so that our principle of action,
, becomes
There is now perfect symmetry between the rôles of space and time.
Now there was still another aspect of the principle of action which was extremely displeasing in classical science: By making the present course of a phenomenon contingent on a condition which could be determined only at some future date, it acquired a teleological character. But, as Planck points out, with the action as given by an energy in a volume of space-time, we may, by placing space and time on the same footing, regard duration as static; and the unsatisfactory teleological aspect of the principle may be obviated.
The space-time theory offers yet further advantages. It suggests that the function of action
must be a space-time invariant. If, then, we know the elementary physical magnitudes which must enter into this invariant, purely formal mathematical considerations enable us to construct all possible invariants, and one of these will represent the function of action. Assuming the world to be built up of electricity and metrical field, the possibility of obtaining the “function of action”