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.