A long time ago, in a galaxy (our galaxy) far, far away, two supermassive black holes found each other and began orbiting one another in a decaying spiral. By the time they collided, about 1.3 billion years ago, they were moving at about half the speed of light. The shockwave from the collision was sent rippling across the universe through the fabric of spacetime, and five months ago washed over the Earth. Humans were ready. Using a pair of special installations built expressly for the task, physicists were able to detect the gravity waves.
The discovery is proof that Einstein had been right. He predicted the existence of gravity waves 100 years ago, and for the past 40 years we’ve been searching for that proof. The detection was made by the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of gigantic instruments in Hanford, Washington, and Livingston, Louisiana. There had been rumors of the detection in circulation, but yesterday those rumors were confirmed at a press conference in Washington, D.C. where the LIGO team made it official. “We did it!” says David Reitze, a physicist and LIGO executive director at the California Institute of Technology (Caltech) in Pasadena. “All the rumors swirling around out there got most of it right.”
On 14 September 2015, at 9:50:45 universal time—4:50 a.m. in Louisiana and 2:50 a.m. in Washington—LIGO’s automated systems detected the signal. It lasted only a quarter of a second. At first it was assumed that the signal was the result of one of the frequent calibration tests routinely run on the system, but some careful cross checking determined that the signal was genuine and had not come from LIGO itself. The oscillation emerged at a frequency of 35 cycles per second, or Hertz, and sped up to 250 Hz before disappearing 0.25 seconds later. The increasing frequency, or chirp, jibes with two massive bodies spiraling into each other. The 0.007-second delay between the signals in Louisiana and Washington is the right timing for a light-speed wave zipping across both detectors.
LIGO researchers detected a gravitational wave that stretched and compressed space itself by one part in 1021, making the entire Earth expand and contract by 1/100,000 of a nanometer, about the width of an atomic nucleus. The observation puts Einstein’s theory of gravity to an exceptionally rigorous test, and provides positive proof of the existence of black holes. Up to now, it was possible to argue that black holes might not exist and could be explained by other observable phenomena. Not anymore. “It will win a Nobel Prize,” says Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland.
The way LIGO is able to detect compression waves in the fabric of spacetime is via two L-shaped contraptions called interferometers with arms 4 kilometers long each. Mirrors at the ends of each arm form a long resonant cavity, in which laser light of a precise wavelength bounces back and forth, just like sound waves would do in a pipe organ. Where the arms meet, the two beams can overlap. If they have traveled different distances along the arms, the wave shapes of the light will drift out of synch and interfere with each other.
Using this technique, researchers can compare the lengths of the light beams in the two arms to within 1/10,000 the width of a proton. That’s sensitive enough to detect a passing gravitational wave as it stretches the arms by different amounts. The system is so sensitive that every possible form of interference has to be either blocked or filtered out somehow. Regular seismic activity, trucks rumbling over nearby roads and waves crashing on distant shores would all be enough to pollute the data.
How do we know what we’re looking at? Computer simulations – comparing what was recorded with computer simulations shows that the two colliding objects were 29 and 36 times as massive as our sun, spiraling to within 210 kilometers of each other before merging. The collision produced an astounding, invisible explosion, and for a tenth of a second the new unifed black hole shone brighter than all the stars in all the galaxies – but only in gravitational waves. Other stellar explosions called gamma-ray bursts can also briefly outshine the stars, but this supermassive dust up is a new record. Kip Thorne, a gravitational theorist at Caltech who played a leading role in LIGO’s development, says, “It is by far the most powerful explosion humans have ever detected except for the big bang,” he says.
Still, LIGO physicists had to rule out every alternative, including the possibility that the reading was a malicious hoax. “We spent about a month looking at the ways that somebody could spoof a signal,” Reitze says, before deciding it was impossible. For González, making the checks “was a heavy responsibility,” she says. “This was the first detection of gravitational waves, so there was no room for a mistake.”
Proving that gravitational waves exist may not be LIGO’s most important legacy, as there has been compelling indirect evidence for them. In 1974, U.S. astronomers Russell Hulse and Joseph Taylor discovered a pair of radio-emitting neutron stars called pulsars orbiting each other. By timing the pulsars, Taylor and colleague Joel Weisberg demonstrated that they are very slowly spiraling toward each other—as they should if they’re radiating gravitational waves.
It is by far the most powerful explosion humans have ever detected except for the big bang.
The detection event corroborates Einstein’s theory of gravitation. Rainer Weiss, a physicist at the Massachusetts Institute of Technology (MIT) in Cambridge, was the man who came up with the original idea for LIGO. “The things you calculate from Einstein’s theory look exactly like the signal,” he says. “To me, that’s a miracle.”
The detection of gravitational waves marks the culmination of a decades-long quest that began in 1972, when Weiss wrote a paper outlining the basic design of LIGO. In 1979, the National Science Foundation funded research and development work at both MIT and Caltech, and LIGO construction began in 1994. The $272 million instruments started taking data in 2001, although it was not until the upgrade that physicists expected a signal.
LIGO researchers are still analyzing data from their first observing run with their newly upgraded detectors, which ended 12 January, but they plan to start taking data again in July. A third interferometer might be added to the abilities of the LIGO array; a team in Italy hopes to turn on its rebuilt VIRGO detector—an interferometer with 3-kilometer arms—later this year.
The elusiveness of the gravity wave has been due primarily to their nature. They are not classical photonic or electromagnetic waves the way light and radio waves are. They are ripples traveling through the foundation of existence itself. Humanity as a species has taken its first step into a larger universe.
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