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How does gravity escape a black hole?

If nothing in the universe can travel faster than light, how come light can't escape a black hole? I mean, Einstein's relativity says nothing can travel faster than light, but yet, light can't escape a black hole. Does this mean that light really isn't the fastest thing? That the pull of the black hole is really faster than light? That Einstein was wrong, even though it's been backed up by scientific evidence? I'm very confused. If anyone would be able to answer my question, I would appreciate it: Why can't light escape a black hole if nothing can travel faster than light?

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marked as duplicate by Qmechanic, David Z Oct 29 '12 at 2:30

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
Essentially a duplicate of physics.stackexchange.com/q/937/2451 –  Qmechanic Oct 28 '12 at 21:09
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If nothing in the universe can travel faster than light, how come light can't escape a black hole? this because nothing can escape the black hole, the answer is in you question, actually, and there is no reason to assume that something should escape out of it anyway. –  TMS Oct 28 '12 at 21:15
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This is a bit like asking, if a Bugatti is the fastest car in the world why can't it travel across the whole of Europe in one hour? Does this mean that the Bugatti isn't the fastest car in the world? –  contrariwise Oct 28 '12 at 21:31
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2 Answers 2

You're assuming that the gravitational pull of the black hole prevents light from escaping because the pull is "faster" than the light. That's not how it works. Gravity exerts a force; it doesn't have a speed.

For an object escaping from a gravity well, that force causes the object to slow down. For light escaping from a gravity well, since the speed of light is constant, it doesn't slow down; instead, it loses energy, shifting toward longer wavelengths. For a black hole's gravity well, the light loses so much energy that it can't escape at all.

(I'm probably oversimplifying this in several ways; a completely accurate explanation would require far more understanding of General Relativity than I possess.)

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The essential idea to grasp here is that, regardless of the fact that light propagates at the "universal speed limit", gravity is curved spacetime.

Within the event horizon, the curvature of spacetime is such that there is no world line (path through spacetime), for light or any physical object, to the exterior of the hole.

Roughly speaking, once inside the horizon, the "direction" to the outside is backwards through time.

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