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Re: Light Faster Than Light?



See Jackson, Sec. 7.8 and thereabouts, for wave propagation in a
dispersive medium, which is what this is all about.
Regards,
Jack

Adam was by constitution and proclivity a scientist; I was the same, and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids run downhill,
not up.
Mark Twain, <Extract from Eve's Autobiography>

On Sat, 2 Sep 2000, Robert B Zannelli wrote:

In an article in the September issue of Scientific American there is an
article about a light pulse which appears to travel FASTER THAN THE SPEED OF
LIGHT. I have read this article several times and must admit I am not clear
about what is going on. Any help would be appreciated. Let me describe what I
do know from this article.
In this experiment a light pulse is transmitted through a cesium gas in a
cell. The Cesium is excited by a laser to a state of "anomalous dispersion "
that unlike the normal case higher frequency light travels at a higher
velocity and lower frequency light travels at a lower velocity through the
cesium medium. This causes the pulse to undergo a rephasing which causes this
effect.
First the authors maintain that no information or energy is transferred
faster than light speed. So Special Relativity is safe. One would assume then
that we are dealing with the same situation we have in the EPR experiment
where apparent superluminal effects cannot be observed until cross
correlated. The authors contend that that it is the group velocity that
exceeds the speed of light and that it is only the "front Velocity" that is
restricted to light speed because it carries all the information of the
pulse. They go on to say that in this experiment because the group velocity
is negative the Pulse actually EXITS THE CELL BEFORE IT ENTERS IT. Physics
text books hold that it is the group velocity that is equated with particle
velocity but the authors maintain that the Physics text books are incorrect.
For me there are some tantalizing hints as to what may be going on. First
every Quantum event is really a combination of time forward and time reversed
interactions. To acquire the probability of a Quantum event we multiply the
probability amplitude (a complex number) by it's conjugate to acquire a
scalar value. In reality, this is the process of taken into account the time
reversed interaction of the event. Also in Maxwell's equations, time reversed
events are an equally correct solution for these equations. Could it be that
in rephasing the light we have reversed the retarded waves (Time Forward) and
the advanced waves (Time reversed) causing this apparently time reversed
event. This is just my take on this experiment and I may far off the mark.
Any help or correction or clarification would be appreciated.

Bob Zannelli