Quantum mechanics studies the dynamics and interactions of entities at very short distance scales, typically less than atomic diameters. These entities, called “particles” for historical reasons, behave according to rules that differ widely from the mechanical laws that constrain the motions and interactions of classical particles.
Historically, the origin of quantum mechanics is usually traced to the discovery that the energy of electromagnetic radiation at a given frequency cannot be subdivided into fractions smaller than certain discrete units, since then called “quanta.” This finding, issuing from the work of Max Planck in the early days of the twentieth century, was followed by other surprising empirical revelations that were markedly at odds with the predictions of classical physics when dealing with processes at the atomic and subatomic scales. In the initial decades of the century the combined efforts of some of the greatest scientists of all time (Bohr, Heisenberg, Einstein, Schrödinger, Born, Pauli, and many others) resulted in the creation of quantum mechanics. This was a novel theoretical framework, a complex generalization of classical physics capable of accounting for previous anomalous findings and of predicting unforeseen new phenomena with unprecedented accuracy.
Quantum mechanics is a foundational discipline: its concepts and methods became instrumental in virtually all other branches of physics in the twentieth century, from cosmology to condensed matter physics.