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Units; philosophy and the vacuum



Hi Gang,

I'm glad to see us getting into some more physics type discussions and want
to offer up a start for another thread. Our semester just started and I've
given my usual discussion concerning the SI system and units and physical
dimensions. And had the following thought, (question, concern).

In the current incarnation of the SI system the speed of light is know with
absolute precision (the uncertainty in measurements being thrown into the
definition of the meter). I.e. the meter is defined to be the distance
light travels in a vacuum in a certain number seconds (1/c seconds), where
c
is the speed of light in a vacuum as expressed SI units.

Philosophically, (logically and perhaps operationally) is this a good
definition? This requires a definition of the vacuum. Is there only one
vacuum? What I'm trying to get at, is what affects does vacuum
polarization
have wrt to this definition. Is the speed of light in the vacuum between
two infinite parallel conducting sheets (casimir, effect) different than
for
other vacuums? If vacuum polarization effects affect this, then can we
assign a non-unity index of fraction to certain vacuums? (in a somewhat
analogous fashion to how some folks assign an impedance to the vacuum).
Etc.

Joel Rauber
Joel_Rauber@sdstate.edu