RADIANT ENERGY 401 



prehensibles, such as the dimensions of atoms and the velocities 

 of electrons, force one to resort to probability summations; for 

 example, if the diameter of an electronic orbit is of the order of 

 10-^ (0.00000001) cm., and electronic velocities approximate 

 10^ cm. (100,000,000 cm. = 1,000 km.) per second, then the 

 time for an electron to make an average revolution would be of 

 the order of IQ-^^ (0.00000000000000001) sec— a period so short 

 that it cannot be separately considered experimentally. If we 

 think in terms of ordinary time and space, then an electron is 

 everywhere at once, remaining only within the confines of its 

 shell. Rather than attempt to give a comprehensible picture of 

 such an atom, the newer quantum mechanics gives only a prob- 

 ability—that of finding the electron in any specified region at 

 any one time. 



The probability, or uncertainty, principle of Heisenberg, based 

 on the concept that the precise path or position of an electron 

 at any one moment is not predictable, led him to state that there 

 must be a renunciation of old and cherished ideas, such as the 

 principle of causahty or the belief that natural phenomena always 

 obey exact laws. In support of Heisenberg's statement, A. H. 

 Compton points out that if the principle of causality is replaced 

 by that of uncertainty, all those paradoxes of atomic physics that 

 have been classed as "quantum phenomena" find a ready solu- 

 tion. This includes the emission of fight, the photoelectric 

 effect, etc. (It should be remembered that classical laws when 

 apphed to large-scale phenomena need not be seriously affected 

 by the principle of uncertainty; in other words, the macroscopic 

 world is predictable, but the microscopic — the atomic — world is 

 said not to be.) 



While the uncertainty principle — the principle that one cannot 

 predict a future position of the electron on the basis of an initial 

 position and velocity, primarily because the present circum- 

 stances are partially unknowable — is now widely accepted by 

 physicists, there are yet those who oppose it, both as a philo- 

 sophical idea and as a mathematical deduction, and maintain 

 that the physical sciences are still based on the strict and univer- 

 sal vafidity of the principle of causality. 



An attempt to formulate a picture of the atom in terms of 

 probabilities has been made by H. E. White. Photographs of a 

 small mechanical model are made which represent pictures that 



