Current experiments that search for dark matter are mostly based on the idea that dark matter should somehow show up, i.e. by interacting with normal matter. Now, however, the only thing we really know about this important part of the universe is that an interaction with normal matter occurs via gravity. This is what first made researchers realize that dark matter exists.

Is it perhaps because the search has so far been fruitless? Then a method could help, which researchers of the NIST and their colleagues presented now. “Our proposal is based purely on gravitational coupling, the only coupling that we know for sure exists between dark matter and ordinary luminous matter,” says study co-author Daniel Carney. The researchers, who include Jacob Taylor of NIST, JQI and QuICS, Sohitri Ghosh of JQI and QuICS, and Gordan Krnjaic of the Fermi National Accelerator Laboratory, calculate that their method can search for dark matter particles with a minimum mass that is about half the mass of a grain of salt or about a billion times the mass of a proton.

Since the only unknown in the experiment is the mass of the dark matter particle and not how it couples to ordinary matter, “someone who builds the experiment we propose will either find dark matter or exclude all dark matter candidates over a wide range of possible masses,” Carney said. The experiment would be sensitive to particles in the range of about 1/5,000 of a milligram to a few milligrams.

This mass scale is particularly interesting because it includes the so-called Planck mass, a mass quantity determined by three fundamental natural constants alone, which corresponds to about 1/5,000 of a milligram. Why is the Planck mass interesting? A black hole of the smallest possible size, whose Schwartzschild radius would correspond to the Planck length, would have the mass of the Planck mass. So if dark matter consists of tiny black holes from the primeval times of the universe, it would be possible to detect it with this method.

Carney, Taylor and their colleagues propose two schemes for their experiment on dark matter with gravity. Both consist of millimeter-sized mechanical devices that function as extremely sensitive gravitational detectors. The sensors would be cooled to temperatures just above absolute zero to minimize heat-induced noise, and shielded from cosmic radiation and other sources of radioactivity.

In one scenario, a myriad of highly sensitive pendulums would each be slightly deflected in response to the pull of a passing dark matter particle. In another strategy, the researchers suggest using spheres floating through a magnetic field or spheres suspended by laser light. In this scheme, the levitation is switched off at the beginning of the experiment, so that the spheres or beads are in free fall. The gravity of a passing dark matter particle would then only slightly disturb the path of the freely falling objects.

“We use the motion of the objects as our signal,” says Taylor. The researchers calculate that an array of about one billion tiny mechanical sensors distributed over a cubic meter is needed to distinguish a real dark matter particle from an ordinary particle or from random electrical noise or “noise” that would trigger a false alarm in the sensors. Ordinary subatomic particles such as neutrons (which interact by a non-gravitational force) would get stuck in the detector. In contrast, scientists expect that a dark matter particle buzzing past the array like a miniature asteroid would shake each detector back and forth through gravity one after the other on its way. As a bonus, the coordinated motion of the billions of detectors would reveal the direction in which the Dark Matter particle is moving.

To produce so many tiny sensors, the team suggests that the researchers could use techniques that the smartphone and automotive industries already use to produce a large number of mechanical detectors. Thanks to the sensitivity of the individual detectors, researchers using this technology do not have to limit themselves to the dark side. A smaller version of the same experiment could detect both the weak forces of seismic waves and the weak forces of the passage of ordinary subatomic particles such as neutrinos and single, low-energy photons.