In classical mechanics, the object of study is clearly defined, endowed with intrin-sic properties, and, most importantly, independent of the act of observation, and even more so, the observer. In quantum mechanics, on the other hand, the act of measurement induces an entanglement—and even an entanglement characterized by the states (14) and (15)—between the object and the very means of its observa-tion, and, consequently, between the object and the subject who controls the obser-vation process. Thus, the observer can no longer position themselves as a mere external spectator: their interaction with the system under study contributes to defining what is being measured. Finally, this interdependence is also manifested through the emergence of macroscopic quantum superpositions (14), a direct consequence of quantum formalism51, which presents a challenge to the theory.

This situation introduces a fundamental element of subjectivity in our under-standing of what could be called “reality”. In this regard, the subjectivist attitude, which reduces the state vector to a mere catalog of our knowledge of possible outcomes, does not resolve the puzzle. On the contrary, it amplifies it by making it even more difficult to identify an objective reality, independent of the observer. This conceptual challenge is one of the most fascinating and troubling aspects of quantum mechanics.

As for the debate about the incompleteness of quantum mechanics—the EPR par-adox, Bell’s inequalities, and their violation—it has undoubtedly been the source of the most spectacular and established conceptual upheavals in this science52.

It has rendered the traditional view, which explains the intrusion of probabilities in quantum mechanics as a reflection of our ignorance of underlying processes, obsolete, in favor of a conception that recognizes the existence of a fundamental indeterminacy at the very core of this science. Thus, quantum mechanics presents a major challenge: to understand the notion of intrinsic probabilities, contrasting it with that of extrinsic probabilities, which simply refer to our lack of knowledge.

Furthermore, this debate led to the emergence of perhaps the most perplexing concept in the theory: that of entanglement, which refers to a situation where two—or more—particles are in a state that is not simply a product of two—or more—individual states. In such a state, correlations between the particles mani-fest instantaneously over seemingly arbitrary distances53. These correlations, known as EPR correlations, constitute a clear indication of a form of “non-locality”. It is important to clarify the use of the term “form”, because true non-locality would involve the transfer of information at a speed faster than light, which is not the case here. Indeed, EPR correlations respect Einsteinian causality.

To grasp the deeply subversive nature of the concept of entanglement, we men-tion some rather recent speculations54, which suggest that “EPR pairs”55 could be connected by a “wormhole”, also called an “Einstein-Rosen bridge” (abbreviated ER), according to an article published the same year as the one on the EPR paradox. Such a “bridge” would be a structure linking two potentially very distant regions of space-time, thus creating a shortcut within it. This hypothetical connection, known as the ER=EPR conjecture, has not yet been confirmed, but it offers a fas-cinating perspective on the nature of these quantum correlations.

As for the debate concerning the issue of measurement in quantum mechanics, it originates from the concept of superposition of states, probably the most emblem-atic of this theory. This concept reflects the fact that, at the quantum scale, a system can exist in a combination of several eigenstates of an operator associated with a given physical quantity. The measurement problem itself, which, in an objective approach to the state vector, leads to considering macroscopic super-positions of states, has given rise to various original, sometimes even extravagant, interpretations of quantum mechanics. To name just a few56: