On September 14, 2015, a signal arrived on Earth, carrying information about a pair of remote black holes that had spiraled together and merged. The signal had traveled about 1.3 billion years to reach us at the speed of light—but it was not made of light. It was a different kind of signal: a quivering of space-time called gravitational waves first predicted by Albert Einstein 100 years prior.
a signal arrived on Earth, carrying information about a pair of remote black holes that had spiraled together and merged. The signal had traveled about 1.3 billion years to reach us at the speed of light—but it was not made of light. It was a different kind of signal: a quivering of space-time called gravitational waves first predicted by Albert Einstein 100 years prior.
On that day 10 years ago, the twin detectors of the US National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO) made the first-ever direct detection of gravitational waves, whispers in the cosmos that had gone unheard until that moment.
The historic discovery meant that researchers could now sense the universe through three different means. While light waves, such as X-rays, optical, radio, and other wavelengths of light as well as high-energy particles called cosmic rays and neutrinos had been captured before, this was the first time anyone had witnessed a cosmic event through its gravitational warping of space-time.
For this achievement, first dreamed up more than 40 years prior, three of the team’s founders won the 2017 Nobel Prize in Physics: MIT’s Rainer Weiss, professor of physics, emeritus (who recently passed away at age 92); Caltech’s Barry Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus; and Caltech’s Kip Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus.
Today, LIGO, which consists of detectors in both Hanford, Washington and Livingston, Louisiana, routinely observes roughly one black hole merger every three days. LIGO now operates in coordination with two international partners, the Virgo gravitational-wave detector in Italy and KAGRA in Japan.
Together, the gravitational-wave-hunting network, known as the LVK (LIGO, Virgo, KAGRA), has captured a total of about 300 black hole mergers, some of which are confirmed while others await further analysis. During the network’s current science run, the fourth since the first run in 2015, the LVK has discovered about 220 candidate black hole mergers, more than double the number caught in the first three runs.
The dramatic rise in the number of LVK discoveries over the past decade is owed to several improvements to their detectors—some of which involve cutting-edge quantum precision engineering. The LVK detectors remain by far the most precise rulers for making measurements ever created by humans.
The space-time distortions induced by gravitational waves are incredibly minuscule. For instance, LIGO detects changes in space-time smaller than 1/10,000 the width of a proton. That’s 700 trillion times smaller than the width of a human hair.
«Rai Weiss proposed the concept of LIGO in 1972, and I thought ‘this doesn’t have much chance at all of working'», recalls Thorne, an expert on the theory of black holes.
«It took me three years of thinking about it on and off and discussing ideas with Rai and Vladimir Braginsky [a Russian physicist], to be convinced this had a significant possibility of success. The technical difficulty of reducing the unwanted noise that interferes with the desired signal was enormous. We had to invent a whole new technology. NSF was just superb at shepherding this project through technical reviews and hurdles.»
MIT’s Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics and dean of the School of Science, says that the challenges the team overcame to make the first discovery are still very much at play.
«From the exquisite precision of the LIGO detectors, to the astrophysical theories of gravitational-wave sources, to the complex data analyses, all these hurdles had to be overcome, and we continue to improve in all of these areas. As the detectors get better, we hunger for farther, fainter sources. LIGO continues to be a technological marvel.»
LIGO’s improved sensitivity is exemplified in a recent discovery of a black hole merger referred to as GW250114 (the numbers denote the date the gravitational-wave signal arrived at Earth: January 14, 2025).
The event was not that different from LIGO’s first-ever detection (called GW150914)—both involve colliding black holes about 1.3 billion light-years away with masses between 30 to 40 times that of our sun. But thanks to 10 years of technological advances reducing instrumental noise, the GW250114 signal is dramatically clearer.
«We can hear it loud and clear, and that lets us test the fundamental laws of physics», says LIGO team member Katerina Chatziioannou, Caltech assistant professor of physics and William H.
