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Waves in a vacum

  1. Oct 10, 2009 #1
    According to relativity, any wave that can travel through vacume can do so only at c, because since vacum looks the same for every observer, so should the mechanism allowing the waves to travel (since that takes place in the vacum) and therefore the speed at which the wave travels (I think), and since all other speeds will be different for observers in other frames. So how can matter waves travel faster than c? I konw wiki says it can because it doesnt carry information, but there has to be a mechanism allowing it to travel......er, right?
  2. jcsd
  3. Oct 10, 2009 #2
    No, matter does not normally travel faster than c. What travels faster than c is not matter.

    There are events independent of each other. In some reference frames they may happen simultaneously but at distant places. Of course they have independent reasons. For example, we cannot and do not say that the event A made happen the event B because of super-luminary transfer of trigging signal.
  4. Oct 10, 2009 #3
    But the reason a wave travels is because the previous state of the wave results in the next state of the wave (eg. light travels because the changing electromagnetic field changes the electromagnetic field around it). So for a wave, state A did make happen state B, er... right?
  5. Oct 10, 2009 #4
    i still don't completely understand your logic, but i think i should point out now that waves of matter can't travel through a vacuum because of what they are, waves of matter. Matter is not a vacuum. Vacuums are the absence of matter. Waves traveling through matter are not traveling through a vacuum because matter is not a vacuum.
  6. Oct 10, 2009 #5
    i think i've figured out your problem! you're making the fundamental error! the error that sparked relativity in the first place:

    if you travel east at 1/2 the speed of light and your shine a flashlight pointing east then the light coming out of the flashlight is traveling at 3/2 the speed of light.

    this is false, because in relativity velocities do not add classically. 1mph + 1 mph ≠ 2 mph. it actually equals 1.999999999999999 and so on mph, because, as you approach the speed of light, it becomes more and more difficult to increase one's velocity.
  7. Oct 11, 2009 #6
    But thats like saying that since electromagnetic waves are waves of light (photons), they dont travel through vacum, either.
  8. Oct 11, 2009 #7
    This has nothing to do with my problem. I have no difficulty with velocity addition, I do have difficulty understanding how relativity can allow waves to travel through vacum at speeds faster (or just different from) c.
  9. Oct 11, 2009 #8


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  10. Oct 11, 2009 #9
    Can you reference the wiki article and explain what "matter waves" you're referring to?
  11. Oct 11, 2009 #10
    The de Broglie relations, mostly.
  12. Oct 12, 2009 #11
    In that case, A.T. made the right point. The real issue is whether you are referring to group velocity or phase velocity.

    The phase velocity of electromagnetic radiation can also exceed c, and just like the phase velocity of "matter waves" exceeding c, no information or energy is carried faster than c.

    The speed limit of c applies to signal velocity, or wave front velocity, (which is normally equal to group velocity), and is basically the speed that information and energy is carried by the wave.
  13. Oct 12, 2009 #12
    Hi Al68

    When considering phase velocity of a single photon is there any difference between wave front velocity and phase velocity? As I understand phase velocity it is simply the velocity of a wave peak. Is this incorrect??
    Does the idea of phase velocity greater than c pertain to groups of photons and the possible phase translation due to interference ?
    Or other??
  14. Oct 12, 2009 #13


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    No, because photons are not matter.
  15. Oct 12, 2009 #14
    That's right. But a single photon is not normally modeled as a wave at all.
    It's about wave dispersion. Here's a decent link: http://en.wikipedia.org/wiki/Phase_velocity.
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