Tests Show We Could Build a Humongous Gravitational Wave Detector in Space

The European Space Agency has shown it has the technology to build a humongous gravitational wave observatory in space—with results more than five times better than required.

Researchers published results from the LISA Pathfinder mission in the Physical Review of Letters on Tuesday, which showed their technology-testing project was a great success. This sets the course to build a gravitational wave detector that could revolutionise astronomy.

LISA Pathfinder is a super-nerdy (and super-exciting) mission to test the tech that will go into building a huge gravitational wave observatory affectionately known as LISA (Laser Interferometer Space Antenna). This would consist of two spacecraft millions of kilometres apart that could pick up the teensy-tiny effects of gravitational waves between two test masses, one in each spacecraft. But in order to test the effect of the waves on the masses, they need to be in complete freefall, unaffected by any other forces. The LISA Pathfinder team showed that they could do this.

"When we turned on our instrument, LISA Pathfinder, the performance was stunning," project scientist Paul McNamara told me. "On the very first day of operation we met the LISA Pathfinder requirements, and then we set about making it better. As of now, we're demonstrating the full performance required for a gravitational wave detector."

The LISA Pathfinder instrument consists of two gold-platinum cube test masses less than half a metre apart inside the same spacecraft. As it was considered a stepping stone mission to LISA, McNamara explained that they were only required to get the cubes into an unperturbed freefall with a precision a factor of ten lower than the ultimate LISA requirements. But they smashed that and got close to what would actually be needed in the real detector.

A graph showing the remaining "noise" compared to the original LISA Pathfinder requirements. Image:

ESA/ATG medialab; data: ESA/LISA Pathfinder Collaboration

"We are exceptionally happy," said McNamara. "We no longer have to do the stepping stone approach to get to the next generation, to the big detector; we're there already."

There was much fanfare when the Laser Interferometry Gravitational Wave Observatory (LIGO) in the US announced the first ever direct detection of gravitational waves earlier this year. Most fundamentally, this discovery showed that gravitational waves—which were predicted by Albert Einstein—actually exist.

A space-based detector like LISA would measure a different part of the gravitational wave spectrum, picking up signals from further away and making observations of some of the earliest moments of the Universe.

Despite its impressive performance, McNamara explained that we can't use LISA Pathfinder itself to observe gravitational waves because it's not big enough—with the test masses so close together, any fluctuations from gravitational waves would be way too small to measure.

What it does show is that it's possible to keep the test masses totally isolated from other disruptions—such as sunlight, radiation, and magnetic forces—by using the spacecraft as a constantly-adjusting shield. The team also showed they could measure the relative positions of the masses with great precision. McNamara said it was only possible to do these tests in space, as using magnetic levitation would add its own disruptors and a drop tower would only give a few seconds' freefall when the measurements need to be taken over hours.

LISA Pathfinder will test a different payload from NASA later this month, and then McNamara hopes to get an extension for the mission until May next year to keep pushing its performance.

The full-size gravitational wave observatory—LISA—is currently hoped to launch in 2034.