(Image: Simon Danaher)
IT’S matter, but not as we know it. In July, an unexpected visitor appeared at CERN’s Large Hadron Collider near Geneva, Switzerland. Dubbed the pentaquark, this peculiar particle represents a fundamentally new way to aggregate the basic building blocks of matter. Although not forbidden by our current understanding of how stuff comes together, it had never been conclusively spotted before.
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This sort of thing is music to ears. A theoretical physicist at Case Western Reserve University in Cleveland, Ohio, Starkman is banging the drum for a bold idea: that there are even more exotic configurations of ordinary matter out there just waiting to be discovered. His audacious proposal is to find traces of this oddball matter by various means, from exhuming data left in mothballed gravitational wave detectors and searching ancient minerals to deploying seismometers on the moon. He even argues that ordinary matter in extraordinary formations could solve one of the greatest cosmological mysteries of our time – dark matter.
For physicists faced with the most difficult conundrums, inventing new particles beyond those we already know has long been the go-to trick. was an early adopter. In 1930, he proposed that the missing energy in certain experiments was carried off by an elusive particle that escaped measurement. Pauli himself was not happy with his invention. “I have done a terrible thing,” he said. “I have postulated a particle that cannot be detected.” He needn’t have worried: what we now know as the neutrino was found in 1956.
Physicists have been busily inventing particles ever since, and in the process they have built up the standard model – the most complete description yet of particles and their interactions. Its crowning glory came in 2012 with the discovery of the Higgs boson, which explains why particles have mass.
But the standard model doesn’t delight physicists. Its mathematical structure appears piecemeal and patched up. And it still contains gaping holes, not least its failure to explain dark matter – the shadowy stuff that accounts for 85 per cent of matter in the universe, yet neither absorbs nor emits light and seems to interact only weakly with other matter.
Faced with this riddle, physicists have followed tradition and contrived dark matter candidates, from weedy WIMPs to massive WIMPZILLAs. But none has been spotted. Contenders suggested by supersymmetry – which predicts that all known particles have a more massive “super-partner” – are also a no-show.
Somewhere along the way, then, the panacea of dreaming up particles has stopped working. That leads Starkman to a provocative conclusion. “Look, the standard model has no experimental failures,” he says. “So the fact that we have philosophical and aesthetic problems with it may be just something we need to get over.” Instead of constantly devising particles, Starkman argues that we should look more closely at those we already know. Perhaps old particles are capable of new tricks.
To form the matter that surrounds us, elementary particles come together in certain standard configurations. Quarks combine in threes to form compound particles known as baryons, for instance. The protons and neutrons that make up atomic nuclei are baryons formed of trios of up and down quarks, the lightest two of the six quark variants. We also know of short-lived combinations of a quark and an antiquark, known as mesons.
But quarks are quirky. They never float around freely thanks to a peculiar property of the strong nuclear force that binds them. When the distance between quarks is small, the force is weak. But as that distance increases, it gets stronger and the quarks are pulled back together. Another strange quality of the strong force is that it is weaker at high energies, such as those produced in collisions at the LHC. Physicists can calculate how quarks interact at these energies but not at lower ones, where the force keeping quarks together strengthens. As a result, physicists still struggle to explain how quarks form mesons and baryons, a process which occurs at lower energies.
This uncertainty has led to proposals that other forms of matter might exist. As early as the 1980s, , a mathematical physicist at Princeton University, suggested that light quarks could combine with their heavier cousins, such as strange quarks, in unusual ways (see diagram).
Unlike in ordinary matter, these combinations of quarks would not form atomic nuclei. Instead they would develop into large amorphous blobs, gathering ever more particles in a small space. Witten called them “quark nuggets”. Bryan Lynn, a theoretical physicist at University College London, and others later expanded this to more examples such as “strange baryon matter” and “chiral liquid drops”.
Such exotic clumps of familiar elementary particles would not contain the enormous spaces between atomic nuclei that we see in normal matter. This would make them as dense as neutron stars, a teaspoon of which weighs as much as a mountain. So even though they might be extremely heavy, they could also be tiny. Some researchers have dubbed them “macros” – a reference to the need to measure their masses in kilograms and tonnes rather than the vanishingly small units usually employed for particles.
“Exotic clumps of familiar particles could be as dense as neutron stars yet much smaller and harder to spot”
And because macros are entirely made up of nuclear matter, without any circulating electrons or empty spaces, they would not be capable of sustaining fusion and therefore could not shine. The high density of the clumps would also make them less likely to interact with incoming light. In short, macros would be diminutive, massive and extremely hard to spot, if not entirely invisible.
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It sounds like the perfect recipe for dark matter. But physicists had previously discounted the idea, for two reasons. First, if macros are compact objects about as heavy as our sun, similar to brown dwarfs or black holes, then they would have to outnumber visible stars in order to account for dark matter. If so, macros would frequently bend the light reaching Earth from stars, an effect known as gravitational lensing. But the amount of bent light we do see is already accounted for by familiar cosmic objects made of normal matter. Second, if nuclear matter were spread out in a thin carpet across the universe, it would interact with itself and other matter, and hinder the formation of galaxies as we know it.
But when Starkman and his colleagues took a closer look, they saw that macros would not have to be so heavy as to cause frequent gravitational lensing, nor spread out thinly enough to regularly interact with anything. Clumped into medium-sized drops, neither huge nor tiny, they would be compatible with existing cosmological observations.
With that in mind, Starkman and his colleagues have begun the search for evidence that such medium-sized macros exist. “This is almost the most silly but definitely the most obvious thing that we should do,” says Starkman’s collaborator David Jacobs, .
They started by trying to figure out where macros at the lighter end of the allowed mass scale might have already showed up. That meant revisiting the work of physicist Paul Buford Price at the University of California, Berkeley, who had searched for signs of massive particles in the Earth’s crust back in the 1980s. Price thought that heavy, weakly interacting particles occasionally passing through would have bumped the crystal lattices of transparent minerals known as micas buried deep underground. But his samples, taken from collections held at the British Museum and the Smithsonian Institution, showed no trace of that.
On to plan B. In the 1970s, researchers mounted polycarbonate plastic sheets on the , hoping to spot particles via the etchings they left on the plastic as the craft orbited Earth. Specifically, they were on the lookout for low-energy cosmic ray ions. Starkman figured that macros too stood a chance of marking the material, but a fresh look at the Skylab plastic did not reveal anything resembling a macro track.
Then there was , an experiment based near Rome, Italy, that started looking for gravitational waves in the 1990s. NAUTILUS consists of a supercooled 2-tonne aluminium cylinder that is closely monitored for any deformations that could signify the passage of ripples stretching space and time. Starkman, Jacobs and Amanda Weltman, also at Cape Town, figured that a macro travelling through the detector could interact with the aluminium, releasing energy that would cause the device to heat up and deform ever so slightly. Alas, they found no sign of macros ().
The striking absence of signals from these experiments allowed Starkman to further constrain the range of allowed macro masses, but the window of possibilities remains large – between roughly 50 grams and the mass of Mount Everest.
Cosmic ray detectors might be tweaked to record oddball matter raining down on Earth (Image: Steven Saffi /Pierre Auger Observatory)
Time to dream up more far-out experiments to narrow the range still further. Jacobs hopes that marine hydrophones, normally employed to study whales or track illegal nuclear weapons tests, may be able to hear the impact vibrations of macros passing through the ocean. He also plans to study data from cosmic ray detectors, designed to look for particle showers created when protons or lightweight nuclei from outer space collide with the upper atmosphere. If a macro interacted with Earth’s atmosphere, it would produce a characteristic light signal, but cosmic ray detectors are not programmed to scan the skies for such hints. Starkman is hoping to persuade the in Argentina to reprogram their detectors.
The best bet might be a little further from home: Earth’s moon. When the last Apollo astronauts departed the moon in 1972, they left behind a network of four seismometers. Over the following five years, these devices recorded thousands of seismic events caused by crashing meteorites, tidal forces or the expansion of the moon’s crust as it warms after a long cold lunar night. Clearly the moon is seismically active, albeit much less so than Earth, with its magma innards and shifting tectonic plates. And it is possible that this quake data could betray macro trails.
In 2002, researchers reported a possible macro signal in Earth’s seismic data, but it turned out to be a false alarm due to an offset clock on a seismometer. Now Starkman and others want to go back to the relative calm of the moon to test the idea that middleweight macros might create a distinctive line of consecutive quakes as they pass through.
The seismometers left on the moon were pretty crude, but planetary geologists are plotting to head back there with better kit. Bruce Banerdt of NASA’s Jet Propulsion Lab in Pasadena, California, and his colleagues have drawn up plans for a more sensitive lunar seismic detection network – and having built one for the InSight mission to Mars, launching next year, he knows what he is doing. “I had no idea just how sensitive a seismometer was until I had to build one,” he says. His Mars instrument is so sensitive that it can pinpoint positions to about the radius of a hydrogen atom – enough to pick up a passing macro.
Is it really worth going to such extremes to hunt for these oddballs from outer space? The difficulty in calculating their behaviour means researchers still can’t be sure that macros could have formed in the necessary amounts to make up dark matter, nor whether they would be stable enough to hold together.
And that’s not the only issue. “I don’t see how it fits into the larger picture,” says , a theoretical physicist at the Massachusetts Institute of Technology. “How do you make this stuff?” He also suggests that if macros exist, they might require less energy to form than ordinary nuclear matter, raising the question of why there is so much ordinary matter around.
Aiming low
What’s more, adds Wilczek, established ideas about dark matter didn’t arise by accident. “I’d be quite disappointed if the dark matter proposals that have evolved logically out of our attempts to improve our theory of fundamentals, and which seem so promising, turn out to be false trails.”
, a cosmologist at the University of Nottingham in the UK, is more pragmatic. “Macro dark matter isn’t perhaps as well theoretically motivated as WIMPs, or other particle dark matter candidates, but it’s not implausible,” she says. “It’s certainly extremely worthwhile to look for signs of macros in existing astrophysical and cosmology data sets.”
Their discovery would certainly make a giant splash. For a start, it would mean the colourful cast of exotic particles previously invented and sought by physicists may well not exist. “If we are right, then this means the LHC won’t see dark matter candidates,” says Jacobs.
“If we are right, then this means the LHC won’t see dark matter candidates”
No one is calling off those searches yet. But rather than reaching for higher and higher energies with particle colliders, perhaps we should seek answers in bringing quarks together at lower energies to better grasp the nature of nuclear matter. Ultimately, this might reveal that the ordinary elementary particles we know and love can come together in some extraordinary ways. The familiar could be about to get very strange indeed.
This article appeared in print under the headline “Strangely familiar”
