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EM radiation appearing to travel faster than C

  1. Dec 16, 2015 #1


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    I just had a flashback to an applied electromagnetic course I took a few years ago. I believe we were learning possibly about waveguides (maybe) or something, but there appeared a phenomenon in the math that made it appear as if the wave was traveling faster than C.

    When asked about it, the professor said this is a sort of 'trick' because actually the information was not traveling faster than C, so it is allowed by the universe.

    I sort of took it for granted at the time and accepted his explanation (it's not the first time I can remember hearing of it) but if anyone knows what I'm talking about I would be grateful to be reminded, now that I have developed a taste for information theory.
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  3. Dec 16, 2015 #2


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    I think you are talking about the phenomenon of anomalous dispersion. In this case the phase velocity of the light can be greater than c. However, information is propagated at the group velocity, not the phase velocity. The group velocity is always <= c.
  4. Dec 16, 2015 #3


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    Interesting... I am curious to try to think of how you could transmit information at phase velocity, > c.
    Changing the phase velocity does not transmit information faster than information can travel via the group velocity? I will have to ponder the physics for awhile I suppose when I get the chance, thanks for reminding me.
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