400 B.C. Democritus discusses the idea that everything is made from
indivisible (fundamental) particles, which he called "atoms."

1803 John Dalton formulates the "law of definite proportions:" That the
relative amounts of the elements which are constituents in a particular
chemical compound are always the same, regardless of origin or method of
preparation.

1859 Kirchhoff and Bunsen measure wavelengths of atomic spectral lines,
establishing that spectra are unique to each element. They and others use
spectral analysis to identify new elements.  About this time, a general
suspicion among researchers develops that the spectra characterize the
atomic species. Several models or pictures of the basic constituents of
matter are suggested.

1861 Maxwell, in a series of papers, describes the interrelation of
electric and magnetic fields, thereby unifying them into electromagnetism.
This leads to the now-famous Maxwell's Equations. One prediction of these
equations is that there are traveling electromagnetic waves in addition to
static electric and magnetic fields.

1867 Kelvin proposes a vortex atom, a geometrical structure that was a
stable assembly of interlocking vortex rings. Kelvin conjectures that a
classifications of knots would yield a classification of the elements.

1869 Mendeleev classifies the known chemical elements in a "periodic
table" according to atomic mass and chemical properties, with gaps for
unknown elements.

1875 Maxwell notes that atoms have a structure that is much more
complicated than that of a rigid body, that is to say, some internal
motion is possible.

1896 Henri Becquerel accidently discovers that uranium emits radiation
that exposes a sensitive film-plate. While investigating how fluorescent
uranium exposes the plate, he recognizes that nonfluorescent uranium has
the same effect, even when wrapped in black paper.  He then studies
further and finds that the radiation could also penetrate through silver
pieces. This was the first recognition of radioactivity.

1897 J.J. Thomson, in a series of experiments, shows that cathode rays
consist of negatively charged corpuscles. He finds the same charge to mass
ratio no matter how cathode rays are produced. He concludes that the
corpuscles are universal constituents of all atoms and that their mass is
about 1/1000 of that of a hydrogen atom. Thus, he discovers the electron.

1899 It is recognized that there are three different types of rays which
are named alpha, beta and gamma. Rutherford recognizes that the rays
emitted in decay of radium are positively charged particles, which he
names alpha particles. These are now known to be a helium nucleus. It begins
to be clear that one atom can "transmute" through radioactive decay into a
different atom, which destroys the notion that each element is an
"elementary particle" in its own right, though it takes another 40 years
for a clear picture of what the nucleus is to emerge.

1900 J.J. Thomson proposes a model for the atom: a number of electrons
moving in some vaguely specified, positive background of indefinite shape.
As late as 1903, Thomson thought that a hydrogen atom contained about 1000
electrons.   Lorentz thought that an electron was an extended charge
distribution.

1900 Planck, trying to understand the observed spectrum of energy radiated
from a blackbody, that is, any black object at a temperature warmer than
its surroundings, suggests that the radiation is quantized; that is, it
comes in certain discrete amounts. Planck presents his ideas to an
audience of not quite 25 people. Nobody understands much, except the final
formula, which fit measurements that had previously been known but
unexplained. Only a few physicists pay attention to the quantum ideas;
there is much confusion. No obvious connection is made in Planck's
treatment between the quantum radiation and the structure of matter.

1905 Einstein, one of the few physicists then taking Planck's quantum
ideas seriously, proposes that light consists of discrete energy packets
(now called photons) whose energy is proportional to frequency.  This
provides an explanation of the photoelectric effect, in which light
falling on a surface ejects electrons from the surface. But nobody,
including Planck, takes it seriously at first. (This is the work for which
Einstein is awarded the Nobel Prize for 1921; his earlier work, on
relativity, is regarded as too speculative.)

1911 Rutherford proposes the nuclear model of an atom, which is violently
opposed by J.J. Thomson. The reception of Rutherford's ideas is
decidedly mixed.

1913 Bohr succeeds in constructing a theory of atomic electronic structure
based on quantum ideas. His theory introduces the idea of quantum states
(or quantum levels) for the electrons in an atom and quantum jumps (or
quantum transitions) of electrons between these levels as the mechanism
that produces the characteristic spectral lines of radiation from atoms.

1919 Rutherford observes first nuclear transmutation.  This process
provides the first evidence that the object recognized to be the nucleus
of the hydrogen atom is also a constituent of other nuclei.  Two years
later, Rutherford postulates that it is a fundamental particle and names
it the proton. Rutherford also makes a first estimate of nuclear size.

1921 From studies on hydrogen scattering, Chadwick and Bieler conclude
that some kind of strong force (not following the 1/r^2 force law) exists
inside the nucleus.

1923 Compton discovers the particle nature of x-rays, as quanta processing
both energy and momentum.

1924 de Broglie conjectures the wave aspect of matter. He introduces the
wave-particle duality as a universal feature for all types of matter and
radiation.

1925 First experimental demonstration of energy and momentum
conservations in individual atomic processes (by Bothe and Geiger).

1925 Heisenberg invents matrix mechanics and within months Schrodinger
develops wave mechanics; both are formulations of quantum theories to
explain atomic systems. Soon after, Dirac shows that the two theories are
equivalent within more general framework. Born gives a probabilistic,
statistical meaning to Schrodinger's wave function.  G.N. Lewis proposes
the name "photon" for a light quantum.

1927 Certain materials (e.g., radium) are observed to emit electrons, in
the process known as beta decay. The spectrum of electron energies is shown
to be continuous. For a while, it is unclear whether these electrons are
of nuclear or atomic origin. Since both the atom and the nucleus are
thought to have discrete energy levels, it is hard to see how electrons
produced in either type of transition could have a continuous spectrum
(see 1930 for answer).

1927 Heisenberg formulates the uncertainty relations, which state the
impossibility of making simultaneous, arbitrarily precise measurements
of both a particle's momentum in some direction and its coordinate in that
direction.

1928 Dirac derives an equation that combines quantum mechanics and special
relativity to describe the electron; it is found to also require the
existence of corresponding positively charged particles. Dirac, together
with Schrodinger, is awarded Nobel Prize in 1933.

1929 Dirac proposes (incorrectly) that the positively charged particles
required by his equation are protons.

1930 A general consensus is reached. Quantum mechanics and special
relativity are well established. Just three fundamental particles exist:
protons, electrons and photons. Born, after learning about the Dirac
equation (1929), said, "Physics as we know it will be over in six months."

1930 Pauli, in letters and private conversations, suggests that an
additional new type of particle, the neutrino (nu), must be being produced to
explain the continuous electron spectrum for beta decay. (See 1927).  If
two particles are produced in the transition, only the sum of their
energies is discrete.

1931 Dirac realizes that the positively charged particles required by his
equation are new objects (he calls them "positrons"). They are exactly
like electrons, in particular, they have the identical mass, but
positively charged. This is the first example of antiparticles, a new form
of matter called anti-matter.

1931 Anderson observes positively charged particles (produced by cosmic
rays) that have the same mass as electrons. Pauli and Bohr do not believe
that these are Dirac's positrons, but they are later determined to be just
that.

1931 Discovery of the neutron, its spin is determined to be 2h. Chadwick
awarded Nobel Prize in 1935. It is suggested that a neutron is as
elementary as a proton. Nuclei can now be understood as composed of
protons and neutrons. The questions of the mechanisms of nuclear binding
and decay become primary problems. Essentially, all of modern particle
physics is later discovered in the attempt to understand nuclear
interactions.

1933 Fermi proposes a theory of beta decay. First introduction of weak
interactions, first explicit use of neutrinos, and first theory of
processes in which a "fundamental" particle changes type (n -> p + e- +
nu). The introduction of a new interaction, in addition to the familiar
electromagnetic and gravitational interactions, is a very radical step.
The idea that the electron (and neutrino) produced in nuclear beta decay
are in no way present in the nucleus before the decay is also radically
new.

1933 Yukawa speculates about the nature of nuclear forces. Combining
relativity and quantum theory, Yukawa tries to describe nuclear
interactions by an exchange of new particles between protons and neutrons.
From the size of the nucleus, which gives the range of the
new interaction, Yukawa concludes that mass of these conjectured
particles (mesons) is about 200 electron masses. This is the beginning of
the meson theory of nuclear forces. Nobel Prize awarded to Yukawa in
1949.

1937 A particle of mass about 200 electron masses is discovered in cosmic
rays by Neddermeyer and Anderson and by Street and Stevenson.  First
believed to be Yukawa's meson, it is later recognized to be another
entirely new type of particle, now called a muon (see 1946-1947).

1938 Otto Hahn and Fritz Strassmann and Otto Frisch and Lise Meitner
discover that they can induce nuclear fission by bombarding uranium nuclei
with neutrons. By this time, it has become quite clear that atoms
themselves are not the fundamental buiIding blocks of matter.  

1946 Realization that the cosmic ray particle thought to be the Yukawa
meson cannot be any such thing because it does not interact strongly
enough as it passes through matter. It is instead a particle with no
strong interactions, just like an electron, except that it is more massive
and thus unstable. The "muon", the first particle of the second generation
to be found, is completely unexpected. It was the first of many unexpected
particle discoveries; I.I. Rabi comments (like a customer in a restaurant
receiving an unexpected dish), "Who ordered that?" The term "lepton" is
introduced as a generic name for objects that do not interact strongly,
namely, the electron, muon, and neutrinos. Correspondingly, the generic
term "hadron" is introduced for particles that do have strong
interactions, such as the proton and neutron, and Yukawa's still
undiscovered meson.

1947 Powell and collaborators, using sensitive nuclear emulsions exposed
to cosmic radiation, discover another type of particle with mass a little
greater than a muon, which does interact strongly. It decays into a muon
and a neutrino. It is Yukawa's strongly interacting meson, now called the
pion.

1947 Development of calculational procedures for quantum electrodynamics
(QED), the relativistic quantum theory of the electromagnetic interactions
of electrons, positrons, photons. Introduction of Feynman diagrams.

1949 A new type of meson, now called K+, is discovered by the Bristol
group (Powell and collaborators) via its decay.  (As early as 1944,
Leprince-Ringuet and Lheritier had seen less substantial evidence for the
K+.)

1951 Rochester and Butler discover two new types of particles in tracks
produced in a bubble chamber in processes initiated by cosmic rays.  They
looked for V-like tracks, which can be interpreted as the diverging tracks
of two charged particles produced by the decay of a "parent" electrically
neutral particle (which leaves no track). The mass of the parent particle
is deduced by reconstructing its energy and momentum from those of the
(known mass) products. The new electrically neutral particles were at
first called V particles. The two types found in this way are now called Lambda_o
and K.  The Lambda_o was the first particle more massive than a neutron to be
discovered.

1953 Beginnings of a "particle explosion," a true proliferation of 'parti-
cles." Classification of particles and their decays becomes a major
activity. Particle decays are classified on the basis of their half-life
into two groupings; decays by strong interaction processes have a
half-life of order 10^-24 seconds while decays via weak processes have
half lives of 10^-13 seconds and even much longer. These can only be
observed when no strong decay is possible. Particles are classified by
spin (fermions or bosons) and by the types of interactions in which they
participate (hadrons or leptons).

1953 Introduction by Gell-Mann and Nishijima of a new particle attribute,
called "strangeness," to explain the disparity between the copious
production of Lambda's and K's and their slow decay. It is recognized that
particles with strangeness decay by only relatively slow weak
interactions, but are produced in pairs of the opposite sign of
strangeness. The modern interpretation of this property is that the Lambda
contains a strange quark, and the K+ contains a strange antiquark. 
A strange quark and its antiquark can be produced together
in a strong interaction, but each decays only by weak interactions.

1953 Scattering of electrons by nuclei reveals that the electric charge
inside protons has a distribution of varying density: and that even
neutrons have some internal charge density distribution. 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.

1957 Separate papers by Schwinger, Bludman, and Glashow suggest that all
weak interactions are mediated by charged, very massive bosons, later
called W+ and W-. This idea has a similarity to that of Yukawa who first
discussed boson exchange 20 years earlier, when he proposed the pion as
the mediator of the strong force. The name Wi was first used by Lee and
Yang in 1960.

1961 Gell-Mann exploits the patterns of particles of similar mass and
spin, but differing charge and strangeness to create a classification
scheme [based on the group SU(3)] now called flavor symmetry, for the
ever-increasing number of known particles. The scheme predicts a new
particle type, Omega, whose discovery shortly thereafter gives great validity
to this idea. For his earlier work on strangeness, this classification
scheme, and the later work on quarks (see 1964), Gell-Mann was awarded the
Nobel Prize in physics in 1969.

1964 First tentative introduction of quarks, by Gell-Mann and
independently by Zweig. Quarks give a basis in terms of the particle
structure for the classification scheme proposed earlier by Gell- Mann.
All mesons and baryons are composites of three species of quarks and
antiquarks, now called u, d, and s of spin 1/2 and with electric charges
(2/3, -1/3, -1/3) in units in which the proton charge is +1.  The
similarly charged d and s quarks are distinguished by the fact that the s
carries the "strangeness" quantum number +1, while the d has zero for this
quantity.  These fractions of a proton or electron charge had never been
observed, and so the introduction of quarks was generally treated more as
a mathematical explanation of flavor patterns of particle masses (see
1961) than as a postulate of actual physical objects. However, the great
simplification from over 100 "fundamental" hadronic particle types to just
three makes the idea very attractive. Later, theoretical and experimental
developments allow us to now regard the quarks as real physical objects,
even though they cannot be isolated.

1964 Stimulated by the repeated pattern of leptons, several papers suggest
a fourth quark carrying another flavor to give a similar repeated pattern
for the quarks, now seen as flavor generation patterns. Very few
physicists take this suggestion seriously at the time. Glashow and Bjorken
coin the term "charm' for the fourth (c) quark.

1965 Color charge, an additional degree of freedom, is introduced as an
essential property of quarks by Greenberg and by Han and Nambu.  All
observed hadrons are presumably color neutral.

1967 Weinberg and Salam separately propose a theory that unifies
electromagnetic and weak interactions. It is of the Yang-Mills type (see
1954). The theory requires the existence of a neutral, weakly interacting
boson (now called the Z) that mediates certain weak interactions that had
not been observed at that time. They also predict an additional massive
boson called the Higgs Boson, which has not yet been observed, to explain
particle masses. Their idea is mostly ignored; from 1967 to 1971,
Weinberg's and Salam's papers, now regarded as the first suggestion of an
essential part of the Standard Model, were only quoted five times.

1968 Observations at the SLAC laboratory indicate that in inelastic
electron-proton scattering the electrons appear to be bouncing off small
dense objects inside the proton. Bjorken and Feynman analyze these data in
terms of a model of constituent particles inside the proton, without using
the name quark for the constituents. Taylor, Friedman, and Kendall are
awarded Nobel Prize in 1990 for this experimental evidence for quarks.

1972 Definite formulation of a quantum field theory of strong
interactions.  This theory of quarks and gluons (now part of the Standard
Model) is similar in mathematical structure to quantum electrodynamics
(QED), hence, the name quantum chromodynamics (QCD). It is also a
Yang-Mills type theory. Quarks are real particles, carrying a color
charge. Gluons are massless quanta of the strong-interaction field, which
also carry color charges. This strong interaction theory is first
suggested by Fritzsch and Gell-Mann.

1973 Spurred by a prediction of the Weinberg-Salam-Glashow-Iliopoulos-
Maiani theories, the Gargamelle collaboration reanalyzes some old data
from CERN and finds indications of weak interactions with no charge
exchange (those due to a Z exchange). Here the interplay of theory and
experiment is interesting. Before the theoretical prediction of the Z,
everyone just assumed that no weak-interaction processes existed that
conserved both flavor and charge, and the early analysis of the CERN data
did not really look for them. It was stated that the search would be too
difficult because of "background" processes (those from similar-looking,
but nonweak-interaction, processes). After the theoretical prediction, the
experimenters found it was possible to exclude background events.
Theoretical bias often decides how hard we think about how to make a
certain measurement.

1974 In a summary talk for a summer conference, Iliopoulos summarizes, for
the first time in one single report, the view of physics now called the
Standard Model. Only particles containing u, d, and s quarks were known,
with the charm quark predicted by the theory, but particles containing it
not yet discovered. Most physicists are sceptical about the need for
charm, but Glashow declares at an international conference in April that
if charm is not found within two years, he would "eat my hat."

1976 A new charged lepton called tau, of mass about 1.78 GeV, is
recognized by Perl and collaborators at SLAC. It is totally unexpected.
The production of particle-antiparticle pairs of this lepton at almost the
same energy as the charm quark and antiquark production threshold had led
to some initial confusion in the interpretation of the J/phi discovery. Only
after the discoveries of the T lepton and the D mesons were sorted out was
the data completely understood. This is a strange repetition of history:
Just as the discovery of a first generation meson (pion) was confused by
the unexpected appearance of second generation lepton (muon), so now the
interpretation of the second generation mesons (J/phi and D) is confused by
the appearance of an equally unexpected third generation lepton (tau).  The
fact that the e and mu leptons possess different associated neutrinos
strongly suggests that the tau lepton also possesses an associated neutrino,
giving six leptons in all.

1977 Discovery of particles, called Upsilon, containing yet another quark
(and its antiquark), by Lederman and collaborators at Fermilab. It is
called the "bottom" quark, charge 3. It gives added impetus to the search
for a sixth quark ("top"), so that the number of quarks would equal the
number of leptons and the third repeat of the pattern of particles in the
Standard Model (third generation) be complete.

1979 The Nobel Prize is awarded to Glashow, Salam, and Weinberg for their
role in the development of the electroweak theory, four years before the
observation of the W+, W- and Z bosons predicted by their theory, but after
the discovery of weak neutral currents, that is, processes mediated by a Z
boson. This indirect evidence was sufficiently convincing to physicists
that the theory must be right.

1983 The W+/- and Z intermediate bosons demanded by the electroweak theory
are observed by two experiments using the CERN synchrotron, converted into
a proton-antiproton collider by van der Meer and his team. The
observations are in excellent agreement with the theory. The spin of the
W+/- could be measured: It is 1h, as required by the Standard Model. The
1984 Nobel Prize was awarded to Rubbia and van der Meer for this work.

1989 Studies at CERN and SLAC of properties for the Z in e+e- collisions
establish the existence of exactly three low-mass neutrino species with
standard interactions, strongly implying that only three families of
fundamental particles exist.

1989 Millions of Z's produced by LEP collider at CERN permit study of the
properties of the Z and its decay products in great detail and provide
precise tests of the Standard Model.

1989 Measurements of the mass and width of the W+, at CERN and at
Fermilab, together with the properties of the Z, provide further tests of
the electroweak aspects of the Standard Model. Other tests, for example,
the energy dependence of the rate of producing multiple particle clusters,
provide equally strong support for the strong interaction (QCD) aspect of
the Standard Model.

1995 After 18 years of searching at many accelerators, the CDF and D0
experiments at Fermilab discover the top quark at the mass of about 175
GeV. No one understands why the mass is so different from the other five
quarks. In fact, no one understands any of the patterns of quark and
lepton masses. The Standard Model can fit them but does not predict them.