The most common relative directions are left,[1] right,[2] forward(s),[3] backward(s),[4] up,[5] and down.[6] No absolute direction corresponds to any of the relative directions. This is a consequence of the translational invariance of the laws of physics: nature, loosely speaking, behaves the same no matter what direction one moves. As demonstrated by the Michelson-Morley null result, there is no absolute inertial frame of reference.