hamster143 said:
Gravity is not part of standard model. There's no coupling constant for gravity in the same sense as we have for electroweak and strong interactions.
I don't mean to be pedantic, but I'm not sure if this is true. One can certainly write down a Lagrangian for gravity, the Einstein-Hilbert action,
\mathcal L = \sqrt{-g}(M_{Pl}^2 R)
Further, one could go ahead and quantize this as a theory for a spin-2 graviton, i.e. by writing the graviton as the perturbation on the flat Minkowski metric:
g_{\mu\nu} = \eta_{\mu\nu} + h(x)_{\mu\nu}
Upon expanding the scalar curvature, one finds (schematically)
\mathcal L = M_{Pl}^2(\partial h\partial h + h\partial h \partial h + h^2 \partial h\partial h + \cdots)
One can then canonically normalize and read off coupling constants. In fact, one can do low-energy (weak-field) calculations of gravitons. A very pedagogical example can be found in Zee, chapter VIII.1 where he calculates the gravitational interaction between two photons. (He also does the corresponding classical GR calculation to show that the answers match.)
The theory is non-renormalizable, but one can still perform calculations with the understanding that it is a low-energy effective theory for some quantum theory of gravity.
Cheers,
Joe
