From: Paul Doherty (pauld@exploratorium.edu)
Date: Thu Oct 12 2000 - 13:36:14 PDT
Message-Id: <l0311071bb60bcd686320@[192.174.2.173]> Date: Thu, 12 Oct 2000 12:36:14 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Speed of light exceeded
Hi Sally
The language is confusing about photons.
You are right that they have no rest mass but they do have mass since they
have energy and via E = mc^2 they also have mass.
There are two speeds associated with waves, the phase velocity and the
group velocity.
Think of a group of bicyclists in a line, as the front rider gets tired he
pulls to the side and joins the back of the group. Then he works his way to
the front again. Each rider moving up through the line is going faster than
the group. The speed of the riders moving up is the phase velocity the
speed of the group of riders is the group velocity. (In this model I'm
ignoring the riders when they slow down to go to the rear.)
Information is carried by modulated energy, the modulations travel at the
group velocity. The speed of light is the limit to the speed of information
flow,group velocity cannot be greater
What the researchers measured was a phase velocity there is no speed of
light limit to the phase velocity. The phase velocity does determine the
index of refraction and the angle light bends when it goes between two
materials.
Paul Doherty
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld
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