Number of words: 301

The crucial feature of atomic physics is that the human observer is not only necessary to observe the properties of an object, but is necessary even to define these properties. In atomic physics, we cannot talk about the properties of an object as such. They are only meaningful in the context of the object’s interaction with the observer. In the words of Heisenberg, ‘What we observe is not nature itself, but nature exposed to our method of questioning.‘ The observer decides how he is going to set up the measurement and this arrangement will determine, to some extent, the properties of the observed object. If the experimental arrangement is modified, the properties of the observed object will change in turn. 

This can be illustrated with the simple case of a subatomic particle. When observing such a particle, one may choose to measure-among other quantities-the particle’s position and its momentum (a quantity defined as the particle’s mass times its velocity). We shall see in the next chapter that an important law of quantum theory-Heisenberg’s uncertainty principle says that these two quantities can never be measured simultaneously with precision- We can either obtain a precise knowledge about the particle’s position and remain completely ignorant about its momentum (and thus about its velocity), or vice versa; or we can have a rough and imprecise knowledge about both quantities. The important point now is that this limitation has nothing to do with the imperfection of our measuring techniques. It is a principle limitation which is inherent in the atomic reality. If we decide to measure the particle’s position precisely, the particle simply does not have a well-defined momentum, and if we decide to measure the momentum, it does not have a well-defined position. 

Excerpted from Pages 140-141 of ‘The Tao of Physics’ by Fritjof Capra