It has been nearly a century since Max Planck proposed that electromagnetic radiation comes in tidy bundles of energy called quanta. Building on this seemingly fragile supposition, scientists erected what is by far the most successful theory in the history. In addition to yielding theories for all the fundamental forces of nature (except gravity), quantum mechanics has accounted for such disparate phenomena as the shining of stars and the order of the periodic table. From it have sprung technologies ranging from nuclear reactors to lasers.

Quantum mechanics is a theory which tries to describe, and predict the behavior of physical systems. The theory was originally designed to make predictions about the behavior of sub-microscopic systems, like atoms and electrons, but since even large-scale systems are composed of atoms, quantum mechanics necessarily comprises a fundamental physical theory--a theory which attempts (at least in principle) to offer as complete an account as can be had of the nature and behavior

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