A tiny wobble in the laser beams confirmed that black holes emit enormous, ringing energy when they collide.
“This is the most powerful event in the universe because in a fifth of a second you pulverize the equivalent of three entire suns and turn them into energy,” says Max Isi, an astronomer at Columbia University and the Simons Foundation, a nonprofit that funds science.
These discoveries have helped hundreds of scientists test the fundamental theories of relativity and black holes proposed by Albert Einstein and Stephen Hawking.
“This process follows a very important principle of what we think black holes should do,” Isi says.
Read more: This is what would happen if you walked through a black hole
Ringing black hole
The discoveries were made thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is actually two different facilities in Washington state and Louisiana. Both sites are located in remote areas, isolated from noise. They work in the USA firing lasers at the beam splitterbased on quantum optics that redirects them into L-shapes. The split beams travel down corridors and hit mirrors located about 2.5 miles (4 kilometers) apart.
If there are no gravitational waves, light does not pass through from the other side. However, as gravitational waves pass through them, one of them will become momentarily and measurably shorter than the other.
In 2015, LIGO scientists first discovered a kind of ringing signal that could result from the collision of two black holes.
“In less than 10 years, we see them almost every other day, and it’s amazing,” Isi says.
Ultimately, on January 14, 2025, scientists at the site detected the clearest signal of black hole collisions they had ever seen. It was closest to the others, at a distance of about one billion light years.
IN study recently published in Letters about physical checks, Isi and his collaborators described how this fluctuating signal is related to the theories of Hawking and Roy Kerr, another physicist who worked on the theory of black holes. The study involved nearly 1,000 authors from LIGO and two complementary facilities, the Kamioka Gravitational Wave Detector (KAGRA) and the Virgo Interferometer in Italy.
What is a ringing black hole?
Black holes have enormous amounts of energy. When they collide with each other, space and time around them are distorted, creating ripples that reverberate like the sound waves created by the ringing of a bell.
They are expressed in gravitational waves that have very specific heights and durations, which scientists have learned to detect in objects such as LIGO, KAGRA and Virgo. Just as sound waves can tell listeners something about the size of a bell, gravitational waves can tell scientists something about the properties of whatever is emitting them.
According to Kerr, black holes are simple objects that can only be described by mass and spin. Analysis of the ringing of these gravitational waves has provided compelling evidence that this is the case, Isi and his colleagues say.
Based on the switching off of lasers on these objects, the researchers discovered two black holes that existed before the January 2025 collision, as well as a larger black hole that remained after their approach. And all this happens in about one-fifth of a second, followed by a ringing sound that lasts about 10 milliseconds.
“These black holes seem to match what we see in theory,” Isi says.
Proof of Einstein and Hawking's theories
Einstein never directly predicted the existence of black holes, but other scientists have used his general theory of relativity to predict their existence.
Meanwhile, Hawking believed that black holes could only increase in size. Essentially, this meant that the area inside their event horizons—the boundary through which no penetrating light could return—would only grow larger.
Hawking himself believed that detection of black hole mergers, which began at LIGO in 2015, could eventually confirm his theory, but he died in 2018. In subsequent years, measurement methods were improved. Then, on January 14, 2025, when a signal of a relatively close black hole collision reached Earth, Isi and his colleagues were able to confirm it.
In this case, each of the two colliding black holes had a mass of about 33 suns. When they merged, the new black hole had the mass of about 63 suns—the mass of about three of the suns it lost was what was emitted in the ringing gravitational waves, Isi says.
Hawking's law of areas is also related to the second law of thermodynamics, which states that entropy—essentially disorder—will always increase if left alone. Isi says that in black holes, the area inside the event horizon is actually a measure of disorder, showing how Hawking's law of area and the second law of thermodynamics are related.
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