Few physicists believe there is much to see or feel at the event horizon of a large black hole, but the horizon is of huge significance to the causal structure of black holes. Inside the horizon surface, space and time in some sense switch identities. If an intrepid astronaut were to venture inside the horizon of a black hole, the ever-increasing tilting of the light cones means he would necessarily have to keep moving toward the center. That is, the radial dimension of space inside acquires the properties of a time dimension, a direction in which one can’t stop or reverse but must move
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