Is there an "aether" to determine if an object rotates? Hi, In modern physics aether theories are obsolete, all inertial systems are equivalent. For rotational motion, the situation is different: In a rotating system, it is possible to measure the rotation without any outside reference (e.g. by measuring the centripetal force needed to keep a body on its orbit). So there is an objective rest state where a body does not rotate. Does this somehow imply that there must be a "directional aether" throughout the universe that determines when an object does not rotate?