On Thu, 1 Nov 2001, EdinburghPM - Geoff wrote:
> Many thanks for your time!
> Presumably your email (see FAQ #9) talks about so called 'vacuum energy'
> where two large
>parallel plates with a sufficiently small gap (ie quantum level) will be
>forced together by the pressure from excess virtual particles outside the
>gap?  Is this related to dark energy?
> Best Wishes
> Geoff R******s
> Institute Of Bartender Physicists

That "vacuum energy" to which you refer is actually conceptually related to the
"dark energy", but in a round-about way.  So, we know that the vacuum
of space-time has fluctuations in it.  If you set up two parallel plates
close together, only certain fluctuations are allowed between the plates.
Specifically, the "modes" of vibration which are allowed can be visualized
as those which from trough to next trough fit neatly within the space of
the plates.  Now, the space on the other side of the plates is bounded
only by the deepest reaches of space (or the wall of the laboratory).
More modes are allowed to vibrate there, connecting to the outside of
the plates.  So, the energy per every bit of volume between the plates
is smaller since fewer fluctuation modes are allowed.  Energy density
is the same as pressure, physically speaking.  So, one can imagine that
the pressure between the plates is smaller than the pressure outside the
plates.  That forces them together.  This is known as the Casimir Effect.

Now, this property of the vacuum, to generate quantum fluctuations, is
distinct from the property of the vacuum which is involved in the
Dark Energy issue, but similar nonetheless.  It is as if the fabric of
space itself is exerting a mini-Casimir experiment within its own little
cells or building blocks.  If space is atomized on some incredibly small
scale, then there may be some Casimir-type forces pulling each individual
cells together.  The problem is that if we look at the scale of the
size of the tiny  building blocks of space (Planck Space), and the Casimir
force which should result is many orders of magnitude higher than the
observed Dark Energy density.  The observed value is small.  The natural
theoretical value is HUGE.  We expect the observed value either to be
very big (as the thoery suggests) or zero.  But, it is some sloppy
small value with no justification!

That is the mystery...

Keep up the bar physics!  I think I need to do some more pubbing
here -- But Berkeley is more for cappuccinos...

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