Science history: Gravitational waves detected, proving Einstein right — Sept. 14, 2015

Ten years ago today, on Sept. 14, physicists detected gravitational waves rippling through the cosmos for the first time.

The roots of this discovery date back a century. Albert Einstein's general relativity predicted that massive objects would warp space-time. When such massive objects accelerate — such as when two black holes collide — they would send ripples through the cosmos, called gravitational waves, he posited.

Einstein never thought we could detect them, because the distortion of space-time caused by these waves would be far tinier than a single atom.

However, in the 1970s, MIT physicist Rainer Weiss, who died in August, proposed it might be possible to detect these tiny ripples from colliding massive black holes.

Key to his scheme was the interferometer, which would split a beam of laser light. From there, the light would travel down two separate paths before bouncing off hanging mirrors and recombining at their source, where a light detector would measure their arrival. Ordinarily, if the paths were the same lengths, these two beams would return at the same time.

But if a gravitational wave was passing by, Weiss reasoned, these beams would be ever-so-slightly out of phase. That's because gravitational waves temporarily smoosh and stretch space-time, thereby creating fluctuations in the length of the passageways through which the laser beams travel.

Weiss, along with Caltech physicist Kip Thorne, proposed the idea of trying to measure these elusive waves. The detector pathways, they argued, needed to be very long to detect such tiny signals. And the project would need two widely spaced detectors to eliminate the possibility that signals came from local disturbances, and to help localize the source of cosmic collisions.

By 1990, the Laser Interferometer Gravitational-Wave Observatory (LIGO) project had been approved, and two identical L-shaped detectors, with arms 2.5 miles (4 kilometers) long, were built in Hanford, Washington and Livingston, Louisiana, respectively.

For years, the detectors found nothing. So LIGO was upgraded to become more sensitive to ever-tinier signals. Much of that entailed protecting the equipment from vibrations caused by nearby traffic, planes or distant earthquakes, which could obscure the signals from the distant universe.

In September 2015, the scientists turned on the upgraded instruments.

Overnight on Sept. 14, researchers at both LIGO sites detected something interesting.

"I got to the computer and I looked at the screen. And lo and behold, there is this incredible picture of the waveform, and it looked like exactly the thing that had been imagined by Einstein," Weiss said in a documentary about the discovery.

It was a strong "chirp," or a fluctuation in the length of the detector arms, and it was a thousand times smaller than the diameter of a nucleus.

On Feb. 11, 2016, scientists announced that the event they'd detected came from the smashup of two massive black holes that collided about 1.3 billion years ago. Europe's gravitational wave experiment, called Virgo, detected the same event.

The discovery ushered in a whole new way to study the universe's most extreme events. Since that first detection, LIGO's detectors, along with its European counterpart experiment Virgo and the Japanese Kamioka Gravitational Wave Detector (KAGRA), have detected around 300 collisions, including triple black hole mergers and the collision of black holes and neutron stars. In June 2023, a team of scientists announced that a faint "gravitational wave background" permeates the universe thanks to pairs of black holes veering toward collision all across space and time. And in September 2025, scientists from the LIGO Collaboration validated Stephen Hawking's decades-old theory about black holes, linking quantum mechanics and general relativity.

Weiss and Thorne, along with their colleague Barry Barish, were awarded the 2017 Nobel Prize for their work.