About 1.3 billion years ago, two black holes in a remote part of the universe collided with one another. The two objects were so massive that the interaction of their gravities distorted the space and time around them. If you could witness the event up close, it would be like living in a funhouse mirror.
One hundred years ago, Albert Einstein predicted that the distortion of spacetime caused by such a collision wouldn't stop at the site of the collision. Like a ripple on a pond, it would propagate outward in gravitational waves.
Today, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced, for the very first time, that they've directly observed a gravitational wave — proving Einstein right yet again.
"We have detected gravitational waves; we did it," David Reitze, executive director of LIGO said in a press conference Thursday to a cheering crowd. "It's exactly what Einstein's theory of general relativity predicted."
"It’s huge; it’s up there with the discovery of the Higgs particle," Cliff Burgess, a particle physicist at McMaster University who is unaffiliated with the LIGO project, tells me. "I bet it will be a Nobel Prize this year, maybe next year. It’s really a first class discovery."
We haven't been able to detect gravitational waves until now because the waves are so incredibly tiny
Up until now, it's been hard to detect gravitational waves because gravity is a relatively feeble force in the universe. (Consider how you can overcome the entire pull of the Earth momentarily by jumping.) And feeble forces produce feeble waves. "This is like a really small ripple on the surface of the water," Burgess says.
Gravitational waves pass through the Earth all the time. We just don't notice them distorting the space around us because by the time they arrive here, their vibrations are on a scale smaller than an atom.
But the LIGO detector is so precisely calibrated that it can detect shifts of less than 1/10,000th the width of an atom. "It’s amazing how they can see anything at all," Burgess says.
LIGO, which is funded by the National Science Foundation, consists of two enormous science experiments: One is in Louisiana; the other is located in Washington state. Both are massive L-shaped tubes. Each arm of the tube is 2.5 miles long.
During the experiments, a laser beam is equally split between the two arms. At the end of each arm is a mirror, which reflects the laser back to the starting point. What LIGO is looking for is evidence that gravitational waves are distorting spacetime enough that one of the arms becomes temporarily longer than the other."You can think of LIGO as the most sensitive ruler ever made," Cosmos Magazine explains.
If a wave is detected in one of the sites, it has to be corroborated with the other (to make sure it's not just a false signal sent by local automobile traffic or other disturbances).
LIGO was first set up in 2002, and for years it found nothing. In 2010 it was shut down for an upgrade, which extended its range. Before the upgrade, it could listen for gravitational waves up to 65 million light-years away. When it was turned back on in 2015, its range was bumped up to 225 million light-years.
On September 14, 2015, just when the upgraded LIGO was just in the process of being turned on, it heard something.
"It's as though we only had seen the surface of the ocean on a very calm day" Kip Thorne, a LIGO co-founder said during the press conference. "All of that changed on September 14. The colliding black holes that produced these gravitational waves created a violent storm in the fabric of space and time."
By listening in on these waves, the scientists were able to reconstruct the cataclysmic event.
"It’s the beginning of a whole new kind of astronomy"
The finding doesn't just confirm a 100-year-old theory — it also opens up the possibility for a "whole new kind of astronomy," Burgess says. People have been looking at the visible light in the night sky for thousands of years. In the past 100 years, we've started to look at it in terms of other types of energy — radio waves, X-rays, ultraviolet, and so on. Each type of energy reveals something new in the night sky.
Physicists are excited by the discovery because it opens the door for telescopes that can "see" or "hear" gravity.
At the press conference, Reitze said that the gravitational waves the scientists recorded from the colliding black holes "proves that binary black holes exist in the universe." And that hasn't been done before. "It's the first time the universe has spoken to us through gravitational waves," Reitze said. "We're going to hear more of these things."
Up until now, "Everything else in astronomy [has been] like the eye," Szabolcs Marka, a LIGO collaborator told the New York Times. "Finally, astronomy grew ears. We never had ears before."
Gravitational telescopes could even allow us to study the gravitational waves left over from the Big Bang. This method of detection would also allow physicists to better understand black holes — which don't radiate light but are thought to radiate waves of gravity.
"If you look with visible light as far as we can look in the universe, the universe is no longer transparent; it becomes opaque," Burgess says. "Almost nothing is opaque to gravity."
Of the discovery, Burgess says, "It’s not the end of something. It’s the beginning of a whole new kind of astronomy."