When astronaut Scott Kelly returned from nearly a year on the International Space Station, he was about 8.6 milliseconds younger than his identical twin Mark. This deep dive scales that real effect up to the most famous thought experiment in special relativity: one twin stays on Earth while the other travels to a star four light-years away at 80 percent of the speed of light. Using the Lorentz factor and length contraction, we show why the traveler ages six years while ten years pass at home, and why both twins get the same answer despite doing completely different math.

Then we tackle the actual paradox, first named by Max von Laue in 1911: if all uniform motion is relative, why can't the traveler claim Earth did the moving? We walk through the frame-switch resolution, Emile Wiechert's 1921 relay version that removes acceleration entirely, ...