Shadows of two failed searches loom over physics
Scientists, like athletes, are obsessed with experiencing the thrill of victory. Just as they fear the agony of defeat. And in the wide world of science, thrills make news much more often than the agony. Winners get the publicity, losers can’t get published.
But sometimes the defeats deserve to make news too, especially when highly publicized experiments fail in their quest. Data reported in 2016 have forced physicists to face the prospect of just such a failure — not once, but twice. Dark matter, supposedly the most abundant form of mass in the cosmos, declines to show up in devices designed to detect it. And it refuses to appear in experiments constructed to make it.
For decades, physicists specializing in subatomic particles have expected to find an entirely new species of matter, a type never seen on Earth, swarming throughout the vastness of space. Galaxies rotate too rapidly and clump too closely if the only source of gravitational force is the matter that glows in visible light. Something else must be out there — an invisible, unidentified source of gravity that does not glow like stars or gas. In fact, most (roughly 85 percent) of the matter in the cosmos, astronomers have long known, must be dark.
Billions of these dark matter particles ought to be passing through your body every second. Your body wouldn’t notice, but large, sophisticated detectors should record a vibration or flash of light when a dark matter particle collides with an atomic nucleus in the detecting material.And yet such experiments repeatedly come up empty. In August and September, for instance, three search teams reported no luck detecting dark matter particles (SN: 11/12/16, p. 14). These were just the latest disappointing reports from similar searches over the last two decades. (One search, from a detector in Italy called DAMA/Libra, does claim dark matter detection, but nobody can confirm it and hardly anybody believes it.)
Still, physicists continue the search, largely because they have a second motivation for believing that dark matter is made of a new kind of particle—a theoretical concept known as supersymmetry.
Supersymmetry appeals to physicists because it hints at ways to solve unsolved problems, such as incorporating gravity into the theory explaining other forces. It originated in physicists’ efforts to understand symmetries connecting force and matter, just as Einstein had exploited symmetries of space and time to develop his theory of relativity. Supersymmetry’s equations imply the existence of “superpartner” particles heavier than particles now known: a force particle partner for every known matter particle, and a matter particle partner for every known force particle. A massive superpartner should have precisely the properties needed to account for the dark matter in space; it would interact only weakly with ordinary matter, inspiring the nickname of WIMP (weakly interacting massive particle).
To many physicists, this confluence of motivations seemed sufficient justification to invoke Gibbs’ Rule No. 39 (for those who watch NCIS on TV): There is no such thing as a coincidence. It was called the “WIMP miracle.” Independently of any theoretical forecasts, astronomers had observed clear signs of a mysterious source of gravity, most likely particles unknown on Earth. Independently of gravitational anomalies in space, theorists had forecast exotic new massive particles permeating the cosmos. One reinforced the other, just as centuries ago Isaac Newton’s law of gravity gained credibility because it explained both the orbits of the planets in space and falling apples on Earth.
Many physicists fully expected the world’s most powerful particle collider — the Large Hadron Collider outside Geneva — to produce WIMPs. But just as direct dark matter detection experiments have failed to spot them, the LHC has reported no sign of creating them (SN: 10/1/16, p. 12).
Story continues after graphicThere’s still hope. LHC experiments might yet create superpartners; dark matter detectors might yet snatch a WIMP from the sky. It’s like a football game late in the fourth quarter, says cosmologist Rocky Kolb of the University of Chicago. “The game is not over yet,” he says. “The clock is ticking, but they have a couple of more years of exploration ahead.”
Nevertheless this convergence of failures hints at a dual crisis in the quest to understand the cosmos. If WIMPs don’t exist, two huge gaps in that understanding persist. Something else must be messing with the motion of galaxies. And something other than supersymmetry will be needed to help physicists incorporate gravity into, and solve other problems with, their standard model of particles and forces.
At a deeper level, the double failure calls into question the very strategies for success that 20th century physics established. Perhaps the power of symmetry principles to reveal nature’s secrets has been drained, and a novel insight into how to pry secrets from nature awaits discovery. And the confidence provided by converging motivations may turn out to be more like wishful thinking than rigorous reasoning. Advocates of a multiplicity of universes, for instance, cite two independent arguments: One, that the best theory for explaining the observed universe implies the existence of others; two, that mathematical formulations (embodied in superstring theory) describe a vast number of different potential vacuum states. Those many states can be interpreted as descriptions of multiple universes. But the dual dark matter failures would suggest that convergent motivations are no guarantee of correctness. Reasoning based on Rule 39 might not be so solid.
So maybe something extraordinarily revolutionary is lurking behind today’s failures. Or maybe not. The dark source of gravity distorting the motion of galaxies might simply be particles other than WIMPs —perhaps a very light, wispy hypothetical particle called the axion. Or it might consist of black holes littered in and around galaxies.
In any event, failure to find or make dark matter particles does avoid one snafu that Kolb had worried about.
“Five years ago, I was concerned that we would have indications of new physics from LHC and different signals from direct detection experiments, and we would be in a period of confusion trying to reconcile the signals,” he says. “Well, we don’t have that problem.”
This article appears in the December 24, 2016/January 7, 2017 issue with the headline, "Double darkness: Shadows of two failed searches loom over physics."
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