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Black Holes are regions of space where things get weird [Citation Needed]. Past the event horizon of a black hole, any moving particle instantaneously experiences a gravitational acceleration towards the black hole that will cancel out it's current velocity, even light. That means that the gravity well of the black hole must be able to accelerate from -C* to 0 instantly✝. Given that fact, we can assume the gravitational acceleration of black holes is C/instant**. Given this, it stands to reason that in successive instants, the particle will be moving at speeds greater than C, because it is experiencing greater gravitational forces and continuous gravitational acceleration.

Does this actually make sense? Is there something I'm missing here? By this logic, it seems like anything inside of the event horizon of a black hole could and should move faster than C due to gravitational acceleration.

Edit: I showed this question to a friend and he questioned if the hypothetical particles that were radiating from the singularity (The photon traveling exactly away from the black hole) might be hawking radiation; that is, the gravitation acceleration of a black hole is only strong enough to curve the path of light around a non-zero radius (thus not actually stopping it, but altering it's course), and not powerful enough to decelerate light. Is this actually what hawking radiation is, or is he as confused as I am?


*Where movement towards the singularity would be considered a positive value, movement away from the singularity is a negative value, that is, anything moving at the speed of light away from the singularity would be moving with a velocity of -C relative to the singularity.

✝If it couldn't accelerate from -C to 0 instantly, any photon traveling exactly away from the black hole would be able to escape the event horizon.

**An instant is an arbitrary amount of time, it could be a fraction of a second, a second, a minute....

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    $\begingroup$ No. You're making A LOT of assumptions about how a BH works, but reality is very different. It's a difficult topic to figure out, short of actually taking a General Relativity class - but that's what's needed to truly understand these objects. $\endgroup$ – Florin Andrei Aug 27 '15 at 18:58
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I think your initial question is a good one, but the text gets a bit more jumbled and covers a few different points.

Can things move faster than light inside the event horizon of a black hole?

Nice question.

Black Holes are regions of space where things get weird.

I'm 100% OK with this statement. I think it's a true enough summary and I'm sure I've heard physicists say this too. Even if "Weird" isn't a clearly defined scientific term, I'm 100% fine with this (even without a citation).

Past the event horizon of a black hole, any moving particle instantaneously experiences a gravitational acceleration towards the black hole that will cancel out it's current velocity, even light. That means that the gravity well of the black hole must be able to accelerate from -C* to 0 instantly✝.

Are you quoting somebody here? Anyway, this isn't quite true. Black holes don't accelerate things from -c (which I'm guessing would be a light beam trying to fly away from the singularity but inside the event horizon), to 0 "instantaneously".

Perhaps a better way to look at it is to consider curvature of space, and inside a black hole, space curves so much that all directions point to the singularity. It's the "all roads go to Rome" scenario, even if you do a complete 180, you're still on a road that leads to the singularity.

I understand the temptation to look at that as deceleration, but I think that's a bad way to think about it. Light doesn't decelerate, it follows the curvature of space.

Given that fact, we can assume the gravitational acceleration of black holes is C/instant**. Given this, it stands to reason that in successive instants, the particle will be moving at speeds greater than C, because it is experiencing greater gravitational forces and continuous gravitational acceleration. Does this actually make sense? Is there something I'm missing here? By this logic, it seems like anything inside of the event horizon of a black hole could and should move faster than C due to gravitational acceleration.

outside of a black hole, continuous acceleration would never lead to a speed greater than C. You can accelerate for billions and trillions of years and all you'd do is just add more 9s to the right of the decimal point.

You seem to be assuming that inside a black hole this can happen, but I'm not sure why you'd assume that.

"continuous gravitational acceleration" - no matter how strong, is no guarantee for faster than light travel. That's logically inconsistent with the laws of relativity.

Edit: I showed this question to a friend and he questioned if the hypothetical particles that were radiating from the singularity (The photon traveling exactly away from the black hole) might be hawking radiation; that is, the gravitation acceleration of a black hole is only strong enough to curve the path of light around a non-zero radius (thus not actually stopping it, but altering it's course), and not powerful enough to decelerate light. Is this actually what hawking radiation is, or is he as confused as I am?

I think, a more correct way to look at hawking radiation is to see it as something that forms just outside of the black hole, a particle/anti particle pair and one escapes and the other falls inside, and that's probably not 100% correct either, but the singularity itself doesn't send out particles. Hawking radiation has to do with quantum properties of space. It's not a property of black holes. The black hole just happens to be unique in that it can capture one half of a virtual particle pair and the other half can escape.

This also is a pretty different topic than your original question.

*Where movement towards the singularity would be considered a positive value, movement away from the singularity is a negative value, that is, anything moving at the speed of light away from the singularity would be moving with a velocity of -C relative to the singularity. ✝If it couldn't accelerate from -C to 0 instantly, any photon traveling exactly away from the black hole would be able to escape the event horizon.

**An instant is an arbitrary amount of time, it could be a fraction of a second, a second, a minute....

I think it's a good idea to differentiate mass-less pure energy particles and particles with mass. You seem to be saying that a ray of light can be traveling away from a black hole at the speed of light, get caught in the gravity, slow down and then fall back into the black hole like a ball that's tossed straight up into the air from the surface of the Earth. That's probably not what happens. The ray of light follows the path of space time ahead of it, which happens to be curved so much that it points into the black hole, even if, in the classical sense, the light begins by pointing away. All space curves into the singularity once you're inside the event horizon, so there is no "away from" anymore. At least, that's how I think it works.

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    $\begingroup$ If I could upvote this answer twice I would. Every once in a while I got new activity on this question and reread this answer and am amazed at how clearly it takes the original assumptions about black holes and explains why they are wrong, and how things actually behave. $\endgroup$ – Sidney Mar 7 '16 at 16:15
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The short answer to your top-level question "Can things move faster than light inside the event horizon of a black hole?" is no. The text of your question gets confusing and contains a lot of assumptions that are not correct. There's no "special" acceleration happening at or near the event horizon. If things could travel faster than the speed of light there, then they would potentially be able to escape the black hole. The hole is "black" because things are bounded by the speed of light and because of the spacetime curvature.

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This is the very question I've been searching for an answer to. Having watched a lot of Lenny Susskind & Space-time videos, I'd never heard this question posed - so thanks to those who asked and answered.

I began by wondering how gravity could have an effect on a massless particle - as witnessed with 'gravitational lensing', but I worked out for myself that it's not the light that's been bent, but the space through which the light is travelling. I can just about get my head around how space and time bends under the influence of a massive body. But when it comes to space/time being so bent to the degree that light gets trapped, I can't visualise it - but I accept that that's what happens.

Here's the thing though: If an observer at a distance from a black hole, watches a pebble fall towards the event horizon, to the observer, the pebble appears to slow as it approaches. When it finally reaches the e/v, light from it is frozen, such that the pebble appears motionless (I get that - and the reasons why that is). But let's imagine there is no accretion disc to confuse matters, and that the pebble approaches the singularity directly (perpendicular to the e/v). From the point of view of the pebble at the e/v, the speed of light leaving it back towards the observer is C - C (speed of light minus speed of light) - have I got that right?

If so, and given that the gravitational effect is only going to increase beyond the e/v, isn't it fair to assume that the pebble will continue to accelerate en route to the singularity?

Imagine a second observer 90 degrees North, South, East, West, up or down from the first observer, whilst he wouldn't be able to see the pebble beyond the point it vanished, at what speed would the pebble be travelling after it vanished between the e/v and the singularity?

Jim Parry, Torquay, South-West Engaland.

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Jim Parry is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
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  • $\begingroup$ Hi Jim! I'm glad you got some good information from the answers to my question! You also ask some great questions of your own -- That said Stack Exchange isn't like a traditional forum (where a comment thread extends unboundedly); we're a question and answer site -- a singular question is asked and a definite answer is provided. As such you're unlikely to get answers to your questions posing them in an answer to my question. If you have further questions, I would suggest using the new question functionality, and linking to my question. $\endgroup$ – Sidney 6 hours ago
  • $\begingroup$ In addition, if you're unclear on how the Stack Exchange network works, I'd take a few minutes to go through the tour section. It's about a 5-10 minute read that helps you get a lot more out of the Stack Exchange network. :) $\endgroup$ – Sidney 6 hours ago
  • $\begingroup$ This is not an answer to the question posted. The speed of light is always exactly the same when measured locally. It is never different. $\endgroup$ – Rob Jeffries 2 hours ago

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