SIZE AND AGE OF THE UNIVERSE— JEANS 129 



we live in a part of the universe whicli is only sparsely filled with 

 nebulae, so that we come to a greater density of nebulae when we go 

 far from home. But a more likely interpretation is that the present 

 observational material is inadequate for statistical treatment. We 

 may hope that the new 200-incli telescope will solve the problem for 

 us in due course. In the meantime, the only inference that we may 

 legitimately draw from our present telescopic observations is that, in 

 all probability, the nebulae extend very much further than the 240 

 million light-years to which our telescopes can penetrate. 



If the effect just mentioned had been observed, it might have been 

 possible to form an estimate of the total volume of space. As this 

 method is not available, we must fall back on other, and less reliable, 

 methods. 



According to an earlier form of the theory of relativity, there was 

 a quite simple relation betw^een the total volume of space and the 

 average density of matter in space. Unhappily, it is not easy to 

 estimate the density of matter in space with any accuracy, but it is 

 at least possible to assign upper and lower limits between which it 

 must lie. This, of course, leads to upper and lower limits for the 

 total volume of space ; and calculation showed that if this theory were 

 sound, space was immense in comparison with that part to which our 

 telescopes can reach. The 240 million light-years to the farthest 

 visible nebulae is only a minute fraction — perhaps a three hundredth 

 part — of the radius of space. Or, to say the same thing in another 

 way, light takes 240 million years to travel from the farthest visible 

 nebulae to us, but would take 500,000 million years to complete the 

 journey round space and get back to its starting point. 



This particular development of the theory of relativity has fallen 

 into disfavor of recent years; it is still possible that it may give a 

 rough approximation to the truth, but it seems quite certain that it is 

 not the whole truth. Other theories have suggested radii of space of 

 about 2,000 and 10,000 million light-years respectively, but it is hard 

 to feel much confidence in these estimates. All that we can say with 

 any confidence is that the dimensions of space are probably far greater 

 than the 240 million light-years to which our telescopic eyes can see. 

 Einstein's latest conjecture is that space may after all, his earlier 

 theories notwithstanding, be of literally infinite dimensions. 



THE AGE OF THE UNIVERSE 



The question of the age of the universe is of a somewhat different 

 nature. There are a great number of different ways of estimating 

 this age; none of them are completely trustworthy, and unhappily 

 they appear to lead to inconsistent results. Stated in its crudest 

 and most obvious form, the problem is, of course, that of examining 



