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Science News

Electrical Pulses Break Light Speed Record 68

Posted by chrisd 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.'"
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Electrical Pulses Break Light Speed Record More

Electrical Pulses Break Light Speed Record

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  • They got gigabit off of fiberoptic and onto copper
    (which some said wasn't feasable), and modems up to 56k (which we all said was impossible)
    so we just have to wait a few years until they make ethernet cards out of this ;)

    terabit ethernet anyone?

    /me thinks his pci bus might not handle the throughput this would offer....

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

      by PhuCknuT ( 1703 ) on Friday January 25, 2002 @01:37AM (#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.

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

        by PhuCknuT ( 1703 ) on Friday January 25, 2002 @01:42AM (#2899509) Homepage
        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.
        • But this is a way of sending more data. By sending data faster, more is able to be transmitted in a constant time.

          What I find most impressive about this is the possibility it has with interstellar communication. (Which of course won't be usefull until we have someone to communicate with, but it's a start.) If a message can be sent faster than light, then one of the bigger problems with exploring anything further away than our solar system would be lessened.

          Of course, the real trick would be to figure out how to send solid energy (matter) at such speeds.

          • Re:Well.... (Score:2, Insightful)

            by MindStalker ( 22827 )
            Umm this example decreases the total time by x regardless of the distance. So you would be able to send the signal 1 nanosecond sooner to the next galaxy that if you used light itself. Its not what your thinking. Basically your creating the wave, then spreading the wave, so that the front of the wave gets there sooner than light, and the back of the wave gets there that same amount of time less than light. Maybe we could figure out how to stretch that wave thousands of light years wide? HAHA
            • Well it's kinda like this:

              if you had a ball that was rolling under a rug, the end of the curvature of the rug at the position of said ball would reach the outside of a rug before said ball came out.

              If you had a 2x4 1 light year long and moved it back and forth you could get a message there faster than light.
        • Since they're using a photonic bandgap fiber, the pulse is already traveling at or very near c. Also, you can't actually use this technique to send information because a) that would actually violate relativity, and b) modulating the pulse screws up the effect by adding or subtracting frequency components to the signal. It's a neat trick, but with no practical use that I can think of...
          • Who says that we can't violate relativity? After all, it is still a theory. A damn important one, mind you, but still unproven.

            Remember -- an example of relativity working of not proof, but a counterexample is enough to discard the entire theory.

            --ravyn
            • That's why I qualified statement (a) with statement (b). Trust me, I want superluminal fiber-optic transmission as much as the next geek, maybe even more because that would improve my job outlook as a new graduate optical engineering tremendously, but based on having read the articles and taken quite a few optics classes, all the evidence suggests that this technique will break down upon trying to transmit data because it is basically a trick of wave superposition.

              One of my professors last semester had a really good electrical engineering example of something that works in sort of the same way and also explained why this wouldn't work for data transmission, but unfortunately I can't remember what it was...
      • "a sine wave, and it shifts it 90 degrees out of phase"

        Uh, wouldn't it be easier to express this as a cosine curve, then?
        • No, I was saying the input looks like a sine wave, and the output looks like a cosine. The peaks of the sine wave get to the output ahead of where they should be.
  • 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.
    • 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.
      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?


      For example, Bob wants to tell Mary whether he got a research grant today. He has arranged to send her FTL waves at amplitude A if he got it, or at amplitude B if he didn't. If those waves are travelling FTL, so is the information.

      • Re:Uhhh, this is nothing new. (Score:4, Informative)

        by mmontour ( 2208 ) <mail@mmontour.net> on Friday January 25, 2002 @12:04PM (#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.
      • I think the point is, you can create the signal faster than light, but you simply can't manipulate the signal to put information in it faster than light.
        • No, you can put information on it, but the information moves allong the FTL wave at light speed. Odd, huh?
          • OK, now IANAP (IANA Physicist,) but I think it goes something like this:

            Think of yourself as a piece of information. You are standing on a train. Think of the train as the light wave, travelling at the speed of light. You are now walking from the back of the train to the front. Relative to the earth, you are travelling faster than the train, thus faster than the speed of light. However, relative to the train, you are travelling at walking speed. The problem is, in order to minimize your time on the train, you start your journey by jumping onto the last car as the train leaves the terminal, and jumping out of the first car as the train approaches the destination platform. Given a sufficiently long train, you could send a large amount of data that would begin to arrive quickly, however you must first establish the carrier (the train) and then you must offload the information. Your net gain should be a faster arrival of the data, but most likely the data will be offloaded at the same speed as usual. Ar you as confused as I am yet?

            OK, I think I'll go read the article now. :^)

  • Sounds familiar (Score:4, Funny)

    by codeButcher ( 223668 ) on Friday January 25, 2002 @05:11AM (#2899813)
    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 /.

  • As I am not a physicist, I wonder how you can measure something that's faster than light???

    D

    • Time to destination, I think.

      if object A is sent from 1 LY away and takes 1 LY to get here, it's traveling at the speed of light.

      if object B is sent from 1 LY away and takes .9 LY to get here, its traveling faster than the speed of light

      I think.
      • You mean "1 year" and ".9 year" to get here. cause A light year is a distance, not a time. So, um. Yeah. Agreeing with alot of the above, this will just decrease ping times. We still can't send information faster than c.

    • Beating Carrier Waves Against One Another (Score:1, Informative)

      by Anonymous Coward
      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.
    • As a bit of a physicist I can't help but wonder why you think there's a difficulty with measuring a velocity faster than that of light???

  • must've been.... (Score:1, Flamebait)

    by Snafoo ( 38566 )
    carrying a Microsoft press release, or other bad news ;)

    Either that, or the scientists crunched the numbers over a nice Italian meal at some bistro...
  • The other article linked to from the one in the submission says:

    Next they send a 3.7-microsecond long laser pulse into the caesium cell, which is 6 centimetres long, and show that, at the correct wavelength, it emerges from the cell 62 nanoseconds sooner than would be expected if it had travelled at the speed of light. 62 nanoseconds might not sound like much, but since it should only take 0.2 nanoseconds for the pulse to pass through the cell, this means that the pulse has been travelling at 310 times the speed of light. Moreover, unlike previous superluminal experiments, the input and output pulse shapes are essentially the same.

    Correct me when I'm wrong, but doesn't this mean that the pulse went out of the cell 61.8 ns before it went in? When I try to picture this phenomena my brain just overloads and dumps the core.

    • No, under normal circumstances the pulse would have gotten there many ns later. It's an increase in speed, not time travel. There is still a delay involved in getting the photons to their destination, it's just shorter.

      If you put one of these setups next to a light bulb and turned them on at the same time, the laser pulse would get there first.

      Justin

  • What bugs me about this (Score:3, Interesting)

    by renehollan ( 138013 ) <<ten.eriwraelc> <ta> <nallohr>> on Friday January 25, 2002 @09:44AM (#2900471) Homepage Journal
    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

  • at the group velocity reached three times the speed of light for frequencies in the absorption band.
    Great. Just when they get a signal to travel faster than light, it gets absorbed.


  • Cerenkov radiation, that's been known for decades?

    • Cerenkov radiation


      I believe it is not at all related. Cerenkov radiation is the result of a charged particle moving through a medium at a speed faster then the speed of light in that medium, like a proton moving through water at 0.9c. This generates photons, but they move at the speed of light in the medium.

  • It's the shape of the pulse... (Score:5, Informative)

    by Crispy Critters ( 226798 ) on Friday January 25, 2002 @03:11PM (#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.

  • I think the Science Times ran something about moving the group velocity faster than c a year ago.

    Plus my reading of this article leaves me thinking they were actually moving their signal at 2/3 c since they were working in a medium where you'd only expect light to travel 8"/ns instead of the 12"/ns in vacum.
  • If time is a fourth dimension, then we can set up the equation x^2 + y^2 + z^2 + t^2 = C^2, where x,y,z, and t are the magnitudes of the vectors in each dimension and C is the speed of light. So all we need to do is travel at an imaginary speed in x, y, and z and x^2 + y^2 + z^2 will be negative so we'll be able to speed up time. Woohoo!
    • FWIW you're close, but no cigar, actually the invariant relativistic equation is:

      Dx^2 + Dy^2 + Dz^2 - Dt^2 = constant

      Where Dx, Dy, Dz and Dt are distances in x,y,z,t directions.

      But noone knows how to get an imaginary velocity...

  • 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 )
    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.

  • Brain Teaser (Score:3, Interesting)

    by Mignon ( 34109 ) <satan@programmer.net> on Sunday January 27, 2002 @04:03PM (#2910610)
    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 [wiskit.com]: 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?

    • This was mentioned and illustrated in Scientific American years ago. I think you are right: the point moves FTL. -- Can any information move faster than light? Suppose you put a four inch long wooden dowel between your finger and the doorbell, and push the bell. Pressure from your finger is passed to the doorbell button. Now imagine a dowel a few million kilometers long; push it, and -- will the push arrive at the doorbell instantaneously, as if the dowel were not there, or at the speed of light? Anyone?
    • Actually the scissors dont close instantly. The clsoing action propergates along the metal at the speed of sound in the metal. Same with waggling a long rod. Speed of sound in steel = 5000m/s ish right? (i dunno the real number)

      Same with tugging on a lightyear long rope doesn't "instantly" send information to the otherend
      • That's kinda missing the point. You could push the blade simultaneously (in the scissors rest frame) all the way along its length rather than have a signal from the handles propagate at the speed of sound in steel.
    • That's where the whole time-slowdown comes in.

      There are no laws stating that two things cannot appear to move faster than light relative to each other to *an observer*. Two spaceships moving at the speed of light towards each other, and starting 2 light seconds apart, will hit in 1 second. From your point of view.

      On the interior of the spaceship, time dilation will take effect, and it will be much longer.

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