30 Journal of the Mitchell Society [September 



Relativity must include the laws of gravitation, and actual pursuit of 

 the conception has justified the hope." So his third achievement -is 

 his new theory of gravitation, which diflfers widely in some respects 

 from that of Newton. To understand it is not easy. 



First, let me remind you that gravitation has always been a phys- 

 ical mystery, and numerous theories to account for it have been de- 

 veloped without success. For one thing the speed of gravitation 

 seems to be infinite, the only thing in nature which has a speed greater 

 than light. Again, we say the sun "attracts" the earth and holds it 

 in its orbit, but when we find that this so-called attraction is a force 

 sufficient to break a million millon rods of the best tensile steel, each 

 seventeen feet in diameter, we are amazed that any ethereal medium 

 can transmit it. And if we let the ether go, we are still more perplexed. 

 Let us, however, start with the conception that all pulls are really 

 pushes. When lemonade is sucked or pulled through a straw, it is 

 really the pressure of the atmosphere which pushes it through the 

 straw from the other end. Perhaps gravitation is a push, rather than 

 a pull. 



When a baseball curves, we do not imagine something pulling it 

 around in a curve, but we think of the bank of air in front and on the 

 side of it, due to its twisting motion, as pushing it around. In like 

 manner a railroad train is pushed by the reaction of the outer rail 

 around the curve in the track. So we may imagine gravitation as 

 pushing the earth around in its curved orbit. From this conception 

 let us proceed to another, — the principle of Least Action. It may 

 be familiarly expressed in this way ; every moving body holds to the 

 line of least resistance. That is what the curving baseball does; per- 

 haps that is what the curving earth does. If so, the first thing to 

 ascertain is what the line of least resistance is in space, and why it 

 is so, if we can ; at any rate, find out what a body will do, and how 

 it will move under certain conditions. 



This Einstein has done, and as he worked he came more and more 

 to the conclusion that the idea of gravitation, like the ideas of time 

 and space, is only part of the mental scaffolding we have erected to 

 explain the phenomena of nature, and has no existence apart from 

 our brains. To understand this let us imagine ourselves in an elevator 

 high up in the Woolworth building. We 'feel the weight of our bodies 

 pressing the soles of our feet ; we feel the weight of the package we 

 are carrying; perhaps a pendulum is swinging in the elevator. Then 



