Anton Sherwood wrote:
> Nobody has yet mentioned the *quantitative* demonstration that the 
> gravity of an uniform hollow sphere is cancelled inside.
> 
> Consider a particle within the sphere and a pair of equal and opposite 
> narrow cones whose apex is that particle.  Each cone meets the sphere in 
> an ellipse.  (The ellipses have similar shape because the axis of the 
> cones meets the sphere at the same angle on both sides.)  The area of 
> the ellipse, and thus the amount of mass pulling the particle in that 
> direction, is proportional to the square of the distance from that part 
> of the sphere to the particle.  But to get the amount of force you must 
> then divide by, guess what, the square of that same distance.  Thus the 
> forces on the particle from the two opposite cones are equal and opposite.

Thanks everyone for this great discussion. I've been following along, 
and sticking my two cents in when I can, but by the time I got as far as 
Anton's post (above) I realised I was getting out of my depth. So I 
released the tension this had caused with a quick render!

Thanks again! (and don't stop!)
Rick Measham

(P.S. Anton: please don't take offense as none is intended)

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Holy shit! That's what I get for posting something bizarre and then staying
away from an internet connection for a couple of days.

Without being too far behind on this cavalcade of discourse, I can reveal
the following:
Yes, it's a cylindrical space habitat which rotates for centrifugal gravity.
It is mostly based on descriptions from Peter F Hamilton's 'Night's Dawn'
series, which featured both hollow-asteroid and 'bitek' habitats (the
latter grown from seeds in orbit around convenient gas giants). However, as
discussed wholesale, this is simply one of the more recent incarnations of
an old idea.

Incidentally, I re-read 'Ringworld' only last week, which was the partial
inspiration for this WIP. Oh, and in response to the unnecessarily heated
Dyson sphere gravity debate, surely if you were sufficiently advanced to be
able to hammer flat everything in your system larger than a pebble and wrap
it all round your sun, gravity wouldn't be a problem? You'd just bolt a few
trillion no.8 gravity generators to the outer surface and hey presto! Or at
least follow the Ringworld lesson - spin the sphere up and live on the
equator.

Anyway, don't think all these allusions have gone unnoticed. Rama might be
the earliest version of this idea - I personally haven't read anything
earlier. Greg Bear's Thistledown provided some of the best descriptions
I've ever read of cylindrical habitats like these, although IIRC that
cavern-space was extremely large. Iain Banks makes reference to similar
stuff, but only as background ammo, most of his action tends to take place
in deep space, or on planets or orbitals.

My habitat is about 20km long and 10km in diameter. There is a light source
at the centre, which will eventually be a 10km plasma tube along the axis.
It's bloody difficult making landscapes that look good and detailed from
above, so I don't know how far this will go, but I'm going to put some
buildings etc in to help with the sense of scale.

Anyway, here's a better version, a bit bigger, and a bit hazier.

Bill

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"Bill Pragnell" <bil### [at] hotmailcom> schreef in bericht
news:web.4350e3b54ff5b998731f01d10@news.povray.org...
> Anyway, don't think all these allusions have gone unnoticed. Rama might be
> the earliest version of this idea - I personally haven't read anything
> earlier. Greg Bear's Thistledown provided some of the best descriptions
> I've ever read of cylindrical habitats like these, although IIRC that
> cavern-space was extremely large. Iain Banks makes reference to similar
> stuff, but only as background ammo, most of his action tends to take place
> in deep space, or on planets or orbitals.
>
> My habitat is about 20km long and 10km in diameter. There is a light
source
> at the centre, which will eventually be a 10km plasma tube along the axis.
> It's bloody difficult making landscapes that look good and detailed from
> above, so I don't know how far this will go, but I'm going to put some
> buildings etc in to help with the sense of scale.
>
> Anyway, here's a better version, a bit bigger, and a bit hazier.
>

I like it!! This starts to be more believable (?), although, from the
landscape features, I would guess that the diameter is much more than 10 km!
More like 100.

AFAIK, Rama is *the* earliest worked-out concept. Like Ringworld being it
for its own. An interesting side track from Ringworld is Gaea (in Titan, by
John Varley). It is a self-sustaining world, but more asteroid to planet
sized.

Thomas

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> I like it!! This starts to be more believable (?), although, from the
> landscape features, I would guess that the diameter is much more than 10 km!
> More like 100.
Difficult to tell, I think. If the land masses are only the size of small
islands (as intended), not continents, it looks fine. The problem is,
shorelines are fractal on all scales so without other visual clues it's
impossible to tell how big they are!

> An interesting side track from Ringworld is Gaea (in Titan, by
> John Varley). It is a self-sustaining world, but more asteroid to planet
> sized
Not heard of that. I shall have to keep my eyes open. Speaking of Ringworld,

http://www.infradead.org/~wmp/gallery4/ringworld.jpg

One of my earlier attempts at world-building. It is exactly the dimensions
laid down by one L. Niven - including the shadow squares! However, being at
the extreme of precision, it has a tendency to munge itself occasionally if
I change the viewpoint etc. One day I'll sort it out.

(I was building a description-accurate model of the Lying Bastard, too, but
this project too has fallen by the wayside - although I was quite proud of
my No.2 General Products hull!)

Bill

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Anton Sherwood wrote:

 > I don't see why not.

Because the earth isn't "pulled away" from the water. I know the 
analysis Larry is talking about. It's bogus. The center of the earth 
isn't pulled towards the moon and away from the water on the surface 
facing away from the moon, except for very vague and imprecise meanings 
of the word "pulled away". The explanation is sufficiently misleading 
that one can safely say it's wrong.

Consider, for example, the earth and moon on big sticks, held still. The 
centers wouldn't be going anywhere. The moon would still be pulling on 
the center of the earth, but all the water would be on the side with the 
  moon.

Now consider that the center of the earth follows the same path it would 
were there no tides. Clearly, the moon isn't pulling the center of the 
earth away from the water on the far side.

Plus, such an explanation doesn't account for tidal locking, wherein one 
face of the moon always points towards the earth, for example.

 > So if the Moon were a point mass Earth would not experience tides?
 > Bzzt, try again.

If the moon were a point mass the moon would not experience tides.
If the earth were a point mass, the earth would not experience tides.

The earth orbits the moon in the same path it would were it a point 
mass, so tides aren't caused by a change in the motion of the earth.

 > Newton showed that a spherically symmetric body - i.e. one whose 
density varies only with radius - produces the same g-field (outside its 
surface) as a point mass.

The water on the earth is not outside the surface of the earth. That's 
what I mean by "inside a system jazz".

-- 
   Darren New / San Diego, CA, USA (PST)
    Neither rocks nor slush nor salted rims
    shall keep us from our appointed rounds.

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Bill Pragnell nous apporta ses lumieres en ce 2005-10-15 08:20:
>>I like it!! This starts to be more believable (?), although, from the
>>landscape features, I would guess that the diameter is much more than 10 km!
>>More like 100.
> 
> Difficult to tell, I think. If the land masses are only the size of small
> islands (as intended), not continents, it looks fine. The problem is,
> shorelines are fractal on all scales so without other visual clues it's
> impossible to tell how big they are!
> 
> 
>>An interesting side track from Ringworld is Gaea (in Titan, by
>>John Varley). It is a self-sustaining world, but more asteroid to planet
>>sized
> 
> Not heard of that. I shall have to keep my eyes open. Speaking of Ringworld,
y No.2 General Products hull!)
> 
> Bill
> 
Gaea is a wheel space-station like living and centient hollow world orbiting Saturn.
It have a main 
"brain" in the central hub and 6 secondary "brains" around the rim. All have ther own
parsonalities. 
Tha main Gaea is obsessed with old movies. The interior environment is subtropical
earth like thank 
to the mirror pannels and large transparent areas on the inside ot the rim.

-- 
Alain
-------------------------------------------------
Taoism: Shit happens.

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Rick Measham nous apporta ses lumieres en ce 2005-10-15 07:03:

> Thanks again! (and don't stop!)
> Rick Measham
> 
> (P.S. Anton: please don't take offense as none is intended)
> 
> 
> 
> ------------------------------------------------------------------------
> 
I like that nerd meeter!

-- 
Alain
-------------------------------------------------
Believe those who are seeking the truth; doubt those who find it.

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Thomas de Groot wrote:
> "Mike Andrews" <nomail@nomail> schreef in bericht
> news:web.434fc6b84ff5b998c717c9af0@news.povray.org...
> 
>>For practicality I prefer Banks' 'Culture' ring model which is to have a
>>smaller ring orbiting in a star's life zone, spinning once per day to give
>>one g of centripetal accelleration and with a small rotation plane offset
>>from the sun. If I got my calculation right (a = lw^2) the radius is about
>>1.85 million km,  which still gives a huge surface area for a reasonable
>>ring width - and you can build several in one system :-)

There may be a problem with tidal stresses at that size.

>>(Wanders off, dreaming of how to reduce Jupiter to workable material ...)
> 
> And there you get the Ringworld described by Larry Niven!!

Not exactly.

What Mike described is a small ring, the center of which is at a 
distance (150Gm or so) from the star.  Ringworld is a huge ring (150Gm 
or so radius), with the star at the center of the ring.

Regards,
John

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Anton Sherwood wrote:
>> Larry Hudson wrote:
>>
>>> No, it's not a resonnance.  The way I've heard the two tides 
>>> explained is that the ocean is raised by the moon's gravity on that 
>>> side of the earth, but it also pulls the _earth_ away from the water 
>>> on the far side.
> 
> 
> Darren New wrote:
> 
>> Um, not really.
> 
> 
> I don't see why not.
> 
>> Tides are caused when any large body orbits a point. Consider two 
>> rocks on the moon, one on the ground very close to the Earth, one on 
>> the ground on the side we never see.  [snip orbital mechanics]
> 
> 
> This does not contradict Larry's version.
> 
 > ,,, [much quoting left out] ...

It's not really "my" version, I don't have any expertise on the subject. 
  And as I stated in my original post, this was the explanation I had 
heard.  It was quite some time ago that I heard this explained, and I 
don't recall what the source was (possibly a science program on TV, but 
it might have been a book or magazine.  Hmmm, thinking back again, it 
probably was TV, because I seem to vaguely recall it being illustrated 
with animated pictures.)  However, I did believe it was an authorative 
source at the time, and it did sound like a reasonable explanation, so I 
accepted it.

But I'm not going to argue about it...    ;-)

      -=- Larry -=-

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Alain <ele### [at] netscapenet> wrote:
> Gaea is a wheel space-station like living and centient hollow world orbiting Saturn.
It have a main
> "brain" in the central hub and 6 secondary "brains" around the rim. All have ther
own parsonalities.
> Tha main Gaea is obsessed with old movies. The interior environment is subtropical
earth like thank
> to the mirror pannels and large transparent areas on the inside ot the rim.

It would therefore appear that Hamilton plundered both this idea and the
original Rama, combining them for his bitek habitats*. They are closed
environments, like asteroids, with extensive cities on the exterior and
garden space on the interior. They are composed of a versatile polyp
material; almost everything is part of a huge single 'creature' - energy,
plumbing, recycling etc. They have superconductor spines which sweep
through their primaries' magnetospheres for energy. Their personalities can
be quirky too. One of them is even a main character!

*Not that this is a bad thing - Hamilton's work weaves virtually every large
sci-fi idea from the last 50 years into his universe, with a generous
helping of his own ideas.

I'll keep my eyes open for some Varley.

B

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