The discovery of the quantum of action in 1900 heralded a radically new departure in physics; it led in 1926 to the theory of quantum mechanics which was widely acclaimed as a revolution. Quantum theory has created a fundamental change in our ideas as relativity theory had done before. But while relativity theory is essentially classical in its conceptions--it is in fact the culmination of classical physics--quantum mechanics is founded on some new principles. The quantum laws are very different in character from the laws we find in classical theory. In particular, quantum mechanics is a strictly statistical type of theory: from this arises the different conception of causality that marks the new outlook of modern physics.
The novel feature of this theory is that energy is regarded as existing in discrete amounts, i.e. quanta. Any given amount of energy may be thought of as an integral multiple of an isolated unit while, in classical physics, energy is something which varies in a continuous manner. And, from the logical viewpoint, the strange and 'abstract', not to say abstruse, nature of the new concepts made it necessary to be more critical about the construction of our theories; it resulted, ultimately, in a better understanding of all physics.
Planck introduced the idea of a discrete quantum of energy in order to overcome the failure of the classical laws of radiation. Einstein, in 1905, extended this idea to the description of the phenomena of light; and he formulated his famous photon theory which, in a different form, re-created the corpuscular theory of light as held by Newton. Finally, basing his work upon the experimental research of Rutherford into the structure of the atom, Bohr in 1913 applied the conception of energy quanta to the emission and absorption of light by atoms. He invented a theory about the constitution of atoms which, immediately, led to great success. In this way a radically new type of theory was constructed in order to describe atomic processes on the microscopic scale.
As a result of Einstein's hypothesis which ascribes a kind of