 General relativity tells us that clocks running at different locations in a gravitational field will, in a sense that can be made precise, generally not agree with one another. In the case of a black hole, this manifests itself in the following way. Imagine someone falls into a black hole, and, while falling, she flashes a light signal to us every time her watch hand ticks. Observing from a safe distance outside the black hole, we would find the times between the arrival of successive light signals to grow larger without limit, because it takes longer for the light to escape the black hole's gravitational potential well the closer to the event horizon the light is emitted. (This is the red-shifting of light close to the event horizon.) That is, it would appear to us that time were slowing down for the falling person as she approached the event horizon. The ticking of her watch (and every other process as well) would seem to go ever more slowly as she approached ever more closely to the event horizon. We would never actually see the light signals she emits when she crosses the event horizon; instead, she would seem to be eternally “frozen” just above the horizon. (This talk of seeing the person is somewhat misleading, because the light coming from the person would rapidly become severely red-shifted, and soon would not be practically detectable.) 