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Possible Duplicate:
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?

Community
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Simeon
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  • Essentially a duplicate of http://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
  • This is not a duplicate. It actually asks why light can't escape from a black hole whereas the other question is for people who already accept that nothing can escape a black hole and asks how it's possible that a black hole has gravity. – Timothy Apr 21 '18 at 23:34

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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.)

Keith Thompson
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  • Would there downvoter care to elaborate? – Keith Thompson Jul 13 '16 at 06:47
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    I upvoted to off-set their downvote. Good answers will always have haters, or those who cannot conform to a reality. – Jossie Calderon Aug 11 '18 at 20:06
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    Where does that lost energy go, by the way? – Jossie Calderon Aug 11 '18 at 20:09
  • @JossieCalderon: That's a good question. I hope somebody else can answer it better than I can. (You might consider posting it as a new question.) I'm confident that the answer is that no energy is actually lost (black holes don't violate energy conservation), but at least at the moment, I can't explain in more depth than that. – Keith Thompson Aug 11 '18 at 20:24
  • https://physics.stackexchange.com/a/194814/127599 – Jossie Calderon Aug 11 '18 at 20:39
  • The light may be redshifted, but that energy was used to expand the universe. It has therefore gained potential energy. Light escaping from a gravity well would appear to be redshifted only because of the time it takes to get out of there, but it's also gained gravitational potential energy. – Jossie Calderon Aug 13 '18 at 02:29
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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.

Alfred Centauri
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  • Is this because spacetime is so curved that, for every second light travels, the spacetime it is travelling in has curved to create a greater distance than a light second? – userManyNumbers Aug 26 '18 at 20:19
  • @userManyNumbers no, even if the spacetime was "still" (constant w.r.t. time) and there was just one black hole in an otherwise uniform (w.r.t. space) gravitational field, the light still could not escape it.

    Once entering the black hole, a photon finds itself in a situation where no geodesic it can travel on exits the event horizon. There are paths exiting the black hole, but these are not geodesics. Also I should note that this whole scenario I just described only applies to "static" black holes https://astronomy.swin.edu.au/cosmos/b/black+hole energy can escape some rotating black holes

     – cmarangu Dec 18 '23 at 18:11
  • @Alfred Centauri nice explaination! I love how you point out that traveling backwards along the light's path is still part of a geodesic and "exits" the black hole, it expanded the way I think about spacetime – cmarangu Dec 18 '23 at 18:16