{♫Intro♫}
In 1956, a team of scientists was working
on an experiment that, at first glance, seemed
kind of trivial.
They had lined up a bunch of cobalt atoms
and were patiently waiting to see which direction
they would spit out some electrons.
According to the knowledge of the time,
theelectrons should have come out
in random directions
— but that’s not what happened.
Instead, the electrons tended to favor
a specific direction.
And to me, and probably you,
this doesn’t seem like that big of a deal.
But the implications
of this tiny, quiet experiment were groundbreaking.
Because these results weren’t just about
some specific atoms:
They challenged one of our fundamental beliefs
about the entire universe
and ultimately opened the door to some of
the biggest mysteries in physics.
Here’s why this little experiment turned out
to be such a big deal.
The reason these results threw physicists for a loop
is because they violate something called
parity symmetry.
At its heart, parity symmetry says
that the laws of physics shouldn’t differentiate
between left and right, or up and down,
or backward and forward.
Because, really, those directions just depend on your perspective.
Take gravity, for example.
Normally, you might say gravity pulls things down. But of course, we only call it down
because that’s where gravity is pulling things.
But if you change your perspective and stand on your head, that doesn’t mean gravity
is going to start pulling everything “down” toward your feet and toward the sky.
It’s going to keep pulling stuff toward the Earth, no matter what direction you say
that is.
And if that sounds obvious… well, yeah.
For decades, parity symmetry was this reasonable, inarguable thing, one that physicists had
used countless times to predict — correctly! — the results of experiments.
It felt like common sense, and was a major assumption we relied on when figuring out
how stuff should work.
But by the 1950s, some researchers had begun to realize that maybe we shouldn’t always
be making this assumption.
In particular,
two researchers pointed out that parity symmetry had been tested, but not in all circumstances.
Like, it had never been tested in certain particles decays.
So three teams of researchers decided to tackle this question — and one of them was responsible
for that now-famous cobalt experiment.
This group was headed by a researcher named Chien-Shiung Wu, and they studied a type of
radioactive cobalt called cobalt-60.
When the cobalt decayed, it spat out electrons. And if parity symmetry were true, those electrons
should have come out about equally in all directions.
It should have happened like this mainly because these atoms are basically sitting still, and
you can more or less ignore gravity when it comes to particle physics.
So it’s not like there’s some force on this cobalt that would cause it to decay in
a specific direction.
Instead, if you did happen to see more electrons coming out a certain way, it would mean you
had a problem on your hands.
If, say, more electrons came out the tops of the cobalt atoms rather than the bottoms,
it would mean that electrons — for some reason — had an easier time moving up than
down.
And according to parity symmetry, there shouldn't be any difference between those directions.
After all, what’s up from one perspective is down from another.
To test if all of this were true, Wu’s team used a magnetic field to line up all their
cobalt in the same way. Then, they set up equipment to figure out how many electrons
came out, and in which directions.
And their results were a bit of a shock.
Instead of seeing the particles come out in random directions, they found that the electrons
tended to come flying out in the opposite direction of the atom’s spin.
So if you swapped left and right in this experiment — in other words, if you gave the atoms
clockwise spin instead of counter-clockwise, you would get a different result: You would
see the electrons fly out in a different direction.
And that’s not supposed to happen!
These results suggested that there is some kind of fundamental difference between left
and right. The universe somehow has a sense of direction.
So, yeah, something was very wrong with parity symmetry: It didn’t exist!
Wu’s team published their findings in January 1957, as did two other teams that had done
similar research on other particles.
But just because we had made this discovery didn’t mean we were out of the woods.
We might have figured one thing out, but a gigantic can of physics-worms had also been
opened.
For one thing, researchers had to grapple with the fact that directions might not be
as arbitrary as they once thought. Because apparently, there’s an innate left and right
to the universe
— which is absolutely bizarre.
But they also had to figure out why this happened. What was so special about cobalt that made
it act this way?
Well, as research went on, it turned out that the cobalt wasn’t necessarily the problem.
It was something called the weak nuclear force, which is the force that governs how atoms
— including cobalt — decay.
For some reason,
the weak force tends to act differently than the other fundamental forces of physics.
And figuring out why is one of the most ambitious projects in the field right now.
Because the weak force doesn’t just treat left and right differently. It also treats
matter differently than antimatter, which we believe shouldn’t happen.
And it even treats time differently than all the other forces — which is exactly as bizarre
as it sounds.
Scientists believe that if we figure one one more weird thing about the weak force, it
could potentially break our understanding of physics.
But then again… it could also help us understand why the universe looks like it does.
So a tiny experiment from 1956 didn’t just affect how we saw a handful of atoms.
It taught us that one of the four fundamental forces of physics is more strange then we
could have ever imagined. And it set us on a path to understanding the laws of the universe
— a path we’re still walking down today.
But that’s a much bigger story.
And if you want to learn more,
we have whole other video
on it!
But as always, thanks for watching
this episode of SciShow.
{♫Outro♫}