Now all the conclusions we have reached up to this point are quite irrelevant to Einstein’s theory proper. They embody the discoveries of classical science and, while they still hold in the relativity theory, we shall see that Einstein’s contributions to the problem of simultaneity lie in another direction entirely. If these points are understood we may pass to a definite example and investigate how the physicist will proceed when he wishes to determine whether or not two external events occurring in different places are simultaneous in the physical sense. Suppose that two instantaneous flashes of light occur at two points, and , on the road; and that we wish to discover whether or not the two flashes were produced simultaneously (always in the physical sense). The mere fact that we might perceive the two flashes simultaneously or in succession would not of itself permit us to settle the question, for the order of our perceptions would be found to vary with our position (of course only to a very small degree in the case of visual perceptions). But suppose now that we stood at the midpoint between and , as measured with our rigid rods. If we perceived the two flashes simultaneously (in the psychological sense), should we be justified in asserting that the two flashes had occurred simultaneously in the physical sense? Classical science answered no; for we were not at all certain that the wave of light was propagated from to with the same speed as the wave propagated in the opposite direction from to . Classical science assumed that the unknown velocity of the ether drift caused by the earth’s absolute velocity would interfere with our determinations, so that nothing definite could be said. But the entire situation changes as soon as we accept Einstein’s principle of relativity and his postulate of the invariant velocity of light. We are then in a position to assert that the ether drift, whether it exists or not, can have no physical significance; and that we are therefore justified in assuming that the rays of light are propagated with the same speed in all directions. If this is the case, we have a right to conclude that the simultaneity of our perceptions of the two light flashes (in the psychological sense) ensures the simultaneous occurrence (in the physical sense) of the two external events, namely, the two flashes on the road. [Pg 169] Now, in the particular example we have considered, the observer was able to station himself at a midpoint between the two points and . But situations may arise where a procedure of this sort would be impossible. Suppose, for instance, that we wish to ascertain whether two events occurring one on the sun and the other on the earth by our sides are simultaneous or not. How shall we proceed?