The Cat and the Fox

The action at a distance discussed in the paper by A. Einstein[aut]Einstein, Albert, B. Podolsky[aut]Podolsky, Boris Yakovlevich and N. Rosen[aut]Rosen, Nathan in 1935 generated a cascade of profound theoretical reconsiderations that accompanied the foundational issues of quantum mechanics throughout the twentieth century, even though it remained often somewhat “hidden” with respect to mainstream physics. In this chapter, we will start by introducing the fundamental concept of entanglement, as proposed by E. Schrödinger[aut]Schrödinger, Erwin in 1935. Subsequently, we will provide an overview of the key aspects of entanglement, both from a theoretical and experimental standpoint, spanning the period from 1935 to 2022. We will explore the repercussions of the Second World War on the field of physics and some aspects of nuclear physics, followed by a discussion on the Bohmian approach to quantum mechanics. We will face the question of the so-called “hidden variables”, the Bell variables”, the Bell[aut]Bell, John inequalities and the experimental validations of the non-local structure of quantum mechanics that led to the Nobel Prizes in 2022, awarded to A. Aspect[aut]Aspect, Alain, J. Clauser[aut]Clauser, John Francis and A. Zeilinger[aut]Zeilinger, Anthon.