# How Can Gravitons Escape A Black Hole?

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1. Aug 15, 2015

### Ryan Reed

Past the event horizon of a black hole, gravity is so immense that even light can't escape. Wouldn't this cause the the gravitons, which travel the speed of light, to be trapped, making a singularity?

2. Aug 15, 2015

### phinds

I don't know what you mean. There is ALREADY as "singularity" at the center of every black hole.

3. Aug 15, 2015

### Ryan Reed

Duly noted, though you didn't answer my actual question.

4. Aug 15, 2015

### Staff: Mentor

5. Aug 15, 2015

### Ryan Reed

I looked at that link and have read it but I don't understand; the curvature in space-time is caused by gravity, which is caused by gravitons. If gravitons can't escape the black hole, then the gravity well of the black hole should look like exactly that, a well. It shouldn't be bent gradually, it should be flat until it gets to the black hole and then becomes a pit. This is obviously not what happens.

Last edited: Aug 15, 2015
6. Aug 16, 2015

### Staff: Mentor

A graviton is a hypothetical particle that appears in some quantum mechanical descriptions of gravity. Even if gravitons do exist in the form predicted by those theories, the gravitational field of an object is not caused by gravitons (or anything else for that matter) flowing away from that object the way that light flows out from the sun. Thus, we can and do have gravity outside the black hole even though nothing can escape from it.

7. Aug 16, 2015

### Markus Hanke

The curvature of space-time is gravity; there is no cause-and-effect between the two.

Static gravitational fields do not require the exchange of gravitons; only changes in the gravitational field would be mediated by gravitons. In that picture, you can regard gravitational waves as "packets" of gravitons, somewhat similar to the relationship between photons and electromagnetic waves.

8. Aug 16, 2015

### stevebd1

9. Aug 16, 2015

### Markus Hanke

Interesting, but the notion of superluminal propagation doesn't sit well with me, even though we are dealing with virtual particles. Wouldn't it be more consistent and less problematic to associate the "hairs" of the black hole with the event horizon, rather than the singularity ? Or better still, consider them properties of the entire space-time ?

10. Aug 16, 2015

### DC1

It does not.

11. Aug 16, 2015

### bahamagreen

I had some correspondence with Saul Perlmutter back in the mid 90s about this very question. His answer was along the lines of those pointing to the external curvature outside the event horizon as the source...

Since then I have read that it is the lingering effect (the external curvature) of the matter that was "inside" at the time of the event horizon formation.

This does raise a couple of other questions;

- assuming that the external gravitational attraction of the black hole is from the curvature outside the event horizon, how does addition of more matter to a black hole increase this external attraction? Since the outside observer sees additional matter indefinitely approaching the event horizon, is it this matter at the event horizon that is causing the additional external curvature and additional attraction?

- if so, it seems like from the perspective of the black hole's interior this matter does enter the event horizon and might net the same external observation...

- but if not (if additional attraction is not from the matter approaching the event horizon), then why do black holes grow? That is, similarly to those novae that are fed matter until they pop (and all do so when at the same conditions and subsequent intrinsic brightness), why wouldn't slowly fed pre-black holes all convert to black holes and present an event horison all at the same conditions, and then not appear from the outside to accept any further matter within the event horizon... and not increase their attraction in spite of additional matter "falling in"?

12. Aug 17, 2015

### Demystifier

There may be gravity without gravitons. While gravitons (or their classical counterpart - gravitational waves) correspond to a traveling gravitational field, there are also gravitational fields which do not travel. The non-traveling gravitational fields are static. Indeed, outside of the black hole there is a static gravitational field, so gravity is there without escaping from the black hole.

13. Aug 17, 2015

### Staff: Mentor

It makes no difference whether the matter is inside or outside the event horizon. The Schwarzschild metric is the solution for the vacuum region outside of any static and spherically symmetric mass distribution - the presence or absence of an event horizon and the distribution of matter on each side of the horizon is irrelevant.

14. Aug 18, 2015

### Ryan Reed

If gravitons only need to be exchanged if the gravitational field changes, then wouldn't a black hole not be able to move since that would need to change the position that its gravity originates?

15. Aug 18, 2015

### Staff: Mentor

You're still thinking as if the gravitational force is caused by gravitons flowing from the source mass to the object being acted on by the gravitational force. That's not the right picture - the gravitons aren't being exchanged between the two masses, they're being exchanged between the gravitational field at a given point and whatever matter happens to be at that point.

One implication of this is that you have to understand the gravitational field and general relativity before you can even start to think in terms of gravitons. There's just no way of making sense of the behavior of gravitons until you understand the behavior of the gravitational field.