IF you take a
charged parallel plate capacitor
and
increase the distance between its plates, you will
find that you have to have to do
work to overcome the attractive Coulomb
force between the plates.
If the capacitor plates have area A, and a surface charge density sigma,
the amount of work you have to do to increase the plate separation by an amount
delta d is:
If the capacitor is
uncharged, you would expect that no work
is required to increase the plate separation (you may assume the ideal case of massless,
perfectly conducting plates). However, in 1948, Hendrik Casimir realized, amazingly
enough, that is not true.
Casimir showed that an attractive (non-gravitational) quantum
force exists between two
uncharged perfectly conducting parallel plates, and this
result is called the
Casimir Effect. For a plate separation of
d, the Casimir force per unit area is given by:
The exact derivation of the Casimir force equation above is a little involved, but
a heuristic argument, plus or minus a multiplicative constant,
is possible via dimensional analysis.
The first step in any dimensional analysis argument is
to specify on what physical quantities the object in question should depend. As
the Casimir force is a quantum effect, it must depend on Planck's constant h.
It should also depend on d, the plate separation. If the force is mediated by particles
that travel at c (as is the case of the electromagnetic force and photons), then
it should also depend on c.
So we want Force/area = F(h,d,c) as the required functional dependence, the exact
form of which we must determine.
The only combination of h [J T], d [L] and c [L/T
2] that has units of
Force/area is:
To get the minus sign and hence show the Casimir force is attractive,
we have 2 choices. One is to do the full blown quantum field theory calculation.
The second is note that while classically, the vacuum consists of empty space, in
quantum theory, due to the Heisenberg time-energy uncertainty principle, vacuum
fluctations are allowed, and the vacuum is anything but empty.
In quantum theory, the vacuum is filled with virtual particles,
and in particular, virtual photons. It is these virtual photons that impart momentum
to the plates when they are reflected from them. Note that in between the plates, only a countable infinity (n=1,2,3,...) of virtual photon frequencies are allowed due to the standing wave condition:
On the other hand, outside the plates, there is no such constraint,
and an aleph nought infinity of frequencies are allowed. So, the pressure on the
outside of the plates must be greater than on the inside by virtue of the larger
number of frequencies and virtual photons, and we can conclude the plates are being
squeezed together...i.e., the force is attractive, which gives the minus sign.