Timeline of Quantum Physics

1670 - 1900: Wave Theory of Light §

• 1670: Sir Isaac Newton develops the corpuscular theory, which suggests that light travelled in small particles he called ‘corpuscles’ (literally, little particles in Latin). At that time, no known theory of waves existed, so this was taken as fact.
• 1678: Around the same time Christiaan Huygens formulates an opposing theory that light must form as a wave, but it doesn’t gain much traction, at least not compared to Newton’s corpuscular theory of light.
• 1801: Thomas Young performs the double slit experiment with light, which highlights the interfacing of waves, something that Newton’s theory could not explain.
• 1818: Huygens’ theory is further improved by Augustin-Jean Fresnel, who shows that the wave model of light can also explain diffraction of waves (of which light would be). Now, the theory of the wave model of light started gaining more traction.
• 1850s-1870s: James Clark Maxwell develops Maxwell’s Equations to summarise the works of:
  • (Around) 1780:: Charles-Augustin de Coulomb’s work on Coulomb’s Law, which formed the fundamentals of electrostatics,
  • 1825: André-Marie Ampère publishes the Ampère Force Law (an early version of Ampère’s Law for circuits).
  • 1831: Michael Faraday discovers Electromagnetic Induction, and solves for the magnitude of Lenz’s Law (but not the negative sign!)
  • 1834: Emil Lenz discovers that a change in Magnetic Flux results in force opposing the change (and adds the negative sign to Lenz’s Law).
• 1864: Maxwell uses his equations to hypothesise that light must be an Electromagnetic Wave, however, he is unable to prove it. It does, however, open the idea of the Electromagnetic Spectrum to physicists.

1800s - 1900s: Early Atomic Theory §

Also see Timeline of Atomic Theory

• 1804-1811: John Dalton proposes that all matter is made up of tiny, indivisible particles, he called atoms. Amedeo Avogadro, another chemist, builds upon this and this results in the early atomic theory of elements and compounds being made up of various atoms.
• 1871-1873: Maxwell and Ludwig Boltzmann formulate the kinetic theory of gasses, which further supports the atomic theory, as they deduce that gasses must be formed from tiny particles colliding randomly.
• 1877: Boltzmann suggests that energy levels of a molecule must be discrete rather than continuous (something that quantum theory proves!). However, at the time he had no evidence to back it up.
• 1879: The Stefan-Boltzmann Law, from Josef Stefan and Ludwig Boltzmann, defines how an object can radiate energy in the form of thermal radiation. This is very important to the beginning of quantum theory.
• 1870: William Crookes discovers that by removing lots of air from a tube (until the pressure drops to around $10^{-6}\text{ atm}$) and then connecting it to a circuit, a dark space begins to emerge from the cathode (negative end) and goes towards the anode (positive end). These were called cathode ray tubes.
• 1897: Joseph John Thomson discovers the first subatomic particle: the electron, by fixing a magnetic field perpendicular to an electric field, see Lorentz Force. Originally, he called these particles corpuscles (from Newton, I think), but later changed the name to electron. He finds the charge-to-mass ratio, $\frac{q}{m}$ of the electron, and is able to correctly state that it is negatively charged, but does not actually find the charge, nor the mass. See the full experiment: Discovery of the Electron ^d663ab
• 1909: Robert Millikan conducts the oil-drop experiment which accurately determines the value of the charge of an electron, $q_e$.

1900 - 1950: Early Quantum Theory §

• 1900: Max Planck explains the Ultraviolet Catastrophe by proposing that energy is not, in fact, continuous, but is discrete. He suggested that the thermal radiation can be expressed as a frequency of some oscillating wave, multiplied by a proportionality constant, known as Planck’s constant, $h\approx 6.63\times 10^{-24}\text{ Js}$. The idea of quantum theory, comes from stating that the quanta (smallest form) of energy is given by these multiples of $h$. This gives rise to the Planck’s Law
• 1905: Albert Einstein explains the Photoelectric Effect, specifically the clash with Classical Physics as the intensity of the light is not proportional to the energy generated. He states that light behaves as a particle, with discrete packets of energy and that Planck’s constant can be used to find the energy: $E=hf$ is the Planck-Einstein Equation. He predicts that these ‘packets’ of energy can be treated as particles, giving rise to photons during instantaneous particle transfers, but can be seen as a wave during longer intervals, agreeing with Maxwell’s deduction (that light is a wave). This is the first case of the wave-particle duality of Light.
• 1906: The atomic nucleus is discovered, by Ernest Rutherford’s Gold Foil Experiment. At the time, it was not known what it was made up of, but it was found to be much smaller than the atom itself, and be very strongly positively charged.
• 1911: A new atomic model, known as the Rutherford model, is proposed by Ernest Rutherford after the scattering experiments, which is accepted and replaces the plum pudding model. In the Rutherford model, electrons orbit the nucleus.
• 1913: Niels Bohr offers a new atomic model: The Bohr Model. Here the electrons still orbit the nucleus, but only in fixed orbits, given by $hf$
• 1922: Electron spin is discovered#todo
• 1922: Arthur Holly Compton discovers the Compton Effect, which supports the particle theory of light.
• 1924: Louis de Broglie suggests that all matter has a Wavelength, known as the De Broglie wavelength. He also expanded the Bohr Model#todo
• 1927: The double slit experiment is repeated again, this time with electrons instead of light, and