134 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



tions, determinism is apparently maintained. In a way, the older 

 physics remains approximately exact for man and for heavenly bodies; 

 but it is powerless to determine at the same time with exactitude the 

 position and velocity of an electron at a given moment and to predict 

 with certainty its rigorously determined motion; discontinuity and 

 indeterminism are integral parts of the atomic world. 



Astrophysicists and physicists who study atomic structure must 

 use their imagination ; it is imagination which leads to experimentation 

 and interpretation. But it is necessary at the same time to be prudent 

 in this respect; the ascribed interpretations are usually afterward 



rejected. 



New ideas can only be accepted as a result of conclusive and repeated 

 experiment. Especially is it necessary always to distinguish care- 

 fully between the results of experiments and the interpretation given 

 these results. 



Thus, in astrophysics, since the work of Hubble in 1925, we con- 

 sider that the spiral nebulae visible through our telescopes are island 

 universes, analogous to the universe of which the earth, the sun, the 

 Milky Way, and all the ordinary stars are a part and which we call 

 "the galaxy." 



The velocities of about 100 of these distant spiral nebulae are 

 believed to have been determined, principally with the aid of the large 

 Mount Wilson telescope; it appears from this work that all are moving 

 away from us and with velocities as great as one-seventh of the velocity 

 of light. Moreover, Hubble presented in 1929 a law according to 

 which the velocity of recession is proportional to the distance; each 

 increase of 1 million light-years in distance increases the velocity of 

 recession by 170 kilometers per second. Thus all the galaxies appear 

 to be running away from each other; they are fleeing from each other 

 with increasing speed; it is this conclusion which gives us the term 

 "the expanding universe," and suggests to us that the universe has 

 doubled its radius in less than 2 billion years. 12 But when the galaxies 

 will have attained a velocity as great as that of light, they can never 

 be seen by man, for light rays from them will be unable to reach the 

 earth. This will mean an unfathomable universe. 



Of course, this interpretation rests upon the shift of lines in the 

 spectra of spiral nebulae examined through the Mount Wilson tele- 

 scope. Who can say that this shift of spectral lines will not soon be 

 interpreted differently? The study of spectra is sufficiently mysteri- 

 ous for the whole interpretation to be taken with reservations. 13 

 Moreover, the strangest feature about Hubble's law is that it assumes 



» See Paul Couderc, Univers 1937, Paris, Editions rationalistes, 1937. 



u The astronomer Esclangon, for example, does not agree that light is absolutely constant and believes 

 that measures relative to the light of stars are different from terrestrial measures; these conceptions, moreover, 

 may be connected with those relating to interstellar space. 



