Subject: Re: Artificial gravity
From: alien@acheron.amigans.gen.nz (Ross Smith)
Date: Fri, 20 May 94 23:06:30 GMT+12
Organization: The Boyz from the Dwarf
In article <2r20s1$km4@kauri.itd.adelaide.edu.au> kickaha@NEWS_DOMAIN (Sundance Bilson-Thompson) writes:
>Anders Sandberg (asa@scaramanga.nada.kth.se) wrote:
>
>: >If anyone has figures or (preferably) equations relating the density of the
>: >fluid, the radius of the loops, angular velocity, and so on, I would be
>: >really interested to see them. Will the density be
>: >so high that you might
>: >just as well use sheets of neutronium?
>
>: I would also really like to know the actual figures. Its one thing
>: playing with the concepts, another to get some figures.
>
>Anyone? Anyone out there?
In one of Robert Forward's papers (the title is "Guidelines to Antigravity"
and it was published in 1962 or 63; I don't have the specific reference,
but I'm pretty sure it's mentioned in the appendix to _Dragon's Egg_), he
describes several ways of producing non-Newtonian gravitational forces.
One involves a heavy rotating ring. An object near the centre feels an
acceleration with a vertical component pulling it towards the plane of the
ring, and a radial component pushing it outwards. The vertical and radial
accelerations are:
g(z) = - G M w^2 z / c^2 R
g(r) = G M w^2 r / 2 c^2 R
where M and R are the mass and radius of the ring, w is its angular
velocity, r and z are the coordinates of the test body, and G and c have
their usual meanings. These formulae only apply for a test body near the
origin (r and z both << R).
Later he describes "devices based on analogies between electromagnetic
and gravitational fields", i.e. the gravitational equivalent of an
electromagnet. The one he describes in detail (and I think this is what
someone earlier in this thread was thinking of) involves a ring-shaped
coil of tubes carrying a heavy, fast-moving fluid (imagine a hosepipe
wrapped around a tyre, so it repeatedly passes up through the middle and
then down around the outside). This generates a gravitational field through
its centre, pointing in the opposite direction to the fluid passing through
the centre (downwards in my hosepipe-and-tyre description). The strength
of the field is:
g(z) = - d ( 4 G N T r^2 )
-- -----------
dt c^2 R^2
where N is the number of turns of the coil, T is the mass flow rate, and
R and r are the major and minor radii of the toroidal coil. Notice the time
derivative involved: a steady-state flow produces no field at the centre;
you need a continuously varying rate of flow (or some other variable in
the equation, but flow rate would presumably be the easiest to control).
Plugging numbers into these equations shows (as Forward points out in the
paper) that any kind of gravity control using general relativity principles
requires enormous densities and speeds ("It is obvious that research in the
field of gravitation will be very difficult since even the most optimistic
calculations indicate that very large devices will be required to create
usable gravitational forces.").
But he also makes another interesting point:
"In studying analogies between electromagnetism and gravitation, it
can be seen that one analogous quantity has not been investigated.
This is the gravitational equivalent to the magnetic permeability.
Electrical power distribution systems depend upon the anomalously
large and nonlinear permeability of iron and other magnetic materials.
Since all atoms have spin, all materials will have a gravitational
permeability which is different from that of free space. Rough
calculations show that this difference is very small, but experimental
investigation may find materials with anomalously large or nonlinear
properties that can be used to enhance time-varying gravitational
fields. Also, since the magnetic moment and the inertial moment are
combined in an atom, it may be possible to use this property to convert
time-varying electromagnetic fields into time-varying gravitational
fields. At present, the only way to search for such materials is to
intersperse wedges of material between gravitational wave generators
and detectors, such as those described by J Weber, and look for a
change in amplitude or direction of propagation. The first efforts
in this direction have been carried out by the Russian workers
Braginsky, Rudenko, and Rukman with negative results."
--
Ross Smith (Wanganui, New Zealand) ... alien@acheron.amigans.gen.nz
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