The twin paradox is a famous thought experiment in special relativity involving identical twins: one stays on Earth while the other travels at near-light speed on a round-trip journey. When they reunite, the traveling twin is younger than the one who stayed behind. At first, this seems paradoxical, since each twin could view the other as moving—so why should one age less?
The resolution lies in the asymmetry of the situation: the traveling twin undergoes acceleration and deceleration when turning around to return to Earth, while the stay-at-home twin remains in an inertial (non-accelerating) frame. Special relativity applies symmetrically only to inertial frames, so once acceleration is involved, the symmetry breaks.
During acceleration, the traveling twin changes frames, and the relativistic effects on time are no longer reciprocal. From the Earth twin’s inertial perspective, the traveling twin’s clock runs slower throughout the journey. General relativity also helps describe the role of acceleration more fully.
Thus, the paradox is resolved by recognizing that the twins do not experience the journey equally—the traveling twin’s path through spacetime is shorter in terms of proper time, leading to less aging.