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Can light be a medium for sound?

  1. Jan 15, 2004 #1
    I guess I have rather a simple question: Can light be a medium for sound waves? I dont know where this question should be placed so I thought general physics was a good spot.
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  3. Jan 15, 2004 #2


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    No. Photons are not charged, and do not interact with each other. They thus cannot carry pressure waves.

    - Warren
  4. Jan 15, 2004 #3


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    Re: Re: Can light be a medium for sound?


    The intensity of a light beam can be modulated with acoustic frequency. This signal can then be transmitted via optic fiber or even through the air (or vacuum) to be demodulated by a receiver. Thus you have transmitted audio signals via a light beam.
    This signal and be either analog or digital. This is the method used in long distance fiber optic communications or between your DVD player and receiver when an optical connection is available.
  5. Jan 17, 2004 #4
    CHROOT - question: Is the only reason you say photons don't interact is because they have no charge?

    Integral - when you "modulate/demodulate" you are converting information signals, the sound wave is not riding on the light wave.

    ARandomNoun - the main reason that wouldn't work is because light has 0 density, and the speed of the sound wave in o density would be 0. There is a theoretical "magic" density (nothing on this planet*) where light would slow down enough, and sound would speed up enough for their harmonic capabilities to be harnessed.

    * iron or steel increase sound to around 5121m/sec, still 51,500 times less than the amount needed. Perhaps this was the density of the Universe @ BIG bang.

  6. Jan 17, 2004 #5
    I think they could, though it wouldn't be very efficient. Ater all, light can transfer momentum. Imagine a big solar sail being hit with a nonconstant light source. It'd be a hugedspeaker (though, being in a vacuum, it woulnd't be very loud).
  7. Jan 18, 2004 #6
    Photons do not interact because there is no potential croos-term in the QED lagrangian (and hence no photon-photon vertex in the Feynman diagrams). This is deeper than saying they have no charge, since you do have cross-terms of the vector potential and the electrons (spinors).
  8. Jan 18, 2004 #7


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    Light can be made to oscilate in such a fassion as to re-create sound waves. This is the technique used in laser-surveilance equipment, wherein a laser beam is pointed at a winow of a room and reflected off that surface back to a receiver. Sound waves in the room cuase the winow to vibrate, and that vibration is carried through the beam to the receiver, where the original sound waves can be reproduced. But that requires modulation/demodulation as Integral spoke of earlier.

    However, if a person had need to, they might be able to reproduce sound waves using only the expansion/contraction of air caused by the thermal energy of a laser or other light source.
  9. Jan 19, 2004 #8
    chroot & GRQC

    I was mainly concerned with the statement that photons do not interact. Wouldn't two photons travelling side by side increase their amplitude/intensity? And photons with x wavelength and y wavelength combining to make photon w/ z wavelength?

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