Re: philosophy and the vacuum

[John Denker <jsd@MONMOUTH.COM>]

At 02:02 PM 9/8/00 -0500, Joel Rauber wrote:

> the meter is defined to be the distance light travels in a vacuum in a
> certain number seconds

OK

> Philosophically, (logically and perhaps operationally) is this a good
> definition?

Logically and operationally, yes.

Philosophically, it depends on one's philosophy, Horatio.

> This requires a definition of the vacuum.

OK

>   Is there only one vacuum?

Logically and operationally, the requirement is that you and I need to be
able to set up the speed-of-light experiment (i.e. the
meter-stick-calibration experiment) and agree on the
results.  Operationally, it is not particularly hard to do this.

>  Is the speed of light in the vacuum between two infinite parallel
> conducting sheets (casimir, effect) different than for other vacuums?

Yes.  Think of it as a waveguide.  For frequencies near waveguide cutoff,
the wave propagation speed goes to zero.  For frequencies greatly higher
than cutoff, there is a correction which is
 -- small, and
 -- easy to account for.

> If vacuum polarization effects affect this, then can we
> assign a non-unity index of fraction to certain vacuums?

Yes.  As an application: the index formulation is an acceptable way to
describe the scattering when waveguides are joined with an impedance
mismatch at the joint.

> (in a somewhat analogous fashion to how some folks assign an impedance to
> the vacuum).

There's no "somewhat" about it.  The index and the impedance are more than
analogous;  they are complementary concepts.  The vacuum does indeed have
an impedance.  In a waveguide or lumped circuit that has inductance per
unit length (L) and capacitance per unit length (C) the impedance is
  Z = sqrt(L/C)
and the speed of waves is
  c_1 = 1/sqrt(LC)
which means the index is
  n = c/c_1 = c*sqrt(LC)