Flat Universe: What Does It Mean to be 3-Dimensional?

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SUMMARY

The discussion clarifies the concept of a flat universe within the context of three-dimensional space. A flat universe means that the geometry of space is Euclidean, where the angles of triangles sum to 180 degrees, circles adhere to the formula for circumference (2πr), and parallel lines remain parallel. This definition contrasts with curved geometries, where these properties do not hold. The analogy of a flat sheet of paper is used to illustrate how a flat universe can exist in a three-dimensional framework.

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ChrisisC
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This may be a very trivial question. I would like to know how a universe could be flat if we are 3 dimensional. When i think of "flat" i think of something similar to a sheet of paper. Although technically 3 dimensional, the paper looks pretty 2 dimensional. So how is 3 dimensional space time "flat"?
 
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Flat in this context means not curved. That is, the geometry of space is such that angles in all triangles add up to 180 degrees, all circles have circumferences equal to 2πr, and all parallel lines stay parallel.
 
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