224 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



can wave his ' now ' through the continuum until its intersection 

 with the world line of our path passes instantaneously from 1932 

 to 1942." In fact, the theory of relativity does not imply that the 

 observer on a distant nebula can experience or observe the event, 

 near ourselves, which we shall experience in 1942, at an epoch 

 defined by an event occurring in our reckoning as 1932. In the 

 definite meanings it is possible to attach to the words, we shall 

 experience the events of 1942 sooner than anybody else can have 

 experience of them. We ourselves are first in the field. But the 

 evolution difficulty still remains. If time means different things for 

 two different observers, how can the universe be said to evolve in 

 time ? For no meaning can be attached to a unique temporal ordering 

 of the totality of events. 



Yet there is one characteristic of the world frequently adduced 

 to show that it is indubitably evolving. That characteristic is an 

 apparent steady increase of entropy. As each observer's time goes 

 on, the universe appears to him to be moving from a less probable 

 state to a more probable state ; it appears to be tending to a change- 

 less state, of uniform temperature, a state called " heat death ", in 

 which all available mechanical energy has been transformed into 

 forms not available at this final constant temperature. In a section 

 called " The Final State of Maximum Entropy " Jeans says that 

 though general considerations cannot indicate the road by which 

 the final end of the universe is reached, they can tell us something 

 as to the nature of this final state. Though the entropy might de- 

 crease momentarily, this is enormously improbable, and it is still 

 more improbable that it should go on decreasing for any measurable 

 period. Jeans draws explicit attention to the fact that his examples 

 of change of entropy are confined to finite portions of the universe, 

 such as a boiling kettle or red ink mixing with water, but he applies 

 the same conclusions to the whole universe. " There can be no end 

 to the increase of entropy until these regions [regions of the universe 

 at different temperatures] are all at the same temperature, with 

 radiant energy diffused uniformly through space. Then and only 

 then will the universe have reached its final state, the perfect quiet 

 and perfect darkness of eternal night." 



Thus we have the two results : relativity suggests that the flux of 

 time is meaningless, whilst thermodynamics suggests that it is highly 

 significant. These are contradictory. If there is a limiting state 

 of the universe to which we are approaching, then states that are 

 old can be distinguished from states that are young, by the degree 

 of their approximation toward the final state. Not only distin- 

 guished, but labeled the one old and the other young. But accord- 

 ing to relativity different observers assign different times to dif- 



