What happens when two black holes collide?

FOR THE FIRST TIME, WE'VE OBSERVED A 50-YEAR-OLD theorem from Stephen Hawking in the natural world. Scientists from the Massachusetts Institute of Technology (MIT), California Institute of Technology, Cornell University, and Stony Brook University analysed the gravitational wave data of two inspiralling black holes. What they found in the collision proved Hawking's Area Theorem: The area of a black hole's event horizon cannot shrink.

When a binary black hole system inspirals, the orbits of the two black holes shrink, and the system begins emitting gravitational waves. The emission rate increases as the orbits shrink further and gain speed, and when a merger finally occurs, wave emission peaks. The two black holes become a single, new black hole, and according to Hawking's theorem, that new black hole should be equal in area to the total area of the original two black holes. That's exactly what the researchers found per the wave data of the observed inspiralling system.

The confirmation brings us closer to understanding the intricacies of black hole functions, says Riccardo Penco, PhD, assistant Professor of High Energy Physics Theory at Carnegie Mellon University. "The detection of gravitational waves allows us to test black holes directly (as opposed to just inferring their existence indirectly by studying the motion of other objects around them). For the longest time, we have studied black holes as mathematical objects endowed with a remarkable set of properties (Hawking's area law being one of them). We are now in a position to test these properties experimentally.'