Different worldlines
The twins begin and reunite at the same events, but follow different paths through spacetime. Those paths contain different amounts of proper time.
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RELATIVITY & SPACE · INTERACTIVE LAB 54
The twin paradox
Send one twin on a relativistic round trip and compare the proper time accumulated along two different paths through spacetime.EARTH TWIN30.00 yearsElapsed 0.00 y
TRAVELLING TWIN30.00 yearsProper time 0.00 y
AGE DIFFERENCE0.00 yearsCompared at the current Earth-frame event
Accumulated time
Spacetime worldlines
The turnaround matters
The travelling twin changes inertial frames at turnaround. Their lines of simultaneity change, so the round trip is not symmetrical.
Not a clock illusion
Each twin experiences their own clock running normally. When they reunite, the elapsed-time difference can be compared directly and is real.
Model note: This version uses constant-speed outbound and return legs with instantaneous turnaround. Acceleration is idealised; Lorentz time dilation and accumulated proper time are calculated.