[1] The proper-length (p. 25) is unaltered; but the relative length is shortened. We have already seen that the word “length” as currently used refers to relative length, and in confirming the statement that the moving rod changes its length we are, of course, assuming that the word is used with its current meaning. [Pg 36] Chapter III TIME Astronomer Royal’s Time. I have sometimes thought it would be very entertaining to hear a discussion between the Astronomer Royal and, let us say, Prof. Bergson on the nature of time. Prof. Bergson’s authority on the subject is well known; and I may remind you that the Astronomer Royal is entrusted with the duty of finding out time for our everyday use, so presumably he has some idea of what he has to find. I must date the discussion some twenty years back, before the spread of Einstein’s ideas brought about a rapprochement. There would then probably have been a keen disagreement, and I rather think that the philosopher would have had the best of the verbal argument. After showing that the Astronomer Royal’s idea of time was quite nonsensical, Prof. Bergson would probably end the discussion by looking at his watch and rushing off to catch a train which was starting by the Astronomer Royal’s time. Whatever may be time de jure, the Astronomer Royal’s time is time de facto. His time permeates every corner of physics. It stands in no need of logical defence; it is in the much stronger position of a vested interest. It has been woven into the structure of the classical physical scheme. “Time” in physics means Astronomer Royal’s time. You may be aware that it is revealed to us in Einstein’s theory that time and space are mixed up in a rather strange way. This is a great stumbling-block to the beginner. He is inclined to say, “That is impossible. I feel it in my bones that time and space must be [Pg 37] of entirely different nature. They cannot possibly be mixed up.” The Astronomer Royal complacently retorts, “It is not impossible. I have mixed them up.” Well, that settles it. If the Astronomer Royal has mixed them, then his mixture will be the groundwork of present-day physics. We have to distinguish two questions which are not necessarily identical. First, what is the true nature of time? Second, what is the nature of that quantity which has under the name of time become a fundamental part of the structure of classical physics? By long history of experiment and theory the results of physical investigation have been woven into a scheme which has on the whole proved wonderfully successful. Time—the Astronomer Royal’s time—has its importance from the fact that it is a constituent of that scheme, the binding material or mortar of it. That importance is not lessened if it should prove to be only imperfectly representative of the time familiar to our consciousness. We therefore give priority to the second question.