Does theoretical physics suggest that gravity is the exchange of gravitons or deformation/bending of spacetime?

Question

Throughout my life, I have always been taught that gravity is a simple force, however now I struggle to see that being strictly true.

Hence I wanted to ask what modern theoretical physics suggests about this: is gravity the exchange of the theoretical particle graviton or rather a 'bend' in space due to the presence of matter?

I don't need a concrete answer, but rather which side the modern physics and research is leaning to.

I think that whichever model suits your need the best should get the job done. But then again I am not a Physicist. — Harshit Joshi, Feb 27 '19 at 16:18
Well, even if you are not a physicist this is still a good point, so thank you. — steve_just_steve, Feb 27 '19 at 16:20
@steve_jyst_steve: I was struggling to understand what was being asked here, so I took a moment to correct the grammar and phrasing. If you’re not happy with that, please just roll-back. — Mozibur Ullah, Feb 27 '19 at 20:57
@MoziburUllah Hello, and thank you very much for doing that, definitely sound better now! — steve_just_steve, Feb 27 '19 at 21:00
An article in the New Yorker about looking at the same "thing" using different stances: A Different Kind of Theory of Everything (via Peter Woit's blog) — David Tonhofer, Feb 27 '19 at 21:31
@TomášZato Hey Tomas, from what I learned from the answers it seems to be just that, both! — steve_just_steve, Feb 28 '19 at 07:41
"I have always been taught that gravity is a simple force", then have you ever asked what is a force ? — Our, Mar 02 '19 at 04:19
@harshit54 how do you define a physicist ? Some who has a Physiscs degree, or as someone who does Physics ? — Our, Mar 02 '19 at 04:23

score 40 · Accepted Answer · answered Feb 27 '19 at 18:24

40

Both.

General relativity describes gravity as curvature of spacetime, and general relativity is an extremely successful theory. Its correct predictions about gravitational waves, as verified directly by LIGO, are especially severe tests.

Gravity also has to be quantum-mechanical, because all the other forces of nature are quantum-mechanical, and when you try to couple a classical (i.e., non-quantum-mechanical) system to a quantum-mechanical one, it doesn't work. See Carlip and Adelman for a discussion of this.

So we know that gravity has to be described both as curvature of spacetime and as the exchange of gravitons. That's not inherently a contradiction. We do similar things with the other forces. We just haven't been able to make it work for gravity.

Carlip, "Is Quantum Gravity Necessary?," http://arxiv.org/abs/0803.3456

Adelman, "The Necessity of Quantizing Gravity," http://arxiv.org/abs/1510.07195

answered Feb 27 '19 at 18:24

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Hey man, the links you sent are very useful and this really helped clear up certain misconceptions I had about gravity, so it can and in fact is very likely to be both. Thank you very much :-) !! – steve_just_steve Feb 27 '19 at 19:16
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This is probably a naive question, but wouldn't gravitons also produce gravitational waves, since the other forces do, e.g. EM waves? – gardenhead Feb 27 '19 at 20:48
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@gardenhead https://physics.stackexchange.com/questions/215173/what-is-the-difference-between-gravitons-and-gravitational-waves This suggests that gravitons are to gravitational waves as photons are to electromagnetic waves. So if my understanding is correct (don't bet on it, I mostly know classical physics), it is more like saying the graviton is the carrier on the quantum level for the gravatational waves. – JMac Feb 27 '19 at 21:13
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@JMac Your understanding is correct indeed. – Avantgarde Feb 27 '19 at 23:25
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" We do similar things with the other forces..." though it is clear what you mean (namely that force carriers particles naturally arise from gauge theories) this sentence is per sé slightly incorrect. The other forces live on a fixed (local) geometry at most coupling to it; they have the natural units to be (perturbatively) renormalisable - which is arguably not the case for the gravitational field (so in this respect it is indeed different from the pack). – gented Feb 28 '19 at 00:38
The phrase "because all the other forces of nature are quantum-mechanical" begs the question. It presumes that gravity is a force, rather than a curvature of spacetime. – Monty Harder Feb 28 '19 at 21:07
@MontyHarder You are also begging the question, by presuming that “a force” and “a curvature of space time” are different things rather than different mathematical treatments of the same underlying phenomenon. – Mike Scott Mar 08 '21 at 11:52
@MikeScott A force is a field that causes affected particles to accelerate, thus altering their trajectories in spacetime, and has a boson that carries the force (at the speed of light if that boson be massless). Gravity does not do that. It changes spacetime itself, and there is no distinction between affected/unaffacted particles. (The particles per se are not affected by gravity, but they appear to be, because spacetime is affected.) – Monty Harder Mar 10 '21 at 15:56
@MontyHarder You’re still describing a difference in the mathematical treatment of gravity considered as a field or as spacetime geometry. Since we don’t know what spacetime really is, we don’t know what is actually being described by the maths. – Mike Scott Mar 10 '21 at 19:24

score 10 · Answer 2 · answered Feb 27 '19 at 20:44

Gravity is as simple a force as all the others, which means that its simple when not looked at too closely, and far more sophisticated when it is. Given that it’s generally supposed all forces are merely low energy relics of a single high energy one, we might suppose that all the forces are as complicated as each other when looked at closely.

Popularly, Gravity is seen as different from the other forces in that its geometric. It turns out that the other forces are also geometric. Nevertheless, the main difference is that in gravity, the metric tensor, which tells us how to measure distances, times and angles is directly implicated in a way that it isn’t in the other forces. For example, there are two equations in EM, one of which does not involve the metric and hence seen as topological, and the other, which does (via the Hodge star) and hence, is coupled with gravity. The other two forces, the weak and strong force are modelled as gauge theories of the Yang-Mills type and hence directly generalising the EM equations. So similarly, they also have a topological and metric aspect, and the latter means it couples to gravity.

Now, whilst gravity hasn’t yet been quantised with several ongoing major projects that attempt this there are several partial semi-classical results which are used to help orientate research into this. One such result is that the quanta of gravity, the graviton, is a massless spin-2 particle. This is understood by looking at a linearisation of gravity which is used in the theory of gravitational radiation, and then by quantising this to show we have a massless spin-2 particle, aka the graviton.

Thank you very much for your answer! Linearisation of gravity is definitely something to look into for me :-)! — steve_just_steve, Feb 27 '19 at 20:50
:-) I thought this was the better answer, (of two good answers). I found it very interesting. Thanks. — Ben, Feb 28 '19 at 20:25
@Ben: You’re welcome. It’s nice to know that an answer is appreciated. — Mozibur Ullah, Mar 01 '19 at 03:59

score 0 · Answer 3 · answered Feb 28 '19 at 20:49

Exchange of particles (gravitons) is a mechanism. Bending of space is a phenomenon for which, the mechanism is not known yet. If that mechanism is known, we can likely manipulate gravity. It is simple to understand repulsion in terms of particle exchange, but I struggle to understand attraction via particle exchange even considering particle exchange in round about path. Moreover, graviton has not been discovered, and even other forces involve virtual particles. Does virtual mean imaginary, or it means something comes into existence, then disappears and on and on?

So, in my opinion - Answer is Bending of space, whatever the mechanism is.

Does theoretical physics suggest that gravity is the exchange of gravitons or deformation/bending of spacetime?

3 Answers3

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