Scientists have used a gravitational wave detector to “hear” two black holes getting bigger as they merged into a single, gigantic entity.
The detection, made by the Laser Interferometer Gravitational-wave Observatory (LIGO) on Jan. 14, provides the best evidence yet for a theory put forth by famed physicist Stephen Hawking more than half a century ago, but which was never proven in his lifetime.
A study based on the research was published Wednesday (Sept. 10) in the journal Physical Review Letters and was led by Adrian G. Abac, a doctoral student at the Max Planck Institute for Gravitational Physics in Potsdam, Germany.
The rippling cosmos
LIGO detects gravitational waves — ripples in the fabric of space-time released during the most extreme events in the cosmos, such as collisions of black holes or neutron stars (the remnants of giant stars). Its first direct detection of gravitational waves, made almost exactly 10 years ago on Sept. 14, 2015, confirmed Albert Einstein’s predictions of general relativity by observing two black holes merging.
Now with a decade of experience under their belts, LIGO collaborators have made many improvements to the detectors — such that black hole mergers are now spotted about once every three days instead of once a month, according to a statement from Caltech, which jointly operates LIGO along with MIT.
During the event detected Jan. 14, LIGO witnessed two black holes merging, with the resulting black hole measuring significantly bigger than the two objects entering into the collision.
Before the merger, the combined surface area of the two black holes was about 93,700 square miles (243,000 square kilometers) — roughly the size of Oregon. After the merger, by contrast, the newly formed and single black hole had a surface area of roughly 154,500 square miles (400,000 square km) — about the size of California. In other words, the newly merged black hole was larger than the sum of its parts.
The detection of the growing black hole confirms a prediction that Hawking put forward in 1971: that the “event horizon — the outer boundary” of a black hole beyond which nothing can escape — can never decrease in size, researchers at Columbia University, which is part of the LIGO Collaboration, said in a separate statement.
“Even though it’s a very simple statement, ‘areas can only increase,’ it has immense implications,” study co-author Maximiliano Isi, assistant professor at Columbia University and associate research scientist at the Flatiron Institute said in a statement from the American Physical Society (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 (disorder) can only increase over time.
This theory now has scientists treating black holes as “thermodynamic objects”, APS continued in the statement, “a paradigm shift cemented by Hawking’s discovery that they have entropy, and emit radiation due to quantum effects near the event horizon.”
“It tells us that general relativity knows something about the quantum nature of these objects and that the information, or entropy, contained in a black hole is proportional to its area,” Isi added.
‘Ringing’ like a bell
It isn’t the first time LIGO put Hawking’s theory to the test; a 2021 observation tentatively confirmed his prediction. The new results, however, “confirm this earlier result with a much higher precision,” Columbia officials added.
The study achieved this precision by examining the pitch and duration of the gravitational waves emitted as the black holes merged. Researchers can make inferences about the black holes through their waves because a black hole’s size and shape influence these waves, in much the same way a musical instrument’s size and shape affect the sound it makes.
The newly detected event, known as GW250114, produced a “ringing” in space-time as the new black hole quieted down after the merger.
“The ringdown is what happens when a black hole is perturbed, just as a bell rings when you strike it,” study co-author Katerina Chatziioannou, an assistant professor of physics at Caltech, said in the APS statement.
The “ringdown” allowed the researchers to confirm that the remaining black hole had a larger surface area than the two black holes that combined to form it.
The findings also prove another theory described by mathematician Roy Kerr about six decades ago. Called the Kerr metric, the theory describes how Einstein’s field equations for general relativity work in a rotating black hole. In other words, “Two black holes with the same mass and spin are mathematically identical,” Isi said. “It’s very unique to black holes.”
LIGO currently includes two detectors — one in Hanford, Washington, and one in Livingston, Louisiana — and the twin interferometers commonly work with Europe’s Virgo and Japan’s Kamioka Gravitational Wave Detector interferometers as part of the Ligo-Virgo-KAGRA (LVK) Collaboration.
As researchers continue to fine-tune LIGO’s twin detectors, at least one other detector is planned. When LIGO-India comes online in 2030 or so, it would “greatly improve the precision with which the LVK network can localize gravitational-wave sources,” Caltech representatives noted. More detectors could come later, as the team seeks to “hear the earliest black hole mergers in the universe,” the statement added.
Cosmic Explorer, a concept for a bigger interferometer in the U.S., would have detectors with “arms” 10 times the length of the current LIGO observatories (each of which is 2.5 miles (4 km) long, to contain lasers inside steel vacuum tubes). Europe also has a proposed project called the Einstein Telescope, which would have one or two underground detectors with arms more than 6 miles (10 km) long.
