- If motion is relative, why isn't the situation symmetric? Why can't we say the Earth moved away from the spaceship and back?
- The key is that the traveling twin experiences non-inertial motion—they must accelerate to turn around and return. Special relativity's postulates, including the symmetry of time dilation, apply strictly to inertial (non-accelerating) frames. The twin who feels the forces of acceleration is unambiguously the one who changes velocity, breaking the symmetry. The Earth frame, to a very good approximation, remains inertial throughout.
- Does the simulator include the effects of acceleration on time?
- No, for clarity and simplicity, this model treats the acceleration phases at the turnaround point as instantaneous. The focus is on the constant-velocity legs where special relativity's time dilation formula applies. In a fully detailed calculation, the proper time during acceleration would be computed, but it contributes minimally to the total effect for realistic accelerations compared to the high-speed cruise duration.
- Is the Twin Paradox just a thought experiment, or has it been tested?
- It has been experimentally verified using precise clocks. In the Hafele-Keating experiment (1971), atomic clocks flown on airplanes around the world showed a measurable time difference compared to stationary clocks, accounting for both special and general relativistic effects. Modern particle accelerators and GPS systems provide daily confirmation of time dilation for moving systems.
- What is 'proper time' and why is it fundamental?
- Proper time (τ) is the time interval measured by a clock following a specific path through spacetime. It is an invariant quantity in relativity, meaning all observers, regardless of their motion, agree on its value for a given worldline. It is analogous to the 'length' of that worldline. The 'paradox' resolves by showing the two twins traverse worldlines of different proper time lengths between the same two spacetime events (separation and reunion).