Talk:Alderson disk
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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 9m / s2 of acceleration 'downward', there will be a corresponding acceleration of several hundred thousand m / s2 '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.
[edit] 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)
