measurement device is replaced by a cat, which is also macroscopic. A quantum system, in a superposition of two states ∣v⟩ and ∣m⟩, is coupled to a mechanism that, depending on the state of the system, either triggers or does not trigger the release of poison from a vial. This then determines the fate of the cat: whether it lives or dies. Formally, according to the unitary evolution postulated by quantum mechanics, the system is described by the following correlated state, analogous to (14):

|Φc⟩ = α|v⟩ ⊗ |alive cat⟩ + β|m⟩ ⊗ |dead cat⟩(15)

which corresponds to a superposition of the states ∣alive cat⟩ and ∣dead cat⟩48. Naturally, in reality, this superposition makes no sense; we observe a well-defined state of the cat49: ∣alive cat⟩ or ∣dead cat⟩. In the framework of the measurement process we described earlier, the well-defined state of the cat corresponds to a well-defined state |Mk⟩ of the measuring device:

.(16)

Here, the practical necessity of the corresponding postulate (6) is clearly evident. Moreover, within the context of the orthodox interpretation, this does not raise any conceptual difficulty, as the state vector is considered a condensation of the information about the system. Thus, the transition from one mode of state vector evolution to another simply represents a change in our knowledge of the system at the moment a measurement is made50. On the other hand, some defend an approach where the state vector is viewed as an intrinsic and objective description of the physical reality of the system. In such a context, the reduction of the state vector at the time of measurement requires a true explanation, leading to the varied interpretations, which lean more or less to one side or the other (see below). Einstein, for his part, believed in the existence of an objective and deterministic reality. His famous maxim, “God does not play dice”, reflects his rejection of fundamental chance: according to Einstein, nature cannot be governed by gen-uine indeterminism. In other words, for him, the result of a measurement must be causally determined. This conviction led him to postulate the existence of hidden variables that would restore a causal description of physical phenomena. However, as mentioned, this hypothesis was invalidated by Bell’s inequalities and their violation in what are commonly referred to as the “Aspect experiments”.

Conclusion: Multiple Interpretations

Answering the question of the nature of the revolution caused by the advent of quantum mechanics can only be done partially and temporarily. Indeed, in nearly a hundred years, quantum mechanics has developed spectacularly, both in eluci-dating complex physical phenomena—such as superconductivity, Bose-Einstein condensates, quantum magnetism, spintronics, the Hall effect, etc.—and in the development of new technologies like materials science, cryptography, metrology, and quantum information. This intense activity demonstrates the impressive and undeniable effectiveness of quantum mechanics, not only as a field of fundamen-tal research but also as a driver of concrete technological innovations. If there is a revolution, it is already this one.

On the other hand, when it comes to the conceptual revolution brought about by quantum mechanics, the advances have not been as spectacular as its practical applications, and it appears that the core of the problem remains out of reach. It is precisely in this elusive nature of its profound meaning that quantum mechanics distinguishes itself from other areas of physics. This uniqueness primarily rests on the dissolution of the traditional boundary between the subject and the object, a distinction that quantum mechanics radically questions.