In particular, Antipas’ team is using their experiment to search for a class of dark matter known as ultralight dark matter. At its heaviest, an ultralight dark matter particle is still about a trillion times lighter than an electron. According to quantum mechanics, all matter has particle-like and wave-like qualities, with larger objects typically containing more particle-like qualities and smaller ones more wave-like qualities. “When people talk about ultralight dark matter, they mean that dark matter is more like a wave,” says physicist Kathryn Zurek of the California Institute of Technology, who was not involved in the experiment.
Like all other dark matter experiments so far, Antipas’ search has turned up nothing. But their absence of a discovery helps constrain the properties of dark matter, as the experiment shows what dark matter is not. In addition, the team’s approach is distinctive compared to better-known dark matter experiments, which search for particles known as WIMPs (weakly interacting massive particles). These experiments commonly involve collaborations of 100 scientists or more, and the detectors have dramatic technical demands. For example, the LZ detector in South Dakota contains 7 tons of liquid xenon, a rare element found in the atmosphere at less than 1 part per million. 10 million To protect the detectors from unwanted radiation, physicists place them in laboratories deep inside mountains or underground in former mines.
In contrast, Antipas’ entire experiment fits on a tabletop, and his collaboration consisted of 11 scientists. Looking for dark matter was actually a side project for his lab. They usually use the equipment to study the weak nuclear force in atoms, which is responsible for radioactive decay. “This was a quick and interesting thing for us to do,” says Antipas. “We use these methods for other applications.” Compared to WIMP detectors, the tabletop experiments are simple and cost-effective, says Gehrlein.
Over the past decade or so, these tabletop approaches have become increasingly popular for dark matter searches, Zurek says. Physicists who first developed super-precise tools and lasers to study and control single atoms and molecules looked for more ways to use their new machines. “More people were moving into the field, not as their primary discipline, but as a way to find new creative applications for their measurements,” Zurek says. “They can translate their experiments into looking for dark matter.”
In a notable example, physicists recast atomic clocks to look for dark matter instead of timekeeping. These precise machines, which do not lose or gain a second over millions of years, rely on the energy levels of atoms, which are determined by interactions between their nuclei and electrons, which depend on the fundamental constants. Similar to Antipas’ experiment, these researchers searched for dark matter by measuring the energy levels of atoms precisely to look for changes in the values of fundamental constants. (They found none.)
But these relatively minimalist experiments will not replace more conventional dark matter experiments, as the two kinds are sensitive to different hypothetical types – and masses – of dark matter. Theorists have posited a variety of dark matter particles whose masses span more than 75 orders of magnitude, Gehrlein says. At their lightest, the particles can be more than a quadrillion times lighter than even the ultralight dark matter Antipas is looking for. The heaviest dark matter candidates are actually astrophysical objects as large as black holes.
Unfortunately for physicists, their experiments have not provided any hints that make one range of mass more likely than others. “This tells us to look everywhere,” says Gehrlein. With so few leads, dark drug hunters need all the reinforcements they can get.