Re: the Classical Casimir effect

On Feb 11, 11:17 pm, "Sue..." <suzysewns...@xxxxxxxxxxxx> wrote:
On Feb 11, 10:29 pm, "Edward Green" <spamspamsp...@xxxxxxxxxxx> wrote:

[I myself hear fallacy yapping all around, but just out of the field
of vision]

I am with you on this.

That referred to my own arguments, BTW. Not, it seems to me, that
this lets us off the hook in explaining why or why not we expect to be
able to predict something analogous to a Casimir effect starting with
classical cavity radiation. I don't think the effect is as purely
quantum in origin as usually suggested, except perhaps that...

The referenced page is in terms of zero point energy.

I thought that was probably the case. Hence lack of reference to the
temperature. That would possibly be a big difference -- classically,
any effect should disappear as T -> 0K (no field excitation),
quantally, we have zero point energy. If the effect of zero point
energy predominates over the classical prediction at room T, then I
guess we may be justified in saying this was a fundamentally quantum
effect.

One thing I really fudged -- you might have noticed -- was the sign of
the effect! I knew what sign was expected (attractive), and this
makes some intuitive sense (greater radiation pressure outside the
plates than inside, because of excluded modes), but it _doesn't_
follow from a simple minded energy argument, twist things as one
might: if moving the plates inward increases the volume of higher
energy field excitation, we would predict doing work _on_ the system
-- a repulsive force.

The reason I think this is wrong (even within the context of a
classical field argument) is that we are ignoring those two other
friends of systems with many modes: heat and entropy. It's quite
possible the direction of spontaneous change is to draw the plates
inward -- even if this increases net field energy -- if this direction
increases entropy: the system will simply suck heat out of the
surroundings to compensate.

Now I notice the quantum calculation also seems to ignore entropy, and
indeed I am unsure how to handle entropy as it relates to zero point
excitation.

Another wrong thing about my "derivation" is that my cutoff was too
simple-minded: some attempt should be made to integrate over angle,
no? (longer wavelengths could fit slantwise in the gap than straight

The effect is not that different from
Van der Waals or London force. Better analysed as
a classical force.

I've seen this comment before (simularity to Van der Waals force). I
can't say I really grasp it. Classically, anyway, the calculation
would seem to treat the metal plates as ideal featureless conductors.
A Van der Waals force OTOH would seem to rely on the plates containing
swarms of discrete electrons, which, on average, stayed out of each
others vicinity across the gap. Even quantally, if I understand it
correctly, the effect relies on the geometry of the space, not the
structure of the surfaces.

Casimir effects result from changes in the ground-state
fluctuations of a quantized field that occur due to the
boundary conditions.

Yes... the squeezing of the field, right?

Casimir effects occur for all quantum
fields and can also arise from the choice of topology. In
the special case of the vacuum electromagnetic field with
dielectric or conductive boundaries, various approaches
suggest that Casimir forces can be regarded as macroscopic
manisfestations of manybody retarded van der Waals
forces [7], [9].

Hmm... I guess I should read [7],[9].

Zero-point field energy density is a simple and inexorable
consequence of quantum theory, but it brings puzzling
inconsistencies with another well [?] verified theory,
general relativity. >>http://arxiv.org/abs/quant-ph/0407153

Again... the mass (or energy equivalent) that pushes
back when you push a car is not the air surrounding
or the remaining vacuum if the air is removed.

When you localise the the true agent of inertia
in a pseudo-space, you can't experimentally
detect the agent. It is the moon and Jupiter ect.

I have no idea what that means.

Zero point energy is a big goose-egg in my book. ;-)

I have no axe to grind. Though I would think at least that it's not
obvious whether it would qualify as stress-energy for GR. No point
philosophizing about it: we would simply need a better joint theory to
make the answer obvious, the independent theories are silent on the
question.

Thanks as always for your feedback.

.

Relevant Pages

  • Re: the Classical Casimir effect
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