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

  • Beta decay
  • nuclear fusion
  • 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 nucleons.

    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 together.

    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 alike.

    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 effort.

    Sources: (my own prose)

    Quarks: The Stuff of Matter by Harald Fritzsch (1983 English Translation) Basic Books, New York.


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