In 1915, Albert Einstein proposed one of his most revolutionary ideas, the Theory of General Relativity. According to this theory, massive objects like stars and planets can bend spacetime, warping the very fabric of reality. A year later, he predicted that very large disturbances, like black holes and supernovas, can produce waves in spacetime that can spread for millions of light years. Scientists have been searching for these “gravitational waves” for decades, but have never seen them directly. Until now.
“Ladies and gentlemen, we have detected gravitational waves! We did it!” said David Reitze, Executive Director of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in a press conference this morning.
The announcement that the LIGO experiment had made the first ever confirmed detection of a gravitational wave might be the biggest scientific announcement since the discovery of the Higgs boson in 2012. Effectively, this announcement provides the first direct proof that gravitational waves exist, and that gravitational wave astronomy is possible.
The LIGO discovery was initially made in September, very soon after the LIGO observatories had undergone a massive upgrade, and the results were published today in a paper in the journal Physical Review Letters, with follow-up papers planned later.
The Hunt for Gravitational Waves
Gravitational waves are distortions in spacetime caused by extreme astronomical events, like black holes merging or supernova explosions. These events cause spacetime to stretch and compress itself, producing vibrations that can be detected with very sensitive equipment. These gravitational waves were first indirectly measured in 1974, when researchers found a pair of orbiting neutron stars that were losing energy at the same rate predicted by Einstein’s theory of relativity, but they were unable to measure the waves themselves.
Finding gravitational waves requires a lot of very precise equipment. In the 1990’s, the NSF funded a joint project between MIT and Caltech, to build two gigantic gravitational wave observatories in Washington and Louisiana. These observatories, called LIGO-Hartford and LIGO-Livingston, respectively, set out to find the smallest signal in the universe.
The LIGO detectors work by shining a laser beam through a beam-splitter, which splits the beam in two, and sends the laser beams down two paths at a right angle. Each beam hits a mirror and gets reflected back to the source, where the beam-splitter works in reverse to recombine them. If the two laser beams have travelled exactly the same distance when they’re recombined, they cancel each other out at the detector. But if a gravitational wave distorts spacetime, then one of the beams will travel slightly farther than the other, and the detector will measure a signal.
The scale of the distortion is incredibly small, so the detector has to be incredibly large in order to compensate. The LIGO observatories are the largest interferometers ever built, measuring 4 kilometers on each side. Even with such large interferometers, though, the distortion the LIGO team had to measure is extremely small, less than a billionth of the width of an atom. To get the sensitivity required to make these kind of measurements, the observatories have the smoothest mirrors ever made, and the lasers travel down tunnels that have been pumped almost completely free of air, to make some of the largest vacuum chambers in the world.
Finding the Signal
The LIGO experiment ran for almost a decade without finding any gravitational waves at all, before it got a massive upgrade starting in 2010. When the upgrades were finished in May of last year, the scientists were confident that they would soon find what they were looking for, but nobody knew just how soon that would be.
On September 14, while the detectors were still in the testing phase, both the Hartford and Livingston observatories recorded a strong signal. The signal was spotted first by the Livingston observatory, at about 4 am local time, and then by the Hartford site a few milliseconds later. This slight delay allowed the scientists to determine roughly where the signal was coming from.
“Having two detectors is like having two ears,” said Gabriela González, spokesperson for the LIGO Scientific Collaboration.
This signal was soon determined to be from a pair of rotating black holes. The black holes are located somewhere in the southern part of the sky, about 1.3 billion light years away. Both black holes are about 150 kilometers (about 90 miles) in diameter, and have a mass of more than twenty times that of our Sun.
The black holes orbited around each other, spiraling closer and closer until they collided and merged into a single black hole about sixty times our Sun’s mass. This entire process took about 20 milliseconds, and the black holes were travelling at about half the speed of light. For that brief moment, the black hole collision produced a power output fifty times that of the entire visible universe.
What Does This Mean?
This discovery provides yet another proof of Einstein’s theory, and launches astronomy into a brand new era. “As we open a new window in astronomy, we may see things we’ve never seen before,” says Reitze. Many others in the press conference compared this discovery to the moment when Galileo first looked through a telescope.
Gravitational wave astronomy offers a number of advantages to scientists over traditional astronomy. Gravitational waves aren’t blocked by dust or gas clouds, allowing astronomers to see right through them into places they couldn’t before. And gravitational waves let astronomers “see” objects that don’t give off light, like dark matter and black holes, and could help us understand them better. Ultimately, however, the full implications of this discovery are impossible to predict. This “new window” is just opening, and it’s hard to say what we’ll find on the other side.