At 4 a.m. on Sept. 14, 2015, in both the desert of eastern Washington State and the backwoods of Louisiana, two beams of light began quivering in distant synchrony as the space through which they were traveling stretched and shrank at a rate of 250 times a second.
These were the twin antennas of the Laser Interferometry Gravitational-wave Observatory, or LIGO. Far out there in time and space a pair of black holes, gargantuan pits of eternally dark nothingness, had collided and merged, sending gravitational waves rippling through the universe and across the paths of the two antennas.
The whole encounter lasted one-fifth of a second. But that instant changed astrophysics, opening a window onto previously inaccessible realms of nature in which space could rip, bend, puff up, crumple and even vanish.
It was the first direct proof that ripples in space-time, predicted by Albert Einstein a century earlier, actually existed. In the decade since, LIGO and other experiments have logged more than 300 of these violent collisions, providing astronomers with clues regarding the evolution of black holes across cosmic history.
That black holes are ubiquitous in the universe is now beyond doubt. The LIGO antennas have justified their once contentious title as observatories, and similar observatories — VIRGO in Italy, KAGRA in Japan — have been started; some 1,600 astronomers and physicists worldwide are doing business as the LIGO/VIRGO/KAGRA, or LVK, collaboration. LIGO’s founders, Rainer Weiss of M.I.T. and Kip Thorne and Barry Barish of the California Institute of Technology, were awarded the Nobel Prize in Physics in 2017.
The anniversary festivities commence on Sept. 13 with parties and open houses at the LIGO observatories in Hanford, Wash., and Livingston, La., A symposium follows on Oct. 10 at Caltech, with speeches by Nobel laureates.
As an opening drumroll, on Sept. 10, a team of more than 1,000 astrophysicists led by Katerina Chatziioannou of Caltech announced that LIGO had confirmed a groundbreaking conjecture, first enunciated more than 50 years ago by Stephen Hawking, that black holes could only grow, a notion that has revolutionized theoretical physics.
But the celebrations have fallen under a shadow. In late August, Dr. Weiss, a garrulous lab rat and tinkerer, died at age 92, less than a month before the anniversary of his greatest achievement. Without him, scientists say, there would have been no gravitational wave observatory.
“I don’t know anybody who had any substantial interactions with Rai that didn’t come away from them a better person in some way,” said David Reitze, a physicist at Caltech and director of the LIGO Laboratory, which runs the two antennas.
More ominously, President Trump has proposed slashing LIGO’s operating budget in 2026 by 40 percent, to $29 million from $48 million, and eliminating one of the antennas. That could spell disaster, as it takes two antennas to triangulate the origins of gravitational waves.
Dr. Reitze, who spent an agonizing summer studying budget scenarios, said he thought the observatory could be run on the budget suggested, but barely. “It’s going to be ugly,” he said. Most immediately affected would be the 200 or so scientists and technicians at the LIGO Laboratory who run and maintain the antennas on behalf of the larger LVK community. Dr. Reitze described them as “very special scientists and engineers with very specialized skills who have mortgages to pay and kids to put through college.”
The impact on performance and future upgrades would be devastating, he added: “It would be nearly impossible for LIGO to recover from a cut of this magnitude.”
A gadget with gravity
Dr. Weiss liked to say that he learned the most by building a gadget and then trying to get it to work. That’s the story of LIGO, in a nutshell.
In the 1970s, in a ramshackle M.I.T. building where radar had been invented three decades earlier, Dr. Weiss began assembling a device to detect the subtle squeezing and stretching of space-time thought to be produced by gravitational waves, still hypothetical at the time. Only the densest, most extreme objects in the universe — black holes and neutron stars — were expected to be able to produce faint waves of this sort. Many astronomers, especially at M.I.T., doubted that black holes even existed.
In time, the National Science Foundation merged Dr. Weiss’s efforts with a similar project at Caltech, where black holes were more in fashion. The result was a pair of L-shaped buildings in Hanford and Livingston; each housed laser beams that bounced back and forth between mirrors and would reveal any incoming fluctuations.
For years the scientists heard nothing, even as they and increased their analytical and computational powers and improved the sensitivity of the antennas. Then, in September 2015, they turned on their latest version of the device, called Advanced LIGO. The detection bells sounded.
“It was waving hello,” Dr. Weiss later said. “It was amazing. The signal was so big, I didn’t believe it.”
In the void 1.3 billion light-years away, two black holes, 29 and 36 times as massive as the sun, had collided and merged to become a black hole with the mass of 62 suns. Three suns worth of mass and energy, equivalent to the light from a billion trillion stars, had disappeared into the rippling of space-time.
That was enough energy to budge LIGO’s mirrors and stretched the distance that the laser beams traveled by four one-thousandths the diameter of the nucleus of a hydrogen atom.
As it happens, the ringing of space-time caused by the collision of these particular black holes produced waves at the same frequencies that humans hear. Translated into acoustical waves, those first gravitational waves were a brief chirp, ending in a middle C.
Such chirps are now a regular feature of the science; if you have the Gravitational Waves Events app on your smartphone, you hear them every three days or so. The most recent compilation of gravitational wave events, released this week, included at least one object not massive enough to be a black hole, and a couple that were too massive, according to standard astrophysical lore.
One disappointment has been the lack of opportunity to engage in what the community calls multi-messenger astronomy, combining gravitational-wave studies with traditional telescopic observations. Black holes being invisible, there is nothing to see.
Multi-messenger astronomy reached its full potential in 2017, when a pair of neutron stars — the dense remnants of collapsed stars — collided, producing a new kind of explosion called a kilonova. Neutron stars are full of stuff that generates both light and noise, and the collision rang the antennas with a chirp that drew more than 4,000 astronomers who would later sign on to one of the many scientific papers on the event. Astronomers calculated that the blast generated 40 to 100 Earth masses worth of gold in a fraction of a second — future cosmic bling.
Sadly, nothing similar has happened since. “It appears that we just got REALLY lucky with GW170817,” Daniel Holz, an astrophysicist at the University of Chicago, wrote in an email. “And that set up completely unrealistic expectations for the rate of these systems.”
But hope springs eternal out there, and strange, novel events could occur any time. “The excitement in this field is that we may be discovering new things,” Dr. Reitze said. “We may be pointing ourselves, telling our astronomer friends to point their telescopes, in places where they even discover new things that aren’t associated with gravitational waves. So we’ll have to see how this plays out over probably the next weeks or months.”
How black holes grow
At the same time, there is the fulfillment of a longstanding promise made to Stephen Hawking, the celebrated English cosmologist and black hole expert.
In 1970, Dr. Hawking made a bold conjecture after studying the equations that describe black holes: The area of a black hole’s event horizon — the invisible bubble in space that defines its point of no return — must always increase. Nature didn’t have to work that way. Why couldn’t black holes split in two, or splatter off each other and disappear like soap bubbles? According to Hawking, these bubbles of nothingness could only grow.
Dr. Hawking’s insight became a keystone of a 1973 paper, “The Four Laws of Black Hole Mechanics,” which he wrote with James Bardeen, a physicist at the University of Washington, and Brandon Carter, now at the French National Center for Scientific Research. They compared the idea to a famous law in thermodynamics, which stated that entropy — the amount of wasted heat or energy in a system — always increased. (Entropy is why you can never build a perpetual motion machine.)
This idea implied that black holes had entropy — as Jacob Bekenstein, then a graduate student at Princeton, first argued — and heat, which suggested that they were not so black after all. They could be hot and even explode, as Hawking predicted a few years later. Among other things it suggested that the growth of black holes is as ineluctable as rust, and that the entropy and disorder in the universe will continue to grow.
A half-century later, some of the best minds in the world are still arguing over how that would work, and what it might mean. At stake is the question of whether Einsteinian gravity, which shapes the larger universe, plays by the same rules as quantum mechanics, the paradoxical laws that prevail inside the atom.
In 2003, Dr. Thorne predicted that LIGO would be able to test Hawking’s theory by actually measuring the sizes and other properties of black holes. That would be his present to his friend Hawking on his 70th birthday in 2012, Dr. Thorne told him.
But the data from the first recorded black hole collision, in 2015, was too noisy and uncertain to draw any immediate conclusions, and Dr. Hawking died three years later. In 2021, after years of computer simulations and data wrangling, a team led by Maximiliano Isi, a physicist at M.I.T., announced that there was a 97 percent chance that Dr. Hawking had been right: The total area of the black holes had increased during the merger, at least for this particular black hole collision.
Now the team led by Dr. Chatziioannou has gone Dr. Isi one better, providing what they say is the best and clearest evidence yet, at a more than 99 percent level of confidence, that Dr. Hawking was right.
The evidence emerged in January, when LIGO recorded the collision of two evenly matched black holes, 33.6 and 32.2 times as massive as the sun.
“It was twice as loud as the next loudest thing we have seen,” Dr. Chatziioannou said in a telephone interview. This was mainly because the LIGO detectors had been improved greatly since the first collision in 2015. “We immediately understood that you could do interesting things with it,” he said.
The merged black hole was 62.7 times as massive as the sun, which meant that three suns worth of mass had disappeared into gravitational waves.
Teasing out these details of the collision required a deep dive into the last few moments of black-hole annihilation and creation. When a newly merged black hole forms, it vibrates like a drum or a bell, generating a fundamental tone and a raft of overtones and undertones. As a result, Dr. Chatziioannou said, “Space-time, the gravity around it, is a mess.”
Because the chirp was so loud, Dr. Chatziioannou and her team were able to neglect the messiness and identify the final reverberations produced as the new black hole settled down. These revealed that the two black holes had begun with a combined area of 240,000 square kilometers, but merged into one with an area of 400,000 — a resounding growth.
“Our results suggest that astrophysical black holes are indeed extremely simple objects that follow general relativity,” Dr. Chatziioannou and her colleagues concluded. And with that, Dr. Hawking’s mortal remains can continue to rest quietly among those of Britain’s other scientific heroes in London’s Westminster Abbey.
Dennis Overbye is the cosmic affairs correspondent for The Times, covering physics and astronomy.
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