One twin stays on Earth. The other flies at a constant cruise speed to a turnaround marker and returns. When they meet again, the traveler has accumulated less proper time than the Earth twin. Twenty animation seconds represent the whole trip, and all ages are displayed in years.
• The Earth twin and clock, a turnaround marker, the traveling spacecraft, a twin worldline comparison and a reunion clock ledger. • Sliders for cruise speed (0.1 to 0.95c) and one-way laboratory distance (1 to 10 light-years). • Readouts: Earth twin age gained, traveling twin age gained, their difference, ship position from Earth, total Earth trip duration and total traveler duration. • The Classic 0.8c trip experiment: Earth gains 10 years while the traveler gains 6. The slower-cruise experiment shows a smaller fractional difference over a longer trip.
The Earth twin remains in one inertial frame, while the traveler must change frames at the turnaround. Along each cruise segment dτ = dt/γ, giving T_earth = 2D/(βc) and T_traveler = T_earth/γ, so the age difference is T_earth(1 − 1/γ). The reversal is what breaks the symmetry; the two ages can only be compared directly at the reunion, where both twins are at the same place.
The model is flat spacetime with a symmetric constant-speed cruise and an idealized instantaneous turnaround. Accelerated transitions are neglected as a limiting case; they explain why the two paths are not interchangeable. It is an educational model of proper-time accumulation and not a spaceflight or life-support calculation.
The traveling twin changes inertial frames at the turnaround, while the Earth twin stays in one. This frame change makes the worldlines physically different, and only the traveler's path has the shorter total proper time.
With a 4 light-year one-way distance the Earth twin ages 10 years and the traveler 6, because γ = 5/3. This is the Classic 0.8c trip experiment in the simulator.
Only when they are at the same place, at the reunion. During the trip, each twin's readout is tied to that twin's own clock and frame, and the ledger in the scene tracks both.
It reduces the fractional difference but the trip takes longer. The Slower cruise experiment shows a smaller age ratio even though the round trip lasts more years.