Here’s a question that most of us have been asked at some point in our lives, “what’s the shortest distance between two points?” By default, most of us will give the same answer that Archimedes gave more than 2,000 years ago: a straight line. If you take a flat sheet of paper and put two points down on it absolutely anywhere, you can connect those two points with any line, curve, or geometrical path you can imagine. So long as the paper remains flat, uncurved, and unbent in any way, the straight line connecting those two points will be the shortest way to connect them.

This is precisely how the three dimensions of space work in our Universe: in flat space, the shortest distance between any two points is a straight line. This is true regardless of how you rotate, orient, or otherwise position those two points. But our Universe isn’t made up merely of three space dimensions, but of four spacetime dimensions. It’s easy to look at that and say, “oh, well, three of them are space and one of them is time, and that’s where we get spacetime,” and that’s true, but not the full story. After all, the shortest distance between two spacetime events isn’t a straight line any longer. Here’s the science of why.

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