“So what’s the relevance of this drain hole to a black hole? The analogy works, roughly, as follows: The drain hole sucking water toward it is equivalent to the singularity at the center of a black hole sucking space toward it. Just as water starts to flow faster than the speed of sound at a circle around the center of the drain hole, so too, there is a spherical surface around the singularity at the center of a black hole at which the speed of the flow of space exceeds the speed of light—yes, think here of empty space as you would a flowing liquid. Because no objects or signals in our universe can move through space faster than the speed of light, everything within this spherical surface is doomed to stay within it. Just as Bob could not go back across the sonic boundary because he could not swim faster than the speed at which the water is flowing inward, so, too, anything within the spherical surface around the singularity cannot go back across it.”