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Quantum Mechanics

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c.600 BC: Thales is the first to record statements about what would later be called static electricity; that, amber (Greek word “elektron”) rubbed with animal fur attracts light-weight objects. Subsequently, anything displaying characteristics of static electricity is associated with amber (“elektron”).

c.440 BC: Democritus and Leucippus assert that things of the world are constructed of atoms (Greek word “uncuttable”) and "the void".

1676: Danish astronomer Ole Rømer is the first to demonstrate that light travels at a finite speed (non-instantaneously) with his studies of Jupiter's moon Io. He estimated light's speed to be 140,000 miles per second, very close to the actual number (186,000).

1690: Christiaan Huygens theorizes that light is a wave

1704: Isaac Newton posits that light is composed of particles

1734: Charles François du Fay discovers unlike charges attract and like charges repel; also, the existence of what would later be called positive and negative charges

c.1745: Ben Franklin conducts experiments on the nature of electricity and coins many terms still in use today:  battery, charge, condenser, conductor, plus, minus, positive, negative, armature.

1804: Thomas Young conducts the first double-slit experiment: light is a wave 

1808: John Dalton's atomic theory: the first scientific evidence that matter is made of atoms, which combine according to specific ratios

c.1840: Michael Faraday invents the concept of energy "fields," and perceives the connection between electricity and magnetism, the basis of light as electromagnetic field

1860: German physicist Gustav Kirchhoff speaks of "blackbody" radiation, the issue Max Planck would address.

1865: James Clerk Maxwell publishes "A Dynamical Theory of the Electromagnetic Field" which demonstrates, via the four “Maxwell equations,” that electric and magnetic fields propagate through space as waves and move at the speed of light. With this paper, Maxwell offered solid mathematical basis for Faraday’s earlier conjectures.

1887: Albert Michelson and Edward Morley at Case Western Reserve University in Cleveland determine the non-existence of a supposed “luminiferous aether” through which light was believed to travel. The Michelson–Morley experiment showed that there is no “aether,” light waves do not move through a medium as do water waves.

1897: J.J. Thompson discovers the electron; atoms are not solid after all but have parts

1900: Max Planck's "constant" requires light to be a particle

1904: J.J. Thompson puts forward his "plum pudding" atomic model

1905: Albert Einstein and the Photoelectric Effect: light is made of particles

1909: Fletcher and Millikan measure the charge of an electron with the famous "oil drop" experiment

1911: Rutherford’s atomic model speaks of a “nucleus.” Unlike the diffused “plum pudding” view, the atom’s positive charge, and almost all of its mass, is concentrated in an extremely small central region

1911: The first Solvay Conference: 18 leading scientists discuss recent massive changes in physics, Newtonianism giving way to Quantum Mechanics

1913: Bohr’s atomic model puts electrons in fixed orbital shells, definite energy levels, which encircle, like planets in a mini solar system, a nucleus

1920: The proton discovered by Ernest Rutherford

1922: The Stern-Gerlach experiment offers the first hint that quantum particles possess an unrecognized property of magnetism, later known as “spin”

1923: Arthur Compton confirms the Photoelectric Effect

1923: Louis de Broglie asserts that the electron particle, matter, can also be a wave

1925: Wolfgang Pauli formulates his Exclusion Principle

1925: Max Born publishes his matrix-math paper supporting Heisenberg

1926: Erwin Schrodinger publishes the wave-function equation

1927: Werner Heisenberg puts forward his Uncertainty Principle

1927: Davisson and Germer empirically confirm, the first experimental verification of quantum mechanics, that the electron particle can exist as a matter-wave

1927: The fifth Solvay Conference: 29 leading scientists discuss "electrons and photons." Debates ensued concerning the meaning and nature of the newly-constructed quantum theory. Albert Einstein and Niels Bohr held center stage

1927-1935: The Bohr-Einstein debates

1928: Paul Dirac formulates his equation, describes the behavior of electrons and quarks, predicts the existence of anti-matter as positron

1930: The Bohr-Einstein debates: Einstein offers his photon-box thought-experiment

1932: The neutron discovered by James Chadwick 

1935: The Bohr-Einstein debates: Einstein presents his "EPR paradox" thought-experiment

1935: Schrodinger, in discussions with Einstein, speaks of the "Schrodinger Cat" thought-experiment

1964: John Bell, physicist from Northern Ireland, proposes a mathematical test for the existence of “hidden variables” via his now-famous inequality theorem

1972: experimental evidence suggests a violation of Bell's inequality, dealing a blow to EPR's "hidden variables"

1978: John Wheeler's version of a delayed-choice thought-experiment

1982: French physicist Alain Aspect (with the collaboration of Philippe Grangier, Gérard Roger and Jean Dalibard) performs the first quantum mechanics experiment to demonstrate a violation of Bell's theorem, an underminding of EPR's "locality"