Twin Paradox
A famous thought experiment in special relativity where one twin travels into space at high speed and returns younger than the twin who stayed on Earth.
Governing formula
τ_traveler = τ_stay / γ (Asymmetric acceleration worldlines)
SI unit
Years (Elapsed Proper Time)
In depth
Resolved because the situation is asymmetric: the traveling twin must accelerate to turn around, changing inertial frames, while the stay-at-home twin remains in a single inertial frame. The traveling twin traces a shorter proper time worldline.
Examples in the real world
Verifying asymmetric proper time aging experimentally using atomic clocks flown on commercial jet airliners (Hafele-Keating experiment).