The role of gravity

The view of physics that is most generally accepted at the moment is that one can divide the discussion of the universe into two parts. First, there is the question of the local laws satisfied by the various physical fields. These are usually expressed in the form of differential equations. Secondly, there is the problem of the boundary conditions for these equations, and the global nature of their solutions. This involves thinking about the edge of space-time in some sense. These two parts may not be independent. Indeed it has been held that the local laws are determined by the large scale structure of the universe. This view is generally connected with the name of Mach, and has more recently been developed by Dirac (1938), Sciama (1953), Dicke (1964), Hoyle and Narlikar (1964), and others. We shall adopt a less ambitious approach: we shall take the local physical laws that have been experimentally determined, and shall see what these laws imply about the large scale structure of the universe.

There is of course a large extrapolation in the assumption that the physical laws one determines in the laboratory should apply at other points of space-time where conditions may be very different. If they failed to hold we should take the view that there was some other physical field which entered into the local physical laws but whose existence had not yet be jn detected in our experiments, because it varies very little over a region such as the solar system. In fact most of our results will be independent of the detailed nature of the physical laws, but will merely involve certain general properties such as the description of space-time by a pseudo-Riemannian geometry and the positive definiteness of energy density.

The fundamental interactions at present known to physics can be divided into four classes: the strong and weak nuclear interactions, electromagnetism, and gravity. Of these, gravity is by far the weakest (the ratio Gm2/e2 of the gravitational to electric force between two electrons is about 10~40). Nevertheless it plays the dominant role in shaping the large scale structure of the universe. This is because the

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