How does gravity get out of a black hole? If gravity is mediated
by a particle, say graviton, and the particle is subject to the usual
limitation that nothing can travel faster than light, how do gravitons get
across the event horizon? The part of the gravity of a black hole that is due
to stress of space time outside the event horizon can certainly be mediated by
gravitons without them needing to exceed the speed of light, but that just
postpones the question: Why is space time stressed outside the event horizon,
if gravitons cannot escape from it? Do they perhaps escape by Hawking-Penrose
radiation? That does not seem right, because the intensity of Hawking-Penrose
radiation decreases with increasing mass of the black hole.
AFAIK a graviton is not proven to exist. Also it is not necessary to exceed the sol to get out of a black hole you just have to always move away from it - now this gets tricky as space-time is warped but for a theoretical partical it should be no problem.
Simularly you don’t have to achieve escape velocity to leave earth’s orbit - you can just keep going up at 1m/s till there is no more up.
Yep, this is one of those phyics problems that has always bothered me. If nothing can escape a black hole, how can gravity escape a black hole?
Perhaps gravity propagates faster than light. OK, let’s ask a slightly different question. If we have a gravitaional field, and that field changes, it seems to me that we couldn’t notice the changes faster than c. Otherwise we could send information faster than c by sending gravity waves.
Maybe the solution is that gravitons don’t interact with each other, but only with other particles. If there are gravitons.
Well, what I know is this: Mass produces gravity (I know it doesn’t really produce it, but it’s the best word I can think of right now to use)
Black holes have mass, so they produce gravity.
Of course, this ignores a lot of theoritical physics, but it has seemed to hold true for a lot of things so far.
The FAQ at http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html
says:
Purely in terms of general relativity, there is no problem here. The gravity doesn’t have to get out of the black hole. General relativity is a local theory, which means that the field at a certain point in spacetime is determined entirely by things going on at places that can communicate with it at speeds less than or equal to c. If a star collapses into a black hole, the gravitational field
outside the black hole may be calculated entirely from the properties of the star and its external gravitational field before it becomes a black hole. Just as the light registering late stages in my fall takes longer and longer to get out to you at a large distance, the gravitational consequences of events late in the star’s collapse take longer and longer to ripple out to the world at large. In this sense the black hole is a kind of “frozen star”: the gravitational field is a fossil field. The same is true of the electromagnetic field that a black hole may possess.
Often this question is phrased in terms of gravitons, the hypothetical quanta of spacetime distortion. If things like gravity correspond to the exchange of “particles” like gravitons, how can they get out of the event horizon to do their job?
Gravitons don’t exist in general relativity, because GR is not a quantum theory. They might be part of a theory of quantum gravity when it is completely developed, but even then it might not be best to describe gravitational attraction as produced by virtual gravitons. See the FAQ on virtual particles for a discussion of this.
Nevertheless, the question in this form is still worth asking, because black holes can have static electric fields, and we know that these may be described in terms of virtual photons. So how do the virtual photons get out of the event horizon? Well, for one thing, they can come from the charged matter prior to collapse, just like classical effects. In addition, however, virtual particles aren’t confined to the interiors of light cones: they can go faster than light! Consequently the event horizon, which is
really just a surface that moves at the speed of light, presents no barrier.
I couldn’t use these virtual photons after falling into the hole to communicate with you outside the hole; nor could I escape from the hole by somehow turning myself into virtual particles. The reason is that virtual particles don’t carry any information outside the light cone. See the FAQ on virtual particles for details.
[technical point]
Oh, if there are gravitons, then they would indeed interact with each other. Really, that’s one of the biggest problems with quantizing gravity; gravitons would not only interact with each other, they’d do so quite extensively. Since they’d be massless, they’d also move at c.
[/technical point]
As it says above, gravity does not need to get out of a black hole; it is merely the geometry of space-time in the vicinity of the black hole as described by the most recent rigorous theory of gravity: the classical physics theory known as General Relativity. What goes on within the event horizon no one knows (it could be a singularity or it could be a tea party). Gravity is geometry.
But, they say, gravity is a force mediated by the graviton. The concept of gravity as a force mediated by the graviton is part of a hypothesized quantum theory of gravitation (and has also fortuitously popped up in some versions of string theory; but the less said about those fantasies, the better). Unfortunately, that is about as far as a quantum theory of gravitation has gone (outside of works of fiction). It rapidly loses any coherence once you try to develop a rigorous description of the world (or even a very small part of it). Gravity as a force radiating out of a black hole is presumed to be mediated by virtual gravitons (the virtual part allowing them to ignore speed of light limitations for a brief time) in a method that is not nor analogous to Hawking radiation. But, since the existence of gravitons is sort of iffy, I think it is safe to say the mechanics of virtual gravitons is iffy-squared.
The last 85 years have been spent comprehending the implications and intricacies of general relativity, and trying to go beyond it. The ‘going beyond it’ part hasn’t really amounted to much. It took about 60 years to just get a good understanding of general relativity. The really big experimental problem with gravity is that its affect is incredibly weak: lack of experimental results hinder determining which direction to take in successful theorizing (just look at string theory). A lot of work has been done based upon the guess that since electromagnetism is a force we understand, we can build a force theory of quantum gravity that is analogous to it. But, perhaps, gravity may not be a force as we understand it at all but…[sub]at this point, our humble commentator is hustled back into the mothership by his compadres…[/sub]
Here’s a riddle: If you have a hole two feed wide, three feet deep, and one foot long, how much dirt is in the hole?
Methinks y’all are thinking of this backwards… it’s not “How does gravity get out of a Black Hole?”, since gravity doesn’t need to “get out” of anything. It’s always out.
I agree with SPOOFE.
Maybe the way quantum gravity is like this:
The singularity does not emit gravitons, but instead absorbs gravitons from the rest of the universe.
A hole is an innie, not an outie.
Here’s another wag - I just came up with. Since no one has found a graviton I made this theory up:
1 The graviton is constant throuout space-time and is a property of spacetime
2 gravitons are attracted to mass and move towards them
3 when they collide with mass they disappear and reappear in another part of space-time - perhaps their original starting point.
Sounds like it could work
There’s no doubt as to the existance of the graviton, and depending on what you mean by a detection, we have detected them: A gravitational wave can be regarded as a stream of gravitons, just as a light wave can be regarded as a stream of photons. We have not yet detected an individual graviton, and considering the typical energies predicted for them, it’ll be a long time indeed before we ever do. What we think of gravitons is somewhat moot, however, since we know plenty about photons, which mediate electromagnetic effects, and as maralinn’s link points out, black holes can also have electric charge and fields. The key is that particles which are serving to mediate forces are as a rule virtual, and the rules are a bit more relaxed for virtual particles.