scrith, neutronium, etc., (fwd)

[Kragen Sitaker]

-- 
<kragen@pobox.com>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
Wed Aug 04 1999
95 days until the Internet stock bubble bursts on Monday, 1999-11-08.
<URL:http://www.pobox.com/~kragen/bubble.html>

---------- Forwarded message ----------
Date: Wed, 4 Aug 1999 13:52:11 -0400 (EDT)
From: Kragen <kragen@kirk.dnaco.net>
To: spamisBAD@mikeash.com
Subject: Re: scrith, neutronium, etc., 

Mike Ash posted on alt.games.marathon:
> The Ringworld HAD to be made out of scrith, there is simply no other
> way. You cannot just pile on the mass to make up for the lack of
> strength; this extra mass increases the amount of strength you require
> to hold it together, and you run around in circles, adding mass for
> more strength, then adding strength for more mass, etc.

Well, if you built a stationary ringworld suspended in space -- not
spinning -- you wouldn't need any tensile strength at all, and
relatively little compressive strength (to keep it from collapsing
under the weight of its own gravity).  (Ignoring the buckling problem,
that is.)

You could actually start it spinning so that the compression from
gravity was exactly equaled by tension from holding the ring together,
and then you would need exactly 0 strength, tensile or compressive,
because it wouldn't be under any stress; you could build the thing out
of cigarette ashes.  Of course, if you stepped on it, you'd break it :)

So you could add just enough strength to deal with random events like
people walking around and meteoroids hitting the thing.  (And since not
all of the ring is the same diameter since the thickness is nonzero,
even if the ring as a whole is under no stress, the outside parts will
be in tension while the inner parts are in compression.)

You would have a normal gravitational field on the inside surface of
the ring; the centripetal force applied by the ground on your feet to
keep you moving with the ring (as opposed to in a straight line through
the ring and off into space) would be in the same direction as the
actual gravitation from the ring's material.  (Gravitation in a ring is
toward the outside, which poses some problems for orbital stability.)

I haven't done the integrals to figure out how much material you need
to get how much gravitational compression stress.

> However, for a world like Halo that is so much smaller, it may be
> possible to use more normal materials.

How big is Halo?  I have calculated that you could build a ring of
Kevlar-149 (a real material) spinning enough to give an acceleration of
1 G with a radius of up to about 2.4 billion meters, assuming
gravitation was negligible.  (As described above, you can increase the
possible diameter by using gravitation.)  The Earth's orbit is about
150 billion meters in radius; the Moon's orbit around the Earth is
about 0.384 billion meters in radius.

(I'll send you the calculations if you like; I'm preparing them for
publication on the kragen-tol mailing list, to which I am also sending
this email.)

> Neutronium is out. Its dense, but so what? It doesn't have any tensile
> strength aside from what gravity gives it. Also, neutronium at
> sub-stellar masses is highly unstable. Technology to make it stable and
> useful and strong is about as easy to come by as technology to make
> scrith.

I'm curious what universe you're referring to here.  Are you saying
that, in Niven's Known Space milieu, neutronium has no tensile strengh
and is highly unstable?  Or are you making these assertions about the
real world?

Nobody has produced neutronium in a laboratory, so it is unknown what
its tensile strength really is or what its stability is.  I'm no
physics god, but I'm not even familiar with mathematical predictions of
these things; maybe you can introduce me to some.

Intuitively, I would expect that neutronium would have tensile strength
coming from both gravity and the strong nuclear force.  I believe
strong bonds at nuclear distances have a strength on the order of tons,
and I think 'nuclear distances' means on the order of 10^-15 meters.
(http://www1.mhv.net/~lepore/nucleus.htm says mean electromagnetic
radius of a nucleus is (1.07 +/- 0.02)A^(1/3) fm, where A is the number
of nucleons, so a piece of neutronium containing a billion neutrons
would have a mean electromagnetic radius of 1.07 picometers.  (Of
course, it's relatively silly to talk about its mean electromagnetic
radius, because it doesn't have an electromagnetic field, right?)  So
we can say that it has on the order of 500 neutrons from center to
outside (of course, it's a sphere, and it's packed some way, so it's
not exact, but on the order of) so you have roughly 500 neutrons per
picometer, or 250,000 neutrons per square picometer.

Now, in a square-meter cross-section of neutronium, we'd have 10^24
square picometers, so 0.25 x 10^30 neutron-neutron bonds or so.  So we
have a tensile strength of around 10^30 tons per square meter.  A ton
is about  9 x 10^3 newtons, so we have an estimated tensile strength of
about 10^34 pascals.

Now, I might be wrong by a bit, but probably not by as much as three
orders of of magnitude.  But typical tensile strengths for very strong
materials are on the order of 10^9 pascals.  So this material ought to
be stronger than ordinary matter by at least a factor of 10^21, also
known as a sextillion, or a billion trillion (in American numbers).

Stable and useful is another matter.  Heck, even creating the material
is another matter.  :)

-- 
<kragen@pobox.com>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
Wed Aug 04 1999
95 days until the Internet stock bubble bursts on Monday, 1999-11-08.
<URL:http://www.pobox.com/~kragen/bubble.html>