# Homework Help: General Relativity gravitational redshift

1. Oct 14, 2012

### clandarkfire

1. The problem statement, all variables and given/known data
The gravitational redshift tends to decrease the frequency of light as it travels upwards a distance h,$$\frac{\Delta{f}}{f_{0}}=\frac{-gh}{c^2}$$
integrate both sides of this equation (from the surface of the gravitation body out to infinity) to derive the expression for the change in frequency near a high gravitational field:$$\frac{f}{f_{0}}\cong{1-\frac{GM}{Rc^2}}$$
2. Relevant equations
Given above. A photon is emitted at the surface of the gravitational body (M) with radius R. It's frequency is measured distance h above the gravitational body to be f, while its frequency at the gravitational body is f0. g is the gravitational attraction of the body on the photon.
3. The attempt at a solution
Well, I've gotten far enough to see that $$\frac{f}{f_{0}}-1=\frac{-gh}{c^2}$$, which makes sense because gh is the increase in gravitational potential energy.
However, I don't know how to express g. I would use $$F_{g}=G\frac{Mm}{r^2}$$, but because a photon's mass is zero, I don't know what to do.
I guess I also need to integrate with respect to h.
Help!?

2. Oct 14, 2012

Anyone?