| 1900 | Max Planck suggests that radiation is quantized (it comes in discrete amounts.) |
| 1905 | 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. Einstein's other theories explained the equivalence of mass and energy, the particle-wave duality of photons, the equivalence principle, and special relativity. |
| 1909 | 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. |
| 1911 | Ernest Rutherford infers the nucleus as the result of the alpha-scattering experiment performed by Hans Geiger and Ernest Marsden. |
| 1912 | Albert Einstein explains the curvature of space-time. |
| 1913 | Niels Bohr succeeds in constructing a theory of atomic structure based on quantum ideas. |
| 1919 | Ernest Rutherford finds the first evidence for a proton. |
| 1921 | James Chadwick and E.S. Bieler conclude that some strong force holds the nucleus together. |
| 1923 | Arthur Compton discovers the quantum (particle) nature of x rays, thus confirming photons as particles. |
| 1924 | Louis de Broglie proposes that matter has wave properties. |
| 1925 (Jan) | Wolfgang Pauli formulates the exclusion principle for electrons in an atom. |
| 1925 (April) | Walther Bothe and Hans Geiger demonstrate that energy and mass are conserved in atomic processes. |
| 1926 | Erwin Schroedinger develops wave mechanics, which describes the behavior of quantum systems for bosons. Max Born gives a probability interpretation of quantum mechanics. G.N. Lewis proposes the name "photon" for a light quantum. |
| 1927 | Certain materials had been observed to emit electrons (beta decay). Since both the atom and the nucleus have discrete energy levels, it is hard to see how electrons produced in transition could have a continuous spectrum (see 1930 for an answer.) |
| 1927 | 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. |
| 1928 | Paul Dirac combines quantum mechanics and special relativity to describe the electron. |
| 1930 | Quantum mechanics and special relativity are well established. There are just three fundamental particles: protons, electrons, and photons. Max Born, after learning of the Dirac equation, said, "Physics as we know it will be over in six months." |
| 1930 | Wolfgang Pauli suggests the neutrino to explain the continuous electron spectrum for beta decay. |
| 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 | James Chadwick discovers the neutron. The mechanisms of nuclear binding and decay become primary problems. |
| 1933-34 | 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. |
| 1933-34 | Hideki Yukawa combines relativity and quantum theory to describe nuclear interactions by an exchange of new particles (mesons called "pions") between protons and neutrons. From the size of the nucleus, Yukawa concludes that the mass of the conjectured particles (mesons) is about 200 electron masses. This is the beginning of the meson theory of nuclear forces. |
| 1937 | A particle of 200 electron masses is discovered in cosmic rays. While at first physicists thought it was Yukawa's pion, it was later discovered to be a muon. |
| 1938 | E.C.G. Stuckelberg observes that protons and neutrons do not decay into any combination of electrons, neutrinos, muons, or their antiparticles. The stability of the proton cannot be explained in terms of energy or charge conservation; he proposes that heavy particles are independently conserved. |
| 1941 | C. Moller and Abraham Pais introduce the term "nucleon" as a generic term for protons and neutrons. |
| 1946-47 | 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 -- I.I. Rabi comments "who ordered that?" The term "lepton" is introduced to describe objects that do not interact too strongly (electrons and muons are both leptons). |
| 1947 | A meson that does interact strongly is found in cosmic rays, and is determined to be the pion. |
| 1947 | Physicists develop procedures to calculate electromagnetic properties of electrons, positrons, and photons. Introduction of Feynman diagrams. |
| 1948 | The Berkeley synchro-cyclotron produces the first artificial pions. |
| 1949 | 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 | Discovery of K+ via its decay. |
| 1950 | The neutral pion is discovered. |
| 1951 | Two new types of particles are discovered in cosmic rays. They are discovered by looking a V-like tracks and reconstructing the electrically-neutral object that must have decayed to produce the two charged objects that left the tracks. The particles were named the lambda0 and the K0. |
| 1952 | Discovery of particle called delta: there were four similar particles (delta++, delta+, delta0, and delta-.) |
| 1952 | Donald Glaser invents the bubble chamber. The Brookhaven Cosmotron, a 1.3 GeV accelerator, starts operation. |
| 1953 | The beginning of a "particle explosion" -- a true proliferation of particles. |
| 1953 - 57 | Scattering of electrons off nuclei reveals a charge density distribution inside protons, and even neutrons. Description of this electromagnetic structure of protons and neutrons suggests some kind of internal structure to these objects, though they are still regarded as fundamental particles. |
| 1954 | 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. |
| 1957 | Julian Schwinger writes a paper proposing unification of weak and electromagnetic interactions. |
| 1957-59 | 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-. Actually, it was Yukawa who first discussed boson exchange twenty years earlier, but he proposed the pion as the mediator of the weak force. |
| 1961 | As the number of known particles keep increasing, a mathematical classification scheme to organize the particles (the group SU(3)) helps physicists recognize patterns of particle types. |
| 1962 | Experiments verify that there are two distinct types of neutrinos (electron and muon neutrinos). This was earlier inferred from theoretical considerations. |