Previous: The IPBES Report with Brit Garner | SciShow Talk Show
Next: A Cancer Gene May Be More Friendly Than We Thought | SciShow News



View count:242,408
Last sync:2022-11-20 11:15
In 1956, a team of scientists conducted an experiment that, seemed kind of trivial, but the results would challenge one of our fundamental beliefs about the entire universe.

Hosted by: Hank Green

SciShow has a spinoff podcast! It's called SciShow Tangents. Check it out at
Support SciShow by becoming a patron on Patreon:
Huge thanks go to the following Patreon supporters for helping us keep SciShow free for everyone forever:

Avi Yashchin, Adam Brainard, Greg, Alex Hackman, Sam Lutfi, D.A. Noe, Piya Shedden, KatieMarie Magnone, Scott Satovsky Jr, Charles Southerland, Patrick D. Ashmore, charles george, Kevin Bealer, Chris Peters
Looking for SciShow elsewhere on the internet?
Sources: [PDF] [PDF] [PDF] [PDF] [PDF]

Image Sources:

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 at the time, the electrons should have come out in random directions, but that is 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 violated 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.  Like, normally, you would say that gravity pulls things down, but of course, we only call it down because that's where gravity is pulling things.  If you change your perspective and stand on your head, that doesn't mean that gravity is going to start pulling everything, like, down toward your feet and up toward the sky.  It's gonna 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 very 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 particle 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 was 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 the cobalt that would cause it to decay in a specific direction.  Instead, if you did happen to see more electrons coming out in a certain way, it would mean you had a problem on your hands.

If, say, more electrons came out of 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 some equipment to figure out how many electrons came out and in which directions and the 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 in the opposite direction of the atoms' spin.  So if you swapped left and right in this experiment, in other words, if you gave the atoms clockwise spin instead of counterclockwise spin, 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 of 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 now we had opened a gigantic can of physics worms.

For one thing, researchers had to grapple with the fact that directions might not be as arbitrary as they once thought, because apparently, there is 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 anti-matter, which we believe should not happen, and it even treats time differently than the other forces, which is exactly as bizarre as it sounds.  

Scientists believe that if we figure out 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 than we could have ever imagined and it set us on a path to understanding the laws of the universe, a path that we are still walking down today.  

But that is a much bigger story and if you want to learn more about it, we have a whole other video on it, which you can watch right now, and as always, thank you for watching this episode of SciShow.