Hawking's theory says that the final area of the black hole has to be bigger than the sum of the two initial areas, says Chatziioannou, "and this is what we demonstrated observationally with that signal."
This kind of proof is just what Hawking hoped for a decade ago, when the first gravitational wave detection was announced. He actually reached out to one of the scientists involved in that effort to see if gravitational waves could be used to test this prediction, says Isi.