A similar situation was created by Paul Langevin in 1911 with what was later called the " twin paradox ", where he replaced the clocks by persons (Langevin never used the word "twins" but his description contained all other features of the paradox). Langevin solved the paradox by alluding to the fact that one twin accelerates and changes direction, so Langevin could show that the symmetry is broken and the accelerated twin is younger. However, Langevin himself interpreted this as a hint as to the existence of an aether. Although Langevin's explanation is still accepted by some, his conclusions regarding the aether were not generally accepted. Laue (1913) pointed out that any acceleration can be made arbitrarily small in relation to the inertial motion of the twin, and that the real explanation is that one twin is at rest in two different inertial frames during his journey, while the other twin is at rest in a single inertial frame. Laue was also the first to analyze the situation based on Minkowski's spacetime model for special relativity – showing how the world lines of inertially moving bodies maximize the proper time elapsed between two events.