The entire universe can be represented by the few particles included in the
table shown on this page.* It would be even more amazing, then, if these 19
were made of the same elemental constituent. If superstring theory turns out to
be true, this will be the case. According to the theory, every one of the
"fundamental" particles are made of identical strings, with the only difference
between the strings being their vibrational patterns (see Resonance in
Strings for more on this).
The Standard Model, Plus Gravity
On one hand there is quantum mechanics and the standard model, which describes
the world of the very small. On the other hand there is general relativity,
which describes gravity and the world of the very large. With one exception,
all of the particles listed on the table are explained by the standard model.
The exception is the graviton, the force particle behind gravity.
Superstring theorists believe they are on their way to developing a framework
that incorporates both general relativity and quantum mechanics. If they
succeed, they will have discovered a theory that solves the greatest problem in
physics—that of unifying the laws of nature.
The graviton is the theorized force carrier of gravity. Gravity is by far the
weakest of the four forces—much weaker even than the weak force. The reason
gravity can seem like a substantial force is that it can only attract, and the
more the mass is concentrated in an area, the stronger the gravitational force.
Electromagnetism, on the other hand, is a repulsive force as well as an
attractive force. Its positive and negative charges tend to cancel each other
out, which neutralizes the overall force.
The graviton has not yet been observed or otherwise proven to exist. However,
the rate at which the spin of neutron stars slows down is consistent with
calculations that include the radiation of gravitons.
The photon is the carrier of all electromagnetic radiation, including radio
waves, light, X-rays, and gamma rays.
Photons and their associated electromagnetic field also keep atoms together from
the attraction among negatively charged electrons and positively charged atomic
nuclei. The electromagnetic force is also responsible for the interaction
between atoms and molecules—without this force, your finger would pass
through your computer mouse (not to mention the atoms and molecules that make
up your body would no longer be bound together).
Weak gauge bosons
W+, W-, and Z0 are the weak gauge bosons,
carriers of the weak force.
The weak force triggers radioactive decay by causing a down quark within a
neutron to change to an up quark. This change transforms the neutron into a
proton, releasing an electron in the process. The weak force can also transform
a proton into a neutron.
Carlo Rubbia, Simon Van der Meer, and a team of 130 physicists
discovered the weak gauge bosons in 1983.
The gluon, which is the carrier of the strong force, works at tiny distances—10-13 cm or less. The strong force works like glue, tightly binding
quarks together in groups of threes to form protons and neutrons and in other
combinations to form other particles.
The strong force also holds together the protons and neutrons that comprise an
atomic nucleus. Without the strong force, the protons would move away from each
other due to the repulsion caused by their like electrical charges.
Indirect evidence of the gluon was first detected at the Deutsches
Elektronen Synchrotron in Hamburg, Germany, in 1979.
Particle physicists expected the weak gauge bosons to be massless particles.
Instead, these bosons have been shown by experiment to have substantial mass.
The Higgs boson was postulated to explain this mass.
According to the hypothesis, the Higgs field slows down the weak gauge bosons,
which would otherwise travel at the speed of light. This slowing down gives the
bosons an effective mass. At high enough temperatures, the effects of the Higgs
field disappear, allowing the weak gauge bosons to travel at the speed of light
and to become massless.
The Higgs boson, named after the man who postulated its existence,
Peter Higgs, explains why the weak gauge bosons are not massless particles.
Family 1 Leptons and Quarks
The electron is a negatively charged particle that surrounds the nucleus of an
atom. Because of its negative electrical charge, the electron is attracted to
the positively charged protons within the atom's nucleus, keeping them bound to
the nucleus. The way in which electrons assemble in the shells around an atom
helps to determine the chemical characteristics of that atom.
The electron's negative charge also keeps atoms from passing through one
another—the repulsion caused by the like charges keeps the atoms apart.
The electron, discovered in 1897 by J. J. Thomson, has the
distinction of being the first subatomic particle detected.
The neutrino is able to travel great distances through matter without
interacting with it—about 600 trillion neutrinos from the sun passes
through your body every second. Neutrinos shoot through the Earth virtually
unimpeded, so the number moving through you is the same even if it is
nighttime and the sun is on the other side of the Earth. The reason is that
they are massless or nearly massless and that they interact with matter only
through gravity and the weak force.
In 1930 Wolfgang Pauli predicted the existence of the neutrino. The
particle was shown to exist by Clyde Cowan and Fred Reines in 1956.
The least massive of the six quarks, the up quark combines with the down quark
to make up the matter that we experience in daily life.
Although individual quarks have never been observed, there is indirect evidence
that convinces particle physicists that they do exist.
In 1964 Murray Gell-Mann and George Zweig independently announced
the theoretical existence of quarks.
Down quarks and up quarks make up the ´protons and neutrons that form
atomic nuclei and therefore they are responsible for the vast bulk of all matter
that we experience and see.
Two down quarks and an up quark comprise a neutron (-1/3 + -1/3 + 2/3 = 0).
One down quark and two up quarks comprise a proton (-1/3 + 2/3 + 2/3 = 1).
Murray Gell-Mann's and George Zweig's quark hypothesis was confirmed
in 1968 at the Stanford Linear Accelerator Center.
Family 2 Leptons and Quarks
Like the electron, the muon is a charged particle. Unlike the electron, it is
more massive and it is unstable; two-thirds of all muons decay into an
electron, a muon neutrino, and an electron anti-neutrino within about two
microseconds after they come into existence.
The muon was the next elementary particle discovered after the discoveries of
the electron, the proton, and the neutron.
Jabez C. Street and Edward C. Stevenson found evidence of the muon
Discovered in 1961, the muon neutrino was shown to be a different particle than
the electron neutrino.
Electron neutrinos are associated with a type of radioactive decay that
produces an electron along with a neutrino. The type of radioactive decay that
produces muon neutrinos also produces muons, which are elementary particles that
are like electrons but more massive.
Credit for the discovery of the muon neutrino goes to Jack
Steinberger, Melvin Schwartz, and Leon Lederman.
The charm quark, which is similar to the up quark but more massive, was the
fourth quark found, following the discoveries of the up, down, and strange
The charm quark was indirectly discovered simultaneously in 1974 at
the Brookhaven National Laboratory and at the Stanford Linear Accelerator
The strange quark was given its name because of its "strange" behavior. When it
was first discovered, it lived much longer than had been predicted.
Like other quarks, evidence of the strange quark's existence was indirect—it
was found within a larger particle that contained it along with an up quark and
a down quark.
In 1947 George Rochester and C.C Butler discovered the "V" particle
(from a cosmic ray), which was later shown to be associated with the strange
Family 3 Leptons and Quarks
The tau lepton is identical to the electron, except that it is 3,500 times
heavier and unstable. It exists for less than a trillionth of a second before
it decays into other particles.
The tau lepton was discovered in 1975 by Martin Perl and a team of
30 physicists at the Stanford Positron-Electron Asymmetric Ring.
Only recently discovered by an international collaboration of physicists, the
tau neutrino is the most massive of the three types of neutrinos. Its existence
was confirmed when a particle that could only have been a tau neutrino hit the
nucleus of an atom and created a tau lepton.
The first direct evidence for the tau neutrino was discovered in
2000 at the Fermi National Accelerator Laboratory in Illinois.
The top quark was the sixth and final quark to be detected. The mass of a
single top quark is equivalent to that of the nucleus of an atom of gold, which
contains 197 protons and neutrons. (These 197 protons and neutrons are made up
of 591 up and down quarks!)
The top quark was discovered in 1995 at the Fermi National
The bottom quark was the fifth quark detected. Its discovery led physicists to
predict that there would be a sixth quark (the top quark), which was later
discovered in 1995.
The bottom quark was discovered in 1976 at the Fermi National
More About this Table
Listed here are the main forces, matter, and actions that each particle is
generally associated with. Exceptions are the fermions that comprise Family 2
and Family 3 particles. These fermions are, for the most part, detected only in
According to superstring theory, each elementary particle is made of a tiny,
fundamental string. And because mass and energy are equivalent (as described by
Einstein's famous equation, E=Mc2), the mass of each "particle" is
determined by the string's energy—the higher the energy of the string, the
greater the mass of the particle. The figures displayed here are in units of
millions of electron volts (MeV) when the particles are at rest.
A charge of 1 means that the particle has a positive electrical charge, a
charge of -1 indicates that the particle has a negative charge, and a charge of
0 means that the particle does not interact with the electromagnetic force.
Quarks, which have fractional charges, can combine in threes to form protons
and neutrons, which have charges of 1 and 0, respectively. Quarks can also
combine in pairs to create particles with charges of +1, 0, or -1.
Experiments reveal that every elementary particle has an intrinsic angular
momentum—or a property called spin. All fermions have a spin of 1/2, while
all bosons have a spin that is a whole integer. According to string theory,
each elementary particle is made of a tiny string, and each string's spin is
associated with the string's vibration.
The four forces, which are carried by bosons and operate over different
distances, vary in strength, with the strong force being the strongest and
gravity weakest. The figures displayed here are all relative to the strong
force (which is why its strength is listed as "1") at a distance of
10-13 cm. Strength is not applicable to fermions because they do not
Some force-carrying particles—such as the photon and the graviton—have
infinite range, while others work over only very short distances. The values
shown are in centimeters. Range is not applicable to fermions because they do
not carry forces.
With the help of instruments such as particle accelerators, physicists are able
to see deep within the sub-atomic world and verify the existence of various
particles. Sometimes there is only indirect evidence of particles, as is the
case with all types of quarks and the gluon.
Unlike the other data displayed in this column, the information shown here does
not show attributes of the elementary particles but rather the "super
particles" (sparticles) associated with each particle. Sparticles are
alternate, perhaps more massive, versions of elementary particles, predicted by
supersymmetry. Physicists have not yet verified the existence of these
*The table lists 19 known particles (17 of which have been experimentally
verified, though some of these indirectly). The matter and forces we
experience, however, can be represented by about half that number. This table
does not list any antiparticles.