204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 5 



processes, being itself unknown, no definite calculations of the time 

 intervals involved could be made. 



When going back in time, the second law leads to ever-increasing 

 energy concentrations in the past; an unlimited past would lead to 

 infinite energy concentrations, a concept which is physically unaccept- 

 able. Certainly some uneasiness was felt in this respect by those who 

 did not want to draw the logical conclusion of a finite age for the 

 present universe. But, then, there was Maxwell's demon, an imagi- 

 nary intelligent being who could, at will, regulate molecular processes 

 and thus do away with the law of entropy. This sufficed to show that 

 the law is not absolute. The law is only of a statistical nature, excep- 

 tions being always possible although more or less improbable. Fur- 

 ther, its validity for unlimited intervals of space and time was ques- 

 tioned. A perhaps not very justifiable complacency about the begin- 

 nings and ends of the world was thus sustained. 



During the second quarter of this century a great change in the 

 scientific outlook in this respect took place. The recession of the 

 extragalactic nebulae, coupled with the finite age of the radioactive 

 elements, suggested that there was a beginning a few thousand million 

 years ago, the same for the galaxies and for the atomic material of 

 which our planet is built. Following the above-mentioned phenomena 

 back in time, moments could be reached beyond which the recession 

 of nebulae and the decay of radioactive isotopes could not continue in 

 the same manner as they do now. The two time limits were not found 

 to be equal although they were of the same order of magnitude : but, 

 within the uncertainties of theory and observation, they could be 

 adjusted to each other. The idea of a finite age for the universe 

 emerged. A stage, some 3,000 million years ago, was visualized at 

 which the universe was closely packed together, when the temperature 

 and density were high enough to invert the radioactive processes and 

 to cause the building up of the heavy unstable isotopes at a rate equal 

 to, or faster than, their total rate of decay (spontaneous 4- induced) 

 in these conditions. 



One view considered this stage merely the remotest phase of evolu- 

 tion of our world, beyond which extrapolation from the present state 

 is not possible. It was not meant to be necessarily an absolute begin- 

 ning — more likely it was not. The concept of age is thus reduced to 

 that of a time scale, or a time interval during which the properties of 

 the universe have radically changed. Tliis definition appears to be 

 somewhat vague; but it would imply nothing short of a complete ab- 

 sence, at the early stage, of all the classes of celestial bodies which are 

 familiar to us now. In such a form the definition is stringent enough. 

 Therefore, even if we could assign an upper limit of age to all existing 

 stars, this would be only a subordinate time scale — that of stellar evo- 



