wrote in message news: ...
I'll add here as a comment that the issue of group velocity is
generally misunderstood, perhaps due to the fact that lower level
textbooks don't explain it well. Group velocity *is not* signal
velocity. Under some circumstances, when the dependence of phase
velocity on frequency over the bandwidth of the signal is weak, group
velocity is a good approximation to signal velocity over distances
short enough so that the pulse shape does not change appreciably in
propagation. That's all. The conditions listed above are reasonably
well satisfied in most practical situations, but they totally fail
under anomalous dispersion situation.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

You're aware of the discussions on sci.physics.relativity that to
determine if a signal travelled superluminally what would be required
is a round-trip measurement. This is because of the uncertainty of
synchronizing clocks in two different locations. The standard SR
method of using light-signals requires the assumption of the constancy
of the one-way speed of light. [1]
I was interested to see that in some descriptions of the experiment
of Wang et.al. using lasers in cesium gas, that the light pulse was
said to exit the chamber before it entered:

"Can c, the speed limit of the universe, the speed of light in vacuum,
be exceeded? In July, 2000, the science-oriented news media were full
of reports that pulses of laser light had broken the speed-of-light
barrier. Physicists L. J. Wang, A. Kuzmich, and A. Dogarliu of the
NEC Research Institute in Princeton, NJ, had a paper about to be
published in the prestigious journal Nature describing an experiment
in which not only had laser pulses traveled faster than light, but had
actually emerged from the apparatus before they had entered it." [2]

It occurs to me that this is what would appear to happen if the
entrance and exit were synchronized using light signals but the actual
signal to be measured was traveling superluminally or if light signals
provided an inaccurate means of synchronizing clocks. What would be
needed to see if the signal pulse was being propagated superluminally
would be to have the pulse reflected back to the source and seeing if
the total travel time was less than the time for light to make the
round trip.

Another experiment might have made such round trip measurements.
These were experiments by Ranfagni et.al. of microwaves propagating in
a wave guide:

"In the new experiments, led by Anedio Ranfagni of the Italian
National Research Council in Firenze, the setup looks innocent enough:
The team sent microwaves (3.5 cm wavelength) through a narrow,
ring-shaped opening onto a large and nearby focusing mirror, which
collimated the waves into a beam propagating back from the mirror,
beyond and behind the source. They "modulated" the microwaves with
rectangular pulses (sharp "amplify" and "attenuate" commands, in rapid
succession) and detected the pulses at positions between 30 and 140 cm
from the source, along the beam axis. The slope of their plot of
arrival times vs. distance led to an apparent propagation speed of 5
to 7% above c, although beyond about 1 m, the speed approached c, all
of which agreed with previous predictions." [3]

The experimenters claim some speeds exceeding c in the reflected
waves but it is difficult to tell here if any of these speeds are for
a round trip signal back to the source.


Bob Clark



1.)Conventionality of Simultaneity
http://plato.stanford.edu/entries/sp...e-convensimul/

2.)Faster-than-Light Laser Pulses?
by John G. Cramer
http://www.npl.washington.edu/AV/altvw105.html

3.)Faster than a Speeding Light
http://focus.aps.org/story/v5/st23