January 18, 1918] 



SCIENCE 



59 



molecular and statistical theories. The 

 converse is also true, as Boltzmann so 

 stoutly maintained; and I think we must 

 recognize that the progress of thermody- 

 namics has been greatly facilitated by the 

 interplay and mutual reaction of both types 

 of theorj-. 



The second example is a more direct one ; 

 it is the remarkable theory of gravitation 

 in which the highly individual genius of 

 Einstein has again manifested itself. It 

 is too early to come to a definite conclusion 

 as to its validity. It has had one striking 

 verification in the deduction of the correct 

 value for the unexplained motion of Mer- 

 cury's perihelion; but this agreement may 

 conceivably be due to accident and, in any 

 case, its evidence is too slender to be re- 

 garded as establishing the theory. But we 

 must face the distinct possibility of its 

 ultimate success; and, in that case, we can 

 not fail to recognize it as a brilliant tri- 

 umph of the descriptive method. It is 

 difScult to believe that any living physicist 

 except Einstein could have constructed this 

 theory even with the help of Minkowski's 

 highly simplified method of description by 

 means of four-dimensional geometry; but 

 it is quite beyond belief that such a theory 

 could have arisen at the present time by 

 the use of any of the more usual methods 

 of theoretical phj'sics. 



There is one further matter in this con- 

 nection to which I should like to invite 

 your attention. It is the question of the 

 complete validity of Einstein's original 

 postulate of relativity. There can be little 

 doubt of its correctness when applied to 

 motions of translation ; speaking in terms 

 of the ether, we may be reasonably confi- 

 dent that it is impossible to detect the ef- 

 fects of uniform translation relative to the 

 ether. But little has been accomplished 

 in extending the theory to motions of ro- 

 tation ; indeed, rotation has alwaj's been a 



stumbling-block to a purely relative theory 

 of motion, as soon as dynamical considera- 

 tions are introduced. As Maxwell says:' 



So far as regards the geometrical configuration 

 of the earth and the heavenly bodies, it is evidently 

 all the same 



' ' Whether the sun predominant in heaven 

 Rise on the earth, or earth rise on the sun; 

 He from the east his flaming road begin. 

 Or she from west her silent course advance 

 With inoffensive pace that spinning sleeps 

 On her soft axle, while she paces even, 

 And bears thee soft with the smooth air along.' '» 



But, as we all know, the plane of Fou- 

 cault's pendulum remains fiixed with refer- 

 ence to the stars, and this has usually been 

 interpreted as proving by dynamical means 

 the absolute rotation of the earth. The 

 thoroughgoing relativist replies, however, 

 that the contrary supposition is equally 

 possible ; it would merely require a restate- 

 ment of the principles of mechanics which 

 happen (for some unknown reason) to 

 take on their simplest form when referred 

 to axes fixed with respect to the stars. The 

 new statement of the laws of motion would 

 seem to us very unnatural, but the essential 

 point is not their strangeness, but that they 

 would be different. To cause them to 

 transform into themselves, as Maxwell's 

 equations do when subjected to the 

 Lorentz-Einstein transformation, would 

 apparently require curious assumptions of 

 cur\'ed space, and of time recurrent after 

 twenty-four-hour periods, which would cer- 

 tainly be very foreign to the ordinary 

 habits and preferences of the human mind, 

 whether we assume that these habits are 

 inherent or acquired. Even from the point 

 of view of convenient description it seems 

 likely that we shall do better by adhering 

 to the belief that the stars are fixed and 

 that the earth rotates. We must, however, 



8 "Matter and Motion," p. 154 (Van Nostrand, 

 1878). 



9 ' ' Paradise Lost, ' ' Book 8, U. 160 et seq. 



