September 2015: Physicists detect gravitational waves for the first time

1.3 billion years ago, two black holes collided, each losing part of its mass in a violent burst of energy. That energy rippled through spacetime as a gravitational wave, arriving Earthside on September 14, 2015. Luckily, recently upgraded detectors were online at the Laser Interferometry Gravitational-Wave Observatory’s two sites in the United States, and the remnant of the distant cataclysm was heard.

LIGO scientists were skeptical of the signal at first. No one had ever detected gravitational waves before, but they had been theorized for about 100 years, and the team had spent decades designing and then building, testing, and improving their detectors. The characteristic rising chirp of the black hole collision heralded a new era in astrophysics, proof that black holes exist, and a long-missing piece of experimental proof for the theory of relativity. But first they had to make sure it was real.

Gravitational waves were predicted by Einstein’s general theory of relativity. According to the theory, when masses accelerate, they generate ripples in the stretchy, four-dimensional medium of space-time. But Einstein and many other scientists weren’t sure gravitational waves could ever be measured, or whether they were even real.

Claims of gravitational wave detections in the 1960s were discredited. But in the 1970s, observations of a double pulsar provided powerful indirect evidence. These paired stars emit radiation from their poles as they rotate around each other, growing closer and rotating faster and faster. Joseph Taylor and Russell Hulse showed that as the stars moved closer together, they lost energy at a rate consistent with predictions of how much energy would be lost to gravitational waves in such a system. The two physicists shared the 1993 Nobel Prize in Physics for this work.

Starting in the 1970s, MIT physicist Rainer Weiss made great strides in designing laser interferometers to detect the waves. Weiss collaborated with Caltech’s Kip Thorne and Ronald Drever to hone these designs, improving the sensitivity of the interferometers, learning how to isolate them from noise, and scaling them up.

Their work was made possible by National Science Foundation funding, starting with grants to the Caltech and MIT labs in 1979 and continuing through site selection in 1992, the Advanced LIGO upgrades that extended the observatory’s range and made possible the first gravitational wave detections, and to the present day. “The NSF was the only option,” says MIT physicist Peter Fritschel.

LIGO’s detectors in Washington and Louisiana split laser light at the corner of an L-shaped set of arms, sending identical beams down each arm. At the ends of the arms, mirrors reflect the light back to a detector near the source. If the system is perfectly isolated from surrounding vibration and noise, the two incoming light beams should be identical. But if a ripple in spacetime passes through, it will stretch and squeeze the beams travelling on each arm differently, and each light beam will have travelled a different distance — a difference that’s picked up by the detector.

Detecting gravitational waves directly is devilishly difficult. The spacetime strains caused by gravitational waves like those produced by colliding black holes are very small, on the order of 10-21, says Fritschel, the MIT physicist, who was also the systems architect for Advanced LIGO in 2015 and is currently its chief detector scientist. A gravitational wave passing directly through a 1-kilometer-long detector will change the detector’s length by only 10-17 meters, says Fritschel. And that detector has to be isolated from other vibrations on Earth that are much stronger.

On that busy September day, the LIGO team was still testing their system, “injecting” false gravitational wave signals to make sure their analytics worked. “I happened to have written the software that sent signals in,” says Peter Shawhan, a physicist at the University of Maryland who joined LIGO in 1999. The black hole collision signal, later named GW150914, looked exactly like physicists’ models of gravitational waves produced by such a celestial event. “It seemed too good to be true,” he recalls. Shawhan and others spent the day verifying that no one had injected this apparently perfect signal by accident or as a prank.

The large LIGO team and their collaborators around the world kept the detection under wraps until they were ready to submit their landmark results to Physical Review Letters. The team submitted their paper in January, and it was published less than three weeks later, in February. “It was very stressful in the months we were doing the checks,” says Shawhan. But it was true: GW150914 proved that gravitational waves are real. For this work, Thorne and Weiss would go on to share the 2017 Nobel Prize in Physics with Barry Barish, who joined LIGO in 1994 and shepherded the large and complex building project to its completion. Drever died earlier that year.

Leading up to LIGO’s first detection, physicists weren’t sure what they would see. They had carefully modelled what it would look like when massive binary systems of neutron stars and black holes merged, but they didn’t know how frequent these events were — and they could only estimate how big the black holes would be. In part because they had been observed with other kinds of telescopes, physicists were more focused on neutron star mergers. LIGO has since detected neutron star mergers, but data from the observatory has shown that black holes are more common than they had anticipated, says Shawhan.

If the first signal had been something less clear, perhaps from an unknown and as-yet-unmodelled source, it wouldn’t have grabbed people’s imaginations, says Shawhan. But the first detection looked just like what physicists had predicted for a binary black hole collision, and the wave form could be turned into a sweeping, chirping sound wave that caught people’s imagination. “You can play it through a speaker,” he says. “It was a short, strong signal, and it grabbed people.”

Today, detections have become more routine, but Shawhan says LIGO is “not just another telescope.” LIGO does serve astronomers, but he says, “we’re still testing general relativity, we’re still testing fundamental physics.”

LIGO has now been joined by other detectors Italy’s Virgo observatory and Japan’s KAGRA interferometer, and others, like the Einstein Telescope, are in the works. But LIGO’s interferometers remain the most sensitive. Basic information about collisions of binary black holes and neutron stars is helping physicists understand their populations, how the universe is expanding, and more. But physicists still want to see more. Planned upgrades will help LIGO see events taking place farther away in spacetime, potentially revealing new kinds of signals that haven’t been seen yet.

The success of gravitational wave astronomy depends on collaborations of enormous scale — something Weiss recognized when he accepted the Nobel Prize. The prize, he said, “[recognizes] the work of about 1,000 people, a dedicated effort that’s been going on for, I hate to tell you, as long as 40 years.”

Note on funding: This summer, LIGO’s funding, like that of many NSF projects, now hangs in the balance. The president’s budget request would cut LIGO’s budget by about 40%; the Senate’s appropriations would restore NSF and LIGO funding to 2025 levels, while the House would cut NSF’s budget by 20%, without offering specifics about LIGO. The final numbers will be determined by Congress this year.