Quantum Theory Timeline

Max Planck suggests that radiation is quantized (it comes in discrete amounts.)

1900

Albert Einstein, one of the few scientists to take Planck's ideas seriously, proposes a quantum of light (the photon) which behaves like a particle.

1905

Hans Geiger and Ernest Marsden, under the supervision of Ernest Rutherford, scatter alpha particles off a gold foil and observe large angles of scattering, suggesting that atoms have a small, dense, positively charged nucleus.

1909

Ernest Rutherford infers the nucleus as the result of the alpha-scattering experiment performed by Hans Geiger and Ernest Marsden.

1911

Albert Einstein explains the curvature of space-time.

1912

Niels Bohr succeeds in constructing a theory of atomic structure based on quantum ideas.

1913

Ernest Rutherford finds the first evidence for a proton.

1919

James Chadwick and E.S. Bieler conclude that some strong force holds the nucleus together.

1921

Arthur Compton discovers the quantum (particle) nature of x rays, thus confirming photons as particles.

1923

Louis de Broglie proposes that matter has wave properties.

1924

Walther Bothe and Hans Geiger demonstrate that energy and mass are conserved in atomic processes.

1925

Wolfgang Pauli formulates the exclusion principle for electrons in an atom.

1925

Erwin Schroedinger develops wave mechanics, which describes the behavior of quantum systems for bosons.

1926

Werner Heisenberg formulates the uncertainty principle: the more you know about a particle's energy, the less you know about the time of the energy (and vice versa.) The same uncertainty applies to momenta and coordinates.

1927

Paul Dirac combines quantum mechanics and special relativity to describe the electron.

1928

James Chadwick discovers the neutron. The mechanisms of nuclear binding and decay become primary problems.

1931

Paul Dirac realizes that the positively-charged particles required by his equation are new objects (he calls them "positrons"). They are exactly like electrons, but positively charged. This is the first example of antiparticles.

1931

Wolfgang Pauli suggests the neutrino to explain the continuous electron spectrum for beta decay.

1931

Hideki Yukawa combines relativity and quantum theory to describe nuclear interactions by an exchange of new particles (mesons called "pions") between protons and neutrons.

1933

Enrico Fermi puts forth a theory of beta decay that introduces the weak interaction. This is the first theory to explicitly use neutrinos and particle flavor changes.

1933

E.C.G. Stückelberg observes that protons and neutrons do not decay into any combination of electrons, neutrinos, muons, or their antiparticles.

1938

C. Moller and Abraham Pais introduce the term "nucleon" as a generic term for protons and neutrons.

1941

Physicists realize that the cosmic ray particle thought to be Yukawa's meson is instead a "muon," the first particle of the second generation of matter particles to be found. This discovery was completely unexpected.

1947

Physicists realize that the cosmic ray particle thought to be Yukawa's meson is instead a "muon," the first particle of the second generation of matter particles to be found. This discovery was completely unexpected.

Enrico Fermi and C.N. Yang suggest that a pion is a composite structure of a nucleon and an anti-nucleon. This idea of composite particles is quite radical.

1949

Donald Glaser invents the bubble chamber. The Brookhaven Cosmotron, a 1.3 GeV accelerator, starts operation.

1952

C.N. Yang and Robert Mills develop a new class of theories called "gauge theories." Although not realized at the time, this type of theory now forms the basis of the Standard Model.

1954

Julian Schwinger writes a paper proposing unification of weak and electromagnetic interactions.

1957

Julian Schwinger, Sidney Bludman, and Sheldon Glashow, in separate papers, suggest that all weak interactions are mediated by charged heavy bosons, later called W+ and W-.

1959