Michelson and Morley set out in their famous experiment to measure the absolute velocity of their laboratory, which was, of course, fixed on the earth. The experiment consisted of timing two rays of light over two equal tracks at right angles to each other. When one track was situated in the direction of the earth's motion they expected to get the same result as when two scullers of equal prowess are racing in a river, one up and down the stream and the other across and back; the winner will be the sculler rowing across the stream, as working out an example will convince. Even if the earth had been stationary at the time of one experiment, the earth's motion round the sun would have been reversed 6 months later and would then have given double the effect. They found, however, that the two rays of light arrived always an exact dead heat. All experimenters who have tried since have arrived at the same result and found it impossible to detect absolute motion. The principle of relativity has its foundation in fact on these failures to detect absolute motion. This principle states that the only motion we can ever know about is relative motion. If we devise an experiment which ought to reveal absolute motion, nature will enter into a conspiracy to defeat us. In the Michelson and Morley experiment the conspiracy was that the track in the direction of the earth's absolute motion should contract its length by just so much as would allow the ray of light along it to arrive up to time. We see, therefore, that according to the principle of relativity motion must always remain a relative term, in much the same way as vertical and horizontal, right and left, are relative terms having only meaning when referred to some observer. We do not expect to find an absolute vertical and are wise enough not to attempt it; in seeking to find absolute motion physicists were not so wise and only found themselves baffled. The principle that all motion is relative now requires to be worked out to all its consequences, as has been done by Einstein, and we have his theory of relativity. Einstein conceives a world of four dimensions built up of the three dimensions of space, namely up and down, backwards and forwards, right and left, with time as the fourth dimension. This is an unusual conception to most of us, so let us simplify it into something which we can more easily picture but which will still allow us to grasp Einstein's ideas. Let us confine ourselves for the present to events which happen on this sheet of paper, i.e., to space of two dimensions only and take time as our third dimension at right angles to the plane of the paper. We have thus built up a three dimensional world of space-time which is every bit as useful to us as a four dimensional representation so long as we only need study objects moving over the sheet of paper.