(TheScientificTruth) writes:

Note that since this last term is what provides the "binding energy"
that holds that atoms and molecules together, its sign is actually
negative, not positive --- i.e., unlike the previous three terms, it
_decreases_ the effective mass of the molecule, rather than increasing it.

Ok, so I think I understand the molecular rest mass (restmass of
particles+kinetic energy of particles-potential energy of electromagnetic
interaction). I also think you answered something else that has been
bothering me about particle/nuclear/atomic/molecular(yes, I like them
all) physics, which is:

Why is the rest mass of a nucleon greater than the sum of the rest
mass of its constituents, but the rest mass of a nucleus is less than
the sum of the rest mass of its constituents.

Based on your answers to my previous posts, I take it that the reason
the above is so is that the combined kinetic energies of quarks are
far greater than the potential energy holding them together, but the
potential energy holding a nucleus together is greater than the
kinetic energy of its constituents.

That is correct. However, please not that the two cases are not directly
comparable, because the "nuclear force" is a _short-ranged_ effective
force, whereas the "color force" between quarks is thought to be a
nonlinear, long-ranged force that has an unusually property called
"confinement," which means that the potential energy of a pair of particles
carrying different "colors" increases without bound as they are separated,
so that, theoretically, an "isolated" free quark would have an _infinite_
color potential energy! Also, the "color" potential energy of a group of
particles is _NOT_ just a simple "linear superposition" sum of pairwise
interactions, like the electromagnetic force, but a nonlinear interaction
that depends on _all_ the particles in a "cluster." (In that respect,
the "color force" is vaguely like General Relativistic gravitation, which,
while not "confining," also becomes nonlinear in the "strong field" limit,
and fails to satisfy either "superposition" or "cluster decomposability.")

Thus, unlike a nucleus, which will "blow apart" if one adds enough kinetic
energy to it, if one adds lots of energy to the quarks in a nucleon, it is
energetically cheaper to instead convert some of that kinetic and potential
energy into one or more quark/antiquark pairs, which will ultimately result
in some number of "colorless" hadrons instead of "free quarks."


-- Gordon D. Pusch

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