While reading P.W.Bridgman's A Sophisticates Primer of Relativity I came acorss an unusual statement. The book deals in some detail with the very basics of SR and while it does approach some things from an unusual angle it is by no means controversial. In the extract below he is talking of various possible ways of measuring velocity. The full section is quite long but I think I have included enough to make the context clear. The bit I do not understand is the second paragraph of the extract. ----It cannot be emphasized too strongly that all these different velocities have something in common, they are all velocities relative to something. We may talk of the velocity of one member of a pair of particles with reference to the other member of the pair, or we may talk of the velocity of a car with respect to the road. Or if we define velocity as change of position in unit time, the change of position has to be with respect to some frame of reference in which position is determined. Velocity is a relative concept, and whenever the physicist allows himself to speak of velocity with an application of absoluteness, he is either forgetting something or is tacitly implying something he has not taken the trouble to make explicit. Sometimes, for example, the MM experiment is described as showing that absolute velocity does not exist. Of course it does not exist, because it is not that sort of thing by definition. What the physicist is actually saying here is that there is no evidence for the existence of the old fashioned ether, which if it existed could be taken as a universal frame with respect to which velocities could be measured. One of the most insidious, and because it is so insidious, one of the most vicious formulations of this point of view is: “Relativity theory says that if two frames of reference are moving with respect to each other, it is impossible to say which frame is ‘really’ moving”. The usual implication here is that nature is so constructed that it is impossible to make the decision. The impossibility is entirely man-made. This point of view is behind some of the intuitive difficulties exploited in some recent discussions of the paradox of the “space traveller”. Can anyone throw any light on his intended meaning? Matheinste.