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How Can Blackholes Influence Anything Outside Their Event Horizon?

  1. Oct 18, 2013 #1
    I just registered and am not entirely sure this is the right section of the forum to ask, but regardless...

    My understanding of gravity is that it is a wavelike distortion in spacetime which travels not instantaneously but at the speed of light, and may also be describable in terms of a stream of graviton particles very similar to photons, also travelling at the speed of light. My understanding of black holes is that in simple terms they are essentially an object which has had the radius to its center of mass reduced to the point of having an escape velocity greater than the speed of light, and the point at which its gravitational pull becomes greater than the speed of light is its event horizon.

    Now, if the event horizon is a boundary at which photons can no longer escape their fall into the curved space of a super dense object because their rate of falling into space is faster than their rate of radiating out of it, how is it that gravitons are able to avoid this inescapably fast escape velocity if they, like photons, travel at light speed? In other words, if everything is falling into a black hole at a rate faster than the speed of light, and gravity travels at the speed of light, how can a black hole exert any gravitational influence on anything outside its event horizon? How can light-speed gravity reach out of the event horizon and exert influence on other bodies if nothing traveling the speed of light can escape the horizon? Is this not a contradiction?

    I imagine this is a subject which some theoretical physicists have stumbled upon and proposed possible solutions, but I'm generally ignorant as to what possible discussion there has been regarding this problem in the scientific community (or if it's even a problem at all, rather than some misunderstanding on my part). I'd like to see what others may have to say on this subject.
  2. jcsd
  3. Oct 19, 2013 #2


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    Not really. Gravity is the result of the geometry of spacetime and is described by what's known as a metric tensor (More on that below). Similar to a static electric field, a gravitational field doesn't travel anywhere. Changes in field do, however, and they travel at a velocity of c.

    As for gravitons, your best bet is to simply forget about them for the purposes of understanding gravity. In the context of quantum physics they would "force carriers", but only as virtual particles. And since talk about virtual particles is VERY advanced stuff and highly prone to misunderstandings and disagreements, it's far better to just leave them until you understand a lot more.

    Pretty much. Another way of looking at it is that beyond the even horizons, all paths through spacetime lead towards the center of the black hole and none lead away from it. So there isn't even a "path" to get out of it at that point.

    Nope. First, gravity is explained and described according the the Theory of General Relativity, or GR. In GR, we have what's called a "metric tensor" that describes the geometry of spacetime, which in turn tells us how gravity behaves since gravity is a result of curved spacetime. It's important to understand that this metric is NOT matter nor is it radiation. It isn't something that can get sucked into a black hole. It is literally the way this metric is bent and curved that gives us gravity. And remember what I said above. Gravity is not "moving" anywhere. Only changes in the field, or metric, move as waves.
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