The timeline of quantum mechanics is a list of key events in the history of quantum mechanics, quantum field theories and quantum chemistry.
The initiation of quantum science occurred in 1900, originating from the problem of the oscillator beginning during the mid-19th century.
19th century
1801 – Thomas Young establishes the wave nature of light with his double-slit experiment. 1859 – Gustav Kirchhoff introduces the concept of a blackbody and proves that its emission spectrum depends only on its temperature. 1860–1900 – Ludwig Eduard Boltzmann, James Clerk Maxwell and others develop the theory of statistical mechanics. Boltzmann argues that entropy is a measure of disorder. 1877 – Boltzmann suggests that the energy levels of a physical system could be discrete based on statistical mechanics and mathematical arguments; also produces the first circle diagram representation, or atomic model of a molecule (such as an iodine gas molecule) in terms of the overlapping terms α and β, later (in 1928) called molecular orbitals, of the constituting atoms. 1885 – Johann Jakob Balmer discovers a numerical relationship between visible spectral lines of hydrogen, the Balmer series. 1887 – Heinrich Hertz discovers the photoelectric effect, shown by Einstein in 1905 to involve quanta of light. 1888 – Hertz demonstrates experimentally that electromagnetic waves exist, as predicted by Maxwell. 1888 – Johannes Rydberg modifies the Balmer formula to include all spectral series of lines for the hydrogen atom, producing the Rydberg formula that is employed later by Niels Bohr and others to verify Bohr's first quantum model of the atom. 1895 – Wilhelm Conrad Röntgen discovers X-rays in experiments with electron beams in plasma. 1896 – Antoine Henri Becquerel accidentally discovers radioactivity while investigating the work of Wilhelm Conrad Röntgen; he finds that uranium salts emit radiation that resembled Röntgen's X-rays in their penetrating power. In one experiment, Becquerel wraps a sample of a phosphorescent substance, potassium uranyl sulfate, in photographic plates surrounded by very thick black paper in preparation for an experiment with bright sunlight; then, to his surprise, the photographic plates are already exposed before the experiment starts, showing a projected image of his sample. 1896–1897 – Pieter Zeeman first observes the Zeeman splitting effect by applying a magnetic field to light sources. 1896–1897 – Marie Curie (née Skłodowska, Becquerel's doctoral student) investigates uranium salt samples using a very sensitive electrometer device that was invented 15 years before by her husband and his brother Jacques Curie to measure electrical charge. She discovers that rays emitted by the uranium salt samples make the surrounding air electrically conductive, and measures the emitted rays' intensity. In April 1898, through a systematic search of substances, she finds that thorium compounds, like those of uranium, emitted "Becquerel rays", thus preceding the work of Frederick Soddy and Ernest Rutherford on the nuclear decay of thorium to radium by three years.Marie Curie and the Science of Radioactivity: Research Breakthroughs (1897–1904) . Aip.org. Retrieved on 2012-05-17. 1897: Ivan Borgman demonstrates that X-rays and radioactive materials induce thermoluminescence. J. J. Thomson's experimentation with cathode rays led him to suggest a fundamental unit more than a 1,000 times smaller than an atom, based on the high charge-to-mass ratio. He called the particle a "corpuscle", but later scientists preferred the term electron. Joseph Larmor explained the splitting of the spectral lines in a magnetic field by the oscillation of electrons.Histories of the Electron: The Birth of Microphysics edited by Jed Z. Buchwald, Andrew Warwick Larmor, created the first solar system model of the atom in 1897. He also postulated the proton, calling it a "positive electron". He said the destruction of this type of atom making up matter "is an occurrence of infinitely small probability". Quotes from one of Larmor's voluminous work include: "while atoms of matter are in whole or in part aggregations of electrons in stable orbital motion. In particular, this scheme provides a consistent foundation for the electrodynamic laws, and agrees with the actual relations between radiation and moving matter." "A formula for optical dispersion was obtained in § 11 of the second part of this memoir, on the simple hypothesis that the electric polarization of the molecules vibrated as a whole in unison with the electric field of the radiation." "... that of the transmission of radiation across a medium permeated by molecules, each consisting of a system of electrons in steady orbital motion, and each capable of free oscillations about the steady state of motion with definite free periods analogous to those of the planetary inequalities of the Solar System" "'A' will be a positive electron in the medium, and 'B' will be the complementary negative one...We shall thus have created two permanent conjugate electrons 'A' and 'B'; each of them can be moved about through the medium, but they will both persist until they are destroyed by an extraneous process the reverse of that by which they are formed." 1899–1903 – Ernest Rutherford investigates radioactivity. He coins the terms alpha and beta rays in 1899 to describe the two distinct types of radiation emitted by thorium and uranium salts. Rutherford is joined at McGill University in 1900 by Frederick Soddy and together they discover nuclear transmutation when they find in 1902 that radioactive thorium is converting itself into radium through a process of nuclear decay and a gas (later found to be ); they report their interpretation of radioactivity in 1903. Rutherford becomes known as the "father of nuclear physics" with his nuclear atom model of 1911.
20th century
1900–1909
1900 – To explain black-body radiation (1862), Max Planck suggests that electromagnetic energy could only be emitted in quantized form, i.e. the energy could only be a multiple of an elementary unit , where is the Planck constant and (the Greek letter nu) is the frequency of the radiation. 1902 – To explain the octet rule (1893), Gilbert N. Lewis develops the "cubical atom" theory in which electrons in the form of dots are positioned at the corner of a cube. Predicts that single, double, or triple "bonds" result when two atoms are held together by multiple pairs of electrons (one pair for each bond) located between the two atoms. 1903 – Antoine Becquerel, Pierre Curie and Marie Curie share the 1903 Nobel Prize in Physics for their work on spontaneous radioactivity. 1904 – Richard Abegg notes the pattern that the numerical difference between the maximum positive valence, such as +6 for H2SO4, and the maximum negative valence, such as −2 for H2S, of an element tends to be eight (Abegg's rule). 1905 : Albert Einstein explains the photoelectric effect (reported in 1887 by Heinrich Hertz), i.e. that shining light on certain materials can function to eject electrons from the material. He postulates, as based on Planck's quantum hypothesis (1900), that light itself consists of individual quantum particles (photons). Einstein explains the effects of Brownian motion as caused by the kinetic energy (i.e., movement) of atoms, which was subsequently, experimentally verified by Jean Baptiste Perrin, thereby settling the century-long dispute about the validity of John Dalton's atomic theory. Einstein publishes his special theory of relativity Einstein theoretically derives the equivalence of matter and energy. 1907 to 1917 – Ernest Rutherford: To test his planetary model of 1904, later known as the Rutherford model, he sent a beam of positively charged alpha particles onto a gold foil and noticed that some bounced back, thus showing that an atom has a small-sized positively charged atomic nucleus at its center. However, he received in 1908 the Nobel Prize in Chemistry "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances",The Nobel Prize in Chemistry 1908: Ernest Rutherford. nobelprize.org which followed on the work of Marie Curie, not for his planetary model of the atom; he is also widely credited with first "splitting the atom" in 1917. In 1911 Ernest Rutherford explained the Geiger–Marsden experiment by invoking a nuclear atom model and derived the Rutherford cross section. 1909 – Geoffrey Ingram Taylor demonstrates that interference patterns of light were generated even when the light energy introduced consisted of only one photon. This discovery of the wave–particle duality of matter and energy is fundamental to the later development of quantum field theory. 1909 and 1916 – Einstein shows that, if Planck's law of black-body radiation is accepted, the energy quanta must also carry momentum , making them full-fledged particles.
1910–1919
1911: Lise Meitner and Otto Hahn perform an experiment that shows that the energies of electrons emitted by beta decay had a continuous rather than discrete spectrum. This is in apparent contradiction to the law of conservation of energy, as it appeared that energy was lost in the beta decay process. A second problem is that the spin of the nitrogen-14 atom was 1, in contradiction to the Rutherford prediction of ½. These anomalies are later explained by the discoveries of the neutrino and the neutron. Ștefan Procopiu performs experiments in which he determines the correct value of electron's magnetic dipole moment, (in 1913 he is also able to calculate a theoretical value of the Bohr magneton based on Planck's quantum theory). John William Nicholson is noted as the first to create an atomic model that quantized angular momentum as h/2π.J. W. Nicholson, Month. Not. Roy. Astr. Soc. lxxii. pp. 49,130, 677, 693, 729 (1912).The Atomic Theory of John William Nicholson, Russell McCormmach, Archive for History of Exact Sciences, Vol. 3, No. 2 (25.8.1966), pp. 160–184 (25 pages), Springer. Niels Bohr quoted him in his 1913 paper of the Bohr model of the atom.On the Constitution of Atoms and Molecules Niels Bohr, Philosophical Magazine, Series 6, Volume 26 July 1913, pp. 1–25 1912 – Victor Hess discovers the existence of cosmic radiation. 1912 – Henri Poincaré publishes an influential mathematical argument in support of the essential nature of energy quanta. 1913: Robert Andrews Millikan publishes the results of his oil drop experiment, in which he precisely determines the electric charge of the electron, and therefrom the Avogadro constant (which is the number of atoms or molecules in one mole of any substance). Niels Bohr publishes his 1913 paper of the Bohr model of the atom.On the Constitution of Atoms and Molecules, Niels Bohr, Philosophical Magazine, Series 6, Volume 26 July 1913, pp. 1–25 Ștefan Procopiu publishes a theoretical paper with the correct value of the electron's magnetic dipole moment . Niels Bohr obtains theoretically the value of the electron's magnetic dipole moment as a consequence of his atom model Johannes Stark and Antonino Lo Surdo independently discover the shifting and splitting of the spectral lines of atoms and molecules due to the presence of the light source in an external static electric field. To explain the Rydberg formula (1888), which correctly modeled the light emission spectra of atomic hydrogen, Bohr hypothesizes that negatively charged electrons revolve around a positively charged nucleus at certain fixed "quantum" distances and that each of these "spherical orbits" has a specific energy associated with it such that electron movements between orbits requires "quantum" emissions or absorptions of energy. 1914 – James Franck and Gustav Hertz report their experiment on electron collisions with mercury atoms, which provides a new test of Bohr's quantized model of atomic energy levels.