Confirmed: the event horizon of a black hole does not shrink

Theoretical physicist Stephen Hawking proposed in 1971 that the boundary of black holes, known as the event horizon, never shrinks over time. Now, 50 years after Hawking proposed this theory about black holes, it has been confirmed by a team of scientists led by the Massachusetts Institute of Technology (MIT). The confirmation of the area theorem of the deceased physicist in 2018, was given with more than 95% precision using the observation of gravitational waves.

The event horizon is the radius at which even the speed of light in vacuum is insufficient to reach the escape velocity of the gravitational field generated by the singularity of the black hole. It is proportional to the mass of it and, in theory, it should only get bigger.

The experts used data from GW150914, the first gravitational waves ever detected, which were created by two black holes that formed a new one, an event that released a large amount of rippling energy through space-time. To answer the question about Hawking’s theorem, the team split the GW150914 signal at its maximum point, or the merger, and then developed a model to analyze the signal before identifying the mass and spin of both involved black holes. Thus, they re-analyzed the gravitational wave signal before and after the cosmic collision and determined that the area of the event horizon did not decrease after the merger.

“The data show with overwhelming confidence that the area of ​​the horizon increased after the merger, and that the area law is met with a very high probability,” explains Maximiliano Isi of MIT’s Kavli Institute for Astrophysics and Space Research. “It was a relief that our result agrees with the paradigm we expect, and confirms our understanding of these complicated black hole mergers.”

Hawking claimed that according to classical physics, the area of ​​the event horizon of a black hole can only get bigger, never get smaller. Mathematically it was valid, but it has been difficult to confirm by observation, mainly because black holes are extremely difficult to observe directly because they do not emit detectable radiation. Now, thanks to this experiment with gravitational waves, we know that it is. Even black holes have to comply with some of the laws of physics.

“There may be a zoo of different compact objects, and while some of them are black holes that follow Einstein and Hawking’s laws, others may be slightly different beasts. So it’s not like you did this test once and it’s over. You do this once, and it’s the beginning, ”says Isi.

The team plans to further test the Hawking area theorem and other theories regarding the mechanics of black holes, using data from LIGO and Virgo, its Italian counterpart. “It’s encouraging that we can think in new and creative ways about gravitational wave data and come up with questions we previously thought we couldn’t. We can continue to unravel pieces of information that speak directly to the pillars of what we think we understand. Someday, these data can reveal something that we did not expect “, concludes the expert.

Reference: Testing the black-hole area law with GW150914, Physical Review Letters (2021).… 4336d883136eb53c122b

On Arxiv: