In many ways, quantum mechanics is a surprising theory. It is known to be nonintuitive, and leads to representations of physical phenomena that are very different from what our daily experience could suggest. But it is also very surprising because it creates a big contrast between its triumphs and difficulties.

On the one hand, among all theories, quantum mechanics is probably one of the most successful achievements of science. It was initially invented in the context of atomic physics, but it has now expanded into many domains of physics, giving access to an enormous number of results in optics, solid-state physics, astrophysics, etc. It has actually now become a general method, a frame in which many theories can be developed, for instance to understand the properties of fluids and solids, fields, elementary particles, and leading to a unification of interactions in physics. Its range extends much further than the initial objectives of its inventors and, what is remarkable, this turned out to be possible without changing the general principles of the theory. The applications of quantum mechanics are everywhere in our twenty-first century environment, with all sorts of devices that would have been unthinkable 50 years ago.

On the other hand, conceptually this theory remains relatively fragile because of its delicate interpretation – fortunately, this fragility has little consequence for its efficiency. The reason why difficulties persist is certainly not that physicists have tried to ignore them or put them under the rug! Actually, a large number of interpretations have been proposed over the decades, involving various methods and mathematical techniques.