Let’s say Alice is watching a black hole from a safe distance, and she sees an elephant foolishly headed straight into gravity’s grip. As she continues to watch, she will see it get closer and closer to the event horizon, slowing down because of the time-stretching effects of gravity in general relativity. However, she will never see it cross the horizon. Instead she sees it stop just short, where sadly Dumbo is thermalised by Hawking radiation and reduced to a pile of ashes streaming back out. From Alice’s point of view, the elephant’s information is contained in those ashes.
There is a twist to the story. Little did Alice realise that her friend Bob was riding on the elephant’s back as it plunged toward the black hole. When Bob crosses the event horizon, though, he doesn’t even notice, thanks to relativity. The horizon is not a brick wall in space. It is simply the point beyond which an observer outside the black hole can’t see light escaping. To Bob, who is in free fall, it looks like any other place in the universe; even the pull of gravity won’t be noticeable for perhaps millions of years. Eventually as he nears the singularity, where the curvature of space-time runs amok, gravity will overpower Bob, and he and his elephant will be torn apart. Until then, he too sees information conserved.
Neither story is pretty, but which one is right? According to Alice, the elephant never crossed the horizon; she watched it approach the black hole and merge with the Hawking radiation. According to Bob, the elephant went through and floated along happily for eons until it turned into spaghetti. The laws of physics demand that both stories be true, yet they contradict one another. So where is the elephant, inside or out?
The answer Susskind has come up with is – you guessed it – both.
NS Space
