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Phase velocity faster than light?

  1. Mar 9, 2014 #1
    I am currently taking an undergrads course in atomic physics, and I have just grazed the concepts of matter wave functions, group waves, group velocity, and phase velocity. For the latter, it has been stated that phase velocity can exceed the speed of light. I don't fathom this. Could anyone please help me with this concept?
  2. jcsd
  3. Mar 9, 2014 #2


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    Staff Emeritus
    Science Advisor

    Does this help?

    Per wiki: http://en.wikipedia.org/wiki/Faster-than-light#Phase_velocities_above_c

    The phase velocity of an electromagnetic wave, when traveling through a medium, can routinely exceed c, the vacuum velocity of light. For example, this occurs in most glasses at X-ray frequencies.[11] However, the phase velocity of a wave corresponds to the propagation speed of a theoretical single-frequency (purely monochromatic) component of the wave at that frequency. Such a wave component must be infinite in extent and of constant amplitude (otherwise it is not truly monochromatic), and so cannot convey any information.[12] Thus a phase velocity above c does not imply the propagation of signals with a velocity above c.[13]
  4. Mar 9, 2014 #3
  5. Mar 9, 2014 #4


    Staff: Mentor

    Its purely a mathematical artefact - nothing is really travelling FTL.

    Also matter waves was simply an interim theory on the way to full blown Quantum Mechanics that appeared when Dirac published his transformation theory.

    Don't spend too long on what is just really of historical interest. Try, as soon as possible, to come to grips with the correct theory. So you understand its REAL basis check out:

    Your course may not get to this for a while, or indeed at all, depending on its depth, but just keep in the back of your mind this is what QM is REALLY about - the rest is basically historical background.

    Last edited: Mar 9, 2014
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