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Homework Help: Relativity: Gravitational Redshifting

  1. Dec 1, 2009 #1
    1. The problem statement, all variables and given/known data
    A signal of frequency 500Ghz leaves the surface of the star Alpha Centauri,
    travels through space, and arrives at the surface of the Earth. At what frequency is the signal
    received on planet Earth?
    (Hint: For this problem, treat Earth and Alpha Centauri as if the separation were infinite. Also,
    ignore any cosmological effects, motion of the star or planet, and any interstellar scattering effects)

    2. Relevant equations

    Gravitational redshifting: 1/sqrt(rs/r) -1

    3. The attempt at a solution

    The equation for gravitational redshifting doesn't seem to be useful in this situation since earth is taken to be infinitely far away. Just be thinking about it one could deduce that the signal is redshifted at first and then blueshifted at earth. But if this is correct I don't know of an equation for shifting from a low gravitational potential to a higher one. Anyone have some hints to get me thinking in the right direction?
  2. jcsd
  3. Dec 2, 2009 #2


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    Science Advisor

    Make sure you know what the variables r and rs represent. You may need to apply this equation more than once.

    Cheers -- sylas
  4. Dec 2, 2009 #3
    ah..i was labouring under an obviously wrong idea of what the r stood for. thanks
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