What is it with gravitons and black holes?

I occasionally wonder about what physicist really think regarding the hypothetical particles, gravitons, those carriers of the gravitational force mandated by the need* to quantize all the forces of nature.  Specifically, I wonder how they behave in and around black holes.

I know, from my understanding of General Relativity, that the influence of gravity travels at the speed of light, and the recent LIGO results, and all other experimental results of which I’ve heard, are consistent with that.  This must surely mean that the proposed gravitons travel at the speed of light, and are thus mass-less particles.  And if they are carrying a force, they must have some form of inherent energy, which means that, according to Einstein at least, their path would be affected by gravity.  This seems contradictory in some ways, but it’s my understanding that the electrical force produced by a moving electron also acts backward on itself, so I guess that’s not completely unreasonable…though here I’m veering further away from any deep knowledge, much to my sorrow.

My real question applies to the surface of an event horizon, that boundary in space-time within which all things are separated from the outside by the strength of the gravitational force – more particularly, according to Einstein, by the degree of curvature of space-time.  If gravitons are particles, carrying the gravitational force, are they constrained by the effects of the event horizon, or –  presumably because they wouldn’t be self-interacting – do they simply pass through it, it being irrelevant to their motion, unlike all other things with finite speeds…which means everything.  That sometimes seems contradictory to me, though by no means am I certain that I’m thinking correctly about this.  Could it be that the gravitons within and outside of an event horizon are two separate populations of gravitons, with the external ones somehow being generated at the horizon?  If not, then how can a particle ignore the degree of gravity, unless, of course, as a mentioned above, they are not self-interacting – which wouldn’t be unusual, since, if I understand correctly, photons also don’t interact with other photons.  But photons would, obviously, interact with gravitons, of course, otherwise they wouldn’t be effected by gravity, as we know they are…the most extreme example of this being at a black hole.

I know that a possible explanation for this might be found in M theory, in which we exist in a 3-brane that floats in a larger, higher-dimensional “bulk,” and that gravitons, unlike all the more “ordinary” particles are not constrained to remain within that brane, but can go above and below it, so to speak, thus bypassing any barrier that is exclusive to the brane.  But I don’t know if this really deals with the issue.

And, of course, how can the idea of gravity as a force, mediated by a quantum particle, be reconciled with the convincing and highly fruitful model of gravity as the consequence of the curvature of space-time?  Obviously, I don’t expect anyone to know the deep answer to this question, since it’s the biggest, most fundamental problem in modern physics:  our two best, most powerful theories of the world don’t work when brought together.  But if anyone out there has any idea of at least the form of such a possible reconciliation – i.e. do proponents of quantum gravity think that it will eliminate the notion of curved space-time, or do they think, somehow, that it will be an expression thereof – I would be delighted to hear from you.  My best reading to date on things like string theory hasn’t given me any real insight into the possible shape of such a unification.

Anyway, these are some of the thoughts that are troubling me this Monday morning.  I’d love to know any of your thoughts in response, or if you have any recommendations on further study materials, I would welcome those as well.


* due to the Uncertainty Principle, among other things.

What Are Black Holes?

Originally posted February 23, 2012

A very old friend of mine—that is, one I’ve known a long time, he’s no older than I am, and I hope I don’t yet count as “very” old—suggested that I write an article about what exactly black holes are. So, I thought about it, for all of about two seconds, and realized that black holes would be a great topic for a general science article. Continue reading “What Are Black Holes?”