Physicists confirm Hawking’s theorem about black holes for the first time

There are certain rules that even the most extreme objects in the universe must obey. A central law for black holes predicts that the span of their event horizon – the limit beyond which nothing can escape – will never shrink. This law is Hawking’s area theorem, named after the physicist Stephen Hawking, who came up with the theorem in 1971.

Fifty years later, physicists at MIT and elsewhere are now confirming Hawking’s surface theorem for the first time by observing gravitational waves. Your results show up today Physical review letter.

In the study, the researchers took a closer look at GW150914, the first gravitational wave signal discovered by the Laser Interferometer Gravitational-wave Observatory (LIGO) in 2015. The signal is the product of two inspirational black holes creating a new black hole, along with a large number of black holes. energy that flows through space-time as gravitational waves.

If the Hawking area theorem holds, then the horizon area of ​​the new black hole cannot be less than the total horizon area of ​​the parent black hole. In the new study, physicists re-analyzed signals from GW150914 before and after the cosmic collision and found that the entire area of ​​the event horizon did not actually decrease after merging – a result they reported with 95 percent confidence.

Their results marked the first direct observational confirmation of Hawking’s area theorem, which had been proven mathematically but had never been observed in nature before. The team plans to test future gravitational wave signals to see if they further confirm Hawking’s theorem or are a sign of illegal new physics.

“It is possible that there are zoos with different compact objects and while some of these are black holes obeying Einstein and Hawking’s laws, others could be slightly different animals,” said lead author Maximiliano Isi, NASA Einstein postdoctoral fellow at the MIT Institute. Kavli for Astrophysics and Space Research. “Well, it’s not like you take this test once and it’s over. You do it once and that’s the start.”

Isis’s co-authors on the work are Will Farr of Stony Brook University and the Center for Computational Astrophysics at the Flatiron Institute, Matthew Giesler of Cornell University, Mark Scheel of Caltech, and Saul Teukolsky of Cornell University and Caltech.

An age of insight

In 1971 Stephen Hawking proposed the area theorem, which gave rise to a number of fundamental insights into the mechanics of black holes. The theorem predicts that the total area of ​​the black hole’s event horizon – and coincidentally all black holes in the universe – will never decrease. That statement is a strange parallel to the second law of thermodynamics, which says that the entropy or degree of disorder in an object must never decrease.

The similarities between the two theories suggest that black holes may behave like thermal and exothermic objects – a puzzling proposition because black holes, by their very nature, never let energy escape or radiate. Hawking finally squared the two ideas in 1974, showing that black holes have entropy and can emit radiation for very long periods of time if quantum effects are taken into account. This phenomenon is called “Hawking Radiation” and remains one of the most fundamental revelations about black holes.

“It all started with Hawking’s realization that the entire area of ​​the horizon would never sink into a black hole,” Isi said. “The Territory Act sums up the golden age of the 1970s when all this knowledge was acquired.”

Hawking and others have since shown the area theorem to work mathematically, but there was no way to compare it to nature until LIGO. first evidence of gravitational waves.

Upon hearing the results, Hawking immediately contacted LIGO’s co-founder, Kip Thorne, Feynman professor of theoretical physics at Caltech. The question: can detection confirm the area theorem?

At that time, the researchers were unable to sift through the necessary information in the signals before and after merging to determine whether the final horizon area was not decreasing, as suggested by Hawking’s theorem. It was only a few years later, and the development of the technique by Isi and his colleagues, that an examination of territorial law became possible.

Before and after

In 2019, Isi and his colleagues developed a technique for extract the echo immediately after the peak of GW150914 – the moment when two parent black holes collide to form a new black hole. The team used a technique to select specific frequencies or tones from a sequence of noises that they could use to calculate the final mass and spin of the black hole.

The mass and spin of a black hole are directly related to the area of ​​its event horizon, and Thorne remembers and approaches Hawking’s question with a follow-up: can they use the same technique to get the signals before and after merging to compare and confirm? defined area?

The researchers accepted the challenge and shared the GW150914 signal again at its peak. They developed a model to analyze the signal before the peak corresponding to the two inspirational black holes and to identify the mass and spin of the two black holes before they merge. From this estimate, they calculated the total area of ​​their horizon – an estimate of about 235,000 square kilometers, or about nine times the area of ​​Massachusetts.

Then they used their previous technique to extract the ringdown, or echo, of the newly formed black hole, from which they calculated its mass and spin, and finally its horizon area, which is 367,000 square kilometers (about 13 times) its area. Gulf States).

“The data show with great confidence that the area of ​​the horizon has increased after the merger and that the broad law has been met with a very high degree of probability,” Isi said. “It is a relief that our results match our expected paradigm and confirm our understanding of this complex black hole merger.”

The team plans to further test Hawking’s areal theorem and other long-standing theories about black hole mechanics with data from LIGO and Virgo, its Italian partners.

“It’s exciting that we can think about gravitational wave data in creative new ways and ask questions that we previously thought we couldn’t,” said Isis. “We can always put out information that speaks directly to the pillars of what we think we understand. One day this data could reveal something we didn’t expect.”

This research was supported in part by NASA, the Simons Foundation, and the National Science Foundation.

–