Bell’s Theorem: The Naive View of an Experimentalist

  • Alain Aspect


It is a very emotional experience to contribute to this book commemorating of John Bell. I first met John in 1975, a few months after reading his famous paper [1]. I had been so strongly impressed by this paper, that I had immediately decided to do my “thèse d’état” — which at that time, in France, could be a really long work — on this fascinating problem. I definitely wanted to carry out an experiment “in which the settings are changed during the flight of the particles”, as suggested in the paper, and I had convinced a young professor at the Institut d’Optique, Christian Imbert, to support my project and to act as my thesis advisor. He had advised me to go first to Geneva, to discuss my proposal with John Bell. I got an appointment without delay, and I showed up in John’s office at CERN, quite nervous. While I explained my planned experiment, he listened silently. Eventually, I stopped talking, and the first question came: “Have you a permanent position?” After my positive answer, he started talking of physics, and he definitely encouraged me, making it clear that he would consider the implementation of variable analysers a fundamental improvement. Remembering this first question reminds me both of his celebrated sense of humour and of the general atmosphere at that time about raising questions on the foundations of quantum mechanics. Quite frequently there was open hostility, and in the best case, irony: “quantum mechanics has been vindicated by such a large amount of work by the smartest theorists and experimentalists; how can you hope to find anything with such a simple scheme, in optics, a science of the 19th century?” In addition to starting the experiment, I had then to develop a line of argument to try to convince the physicists I met (and among them some had to give their opinion about funding my project). After some not so successful attempts at quite sophisticated pleas, I eventually found out that it was much more efficient to explain the very simple and naive way in which I had understood Bell’s theorem. And to my great surprise, that simple presentation was very convincing even with the most theoretically inclined interlocutors. I was lucky enough to be able to present it in front of John Bell himself, and he apparently appreciated it. I am therefore going to explain now how I understood Bell’s theorem twenty five years ago, and I hope to be able to communicate the shock I received, which was so strong that I spent eight years of my life working on this problem.


Quantum Mechanic Bell Inequality Coincidence Rate Polarization Correlation Atomic Cascade 
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  • Alain Aspect

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