What do gravitational wave detectors measure?

What are the gravitational wave detectors actually recording? Seems like a nonsensical question at first. LIGO and the follow on detectors are nothing short of remarkable, both in their engineering and their science discoveries. I just do not understand the moment they are recording. My question in more detail is this, are the waves of blackhole mergers that they are detecting from the moment the two event horizons impact/merge or from when the masses at the centre of the blackholes are merging? 

– Marco

For those of us who skipped high school physics, let’s start at the beginning: what is a gravitational wave?

“They’re really kind of like a ripple in the fabric of spacetime,” says Adam Deller, an astrophysicist from Swinburne University of Technology.

Our current understanding of how gravity works is based on Einstein’s general theory of relativity – which turned 100 years old back in 2015. The theory is the best way scientists currently have of describing the relationship between mass, spacetime and gravity.

According to Einstein’s theory, massive objects cause spacetime to curve, which then affects how the massive objects can move through it.

“People often summarise it as ‘mass tells spacetime how to curve, and curved spacetime tells mass how to move’,” Deller explains.

He says to think of spacetime as a rubber sheet with a massive object – say, a heavy ball – sitting on top of it. The ball will weigh down the sheet and cause it to buckle or curve. Then, if you try to roll another ball across the sheet, it won’t travel in a straight line, but move in a pathway that follows the curve created by the mass of the first ball. That’s gravity.

Gravitational waves are a natural consequence of this relationship between spacetime and mass.

“If you start the masses moving around, then the curved spacetime starts moving around as well, and you get these kind of little wobbles, which are gravitational waves,” Deller says.

Everything that has mass and accelerates produces gravitational waves, but unless the mass and acceleration is very large, the waves that are produced are too tiny to detect.

Luckily for physicists, space is full of very massive objects that do undergo tremendous accelerations – think supernovae or indeed black hole mergers, which is when two black holes orbiting each other in a binary system eventually collide and merge to form a single black hole.

“Those two black holes in the process of merging lose a lot of energy radiated away as gravitational waves,” Deller says.

“There’s nothing for free – you’re creating something like a very powerful gravitational wave [and] that comes from the energy of the two black holes.”

The event horizon can be thought of as the ‘edge’ of the black hole – it’s the distance at which you would need to be moving at the speed of light to get away from the black hole.

At the very centre of a black hole, mass is crammed into a tiny point – but since nothing that passes the event horizon can ever escape from a black hole, we’re a little unsure as to what it looks like.

So, back to Marco’s question: do the gravitational waves detected from black hole mergers come from when the event horizons meet, or when the masses at the centres of the black holes merge? It turns out that it’s a bit of both.

“The answer to that question is that it happens all along,” Deller explains.

“As the two black holes are circling around each other, kind of like a death spiral, the gravitational waves get emitted the whole time,” Deller explains.

The size of the gravitational waves peaks when the event horizons touch, and then decays in a process known as “the ring-down”, explains Matthew Bailes, a professor at Swinburne University of Technology and director of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

And how do gravitational wave detectors actually measure this happening?

The key technique for detecting gravitational waves that reach the Earth is called interferometry – that’s where places like the Laser Interferometer Gravitational-Wave Observatory, or LIGO, come in.

In brief, a laser beam is split in half and shone down two long tunnels positioned at right angles. At the ends of the tunnels, the light is bounced off a mirror and reflected back to where it came from, where the two halves are recombined.

Gravitational waves cause spacetime to stretch and compress a bit, slightly moving the mirrors and causing one half of the laser beam to appear to travel a slightly longer distance, and the other half a bit shorter. If there are no gravitational waves, the paths of two halves of the laser will look identical.

Because the gravitational waves come from so far away and are so tiny by the time they reach Earth, the interferometers must be built in a vacuum so that nothing else disturbs the mirrors.

“What you’re looking for is a change is the position of the mirror that’s a fraction of a width of a proton,” Deller says.

So now we know: that’s what gravitational wave detectors are recording.

More on black holes and gravitational waves:

Gravitational waves from black holes swallowing neutron stars

More gravitational waves detected than ever before

Could gravitational waves help us find dark matter?

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