Scientists were able to use gravitational wave detectors to “listen” the two black holes grow larger as they merge into a single giant being.
The detection, made by the laser interferometer Gravitational Wave Observatory (LIGO), on January 14th, provided the best evidence of a theory published by well-known physicist Stephen Hawking more than half a century ago, but was never proven in his lifetime.
The research-based study was published in a Journal Physics Review Letter on Wednesday (September 10) and was led by Adrian G. Abac, a doctoral student at the Max Planck Institute for Gravitational Physics in Potsdam, Germany.
The rippling universe
Ligo detects gravitational waves. Ripples of space-time fabric emitted during the most extreme events in the universe, such as the collision of black holes and neutron stars (remains of giant stars). The first direct detection of gravitational waves, which took place almost ten years ago on September 14, 2015, confirmed the predictions of Albert Einstein’s general theory of relativity by observing the merger of two black holes.
Now, he has 10 years of experience, and Ligo’s collaborators have made many improvements to the detector. This has led to the discovery of black holes about once every three days, rather than once a month, according to a statement from Caltech, which is operating in collaboration with MIT.
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During the event detected on January 14th, Ligo witnessed the fusion of two black holes, and the resulting black holes were significantly larger than the two objects entering the collision.
Before the merger, the total surface area of the two black holes was approximately 93,700 square miles (243,000 square kilometers). In contrast, after the merger, the surface area of the newly formed single black hole had a surface area of approximately 154,500 square miles (400,000 square km) of California. In other words, the newly merged black hole was greater than the sum of the portions.
The detection of growing black holes confirms the prediction Hawking proposed in 1971. The “horizon of events” of black holes that can’t escape beyond that can never be reduced in size, researchers at Columbia University, part of the Ligo Collaboration, said in a separate statement.
“Even though it’s a very simple statement, “a region can increase,” which has immeasurable meaning,” Maxilliano Ishi, an assistant professor at Columbia University and an associate researcher at the Flatiron Institute, said in a statement from the American Physics Association (APS). Hawking’s theorem is known as the second law of black hole mechanics, and is similar to the second law of thermodynamics, which states that entropy (impair) can increase over time.
The theory continues in a statement by scientists who currently treat black holes as “thermodynamic objects,” saying, “a paradigm shift cemented by Hawking’s discovery of entropy, emitting radiation from quantum effects near event horizon.”
“The theory of general relativity knows something about the quantum nature of these objects and tells us that the information contained in a black hole, or the entropy, is proportional to that region,” ISI added.
“Ring” like a bell
It’s not the first time Ligo has tested Hawking’s theory. Observations in 2021 have tentatively confirmed his predictions. However, the new results “please check this previous result with much higher accuracy,” Colombian officials added.
This study achieved this accuracy by examining the pitch and duration of the gravitational waves emitted when black holes are merged. Because the size and shape of the black holes affect these waves, researchers can infer about black holes through the waves. This affects the size and shape of the instrument in almost the same way that it affects the sound it makes.
The newly detected event, known as GW250114, produced a “rance” in space-time as new black holes became quieter after the merger.
“Ringdown is what happens when a black hole is disturbed, like when you hit it, the bell rings,” Katerina Chatziioannou, assistant professor of physics at Caltech, said in an APS statement.
“Ringdown” allowed researchers to confirm that the remaining black holes had a larger surface area than the two black holes.
The findings also prove another theory written by mathematician Roy Kerr about 60 years ago. A theory called KERR metrics explains how Einstein’s field equations in general theory of relativity work in rotating black holes. In other words, “two black holes with the same mass and spin are mathematically identical,” ISI said. “It’s very unique to a black hole.”
Ligo currently includes two detectors in Hanford, Washington, and the other in Livingston, Louisiana, with twin interferometers generally acting as part of the Ligo-Virgo-Kagra (LVK) collaboration, in collaboration with the European Virgo and Japan’s kamioka gravitational wave detector interferometer.
At least one other detector is planned as researchers continue to fine-tune Ligo’s twin detectors. When Ligo-India comes online in around 2030, a representative from Caltech said, “it will significantly improve the accuracy in which LVK networks can localize gravitational wave sources.” More detectors could come later, the statement added, as the team is “trying to hear the earliest black hole merger in space.”
The US larger interferometer concept, Cosmic Explorer, has a detector with a “arm” 10 times the length of the current Ligo Observatories (each of which contains a laser in a vacuum tube of steel). There is also a proposed project in Europe called the Einstein Telescope. This has one or two underground detectors with arms that are more than 6 miles long (10 km).
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