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Thread: Question about Pound-Rebka?

  1. #1
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    Question about Pound-Rebka?

    Question about Pound-Rebka...


    It is my understanding that...

    An Fe-57 Emitter was placed on the ceiling, inside a speaker cone,
    and an Fe-57 Receiver was placed on the floor (stationary).

    The speaker's phase was manipulated to cause a Doppler Redshift,
    to offset the Gravitational Blueshift from the gravity of the Earth,
    in order to achieve the necessary resonance for photon emission and absorption.

    Is this correct so far, or...
    Was the Doppler Redshift caused by either:
    the emitter having been moved away from the receiver?, or
    the receiver having been moved away from the emitter?


    Then the experiment was reversed...
    Whereby the Gravitational Redshift was offset by a Doppler Blueshift.

    My follow-up question(s)...
    Was the emitter placed on a speaker cone on the floor, with a stationary receiver on the ceiling?, or
    Was the emitter stationary on the floor, with a receiver placed inside a vibrating speaker cone on the ceiling?, or
    Was the emitter moved towards the receiver?, or
    Was the receiver moved towards the emitter?


    Thanks.

  2. #2
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    The redshift was caused by the vibration of the speaker cone - during each vibration cycle, it would hit a state of motion that carried the emitter away from the detector at just the right speed to counteract the gravitational blueshift at the detector. By checking the phase of the speaker cycle at which the detector absorbed gamma rays this velocity could be determined, converted to a redshift, and the corresponding gravitational blueshift derived.

    Grant Hutchison

  3. #3
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    Re the follow-up questions - the original paper is here.
    The source was Co57 and the absorber Fe57. The paper isn't entirely clear, but given that the absorber assembly was "integrally mounted" on a Dumont 6363 multiplier phototube, I'd assume the reversal of the experiment involved swapping speaker+emitter with absorber+detector, rather than mounting the absorber+detector on the speaker cone.

    Grant Hutchison

  4. #4
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    Thank you for your responses Grant.

    As a follow-up question...

    I would assume that in the second case where the Emitter was shot upwards,
    that the Vibration of the Speaker Cone would cause a movement of the emitter, such that the Emitter's clock would slow (moving clocks slow in SR)...i.e a Redshift of the emitted photon.

    This movement effect (Redshift) should somewhat offset the Doppler Blueshift caused by the Emitter moving upwards toward the Detector.
    Is this correct?
    Would the speaker's vibration need to be increased to compensate for the difference?

    To clarify...
    When the emitter moves away from the detector (gamma goes up), there is a Doppler Redshift and a Redshift from moving clocks,
    and when the emitter moves towards the detector (gamma goes down), there is a Doppler Blueshift plus a Redshift from moving clocks,...Correct?

    Thanks!

  5. #5
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    In principle, you would need to allow for the effects of Special Relativity as well as classic Doppler (which combine to produce "relativistic Doppler"), as you describe. In practice, the velocity of the speaker cone is so low compared to the speed of light that classical Doppler would be indistinguishable from relativistic Doppler.

    Grant Hutchison

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