Editing Relativistic wave equations (section)

=== Early 1920s: Classical and quantum mechanics ===

The failure of [[classical mechanics]] applied to [[molecule|molecular]], [[atom]]ic, and [[Atomic nucleus|nuclear]] systems and smaller induced the need for a new mechanics: ''[[quantum mechanics]]''. The mathematical formulation was led by [[Louis de Broglie|De Broglie]], [[Niels Bohr|Bohr]], [[Erwin Schrödinger|Schrödinger]], [[Wolfgang Pauli|Pauli]], and [[Werner Heisenberg|Heisenberg]], and others, around the mid-1920s, and at that time was analogous to that of classical mechanics. The Schrödinger equation and the [[Heisenberg picture]] resemble the classical [[equations of motion]] in the limit of large [[quantum number]]s and as the reduced [[Planck constant]] {{math|''ħ''}}, the quantum of [[action (physics)|action]], tends to zero. This is the [[correspondence principle]]. At this point, [[special relativity]] was not fully combined with quantum mechanics, so the Schrödinger and Heisenberg formulations, as originally proposed, could not be used in situations where the particles travel near the [[speed of light]], or when the number of each type of particle changes (this happens in real [[fundamental interaction|particle interaction]]s; the numerous forms of [[particle decay]]s, [[annihilation]], [[matter creation]], [[pair production]], and so on).