Could we hunt for gravitational waves from the Moon?

Such an observatory could unlock the cosmos’ secrets

NASA is working to establish a permanent human presence on and around the Moon by the end of the 2020s via a program known as Artemis. That presence may eventually include radio telescopes on the Moon’s exceptionally quiet far side, and perhaps even more ambitious off-Earth science facilities.

A recent study makes the case for building a gravitational-wave observatory on the Moon. Gravitational waves are ripples in space-time created by massive objects. They were predicted by Einstein’s general theory of relativity in 1915, and first directly detected a century later by the Laser Interferometer Gravitational-Wave Observatory (LIGO) consortium.

LIGO has detectors at two sites in the US: one in Louisiana and one in Washington. Each detector is an L-shaped structure with arms four kilometres (2.5 miles) long, with a laser at the centre of the array. The laser shines light down each arm, and mirrors reflect the light back. If the light from one arm is beamed back a little late, it’s evidence of a possible gravitational-wave-induced distortion.

The LIGO team has detected dozens of gravitational-wave events to date, most of them caused by mergers between two black holes. But this is exacting work, and our noisy, active Earth makes spotting signals tough. The Moon, on the other hand, is an exceptionally quiet place.

“The Moon offers an ideal backdrop for the ultimate gravitational-wave observatory since it lacks an atmosphere and noticeable seismic noise, which we must mitigate at great cost for laser interferometers on Earth,” said Avi Loeb, an astronomer at Harvard University. “A lunar observatory would provide unprecedented sensitivity for discovering sources that we do not anticipate and that could inform us of new physics. GLOC could be the jewel in the crown of science on the surface of the Moon.”

‘GLOC’ is short for the Gravitational-wave Lunar Observatory for Cosmology, the name

Loeb and Karan Jani propose for the Moon facility. GLOC would be huge compared to LIGO and other detectors on Earth, featuring arms 40 kilometres (25 miles) long. And it would be incredibly sensitive, capable of spotting gravitational-wave events in nearly 70 per cent of the observable volume of the universe.

GLOC is just an idea at the moment, but Loeb and Jani say they hope to develop a pathfinder mission on the Moon that would test required technologies in the coming years. And if GLOC – or something like it – does end up getting built, it will pay scientific dividends for decades to come.