Re: Magnetic lines like these ?

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Joel Rauber asked:
> I recall one of my E&M courses where our instructor calculated the 
> resistivity of the vacuum; this was related to looking at wave propagation 
> through the space.  It was a most pedestrian looking number (I forgot what, 
> and details;) But it wasn't zero or even very small (wasn't very large 
> either); i.e. not a perfect conductor.
>
> Does anybody know what I'm remembering? or recall discussions along these 
> lines?

Joel, you are, I believe, thinking of the so-called *impedance* of free
space.  This quantity has *nothing* to do with any conduction phenomenon
pertaining to the vacuum.  This quantity is a purely unit system-dependent
measure of the relative ratio of the amplitude of the magnetic field to
the amplitude of the electric field for EM radiation propagating in a
vacuum.  It's called an impedence because that is the engineering dimension
of the quantity.  In SI units the impedence of free space has the value:
sqrt([mu]_0/[epsilon]_0) == 4*[pi]*299792458*10^(-7) ohm ~= 376.7303135 ohm.
This crazy number is just one of the many asthetic problems with the SI
system in its treatment of EM quantities.  This quantity has various values
and dimensions depending on the system of units chosen for doing E&M.
(In my personal 'sensible units' SU system this quantiity comes out being
the pure dimensionless & unitless pure number 1).

BTW, even though the vacuum is a perfect insulator, it does have a *finite*
dielectric strength.  Once an electric field exceeds the order of magnitude
of about 10^(18) V/m then the vacuum becomes unstable against electron-
positron pair production due to the field polarizing the vacuum so much
that the virtual positrons are pulled in the opposite direction as the
virtual electrons to such an extent that they separate too much to be able
to recombine in the time allowed by the uncertainty principle, thus
producing pairs of real particles.  This effect effectively places a hard
theoretical upper bound on the largest field strength that can exist for a
static electric field.

David Bowman
dbowman@gtc.georgetown.ky.us