180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
excursion. Dirac showed great courage in even trying to give a 
physical picture of his mathematical theory. The fact is that in 
the microscopie field things may behave in a way entirely foreign to 
the way in which we have always seen large objects behave, hence they 
cannot be explained in the old familiar ways. 
There is in most people a strong tendency to label as “bunk” that 
which is not understood. This tendency is, on the whole, a healthy 
one. Skepticism is preferable to credulity if one is thinking in terms 
of the struggle for existence. The radio listeners who believe all the 
remarkable statements made about cough syrups, breakfast foods, 
cigarettes, etc., must certainly be struggling very hard for existence. 
However, skepticism based upon a lack of understanding is a danger- 
ous attitude of mind. Prof. P. W. Bridgman of Harvard has this to 
say in his book, “The Logic of Modern Physics”: 
It is difficult to conceive anything more scientifically bigoted than to postulate 
that all possible experience conforms to the same type as that with which we 
are already familiar, and therefore to demand that explanations use only elements 
familiar in everyday experience. Such an attitude bespeaks an unimaginative- 
ness, a mental obtuseness and obstinacy which might be expected to have 
exhausted their pragmatic justification at a lower plane of mental activity. 
The explanation of microscopic phenomena, then, utilizes concepts 
which are not familiar to everyday experience. For that reason the 
microscopic tends to undermine any smug complacency we may have 
regarding our knowledge of nature and the universe. Take, for 
example, the Heisenberg uncertainty principle. This principle states 
that we can never know accurately both the position and the velocity 
of asmall particle. It is easy to see why this is true. We can see the 
small particle because light has bounced off of it into our eye. We see 
it in the direction from which the light bounced. 
But the light, in bouncing from the particle, must have given it a 
push so that either its position or its velocity will have been changed 
by the mere fact that light must be used to observe it. By the time the 
light photon gets to the eye of the observer the particle will not be at 
exactly the spot from which the photon appeared to bounce. 
This uncertainty principle has been given an exact mathematical 
formulation. It turns out that if the position of an electron is known 
to within 0.004 inch then the speed of its motion is uncertain to within 
about 3 feet per second—the speed of a slow walk. 
The tendency, at first, is to consider this as rather a superficial 
principle. I can easily imagine a particle to have both position and 
momentum simultaneously; why bother so much about a mechanism 
for determining them? “However, a thorough study of the situation, 
with an analysis of every conceivable means afforded by nature for 
making determinations, impresses one with a feeling that here is a 
