A salmon and a photon share an important property. A salmon swimming upstream to spawn is unable to reach its destination if the current is too strong. Now suppose the current in the river increases in the downstream direction. There could then be a sharp divide where the current’s speed equals the speed at which a salmon can swim. A salmon placed on the upstream side of the divide will spawn, but a salmon on the other side will inevitably be swept farther downstream.

Just as the river with varying current can have a sharp boundary that is of crucial importance for the salmon, so too a black hole, with its varying gravitational field, has a horizon that separates two distinct possibilities for the behavior of light. A photon that is farther from the black hole than the horizon can escape to infinity. Once inside the horizon, though, light (and...

You do not currently have access to this content.