energy, into a disorganized form of energy, such as thermal energy (or heat), dur-ing a thermodynamic transformation. The growth of entropy also characterizes the quasi-irreversibility of these transformations, since the system can almost never return to its initial, ordered state. Thus, statistical mechanics not only explains a multitude of physical phenomena but also sheds light on the asymmetry between the past and the future, in other words, the existence of an arrow of time.

At the beginning of the 20th century, two theories would form the basis of con-temporary physics: Einstein’s theory of relativity, improperly called special rel-ativity2, and quantum mechanics, through the works of Planck, Einstein, Bohr, Heisenberg, Schrödinger, de Broglie, Dirac, Pauli, Born, von Neumann, and oth-ers. More than simple theories, relativity and quantum mechanics3 are considered “framework theories” because they each provide a global formal structure that groups general concepts and principles that more specific theories must respect. Those that do not strictly adhere to these principles are considered valid approx-imations only within appropriate limits.

Einstein’s relativity, by associating the concept of the relativity of motion4 with the invariance of the speed of light5, merges the notions of space and time. These notions, which were absolute in Galilean or Newtonian physics, then become relative. It brings forth the notion of space-time, which becomes the new frame-work within which all motion must be considered. It also imposes a principle of invariance, called Einsteinian relativistic invariance6, which any theory describing phenomena involving speeds close to that of light must obey, as is the case with Maxwell’s electromagnetism.

If the theory of relativity modifies the spatio-temporal framework in which matter resides and evolves, quantum mechanics, whose objective is to describe the micro-scopic components of this matter, overturns our very conception of it. It presents, in particular, these components as entities that combine, although not simulta-neously, both particle-like and wave-like characteristics, two aspects incompatible within Newtonian physics. It makes the notion of trajectory obsolete and inval-idates the ideas of determinism and locality, which are foundational to classical physics. However, this theory, which turns out to be the most precise, both in its original form and in its relativistic version7, also provides the general framework into which any theory of the microscopic world must integrate.

Shortly after the advent of the theory of special relativity, Einstein developed the theory of general relativity, which aims to provide a relativistic framework for gravitation. In this framework, gravitation loses its status as a force, as defined in Newtonian physics, to become a purely geometric notion: the curvature of space-time, generated by the presence of matter and energy.

These revolutions continued in the second half of the 20th century with the discovery of two fundamental interactions: the “strong” interaction, which acts between quarks as well as between protons and neutrons (called “hadrons”), which they constitute8. This force ensures the cohesion of the atomic nucleus, in the same way that the electromagnetic interaction, which acts between the nucleus and the electrons, ensures the cohesion of atoms and molecules. The other inter-action, called “weak”,9 acts on quarks and leptons, such as electrons and neutrinos. It governs various radioactive processes and allows, in particular, the conversion of different types of quarks, and thus hadrons, into one another. This phenomenon is crucial, especially in the context of nuclear fusion that occurs in the hearts of stars.

With electromagnetism and gravity, the strong and weak interactions constitute the four fundamental interactions known to date. The latter two have formulations that inherently respect both the requirements of Einsteinian relativistic invariance