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How Does Gravity Escape a Black Hole?

We recently received the following question:

If, by definition, light cannot escape from a black hole, then how do gravity waves, or gravitons, escape?

Bob M.
Switzerland

Thanks for the question Bob.
This is an interesting question that I often hear.

As often happens at this scale, there are two legitimate ways to look at this problem. From a classical or general relativity perspective, this is actually a non-issue. Gravitation is described in General Relativity as the curvature of space time. Since anything with mass (energy) curves space-time there is no contradiction caused by black holes constraining photons but not gravity. This is because gravity is merely the structure of space itself.

Looking at this question through the lens of Quantum Mechanics is problematic though. This would require a unification of General Relativity and Quantum Field Theory which is one of the holy grails of physics. Since we don’t yet have this theory of quantum gravity no one can definitely answer your question Bob.

Scientists can make some educated guesses though and my take on the consesus on this is that gravitons would have no problem escaping from a black hole because they would likely be VIRTUAL gravitons and virtual particles have a get out of black-hole free card. In fact virtual particles can seemingly break many laws of physics like traveling faster than light or perhaps even going back in time!

This may sound like scientific heresy but this is actually permitted by the laws of physics as long as we can’t directly observe them and they disappear before they violate the Uncertainty principle.

Look at hawking radiation. Photons of this type of radiation are emitted from black holes in just this way.

If these virtual particles sound interesting to you I recommend a deeper look. They are one of the most fascinating revelations of modern physics.

The idea then is that the virtual gravitons can escape a back hole because they can temporarily exceed the speed of light.

Keep in mind though that once quantum gravity is fleshed-out, we may find that there actually is no need for gravitons nor virtual gravitons.

If and when this happens, please check back here….I’m sure I’ll be talking about it.

3 comments to How Does Gravity Escape a Black Hole?

  • pzeimet

    Great graphic. I remember you explaining this on a recent episode of SGU, but it’s nice to have it in writing.

  • Thanks Pzeimet,

    I appreciate that. I try to add some more details plus having the links right there make it easy to delve deeper.

  • tortorific

    Gravitons if they exist are force carrier particles. Time and distance are not constant, they vary according to frame of reference so you can’t have forces acting at a distance there has to be some kind of medium. Force carrier particles, gauge bosons (boson just means it has integer spin), provide this function. Gravity would be caused by gravitons interacting with the higgs boson (or whatever else it is that causes mass). We are pretty deep into theory at this stage.

    Gravitons would not be attracted to black holes they would be the thing causing the attraction. In a theory of quantum gravity it would not be curvature of space time that causes gravitational attraction.

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