PHYSICS 73 



I have mentioned a number of cases and illustrations of 

 the unification and correlation that is now going on in physics. 

 But the work is not complete and I wish to call attention to 

 a few such problems. We have been led to suppose that 

 there is a medium called the ether which fills all space. The 

 reason for this supposition may be briefly stated. Light and 

 heat are transmitted across space as wave motions traveling 

 with a finite and definite speed. Energy in the form of 

 waves leaves the sun; some minutes later it arrives here. 

 We believe that there must be some medium capable of trans- 

 mitting energy by this wave motion. Other facts add to 

 this. For example, in the region around a magnet or an 

 electrified body there exists energy, even when all ponderable 

 matter has been removed from this region. Moreover, it 

 was found as a result of the work of Maxwell and Hertz 

 that this electric and magnetic energy could be propagated 

 through space, traveling with the same speed as light. We 

 conclude then that this ether is something capable of trans- 

 mitting by wave motion not only light but electric and mag- 

 netic energy. 



Many important problems arise in connection with the 

 ether. For example, is the ether stationary in space or is it 

 dragged by moving bodies? Does the earth in its motion 

 carry the ether with it, as, for example, a moving train carries 

 air along with it, or does the earth slip through the ether 

 without disturbing it? Many experiments lead to the idea 

 that the ether is fixed in space and that we are slipping 

 through it. But contradictions arise. If the earth by 

 virtue of its rotation is moving eastward through the ether, 

 the ether remaining stationary, we should find that light 

 travels slower when going towards the east than when moving 

 westward. But no such difference has been detected. The 

 explanation of this contradiction is one of the great problems 

 of today. 



