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Science News

Electrical Pulses Break Light Speed Record 68

J'raxis writes "PhysicsWeb writes that 'Pulses that travel faster than light have been sent over a significant distance for the first time. Alain Haché and Louis Poirier of the University of Moncton in Canada transmitted the pulses through a 120-metre cable made from a coaxial 'photonic crystal.' Haché and Poirier emphasize that their experiment does not break any laws of physics. Although the group velocity exceeds the speed of light - an effect permitted by relativity -- each component of the pulse travels slower than light.'"
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Electrical Pulses Break Light Speed Record

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  • Re:Well.... (Score:4, Informative)

    by PhuCknuT ( 1703 ) on Friday January 25, 2002 @02:37AM (#2899498) Homepage
    This actually has nothing to do with bandwidth, nor will it make ftl communications possible. Think of it this way. Put in a sine wave, and it shifts it 90 degrees out of phase. So when the leading edge of the wave hits the other end, it makes it a peak, and when the peak gets there, it is at the trough of the output. It looks like the peak got to the output faster than light, but in reality it was just the leading edge of the wave being amplified.
  • by Mt._Honkey ( 514673 ) on Friday January 25, 2002 @04:16AM (#2899668)
    We have always known that we could send waves "piggybacking" on light that move FTL. When light enters a plasma, such as the ionosphere, the free electrons can cause little ripples to travel along the light wave at significant FTL. However, while you can send information on these waves, the information itself does not move FTL, but at c. This has been known for quite some time, this is just the first time I know of that it has been done in a cable.
  • by Anonymous Coward on Friday January 25, 2002 @10:02AM (#2900293)
    Send a signal. Compare it with a reference signal. Compare times. The comparision of one against the other is "beating" one signal against another. So comparing sines. The two signals combine and/or cancel, and produce a new signal. That signal correlates to a time.

    That's the same way that radar worked back when it was just an oscilloscope hooked to a radio. (Oscillation Scope.) You don't actually run a clock to see how far the signal has travelled, rather you compare it against another signal for a time difference. Very easy to do with analog.
  • by Captoo ( 103399 ) on Friday January 25, 2002 @11:47AM (#2900781)
    If you look closely at the equation used to describe time dialation in the theory of reletivity, you will see that it is simply a variation of the famous a^2 + b^2 = c^2, where a is your velocity through space and b is your velocity through time. What it boils down to is that your speed in four dimensions always equals c.
  • by SIGFPE ( 97527 ) on Friday January 25, 2002 @12:18PM (#2900953) Homepage
    There's nothing unusual or fantastical about this claim. Group velocity/phase velocity 'n' all that stuff is basic undergrad material.
  • by hubie ( 108345 ) on Friday January 25, 2002 @12:56PM (#2901209)
    This is straight-up physics of waves, and as such is not unexpected. From time to time another experiment is done and it gets widely reported and misunderstood (even by scientists that should know better, but who have forgot their freshman/sophomore level physics).

    This is the basic misunderstanding of what the phase, group, and signal velocities of a wave system are. The bottom line is that you cannot send information using these superluminal signals, so there are no time travel/relativity problems. A nice Java applet showing this is here [netspace.net.au].

  • by mmontour ( 2208 ) <mail@mmontour.net> on Friday January 25, 2002 @01:04PM (#2901288)
    Please forgive my lack of knowledge in this area, but what you said just doesn't make sense to me. If you can send waves FTL, and send information on those waves, then it logically follows that you can send information FTL... What am I missing?

    Take a look at this applet [netspace.net.au] and this page [netspace.net.au]. They give a good illustration of the concept:

    [...]If dn(v)/dv is sufficiently negative, it can reduce the denominator in Equation (3) to less than one, yielding a group velocity greater than c. Why is this not a contradiction of special relativity? No energy or information needs to travel at the group velocity in order for the shape of the wave to exhibit features that move at that speed. If you tried to signal someone with a superluminal pulse by dropping a shutter in its path at the last moment, you'd find you were too late: the pulse would happily "pass through" the shutter, because every influence that was actually responsible for its appearance on the other side would have passed through already.
  • by Crispy Critters ( 226798 ) on Friday January 25, 2002 @04:11PM (#2902696)
    What is happening here is that they are sending a pulse of light, and the envelope or shape of the pulse changes as it travels. Previous papers have shown a pulse that starts out as a Gaussian and becomes progressively more skewed as it propagates.

    This allows the peak of the pulse to move faster than light speed. However, the leading edge of the pulse does not.

    This is why this is not a technique for sending information faster than the speed of light.

  • by cmpalmer ( 234347 ) on Friday January 25, 2002 @05:29PM (#2903370) Homepage

    I found this on Greg Egan's (the SF author and programmer) site: Subluminal Applet [netspace.net.au]

  • More FTL "tricks" (Score:1, Informative)

    by Cade144 ( 553696 ) on Friday January 25, 2002 @06:53PM (#2903839) Homepage
    Slightly off topic, but you can perform your own FTL demo at home.
    The classic example uses a bright searchlight reflecting of the clouds at night, but I suppose a laser pointer in a large auditiourm would work well too. The bright spot can be "moved" faster than light accross the clouds, just by moving the light source through a few minutes of arc.
    Unfortunatly the spot is not a physical thing, just an image. No real information is moved FTL.
  • by spike hay ( 534165 ) <.blu_ice. .at. .violate.me.uk.> on Saturday January 26, 2002 @09:40PM (#2908053) Homepage
    There is no such thing as faster than light. In this experiment, nothing is moving faster than light (just ignore common sense, it does not apply here). From my understanding, it's just the peak of the wave that travels FTL. The photon is not going FTL.
    Though this isnt really travel, as such, the only FTL phenomenon we know of is quantum teleportation. This is when you "entangle" two particles. When you entangle 2 particles, they act as one. If you changed the polarity of one, the other would instantly change to the opposite polarity, even if it is accross the galaxy. However, this still does not allow FTL star-trek teleportation or communication. Due to good old Heisenberg's Uncertainty Principle, you cannot measure a particle's properties exactly, because doing so would disrupt the particle.
    If you and your friend Bob both had entangled photons, and you were at Alpha Centauri, you could vertically polarize your photon. Bob's photon back at Earth would instantly become horizontally polarized. But it Bob tried to measure his photon by sending it though a polarizing filter, he would only have a 1 in 4 chance of correctly measuring the photon. It's essentialy random.
    The only way around this is for you to tell Bob that you polarized your photon vertically. This can only be done at light speed with a radio signal. Then Bob can send the photon through a horizontal filter.

The shortest distance between two points is under construction. -- Noelie Alito