Traditionally, scientists say that there are
four fundamental forces in the universe. Actually, that number is fuzzier than you
might think, and, the more that I think about it- there’s probably a video in that. But let’s stick with tradition for now. The four fundamental forces are gravity, electromagnetism,
the strong nuclear force, and the weak nuclear force. I’ve made videos about the first three,
but haven’t really addressed the weak nuclear force. I saved the weak nuclear force for last because,
well, in many respects, it’s really mind-blowing and exhibits behaviors that are completely
shocking. For instance, it’s the only force that is
aware of the spin of the particles involved and whether they are matter or antimatter. So how does that work? Well, to understand that, you need to know
something about subatomic spin. In the classical world, spin is pretty easy. You spin an object around an axis, like you
see me doing here with this ball and you can spin it one way or the other. Now, scientists can define a direction for
the spin, which is a little counterintuitive. The direction for the spin is along the rotation
axis, but there is an ambiguity. After all, the axis extends both above and
below the object. So, to break the ambiguity, you can take your
right hand- by the way, it’s important that you use your right hand- and let your fingers
wrap in the direction that the object is spinning, and the direction of your thumb is the spin
direction. In fact, once you’ve done that, you can
kind of ignore the idea that the object is physically spinning at all and just rely on
the direction of your thumb. And, simplifying a little bit more, you can
replace the thumb with an arrow. The direction of the arrow contains all information
about the spin of the object. In the classical world, the length of the
arrow depends on how fast the object is spinning. A faster spinning object has a bigger arrow
and a slower spinning object has a shorter arrow. In the subatomic world, these classical ideas
aren’t strictly correct, but they are still somewhat useful. The direction of the spin of an object can
be represented by an arrow and the length can represent the amount of spin. There are definitely major differences between
classical and quantum spin. For instance, quantum objects don’t actually
spin. In addition, the spin axis has to be parallel
or antiparallel to the direction of motion. This is obviously not true in the classical
world, but, well, the quantum world is very counterintuitive. Also, in the subatomic world, particles like
electrons and neutrinos can have only a spin of plus or minus a half. The plus or minus half is used to indicate
whether the “thumb” of the spin direction is in the direction or opposite the direction
of the particle’s motion. Things get even more complicated if the object
isn’t moving, but in the case I’m about to talk about, that particular situation isn’t
relevant. So in the 1950s, scientists had tested interactions
using both the strong force and electromagnetism and had shown that neither of them cared a
hill of beans about the direction of the spin of the particles involved. However, in 1956, two Chinese theoretical
physicists, Tsung-Dao Lee and Chen-Ning Yang, dug through the literature and found that
nobody had tested whether the weak force cared. Now neither of these guys knew how to test
this because they were- well- theoretical physicists. So they turned to a female experimental colleague
by the name of Chien Shiung Wu. The female thing will be relevant in a moment. Wu was an impressive scientist. She was the best in the business when it came
to studying the spin of atomic nuclei. So what she decided to do was to set up an
experiment in which Cobalt 60 would decay via the weak force into Nickel 60, an electron,
and an electron antimatter neutrino. In order to test whether the weak force cared
about spin and spin directions, Wu had to set up a very strong magnetic field to align
the spin of the cobalt nuclei. Spin is one of those things that is conserved,
which means that it has to be the same before and after an interaction. The spin of cobalt 60 is 5 and the spin of
the form of nickel 60 into which it decays is 4. The spin of both the electron and neutrino
is a half. So we can see in this diagram what has to
happen in terms of spin. We can even put in some plus signs and an
equal sign to really make the point. In terms of the decay, what you’d expect
to see is that during the decay, the electron and antineutrino would fly away along the
direction of spin of the cobalt nucleus or the opposite direction. And, if the weak force doesn’t care about
spin, the electron moving up and antineutrino moving down configuration would occur as often
as the electron down and antineutrino up configuration. Because it’s very hard to detect neutrinos,
she just looked for the direction the electron flew. So what did she see? Well she saw the most unexpected thing. The electron always flew away in the direction
opposite of the spin of the cobalt nucleus. Because momentum is conserved, that meant
that the antineutrino always travelled in other direction. So what did that mean? It meant that the weak force had a preference
for certain spin configurations. After some thought and careful study, what
we now know is Wu’s observation originates from a very peculiar property of the weak
force. This feature is the following thing- there
is a very strict rule about the spin of neutrinos and the direction they’re traveling. The rule is that the spin of a neutrino is
opposite the direction the neutrino is traveling and the spin of antineutrinos is parallel
to the direction they’re traveling. We physicists have a jargon for this sort
of situation using hands. Take both hands and point your thumbs in the
direction a particle is moving. If the fingers of your right hand curl in
the sense that the particle is spinning, we say that it is a right handed particle. If the fingers of your left hand curl in the
direction of spin, it is a left handed particle. And you can see here how the right/left handed
jargon is related to the arrow method of labelling motion and spin directions. Okay- now we’re ready to talk about what
this all means. An incautious individual would say that Wu
found that all neutrinos are left handed and antineutrinos are right handed. But that would be a hasty statement. The more accurate statement is that since
neutrinos are only observed via weak force interactions, we can say that the weak force
only interacts with left handed neutrinos and right handed antineutrinos. And, generalizing further, the weak force
interacts with left handed particles and right handed antiparticles. So this is a big deal. The other forces don’t care about the handedness
of the particles they interact with, but the weak force does. This discovery blindsided the scientific community
and required them to entirely rethink the theory of the weak force. It also raises the question of right handed
neutrinos. Do they not exist? Or do they exist, but we just can’t see
them because the weak force refuses to interact with them? The story of the search right handed neutrinos
is an interesting one and worth a video in and of itself. If only there were someone with a history
of creating videos about particle physics who could make one… hmm… And, about that female thing. I sketched out the most basic description
of Chien Shiung Wu’s experiment, but the reality was unbelievably subtler and more
complex. It was an experimental masterpiece. Plus, the result overturned our understanding
of the weak force. If you’re a serious student of science history,
it’s worth looking up. With such an accomplishment, you’d think
that she’d be a shoo-in for science’s highest honor. And, indeed, the 1957 Nobel Prize in physics
was awarded for this line of work to- you got it- Yang and Lee. Wu was entirely overlooked. Now, I wasn’t on the committee at the time-
in fact, I wasn’t even born- and maybe the committee had a strong bias for theoretical
instead of experimental work, but I can’t help but wonder if gender didn’t play a
role. Maybe not. I don’t know. But there is no question in my mind that Wu
should have received part of that Nobel prize. Anyone who can revolutionize our understanding
of the universe as much as she did is simply a magnificent scientist.
