Through a complex series of equations, Albert Einstein worked out that the acceleration of massive objects should create gravitational waves, or ripples in the fabric of space-time. Such waves propagate across the universe at the speed of light, causing tiny tremors in the atoms that make up matter. Even as Einstein laid out his general theory of relativity a century ago, rewriting rules of the cosmos that had been in place for more than 200 years, he was not entirely certain about these gravitational waves. He said they did not exist, then took it back; 20 years later, he flip-flopped again.
Over generations spent studying Einstein’s theory and equations and developing new technologies, scientists inched closer and closer to confirming the existence of the gravitational waves he predicted. The Laser Interferometer Gravitational Wave Observatory (LIGO) launched its first experiment in 2000, using two detectors located in Livingston, Louisiana, and Hanford, Washington. But the detectors initially weren’t sensitive enough to find evidence of the miniscule disruptions caused by gravitational waves. After undergoing a series of upgrades from 2010-2015, LIGO’s technology went back online, with more powerful lasers and a better system of isolating the detectors from vibrations on the ground. At the time, scientists predicted they would be able to detect the first gravitational waves by 2016.
In fact, early on the morning of September 14, 2015, nearly as soon as the system was up and running, loud signals came through the detectors in both Louisiana and Washington. Each LIGO detector has an L-shaped antenna with two arms, each measuring some two-and-half miles long. At the end of each arm are mirrors of ultra-pure glass isolated from all outside light and vibrations. Near the point where the two arms meet, a beam of laser light is sent out down each arm simultaneously, then bounces off the mirror and is reflected back. If the arms are precisely the same length, the returning beams cancel each other out, and LIGO’s detector sees no light. But a passing gravitational wave will compress one arm of the detector and stretch another. The disruption is tiny—only a fraction of the width of a proton—but it is enough to misalign the reflected laser beams and light up the detector in a rhythmic pattern.
That flickering light, turned into sound waves, produced the distinctive audio chirps that LIGO scientists heard on September 14. The researchers spent the next several months painstakingly investigating potential environmental and instrumental disturbances in order to confirm that the gravitational waves detected were real. Finally, on February 11, they made the official announcement.
What caused the gravitational waves observed by LIGO’s detectors? Scientists say that some 1.4 billion years ago a pair of black holes circled each other in a distant galaxy before finally colliding. The collision of the two black holes—about 29 and 36 times more massive than the sun, respectively—produced a gigantic amount of energy in a fraction of a second, the equivalent of about 50 times the power of the entire visible universe. This energy, in the form of gravitational waves, is still spreading outwards today.
LIGO team member Rainer Weiss, an emeritus professor of physics at the Massachusetts Institute of Technology, first proposed the technology as a means of detecting gravitational waves back in the 1980s. “The description of this observation is beautifully described in the Einstein theory of general relativity formulated 100 years ago,” Weiss said in a statement heralding his team’s historic discovery. “It would have been wonderful to watch Einstein’s face had we been able to tell him.”