In 1964, John Stewart Bell published an important result that was later called Bell's theorem [1]. It states that certain predictions of quantum mechanics cannot be accounted for by any local realistic theory. Bell's theorem has been called “the most profound discovery of science” [2]. By introducing a notion of quantum nonlocality, it not only transforms the study of the foundations of quantum mechanics, but also paves the way to many quantum technologies that have been developed in the last decades. It can be expected that Bell's work will play a more important role in the physics of the future. Admittedly, there are still controversies on the underlying assumptions and deep implications of Bell's theorem. This also poses a challenge to us in understanding quantum theory, as well as the physical world at the most fundamental level.

This book is an anthology celebrating the 50th anniversary of Bell's theorem. It contains 26 original essays written by physicists and philosophers of physics, reflecting the latest thoughts of leading experts on the subject. The content includes recollections of John Bell, an introduction to Bell's nonlocality theorem, a review of its experimental tests, analyses of its meaning and implications, investigations of the nature of quantum nonlocality, discussions of possible ways to avoid nonlocality, and last but not least, analyses of various nonlocal realistic theories. The book is accessible to graduate students in physics. It will be of value to students and researchers with an interest in the philosophy of physics and especially to physicists and philosophers working on the foundations of quantum mechanics.

This book is arranged in four parts. The first part introduces John Bell, the great Northern Irish physicist, and it also contains a few physicists’ treasured recollections of him. In Chapter 1, Andrew Whitaker introduces the Irish tradition of physics, Bell's Belfast background, and his studies of physics at Queen's University Belfast. Whitaker argues that there may exist a connection between Bell's work and the Irish tradition, especially concerning his views on the apparent incompatibility between quantum nonlocality and special relativity.