From: palmer@sfu.ca (Leigh Palmer)
Newsgroups: sci.physics
Subject: Re: Physics puzzle - antigravity device!
Date: 23 Nov 92 19:37:12 GMT
Organization: Simon Fraser University

In article <1992Nov22.234125.13222@galois.mit.edu> jbaez@riesz.mit.edu (John C.
Baez) writes:
>I just got back from the University of Utah, where the physics
>department provided excellent hospitality.  In particular, Richard Price
>told me the following puzzle (our of a book of his, apparently).  

... and I was shown excellent hospitality by their Philosophy Department when I
visited in April. Must be a good place!

>Part 1 - Place a conducting hollow (and evacuated) spherical
>shell on the table.  Since (at least some) of the electrons are free to
>move in the conductive shell, gravity will pull them downwards a little
>bit.  Of course, when there are more electrons near the bottom of the
>shell, they produce a repulsive electric field.  Thus the electrons will
>come into an equilibrium where there are slightly more electrons near
>the bottom of the shell - distributed in such a way that the electric
>field they produce is just enough to cancel the gravitational pull on 
>the electrons.  (This effect will be very small.)  Show that the
>resulting configuration has the following nice property: for any
>electron placed *inside* the evacuated shell, the gravitation downwards
>force will be equal and opposite to the electrostatic force created by the
>electrons on the shell.
>
>This part is for real, and according to Price this effect has actually
>been observed.  Note that protons or neutrons would fall in this shell
>since they have a different charge/mass ratio.  

What follows is a legend retained from the dim past. The details may be
incorrect, and I'd appreciate corrections from anyone who Really Knows.

Actually this effect was "discovered" theoretically by Leonard Schiff at
Stanford after a Bill Fairbank inspired experiment to measure g-2 of the
electron failed. The intent of the experiment was to make a pulse of electrons
with a low temperature Maxwell-Boltzmann velocity distribution and fire them up
a (conducting) tube. The slowest of these were to fall back down the tube and
spend sufficient time in a magnetic field to have magnetic resonance
measurements made on them. The expected falling charges were seen (but the
resonance measurements were not made) and the graduate student who made the
source submitted his thesis and left Stanford. Schiff looked at the result and
said "That can't be, because ...". Fairbank (or one of his students) checked,
and sure enough the falling charges were not electrons; their charge to mass
ratio proved to be that of helium ions.

The "proof", of course, depends upon the sagging of electron density in the
isothermal atmosphere of a gas electrons in a rigid lattice of positive ions
having homogeneous charge density. The solution is in the kinetic theory
chapter in any good thermal physics book.

>Part 2 - Now *imagine* that there are 2 kinds of mobile charge-carrying
>particles on the surface of the shell, the electron and the "selectron"
>say, with different charge-mass ratios.   One can use the same argument
>as in the answer to part 1 to show that a particle with *any*
>charge-mass will levitate in the shell.  Thus one has an anti-gravity
>chamber!
>
>This part is bogus.  But when someone gives the answer to number 1 I
>will show how to use that answer to "prove" part 2, and then you can
>have fun trying to spot the error in my argument.
>
>So try part 1 first.

Schiff worked that one out, too, but I don't recall the details. I await the
other shoe, John.

Leigh