482 



THE SCIENTIFIC MONTHLY 



THE PROGRESS OF SCIENCE 



PROFESSOR EINSTEIN'S VISIT 

 TO THE UNITED STATES 



Plans have been under considera- 

 tion for lectures by Professor Ein- 

 stein in the United States, but his 

 arrival at the beginning of April 

 on a mission to promote the Zionist 

 movement was a surprise. He is ac- 

 companied by Professor Chaim 

 Weizmann, director of the chemical 

 research laboratories of the British 

 Admiralty during the war, now head 

 of the World Zionist Organization, 

 and two other leaders in the move- 

 ment. Professor Einstein is reported 

 to be especially interested in the es- 

 tablishment of a University of Jeru- 

 salem and to be ready to take part 

 in its work, but it is not likely that 

 he will leave Berlin permanently. 

 Professor Bergson has denied the re- 

 port that he would leave Paris to 

 become professor at Jerusalem. 



Arrangements were promptly made 

 for scientific lectures by Professor 

 Einstein at several universities, the 

 first being appropriately given at Co- 

 lumbia University, which awarded to 

 him last year the Barnard Medal on 

 the recommendation of the National 

 Academy of Sciences. Four lectures 

 have been given at the College of the 

 City of New York and a series of 

 five lectures is announced to be given 

 at Princeton University from May g 

 to 13. Scientific men are invited to 

 attend the Princeton lectures ; those 

 wishing to do so should write to Pro- 

 fessor H. A. Thompson. 



It is satisfactory that there should 

 be such widespread popular interest 

 in Professor Einstein and his work. 

 In the article on the History of 

 Mathematics by Professor Ernest 

 Brown in the present issue of the 

 Monthly and in the article on the 

 History of Physics by the late Pro- 

 fessor Andrews Henry Bumstead in 

 the last issue will be found state- 

 ments of Professor Einstein's con- 



tributions in their historic continuity. 

 An article on the Theory of Rela- 

 tivity by Professor E. B. Wilson was 

 printed in the issue of the Monthly 

 for March, 1920. In the issue of 

 Nature for February 17 last will be 

 found a series of articles on all as- 

 pects of the theory of relativity. Pro- 

 fessor Einstein himself contributes 

 an article on the development of his 

 theory in which he writes : 



The development of the special 

 theory of relativity consists of two 

 main steps, namely the adaptation of 

 the space-time "metrics" to Max- 

 well's electro-dynamics, and an 

 adaptation of the rest of physics to 

 that altered space-time "metrics." 

 The first of these processes yields the 

 relativity of simultaneity, the in- 

 fluence of motion on measuring-rods 

 and clocks, a modification of kine- 

 matics, and in particular a new 

 theorem of addition of velocities. 

 The second process supplies us with 

 a modification of Newton's law of 

 motion for large velocities, together 

 with information of fundamental im- 

 portance on the nature of inertial 

 mass. 



It was found that inertia is not a 

 fundamental property of matter, nor, 

 indeed, an irreducible magnitude, but 

 a property of energy. If an amount 

 of energy E be given to a body, the 

 inertial mass of the body increases 

 by an amount E/r 2 , where c is the 

 velocity of light in vacuo. On the 

 other hand, a body of mass m is to 

 be regarded as a store of energy of 

 magnitude mc 2 . 



Furthermore, it was soon found 

 impossible to link up the science of 

 gravitation with the special theory of 

 relativity in a natural manner. In 

 this connection I was struck by the 

 fact that the force of gravitation pos- 

 sesses a fundamental property, which 

 distinguishes it from electro-magnetic 

 forces. All bodies fall in a gravita- 

 tional field with the same accelera- 

 tion, or — what is only another form- 

 ulation of the same fact — the gravi- 

 tational and inertial masses of a body 

 are numerically equal to each other. 

 This numerical equality suggests 

 identity in character. Can gravita- 

 tion and inertia be identical? This 

 question leads directly to the General 

 Theory of Relativity. Is it not pos- 



