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Group Velocities

  1. Jul 28, 2014 #1
    A co-worker recently shared an article with me that demonstrated a sound pulse traveling faster than c.

    After doing much research, I am still confused as to how this does not send information faster than light.

    If the leading edge of the pulse arrives before the rest of it, how would that not imply information? If you can detect it, does not it's mere presence imply information?

    I know that I am confused, I'm just looking to be set right.
  2. jcsd
  3. Jul 28, 2014 #2


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    Do you have the article link?
  4. Jul 28, 2014 #3
  5. Jul 28, 2014 #4


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    There's a recent thread https://www.physicsforums.com/showthread.php?t=762019 where the OP demonstrates a group velocity which is ~10x the phase velocity (c) and is negative. I think if you look at how it was done, you can see how the group velocity can be a huge number but it doesn't imply that any information (and/or energy) is actually being transmitted at -10c.
  6. Jul 28, 2014 #5


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  7. Jul 28, 2014 #6
    Thanks, I'll look into the paper. I've been trying to find papers on group velocity but they have all been stuck behind a pay wall.

    On a bit of a tangent, any good sources for scientific journals that are free?
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