Dominique Mouhanna

Knowledge Put to the Test by Quantum Mechanics

“I am a quantum engineer, but on Sundays, I have principles.”

— John Stewart Bell —

Introduction: Physics, a Series of Revolutions

One of the unique aspects of physics lies in the fact that it is the place, not only of technical innovations deeply transforming our material life but also of con-ceptual revolutions overturning our way of apprehending reality. Among these revolutions, the one occurring in the 16th century, known as the “Copernican” revolution, does nothing less than move the “center of the world” from the Earth to the Sun, marking the shift from geocentrism to heliocentrism. The subsequent revolutions, Galilean and Newtonian, in the 17th and 18th centuries, pave the way for the idea of universal physical laws. It is understood, on the one hand, that all objects fall in a vacuum in the same manner, that is, with the same acceleration; on the other hand, that the same law—in this case, the law of universal gravitation—simultaneously governs the fall of bodies on Earth and the movement of celestial bodies in the sky. This rupture marks the abandonment of the particularism underlying geocentrism, which had prevailed since Antiquity. With Newtonian physics, and under the influence of thinkers like Descartes, a mechanistic and deterministic view of the world also emerges, partially relegating the theological vision of the universe.

In the 19th century, a process begins that will continuously obsess physicists: the unification of the phenomena surrounding them. Maxwell, building on the work of Gauss, Faraday, and Ampere, realizes that two apparently disparate categories of phenomena—electrical and magnetic phenomena—actually belong to a single category: electromagnetism. This condenses into a set of four equations, known as Maxwell’s equations. A new paradigm then emerges: that of the field, a dynamic quantity exhibiting both spatial and temporal dependence, like the electric field E⃗(x⃗,t) and magnetic field B⃗(x⃗,t). This mathematical concept allows for the formal description of a disturbance propagating in a physical medium1— in other words, a wave—and complements the concept of variable x⃗(t), used to specify the position of a point or a moving material body.

Simultaneously, from the late 18th century to the end of the following century, thermodynamics develops under the impetus of Watt, Thompson, Carnot, Clausius, and Kelvin, etymologically the “study of heat and work”. Subsequently, thanks to Maxwell, Boltzmann, and Gibbs, statistical mechanics comes into being and establishes its microscopic foundations. These theories also lead to the unifi-cation of two concepts previously considered distinct: heat and work, allowing the formulation of one of the most fundamental principles in physics: the conserva-tion of energy in an isolated system. A central notion is also introduced: entropy, which etymologically means “change within”. Entropy measures the disorder within a system. Its quasi-systematic growth translates the almost inevitable ten-dency of disorder to increase in an isolated system. This phenomenon illustrates the conversion of an organized form of energy, such as mechanical or chemical