Countless experiments around the world hope to reap scientific glory for the first detection of dark matter particles. Usually they do this by seeing if dark matter collides with normal matter or by smashing particles into other particles and hoping that some dark matter will emerge. But what if the dark matter behaves more like a wave?
That’s the intriguing possibility being championed by Asimina Arvanitaki, a theoretical physicist at the Perimeter Institute in Waterloo, Ontario, Canada, where she holds the Aristarchus Chair in Theoretical Physics — the first woman to hold a research chair at the institute. Detecting these hypothetical waves of dark matter requires a bit of experimental ingenuity. So she and her collaborators are applying a wide variety of radically different techniques to the search: atomic clocks and resonating rods originally designed to hunt for gravitational waves — and even lasers shining on walls in the hope that a little dark matter will trickle down to the other side.
“The advances in particle physics over the past 50 years have focused on accelerators, and rightly so, because every time we went to a new energy scale, we found something new,” says Arvanitaki. That focus is starting to shift. To achieve ever higher energies, physicists must build ever larger accelerators – an expensive affair when funding for science is dwindling. There is now more interest in smaller, cheaper options. “These are things that usually fit in the lab, and the turnaround time for results is much shorter than the collider,” says Arvanitaki, admitting, “I’ve been doing this for a long time and it hasn’t always been popular. “
The end of the WIMP?
While most dark matter physicists have focused on looking for weakly interacting massive particles, or WIMPs, Arvanitaki is among a growing number focusing on lesser-known alternatives, such as axions — hypothetical ultralight particles with masses as small as could be as ten thousand trillion trillion times smaller than the mass of the electron. In contrast, the masses of WIMPs would be greater than the mass of the proton.
According to David Kaplan, a theorist at Johns Hopkins University (and producer of the documentary Particle fever). But the WIMP’s dominance in the field so far is also due in part to excitement over the idea of supersymmetry. That model requires every known particle in the Standard Model — be it fermion or boson — to have a super partner that is heavier and in the opposite class. So an electron, which is a fermion, would have a boson super partner, the selectron, and so on.
Physicists suspect that one or more of those unseen superpartners may form dark matter. Supersymmetry not only predicts the existence of dark matter, but also how much of it there should be. That fits neatly into a WIMP scenario. After all, dark matter could be anything, and the supersymmetry mass range seemed like a good place to start the search given the compelling theory behind it.
But in the decades that followed, experiment after experiment came empty. With each null result, the parameter space where WIMPs might be lurking shrinks. This makes distinguishing a potential signal from background noise in the data increasingly difficult.
“We’re about to hit what’s been called the ‘neutrino floor,'” says Kaplan. “All the technology we use to discover WIMPs will soon be sensitive to random neutrinos flying through the Universe. Once there, it becomes a much messier signal and harder to see.
Particles are waves
Despite the momentous discovery of the Higgs boson in 2012, the Large Hadron Collider has yet to find any evidence of supersymmetry. So it shouldn’t surprise us that physicists are turning their attention to alternative dark matter candidates beyond the mass range of WIMPs. “It’s a fishing expedition now,” says Kaplan. “When you go on a fishing expedition, you want to search as broadly as possible, and the WIMP search is narrow and deep.”
Enter Asimina Arvanitaki – “Mina” for short. She grew up in a small Greek village called Koklas, and since her parents were schoolteachers, she grew up with no shortage of books in the house. Arvanitaki excelled in math and physics – at a very young age she calculated the time it takes for light to travel from the Earth to the Sun. Although she briefly considered becoming an auto mechanic in high school because she loved cars, she decided, “I was more interested in why things are the way they are, not how to make them work.” So she studied physics instead.
Similar reasoning convinced her to shift her focus at the Stanford school from experimental condensed matter physics to theory: She found her course in quantum field theory more sparkling than any of the experimental results she produced in the lab.
Central to Arvanitaki’s approach is a theoretical reimagining of dark matter as more than just a simple particle. A curious quirk of quantum mechanics is that particles exhibit both particle and wave-like behavior, so we’re actually talking about something more akin to a wave packet, according to Arvanitaki. The size of those wave packets is inversely proportional to their mass. “So the elementary particles in our theory don’t have to be small,” she says. “They can be super light, meaning they can be as big as the room or as big as the entire universe.”
Axions are a perfect candidate for dark matter, but they interact so weakly with ordinary matter that they cannot be produced in accelerators. Arvanitaki has proposed several smaller experiments that could succeed in detecting them in ways colliders can’t.
Walls, clocks and bars
One of her experiments is based on atomic clocks – the most accurate timekeeping devices we have, in which the natural frequency oscillations of atoms serve the same purpose as the pendulum in a grandfather clock. An average wristwatch loses about a second every year; atomic clocks are so accurate that the best would lose just one second per age of the universe.
Within its theoretical framework, dark matter particles (including axions) would behave like waves and oscillate at specific frequencies determined by the mass of the particles. Waves of dark matter would also cause the atoms in an atomic clock to oscillate. The effect is very small, but it should be possible to see such oscillations in the data. A pilot study of existing atomic clock data turned up nothing, but Arvanitaki suspects a more dedicated analysis would be more fruitful.
Then there are the so-called “Weber rods,” which are massive aluminum cylinders that Arvanitaki says should ring like a tuning fork when a wavelet of dark matter hits them at just the right frequency. The rods take their name from physicist Joseph Weber, who used them to search for gravitational waves in the 1960s. He claimed to have detected those waves, but no one could replicate his findings, and his scientific reputation never quite recovered from the controversy.
Weber passed away in 2000, but chances are he’d be glad his bars found a new use. Since we don’t know the precise frequency of the dark matter particles we’re hunting, Arvanitaki proposes building a kind of xylophone out of Weber rods. Each bar would be tuned to a different frequency to scan for many different frequencies at once.
Walk through walls
Yet another inventive approach is sending axions through walls. Photons (light) cannot pass through walls – shine a flashlight on a wall and someone on the other side will not be able to see that light. But axions interact so weakly that they can pass through a solid wall. Arvanitaki’s experiment exploits the fact that it should be possible to turn photons into axions and then reverse the process to recover the photons. Place a strong magnetic field in front of that wall and then shine a laser at it. Some photons become axions and pass through the wall. A second magnetic field on the other side of the wall then converts those axions back into photons, which should be easily detected.
This is a new kind of dark matter detection that relies on small lab experiments that are easier to perform (and therefore easier to replicate). They’re also much cheaper than setting up detectors deep underground or trying to produce dark matter particles at the LHC — the largest, most complicated scientific machine ever built, and the most expensive.
“I think this is the future of dark matter detection,” says Kaplan, though both he and Arvanitaki are adamant that this will undermine many ongoing efforts to hunt WIMPs, whether deep underground or in the LHC. , must complement and not replace.
“You have to look everywhere, because there are no guarantees. This is what research is all about,” says Arvanitaki. “What we think is right, and what nature does, can be two different things.”
Jennifer Ouellette is a Los Angeles-based science writer and author of four popular science books.