RIGHT now, roughly half a proton’s worth of dark matter is passing through your thumb. That points to the really exasperating thing about this mysterious stuff. Astronomical observations tell us that the gravity of something huge and unseen is tugging on stars and galaxies, making them whirl faster than they otherwise would. They suggest there is an awful lot of whatever it is – a whopping five times more than ordinary matter. And yes, we can even calculate how much of it should be passing right through us here and now.
Yet open the box, and there’s nothing there. A no-show of dark matter in experiments designed to detect it on Earth increasingly has physicists questioning every notion they have about it. Now, a group of them is proposing we cut to the chase by focusing on the one thing we can say for certain about dark matter: that it interacts through gravity. We can use that, they say, to detect its ghostly gravitational influence as it passes through us and everything else.
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Well, possibly. Gravity is such a ridiculously weak force that detecting dark matter’s touch directly would involve connecting together as many as a head-spinning billion mini-sensors, each one more sensitive to tiny disturbances than the huge set-ups used to spot gravitational waves. Then again, that worked, so why not this? “It’s new, it’s exciting, it’s coming out of left field,” says , a particle physicist at Purdue University in West Lafayette, Indiana. “We all agree that it’s a bit crazy, but I think everybody will give you a different idea of how crazy it is.”
It might seem ridiculous to say gravity is a weak force, given the calamitous effects it has if, say, you step out of an aeroplane in mid-flight without a parachute. That’s true enough – but equally, you buck the gravity of an entire planet with ease every time you pick up your phone. To put a physicist’s measure on that, the gravitational attraction between two protons is roughly 40 orders of magnitude, or 10 thousand billion billion billion billion times, weaker than their electrostatic repulsion.
Gravity’s comparative feebleness is one of its many mysteries. Another is why it is the only one of the four fundamental forces of nature that can’t be described using quantum mechanics. Physicists have searched for decades for an overarching framework that might combine quantum theory with Albert Einstein’s space and time-warping general theory of relativity, which describes gravity, but they have had little success.
This was the ballpark that theoretical physicist at the University of Maryland and his colleagues were playing on in 2017. They were considering how to design an experiment to search for the graviton, a hypothetical quantum particle that would carry gravity. As they were thinking of possible confounding background effects, someone pointed out that dark matter flying through the lab might conceivably introduce noise. “We said, ‘Ha ha, very funny’,” says Carney. “But then we made an estimate and realised that’s not noise – that’s a signal.”
That’s because our best devices can now sense bewilderingly small forces, down to about a zeptonewton. A newton is roughly the force you exert against gravity to hold an apple in your hand. A zeptonewton is 10-21, or a million million billion, times smaller than a newton – roughly the force you would exert picking up a strand of RNA.
Carefully balanced
Sensors that can see the tiny displacements caused by zeptonewton forces take us beyond the precision frontier established by the US Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015 when it made the first direct detection of a gravitational wave. These ripples in space-time are caused by far-off, cataclysmic mergers of massive objects such as black holes and are a key prediction of Einstein’s relativity. Detecting one involved bouncing laser beams many times back and forth through concrete tunnels between two reflecting mirrors, positioned kilometres apart, to measure changes in distance 10,000 times smaller than the width of a proton.
Researchers in many labs are tinkering with zeptonewton sensors. One version that Carney and his colleagues are interested in balances a glass bead a few hundred nanometres across in a laser beam, and cools it to a fraction of a degree above absolute zero in an ultrahigh vacuum. Disconnected from its surroundings, the bead is free to move in response to hidden particles or forces – be they gravitons or, as the researchers calculated, chunks of dark matter. A salt-grain-sized lump of dark matter weighing about a millionth of a gram, for instance, would cause the bead to vibrate measurably if it happened to fly within a millimetre.
“This simple idea is a world away from the complex detectors we rely on now”
This simple idea is a world away from the complex interactions that current, so far unsuccessful, methods of dark-matter detection rely on. These focus on theorists’ favoured guise for dark matter, particles known as WIMPs (see “How (not) to find dark matter”), and probe mass scales far lower than the microgram masses the new detectors would be most sensitive to. The higher range, known as the Planck scale, would instead contain objects that are extremely compact yet incredibly massive, making them subject to both quantum mechanics and general relativity. If new particles and forces existed at this scale, they might not just solve the dark matter puzzle – they could provide crucial hints, too, into how to unify these two theories.
The new detector would be agnostic as to what exactly this dark matter would be. It might be a collection of dark particles similar to visible molecules, a bunch of black hole remnants or a topological defect in space-time. Or it could be a curled up cosmic string as favoured by string theory – a prominent attempt to combine quantum theory and general relativity – or hypothetical extra-large WIMPs created shortly after the big bang, dubbed WIMPzillas.
Were such a detector to find anything, it would be big news. “The first thing you do, which earns you the Nobel prize, is you show that dark matter is real,” says , an experimental particle physicist at Yale University. Even if a detector found nothing, it would tell physicists that the mass range it was looking at is empty of dark matter.
There are still a few hurdles to jump before that becomes reality, however. For a start, there’s how you distinguish the tiny shaking of a bead from other, equivalently sized gravitational effects – say an elevator descending in a distant part of the lab building. Such effects have been known to confound previous attempts to measure the fundamental strength of gravity.
There is a workaround. Earth is ploughing through a thicket of dark matter as it, and the entire solar system, rotates around the centre of the Milky Way at some 800,000 kilometres per hour. Carney and his colleagues showed that if you arranged many of the new zeptonewton sensors in a regular three-dimensional grid they would vibrate in a coordinated manner when a dark matter nugget passed. In a similar way, LIGO and its European partner, VIRGO, require signals from multiple detectors at the same time to ensure their gravitational-waves are real.
Here’s the snag, however. Given the calculated local density of dark matter, an experiment that is a cubic metre in size would need somewhere between a million and a billion hypersensitive sensors spaced between a centimetre and a millimetre apart to uncover dark matter’s signature within a year. With a single zeptonewton sensor currently costing around $1 million, the price would need to come down considerably for it to be feasible. Then you would need ways to skirt less worldly problems, such as the stringent limits that the fuzzy laws of quantum mechanics set on measurement precision at such small scales.
Then again, these are just the sort of problems that experimental physicists love to overcome. “This is really the opening shot in a longer conversation,” says , a theorist at Fermi National Accelerator Laboratory near Batavia, Illinois, who is part of Carney’s team.
“Ultimately, the hope is that these efforts might reveal new aspects of reality”
He and around 30 other physicists gathered at the University of Maryland last October for a two-day brainstorming session to try to fill in the details of the vision. Work is now in full flow. Later this year, Lang and his colleagues hope to take data from a simple prototype of the sensor to see how strong a gravitational response they can get from particles that pass by with masses of a few millionths of a gram. Carney is doing something similar with instruments at the National Institute of Standards and Technology in Washington DC, while Moore wants to build arrays of around 10 ultrasensitive sensors within the coming year, providing insights into constructing set-ups with larger numbers.
Ultimately, the hope is that as these efforts progress, they might reveal not just the identity of dark matter, but entirely new aspects of reality. Visible matter is made from an astounding wealth of particles and forces, and a fair number of physicists suspect dark matter might be as well. The same types of sensors that could feel dark matter’s extremely tiny gravitational kick would be useful to search for this “dark sector”. Several researchers who attended the October meeting are currently looking for a dark force that could subtly affect neutrons at close range – an effect some think we have already seen hints of in controversial recent experiments that observed anomalous effects within helium and beryllium nuclei.
Such an ambitious programme naturally invites scepticism. “It’s not fundamentally impossible,” says , an experimental physicist at Louisiana State University in Baton Rouge. “But it’s really, really hard.” Whether something at the scale of the full array would be the best use of limited funding remains an open question, he adds.
Meanwhile, the WIMP detectors could still turn up trumps, rendering the whole enterprise redundant. Just last month, the XENON1T collaboration at Italy’s Gran Sasso National Laboratory announced a tentative signal, not of WIMPs, but of axions, an alternative guise for dark matter. Or a breakthrough may come from a different direction entirely (see “Anti-detectors”). But such developments wouldn’t exclude the existence of the more massive forms of dark matter that the gravitational experiments are aiming to discover. “There’s a sense that we want to cover all our bases,” says , a theoretical particle physicist at the University of California, San Diego. “Maybe we’re potentially missing out on something that could be detectable.”
For the naysayers, Lang points to LIGO as a role model. “Someone said 30 years ago, ‘We can detect gravitational waves, but we need to measure the distance between two mirrors to a tiny fraction the size of an atom’,” he says. “This is obviously crazy, and yet they did it.”
ANTI-DETECTORS
Run out of options to detect dark matter? Then why not ditch the usual approach, which involves looking for dark matter’s interactions with normal matter (see main story), and instead try your luck with antimatter.
That is the principle behind tests recently performed at the Baryon Antibaryon Symmetry Experiment (BASE) at CERN near Geneva, Switzerland. Antimatter is even more of a mystery than dark matter: a whole mirror world of particles just like normal “baryonic” matter particles, but with opposite electric charge.
We know antimatter exists, because it pops up in the form of transient high-energy cosmic rays and as blips in radioactive decays. But the standard model of particle physics, our current best theory of matter and the forces that work on it – except gravity, which sets its own rules – says that the big bang should have created just as much antimatter as matter. Worse, since matter and antimatter “annihilate” on contact, nothing of either should be left at all.
It is this existential enigma that BASE and other experiments housed at CERN’s Antimatter Factory are hoping to shed some light on. BASE uses complex arrangements of electric and magnetic fields to hold antiprotons in an ultrahigh vacuum so they don’t annihilate. This means you can probe their properties in detail, in the hope of finding some small, unpredicted asymmetry in the properties of antimatter and matter that would explain matter’s dominance.
It could flush out dark matter too. Given that the standard model also fails to explain what dark matter is and how it works, there is a chance its interactions with antimatter and matter might be different. If it interacts more with antimatter, that could provide not just a new source of asymmetries to explain antimatter’s disappearance, but also a way to detect dark matter.
Late last year, Christian Smorra and his BASE colleagues from their searches for disturbances you would expect to see in antiprotons if they brushed up against dark matter made of very light axion particles. They saw nothing, putting a further constraint on the degree to which very light axions can interact with antiprotons. But with many other types of antimatter and dark matter still to be investigated, it is a case of never say never. Richard Webb
HOW (NOT) TO FIND DARK MATTER
Current searches for dark matter concentrate on the form that, until recently, most theorists expected it to come in: as a weakly interacting massive particle, or WIMP. These emerge naturally from a theory called supersymmetry, which extends the current “standard model” of particle physics, and so ties up many of its loose ends.
WIMPs would be expected to have a mass of between 50 and a few thousand times that of a proton, and interact with regular material not just through gravity, but also through the subatomic weak force, hence the name. That supplies a theoretically easier way of looking through them than by their gravitational interaction (see main story): by their weak-force recoil as they encounter the nuclei of normal atoms.
The current state of the art experimental set-up involves large vats containing a tonne or more of liquid xenon, sitting in underground mines to protect the vats from interfering cosmic rays. Smaller versions of such experiments running for the past 30 years have routinely seen a whole lot of nothing. The same goes for the particle-smashing Large Hadron Collider at CERN near Geneva, Switzerland, which should have produced particles predicted by the simplest versions of supersymmetry by now, casting further doubt on whether they exist.
With no WIMPs in the bag so far, a flowering of new dark matter searches is focusing instead on ultralight “a澱DzԲ”, which would weigh a millionth or even a billionth of the mass of an electron. While there have been recent tentative hints, it is still too early to say whether there is anything there either. Adam Mann
