For those interested, Manjit Kumar’s book entitled ‘Quantum’ is an excellent account of the historical issues that surrounded quantum theory, especially in the early years of the 20th century up until the 2nd World War. As such, the following abbreviated timeline is only intended to be representative of just a few key milestones.
- 1900: Max Planck
In an attempt to explain black body radiation, Planck suggests that electromagnetic energy can only be emitted in quantized bundles, which align to multiples of an elementary unit [E=hf], where [h] is Planck's constant and [f] is the frequency of the radiation.
- 1905: Albert Einstein
While the photoelectric effect had been discovered by Heinrich Hertz in 1887, it was Einstein who forwarded the idea, based on Planck’s quantum hypothesis, that light itself consists of individual quantum particles, i.e. photons.
- 1913: Niels Bohr
Bohr outlines the first meaningful model of the atom in which negatively charged electrons are described as revolving around a positively charged nucleus at certain fixed radii determined by quantized momentum. These orbits are also special in the sense that the electrons are now assumed not to radiate while in these defined energy levels and any transition between levels requires the emission or absorption of quantized energy, i.e. photons.
- 1916: Arnold Sommerfeld
The initial Bohr model of the atom, based on circular orbits, could not explain the spectral lines linked to the Zeeman Effect caused by a magnetic field. In response, Sommerfeld introduces the idea of elliptical orbits in addition to circular orbits.
- 1922: Arthur Compton
The Compton effect, or Compton scattering, results when high-energy photons collide with a target, which can then free loosely bound electrons from the outer shell of the surface atom. The scattered radiation experiences a wavelength shift that cannot be explained in terms of classical wave theory and was to add considerable weight to Einstein's photon theory.
- 1923: Louis De Broglie
Forwarded the idea that electron particles also have an associated wavelength given by Planck’s constant [h] divided by it momentum [ρ], i.e. λ=h/mv. Such a suggestion would now reverse the normal wave-particle duality debate to include particles as waves, as well as waves as particles.
- 1925: George Uhlenbeck and Samuel Goudsmit
Forwarded the idea of electron spin, which like Sommerfeld’s extension of the basic Bohr models helped to further explain the splitting of atomic spectral lines.
- 1925: Werner Heisenberg
Develops matrix mechanics formulation of quantum mechanics with the help of Max Born.
- 1925: Wolfgang Pauli
Outlines the ‘Pauli exclusion principle’ that states that no two identical fermions may simultaneously occupy the same quantum state.
- 1926: Erwin Schrödinger
Expands on De Broglie’s electron wave model and develops a wave equation, which is initially thought to represent the distribution of the electron’s charge in space.
- 1927: Werner Heisenberg
Forwards the idea of the quantum uncertainty principle
- 1927: Max Born
Forwards the idea of the probabilistic nature associated with a wave function.
- 1927: Paul Dirac
Dirac develops a relativistic electron quantum wave equation in which the principle of special relativity is linked to quantum electrodynamics. This work leads to the suggested existence of the positron.
- 1930: Wolfgang Pauli
Pauli suggests that, in addition to electrons and protons, atoms also contained a neutral particle, which he calls the ‘neutron’.
- 1932: James Chadwick
Performs a series of experiments that leads to the discovery of the neutron.