# Gravitational red shift question

1. Aug 16, 2005

### asdf1

Why does the frequency of a photon emitted from the surface of a star decrease as it moves away from the star?
if hf=hf(1-GM/(R*c^2)
then as the distance becomes larger, R becomes larger, so fbecomes smaller?

2. Aug 16, 2005

### TD

We use the fact that the total energy must remain constant.
I'm not 100% sure, but I think t hat when leaving the star, the photon has to get out of the gravitation-field of the star, therefore its kinetic energy has to decrease since the potential energy is rising.

3. Aug 17, 2005

### asdf1

hmm... that makes sense, but the mathematical equation looks weird...
it seems to say otherwise~

4. Aug 18, 2005

### asdf1

still a little weird~