The Twin Paradox

The longest path is straight

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• The travelling twin’s proper time is the sum of the intervals along their worldline.
• Bob’s bent worldline has less proper time than Alice’s straight one.
• In spacetime, straight worldlines maximise proper time.

Steps

1. A trip to a star
2. Why not symmetric?
3. Birthday messages
4. A longer trip
5. Nearly light speed
6. Muons in a ring