the nearest approach to a straight line that the neighbourhood permits. The neighbourhood is supposed to be non-Euclidean, that is to say, to contain no straight lines such as Euclid imagined. If a body is moving freely, as the planets do, it observes a certain rule. Perhaps the simplest way to state this rule is as follows: Suppose you take any two events which happen on the earth, and you measure the time between them by ideally accurate clocks which move with the earth. Suppose some traveller on a magic carpet had meanwhile cruised about the universe, leaving the earth at the time of the first event and returning at the time of the second. By his clocks the period elapsed will be less than by the terrestial clocks. This is what is meant by saying that the earth moves in a "geodesic", which is the nearest approach to a straight line to be found in the region in which we live. All this is, so to speak, geometrical, and involves no "forces". It is not the sun that makes the earth go round, but the nature of space-time where the earth is.
Even this is not quite correct. Space-time does not make the earth go round the sun; it makes us say the earth goes round the sun. That is to say, it makes this the shortest way of describing what occurs. We could describe it in other language, which would be equally correct, but less convenient.
The abolition of "force" in astronomy is perhaps connected with the fact that astronomy depends only upon the sense of sight. On the earth, we push and pull, we touch things, and we experience muscular strains. This all gives us a notion of "force", but this notion is anthropomorphic. To imagine the laws of motion of heavenly bodies, think of the motions of objects in a mirror; they may move very fast, although in the mirror world there are no forces.
What we really have to substitute for force is laws of correlation. Events can be collected in groups by their correlations. This is all that is true in the old notion of causality. And this is not a "postulate" or "category", but an observed fact—lucky, not necessary.
As we suggested before, it is these correlations of events
