Quantum Mechanics

Abstract

Bodies and fields are now behind and for good: the things with which the quantum theories are concerned cannot be pictured as classical entities. They are sui generis entities deserving a special name — say quantons — this being why the quantum theories themselves are sui generis. True, the words ‘particle’ and ‘field’ are retained even in the names of some quantum theories, but they designate concepts differing from the classical ones. In particular, a “particle” is in the new context anything that, whether localizable or not, is endowed with mass: it is the referent of quantum mechanics, the basic theory of matter — to be abbreviated QM. Massless entities like photons and neutrinos (which of course are not particles since they have neither a mass nor a narrow localization) are studied by quantum field theories of QFT for short. But these latter theories also study the “particles” at a deeper level (second quantization). Therefore QFT covers the whole realm of quantons and QM is just the elementary part of the theory and the one applying to microsystems endowed with mass. Yet QM gives rise to most of the interpretation puzzles that have ridden the field for nearly half a century. This, and the lack of space, justify our concentration on it.