Real or not real that is the question...

Abstract

My discussions with John Bell about reality in quantum mechanics are recollected. I would like to introduce the reader to Bell’s vision of reality which was for him a natural position for a scientist. Bell had a strong aversion against “quantum jumps” and insisted to be clear in phrasing quantum mechanics, his “words to be forbidden” proclaimed with seriousness and wit – both typical Bell characteristics – became legendary. I will summarize the Bell-type experiments and what Nature responded, and discuss the implications for the physical quantities considered, the real entities and the nonlocality concept due to Bell’s work. Subsequently, I also explain a quite different view of the meaning of a quantum state, this is the information theoretic approach, focusing on the work of Brukner and Zeilinger. Finally, I would like to broaden and contrast the reality discussion with the concept of “virtuality,” with the meaning of virtual particle occurring in quantum field theory. With some of my own thoughts I will conclude the paper which is composed more as a historical article than as a philosophical one.