YESTERDAY’S announcement of a direct detection of gravitational waves by LIGO (the Laser Interferometric Gravitational Observatory) is a triumph for the international community that has been working on this detection for the past 25 years. While rumours have been circulating for the past couple of months, it actually wasn’t immediately clear to physicists that the signal was real. They have been working away in the background in order to make absolutely sure that they have found what they think they’ve found.
Gravitational waves are ripples in the fabric of spacetime caused by huge catastrophic astronomical events like supernova explosions and colliding black holes. They were originally predicted by Albert Einstein, when he came up with his general theory of relativity, a theory that was published 100 years ago.
Einstein said that anything with mass will warp the fabric of spacetime. A planet will cause spacetime to curve around it, a star will curve space even more. We know that curvature as gravity. When a star curves spacetime, it causes planets, like the Earth, to follow that curvature when they move around it in orbit.
The result of spacetime having curvature like this is significant. It means that huge, catastrophic astronomical events, like colliding black holes or supernova explosions, can cause shockwaves. As the shockwaves travel they distort everything in their path by just a tiny amount. The distortions are excruciatingly small, but they’re long lived and, eventually, they can be detected on the Earth, provided you have a sensitive enough detector.
Enter LIGO: This set of three detectors in Washington State and Louisiana in the US is the result of 25 years of development, with thousands of scientists from around the globe. The idea is simple enough: a laser beam is split in two and half of the light is sent in one direction, while the other half is sent in another. These light beams bounce off mirrors four kilometres away from the laser beams’ origin and rush back to meet each other near where they started. Exactly how long it takes for them to get back is very carefully recorded and, in this way, the LIGO scientists know how far away the mirrors are to an extreme level of precision.
When a gravitational wave passes over LIGO, it distorts the detector and moves the mirrors. The change in how long it takes the laser beams to bounce off the mirrors is carefully recorded. It’s that simple, and it would be easy, except that almost anything could cause LIGOs mirrors to move, resulting in a much bigger effect than that which would be caused by an astronomical event. It has taken 25 years of very hard work to isolate those mirrors from everything else going on. It’s only now that the detector, now known as Advanced LIGO, is sensitive enough to pick up the real deal.
This isn’t just an impressive result for precision science, though. The long-awaited discovery will revolutionise astronomy. The gravitational wave that was detected on September 14, came from two black holes merging, but that’s only one phenomenon that can cause them. Up until now, we’ve never had real confirmation that black holes even exist, and that is typical for the kind of event that causes gravitational waves. In the next few months and years, we can expect to learn a lot about our Universe, and our place in it.
Rebecca Douglas is a Science Communicator and Gravitational Wave researcher
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