Proposed dark matter detector watches a billion tiny pendulums swing

Despite hunting for it for decades, scientists have so far been unable to detect any definitive signs of dark matter. But a new detector design, using an array of billions of tiny pendulums, could finally break the silence by searching for the effects of dark matter’s incredibly strong gravitational pull.

Mass and gravity are inextricably linked. The more mass an object has, the stronger its gravitational pull. That’s how we know that dark matter exists in the first place – decades of astronomical observations indicate that there isn’t enough visible mass to account for the gravitational effects we can see. Some other invisible substance must be outnumbering ordinary matter by a ratio of five to one.

Since it doesn’t emit, reflect or interact with light in any way, dark matter has unsurprisingly eluded direct detection so far, but it’s not from a lack of trying. One of the main methods of searching involves a huge tank full of a detector fluid, such as xenon, superfluid helium or supercooled water, and waiting for a reaction that could only mean a dark matter particle has bumped into an atom in the tank.

Other experiments focus on detecting disturbances to electricity and magnetism, which are predicted to be side effects of hypothetical particles called axions. The problem is, these systems rely on presumptions about other properties of dark matter particles. Instead, the new detector design focuses on the one thing we do know about dark matter – its strong gravitational influence.

In the new proposed experiment, a billion tiny pendulums would be suspended in a cubic space some 10 m (32.8 ft) long, with their movement tracked by lasers. The principle sounds simple – if a dark matter particle happens to whiz through the instrument, its mass should briefly pull the nearest pendulums towards it.

Environmental “noise” would make the pendulums wobble around constantly, but a dark matter signal would stand out. Since one of these particles would pass right through unimpeded, you’d see a row of pendulums suddenly disturbed one after the other. Studying how much force a passing particle imparts on each pendulum could also reveal details about dark matter, including the particle’s speed and direction.

The team says this method is promising because it doesn’t rely on dark matter having other properties – only the gravitational influence that’s already known about it. The only unknown factor would be its mass, and this instrument would be sensitive to particles with a wide range of masses, from around one 5,000th of a milligram to a few miligrams. That’s much heavier than the usual range hunted for in experiments.

“Our proposal relies purely on the gravitational coupling, the only coupling we know for sure that exists between dark matter and ordinary luminous matter,” says Daniel Carney, co-author of the study. “if someone builds the experiment we suggest, they either find dark matter or rule out all dark matter candidates over a wide range of possible masses.”

Until somebody does implement the design, the idea remains purely conceptual – but intriguing, nonetheless.

The research was published in the journal Physical Review D. The team describes the concept in the video below.

Source: NIST