= Twin paradox
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The twin paradox compares <proper time> along the different <world lines> of a travelling observer and a stay-at-home observer who reunite. Reciprocal inertial <time dilation> produces an apparent contradiction only if the traveller's change of <inertial frame> is ignored.
The decisive experiment is local: put two clocks together at departure, separate them, then compare their readings together at reunion. Everyone agrees on those two <spacetime events> and on the final difference. The <twin paradox> asks why two clocks can accumulate different <proper times> between the same <spacetime events>, despite reciprocal <time dilation> during uniform motion. Its answer is the geometry of their complete <world lines>, together with the <relativity of simultaneity> when comparing distant clocks.
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