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Proposed in 1909 by Paul Ehrenfest.
Etymology from Wiktionary, CC BY-SA 4.0.
Ehrenfest paradox
Today it means: A relativistic paradox concerning the concept of Born rigidity within special relativity, where an ideally rigid cylinder that is made to rotate about its axis of symmetry. The radius R as seen in the laboratory frame is always perpendicular to its motion and should therefore be equal to its value R₀ when stationary. However, the circumference should appear Lorentz-contracted to a smaller value than at rest. This leads to the contradiction that R = R₀ and R < R₀. Full definition →