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Homework Help: Derivation of gravitational redshift: Mass of a photon?

  1. Mar 22, 2014 #1
    This is not exactly a homework question.

    In a physics textbook, they derive an expression for gravitational redshift of a photon emitted by a star at a large distance from the source by taking photon as a mass travelling up, against a gravitational potential and hence expending its electromagnetic/quantum energy. The mass of the photon is taken to be:
    [itex]m = \frac{h\nu}{c^{2}}.[/itex]​

    According to that equation, the mass of an X-ray photon of [itex]10^{21} Hz[/itex] would be about 8 times mass of an electron.

    Is this treatment appropriate?

    PS: I'm aware that the "accepted" derivation for gravitational redshift involves general relativity, but the expression derived in this text is a special case of that expression.
  2. jcsd
  3. Mar 22, 2014 #2


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    Staff: Mentor

    If you replace m by the energy E, you get a correct approach, so where is the point of using a relativistic mass...
    Anyway, it gives correct results.

    A photon with that frequency (I would call it gamma instead of X-ray, but that does not matter) has more energy than an electron at rest, indeed.
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