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Electrical Pulses Break Light Speed Record

Posted by chrisd on from the canadians-show-us-the-way-again dept.

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.'"

9 of 68 comments (clear)

  1. Re:Well.... by PhuCknuT · · Score: 4, Informative

    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.

  2. Re:Well.... by PhuCknuT · · Score: 4, Interesting

    I forgot to mention how this IS important. What this type of research will lead to is reduced latency. Instead of information traveling down a cable at 2/3 of the speed of light, they can use methods like this to send data at much closer to the speed of light. Not more data, but slightly lower ping times.

  3. Sounds familiar by codeButcher · · Score: 4, Funny
    Although the group velocity exceeds the speed of light ... each component of the pulse travels slower than light.

    My team words just the opposite: Each individual working at breakneck speeds, but the group never gets there fast enough.

    Now if they only could stop posting to /.

    --
    Free, as in your money being freed from the confines of your account.
  4. What bugs me about this by renehollan · · Score: 3, Interesting
    is a problem akin to Shannon's Theorem: you know, the maximum data rate of a channel is related to the bandwidth and noise floor?

    In this case, the effect occurs close to the intentional absorbtion band, where signals get reflected because of impedance mismatch. So, the signal gets strongly attenuated. Gets there faster, but is much weaker, yes?

    The effect of the thermal noise of the receiver in the band of interest thus gets more significant. More relative noise, less bits per pulse (think AM).

    So, what would be a 1 km cable capable of carrying 100 mb/s (for example -- I'm pulling these numbers outa my...) now looks like a 100 m cable capable of carrying 1 Mb/s... great for wire latency, lousy for bandwidth.

    Now, we all know that for typical packet sizes, wire latency is insignificant to data serialization latency: the time it takes for the last bit in a packet to leave the transmitter, compared to the first bit. So, you've cut wire latency by 90% and increased data latency by much more.

    What am I missing here? Or, is there, as I suspect, NSTASFL

    --
    You could've hired me.
  5. Re:FTL Pulse = Science; Perpetual Motion = Hoax??? by SIGFPE · · Score: 3, Informative

    There's nothing unusual or fantastical about this claim. Group velocity/phase velocity 'n' all that stuff is basic undergrad material.

    --
    -- SIGFPE
  6. Re:FTL Pulse = Science; Perpetual Motion = Hoax??? by hubie · · Score: 3, Informative
    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.

  7. Re:Uhhh, this is nothing new. by mmontour · · Score: 4, Informative

    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 and this page. 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.

  8. It's the shape of the pulse... by Crispy+Critters · · Score: 5, Informative
    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.

  9. Brain Teaser by Mignon · · Score: 3, Interesting
    As a youngun', I was told by an adult friend that if you closed a pair of scisors fast enough, the point where the two blades crossed would move faster than the speed of light. It was presented as a sort of paradox about how something could go faster than light. (Note to Marilyn Vos Savant: it's 'cause the thing moving faster than light has no mass, not 'cause Einstein was wrong.)

    Actually, I may be the dope - I never verified if this was true. Anyone know?