Apart from detailed figures, however, it is clear that we cannot go backward in time for ever. Each step back in time involves an increase in the total weight of the matter of the universe, and, just as with individual stars, we cannot go so far back that this total weight becomes infinite. Indeed a limit may quite possibly be set by considerations which we have already mentioned. The complete annihilation of all the matter now in the universe would raise the temperature of the earth’s surface by the six-thousandth part of a degree; the annihilation of a million times as much matter would raise it by 160 degrees. We cannot admit that as much radiation as this can be wandering about space. The earth’s temperature is determined by the amount of radiation it receives from the sun; it adjusts its temperature so that it radiates away just as much energy as it receives. A small correction is required on account of the earth’s own radio-activity, but this need not bother us. What would bother us, and would indeed upset the balance entirely, would be the radiation of a million dead universes if this were for ever streaming on to us out of space; in this event the earth’s surface would have to rise to a temperature well above that of boiling water before it could restore the balance between the radiation it received and that it emitted. In a word, the radiation of a million dead universes would boil our seas, rivers and ourselves. THE CREATION OF MATTER. All this makes it clear that the present matter of the universe cannot have existed for ever: indeed we can probably assign an upper limit to its age of, say, some such round number as 200 million million years. And, wherever we fix it, our next step back in time leads us to contemplate a definite event, or series of events, or continuous process, of creation of matter at some time not infinitely remote. In some way matter which had not previously existed, came, or was brought, into being. If we want a naturalistic interpretation of this creation of matter, we may imagine radiant energy of any wave-length less than 1·3 × 10⁻¹³ cms. being poured into empty space; this is energy of higher “availability” than any known in the present universe, and the running down of such energy might well create a universe similar to our own. The table on p. 144 shews that radiation of the wave-length just mentioned might conceivably crystallise into electrons and protons, and finally form atoms. If we want a concrete picture of such a creation, we may think of the finger of God agitating the ether.