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Quantum Theory Timeline

By C&Y

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

  • 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.

  • 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.

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

  • Albert Einstein explains the curvature of space-time.

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

  • Ernest Rutherford finds the first evidence for a proton.

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

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

  • Louis de Broglie proposes that matter has wave properties.

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

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

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

  • 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.

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

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

  • 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.

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

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

  • 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.

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

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

  • 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.

    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.

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

  • 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.

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

  • 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-.