When scientists first began to realize that atoms were
made of smaller particles, they also realized that there had to be some
force that held those particles together.
The development of QED in the first half of the twentieth century
showed that electrons were bound to atoms by the electromagnetic force
alone, it remained to show how the particles left over, atomic nuclei,
were held together.
All nuclear particles were positive or neutral, and much closer together
than the electrons, which meant a different (and much stronger) force had
to be responsible. And thus was born the "nuclear force".
Later, it was shown that beta decay operated by a different mechanism
from that holding together atomic nuclei (an isolated neutron decays into
a proton). Furthermore, the interaction that controlled beta decay was
shown to be much weaker than the one holding atomic nuclei together, so
the former was named the "weak nuclear force" and the latter the "strong
nuclear force".
The strong nuclear force, then, consisted of interactions between certain
types of particles, named "hadrons". The vast majority of hadrons were
the "nucleons", that is, protons and neutrons in atomic nuclei, interacting
by exchanging pi mesons.
However, particle accelerators kept turning up more and more exotic
hadrons, and there wasn't a very good theory to explain them.
Then, in the 1960's came Murray Gell-Mann and his theory of "quarks".
This wasn't widely accepted at first, but eventually, scientists began
to see that it was the only theory that explained the zoo of particles
they had discovered.
Quark theory eventually developed into its current version, QCD, with
quarks being inextricably bound together by the color force.
However, notice that QCD doesn't directly explain the "strong
nuclear force", meaning the mechanism that holds nucleons together in atomic
nuclei: For hadrons to interact, they can't exchange naked bits of color,
because naked bits of color aren't allowed to exist. They have to exchange
color singlets: mesons or glueballs.
In the end, atomic nuclei exist as a side effect of the color
force:
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A hadron is perturbed away from another one by quantum fluctuations we
can't measure. This builds up "stress" in the color field, that is, potential
energy.
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Sometimes these perturbations cancel each other out.
-
Rarely, perturbations or external events exceed the binding force. Then
we say the nucleus is undergoing some process of radioactive decay.
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Often, however, the system builds up enough potential energy to create
a pi meson, but not enough to break away:
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A pion is created.
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The creation of the pion converts the potential energy to matter and mechanical
energy.
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The pion travels from one nucleon and is consumed by the other one.
-
The system's potential energy being reduced, the two nucleons move closer
together.
I can't explain to you how this interaction would occur using glueballs,
but I suspect it would be analogous.
If you think about the mechanism I just described a minute, you will
realize that the "strong nuclear force" is the QCD analogue of the electromagnetic
"Van der Waals" forces that hold certain molecules together (such as
the layers of graphite).