On Sept. 14, 2015, humanity detected the ripples of the universe for the first time. The Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington state captured gravitational waves from the collision of two black holes about 1.3 billion light-years away (light-year: the distance light travels in one year, about 9.46 trillion km).
The LIGO collaboration, involving about 1,000 scientists from 13 countries including the United States, Korea, Germany and the United Kingdom, held simultaneous press conferences on Feb. 11 the following year in Washington, London and Pisa, Italy, to officially announce the detection of gravitational waves. The moment that sensed the warping of spacetime was a landmark event in the history of science. Ten years after the first detection, humanity is now uncovering the evolutionary process of the universe through gravitational waves. The era of "gravitational-wave astronomy" has dawned.
◇ A cosmic event predicted by relativity
Gravitational waves are a phenomenon predicted 100 years ago by Einstein while explaining the motions of the universe, referring to gravitational energy spreading like waves when gigantic events occur in the cosmos, such as stellar explosions or black hole formation. In 1915, Albert Einstein published the general theory of relativity and predicted that just as a fabric dents when a bowling ball is dropped on it, when celestial bodies act violently, the surrounding spacetime is twisted and gravitational waves are generated.
Lee Hyungmok, a professor in the Department of Physics and Astronomy at Seoul National University and head of the Gravitational Wave Research Collaboration, said, "Even when a person walks by, gravity shakes ever so slightly, but it is too weak to detect," and explained, "When massive stars like black holes or neutron stars merge, the signal comes out relatively strong, and this wave makes space itself heave."
The problem was that gravitational-wave signals are so faint that they are difficult to detect. While it was predicted that when gigantic events occur in the cosmos, such as stellar explosions or black hole formation, gravitational energy would be emitted in the form of gravitational waves, the signals seemed too extremely weak for direct observation.
The turning point was the LIGO project, launched in the mid-1990s with support from the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). Each LIGO observatory has two enormous 4-km-long vacuum tubes extending in an L-shape, with mirrors at both ends. A laser beam is sent back and forth between the mirrors at the ends of the vacuum tubes; when a gravitational wave passes, the tubes warp slightly and the distance the laser beam travels changes a bit. Measuring this change allows detection of minute deformations of spacetime.
◇ Gravitational-wave astronomy, observing black hole mergers every three days
Scientists involved in the LIGO research called the experiment "a challenge bordering on the impossible." Even minute noise from earthquakes, wind or temperature fluctuations could shake the results. The international team kept the experiment going for decades, upgrading equipment and repeating failures.
At last, a century after Einstein predicted the existence of gravitational waves, the first signal was captured. The detected waves came from the process in which two black holes, each about 30 times the mass of the sun, merged to form a giant black hole about 62 times the mass of the sun. Another signal was then captured that December from a separate black hole merger.
A black hole is a celestial object formed by the extreme collapse of a very massive star, with gravity so strong that it swallows not only matter but even light. Its English name, "black hole," comes from that.
Since then, gravitational-wave detection has become not a one-off achievement but a new field known as "gravitational-wave astronomy." Black holes, which cannot be seen in visible light or electromagnetic waves, and neutron stars formed after a star ends its life in an explosion can now be observed through gravitational waves. Observing gravitational waves alongside electromagnetic waves allows much more precise localization of their origin, enabling detailed study of the characteristics of cosmic phenomena.
In recognition of these achievements, the 2017 Nobel Prize in physics was awarded to Rainer Weiss, an emeritus professor at the Massachusetts Institute of Technology (MIT); Barry Barish, an emeritus professor at the California Institute of Technology (Caltech); and Kip Thorne, an emeritus professor at Caltech, for their contributions to the first detection of gravitational waves.
Gravitational-wave detection has now become routine in the scientific community. LIGO has partnered with Virgo in Italy and KAGRA in Japan to more than double sensitivity. The observable range of the universe has now doubled, and the number of galaxies that can be captured has increased eightfold. Since 2015, the three detectors have recorded more than 300 gravitational-wave events. David Reitze, executive director of the LIGO observatories, told the journal Nature, "We're now observing black hole mergers on average every three days, and things are only going to get better from here."
◇ Hawking's black hole theory also verified
Above all, as sensitivity to capture gravitational waves has improved, researchers have recently advanced to observationally testing the "black hole area theorem" proposed by Stephen Hawking in 1971. Hawking's theorem says that when two black holes merge, the surface area of the final black hole must increase. Based on data far clearer than the first observations in 2015, the LIGO team precisely analyzed the "ringdown" of black holes and confirmed that Hawking's theorem holds with astounding accuracy.
Katerina Chatziioannou, a professor at Caltech, said, "We can now clearly hear the quivering sound as a black hole rings like a bell when it merges," and added, "This allows us to test the foundations of the laws of physics."
Today, the LIGO detectors are called the most precise "rulers" ever built by humans. They can detect changes down to one 700-trillionth the thickness of a human hair. With cutting-edge techniques to suppress quantum noise and new artificial intelligence (AI) analysis tools joining in, sensitivity is improving year by year. Nergis Mavalvala of MIT said, "LIGO continues to display technological wonders," adding, "As the detectors advance, we yearn for an ever more distant and fainter universe."
Once a third LIGO observatory is built in India, the precision of the gravitational-wave detector network is expected to improve dramatically. In addition, the United States is envisioning the 40-km-scale Cosmic Explorer, and Europe is planning next-generation observatories such as the Einstein Telescope. When such massive facilities go into operation, humanity is expected to be able to directly observe even the first black hole mergers shortly after the Big Bang (daebakpok).
Rainer Weiss, the professor who first proposed the LIGO concept, said in his lifetime, "The chances of this being realized are almost none," but after more than half a century of effort, the impossible became possible. Aamir Ali, a program director at the National Science Foundation (NSF), said, "Just 10 years ago, we were only beginning to listen to the ripples of the universe for the first time," adding, "From now on, through this new window, we will rewrite the history of a universe we did not know at all."
References
Physical Review Letters (2025), DOI: https://doi.org/10.1103/nwgd-g3zl
Physical Review Letters (2016), DOI: https://doi.org/10.1103/PhysRevLett.116.061102