Remembered another one: gravitational orbits can't occur in 4D space or higher. In 3d space, we have inverse-square laws. Think of a light source, the area th

Remembered another one: gravitational orbits can't occur in 4D space or higher. In 3d space, we have inverse-square laws. Think of a light source, the area that is covers at a given distance is the area of a sphere, so it makes sense that it would decrease with that area, or 4piR*R. Gravity is also inverse-square in 3D space (though relativity makes this much more complicated, I expect). Implication is that in a 4D space, it would be an inverse-cubed relation. It turns out that stable orbits can only happen with inverse squared laws. https://en.wikipedia.org/wiki/Bertrand%27s_theorem https://en.wikipedia.org/wiki/Bertrand%27s_theorem