system
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human
Since gravity according to Einstein is a property of the curvature of space, is there a need for this force to have a corresponding boson?

At the Origins Project presentation at ASU, David Gross brought up the figure of 10^19 GeV as the energy to search for the gravaton. My thought process was that if gravity is the result of the curvature of space itself, does it need a carrier boson (the fabled gravaton)? Would the lack of such a particle require any significant revisions to such theories as the Standard Model and Supersymmetry?
gpt
There's a really interesting proof - by Feynman and some others, IIRC - showing that if you start off with a theory of massless bosons with spin 2 (i.e., twice the spin of a photon) running around and interacting on a *flat* spacetime background, that theory turns out to look *exactly* like Einstein's theory of gravity as the curvature of spacetime.

In other words, those bosons have no choice but to look, on aggregate, like they're describing the curvature of spacetime, and because of their interactions with matter, that matter will *also* act as if it's living in a curved spacetime. This is pretty damn impressive, as is the fact that the theory that comes out of this is uniquely the one that Einstein came up with for completely different reasons.

So while you can have gravity-as-spacetime-curvature without gravitons, the two are pretty naturally linked (you can't have gravitons without spacetime curvature). This link is thought to occur physically through *quantum mechanics*. Every particle in the standard model is described at the non-quantum level in terms of fields, rather than particles. Fields are mathematical objects which have different values at different points in space and time - you might be familiar with the electric and magnetic fields, which are examples of this. Gravity is also a theory of a field, in this case the gravitational field, which describes how much and in which direction spacetime is curved at any given place.

Now, when you apply the rules of quantum mechanics to a theory of fields, then energetic excitations of those fields arise, and some of those excitations have exactly the right properties to be identified as particles. So when you have quantum theory in the mix, fields lead to particles.

We can do this process for every known field in the Standard Model. For example, quantizing the electric and magnetic fields leads to photons, or particles of light. However, we still don't know how to do it for gravity. While it's conceivable that whatever new physics we need in order to do this will require us to drop the fields -> particles picture, it seems most likely that quantizing gravity will lead to gravitons just like quantizing the electric and magnetic fields led to photons.