[extropy-chat] Casimir Torque Project

Hal Finney hal at finney.org
Thu May 5 19:07:29 UTC 2005

Adrian Tymes writes:
> --- Hal Finney <hal at finney.org> wrote:
> > As I understand it, this device consists of a disk that has a metal
> > square in the center and a metal ring around the outside.  Between
> > the
> > disk and the ring are specially shaped black body structures which
> > block
> > the Casimir force.  The whole thing is one piece and rigid.
> Incorrect: the ring is mechanically separate from the rest of the
> device.  The black body structures and the metal square in the center
> are rigid with respect to each other, though.

Okay, I apologize for misunderstanding this part.  So we could imagine
instead fixing the central square, in which case the outer ring would
begin to rotate, right?

I still don't think it will work.  Here is an explanation of Casimir force,
from http://www.du.edu/~jcalvert/phys/casimir.htm

"The Casimir Force is an electric force, but its origin is different
from that of ordinary electric forces. It is a purely quantum-mechanical
effect arising from the zero-point energy of the harmonic oscillators that
are the normal modes of the electromagnetic field. The electromagnetic
field must satisfy certain boundary conditions at the surfaces of our
conducting plates, and these boundary conditions rule out some of the
modes (oscillators) that would otherwise exist in unbounded space. Since
there are fewer oscillators between the plates, there is less zero-point
energy in this region. If the plates are brought closer together, this
volume of smaller energy density is decreased, while the volume of normal
zero-point energy density is increased.  Since this results in an overall
gain of energy to the universe, a force pressing the plates together is
the result."

This is consistent with the concept of a conservative force.  The force
results from a change in the potential energy of the system.  In a
conventional parallel-plates Casimir force test, the ZPE is lower in
the region between the plates, and so moving the plates together is
energetically favorable.  This is what produces the force.

If you apply this concept to your system, you see that your device does
not work.  There is no torque.  Rotating the outer ring does nothing
to change the volume or shape of the altered ZPE region.  The potential
energy will remain exactly the same as the ring rotates.  So there will
be no torque which acts to rotate the ring.

In fact, despite your attempt at geometric arrangement, the pressure
on the outer ring will be entirely central (directed towards the center
of the ring).  The same thing would happen if you considered the ZPE as
gas pressure.  Imagine having high pressure gas outside the system and
low pressure gas in the cavity (white region).  Despite the asymmetrical
shape of the cavity, the pressure on the ring will be perpendicular to it.
The gas pressure will not act to rotate the outer ring.

You keep saying, or implying, that some kind of detailed QM explanation
will justify your torque.  You say that if the device doesn't spin that
this will require re-thinking the theory of quantum mechanics!  It seems
to me that this is all wrong, based on the considerations I have offered
here.  If conventional QM predicted that rings would speed up arbitarily
in a vacuum, it would be a strong argument against the accuracy of QM.

Do you have a more detailed QM model for the Casimir force in your design
that you can offer?  While I am not a QM expert I did have two years of
physics at Caltech and have kept up a modest interest over the years.
I would not be afraid of some equations if you want to offer them.
I am also curious to know the qualifications of the academics who have
endorsed your design, in broad terms - you don't have to name any names
or embarrass anyone.


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