In his 1983 book, God and the New Physics, Paul Davies writes:
Christians believe that God is eternal. The word ‘eternal’ has, however, been used to mean two rather different things. In the simpler version, eternal means everlasting, or existing without beginning or end for an infinite duration. There are grave objections to such an idea of God, however. A God who is in time is subject to change. But what causes that change? If God is the cause of all existing things (as the cosmological argument of Chapter 3 suggests), then does it make sense to talk about that ultimate cause itself changing?
In the earlier chapters we have seen how time is not simply there, but is itself part of the physical universe. It is ‘elastic’ and can stretch or shrink according to well-defined mathematical laws which depend on the behaviour of matter. Also, time is closely linked to space, and space and time together express the operation of the gravitational field. In short, time is involved in all the grubby details of physical processes just as much as matter. Time is not a divine quality, but can be altered, physically, even by human manipulation. A God who is in time is, therefore, in some sense caught up in the operation of the physical universe. Indeed, it is quite likely that time will cease to exist at some stage in the future (as we shall see in Chapter 15). In that case God's own position is obviously insecure. Clearly, God cannot be omnipotent if he is subject to the physics of time, nor can he be considered the creator of the universe if he did not create time. In fact, because time and space are inseparable, a God who did not create time, created space neither. But as we have seen, once spacetime existed, the appearance of matter and order in the universe could have occurred automatically as the result of perfectly natural activity. Thus, many would argue that God is not really needed as a creator at all except to create time (strictly, spacetime).
So we are led to the other meaning of the word eternal — ‘timeless’....
A timeless God is free of the problems mentioned above, but suffers from the shortcomings already discussed on page 38. He cannot be a personal God who thinks, converses, feels, plans, and so on for these are all temporal activities. It is hard to see how a timeless God can act at all in time (although it has been claimed that this is not impossible). We have also seen how the sense of the existence of the self is intimately associated with the experience of a time-flow. A timeless God could not be considered a ‘person’ or individual in any sense that we know. Misgivings of this score have led a number of modern theologians to reject this view of an eternal God. Paul Tillich writes: ‘If we call God a living God, we affirm that he includes temporality and with this a relation to the modes of time.’ 3 The same sentiment is echoed by Karl Barth: ‘Without God's complete temporality the content of the Christian message has no shape.’
There is thus a grave and fundamental difficulty in reconciling all the traditional attributes of God. Modern physics, with its discovery of the mutability of time, drives a wedge between God's omnipotence and the existence of his personality. It is difficult to argue that God can have both these qualities.[1]
