– The “many-worlds” interpretation, proposed by Everett, posits that during a measurement, all results are realized but in “parallel worlds”. The advantage of this interpretation is that it avoids the question of the articulation between two types of evolution, as there is no need to reduce the state vector. However, its... uneconomical nature makes it difficult to pass the test of Occam’s razor: “Entia non sunt multiplicanda sine necessitate”.57

– The objective collapse interpretation proposed by Ghirardi, Rimini, and Weber (GRW), assumes the existence of an internal process that causes the state vector to collapse. This is a hidden-variable (non-local) theory that modifies Schrödinger’s equation ad hoc by adding extra terms.

– The Bohm-de Broglie interpretation, or pilot-wave theory, postulates that the particle—or the collection of particles—is guided by a wave and follows a clas-sical trajectory. This is a deterministic theory consistent with all predictions of quantum mechanics. It resolves, like the GRW theory, the measurement prob-lem, but more naturally, as it does not require modifications to the formalism. However, it proves to be, as it should be, non-local and incompatible with rela-tivistic invariance.

– The relational interpretation, proposed by Carlo Rovelli, maintains that the state vector does not describe an objective property of the system, but rather the relationships between systems, especially between the system and the (or the) observer(s). This interpretation is heavily inspired by Einstein’s theory of relativ-ity, where most quantities are relative to the observer. The measurement problem is “resolved” by the idea that it is no longer associated with an objective reduction of the state vector, but with the establishment of specific relationships between observers, leading to different but compatible interpretations of reality. There are, moreover, several very interesting interpretations in which such an ontological shift occurs, during which the central concept of quantum mechanics shifts from the system to the correlations between systems, thus recognizing the idea, present in many philosophers, of the impossibility of accessing the thing “in itself”.58

– The interpretation via decoherence. Decoherence is a concept introduced by H. Dieter Zeh in 1970 and later developed, notably by Wojciech H. Zurek. It is undoubtedly one of the most fruitful approaches to understanding the behaviors of macroscopic systems and for addressing the question of measurement. Indeed, while relying on quantum formalism as it is, this approach manages to justify the emergence of classical behavior, both for macroscopic systems and for quantum systems interacting with their environment, particularly during measurement. The fundamental idea is based on the fact that, in both cases, the system is not isolated: it interacts continuously with its environment59. This interaction dis-turbs—in fact, delocalizes in the environment—the phase relationships between superposed macroscopic states, thus compromising the coherence and stabil-ity of such a superposition of states60. Decoherence helps partially resolve the measurement problem because it shows how, in practice, the effects of quantum superpositions become unobservable at the macroscopic scale. However, it does not solve the problem of state vector reduction, in the sense that it never leads to the selection, for a given observable, of a value from among others.

We observe that the attempts to interpret quantum mechanics are not only varied but also continuously proliferating, which shows that the subject remains deeply open and far from having revealed all its secrets.