[TML] Alderson disk

[Jerry W Barrington]

On 12/10/07 3:49 PM, Stuart Frew wrote:

>> So . . . a fantasy world that really IS a "flat earth."  The Sun would
>> always be about at dawn, right?  So would you call it Dawnworld?
>> There'd be a permenantly Light and Dark side to everything, wouldn't
>> there?  Wait one.  Shadow squares, like with Ringworld?
>> 
> I believe that the sun could (technically, if you ignore the physical
> effects of motion on the sun) bob up and down through a hole in the
> middle.
> 
> So the sun would always be on one side but it could rise and set.
> 
> How do the weather patterns form, is the disk spinning at all?

Rise & set, yes, but only by a few degrees above the horizon (for any
reasonable "bob").  The outer side of mountains will be permanently
shadowed.

And the form I've seen of this disk before required an inner wall, to keep
the atmosphere from falling into the sun.  This would form another shadow
zone, possibly wider than Earth.

Oh, if your "bob" is on the order of months, that'll give you seasons (keep
the bob small enough not to get months of dark!).  :)

> Have a smaller disk (but still freaking big) face onto a red giant.
> Perpetual dusk at noon.

Requires constant thrust to keep from falling into the star.  Or to keep
proper (face-on) orientation if the disc is orbiting the star.  That's not a
dynamically stable configuration.

On 12/10/07 3:49 PM, Leonard Erickson wrote:

>On 9 Dec 2007 at 20:46, Richard Aiken wrote:
>> I'm thinking the disk would have to be spinning.  The original
>> primordial mass would have been spinning.  Stopping that would have
>> been a needless waste of energy.  Besides, the centripetal force of
>> the spin would be part of what's keeping the disk from collapsing into
>> the star.
> 
> Not even *close*. To do that, it'd have to be spinning faster enough
> to counter the effective "sideways" field. Which would throw
> everything on the surfaces towards the rim.

And don't forget, the disk spins as a unit.  At some speed, the outer rim
approaches the speed that it would fling things off (sets upper limit for
rotation speed, probably less than 1/yr for 10 AU disk).  The inner rim
would be nowhere near orbital velocity.  Of course, the inner and outer edge
gravity would modify this.

> I also need to work out how atmospheric absorption affects the light
> at different times of day and how that affects atmospheric temp at
> various "latitudes" (distances from the center)

When you calculate, remember that the outer reaches suffer from both 1/R^2
*and* the sun always being at lower elevation.  If your 45 degree elevation
is as seen from the inner edge, the outer edge will only see 5.7 degree
elevation.  If it was from the 1 AU circle, that yields 63.4 & 11.3.

Hmm.  I just realized, if the gravity falls off slowly like you mentioned,
the scale height will be much more.  Lot's taller column of air.  More
diffusion of the light?

>> Personally, I'd say escape would be easiest from the rim.  Any pointers to
>> the math on the gravity?
> 
> The gravity at the rim is *worse*, because you have *millions* of KM
> of rock pulling at you...

True, but most of it is millions of KM away too.  :)  Plus the rotation
speed.

>> And why 10 AU with a 1 AU hole?  Seems excessive, even for a broad
>> interpretation of the habitable zone.
> 
> Well, I wanted to have the available areas range as far into "you've
> got to be kidding" climate ranges as wasn't going to be impossible to
> survive, and then go a bit beyond.
> 
> If I use part of it for a D&D campaign, well, it gives "interesting
> places for the Frost Giants and Fire Giants and other such critters
> to come from.
> 
> Besides, who says humans are the only species living there?

Well, unless you're going with quite exotic life, between freezing and
boiling water is a pretty good limit.

For that matter, the Mercury-like regime is going to evaporate atmosphere.
You have to recapture it somehow, or eventually all your air & water go
away.  Same for the outer parts locking it all up in ice.

> Just consider the sort of stuff that might get traded across millions
> of miles of climate zones.

Probably not all that much.  Given that some proportion gets sold in each
increment of distance, it becomes exponentially improbable for any thing to
be available for sale an arbitrary distance from it's origin.  Plus at the
speed you were talking about limiting it too, imagine the wear and tear on
an object traveling that far.  And the price would have to go up at least
linearly with distance to be worth hauling it.  Pretty soon, nobody could
afford it.



On 12/10/07 3:49 PM, Tommy Grav wrote:

> I would have to sit down and look at the numbers, but why not but
> a massive object (like a neutron star or a black hole) in the middle
> of the disk and have a star that orbits the central point mass beyond
> the edge of the disk. It would be hard to find a stable orbit I suppose,
> but given the magi-tech needed to create a diskworld, it might be
> possible to explain such and orbit and why the central mass makes
> the scenario work. Just thinking off the top of my head :)

Assuming the NS or BH of comparable mass to the star we've assumed, the
structure wouldn't need to be any stronger, so that's ok.

But the orbiting star is going to go around in years, or months at best.
That's your day period.  Ouch.



On 12/10/07 3:49 PM, Richard Aiken wrote:

> <shrug>  The same tech that keeps the central hole stable (unmelted)
> would also work for the "vent holes," wouldn't it?  As for the heat,
> you could have free air exchange with both sides of the disk through
> these holes, if you make them wide enough.  How wide is enough?  Ten
> miles?  Fifty?  One hundred?  More?  Of course, I'm not sure how that
> would affect things on the disk surfaces.  Permanent tornadoes at each
> opening?

Shouldn't make tornadoes unless there is flow from one side to the other.
And with lots of vent holes, any pressure differential should go away.

Interestingly, if your holes are wide, there could be water blobs condensing
at the center.  But then, if the air doesn't blow thru, that heat would
build up in the air...  hmm.






By the way, you might want to look at what others have done with the idea.
Here's a few:
<http://www.geocities.com/Area51/Chamber/2838/disk/disk.html>
<http://www.westendgames.com/forum/archive/index.php?t-291.html>  <-- this
includes Larry Niven's description from 33 years ago.
<http://en.wikipedia.org/wiki/Alderson_disk>
<http://groups.google.com.bz/group/rec.arts.sf.science/browse_thread/thread/
7c80739c4e4762b7/b17a62169df56188>  <-- some of math here.


Also, the star is not stable at the center of the hole.  It would tend to
drift towards the disk.