Professor Dave again, let's discuss
quantum chromodynamics.
We saw how quantum electrodynamics takes
the electromagnetic force and explains
it in terms of quanta, an exchange of
particles that mediates the force,
virtual photons in this case. There are
three more fundamental forces to go: the
strong and weak nuclear forces, as well
as gravity, and for a coherent view of
the universe we must be able to explain
each of these forces with its own
quantum field theory. For the nuclear
forces this will require that we
familiarize ourselves with a brand new
type of particle: the quark. In general
chemistry we learned how JJ Thomson's
cathode ray experiment brought about the
realization that atoms were not the
smallest things, and we soon realized
that atoms are made of protons, neutrons,
and electrons. But as it turns out,
protons and neutrons are in turn made up
of smaller particles still, and these are
quarks of different varieties. The name
quark comes from the writings of James
Joyce, who spoke of three quarks for
Muster Mark. As quarks typically come in
groups of three, the title seemed apropos.
These quarks also come in three kinds of
color charge, which refers to the three
kinds of color perceived by humans: red,
green, and blue. This does not have
anything to do with the actual
phenomenon of color, it is just a way of
categorizing quarks, and since chroma
means color in Greek, quantum
chromodynamics became the name of the
quantum field theory that deals with
quarks. Protons and neutrons are each
comprised of three quarks, which are
forever bound. Quarks of different color
charge exert an attractive force between
one another, like the electromagnetic
force between particles of opposite
electrical charge. But the difference is
that the electromagnetic force weakens
as particles of like charge are pulled
further apart, whereas
the attraction between quarks will
strengthen as they are pulled apart, so
they always stick together. Protons are
made of two up quarks and one down quark,
while neutrons are made of one up quark
and two down quarks. Up quarks have a
two-thirds positive charge and down
quarks have a one-third negative charge
so doing the math, we can see how we
arrive at +1 and zero as the charges on
the proton and neutron. There are other
types of quarks beyond the up and down quarks.
There are also top, bottom, charm, and
strange quarks, which have different
properties. We must understand that the
attractive force between quarks is what
keeps the particles of the nucleus together.
It is the strong nuclear force, capable
of keeping positively charged protons
together against their electromagnetic
repulsion. And just like QED did for the
electromagnetic force, QCD shows that the
strong nuclear force is mediated by the
exchange of quanta called gluons. It is
the exchange of these gluons, which come
in eight varieties, that keeps an atomic
nucleus stable. We can see that our
quantum field theories are getting more
complicated, as QED needed just one
particle, the photon. The quantum field
theory that governs the weak nuclear
force needs three particles, in the form
of W and Z bosons. But the strong nuclear
force, as we said, requires eight gluons.
Okay, let's slow down for a minute. Quarks,
bosons, gluons... what is going on? What
happened to just plain old protons,
neutrons, and electrons? As much of a
headache as you may now have, we need to
accept that the development of quantum
theory brought along with it dozens of
new particles that need to be
categorized and understood. Luckily, we
have a model that organizes all of them
nicely, so let's learn the basics about
all these particles next.
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