What do we call the boundary a black hole's light cannot escape?

The event horizon, which is not a surface but a location - the distance at which the escape velocity reaches the speed of light. Nothing is there. It is the boundary of a region from which no information can reach the outside, and for a black hole the mass of the Sun it would sit about three kilometres out.

The strangest part is what it does to time. To someone watching from a safe distance, an object falling in appears to slow, redden and freeze at the horizon, dimming until it fades - it never appears to cross. To the object itself nothing happens at that moment at all; it passes through and continues, with the horizon behind it and no way back.

Stephen Hawking then made it worse and more interesting. In 1974 he showed that quantum effects at the horizon should make a black hole radiate very faintly and eventually evaporate, which raises the question of what happens to the information about everything that fell in. Physicists have been arguing about the answer for fifty years, and the argument is one of the main reasons anyone thinks gravity and quantum mechanics have to be reconciled.