According to Einstein's theory of general relativity, massive objects create distortions in space and time. Near a black hole, these distortions become so strong that time behaves in unexpected ways.

Imagine that we are on a spaceship near a black hole. We drop a clock into the black hole and compare its time to that of our onboard clock. The falling clock runs progressively slower. It never crosses the event horizon, but stays frozen there in space and time. The falling clock also becomes continuously redder, since its light loses energy as it escapes from the black hole's vicinity.

By contrast, if we were falling with the clock, time would appear to behave perfectly normally. We would see no slowdown as we approached the event horizon. We would cross the horizon without any perceptible change, and our color would not appear to change. This is the principle of relativity: things can appear different depending on whether you are moving or standing still.