ABSTRACT
This whole point of view, as we have seen, is superseded by relativity. There are no longer such things as ‘straight lines’ in the old geometrical sense. There are ‘straightest lines’, or geodesics, but these involve time as well as space. A light-ray passing through the solar system does not describe the same orbit as a comet, from a geometrical point of view; nevertheless each moves in a geodesic. The whole imaginative picture is changed. A poet might say that water runs downhill because it is attracted to the sea, but a physicist or an ordinary mortal would say that it moves as it does, at each point, because of the nature of the ground at that point, without regard to what lies ahead of it. Just as the sea does not cause the water to run towards it, so the sun does not cause the planets to move round it. The planets move round the sun because that is the easiest thing to do – in the technical sense of ‘least action’. It is the easiest thing to do because of the nature of the region in which they are, not because of an influence emanating from the sun.
