The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected an unusually strong collision between two black holes, allowing physicists to verify the black hole area theorem proposed by Stephen Hawking in 1971. On September 10, the relevant research results were published in Physical Review Letters.
A diagram of two black holes merging to create gravitational waves traveling through the universe. Image source: Maggie Chiang for Simons Foundation
The theorem states that the black hole event horizon created when two black holes merge, that is, the boundary where even light cannot escape the control of the black hole, cannot be smaller than the sum of the areas of the two original black holes. This theorem echoes the second law of thermodynamics. The second law of thermodynamics states that entropy, or the disorder within an object, never decreases.
Black hole mergers distort the fabric of the universe, creating tiny fluctuations in space-time called gravitational waves that travel across the universe at the speed of light. There are five gravitational wave observatories on Earth looking for waves that are ten thousand times smaller than the nuclei of atoms. They include the two LIGO detectors in the United States, as well as Italy's Virgo detector, Japan's KAGRA and Germany's GEO600, operated by an international collaboration called LIGO-Virgo-KAGRA (LVK).
The latest collision, named GW250114, is nearly identical to the collision that produced gravitational waves first observed in 2015. Both black holes were between 30 and 40 times the mass of the sun and occurred 1.3 billion light-years away.
This time, the upgraded LIGO detectors are three times more sensitive than in 2015, so they can capture the waves produced by the collision in unprecedented detail. This allowed the researchers to calculate and confirm that the event horizon area did become larger after the black holes merged, thereby verifying Hawking's theorem.
Laura Nuttall of the University of Portsmouth in the UK, a member of the LVK team, said that when black holes collide, they produce gravitational waves that sound like bells. Previously, these overtones dissipated too quickly to be observed clearly enough to calculate the area of the event horizon before and after the collision, which is required to test Hawking's theory. A 2021 study of the first detected collision supported the theory with 95% confidence, but the new study increases that confidence to a convincing 99.999%.
In the 10 years scientists have been observing gravitational waves, they have recorded about 300 black hole collisions. But none were captured as strongly and clearly as GW250114, which is twice as loud as other gravitational waves detected so far.
When GW250114's waves reached Earth, only the LIG was operating, not the other detectors monitored by the LVK collaboration. This doesn't affect testing of Hawking's theory, but it does mean researchers can't more clearly determine where these waves originate in the sky.
Ian Harry, also from the LVK team at the University of Portsmouth, said upgrades to LIGO and other planned observatories due to come online in the future will bring greater sensitivity, allowing us to study the physics of black holes in greater depth. "We may not catch all the signals, but we will have events like this again. The next round of upgrades may be in 2028 and we will see something similar, and maybe by then the sensitivity will be to the point where we can really delve into it."
The findings open up new avenues for research into quantum gravity, which physicists hope will unify general relativity and quantum physics. Nuttall said the latest results show that general relativity and quantum mechanics continue to work well together, but some differences are expected to emerge in the future.
"At some point we may start to see that things don't work so well together, and that will be when we get very close signals that will appear extremely loud in our data as the sensitivity of the instrument increases," Nuttall said.
The latest data from LVK also allowed scientists to confirm an equation proposed by mathematician Roy Kerr in the 1960s, which predicts that black holes can be characterized by just two metrics: their mass and spin. Essentially, two black holes with the same mass and spin are mathematically identical. "Thanks to observations of GW250114, we now know this is true."
Related paper information: https://doi.org/10.1103/kw5g-d732