Torus-Earth
(Lest I come across as a complete ass: this is a cool piece, of course.)<p>At least the discussion of gravitation is wrong. Somewhat unintuitively, the gravitational force on the inner part of the torus (the surface closest to the center, on the plane of the axes) is 0.<p>The pathological example is a hollow sphere of dense material. Outside the sphere, it "looks like" (if you just measure g) a solid planet. Inside the sphere, there is no gravitational field whatsoever, no matter how close you come to the surface.<p>Surprisingly, this holds no matter how large the sphere is. Suppose you're sitting on the inside of the surface of the sphere, and you decrease the radius a bit. Now the gravitational pull /away/ from this surface decreases like r^-2, so you would expect the gravity towards the surface (which is essentially unchanged) to increase. The issue is that the amount of material opposite you - the surface area of a sphere, really - also increases like r^2. (This is informal, but the best I can do for an intuitive explanation.)<p>Tough to find a good explanation of this online. <a href="http://physics.stackexchange.com/questions/364/gravity-on-a-doughnut-shaped-mobius-planet" rel="nofollow">http://physics.stackexchange.com/questions/364/gravity-on-a-...</a>