A force which is responsible for all of the byproducts
of nuclear interactions. It is the only mechanism which can convert
nuclear binding energies into electromagnetic energy, and hence heat and
mechanical energy. This includes
the changing of one flavor of quark to another
The weak nuclear force is carried by three vector gauge bosons, the
W+, W-, and Z0
particles. These particles can travel over distances of 10-17
m or less. Certain particles, the various types of
neutrinos, can interact with other particles only via the weak force.
In 1935, Japanese physicist Hideki Yukawa formulated
his theory of the "nuclear force" which binds the
protons and neutrons in an atomic nucleus together. He accepted the
existence of Wolfgang Pauli's theoretical particle, the neutrino, and
proposed the existence of a new particle which transmitted the force between
However, Yukawa's theory went a little too far: he proposed that this
new particle was responsible for all nuclear interactions, including a
type of radiation called beta decay. The forces involved in beta
decay, however, were much weaker than the forces holding atomic nuclei
In 1940, Enrico Fermi suggested an alternate theory for beta decay
that did not involve Yukawa's particle. It explained beta decay better than Yukawa's
theory, and furthermore, explained the existence of a new particle that
had been discovered in the meantime, the muon. Part of this theory
involved a weaker nuclear force which acted on leptons and hadrons
In 1946, Yukawa's new particle was discovered by British physicist Cecil
Powell, and named the pi meson or "pion", showing that Yukawa's explanation
of the strong nuclear force held some merit.
Meanwhile, particle accelerators around the world started producing
new "strange" particles whose masses were betwen that of the pion and the
nucleons. Furthermore, the strange particles would quickly decay
into normal particles. Yukawa's model couldn't explain all of this.
Physicists made up a new quantum property called "strangeness" but were
at a loss to explain it.
In 1956, American physicists Tsung-Dao Lee and Chen Ning Yang
suggested that beta decay could be explained if the parity of a particle's
spin was not conserved during certain interactions. They won the 1957
Nobel Prize for Physics for this theory.
In 1958, Richard Feynman and Murray Gell-Mann unified the concepts
of symmetry-breaking and the weak nuclear force.
Early in the 1960's, Gell-Mann also proposed a new theory of the strong
nuclear force, involving new particles called "quarks". This was
not widely accepted at the time, and we'll get back to it later.
Throughout the 1960's, new theories called "gauge theories" began
to appear, using group theory from mathematics to unite and explain
the phenomena that had been observed. Sheldon Glashow, Abdus Salam,
John Ward, and Stephen Weinberg proposed that the weak nuclear force
was carried by three new "vector boson" particles they called the W+,
W-, and Z0 particles.
This "electroweak theory" also showed that the electromagnetic force
and the weak nuclear force acted like the same force at high-enough energies.
It also explained why spin symmetry was broken.
In the early 1970's, the theory of quarks started gaining acceptance:
This theory was developed into Quantum Chromodynamics, or QCD, involving
a new "color force" which was the real force behind the puppet strong
nuclear force. One consequence was that most of the behaviors of
nuclear decay involved a quark's changing from one flavor to another inside
of a hadron.
Finally, in 1983, researchers at CERN managed to produce Z particles
in their experiments, confirming electroweak theory. Carlo Rubbia
and Simon van der Meer won the 1984 Nobel Prize for Physics for this
Sources: (my own prose)
Quarks: The Stuff of Matter by Harald Fritzsch (1983 English
Translation) Basic Books, New York.