Ask Ethan: How Do Gravitational Waves Escape From A Black Hole? (Synopsis) [Starts With A Bang]

“I think there are a number of experiments that are thinking about how you could look in different frequency bands, and get a glimpse of the primordial gravitational wave background. I think that would be really revolutionary, because that would be your first glimpse at the very first instant of our Universe.” -Dave Reitze, LIGO’s executive director

Black holes are remarkable entities that have puzzled and fascinated us since they were first postulated long before Einstein developed his theory of relativity. One of their fundamental but bizarre properties is the fact that once something crosses or winds up inside the event horizon, it can not only never escape, it heads inevitably towards the central singularity. At that point, the only “information” about the singularity is its mass, charge (of various types), and spin.

Yet when two merging black holes coalesced together, as seen multiple times by LIGO, the mass of the final black hole was approximately 5% less than the sum of the masses of the two black hole progenitors. If nothing massive or massless can escape through the event horizon, how did this energy get out?

Our intuitions might lead us astray, but the mathematics provides a straightforward explanation that’s not so different from other physics you might be used to. Come find out on this edition of Ask Ethan!