How scientists detected gravitational waves
Scientists who have spent decades peering into outer space announced Thursday they have detected gravitational waves, the ripples in spacetime that Albert Einstein long ago predicted.
“We have detected gravitational waves. We did it!” David Reitze, a physicist and executive director of the LIGO Scientific Collaboration, announced to applause at the National Press Club in Washington.
Gravitational waves, often said to look like ripples in a pond, will help scientists answer questions about creation of astronomical phenomena and disturbances, such as the merging of black holes, collision of neutron stars, supernova explosions and more. Gravitational waves were predicted by physicist Albert Einstein’s general theory of relativity 100 years ago.
All of Einstein’s theory had been proved except for the presence of gravitational waves, but that all changed Thursday.
“It’s mind-boggling,” Reitze said.
Einstein was right, said Rainer Weiss, co-founder of LIGO and a professor of physics emeritus at the Massachusetts Institute of Technology.
“His equations have worked so well, in ways he never could have imagined,” Weiss said.
The discovery might be one of the major scientific discoveries in decades, Reitze said.
“As we open a new window into astronomy, we may see things we’ve never seen before,” Reitze said.
Penn State physics theorist Abhay Ashtekar, who wasn’t part of the discovery team, said: “It’s really comparable only to Galileo taking up the telescope and looking at the planets.”
Noted physicist Stephen Hawking congratulated the LIGO team, telling the BBC: “Gravitational waves provide a completely new way of looking at the universe. The ability to detect them has the potential to revolutionize astronomy.”
The waves were detected by LIGO, the Laser Interferometer Gravitational-Wave Observatory, which has facilities in Hanford, Wash., and Livingston, La.
Reitze described how, last Sept. 14, tiny blips of a signal, a “chirp,” were detected seven milliseconds apart by the massive observatories in Louisiana and Washington state. That signal led scientists to the collision of two black holes more than a billion years ago.
“Up until now, we have been deaf to the universe,” Reitze said. “Today we were able to hear for the first time.”
These black holes were each about 93 miles in diameter — roughly 50 miles wider than Washington, D.C.
“Pack 30 times the mass of the sun into that, then accelerate it to about half the speed of light,” and that is just for one black hole, Reitze said.
That collision unnerved nearby stars and caused ripples that spread outward, traveling 1.3 billion light-years, passing through stars and other objects, until they reached Earth and were detected that September day. It was the exact way Einstein had predicted that gravitational waves would be discovered.
“The gravitational waves detected agree perfectly with predictions from Einstein’s theory of relativity,” said Kip Thorne, a co-founder of LIGO and a consultant for the 2014 movie “Interstellar.”
The waves were so tiny, Reitze said, that only LIGO can measure them. “It’s like trying to measure something that is 1/10,000th the diameter of a proton.”
The researchers said they had all been in shock when they got the first reading in Louisiana, and they couldn’t be sure LIGO was reading gravitational waves, not just environmental noise, until they could examine a second reading at the other observatory.
“We know it’s real because seven milliseconds later we saw the same (reading) in the Hanford detector,” said Gabriela Gonzalez, a physicist at Lousiana State University and spokeswoman for the LIGO Scientific Collaboration. “The signals grow in frequency and amplitude and then settle down. That’s the prediction we know from solving Einstein’s theory.”
This detection also proves that binary black holes — a system of two black holes orbiting each other — can exist, Reitze said.
“This is the first time a binary black hole has been directly observed,” Reitze said.
Scientists had found indirect proof of gravitational waves in the 1970s by studying the orbits of two colliding stars, and the work was honored as part of the 1993 Nobel Prize in physics. But now scientists can say they have actually detected a gravitational wave.
“It’s one thing to know soundwaves exist, but it’s another to actually hear Beethoven’s Fifth Symphony,” said Marc Kamionkowski, a physicist at Johns Hopkins University who wasn’t part of the discovery team. “In this case, we’re actually getting to hear black holes merging.”
Detecting gravitational waves is so difficult that when Einstein theorized them, he figured scientists would never be able to hear them. In fact, the greatest scientific mind of the 20th century came to doubt himself in the 1930s and questioned whether such waves really do exist.
In 1979, the National Science Foundation sought a way to detect the waves. Eventually, the two LIGO detectors were built and turned on in 2001. But after years with no luck, scientists realized they had to build a much more sensitive system, which was turned on last September.
Sensitivity is crucial because the stretching and squeezing of spacetime by gravitational waves is incredibly tiny. Essentially, LIGO detects waves that pull and compress the entire Milky Way galaxy “by the width of your thumb,” said team member Chad Hanna of Pennsylvania State University.
Until now, Thorne said, scientists have seen spacetime only as if it were the surface of a calm ocean. Now, he said, they’re seeing a storm: the collision of the black holes, a 20-millisecond event that briefly generated 50 times the power of all stars in the universe put together.
LIGO can measure this astronomical storm using two L-shaped lasers, two mirrors and a detector. The light from the lasers bounces off a mirror to the detector. When a gravitational wave passes by, the path of the laser stretches slightly and hits the detector a little differently.
“All of this technology wasn’t available to Einstein,” Weiss said.
And this detection is just the beginning, Gonzalez said. “Now that we have detectors, now that we know it’s out there, we'll be listening to the universe.”
At best, LIGO in its current state is at a third of its maximum sensitivity, Weiss said. “Over years, the noise level will be brought down, and LIGO will be three times better and see three times farther,” Weiss said.
Scientists all over the world are working on developing laser detectors like LIGO.
“It took a worldwide village to do this,” Gonzalez said.
The more interferometers there are across the world measuring gravitational waves, the easier it will be to find the black holes or other astronomical disturbances in the universe, Gonzalez said.
Thorne, the Cal Tech physicist who co-founded LIGO and has been working on gravitational waves for more than half a century, said he kept the secret even from his wife until just a few days ago. When he heard about the wave, he said, “it was just sort of a sigh of happiness.”
Despite the growing understanding of how warped spacetime behaves, Thorne said, scientists aren’t dabbling in the realm of science fiction yet. “I don’t think (LIGO) is going to bring us any closer to time travel,” Thorne said with a chuckle. “LIGO is heading in a different direction.”
Scientists have been working on detecting gravitational waves for 40 years, largely with the support of national science grants. Reitze thanked “U.S. taxpayers and Congress, who supported this research.”
“We’re seeing our universe through new eyes in an entirely new way,” said France Cordova, National Science Foundation director. “Einstein would have been beaming.”
The Associated Press contributed to this report.