Forces, in the subatomic world, have a different character than we’re used to when we hear
the word “force.”
For instance, when we hear the word “force” we might be thinking about pushing a car,
or military might or maybe even “these aren’t the droids you’re looking for.”
But in the quantum realm, forces occur when a particle carrying some kind of charge emits
another particle transmitting a force.
I made a video about quantum electrodynamics, which is the theory of when a particle carrying
electric charge emits a photon and another video about quantum chromodynamics, when a
particle carrying the strong charge emits a gluon.
But in this video, I’d like to talk more about the strong charge. It turns out to be
more complicated than the familiar positive and negative charges of electromagnetism.
So how did people figure this all out?
It all started back in 1964 when Murray Gell-man and George Zweig independently proposed what
we now call quark theory. We know a lot more about quarks then they did back then, but
they proposed that there were three types of quarks, with the names up, down and strange.
Each quark was a fermion, which means that they carried subatomic spin and that there
were only two kinds of spin: +1/2 and -1/2.
Protons and neutrons, which are examples of a class of particles called baryons, each
contained three quarks. Protons contained two up quarks and a down quark, while neutrons
contained two downs and an up.
But, beyond the fact that baryons contain exactly three quarks, there were no rules
on which quarks could exist in a baryon.
So that means that all other configurations should exist. They are listed here: “up,
up, up”; “up, up down”; “up, down, down”; and “down, down, down” for example.
And then, there are the ones containing strange quarks, like “up, down, strange”, et cetera.
In fact, when the quark model was proposed, the configuration “strange, strange, strange”
hadn’t been found. However, less than a month after the quark theory paper was released,
the Omega minus particle was discovered. The Omega minus was a particle with three strange
quarks. Things were looking good.
There was a problem though. There is a principle of quantum mechanics that governs the behavior
of fermions, which is what quarks are. This rule is called the “Pauli Exclusion Principle,”
named after Austrian born physicist Wolfgang Pauli.
What Pauli’s principle says is that it is impossible for two identical fermions to exist
at the same place. This, by the way, is why electrons surround atoms in the way that they
do. If Pauli was wrong, chemistry would be enormously different.
So what does the exclusion principle have to say about quarks?
Well suppose that we have a baryon with three of the same kind of quarks, say “up, up,
up” for example.
From an exclusion principle point of view, having three up quarks in the same place sounds
ominous. But spin helps us. The first up quark could have a spin of +1/2 and the second could
have a spin of -1/2. The differences in the spin make the two fermions different. So-
so far, so good.
But now, let’s add the third up quark. No matter if it is a spin +1/2 or -1/2, it will
duplicate the spin of an existing quark. And, according to Pauli’s principle, such a particle
cannot exist.
Yet they do. With the discovery of the omega minus, all the possible configurations of
three quarks in a baryon had been found. So either the quark idea was wrong, or someone
needed to work out an answer.
In October 1964, an American physicist named Oscar Greenberg (although he prefers to be
called Wally) supposed that perhaps each quark had another property that we now call color.
Color, by the way, is the name that we use for the strong force charge, but we’re getting
ahead of ourselves.
Color would then come in three varieties. Quarks would have color, but particles like
protons would not. Thus we need to somehow invent an idea where if we added three different
things, they would cancel each other out and effectively add to zero. That’s like adding
a positive and negative number and getting zero, but with three things and not two.
Luckily, there is historical precedent for such a thing. In 1861, legendary physicist
James Clerk Maxwell was working with a photographer by the name of Thomas Sutton. They were playing
around with colors and light. They found that if they projected a red, blue and green light
on a wall, that the combined three colors looked white.
Taking this analogy, we now name the colors of quarks red, blue and green and take as
a metaphor the color white as having no color.
In the context of the quark model, we now can have three up quarks or three down quarks
or three strange quarks. Since each quark has a different color, then no two quarks
are the same and the Pauli Exclusion Principle causes no problems.
Proving that color is a real thing is rather tricky and has to be done indirectly. It boils
down to assuming that there are three different quark strong charges and seeing the implications
of that idea. When we do that, we find that we can explain all the data involving quark
interactions. So scientists are now very confident of the color idea.
The invention of the idea of quantum color is a pretty interesting story. Prior to the
quark model, physicists were discovering all sorts of particles without any sort of unifying
idea. The quark model was a piece of the tale, but the story was made more complete with
the invention of color. And, a little more work resulted in the creation of the full
theory of quantum chromodynamics which, of course, I’ve made another video about.
But in this video, I want to concentrate on Wally Greenberg’s epic idea that made the
world a colorful place.