Quantum mechanics, an introduction

Modern quantum mechanics was born in 1925, when the Schrodinger invented wave mechanics and the German physicist Heisenberg developed matrix mechanics.

The begining of quantum mechanics can be traced way back in 1900, when German physicist Max Planck introduced the idea that energy is quantized, in order to derive a formula for the observed frequency dependence of the energy emitted by a black body. In 1905, at most at the same time Einstein introduced Special Relativity, he also submitted a paper to explain the photoelectric effect by postulating that light energy comes in quanta called photons. The idea that each photon had to consist of energy in terms of quanta was a remarkable achievement as it effectively removed the possibility of black body radiation attaining infinite energy if it were to be explained in terms of wave forms only.

In his paper of July 1913 titled On the Constitution of Atoms and Molecules, Bohr explained the spectral lines of the hydrogen atom, again by using quantization. In 1924, the French physicist Louis de Broglie put forward his theory of matter waves by stating that particles can exhibit wave characteristics and vice versa or known as duality of particle and waves.

The uncertainty princple was formulated by Heisenberg in 1927, and the Copenhagen interpretation took shape at about the same time. Starting around 1927, the English physicist Paul Dirac began the process of unifying quantum mechanics with special relativity developed solely by Einsten by proposing the Dirac equation for the electorn. Dirac also pioneered the use of operator theory, including the influential bra-ket notation which derived its form from the matrix mechanics as described in his famous 1930 textbook.

During the same period, Hungarian polymath von Neumann formulated the rigorous mathematical basis for quantum mechanics as the theory of linear operators on Hilbert spaces, as described in his likewise famous 1932 textbook. These, like many other works from the founding period still stand, and remain widely used.

Beginning in 1927, attempts were made to apply quantum mechanics to fields rather than single particles, resulting in what are known as quantum field theories. The pioneer of this area included, Wolfang Pauli. Paul Dirac, Victor Frederick Weisskopf and Pascual Jordan.