Hardy’s Test of Local Realism

There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

Neils Bohr

This post summarizes my bachelor’s thesis at Reed College, which can be read in its entirety here

If a tree falls in the woods, does it make a sound or not? We intuitively assume that it does. We assume that a basketball continues to be orange when we look away from it. However, the beauty of this question is that we in principle cannot know. We can argue, but it is by definition not testable, and therefore we can never have a definitive scientific resolution of the question.

As quantum physics came of age in the early 20th century, many were uncomfortable about the physical implications of the mathematical formalism. In particular, our observation of a system was an intrusive process which altered the state of the system in a fundamental way. When we were not observing an electron, it simply did not have a well-defined position. But was this really true? As with the tree in the woods, we cannot know what happens when we are not looking.

In 1964, a young Irish physicist named John Bell laid the theoretical groundwork for an experiment which would, amazingly, be able to decisively address this fundamental question, which physicists call the principle of local realism. The test was based on a statistical analysis of pairs of entangled quantum particles. It would be another 20 years before such an experiment could actually be performed, and such experiments are still a highly active topic of research in the quantum physics community.


My thesis was an experiment based on the work of Bell. The particular theoretical formulation was based on the work of Lucien Hardy. The basic idea of this experiment is that we shine a laser on a particular crystal, and some portion of the incident photons will decay into an entangled pair. We can then measure the statistics of these pairs, allowing us to discredit local realism with high confidence.

This project involved a lot of lab work, and also a lot of interesting theory. I was able to write a chapter devoted to the history and philosophy of the theory of local realism, which is fascinating to me. Of course, none of this would have been possible without the patient guidance of my advisor Dr. John Essick. I’m very grateful that I got the opportunity to execute such a complicated experiment under my own steam – it certainly was a big learning experience.