Talk:Alderson disk

old comments
Aysle is, I think described as being more of a Torus, I think. - Nils 19:13, 13 Nov 2004 (UTC)

The Alderson disc is gravitationally unfeasible. The pull of any object taken as a whole is toward its center of mass; in the case of a uniform disc, this is the center of the disc. Thus the pull would still be toward the sun. In fact, from the point of view of a person standing on the disc, the 'horizontal' component of gravity will be as many times greater than the 'vertical' as the disc's radius is greater than its thickness. Thus, if the disc is made massive enough to provide about $$9 m/s^2$$ of acceleration 'downward', there will be a corresponding acceleration of several hundred thousand $$m/s^2$$ 'inward'. (This applies only to a person standing on the outside edge; on the inside edge, the 'horizontal' component of gravity will be zero.) --Ian Maxwell 04:45, 2004 Nov 28 (UTC)

That criticism is not completely unwarranted, however, it does not render the disk infeasible. It is an incorrect assumption that gravity will act directly towards the center of mass of a body. For an infinite sheet (in our case, near infinite), the gravitational field above it (acceleration) I believe is 2*Pi*mu*G = 6.28*mu*G, where mu is the mass per area. Clearly, the sideways component of gravity will be most severe near the edge, and least severe near the middle. If we assume the inner radius to be one AU and the outer to be 2AU, my upper limit calculation for the sideways component of gravity at the 1.5AU position is 6.11*mu*G. That is an extreme upper limit, and it will be much less than that. In other words, at the middle point, gravity will operate at less than a 45 degree angle to the normal. This would create many difficulties and limit the usability of many respective parts of the disk. At some set distance from the sun, gravity would, in fact, be directly normal, right before you reach the inner radius, where gravity will be tilted outwards instead of inwards. I plan on adding much more to this megastructure section later. Alan 07:55, 12 December 2005 (UTC)

If the disk rotates, centripetal acceleration will partially counteract the horizontal pull of the disk, reaching it's maximum value at the outer edge, just as the horizontal component of the disk's gravity does. If one varies the density of the disk with radius, the angle of gravity to the normal at any point can be controlled not entirely arbitrarily, but to a high degree. Thus it ought to be possible to come up with a density function that causes the horizontal component of the disk's gravity to everywhere cancel, or at least come very near to cancelling, with the centripetal acceleration.
 * Yeah, I suppose that might work actually. I wasn't previously considering spin on the disk to be a design parameter.  But that's right, the acceleration due to the spin would be constant while you would have some vector of gravity due to the disk that would be at some angle to the normal, and in the areas where gravity was previously tilted outwards, the density and acceleration could supposedly be adjusted to get gravity that acts normal to the surface.  The gravity vector, however, isn't just a function of the density of the disk at a certain radius, but affected by all mass around it, this just makes the exact calculations more difficult.  I believe that you couldn't make gravity upright for 100% of the disk, but with a lot of work, it would seem that gravity could act directly downwards for a large portion of the disk. theanphibian 07:09, 19 May 2007 (UTC)

None of that addresses the central problem though: that of the sun crashing into the disk. As soon as the sun drifts off-centre even a little bit, it will start to feel more gravity from the closer part of the disk than from the part of the disk which is further away. This will pull the sun closer to the disk, which will increase the effect, which will make the sun accelerate into the disk.

Continuous disc?
Is the Alderson disk necessarily one continuous chunk of solid matter? If it consisted of rings rotating with different velocities, the structural problems would be far less severe. Icek 19:25, 19 May 2006 (UTC)
 * If the design sucks and you have a better one, then oh well, doesn't change the article. But if you want to discus a proposal of rings around a sun, you might best deal with the idea of one ring around a sun.  Will gravity hold it together well?  Can it hold an atmosphere?  Would other rings disrupt it?  I know, I've been through these sort of discussions. theanphibian 06:11, 29 May 2007 (UTC)

If the disk consisted of segments rotating at different velocities, you'd need something (rollers?) to keep them apart. Even then, you'd have things like loss of relative rotation speed through friction and wear-and-tear to deal with. — Preceding unsigned comment added by 80.114.147.138 (talk) 02:35, 18 March 2016 (UTC)

Sun bobbing - mass of the disc wouldn't matter, right?
"This could be solved by forcing the sun to bob up and down within the disk, lighting first one side then the other. (As the disk far outmasses the sun, this is a plausible option.)"

What factor would the mass of the disc play in this? Since the motion of the sun is relative to the disc and vice versa, it wouldn't matter whether the sun bobbed or the disc did. So that note should be removed, if I'm not mistaken? --StarManta 05:28, 29 June 2007 (UTC)
 * I agree, I see no reason for that note to be there, it seems irrelevant.Darkcraft 10:12, 24 September 2007 (UTC)


 * It's probably a paraphrase of a line in Niven's essay. Having previously said that a ringworld could be bobbed for seasons, or to allow coplanar rings without permanent eclipse, he says (roughly): "The sun stays permanently on the horizon, unless we bob it. (This time the sun does the bobbing.)" —Tamfang (talk) 17:52, 18 August 2016 (UTC)

Stars Outside Our Milky Way
In the "In Popular Culture" section, under the Halo 3 paragraph, this line is found: It also had a nearby star, despite being outside the Milky Way, where stars cannot naturally form (...)

I'm going to take this out. Its astronomical incorrectness aside (countless stars have been born outside our galaxy), it doesn't add anything relevant to the article. ZbeeblebroxIV (talk) 04:44, 10 February 2010 (UTC)


 * Stars can also be ejected from galaxies. —Tamfang (talk) 19:30, 18 March 2016 (UTC)

The point of an Alderson disk?
I can see the point of a ring world, even of a Dyson sphere. But the article doesn't say anything about what desirable or useful properties an Alderson disk would offer. Anyone? Sources? Speculation? Anything? --67.180.106.165 (talk) 04:01, 14 March 2012 (UTC)

I came here to post this as well 71.236.2.96 (talk) 02:55, 22 March 2012 (UTC)
 * The only point is to provide a setting for fiction. Alderson, although not a fiction author himself, was an enthusiastic member of the ‘science’ fiction community. — Preceding unsigned comment added by 80.114.147.138 (talk) 02:31, 18 March 2016 (UTC)

Frankly, if you or your species has the capability to build such things (One would probably need to bring another star over, then disassemble it), you probably dont need a point other than "Hmmm...pretty bored with all these dyson spheres, can we do it different this time?".178.15.151.163 (talk) 16:05, 16 August 2016 (UTC)

Removed gravity passage, needs to be made clearer and restored
Hey everyone, thanks for your contributions! I removed this passage:


 * "Close to its surface, the gravity of the disk would closely approximate that of an infinite flat plate, for which gravity is perpendicular to the surface. Near the inner and outer edges of the plate, edge effects would become significant."

Could someone more familiar with the topic please help this this paragraph out? What is meant by "close to the surface"? What is the gravity of an infinite flat plate? What is gravity that is perpendicular to the surface? Are edge effects the same as Coriolis effects? thanks again. Informata ob Iniquitatum (talk) 05:57, 21 June 2017 (UTC)