The subject of this chapter is Einstein’s special relativity theory and what it says about the geometry of flat spacetime. This is not so difficult or abstruse as it sounds; it involves little beyond high school mathematics. A spacetime is simply the mathematical version of a universe that, like our own physical universe, has dimensions both of space and of time. A fiat spacetime is a spacetime with no gravity, since gravitation tends to “bend” a spacetime. Flat spacetimes are the simplest kind of spacetimes; they stand in the same relation to curved spacetimes as a flat Euclidean plane does to a curved surface.
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