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Would Gravity Waves Show Doppler Shift?

  1. Jul 11, 2008 #1
    Random question that I thought of while trying to fall asleep last night. :)

    We all know that galaxies moving very fast away from us exhibit visual redshift.

    If gravity waves / gravitons are real, they must then have a frequency of some kind. Does the frequency of a gravity wave affect its strength? (i.e. does a more massive object emit higher frequency gravity waves?) If so, would an object moving away from us be "red shifted" to have a weaker gravitational effect than if it were the same distance away, but moving towad us?
     
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  3. Jul 11, 2008 #2

    Redbelly98

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    The frequency of the wave indicates the rotation rate of the object emitting it.

    And yes, they would exhibit a Doppler shift. Doppler shift happens for any periodic emission with a finite propagation speed. Light, sound, and gravity waves all have this property.
     
  4. Jul 11, 2008 #3
    Ah, I see. So the frequency of the gravity wave would not affect its strength?

    I was wondering if gravity waves would behave like photons, i.e., the frequency - not amplitude - would determine the force it exerted on a particle.
     
  5. Jul 11, 2008 #4

    Redbelly98

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    Frequency would determine the energy of individual gravitons.

    However, in practice the field of a gravitational wave would contain many gravitons. The amplitude of the field would determine the force exerted on a mass.

    Similarly, it is the electric field amplitude that determines the force that a laser beam exerts on, say, an electron.
     
  6. Jul 11, 2008 #5
    By "rotation", do you mean about an axis through the object's center of gravity, something akin to an orbit, or both?

    Regards,

    Bill
     
  7. Jul 11, 2008 #6
    I think in GR it is incorrect to think that gravitational waves are emitted by objects. Gravitational waves exist due to the changing relationships between multiple objects.
     
  8. Jul 11, 2008 #7
    I think that is sufficiently vague to cover what I was alluding to. :smile:

    Regards,

    Bill
     
  9. Jul 11, 2008 #8

    Redbelly98

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    Antenna and MeJen,

    I should probably have used the word "system" rather than object. At any rate, I was making a general comment to answer the op's question about the meaning of frequency.

    As to the source of gravity waves that people are trying to detect, I'll defer to somebody more knowledgeable than I:
    https://www.physicsforums.com/showpost.php?p=1696992&postcount=5

    Regards,

    Mark
     
  10. Jul 11, 2008 #9
    Well, what I (the op :)) was getting at was would an object moving very fast either toward or away from us exert a different instantaneous force than if it were stationary, due to doppler shift of the gravity waves?

    If I understand Redbelly correctly, the answer is no.
     
  11. Jul 11, 2008 #10
    The "mountain range" on a spinning object is a pretty good example of what I was thinking of when I said "akin to an orbit". I might even wager that such an oscillation would yield the highest frequency waves.

    Regards,

    Bill
     
  12. Jul 11, 2008 #11
    I think E=hf still holds (which would imply the opposite), but I'll defer to someone more knowledgeable than myself.

    Regards,

    Bill
     
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