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Speed of lasers

  1. May 19, 2012 #1
    So from what I understand nothing is faster than the speed of light, but how come lasers are faster than the speed of light? A quick google search shows that a laser set record by going 300 times faster than the speed of light!

    Also, what happens if you shine a regular consumer laser in space? Would it travel years without dissipating?

  2. jcsd
  3. May 19, 2012 #2


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    Staff: Mentor

    Lasers are not faster than the speed of light - they are light. You misread whatever link you found.
  4. May 19, 2012 #3
    Did some research regarding this issue, they seem to talk about group or phase velocities which can be faster than the speed of light. What about my second question, if you point a laser in space that is.
  5. May 21, 2012 #4

    Andy Resnick

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    Science Advisor
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    Yes, the group velocity can formally exceed c_0, and there are also experiments showing that, for example, the leading edge of a pulse can be detected before the pulse was emitted. This is not a conflict with special relativity, because the group and phase velocities are different, and near an absorption line become *very* different.

    As for the diffraction of a laser beam, if it is a "normal" (Gaussian) beam, the product w*θ is constant, where w is the beam waist and θ the divergence angle: in order for a Gaussian beam to be collimated, it must be expanded to a large diameter. Non-Gaussian beams (Bessel beams, optical vortices, etc) do not diffract this way and approach a limit of zero diffraction:

  6. May 21, 2012 #5
    As others have said the group and phase velocities can be faster than c, group velocity in some materials can even be negative but the signal velocity cannot exceed c and this is what carries information.
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