If, then, we wish to emphasise the great distinction between the classical view and the relativistic view, we must say: “According to classical science, the speed of light waves, and of electromagnetic waves generally, is constant in all directions with respect to the stagnant ether, hence also with respect to that particular observer who happens to be at rest in the stagnant ether. According to the relativistic point of view, the speed of light waves is constant throughout empty space when measured by any Galilean observer; that is, by any observer in any non-accelerated frame” (such frames being recognised by the absence of all centrifugal or inertial pushes and pulls). For the present we are in no position to predict what would happen if the observer, instead of being posted in a Galilean frame, were situated in a non-Galilean, i.e., accelerated or rotating, frame. All the statements we have made up to this point concern only Galilean observers. Such are the restrictions which the special principle and the special theory of relativity impose upon us. It is most important to understand this fact, as many of the criticisms levelled at Einstein’s theory are due to a failure to grasp the point. As the theory now stands, acceleration and rotation remain absolute and are therefore excluded from the special principle of relativity, which refers solely to motions in space that are relative. These are Galilean motions. Perhaps a definite illustration will make these points clearer. Consider, for example, Michelson’s latest experiment (not the celebrated one), or, again, consider Sagnac’s experiment. The essence [Pg 154] of both these experiments is to show that a ray of light travelling round the earth in the direction of the earth’s rotation requires a longer time to return to its starting point than would be the case for a ray travelling in the opposite direction. Obviously the velocity of the light waves with reference to the earth is not the same in all directions, so that we are able to detect the rotation of the earth on which we stand. The critic then infers that Einstein’s principle of relativity is upset by experiment. But the critic fails to realise that the motion that has been detected is a rotation, hence an acceleration, and that Einstein’s special principle confines itself to denying any significance to absolute velocities, that is, to motions which are not accelerated. Had this not been the case, Einstein’s principle would have been untenable since it is a fact of common knowledge that a large number of experiments (Foucault’s pendulum, the gyroscope, etc.) are capable of revealing the earth’s rotation. It is absolute velocity and not acceleration that experiment has ever obstinately refused to reveal.