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.
Now, in the particular example we have considered, the observer was able to station himself at a midpoint between the two points
and