What Is a Gravitational Wave Detector, and Why Do We Want to Send One to Space?

By the end of this year, the European Space Agency will launch a demonstration mission that could lead to a whole new way for astrophysicists to observe the Universe. The mission, called "LISA Pathfinder," will test new technology to detect gravitational waves in space.

Gravitational waves are ripples in spacetime. They're a prediction of Albert Einstein's theory of general relativity, but while we've seen indirect evidence they exist, we haven't ever directly detected them.

"We've spent many years observing through the electromagnetic spectrum, and we've learned an enormous amount of information about what's out there," Paul McNamara, project scientist on the LISA Pathfinder mission, told me.

But it's not the full picture. "Imagine watching TV tonight and turning the sound off," Namara said. Being able to study the universe using gravitational waves would be like turning the sound on, adding a new dimension to observations.

Observing the 'Ultraviolent' Universe

McNamara gave the example of two supermassive black at the centre of two galaxies merging, which previously would have been unobservable.

"The energy released in gravitational waves during the last few minutes of the merging of just one pair of supermassive black holes is comparable to the total energy emitted as light by all stars and galaxies across the cosmos over the same time," ESA writes.

"Up until now we've never seen that, because we're looking at it through the electromagnetic spectrum and it gets blocked by intervening galaxy—so dust, the arms, the stars," McNamara explained. "We never see the thing happening at the centre."

"Now, with gravitational waves we can peer right through all that dust and all of the rest of it, and we can see really what's happening in these ultraviolent events in the universe."

Testing at Picometre Precision

But that's getting ahead of the current mission. LISA Pathfinder aims to demonstrate that we can build the necessary technology by testing it on a smaller scale. The eventual LISA (Laser Interferometer Space Antenna) mission—the one that will actually put a functional gravitational wave detector out in space—isn't planned to launch until 2034.

The problem is that, while gravitational waves might originate with extremely violent cosmic events, their measurable effect is teensy. "Because we're measuring such a small effect, if you were to make your detector one metre long, you would have to measure to the order of 10 to the -21 metres change," McNamara said. That's several orders of magnitude less than the size of a single electron. ESA's solution? Build a detector millions of kilometres long. Then they only have to measure changes of "about a millionth of a millionth" of a metre.

These gravitational wave effects will be detected by measuring such tiny changes in the distance between two test masses, which is done using laser interferometry. While the test masses will be about a million kilometres apart in the LISA mission, LISA Pathfinder just aims to demonstrate that it's actually possible to make such tiny measurements, and it will see two test masses launched in the same spacecraft just 38cm apart. The key is to show that the two masses can happily fly through space maintaining their position in relation to each other with picometre precision.

The First Physics Lab in Space

As you can imagine, it's quite a technical challenge. In order to detect the effect of gravitational waves, the test masses can't be disturbed by anything else.

LISA Pathfinder uses two 4.6cm gold-platinum cubes as its test masses. The gold-platinum alloy is used because it's very dense and isn't magnetic. Inside the spacecraft, the cubes are free floating; they're not touching the interior of the craft at all once they're in orbit. The spacecraft around the cubes is basically there to shield them from any interference—we're talking tiny disruptions like photons from sunlight—so they can stay in free fall in the vacuum of space, moved only by gravitational waves.

"We know we can do that very well; we can really isolate it from solar radiation pressure, it's not that difficult," said McNamara. "The big problem is you now have to build a spacecraft which doesn't have its own internal noise."

That means no magnets, no stainless steel (as it's also magnetic), and everything perfectly balanced to avoid external gravity from the spacecraft. "Everything we put on the spacecraft, we know the mass of it," said McNamara. "If we use a cable tie to hold something down, we measure the cable tie before it goes on the spacecraft, we put it on, we cut the bit off you don't want at the end, we measure the bit we cut off, and we subtract it—so you know exactly what you've got left on the spacecraft."

The approach makes the mission more like a physics lab than an astronomy mission, McNamara said. "It will be the first physics lab in space."

LISA Pathfinder is getting packed away in a transport container on Thursday, and will be taken to Europe's spaceport in French Guiana, which it'll launch from towards the end of the year. It will take about two months to get to its final orbit around the L1 Lagrange point, 1.5 million kilometres away between the Earth and the Sun. Once the cubes are released into free fall, there shouldn't be any motion between them if the technology is functioning as planned. The test mission won't actually attempt to detect gravitational waves, as it's not on a large enough scale for them to be measurable.

McNamara's confidence in the mission's success is unshakeable. "I'm very confident; I think we're going to be very surprised how well it works," he said, adding that on-the-ground tests had performed with much greater precision than required.

"We start operating in March," he continued. "I expect in the middle of March we'll be writing papers to say 'look how wonderful we are.'"