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Doppler and enegry conservation

  1. Mar 13, 2007 #1

    malawi_glenn

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    In relativistic doppler shift, the energy of the photon is increasing och decreasing, due to realative motion of observer vs emitter. Let say that the photon is redshiftet, does any energy dissapear? :uhh:
     
  2. jcsd
  3. Mar 13, 2007 #2

    Danger

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    The energy isn't lost. While the frequency decreases, the wavelength increases. Each individual photon retains its initial energy; it just takes a 'longer path' to get here. That's a pretty lousy response. Best wait for ST or other to clear it up.
     
  4. Mar 13, 2007 #3

    malawi_glenn

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    Is this what you mean?

    When the photon is emitted, its wavelenght is less than the wavelenght as the observer see it. The energy must have dissapeared, scince the wave lenght is increasing. There must be a relativistic answer to this. Of course the frequency is increasing if wavelenght is decreasing.. But that is not the anszer to my question i think. You must compare the initial wave lenght and the final wavelenght.

    So i guess that the answer lies in the relativisic area. According to the emitter the photon energy is constant, and according to the observer the photon energy is constant, scince we cant follow a photons path. If I would make a guess. But energy is an invariant of motion.. ? That is why Iam asking.
     
  5. Mar 13, 2007 #4
    There will be difference in energy measured by emitter and receiver and that is fine. But the "change in energy" is not observed respect to emitter(and receiver). Difference in energy measured b/t two reference frame is not the same thing as the change in energy observed by one frame.

    What makes you think Energy is invariant of motion? For one, kinetic energy is obviously a relative quantity respect to different reference frame.
     
  6. Mar 13, 2007 #5

    Mentz114

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    Energy is an invariant of force-free motion, but that energy will be measured differently from different inertial frames.
     
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