How we could turn the entire moon into a gravitational wave detector

Gravitational wave detectors on Earth have already revolutionized astronomy and in the next decade we will have LISA, the first gravity observatory in deep space. But there is room to build a detector between these two types, both in terms of location and frequencies. This can be done by building it on the moon.

The idea for a lunar gravity wave detector was first proposed during the Apollo program. Now, in a new paper, scientists explore three options for building gravitational wave observatories on the moon, including turning the moon itself into a detector. As they note, there are plenty of challenges and the technology isn’t quite there yet, but the potential is enormous – just like the antenna itself.

Gravitational wave detectors can measure the small distortions of space-time caused by gravitational waves created when massive objects move and merge. One of the proposals, from senior author Jan Harms of the Gran Sasso Science Institute, is the Lunar Gravitational Wave Antenna, which aims to use the deformation of the moon as a gravitational wave passes through it using highly sensitive seismometers.

‘The gravitational wave would come from very far away, like the early universe. [I]That would come to the moon and make it vibrate like a bell,” Harms told IFLScience. “Then you can essentially put sensors on the surface of the moon to measure its deformations or vibrations.”

Another proposal suggests using mirrors and lasers instead of seismometers to measure the smallest deformations, while the third proposal focuses not on the moon as a detector, but as an ideal location to build a detector, just like the Ligo and Virgo detectors we have on Earth.

The laser interferometers that LIGO and Virgo use are large L-shaped vacuum tubes through which a laser beam is sent. The light would take the same amount of time to travel through both tubes. However, if a gravitational wave passes through it, it will be shorter than an atom, enough for the detector to confirm the event.

To achieve such delicate precision, the mirrors that reflect the lasers are attached to devices called super dampers, which can reduce the planet’s shaking by a trillion times. The moon may have moonquakes, but it is the most seismically quiet place in the solar system, so the accuracy would be even higher.

To maximize the sensitivity of these devices, they must also be cooled to extreme temperatures. The moon has some incredibly cold areas, such as the bottoms of craters at the poles, where sunlight never reaches. However, this is also one of the challenges identified in the research: if the detector is placed in craters that are always in darkness, how can it be supplied with power? Lunar missions are usually powered by solar panels, and we saw what happened earlier this year when that went wrong with Japan’s SLIM lunar lander.

Controlling the experiment remotely will also be a problem unless we plan many permanent moon bases. There are also challenges in creating seismometers, laser systems and high-performance mechanics that would operate with the high accuracy required. So these detectors are not ready yet, but the team is confident that the challenges can be overcome. It may not be a quick turnaround, but it is certainly possible, and there is a very compelling scientific argument for doing so.

“This attempt to realize detectors closes the gap between what we want to do on Earth and what we can do in space. This is going to open up a huge scientific case that we can’t get any other way,” Harms told IFLScience.

Gravitational wave detectors on the moon would detect gravitational waves in the deciHertz range instead of the 1 to 100s Hertz range on Earth. These waves are produced by the merger of binary neutron stars, months or even years before their collision. It could also help us find more precise locations of massive black hole binaries. Given that gravitational wave detectors on the moon would have long lifetimes and would not be start-and-stop experiments, they could provide broader searches for gravitational wave events and test general relativity even further.

The study was published in the journal Philosophical Transactions Of The Royal Society A.

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