Ethan Siegel in Forbes:
Perhaps the greatest discovery of all announced in 2016 was the direct detection of gravitational waves. Even though they had been predicted by Einstein's general theory of relativity 101 years prior, it took the development of a laser interferometer sensitive to ripples in space that would displace two mirrors separated by multiple kilometers by less than 10^-19 meters, or 1/10,000th the width of a proton. This finally came to pass during LIGO's 2015 data run, and two bona fide black hole-black hole merger events unambiguously popped out of the data. But how does physics actually allow this? Mārtiņš Kalvāns wants to know:
This question has puzzled me for a long time. Articles about LIGO discovery state that some percentage of black hole merger mass was radiated away, leaving [a] resulting black hole smaller than [the] sum of [the] original mergers. Yet it is accepted that nothing escapes black holes […] So my question is: how was energy radiated from black hole mergers?
This is a really deep question, and goes straight to the heart of black hole physics and general relativity.