So experimental physicists must go to extraordinary lengths to find them. They must build supersensitive detectors and shield them from the noisy world, they must ensure they do not fool themselves into seeing something that's not there — and above all, they must be patient.

Eric Adelberger, a physicist at the University of Washington in Seattle, could teach patience to a saint. He has spent the past decade watching moving pendulums in his lab. Adelberger's group uses the pendulums to test the behaviour of gravity at very small scales — where Einstein's laws conflict with quantum mechanics. “There are lots of ideas predicting new forces that would show up in these experiments,” he says.

Rather than swinging back and forth, these pendulums twist as Earth spins. In one system, otherwise identical weights made of lead and copper are tested to see if gravity pulls on them differently. If it did, it would contradict Galileo's legendary finding that two balls of different material fall to the ground at the same rate. It could also point to new physics governing gravity on the smallest scales. For example, string theory, one possible link between gravity and the quantum world, predicts particles that would change the laws of gravitation over short distances.

Small is detectable

Before Adelberger got into gravity he was in nuclear physics, where experiments involve hundreds of researchers. His work on pendulums has given him individuality, he says. “With these kinds of experiments, you're not a little part of a big thing, it's really yours,” he says.

The pendulums took years of patient refinement: “It's kind of like a child,” he says. The test weights are electrically inert, and tonnes of lead are used to shield them from the gravitational fields of nearby objects, such as hills and buildings.

Even with careful planning, Adelberger's group has had to cope with unexpected events. One wet November, a graduate student found that the gravitational field around the pendulums seemed to be growing inexplicably day by day. “It was the rain soaking into the Earth and changing the gravity field in the lab,” Adelberger says.

“The difference in gravitational fields we can measure now is equivalent to the difference between a certain point in front of my nose and a point 1.6 nanometres higher,” he says proudly. But in over ten years, Adelberger's probes have revealed little more than Seattle rain.

That doesn't bother him in the least, and it hasn't hurt his career. Adelberger became a Fellow of the American Physical Society in 1984 and was elected into the National Academy of Sciences in 1994. He credits his success to the effect his pendulums have had on theorists' ideas. “The negative results we are producing have a real impact on physics,” he claims.

Non-event

The even greater impact that a positive result would have means that workers on these experiments must guard against the risk of self-delusion. It's a problem that has preoccupied Rainer Weiss, a physicist at the Massachusetts Institute of Technology in Cambridge. Weiss has devoted his career to detecting gravity waves — tiny ripples in the Universe's gravitational field that would have huge implications for cosmology if they could be seen.

That was what Joseph Weber, a physicist at the University of Maryland, College Park, claimed to have done in the late 1960s. Weber said that gravitational waves striking a giant bar of aluminium in his laboratory caused it to ring like a bell. But critics pointed out that the power needed to generate these waves would wipe out the entire Milky Way. Weber, it seemed, had been fooled by his own statistical analysis, and the controversy set back the field of gravity waves for decades.

Last year, the field entered the mainstream, when the $300 million Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors in Louisiana and Washington state, fired up giant microwave lasers that Weiss hopes will be able to spot gravity waves coming from the nearby Milky Way. With millions of dollars and the careers of some 400 researchers on the line, the group knows it cannot afford to cry wolf. “We have a set of physical conditions that have to be met for us to take an event seriously,” Weiss says. But noise — such as thermal and mechanical vibrations — could blur the signal and trigger events that could be real or just an illusion. “You will then find us playing the same game that Blas Cabrera did,” Weiss says.

Cabrera's Valentine's Day detection left him with a tough decision. Magnets have a north and a south, but for decades some theorists have predicted that a few lone norths or souths might wander the cosmos. These could account for a big part of the Universe's missing matter. They would also make physicists rethink the standard model, which does not predict monopoles.

But Cabrera knew his discovery was problematic. Although it matched theoretical predictions, and there were no obvious experimental errors that could explain the result, it was only a single event — hardly enough to overthrow established thinking.

Dark materials

In the end, Cabrera wrote a paper that was published in the prestigious journal Physical Review Letters (B. Cabrera Phys. Rev. Lett. 48, 1378–1381; 1982). He didn't claim to have found monopoles, he just described the detector and the event. “Many of us said: ‘My God, why the hell did he do that?’” says Weiss. But Cabrera's colleagues came to agree with his approach. “He published what he had, showing exactly what he did,” Weiss says. He has invited Cabrera to talk to the LIGO group about what to do if they find an uncertain event.

Cabrera's device never spotted another monopole. His group has built second- and third-generation detectors, the last of which was thousands of times more sensitive than the original. But they came up empty-handed. “We never again saw an event like this one in any of the individual instruments,” he says. “It seemed less and less likely that the one event we saw was a monopole.”

But the negative results were not fruitless. They challenged theorists, including Alan Guth and Henry Tye, then at the Stanford Linear Accelerator Center, to explain monopoles' rarity. This influenced Guth's work on inflation theory — the idea that the Universe expanded exponentially just after the Big Bang. Astronomical observations have since confirmed this, and the theory has become one of the pillars of modern cosmology.

In the early 1990s, Cabrera switched to seeking other particles that might explain the Universe's missing mass. The resulting Cryogenic Dark Matter Search has run for 14 years: the latest detector, shielded from radiation deep within a Minnesota mine, was switched on last year. Cabrera is unconcerned that so far they have found no new particles, “If they're there, we will see them,” he says.

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Brumfiel, G. The waiting game. Nature 429, 10–11 (2004). https://doi.org/10.1038/429010a

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