232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 



independently of that of the wind surrounding the plane. It is 

 reasoned that since the plane is moving through the earth's magnetic 

 field a potential will be set up between the ends of a wire stretched 

 between the wing's tips. It is only necessary to measure this poten- 

 tial diiFerence in order to calculate the speed of the jjlane with 

 respect to the earth. Careful analysis shows that the concept of the 

 potential difference under these conditions is meaningless except with 

 reference to a particular reference system. If this system is referred 

 to the plane itself, this potential difference is zero quite regardless 

 of what the sj^eed of the plane may be with reference to the earth. 

 A contrary result would conflict with the relativity theory. 



Meaningless questions will assume far greater importance in future 

 years. We shall see that the latest forms of the quantum theory now 

 give us the best of reasons for believing that the identity of separate 

 electrons within atoms or molecules may be partly or wholly lost, so 

 that it may have no meaning to ask whether a particular electron we 

 find as a result of experiment is the same electron which has pre- 

 viously produced an observed phenomenon. Even more far-reaching 

 in its consequences is the Bohr-Heisenberg uncertainty principle ac- 

 cording to which it has no meaning to ask what is the precise position 

 and velocity of an electron or atom. An electron may have a definite 

 position or a definite velocity but it can not in any exact sense have 

 both. This doesn't mean merely that there are experimental difficul- 

 ties in measuring them, it means that the concepts themselves (posi- 

 tion and velocity) are relative to one another in a sense somewhat 

 analogous to that of time and space in the relativity theory. 



One's instinctive reaction when first questioned as to the objective 

 reality of space, time, position, velocity, etc., is to object to such 

 consideration on the grounds that they are too metaphysical. The 

 recent advances in physics demonstrate that these methods of think- 

 ing are eminently practical; they represent, in fact, an attempt to 

 get away from the metaphysical character of much of our thinking 

 in the past. Instead of taking for granted objective realities cor- 

 responding to our concepts, we now deal with things which can be 

 measured in the laboratory, the concrete data that we have to 

 start from. 



It is, however, very useful to retain the concept of reality. Bridg- 

 man suggests that reality should be measured by the number and 

 the accuracy of the independent ways in which we arrive at similar 

 measures of the concept in question. For example, owing to the 

 fact that we have so many concordant methods of measuring the 

 distance between the ends of a base line used for triangulation, we 

 attribute great reality to the concept of length or, rather, to those 

 concepts of length which are applicable in cases of this kind. We 



