The LIGO-Virgo-KAGRA collaboration has decided to mark the 10th anniversary of the discovery of gravitational waves with a very special gift. They have published data on the clearest signal they have yet detected in the observatory, which allowed them to confirm Stephen Hawking’s area theorem of black holes.
The event was seen on January 14, 2025, so it is known as GW250114, and it was not included in the recent release of data. It is similar to the first gravitational wave event ever observed, GW150914. Both involved collisions between black holes of 30 to 40 solar masses, and both happened around 1.3 billion light-years from Earth. The difference is 10 years of improvements in how we measure gravitational waves, making this new signal incredibly clear.
Previous gravitational waves detected by LIGO-Virgo-KAGRA are somewhere between a loud laugh or a shout over the din of the party. GW250114 is akin to a fire alarm going off.
Simona Miller
“GW250114 is the loudest gravitational-wave signal that we have observed so far – by a lot! By “loudest,” I mean that the amplitude of the signal is much larger than the background noise in our detectors,” Simona Miller, a Caltech grad student and co-author of the paper, told IFLScience.
“Think of it like being at a dinner party where everyone is quietly chatting. Previous gravitational waves detected by LIGO-Virgo-KAGRA are somewhere between a loud laugh or a shout over the din of the party. GW250114 is akin to a fire alarm going off.”
Thanks to the clarity of the signal, the team was able to measure the area of the two black holes before the collision and the area of the final black hole product. The total surface area for the two parent black holes was 240,000 square kilometers (93,000 square miles), which is roughly the size of Oregon. The final area shows a clear increase, being about 400,000 square kilometers (about 154,000 square miles) or roughly the size of California.
“Because GW250114 is so loud, we can measure the masses and the spins of the orbiting black holes that generated it with much higher precision than previous signals – as well as, crucially, the mass and spin of the remnant black hole from the collision. Since a black hole’s area is calculated directly from its mass and spin, we also have tighter constraints on the areas themselves, allowing us to verify Hawking’s area theorem with higher confidence than ever before,” Miller continued.
The area of a black hole is linked to its entropy. Just like entropy always increases, so should the area of a black hole. There is a caveat, though. Hawking proposed that black holes can radiate and lose mass, increasing the general entropy of the universe and thus reducing their area.
The signal-to-noise ratio – an indication of the clarity of the signal – of this detection was 80, while the first ever detection was 26. The incredible detectors measure how the laser light sent down two airless tubes changes due to gravitational waves passing through. The change is but a fraction of the size of an atom.
“The measurements we are making now remain the most precise measurements humans have ever achieved in any field of science and engineering,” Professor Vicky Kalogera, one of LIGO’s leading astrophysicists, told IFLScience. “It is beyond what anybody could ever believe was possible. If you’re going to identify a breakthrough, that is an unimaginable breakthrough.”
A second paper also published today performs further general relativity tests. The era of using gravitational wave observations to test Einstein’s theory is upon us. “The gravitational-wave signal GW250114 is a milestone in the decade-long history of gravitational wave science,” the authors write in the paper.
The work is published in Physical Review Letters.