Let us study in more detail this soap-bubble, blown of emptiness, by which modern science portrays the universe. Its surface is richly marked with irregularities and corrugations. Two main kinds may be discerned, which we interpret as radiation and matter, the ingredients of which the universe appears to us to be built.

Markings of the first kind represent radiation. All radiation travels at the same uniform speed of about 186,000 miles a second. If the train in Fig. 2 (p. 88) had travelled at a uniform speed of a mile a minute, its motion would have been represented by a perfectly straight line inclined at an angle of 45° to the vertical. A succession of trains all moving uniformly at a mile a minute would be represented by a lot of lines all parallel to this. Now let us change our standard speed from a mile a minute to 186,000 miles a second, and replace the one direction from London to Plymouth by all the directions in space. The diagram on p. 88 now becomes replaced by the four-dimensional continuum, and radiation is represented by a set of lines all making the same angle (45°) with the direction of time advancing.

Markings of the second kind represent matter. This moves through space at a variety of different speeds, but all are small in comparison with the speed of light.