**1897**

Pieter Zeeman shows that light is radiated by the motion of charged particles in an atom, and Joseph John (J.J.) Thomson discovers the electron.

**1900**

**1905**

Albert Einstein proposes that light, which has wavelike properties, also consists of discrete, quantized bundles of energy, which are later called photons.

**1911**

Ernest Rutherford proposes the nuclear model of the atom.

**1913**

Niels Bohr proposes his planetary model of the atom, along with the concept of stationary energy states, and accounts for the spectrum of hydrogen.

**1914**

James Franck and Gustav Hertz confirm the existence of stationary states through an electron-scattering experiment.

**1923**

Arthur Compton observes that x-rays behave like miniature billiard balls in their interactions with electrons, thereby providing further evidence for the particle nature of light.

**1923**

Louis de Broglie generalizes wave-particle duality by suggesting that particles of matter are also wavelike.

**1924**

Satyendra Nath Bose and Albert Einstein find a new way to count quantum particles, later called Bose-Einstein statistics, and they predict that extremely cold atoms should condense into a single quantum state, later known as a Bose-Einstein condensate.

**1925**

• Wolfgang Pauli enunciates the exclusion principle.

• Werner Heisenberg, Max Born, and Pascual Jordan develop matrix mechanics, the first version of quantum mechanics, and make an initial step toward quantum field theory.

**1926**

• Erwin Schrödinger develops a second description of quantum physics, called wave mechanics. It includes what becomes one of the most famous formulas of science, which is later known as the Schrödinger equation.

• Enrico Fermi and Paul A.M. Dirac find that quantum mechanics requires a second way to count particles, Fermi-Dirac statistics, opening the way to solid-state physics.

• Dirac publishes a seminal paper on the quantum theory of light.

**1927**

Heisenberg states his Uncertainty Principle, that it is impossible to exactly measure the position and momentum of a particle at the same time.

**1928**

Dirac presents a relativistic theory of the electron that includes the prediction of antimatter.

**1932**

Carl David Anderson discovers antimatter, an antielectron called the positron.

**1934**

Hideki Yukawa proposes that nuclear forces are mediated by massive particles called mesons, which are analogous to the photon in mediating electromagnetic forces.

**1946-48**

Experiments by Isidor I. Rabi, Willis Lamb, and Polykarp Kusch reveal discrepancies in the Dirac theory.

**1948**

Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga develop the first complete theory of the interaction of photons and electrons, quantum electrodynamics, which accounts for the discrepancies in the Dirac theory.

**1957**

John Bardeen, Leon Cooper, and J. Robert Schrieffer show that electrons can form pairs whose quantum properties allow them to travel without resistance, providing an explanation for the zero electrical resistance of superconductors. This theory was later termed the BCS theory (after the surname initials of the three physicists).

**1959**

Yakir Aharonov and David Bohm predict that a magnetic field affects the quantum properties of an electron in a way that is forbidden by classical physics. The Aharonov-Bohm effect is observed in 1960 and hints at a wealth of unexpected macroscopic effects.

**1960**

Building on work by Charles Townes, Arthur Schawlow, and others, Theodore Maiman builds the first practical laser.

**1964**

John S. Bell proposes an experimental test, "Bell's inequalities," of whether quantum mechanics provides the most complete possible description of a system.

**1970s**

Foundations are laid for the standard model of particle physics, in which matter is said to be built of quarks and leptons that interact via the four physical forces.

**1982**

Alain Aspect carries out an experimental test of Bell's inequalities and confirms the completeness of quantum mechanics.

**1995**

Eric Cornell, Carl Wieman, and Wolfgang Ketterle trap clouds of metallic atoms cooled to less than a millionth of a degree above absolute zero, producing Bose-Einstein condensates, which were first predicted 70 years earlier. This accomplishment leads to the creation of the atom laser and superfluid gases.

For more extensive timelines of quantum physics, see two of Abraham Pais's
books: *Inward Bound: Of Matter and Forces in the Physical World* and *Niels
Bohr's Times: In Physics, Philosophy, and Polity.*

Also see the APS Century of Physics Timeline.

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