On Fri, 21 Jul 2000, [iso-8859-1] Daniel Davies wrote:
> I heard about this years ago, but never knew how until
> today .....
>
> "The group velocity of a laser pulse in this region
> exceeds c and can even become negative(16, 17), while the shape of
> the pulse is preserved."
>
> And group velocities above c are not ruled out by
> special relativity. Special relativity's restriction
> to c or below is for the "front velocity" of a wave;
> the first stirring that tells you there's a wave
> there.
What this all hinges on is, as far as I can see, the interaction between quantum fluxuations and special relativity. Because a particle is never in "one place" but in fact is smeared over a probabilistic area, plus is continually emitting matched matter and antimatter counterparticles that are quickly reabsorbed - what's called the quantum frenzy at the sub-sub atomic level - there is the possibility of the distance edge of each probability region making a jump on the "center of gravity" of the particle. If there is a dense enough medium to take advantage of this quantum frenzy - the cesium they used in the most recent experiment I guess - information can be conveyed faster than the speed of light in this quantum region.
It is without question interesting theoretically and may have practical uses, but since it only matters for objects travelling at the sub-sub atomic level, it doesn't get us either macro time travel or messaging for objects not embedded in the dense field of quantum material to facilitate the quantum frenzy conveying of information to replicate the wave.
Quantum defies a lot of rules- such as the fact that particles can move through solid objects, escape black holes and do a bunch of things impossible under either special or general relativity rules. There have also been past experiments that in many ways demonstrated information being sent in faster-than-light ways, notably variations on the quantum double-slit experiment by physicists like Wheeler.
This light pulse experiment is interesting, but to me less mindblowing than a lot of those double-slit experiments.
-- Nathan Newman