1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: General Relativity gravitational redshift

  1. Oct 14, 2012 #1
    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,[tex]\frac{\Delta{f}}{f_{0}}=\frac{-gh}{c^2}[/tex]
    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:[tex]\frac{f}{f_{0}}\cong{1-\frac{GM}{Rc^2}}[/tex]
    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 [tex]\frac{f}{f_{0}}-1=\frac{-gh}{c^2}[/tex], which makes sense because gh is the increase in gravitational potential energy.
    However, I don't know how to express g. I would use [tex]F_{g}=G\frac{Mm}{r^2}[/tex], 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.
  2. jcsd
  3. Oct 14, 2012 #2
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook