On Jun 7, 10:19 am, "optman" wrote:
CITE
We at Fermilab deal everyday with particles moving faster than the speed of light!
In fact, we use them to detect some fast particles moving through our detectors.
You might say: "What?? In the previous 12 pages you convinced me that
there is no such thing as an object moving faster than the speed of light!".
Don't worry: you are right. The trick is that I carefully left out in the first
paragraph the words IN A VACUUM when I said "particles moving faster
than the speed of light!" Got it? ...
/CITEhttp://home.fnal.gov/~pompos/light/light_page14.html

It seems to me that this Fermilab scientist is meaning that
the vacuum is rather a limiting environment for the light.

How do you interpret what this scientist is saying?

The "speed of light" is the speed at which
electromagnetic waves propagate. You can calculate
this from Maxwell's Equations to be
c = 1/sqrt(\mu \epsilon)
where \mu and \epsilon are the magnetic permeability
and electric permitivity, respectively. In
a vacuum, these both have their minimum values
(\mu_0 and \epsilon_0), and so you get a maximum
value of 3e8 m/s = c. In any other material, they
will be larger (and, in general, frequency
dependent), and the velocity will be lower.

The ratio of these velocities is the
index of refraction. Typical indices of
refraction for transparent materials are
on the order of 1.5 (meaning light
will propagate at about 2/3c)
http://hyperphysics.phy-astr.gsu.edu...les/indrf.html
Using a gas and controlling the pressure,
you can get an index of refraction arbitrarily
close to 1.

If a charged particle enters such a medium going
*faster* than the local speed of light (but NEVER
faster than c), it will emit a "shock wave",
of a burst of light, with a flat frequency
spectrum. This is called "Cherenkov radiation".

This is a very valuable tool experimentally,
because Cherenkov radiation depends on the
*velocity* of the particle, whereas magnetic
spectrometers measure *momentum*. If you
separately measure the momentum and
velocity, you know the mass, so Cherenkov
detectors play an important role in particle
identification. For example, they are often
used to distinguish pions, kaons, and protons
in high energy physics experiments.

-jc