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Question about phase velocity

  1. Apr 5, 2012 #1

    I am reading about phase velocity vs group velocity and while I get most of it, there is one thing I don't see. How can the phase velocity ever exceed c? At least in a vacuum, I think it never happens. Is that right? And if it does exceed c in a medium, what is the general physics behind it that lets it do that?

    Thank you for your help!
  2. jcsd
  3. Apr 6, 2012 #2
    Have you tried here:


    Phase velocity CAN exceed c...be superluminal.....
    but such does not carry any information. It's group velocity that carries info and cannot go faster than c.... and is what sets limits on information processing in telecommunications, for example...Note that phase and group velocities can even go in opposite directions.....illustration above.

    Why nothing can go faster than c...because all observers always see light going by at 'c'...you just can't ever catch up not matter how fast you peddle.......
  4. Apr 6, 2012 #3

    George Jones

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    In anomalously dispersive materials, group velocity can be larger than c.
  5. Apr 6, 2012 #4
    Yes I did read the wiki article. My question is: Can the phase velocity of an electromagnetic wave in vacuum exceed c? I think it cannot but I whether I am right about that.

    Now, coming to light in media. My picture to understand phase velocity is simply an infinite sine wave of some frequency. So can someone explain exactly what causes this sine wave to travel faster than c in a medium? The Wikipedia articles doesn't seem to cover this (or I have missed it).

    Thank you for your help.
  6. Apr 7, 2012 #5


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    An electromagnetic wave in vacuum travels at c, period. No need to worry about that.

    To understand the other cases, remember that the phase velocity is just a feature of the wave pattern and does not represent the propagation of any physical quantity.

    For example the phase velocity exceeds c for electromagnetic wave propagation down a waveguide. Why? Because the wave fronts of the wave lie at an angle α to the axis of the waveguide. This effectively increases the wavelength of the wave parallel to the waveguide by a factor 1/sin α. Which increases the phase velocity by the same factor. This carries no physical significance at all, it is just geometry.
  7. Apr 7, 2012 #6
    Thank you Bill. That was very clear!
  8. Apr 7, 2012 #7
    Are you familiar with the Lijun Wang of Princeton experiments that supposedly observed vastly superluminal speeds?? I have searched but found only superficial descriptions of their setup and results.

    In any case I don't understand how either a group or phase velocity could be detected as they both seem to be abstract relationships between the frequencies/wave patterns of photons traveling at different speeds in a medium but which, still are individual wave packets and can only be detected as such.
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