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Phase Velocity

  1. Feb 23, 2008 #1

    I'm studying plasma waves now, and we have talked about waves in class that have phase velocities faster than the speed of light. For example, some of the waves from the Appleton-Hartree dispersion relation have this characteristic.

    I asked my professor about it and he said that is wasn't a problem because some of the velocity was with the transverse motion and some with the propagation of the wave. This isn't completely satisfying to me because then I have to think of information traveling faster than light. Is this a quantum mechanical effect or is there some other way I should think of these waves?


  2. jcsd
  3. Feb 23, 2008 #2


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    Staff: Mentor

    You get a similar effect by swiping a laser beam across the moon from earth. The dot it makes moves faster than the speed of light, but no photon ever does, and thus no information can be transmitted that way.
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