Re: Does the speed of light

In response to questions concerning recent reports of light pulses being
transmitted at effective speeds faster than the speed of light in a vacuum,
I would first refer people to an article in the Science section of the New 
York Times, May 30, 2000.  From what I gather in this article, the light
pulse is preceded by a "tail," which you can think of as being like a
ripple coming out from a wave approaching a beach.  In the case of the
experiment being described, the ripple enters a chamber containing cesium
atoms (which have been "excited," or given extra energy, by shining another
beam of light on them).  The ripple triggers the formation of a wave pulse
which uses energy from the excited cesium atoms to mimic the approaching
original pulse.  It is this wave pulse created from the "borrowed" energy
which appears on the outgoing side of the chamber.  A "back-wash" wave then
propagates at a speed of 300 times the speed of light through the chamber
with the cesium atoms and exactly cancels the incoming original wave pulse.
The mechanism(s) involved in the creation of the mimic pulse and the
backward traveling pulse are not covered in the article, and the paper on
which the NY Times article is based is still undergoing review prior to
publication in the general science journal "Nature," so I cannot comment
further on that aspect.  However, it is stated that the shape and size of
the main pulse are derived from information carried in the preceding
ripple, which has already entered the chamber before the mimic
pulse is emitted, which I believe is why researchers assert that this 
experiment does not violate causality principles or the principles of 
relativity.  
 
The New York Times article is available on-line at:
www.nytimes.com/library/national/science/053000sci-physics-light.html
(may need to register for a free account with nytimes.com to access this) 

[note added by MadSci Admin:
The article has been submitted to the journal "Nature", but it
apparently is still in the review process.
For a good general treatment of the subject of group velocity,
which is really what is being measured (I think), see any college-
level Optics textbook.  I recommend "Optics" by Eugene Hecht.]