• RELATIVITY RUSSELL. 207 



EINSTEIN CONCLUDED LIGHT DOES NOT TRAVEL IN A STRAIGHT LINE. 



Einstein on the other hand, assumed, just for the fun of seeing 

 what would come of it, that the principle of relativity still applied in 

 this case, so that it would be impossible to distinguish between the 

 conditions of the observers in the two boxes by any optical experi- 

 ment. 



It can easily be seen that it follows from this new generalized rela- 

 tivity of Einstein that light cannot travel in a straight line in a gravi- 

 tational field. 



Imagine that the first observer sets up three slits, all in a straight 

 line, at considerable distances apart. A ray of light which passes 

 through the first and second will obviously pass exactly through the 

 third. 



Suppose the observer in the freely falling system attempts the 

 same experiment, placing the line of his three slits at right angles to 

 the direction in which he is falling and having them equally spaced. 



The ray of light which has passed the first slit, must, in order to get 

 through the second, move not toward the point where that slit was 

 when it emerged from the first, but toward the point where the 

 second slit will be when the light reaches it. 



It will, therefore, be moving not at right angles to the direction in 

 which the system is falling, but at a slant, so that during the interval 

 in which it has traveled laterally from the first slit to the second, it 

 will have moved downward by a certain fixed amount, namely by the 

 amount through which the system fell in that interval. 



In moving from the second to the third slit, the light will occupy 

 the same interval of time, and, if it moves in a straight line, will go 

 downward by the same amount as before. 



But since the system is falling ever faster and faster, it will during 

 this time interval have dropped farther than it did in the preceding 

 time interval, and carried the third slit with it. 



Hence the ray of light will strike above the third slit and fail to go 

 through it, provided it travel in a straight line in space. 



But on Einstein's assumption it must go through the third slit, 

 since the two conditions are indistinguishable. 



In consequence, the path of the light in space must be curved and 

 not straight when gravitation is present, and the ray of light must 

 bend downward, that is, in the direction of the gravitational force. 



DEFLECTION OF LIGHT EFFECTED BY GRAVITATION. 



This deduction from Einstein's new principle may thus be reached 

 in a very simple fashion, but the further following out of the princi- 

 ple, and the exact calculation of its consequences is far too intricate 

 a matter for me to speak of here. 



