Wednesday, October 1, 2014

Newton versus Heisenberg

Death of a microbe
The Heisenberg Uncertainty Principle has become famous well outside quantum physics, but it is often cited in garbled form. I once found it mentioned in a textbook on social science research methods, where it was defined as the fact that electron microscopes can damage samples when they observe them by electron bombardment. Physicists roll their eyes at such misunderstandings, but I think that physicists themselves often misunderstand the Uncertainty Principle, by confounding Heisenberg’s specifically quantum mechanical relation with a principle of measurement that goes back to Newton. 
What the Uncertainty Principle really means, in practical terms, is that if you construct an apparatus to control one experimental property more tightly, so that the variations in its value between different repetitions of the experiment will be smaller, then past a certain point there will always be some other thing that becomes correspondingly less well controlled, so that its run-to-run variations become wilder. The ‘certain point’ at which this trade-off sets in represents a degree of precise control so high as to be quite unattainable anyway in the macroscopic world. So although the Uncertainty Principle is profound, it is really irrelevant to fields like social science research.

Observing something means letting it act on you.
What even many physicists think the Uncertainty Principle means, however, is something that really is widely relevant: the fact that no measurement can ever be purely passive, but always affects the thing being measured. This principle is both true and important, but it is not specifically quantum mechanical.

Observation is a physical process. A meter can only register the position of an object if there is some interaction that makes the object’s position act on the meter. Newton told us, long before Heisenberg, that this means that any meter is also going to react upon the the thing it measures. Such ‘observer effects’ are apt to be important when large meters measure tiny things; the Heisenberg Principle, however, is not this, but an additional complication in microscopic measurements.

In the early days of quantum mechanics, critics of the new theory tried to argue that the Uncertainty Principle was not self-consistent, by describing hypothetical experiments that would obey the Principle in each individual process, but yet still lead to an indirect violation of the Principle as an end result. These arguments all had subtle flaws, and the most famous flaws involved reaction effects. Thus the only really solid connection between the Uncertainty Principle and measurement reaction is historical.

Newton observes Heisenberg stealing credit for his ideas.
Newtonian reaction in physical measurements is a distinct concept from Heisenberg Uncertainty, but it does at least seem that one must get the former right in order to understand the latter. So perhaps there really is some deep connection between them. Until that connection comes to light, however, anyone who wants to relate observer effects in general to a basic principle of physics should really be citing Newton, not Heisenberg.