4 PHENOMENA, ATOMS, AND MOLECULES 



the uncertainty principle, developed by Bohr and Heisenberg, this conflict 

 has now disappeared. According to this principle it is fundamentally 

 impossible to measure accurately both the velocity and the position of any 

 single elementary particle. It would be possible to measure one or the 

 other accurately but not both simultaneously. Thus it becomes impossible 

 to predict with certainty the movement of a single particle. Therefore, 

 Ampere's estimate of the scope of science has lost its basis. 



According to the uncertainty principle, which is now thoroughly well 

 established, the most that can be said about the future motion of any 

 single atom or electron is that it has a definite probability of acting in any 

 given way. Probability thus becomes a fundamental factor in every ele- 

 mentary process. By changing the conditions of the environment of a given 

 atom, as, for example, by changing the force acting on it, we can change 

 these probabilities. In many cases the probability can be made so great that 

 a given result will be almost certain. But in many important cases the 

 uncertainty becomes the dominating feature just as it is in the tossing of 

 a coin. 



The net result of the modern principles of physics has been to wipe 

 out almost completely the dogma of causation. 



How is it, then, that classical physics has led to such definite clean-cut 

 laws? The simplest answer is that the classical physicist naturally chose 

 as the subjects for his studies those fields which promised greatest success. 

 The aim of the scientist in general was to discover natural laws. He there- 

 fore carried on his experiments in such a way as to find the natural laws, 

 for that is what he was looking for. He was best able to accomplish this 

 by working with phenomena which depended upon the behavior of enor- 

 mous numbers of atoms rather than upon individual atoms. In this way 

 the effects produced by individual atoms averaged out and became imper- 

 ceptible. We have many familiar examples of this effect of averaging — 

 the deaths of individual human beings can not usually be predicted, but 

 the average death rate in any age group is found to come close to expecta- 

 tion. 



Since the discovery of the electron and the quantum and methods of 

 detecting or even counting individual atoms, it has been possible for scien- 

 tists to undertake investigations of the behavior of single atoms. Here they 

 have found unmistakable experimental evidence that these phenomena 

 depend upon the laws of probability and that they are just as unpredictable 

 in detail as the next throw of a coin. If, however, we were dealing with 

 large numbers of such atoms the behavior of the whole group would be 

 definitely determined by the probability of the individual occurrence and 

 therefore would appear to be governed by laws of cause and effect. 



Just as there are two types of physics, classical physics and quantum 



