PHILOSOPHY OP PHYSICS — MILNE 235 



at 99/100 of the velocity of light, 280,000,000 years; for a nebula 

 moving at 999/1,000 of the velocity of light, 90,000,000 years; for a 

 nebula moving at 9,999/10,000 of the velocity of light, 28,000,000 

 years. These figures have a perfectly concrete meaning. If we de- 

 tect an event taking place, as we say, 7iow on a nebula moving with 

 9,999/10,000 of the velocity of light away from us, then the age of 

 the universe reckoned by the moving observer on this nebula, at that 

 event is only 28,000,000 years — comparable with the age formerly 

 assigned by Kelvin to the universe in our own time scale. We see 

 that the phrase " the age of the universe " has no objective content. 

 Given an event, we have to mention an observer in whose " now^ " it 

 occurs, and the ages are different according to the observer chosen. 



It then follows that the universe for the distant observer has only 

 been running down for 28,000,000 years. For us it has been running 

 down for 2,000,000,000 years. And these two estimates relate to the 

 same event, observed by two different observers. Thus for the fast- 

 moving observer the universe has run down less than for us, at the 

 two " nows " corresponding to the same event. Since, with an infinite 

 number of nebulae in the world running away from us with all 

 speed up to that of light, we can always find an example of a nebula 

 with a velocity which is arbitrarily close to that of light, we can 

 therefore specify a nebula for which, at the event on it in our 

 " present ", the running-down of the universe is as little as we please 

 (reckoned from its zero). 



It follows tliat though the universe is running down for each 

 separate observer, no absolute measure of the amount it has run 

 down at any definite instant of our time can be given. For us, it 

 has run down a definite amount. But for other observers, whom we 

 regard as contemporaneous observers, it has run down less, and we 

 can always in principle mention observers for whom it has run 

 down as little as we please. Here there is no sense in which it can 

 be said that the universe is running down independent of an observer. 

 In the ordinary sense of alivays, it always contains observers or states 

 which are practically at the outset of their world careers, ready 

 " wound up." The world system, though decaying and dying at 

 each separate place and for each separate observer, always com- 

 prises other observers, at great distances, for whom this process of 

 decay has hardly gone on at all. It is not a question of cycles of 

 rejuvenation, or of a process of revivification. The fresh start is 

 always present, but it is not really a fresh start, it is the one start 

 that every portion had, but differently reckoned. The world system 

 is like a tree which decays at its center, but lives on just under the 

 bark. Thus no absolute sense can be attached to saying that the 

 world has run down, though everywhere it is running down. It 

 always contains experiences for which the process of running down 



