Detection of Gravitational Waves Opens a “New Window” for Astronomy

The astounding observation of ripples in space-time was predicted by Einstein’s theory of relativity.

Astronomers observed the ripples in space-time caused by gravitational waves from two black holes colliding. (Artist's impression: LIGO)

It’s only February, but we already need to make updates to one of our six space stories to watch in 2016.

“Ladies and gentlemen, we have detected gravitational waves. We did it,” said David Reitze, the executive director of LIGO, to a packed and palpably giddy room at the National Press Club in Washington, D.C. this morning. The two locations that make up the Laser Interferometer Gravitational-wave Observatories—in Richland, Washington and Livingston, Louisiana—detected a signal last September from a binary black hole system that merged 1.3 billion years ago.

That merger was spectacular in itself: Two black holes, each about 30 times the mass of our sun, whipped around each other at half the speed of light, closer and closer until they collided in an impact that generated a peak “power output that was 50 times greater than all of the power put out by all of the stars in the universe put together,” explained Kip Thorne, one of LIGO’s co-founders. But, since the merger happened so quickly—a mere 20 milliseconds—its actual output was about three solar masses of energy. “It’s mind-boggling,” Reitze aptly summed up.

The collision created “a violent storm in space-time,” said Thorne. And it’s the observation of this storm that was the real story today. Scientists have for the first time detected the signal of a black hole collision in the form of gravitational waves. (Read our feature from last summer on gravitational waves and the people and observatories built to find them.) These disturbances in space-time were predicted by Albert Einstein’s theory of relativity, which this astounding observation has now directly confirmed. 

LIGO uses a method called interferometry. A laser is shined into a beam splitter, which sends each half of the beam down perpendicular, two-and-a-half-mile-long arms of the observatory, where it hits a mirror, bounces back to its starting point, and is recombined. Under normal circumstances, the light waves cancel each other out and the detector won’t see anything. But if a gravitational wave passes by Earth, it disturbs space-time and causes a “strain” on the mirror—essentially, it changes the distance the light has to travel, thus shifting the light waves very slightly. When the beams recombine, they no longer cancel each other out, and light appears at the detector. The wave produced by this black hole merger over a billion years ago caused a strain only about 1/1,000th the size of a proton.

Though scientists have known about gravitational waves since 1916—Einstein published his paper on them exactly 100 years ago—not until now has the technology been available to create a detector so sensitive.

And LIGO is only warming up. New modifications are already being implemented; eventually it will be three times as sensitive as it is today.

Scientists have “opened a new window” in the field of astronomy, as Reitze said, like optical, radio, and X-ray astronomy before it, and the incredible discoveries that followed from those observations. Astronomers hope to observe these space-time disturbances made from all sorts of exotic interactions in the universe—supernovas, neutron stars, even from the rapid expansion of spacetime from just after the Big Bang. And this new window might show us something even weirder: “Gravitational waves are so radically different than electromagnetic waves, I think we’re going to be really surprised what we find.”  

Animation showing how LIGO’s interferometers respond to a gravitational wave. (Video: National Science Foundation)
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