Imagine little flat people living on the surface of a piece of paper. Just the surface: their world is not even as thick as the paper, with no vertical dimension at all. Stick a pencil through the paper and all they would see is a circle, a two-dimensional cross section. Fold the paper or roll it up, and they wouldn’t know the difference.
Now you’re ready for brane-world theory, which proposes that our three-dimensional universe lies inside higher spatial dimensions, and we are no more aware of them than those flat people are of our third dimension. Since a membrane separating two spaces is a handy example of a two-dimensional object in three-dimensional space, theorists started referring to a plane as a 2-brane. By analogy, we live in a 3-brane. (Although we have four dimensions if you count time, as physicists do.)
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