You will sometimes hear that time can speed up or slow down according to the theory of relativity. That’s baloney. Or to be more polite about it, it’s a misleading way to describe a real phenomenon. The new feature in relativity is that the total duration experienced by two observers moving in different ways will generally not be equal, even if they begin and end at the same events in spacetime. This isn’t because the rate of time is changing; it’s just because one person moved on a different path, and therefore covered a different amount of spacetime. If one person walks in a straight line
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