Stephen Hawking was immediately on the phone in 2015 when he learned that the Hanford LIGO observatory in Eastern Washington and its twin observatory in Louisiana had made the first-ever direct detection of gravitational waves from the collision of two black holes.
He asked one of the physicists behind the detection if LIGO could test Hawking’s 1971 prediction about black hole mergers.
That first detection of gravitational waves confirmed Einstein’s theory of relativity. And since then, Hawking’s prediction has also been confirmed by both U.S. Laser Interferometer Gravitational-wave Observatories, according to a study published this month in “Physical Review Letters.”
The publication came just days before the 10th anniversary this month of the first wave detection, or a ripple in space and time, created by two black holes orbiting each other until they crashed together.
Artistic representation of a ringing rotating black hole. It is shown from the view of one of the black holes as it spirals toward its cosmic partner.
“It’s the first time the universe has spoken to us through gravitational waves,” said David Reitze, the executive director of the LIGO Scientific Collaboration, in his announcement 10 years ago.
It is not gravitational waves from just mergers of black holes that have been detected, although they make up the majority of observations. The U.S. LIGO observatories also have detected neutron star mergers with other neutron stars or black holes.
The resulting detections are expanding humankind’s understanding of the universe and raising new questions.
This month, the findings from a detection in January 2025 of a black hole merger similar to the first one detected a decade ago allowed scientists to test the fundamental laws of physics and answer Hawking’s question.
But it took decades of scientific innovation to get here.
Hanford LIGO and the Louisiana LIGO operated for eight years starting in 2002 without identifying a ripple in space and time.
Then the next five years were spent on a complete overhaul and update of the observatories, giving them the capability to be 10 times more sensitive.
The upgrade paid off.
During an initial engineering run to test equipment before the start of official operations, the Louisiana LIGO and the Hanford LIGO both detected a gravitational wave moving through the Earth just as Albert Einstein had predicted nearly 100 years earlier.
The detection gave researchers a new way to study the universe, in addition to light waves, such as used by more traditional observatories, or through high-energy neutrinos from black holes hitting the Earth.
Now LIGO is a black-hole hunting machine, say observatory officials.
After more improvements to make the observatories more sensitive, including some relying on cutting-edge quantum precision engineering, the twin U.S. observatories have detected about 300 black hole mergers, some of which are confirmed and some still under analysis.
The Hanford and Livingston LIGOs now observe a black hole merger every three days.
They can measure changes in space-time that are 700 trillion times smaller than the width of a human hair.
Future of gravitational wave detections
Barry Barrish, an American experimental physicist who was one of three scientists awarded the 2017 Nobel Prize in Physics for their contributions to LIGO, was at the observatory last weekend to celebrate the 10th anniversary of the first detection of gravitational waves.
The information gleaned from the detection of a neutron star merger that emitted gamma rays provided confirmation that Einstein’s theory of relativity was correct in its prediction that gravitational waves should travel at the speed of light, Barrish said.
The light-based observations also showed that elements heavier than iron, such as gold and platinum, were created in neutron star collisions and then blown out of the immediate region and far out into space.
Observations show that likely the heavy elements like gold and platinum on Earth came from earlier neutron stars, Barrish said.
This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other.
There is much more to learn from gravitational waves.
Barrish is hopeful that detections that are not possible now could add to knowledge of the Big Bang, or the origin of the universe.
Information about the origin of the universe currently comes from electromagnetic radiation. But the universe was opaque until almost 400,000 years after the Big Bang, leaving no understanding of the first instance of the event.
But future observatories might be able to detect primordial gravitation waves from more than 13 billion years ago, providing insights into the origin of the universe.
While the twin LIGO observatories are making detections that provide insights into the fundamental laws of physics, the Trump administration has recommended shutting down either the Louisiana or Washington LIGO observatory.
Asked about that proposal during his visit to Hanford LIGO, Barrish advised contacting Congressional representatives. Sen. Patty Murray, D-Wash., is leading the fight to save the twin observatories.
Hawking’s theorem confirmed
The latest finding announced based on LIGO detections came from gravity waves that passed through the Earth in January 2025.
Both that gravitational wave event and the first one detected in 2015 involved colliding black holes about 1.3 billion light-years away with masses between 30 to 40 times the mass of our Sun.
But with the upgrades to LIGO, the more recent merger could be heard “loud and clear,” said authors of a study published in Physical Review Letters.
Inside a horizontal access module chamber as Advanced LIGO was being assembled prior to its first observing run in 2015.
Scientists could “hear” two black holes growing as they merged into one, verifying the late Stephen Hawking’s theorem in 1971 that said the total surface area of a black hole formed from a merger will be larger than the combined size of the two black holes that formed them.
Hawking, a theoretical physicist famous for his groundbreaking science and his book “A Brief History of Time,” predicted that the resulting black hole would have a larger surface area, despite losing energy in the form of gravitational waves and increasing spin, which can lead to it having a smaller area.
In the new study, scientists were able to determine not only the mass of the two black holes that merged from the strong signal of gravitational waves as they collided, but also the less clear signal as the final black hole vibrated like a struck bell.
By precisely measuring the details of the ringdown phase, scientists were able to calculate the mass and spin of the black hole and then determine its surface area.
The two black holes had a combined surface area roughly the size of Oregon before merging. The resulting black hole was about the size of California, according to LIGO officials.
In another notable finding revealed this year, the world’s coalition of gravitational wave observatories, which now includes observatories in Italy and Japan, announced the detection of gravitation waves from the most massive collision of black holes ever observed.
It raised the possibility that at least one of the black holes, at 140 times the mass of the Earth’s sun, that merged had formed from a previous merger, which could lead to new information about stellar evolution.
New way to study universe
Arguably the single most scientifically notable detection to date since the initial 2015 detection was the August 2017 detection from gravitational waves from a merger of two neutron stars.
Unlike the ripples from black hole mergers that lasted as little as a fraction of a second, these ripples continued for 100 seconds.
Artist’s rendition of the merger of two neutron stars.
It was an immediate clue that the ripples came from the massive, fiery collision of two neutron stars. Neutron stars are the collapsed cores of large stars and are the smallest, densest stars known to exist.
The two neutron stars observed in 2017 orbited each other and created gravitational waves before they merged into an ultradense object, emitting a fireball of gamma rays.
Because the gravitational waves were observed by the Hanford, Louisiana and Italy gravitational-wave observatories, their information could be combined and triangulated to pinpoint the location of the neutron star merger in the sky.
Over the next 11 hours after the neutron star merger was detected, telescopes saw multiple forms of light, or electromagnetic radiation, including X-ray, ultraviolet, infrared and radio waves.
The mergers of black holes emit no light, making telescopes of no help in studying the sources of previously detected gravitational waves.
The neutron star merger was the first time a cosmic event had been viewed with both gravitational waves and light, giving scientists a new way of learning about the universe.
The LIGO detector in Hanford, Wash., uses lasers to measure the minuscule stretching of space caused by a gravitational wave.
How LIGO detects minute waves
Gravitational waves carry huge amounts of energy, but dissipate to the point that they are barely detectable as they pass through Earth. The first detection in 2015 was of gravitational waves that traveled at the speed of light from a black hole merger 1.3 billion years ago.
As the waves move through objects, they stretch them lengthwise and cause them to compress sideways. A circle becomes an ellipse.
LIGO is a new type of observatory that relies not on a telescope, but observing the infinitesimal movements caused by gravitational waves.
The Hanford LIGO was built with two vacuum tubes that extend at right angles for 2.5 miles across shrub steppe land north of Richland. It is on a part of the federal 580-square-mile Hanford nuclear site that was never used in Hanford’s mission to produce plutonium for the nation’s nuclear weapons program.
At the end of each tube, a mirror is suspended on fine wires.
A high-power laser beam is split to go down each tube, bouncing off the mirrors at each end. If the beam is undisturbed, it will bounce back and recombine perfectly.
But if a gravity wave is pulsing through the Earth, making one of the tubes slightly longer and the other slightly shorter, the beam will not recombine as expected.
To make sure the disturbance detected at Hanford LIGO comes from a gravitational wave, rather than other movements such as a garbage truck passing by or pounding waves along the Pacific Coast at the other side of Washington, the signals can be matched to the signals detected more than 2,000 miles away at the identical LIGO in Louisiana.
There’s also value in having multiple observatories to determine the area of the sky that contains the source of the waves, allowing further study, argue supporters of keeping both U.S. LIGO observatories.