| 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. |
At the start of the twentieth
century, scientists believed that they understood the most fundamental principles of
nature. Atoms were solid building blocks of nature; people trusted Newtonian laws of
motion; most of the problems of physics seemed to be solved. However, starting with
Einstein's theory of relativity which replaced Newtonian mechanics, scientists gradually
realized that their knowledge was far from complete. Of particular interest was the
growing field of quantum mechanics, which completely altered the fundamental precepts of
physics. 