Strong Force
Chalky
When I was a young student, I thought that I had a reasonably solid
understanding of what the strong force (aka strong nuclear force) was,
namely, the force that holds nuclei together against the
electromagnetic repulsion of protons, as expounded again now (imo very
well) at http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html#c2.
However, I have noted in the meantime, that some sources eg
http://en.wikipedia.org/wiki/Strong_interaction have subverted that
original definition, within the context of QCD, to now mean the force
that holds quarks together, with the strong nuclear force now
relegated to the altered name of "residual strong force".
In contrast,
http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html#c2
describes the force that holds quarks together as the color force, so
that the strong nuclear force (aka strong force) can alternatively be
described as the residual color force.
This leaves me a little confused as to which should actually be
considered correct, nowadays.
Can anyone clarify here?
Tom Roberts
> This leaves me a little confused as to which should actually be
> considered correct, nowadays.
Nobody else has responded, so here's the perspective of a particle
physicist (experimental) with distant knowledge of basic nuclear physics
and a working knowledge of the standard model.
First, when you say "holds quarks together" this does NOT mean that
quarks have constituents that must be "held together". Quarks are
fundamental, point-like, and have no internal structure in the standard
model, which is the relevant theory. This is talking about holding
quarks together to form a nucleus.
A proton's size is on the order of 1 fm (10^-15 meter), as is a
neutron's. Typical nuclei are only slightly larger (~8 fm for U238).
Nuclear structure has been reasonably well described by a shell model,
that considers nuclei as collections of protons and nuclei bound by an
unspecified "strong" force that is able to overcome Coulomb repulsion as
long as there are sufficient neutrons present. In this model, n-p, n-n,
and p-p strong forces are approximately equal, but the last also has a
Coulomb repulsion. This model considerably predates both QCD and the
standard model.
Ostensibly in QCD the nucleus could be just a collection of a handful of
"valence quarks" (3*A for a nucleus with atomic number A), plus a bunch
of gluons, transient virtual quarks, and other stuff. But if that is so,
how could the nuclear shell model work at all? After all, neutrons and
protons are color singlets, so there is no direct color force between
them. But it's clear that the neutrons and protons in a nucleus are
separated by distances comparable to their size, so just like EM dipoles
attract, so too can the color force "see" inside the neutrons and
protons, but with less "strength" than the color forces holding the
neutrons and protons together individually -- a "residual color force"
analogous to the EM force between dipoles.
The EM force holding an atom together is the same EM force that attracts
dipoles to each other and makes van der Waals forces between atoms and
molecules. So too the color force holding neutrons and protons together
inside a nucleus is the same color force holding them together to form
the nucleus. But those dipoles and nuclei are held together less
strongly, because it is not the direct force but rather the gradient of
the force that holds them together (this is speaking rather loosely,
especially in the case of nuclei -- asymptotic freedom and quark
confinement imply the color force has a character quite different from
the EM force).
This is obviously all hand-waving. I am not an expert on QCD, nuclear
physics, the nuclear shell model, or the standard model, but as a
particle physicist that's how I see this: there is only one color force,
but apparently it is energetically favorable for nuclei to consist
mostly as a set of color singlets (neutrons and protons, and perhaps
mesons as well) bound together comparatively weakly (but still strongly)
by "residual" color forces between those singlets, because the singlets
are not point-like and are very close together. Remember this is a
quantum world at this scale, and everything is "fuzzy"....
So, for instance, I expect that the valence quarks of
the neutrons and protons in a heavy nucleus are continually
re-arranging themselves, so the individual neutrons and
protons don't last very long, but on average the nuclear
"stuff" arranges itself roughly as neutrons and protons.
(This is more hand-waving, as this is really superposition
of states, not "re-arranging"....)
Tom Roberts