The Twin Paradox
One twin flies away and comes home younger than the other. Yet each of them saw the other's clock running slow.
1905 (implicit in Einstein), 1911 (Langevin's traveller)20 min read
French theoretical and experimental physics
Paul Langevin
1872 – 1946
Professor at the Collège de France, known for the Langevin equation, the theory of paramagnetism and the invention of ultrasonic echo-ranging. At the 1911 philosophy congress in Bologna he described a traveller who leaves Earth at high speed and returns to find that far more time has passed at home than aboard the ship — the thought experiment now called the twin paradox, though Langevin never mentioned twins.