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01/18/2000
Where' the Spin
"Where''s the beef?" was a catch phrase some years ago, both in
the award-winning TV commercial and as a rhetorical question
relating to the substance of a political debate. Now, there''s
another question, this time in physics, "Where''s the spin?" In a
way, this is a follow-up to an earlier column on ballet dancers and
their knack for controlled spinning on and off the floor. In
another way, it''s sort of like the quest for the identity of all that
dark matter out in space. Only in this case it''s like searching for a
missing ingredient in the tiny nucleus of the atom.
But let''s first go back to the prehistoric days of my youth. When
I first learned about atoms, things were really pretty simple. Let''s
take the simplest atom of all, ordinary hydrogen. We were taught
that the center, or nucleus, of the hydrogen atom was this dense,
positively charged proton, essentially a little round ball with a
charge on it. Whirling around this proton was this really teensy
thing with a negative charge called an electron. The reason the
electron was kept in this orbit was that opposite charges attract
each other. When I got to college and graduate school, I found
out there was something called quantum mechanics and things
weren''t quite so simple. Most upsetting to me was that you
couldn''t pin this electron down precisely and it could wander
around a good bit. In fact, it could be treated like a wave and I
learned about an equation called Schrodinger''s (with an umlaut
over the "o") equation and was even fooled into thinking I
understood how to solve it. When solved, the electron was
smeared out like a cloud around the proton, but I contented
myself by thinking that the electron was just flitting hither and yon
so rapidly that if you took a time exposure, it would look like the
calculated cloud.
Through all this, at least the proton was still a simple, solid chunk
of stuff, one of the handful of fundamental particles of physics.
Then the theorists began theorizing and the experimentalists
began bombarding the proton with other particles. Having
nothing better to do, the theorists decided that they should try to
explain the origins of such things as the charge of the proton and,
in addition, the "spin" of the proton. This "spin" is pretty much
like the spinning ballet dancer''s angular momentum except that
the values of the spins of particles such as the proton are limited
to or characterized by numbers like 0, 1/2, 1, 3/2, etc. This has
something to do with Planck''s constant, which we mentioned a
few weeks ago. But no matter, you don''t have to know any more
now because physics community finally decided that our proton
was not so simple after all.
What they came up with was that the proton is actually made of
point-like things called "quarks". It is appropriate that the term
"quark" was coined by physicist Murray Gell-Mann based on a
passage in "Finnegan''s Wake", by James Joyce. I once tried
reading a sentence or two of Joyce''s "Ulysses" and, frankly, I find
quarks to be just as understandable! Well, these quarks have
spins and charges of their own. In fact, the charge of a quark is
either a positive or a negative 1/3 or 2/3 of the charge on a proton
or on an electron. This really bugs me! We were always taught
that you couldn''t have a fraction of a charge. The theorists also
decided that there should be several kinds of quarks. Theorists
can name things anything they want if they''re first to think of
them and they named these new particles up and down quarks,
top and bottom quarks and strange and charmed quarks. In fact,
not too long ago, there were headlines in the daily papers that
physicists had finally found the last of the predicted quarks,
fittingly, the top quark. This of course made everybody happy.
Now that we know quarks are real, let''s get back to our proton.
It turns out, as we said, that the proton is not just a glob of
fundamental stuff. It is actually made up of two up quarks and
one down quark. Each quark has a spin of 1/2. With three
quarks, if two of them were lined up in opposite directions their
spins cancel and the result is that the third quark is responsible for
the 1/2 spin found for the proton. Everything seems hunky dory.
But not for long. These three quarks are flitting around inside the
proton at tremendous speeds, approaching the speed of light.
You might wonder, "Why don''t they just fly apart?" It turns out
the proton is a most stable piece of work. Indeed, there have
been many experiments trying to detect the decay of a proton into
something else, without any success. As I recall, it''s been shown
the typical proton will hang around for at least many billions or
trillions of years. So what holds the quarks together? You know
the answer. You have to put some glue on the quarks to hold
them together. So, what do you call these things? Of course,
you call them "gluons". See, you don''t have to be all that smart
to be a theorist!
Naturally, we wouldn''t be talking about these gluons if there
weren''t confirmation that such things exist. The gluons are sort
of fleeting particles that the quarks toss back and forth and the
result is like a bunch of rubber bands holding the quarks together,
what is called the "strong force". Things get more complicated
because these gluons, of all things, also have spins. In fact, each
has a spin of one. Remember that we want the spins to sum up to
give 1/2, the spin of the proton. Now, with gluons and quarks,
it''s a bit more complicated to figure this out.
This was the picture until the 1980s, when the experimental guys
came up with a real shocker. They found that NONE of the spin
of the proton came from the quark spins! The beginning of
"Where''s the spin?" As the theoretical guys thought some more
and the particle energies got higher and higher, the picture of that
simple proton got even more complex. Today, the picture is that
there are not only these three quarks and various gluons rattling
around in that tiny proton but blinking and dancing in and out are
anti-quarks. Now, I don''t have the foggiest idea how this
happens or what an anti-quark, or even a quark for that matter,
looks like. But I do know that when you have stuff and anti-stuff
of any kind, when they bump into each other they both go "poof"
and are annihilated. A spectacular example of this is found near
the center of our Milky Way, where there is a humongous cloud
of radiation formed by electrons being annihilated by positrons,
the antimatter counterpart of the electron.
In fact, one of the burning questions in physics today is what
happened to all the antimatter that was formed after the Big
Bang? It seems that there should have been equal amounts of
stuff and anti-stuff and you would expect that all the stuff and
anti-stuff would have gotten together and there should be nothing
left, including us! As I understand it, at one point after the Big
Bang there was a "soup" of quarks and anti-quarks. Current
thinking is that there was a very slight, maybe one in one hundred
million, excess of stuff over anti-stuff and that, fortunately for us,
there was enough left-over ordinary stuff to form us and our
universe as we know it.
Well, back to the proton. There is still a major problem trying to
figure out the contribution to the spin of all these exotic things
flying around that make up the proton. Even the most powerful
computers are not enough without some advances in the theory to
allow calculations to be made. However, you might think about
the fact that when you drink those 8 glasses of water every day,
you''re drinking over a trillion trillion hydrogen atoms (not to
mention the oxygen atoms) and heaven knows how many quarks,
gluons and antiquarks! With all that spinning going on, maybe it''s
no wonder your stomach occasionally rebels and growls.
After finishing this column, I read the obituary of Dr. Karl
Strauch, former head of the Harvard physics department, in the
New York Sunday Times. Dr. Strauch, who died at 77 a couple
weeks ago, was cited as being among the first to see evidence that
quarks existed when he was director of the Cambridge Electron
Accelerator. Later, he also worked at the Stanford Linear
Accelerator, where much of the key work on quarks was done. I
visited the Stanford facility a year ago and would highly
recommend if you can arrange a tour to take it if you''re in the San
Francisco area. Even for someone totally unfamiliar with physics,
the size and complexity of the equipment, the arrangements of the
particle detectors and the interpretation of the vast amount of
data are all tremendously impressive. A common feature of
nuclear physics papers is that the list of authors may run to 20 or
more individuals, not surprising when you see the facility in
person.
Note: For anyone wanting to delve further into the proton, a
good place to look is in the article titled "Mystery of the Nucleon
Spin" in the July, 1999 issue of Scientific American. There are
some neat schematic pictures of what''s going on.
Allen F. Bortrum
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