Now imagine this happening in the four-dimensional fabric of space and time.

NPR: Einstein: Relatively Speaking, a Complicated Life

On the next rung up the ladder of difficult mathematics comes doing the same thing in four-dimensional space.

ECONOMIST: Proof of the truth of the Poincar�� conjecture

Einstein's theory of relativity unified these and created a four-dimensional space that is an analogue to our three-dimensional space, except that the "distance" between two points need not be positive.

WSJ: Book Review: Surfaces and Essences