Re: OT: Ringworld

> Date:         Tue, 4 Dec 2001 09:44:39 -0800
> From: Anton Sherwood <bronto@...>
>
> Lars Henrik Mathiesen wrote:
> > The sphere or ring itself will not exert any gravitational attraction
> > on matter inside them ---
>
> True for the sphere, false for the ring, as ought to be obvious from the
> informal geometric argument:
Right... I thought I had an argument from rotational symmetry that the
ring case was like the sphere, but I see where it breaks down now.

[snip clever argument for the sphere case]

> See http://www.ogre.nu/images/gringi.jpg for the contours of the ring's
> g-field (actually 200 point masses on a circle).  The net force is
> perpendicular to the contours.  Unsurprisingly, it's generally toward
> the nearest point on the ring.
Someone should have told Niven --- doesn't his Ringworld spin? He
could just have adjusted the thickness of the unobtainium layer to get
whatever gravity he wanted.

> The central equilibrium point is unstable: a nudge off the axis will
> send your test mass falling to the ring.
Even without flexibility. Oh, that's why it has to spin, of course ---
to keep the flexible ring stable.

Lars Mathiesen (U of Copenhagen CS Dep) <thorinn@...> (Humour NOT marked)

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