Transcript: A fundamental prediction of General Relativity is the fact that time slows down in strong gravitational fields. The ultimate test of this idea would be to observe someone falling into a black hole carrying a clock. In theory, the clock would slow down and come to a complete halt as they reached the event horizon. We can’t do that experiment, but physicists have done other experiments in weaker situations of gravity and seen the effect of time slowing down. Atomic clocks do run slower on the Earth’s surface than they do in a plane five miles up where the gravity is slightly weaker, and a light beam passing by the limb of the Sun slows down by a measurable 250 millionths of a second.