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Is a photon's wavelength relative to the velocity of its source?

  1. Jan 30, 2009 #1
    Is a photon's wavelength dependent on the velocity (or acceleration) of the atom which it was emitted?
    For example, if a photon is emitted from a stationary (or slow moving) molecule, then that molecule is accelerated. Will the photon's wavelength shift wrt to the acceleration (or velocity) of the faster moving molecule?
     
  2. jcsd
  3. Jan 30, 2009 #2
    Yes: it is called the Doppler effect, or often red shift (increasing wavelength) or blue shift (decreasing wavelength).

    If the source is moving away from the observer, a red shift occurs: the wavelength will increase (and thus the frequency will decrease). If it is moving towards the observer, the opposite (blue shift) occurs. By measuring the amount of red/blue shift of the light emitted by distant stars, you can basically calculate their speed relative to your speed.
     
  4. Jan 30, 2009 #3

    jtbell

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    Staff: Mentor

    If you mean the molecule accelerates after emitting the photon, then there is no effect on the photon.
     
  5. Jan 30, 2009 #4
    That's exactly what I meant. Thanks.

    Why would this not occur? Would it violate a physical law?
     
  6. Jan 30, 2009 #5
    Sorry, I misunderstood your question. :)

    Why would it occur? Your question sounds like you may have some idea about why it would and you are wondering why it wouldn't... Why and how would a molecule be able to change the wavelength of a photon that it has already emitted? Maybe something with entanglement but I have never heard about something like this.
     
  7. Jan 30, 2009 #6
    Nick,

    That's basically what I was wondering about.

    Thanks,

    Greg
     
  8. Jan 30, 2009 #7

    JesseM

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    No, entanglement can never be used to actually gain FTL information about what's going on at a distant location, and if what you're proposing was true we could instantaneously learn when a distant collection of atoms was accelerated by looking at the frequency of a beam of photons coming from them (you're saying the frequency would change instantly rather than after a time interval of the distance between us divided by c).
     
  9. Jan 30, 2009 #8
    A photon always has multiple wavelengths...

    Are we talking about the wavelength of an electromagnetic wave or the wavelength of a single photon? Big difference.
     
  10. Jan 30, 2009 #9

    JesseM

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    Not if it has a well-defined energy (in which case it has a single frequency given by E=hf and therefore a single wavelength since f = c/wavelength for a photon).
     
  11. Jan 30, 2009 #10
    c divided by the wavelength of what? The electromagnetic wave in which the photon is carried?

    In order for the photon to act as a particle it must be a set of multiple wavelengths that cancel out over a short distance.

    If the photon was only a single wavelength there would be no particle like nature.
     
  12. Jan 30, 2009 #11

    I'm not sure whether FTL is required for my example or not. Quantum entanglement doesn't necessarily enable FTL transmission, right?
     
  13. Jan 30, 2009 #12
    nuby,

    are you asking if the slight pressure of each photon on the next could change the EM wave in a similar way a string works where each atom contributes from a force oscillating one end of the string?
     
  14. Jan 30, 2009 #13

    jtbell

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    Photons don't interact with each other.

    (Except for a higher-order process called Delbrück scattering that's apparently so rare that it hasn't been observed yet for real photons.)
     
  15. Jan 30, 2009 #14
    I was going to answer that the interaction would have to be FTL and so therefore is impossible, was waiting to see if thats what he was thinking though.

    Interesting link
     
  16. Jan 30, 2009 #15

    JesseM

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    No, it doesn't, but quantum entanglement doesn't imply the frequency of a photon is instantly altered by later motions of the source either. If it did, that would imply the possibility of FTL communication--if your idea was correct then if I wanted to send a message I could just accelerate the source, and distant observers could measure a change in the frequency of a stream of photons emitted by the source instantly.
     
  17. Feb 3, 2009 #16
    If I'm not mistaken, the wavelength observed by the accelerating molecule changes, but the rest of the world sees it as the same wavelength. This can be explained using the dopler effect as previously mentioned.The photon's wavelength will only be impacted by the velocity of the particle when it is emitted.

    I've never heard of this sort of quantum entanglement... could one of you provide me with the appropriate link please?
     
  18. Feb 3, 2009 #17
    I suspect it is; however the effect (if true) would be strongly dependent on wavelenght: you need X-rays to change significantly an electron's speed, so for a transition in the visible spectrum, for example, the speed variation of the atom is very very small.
     
    Last edited: Feb 3, 2009
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