Let?s be semantically careful here, since gedankens like this are how the relativists went awry. In the antecedent, who measured the speed? A? B? another world C? Even if A and B both independently measure 100 mph, still there are problems : * speeds may be different, but the equipment/procedure lacks the precision to detect the difference. What if the speed were a thousand times faster? A million? * Perhaps neither A and B are the absolute frame, but C is. If C were used , then a difference in speed would be seen, but C wasn?t chosen?.. too bad. * Maybe there?s an absolute frame that doesn?t separate out by velocity experiments, but only with higher order motion changes. Then A could be an ARF but not discernible with speed measurements. Robert Philip: In free space if two worlds A and B are coming together to collide at 100mph, no one can say what the real motions are out of an infinite number of variations which may be: A is stationary, A is moving towards B, A is moving away from B or B is stationary, B is moving toward A B is moving away from A. And all this infinite number of variables must have the resultant combination that will be a 100mph collision. There is no fixed point of reference. Thus one is assumed. I who live on A can chose my world as static. You on B can chose that your world is static. Nothing changes mathmatically, it wil be a 100mph collision. But the reality is a presumption in both cases, and niether world is static. Therefore not a proven reality.