In theoretical physics, a branch of physics, the rigidly rotating disk paradox, sometimes called the Ehrenfest paradox, is a paradox concerning a rotating disk. Basically, the paradox says that when a disk (or any circular object in general) is rotated, it will have a smaller circumference than 2πr, because the radius will remain the same, but the circumference is shortened due to the Lorentz contraction. For a more detailed explanation, see below.