Why do moving rulers shrink (length contraction) and moving clocks run slow (time dilation) such that everyone measures the same speed of light c, regardless of their relative motions? Einstein resolved this mystery at the turn of the 20th century in "principle fashion" by turning the question on its head. He invoked the relativity principle (AKA "no preferred reference frame") and argued that since c appears in the equations of physics, no preferred reference frame says that everyone must measure the same value for c regardless of their motions relative to the source (that is, regardless of their reference frames).
Length contraction and time dilation then follow as a result of this "light postulate." Ironically, he missed a chance to use the exact same principle to resolve the mystery of quantum entanglement, which he introduced in 1935, calling it "spooky actions at a distance." Indeed, he died believing that quantum entanglement was evidence that quantum mechanics was "incomplete" when his relativity principle would have told him that quantum mechanics is as complete as possible with respect to quantum entanglement.
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