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.)
        :Peter

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