Scientists just detected gravitational waves—the last big puzzle piece of Einstein’s theory of relativity

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Today (Feb. 11), researchers from the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they have finally detected gravitational waves in space. The research was simultaneously published (paywall) in Physical Review Letters.

Gravitational waves are ripples in space-time. They result from massive—like, black-hole massive—objects accelerating through space, like the waves we see in water when a stone disturbs it. They spread infinitely outward at the speed of light.

Einstein first predicted these waves 100 years ago with his general theory of relativity. Though all the important aspects of this theory, including mass-energy equivalence, have been proven so far, it’s taken researchers until now to show that these waves exist.

The trouble with finding them is that they’re ordinarily too small to detect. As the Guardian puts it, even a gravitational wave from something four light years away would move space by just a fraction of the width of an atomic nucleus. The closest star outside of our own solar system is just 4.24 light years away, but ordinary stars aren’t nearly big or fast-moving enough to generate any gravitational waves we could hope to detect.

The waves the LIGO team picked up were the result of two massive black holes colliding. Each black hole was about 150 km (90 miles) in diameter, 30 times the mass of the sun, and traveling at half the speed of light. The collision occurred 1.3 billion years ago.

To detect them, the LIGO team built two detectors: one each in Livingston, Louisiana and Hanford, Washington. Each detector consisted of two 4-km long tunnels at right angles. Each tunnel was emptied of all air. A laser beam was split in two, and each beam was sent down one tunnel and reflected back, where they were recombined.

Gravitational waves stretch and then contract spacetime as they pass, just as ripples on a pond move the surface up and then down. As they passed through the LIGO team’s lasers, they changed, ever so slightly, the times it took each laser to travel those four kilometers. The distortion would only be as wide as a fraction of the width of a single atom. But that was enough to change the pattern of the laser light where the two beams were recombined.

Each of the detectors in Louisiana and Washington would have spotted this change, with a minor delay in time as the gravitational wave passed from one LIGO detector to another, thus confirming that it wasn’t just a local distortion, but an ancient echo of the universe passing through our planet.

Finding these waves gives scientists a new way to look at the universe. It could, for instance, give us clues about what happened tiny fractions of a second after the Big Bang, an explosion whose waves must still be rippling through the fabric of our reality.