Published as: Aerts, D. and Reignier, J., 1991, "The spin of a quantum entity and problems
of non-locality", in the Proceedings of the Symposium on the Foundations of Modern
Physics 1990, Joensuu, Finland, eds. Lathi, P., et al., World Scientific Publishing
Company, Singapore, 9 - 19.
THE SPIN OF A QUANTUM-ENTITY AND PROBLEMS
D. AERTS and J. REIGNIER
Theoretische Natuurkunde, Vrije Universiteit Brussel, Pleinlaan 2,
1 . ABSTRACT
We introduce a possible definition for the concept of non-locality in the quantum
world, which seems to us a minimal operational definition, taking into account the results of
actually performed experiments and reasonings about possible 'gedanken' experiments.
The definition is the following: An entity is "non local" if it is possible to prepare it in a state
such that it can be influenced from macroscopically separated regions of space by
(macroscopically) local apparatus acting only in one (or several) of these separated regions
at one time. We discuss two examples of spin superposition experiments which clearly
show that quantum entities are non-local. In particular, we show that the familiar Stern-
Gerlach experiment allows a nice illustration of this non-locality.