# Quantum Mechanics - A Historical Survey

• 1879 Josef Stefan published that the energy density emitted by a black body is proportional to the fourth power of temperature. This was followed by a theoretical description of Ludwig Boltzmann. Today the ralation $\omega = \sigma T^4$ is known as Stefan-Boltzmann law
• 1896 13 years later W. Wien found a relation including an additional factor, namely an exponential dependent on temperature and the wave length, Wien's law of radiation
• 1900 Max Plank introduced the constant $h$ to quantise the allowed energy states in black body radiation, which lead to Plank's law, a variation of W. Wien's formula. Plank's law and the need for quantisation was a very controversal topic at this time and continued to be for many years. This is also the reason why others tried to explain the experimental data without quantisation.
• 1905 Jeans corrected the work earlier published by Rayleigh (1900). They tried to explain black body radiation by classical arguments assigning the energy of a classical harmonic oscillator to each mode.
• 1905 Albert Einstein proposed the "Lichtquantenhypothese". He observed that the kinetic energy of an emitted electron depends only on the frequency and can't be explained by classical arguments. In 1921 he received the Nobel Prize for the explanation of this photoelectric effect.
• 1913 Niels Bohr adopts the idea of quantisation and explains the discrete energy spectra in the hydrogen atom (Bohr model).
• 1922 Arthur Compton finds that X-rays can indeed act like dircrete particles and that electrons get scattered by the X-rays (Compton Effect).
• 1924 In his dissertation de Broglie postulates that every particle can be assigned a wave length, the so called de Broglie wave length. He received the Nobel Prize for his work in 1929.
• 1926 Clinton Joseph Davisson and Lester Halbert Germer found the typical interference pattern behind a cristalline metal foil, caused by electrons behaving like waves. The postulate of de Broglie was hence verified.

• To readers interested in the history of Quantum Mechanics I recommend the amusing and insightful book "THIRTY YEARS THAT SHOOK PHYSICS" by George Gamow on the profound change physics has undergone in the early 20th century. He, himself a physicist known for his work on e.g. radio active decay (Gamow factor), knew many influencial physicists personally and enriches the stories by many anecdotes of his own experience on e.g. Niels Bohr or Wolfgang Pauli.
• A short film showing many people that contributed to the development of Quantum Mechanics at the Solvay Physics Conference in 1927, can be found here.
• An extraordinary lecture series was given by Paul Dirac in 1975 (Christchurch - New Zealand) on the quantum mechanical devlopments (video of bad quality, but original).
• David Peat interviews with Werner Heisenberg [1] and other interviews with Heisenberg [2] (German)