COSMOGONY JEANS 167 



tined to <j;() on forever? So far as we can at present see, no; for a 

 general guiding ])rinciple, that of generalized relativity, fixes a 

 limit, which Ave are fast a})proaching. According to this theory, 

 space can not extend forever; it has no limit, but is nevertheless finite 

 like the surface of the earth. AVithout exploring and surveying the 

 whole of the earth's surface, we can make a fair estimate of its total 

 area by measuring its radius, which we can do by measuring its 

 curvature at any one point. In the same way the total volume of 

 space is fixed by a quantity, the curvature of space, which can be 

 determined by measuring the density of distribution of matter in 

 space. Space which contained no matter would go on forever, but 

 the parts of space we can survey with our telescopes contain enough 

 matter to show that Ave already see an appreciable fraction of the 

 Avhole of space. It is as tliough our baby, Avatching ships coming 

 from over the horizon, concluded that the earth's surface Avas curved 

 and formed a general rough conception of its size by imagining the 

 obserA'ed curvature continuing until the earth's surface rounded back 

 on itself. 



Exact figures are impossible, but Hubble has calculated that space 

 is not likely to extend to more than about a thousand times as far 

 as the farthest nebula visible in the biggest telescope. Nothing pre- 

 vents our going on and on in space beyond this distance, but if we 

 do we merely come back to ourselves. The possessor of a sufficiently 

 sensitive Avireless apparatus may emit signals and pick them up a 

 seventh of a second later after they liaA^e traveled round the Avorld. 

 In the same Avay a not inconceivable increase in the size of our tele- 

 scopes Avould take us around the Avhole of space, and Ave should see 

 the stars surrounding our sun by light which had traveled round the 

 universe, not of course as they now are, but as they were 100,000 

 million years ago. 



Such considerations make it improbable that the expansion of the 

 universe can continue at its present rate for much longer. Having 

 grasped that the Avorld is round, the infant speedily forms a fair 

 idea of its size. Our particular infant, mankind, has made the great 

 discovery of the existence of the outer world, has formed some con- 

 ception of its size, and adjusted his ideas, not b}' a process of sIoav 

 revelation, but by a brain flash of the last few seconds. In his 

 mature years and his staid old age he is no doubt destined to make 

 many sensational discoveries, but he can never again liA^e through 

 the immortal moment at which he first grasped the immensity of the 

 ; outer world. We only live through a fcAv ticks of his clock, and 

 fate might haA'e ordained that they should be anyAvhere in the three 

 days that the child haS already lived, or in the 70 long, and possibly 

 tedious years yet to come. The wonderful thing is that she has 



