kpmartin (08/06/82)
It must be noted that there is a distinction between a directional reference, which has at least two distinguishable directed lines (e.g. vectors) which are non-parallel, and a "frame of reference", which in addition has an origin. The first allows one to determine a direction (from the angle between it and each "axis", and the handedness of the (axis1,axis2,direction) triplet. The second one also allows one to fix a position by giving the direction and length of the radius vector from the origin to the object in question. Empty space would appear to have the axes, but no origin. Any acceleration is caused by a force, and one of Newton's laws states that there is an equal and opposite force acting elsewhere (although the speed of light limitation blurs the concept of "is", the opposite force may occur sometime in the "past" or "future", local time). The centre of mass of the two objects can then be used as an inertial frame of reference (provided there are no other external forces, whose reacted-upon objects should also be part of the system), with respect to which the aceleration is perceived. Note that the "absolute" speed and location of this centre of mass cannot be determined. Thus a rocket and its exhaust gases in otherwise empty space form such a system, as do all the parts of the spinning wheel. To spin the wheel about a point on its rim would either require a rocket engine of some sort (in which case the exhaust gases have to be included in the system), or some external force whose reaction has not been accounted for.