# - gravitational coupling constant value?

1. Jan 5, 2009

### karnten07

URGENT- gravitational coupling constant value?

I urgently need to find a value for the gravitational coupling constant since the only on ei have is from wikipedia. Ive looked on the partcicle data group but couldnt find it there. Anyone?

Thanks
Karnten07

2. Jan 5, 2009

Staff Emeritus
Re: URGENT- gravitational coupling constant value?

I assume this is homework? (Why else would it be urgent?)

If you mean Newton's constant, the PDG most certainly does have it.

3. Jan 5, 2009

### humanino

Re: URGENT- gravitational coupling constant value?

Maybe Karnten07 is falling and trying to calculate her impact time.

4. Jan 5, 2009

### hamster143

Re: URGENT- gravitational coupling constant value?

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.

5. Jan 5, 2009

### JosephButler

Re: URGENT- gravitational coupling constant value?

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

6. Jan 5, 2009

### humanino

Re: URGENT- gravitational coupling constant value?

I think you are right, I wanted to post something like this, but I'm not a gravitation aficionados.

So I had a question. We can define the (e.g.) strong coupling constant g/T as the amplitude to emit a gluon in a sphere of radius T. Could I somehow cheat and define similarly the gravitational coupling constant, hiding under the carpet the (or showing thus the equivalent problem of) definition of graviton ?