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Gravitational redshift question

  1. Jul 7, 2012 #1
    A photon emitted from a star with a large mass (strong gravitational field) will be redshifted more than a comparable photon emitted from a star with a weaker gravitational field (comparable meaning the two photons would otherwise be expected to have exactly the same values in both cases, i.e. third to second energy level of Hydrogen, or whatever).

    What I don't understand is that it seems as though the two photons from the two stars are assumed to have the save energy levels at their source. But how exactly can that occur? When a photon of a given energy level enters the system with a high gravitational field, would it not be blue shifted as it entered, and then redshifted as it exits? In that way the redshift and blueshift would cancel and the entire gravitational redshift phenomenon would be hidden from us outside observers when we make the energy measurements. This is how I see it viewing gravity as a conservative force.

    Also as a star forms and increases its mass, doesn't everything is the entire system become more blueshifted under its own increasing gravitation? This would mean that all matter and energy in a larger mass system would be more blueshifted than a lower mass system, assuming they have the same size. Again, it seems as though photons emitted would be redshifted while exiting, and since they began blueshifted, the two effects would cancel and we outside observers wouldn't measure a difference.

    But of course we do measure gravitational redshift, so what am I missing about the nature of star systems and their relative redshift/blueshift effects?
  2. jcsd
  3. Jul 7, 2012 #2


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    As far as the atoms producing the photons are concerned (in their own frames) the frequency / energy is the same. But, to an observer in another frame, the energies will be different.
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