Physicists have just observationally confirmed one of Stephen Hawking’s theories on black holes for the first time. Hawking's area theorem predicted that the area of a black hole's event horizon can never decrease. Thanks to the gravitational waves emitted in a black hole collision, physicists have been able to observationally confirm this, publishing their findings in Physical Review Letters.
The so-called second law of black hole mechanics states that the area of the event horizon of a black hole always increases with time. The event horizon is the last precipice from which something can escape the gravitational pull of a black hole. Once you cross that threshold, not even light can escape.
This law, stated by Hawking in 1971, is akin to the second law of thermodynamics that states that entropy always increases in an isolated system. And in fact, the entropy of a black hole is related to its area.
Testing the black hole area theorem is far from easy but the researchers worked out that they could potentially use the information gathered from black hole mergers. These events release huge amounts of gravitational waves, which when they get to Earth pass as vibrations the size of a fraction of an atom.
From those vibrations, it is possible (thanks to a lot of theoretical work) to extract information on the mass and the spin of a black hole, two parameters related to the area of the black hole. The team looked at the first-ever gravitational wave event detected, GW150914, and estimated the area of the two colliding black holes and the area of the final resulting black hole, and confirmed with 95 percent confidence that the area increased.
“The data show with overwhelming confidence that the horizon area increased after the merger, and that the area law is satisfied with very high probability,” said lead author Maximiliano Isi, a NASA Einstein Postdoctoral Fellow at MIT’s Kavli Institute for Astrophysics and Space Research, in a statement. “It was a relief that our result does agree with the paradigm that we expect, and does confirm our understanding of these complicated black hole mergers.”
The team is planning to test the area theorem and other laws and theories related to black hole mechanics with the data collected by the three gravitational observatories around the world, the US's two LIGO detectors, and Virgo in Italy. This might provide insights into Einstein's general relativity as well, and maybe hint at what lies beyond our current physical theories.
“It is possible that there’s a zoo of different compact objects, and while some of them are the black holes that follow Einstein and Hawking’s laws, others may be slightly different beasts,” explained Isi. “So, it’s not like you do this test once and it’s over. You do this once, and it’s the beginning."