Note 1 — XXI THE EQUIVALENCE HYPOTHESIS

This circumstance, which gives certain bodies an absolute or preferential motion, is unpalatable to the relativist; he would like there to be no difference as regards forces1 between the case when the earth rotates with reference to outside bodies (the stars) considered as fixed, and the case when the earth is considered fixed and all outside bodies rotate around it. This point cannot be investigated by direct experiment; we can spin a top but we cannot keep a top at rest and spin the world round it, to see if the forces are same.