New thread continuing a topic raised in 'Physical Space Properties Question' Okay, so I saw on wikipedia that you can have a 'flat torus' --- at least a 2 dimensional torus inbedded in 4-space. So you can also have a 3-torus, that's flat? And would time just be added on as an additional, orthogonal dimension? I'm a little confused about how/why this is considered flat. In the same wikipedia article, it says that a (2)cylinder is also flat---this is news to me. The analogy they make is that bending a flat piece of paper into a cylinder doesn't require any stretching/deformation of the paper. Okay. And I also realize that a 2cylinder would still have triangles whose angles add to 180 degrees... etc etc. These things definitely aren't true for the standard 2-torus in 3D; I would have assumed the 3-torus was the same. My understanding of differential geometry is rudimentary--only what I've gleamed from attempts at GR. I have no experience with 'topology' per se. None-the-less, equations would be welcome.