----- Original Message -----
From: "Peter Clark" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, August 08, 2002 9:18 AM
Subject: Re: OT: The Geography Of A Discworld and the surrounding universe.


> On Thursday 08 August 2002 17:44, Joe wrote:
> > No, I mean a giant disk in space with a small ball of  flaming gas
orbiting
> > it.  I'm trying to avoid saying how it came into being, I'm just
describing
> > how it is.
>         Ok, setting aside questions of *how* such a structure could
naturally occur,
> there are several more questions that need answering.
>         1. How thick is the disk? Unless it is massively thick, a strong
> gravitational pull will be nearly non-existant. I haven't done the math,
but
> I'm guessing that it would have to be at least 20k kilometers thick in
order
> for there to be any meaningful gravitational pull. Gravity is very
important;
> if it's not strong enough, any atmosphere will just drift off into space,
> along with any inhabitants. Little children would have to be extremely
> careful, lest with one bounce they go flying into space.
>         As for the notion of a gravitational center of the universe--it's
pointless.
> The pull of gravity decreases exponentially with distance. At a large
scale,
> gravity works only on large bodies--stars circling the galactic center,
for
> instance--not on small objects.
>         2. How wide is the disk? I assume that we are talking about some
very large
> structure, but there are some serious concerns to its stability. For one,
a
> disk is a most unusual object in space. There's a reason why large bodies
in
> space are round. If you have a small star orbiting it, that's going to
cause
> gravitational distortion; if the disk is too small, it will gradually
become
> more sphere-like; if the disk is too large, eventually it will
disintergrate
> because of the stress.
>         With our current understanding of physics, a discworld simply
cannot be. One
> could build a disk, but without some means of "roofing" it to keep the
> atmosphere in and providing some form of artificial gravity, it could not
> support land-based life. Such a world would be more fantasy, á la Terry
> Pratchett, rather than vaguely plausible, á la David Niven. (Although even
a
> Ringworld would be unstable without the use of some sort of
auto-correcting
> propulsion system.)


I'm avoiding the 'how' question. ;) I've already explained the gravity
thing: All things are pulled towards a gravitational centre of the universe.
There appears to be some kind of shield-dome keeping the atmosphere in, and
the space-ether out. (I never said this universe was the same as ours, just
the basic physical laws).  The space ether, I think, is in fact liquid
hydrogen.