The faster an object moves, the slower its clock runs and the shorter it appears in the direction of travel. Lorentz factor γ = 1/√(1−v²/c²). At v=0.866c: γ=2 (half aging rate, half length). Cosmic ray muons born 15 km up (lifetime 2.2 µs) reach the surface thanks to time dilation — without relativity they'd decay in ~600 m. GPS satellites correct for both special and general relativistic effects.
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