Ask Ethan: Do Merging Black Holes Create An In…

Ask Ethan: Do Merging Black Holes Create An Information-Loss Paradox?

“When black holes merge they [lose] energy through gravitational waves. Does this pose the same problem as Hawking radiation does, with respect to loss of information? Or is the information on what has gone into the black hole somehow encoded into the gravitational wave? And if it is could we someday hope to decode what went into the black hole using gravitational waves?”

Black holes and entropy have long been a problem. If you calculate a black hole’s temperature and entropy in General Relativity alone, you get an absurd answer: 0. That implies that the pre-existing object that forms a black hole, which definitely has entropy, would see it disappear when it formed a black hole. But the entropy of a system can never decrease, implying that black holes must have an entropy after all: encoded on the surface of the event horizon. So let’s take two black holes with entropy on their event horizon and merge them together. What happens? You form a larger black hole with a larger event horizon, while gravitational waves carry away roughly 5% of that mass in the form of gravitational waves.

Who gets the entropy? Is it still conserved? And where does it go, in theory and in practice? Here’s what we know so far, for the final Ask Ethan of 2018!