Gravity as geometry vs as a force

[Cobalt101]

I'd appreciate some explanation on how does one understand/reconcile the seemingly alternative concepts of gravity as (i) due to the warping of space by matter vs (ii) the exchange of gravitons. Is the latter a construction of how gravity can be considered within a quantum mechanics framework ? And how would exchange of gravitons produce a force of attraction anyway ?

 

[PeterDonis]

First of all, gravitons have not been observed and nobody expects to observe them any time soon. They are just a theoretical concept that many physicists like because of the obvious analogy with other interactions when modeled using quantum field theory. So the answers I will give below are only valid on the assumption that this theoretical concept will actually turn out to be verified experimentally at some point.

how does one understand/reconcile the seemingly alternative concepts of gravity as (i) due to the warping of space by matter vs (ii) the exchange of gravitons

The same way we reconcile the classical and quantum views of other interactions. For example, electromagnetism can be thought of as a classical field, or as a quantum interaction mediated by the exchange of photons. Which model we use depends on the specific scenario; the classical model works well for many scenarios but has limitations; the quantum model is more fundamental but also harder to use.

how would exchange of gravitons produce a force of attraction anyway ?

The same way an exchange of photons between particles of opposite charge produces an attraction. A good brief discussion by John Baez is here:

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

If you want to ask further questions on this particular topic, you should start a new thread in the Quantum Physics forum.

 

[pervect]

I'd appreciate some explanation on how does one understand/reconcile the seemingly alternative concepts of gravity as (i) due to the warping of space by matter vs (ii) the exchange of gravitons. Is the latter a construction of how gravity can be considered within a quantum mechanics framework ? And how would exchange of gravitons produce a force of attraction anyway ?

It's probably too technical, but http://arxiv.org/abs/astro-ph/0006423 does discuss one possible non-geometrical approach to gravity.

A pedagogical description of a simple ungeometrical approach to General Relativity is given, which follows the pattern of well understood field theories, such as electrodynamics. This leads quickly to most of the important weak field predictions, as well as to the radiation damping of binary pulsars. Moreover, certain consistency arguments imply that the theory has to be generally invariant, and therefore one is bound to end up with Einstein's field equations. Although this field theoretic approach, which has been advocated repeatedly by a number of authors, starts with a spin-2 theory on Minkowski spacetime, it turns out in the end that the flat metric is actually unobservable, and that the physical metric is curved and dynamical.

If you take the theory really seriously as a way to teach GR, though , there may be some pitfalls related to topology - which, however, the author doesn't discuss. Basically, if one envision an unobservable flat underlying structure to space-time, one can't imagine multiply-connected topologies (at least, I don't see any way to do it), but with the geometric view, one can imagine multiply connected topologies.

The oversimplified version: If one follow Straumann's approach, one might understand a lot of the predictions of GR, but one probably won't understand black holes in the same manner that traditional GR does.

It's unclear to me how (or even if) this could be experimentally addressed, though.