Amanda Gefter does a good job explaining black holes, evaporation, and the conservation of information, and her illustration of an elephant caught in a black hole is through-provoking.

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. Inside or out?

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.

From the perspective of an elephant, spaceship, planet, or solar system falling into a black hole, it will theoretically appear to take millions of years to actually reach the singularity at the center. During this time, could a falling civilization continue, and perhaps discover a way to escape if one exists? If faster-than-light travel is possible, one might expect that a civilization trapped in a black hole for millions of years would be highly motivated to discover it and use it to escape. We know our universe is full of black holes, and if it is also full of other intelligent lifeforms then some of them have probably fallen into a black hole and potentially discovered how to travel faster than light. If not, then either there aren't any other lifeforms, or FTL travel is impossible.

(HT: GeekPress.)



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