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.'"
Re:Well.... (Score:4, Informative)
Uhhh, this is nothing new. (Score:2, Informative)
Beating Carrier Waves Against One Another (Score:1, Informative)
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.
Re:FTL - information backwards in time (Score:2, Informative)
Re:FTL Pulse = Science; Perpetual Motion = Hoax??? (Score:3, Informative)
Re:FTL Pulse = Science; Perpetual Motion = Hoax??? (Score:3, Informative)
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].
Re:Uhhh, this is nothing new. (Score:4, Informative)
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.
It's the shape of the pulse... (Score:5, Informative)
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.
Applet that Illustrates This (Score:2, Informative)
I found this on Greg Egan's (the SF author and programmer) site: Subluminal Applet [netspace.net.au]
More FTL "tricks" (Score:1, Informative)
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.
Re:FTL - information backwards in time (Score:2, Informative)
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.