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quantum mechanics:
The branch of physics that deals with the structure of the atom and the motion of atomic particles such as electrons and nuclei within atoms. It is also concerned with the interaction between matter and radiation. Because quantum mechanics treats physical events that we cannot directly observe, its concepts are unknown in everyday experience. For example, quantum mechanics describes certain kinds of energy as having the characteristics of both waves and particles. The three scientists most closely associated with the fundamental ideas of quantum mechanics are Max Planck, Erwin Schrödinger, and Werner Heisenberg.