Imagine a scenario where we’ve made  contact with a group of friendly aliens.

We’ve spent a long time sending  messages back and forth, teaching each other the things  we know about the universe. At some point, we get around  to discussing human greetings, like shaking right hands.

But hold on … how will we define right  vs left to our long distance friends? Lucky for us, there’s a physics experiment  that can demonstrate right and left, so we teach them the  experiment and they repeat it. Eventually, we decide our  civilizations should meet, and you’ve been chosen as the human ambassador.

As a gesture of goodwill, the alien  reaches out to shake your hand. But they’ve held out a hand on their left side. Should you shake it?

This scenario was posed by the famous physicist Richard Feynman during a lecture, and it was repeated to me as a homework problem in my quantum field theory class. And the answer to this riddle is one of  the most important discoveries in physics. [intro music] An important lesson that we learn as kids is how to tell our right from our left. But as far as many of the  laws of physics are concerned, it’s a meaningless distinction.

After all, right and left are  just mirror images of each other. Like, if I turn my hands inward, my right hand turns counterclockwise  while the left turns clockwise. The actions are mirrored, but I’m doing the same motion with both hands.

A spiral galaxy looks like it’s  turning in a specific direction. But if you were to look at the galaxy from below,  its rotation would appear to be a mirror image. In this case, all you did  was change your perspective.

No matter which direction it turns,   or where you view the galaxy from,  the laws of physics are the same. The forces that hold the galaxy together, and the forces that make the stars shine, don’t seem to care how you define  clockwise or counterclockwise. This right-left symmetry is  what physicists call parity.

And for a long time, physicists assumed our world would be entirely indistinguishable  from a “mirror world” where right and left were flipped. Everything in this reversed world  would still have all the same physics. In other words, they thought that “right”  and “left” were just human distinctions.

They assumed there was no objective  way to define those things. Experiments had already shown  that gravity and electromagnetism showed no preference for right or left. Neither did the strong force,  which holds atomic nuclei together.

But there was a building  suspicion that the weak force, the one responsible for radioactive  decay, might violate the parity “law”. It took a clever experiment, performed by an expert in radioactive  decay, to set things right. Or, uh, left?

Chien-Shiung Wu was born in China, and came to the US in 1936 for graduate school. She received her PhD from UC  Berkeley in 1940 and then, like many other physicists in America at the time, was recruited to work on the Manhattan project. She ended up as a professor  at Columbia University, and became known as the leading expert  in a certain type of radioactive decay called beta decay.

Beta decay happens when a  neutron turns into a proton, releasing an electron and an  antineutrino in the process. Electrons are sometimes called beta particles, which is how the beta decay process got its name. In 1956, one of Wu’s colleagues approached her with the idea of testing the concept  of parity symmetry using beta decay.

Some theoretical calculations had suggested  that beta decay might not preserve parity… that it might reveal a true difference  between our world and a mirror world. And Wu immediately realized the idea could  be tested using radioactive cobalt atoms. Cobalt-60 atoms have 27 protons, and 33 neutrons.

They are unstable, and decay into Nickel-60 atoms. Her experiment would measure the direction  the electrons were shed during the decay. The key was that Cobalt-60 atoms  can act like little compass needles.

She had to make the experiment really cold, so that the atoms wouldn’t  be jostled around by heat. But then she was able to use a really strong  magnet to align the atoms with a magnetic field. By convention, physicists say  that an atom aligned to the   North pole of a magnetic field is pointing “up” and that it is spinning in a counterclockwise  direction when viewed from above.

So with most of the atoms pointing North, and therefore spinning the same way, Wu was able to see which direction the cobalt  atoms emitted electrons as they decayed. There were two possible outcomes: Either the electrons would fly  to the north and south equally, indicating that parity doesn't matter. Or they would favor one  direction, meaning that it does.

For hundreds of years, physicists would have predicted the  electrons should have no preferred direction. Yet the experiment found that  when cobalt atoms pointed north, the electrons tended to go south. This proved the weak force behaves differently than the other three fundamental forces.

And it proved that nature does care about parity. To start with, this experiment  showed that right versus left was more than just a human construct. If you have all the tools  to build this experiment, then you can define right  versus left in terms of physics.

Put some cobalt atoms in a magnetic field, and tweak the field until electrons are  mostly emitted in the same direction. Whichever direction your cobalt atoms  are spitting out most of their electrons, call that “south”. That makes the opposite direction “north”.

Then you can define “right” versus “left”. The magnetic field is created by  loops of wire carrying a current. If you orient “north” so that it  points from your feet toward your head… or whatever the alien equivalents are… then the current in the wire loop will be  flowing away from you on the right side, and toward you on the left side.

Wu’s experiment forced everyone to do a  hard reset on their expectations of physics. Physicists quickly got down to  the business of reworking their   theories to accommodate the “death of parity”. From this chaos, important theories  such as the Standard Model emerged, as well as electroweak theory, a single equation that can describe the electromagnetic and weak forces.

Wu had revealed deep truths about the Universe, and sparked a million new questions. Her experiment has gone down in history as one of the most important in all of physics. Consequently Wu, known to many  of her colleagues as Madame Wu, was dubbed the “Queen of Nuclear Research”, the “First Lady of Physics”, and was often compared to Marie Curie.

Before I tell you what this has  to do with the left-handed alien, here is a quick ad. Thank you for watching SciShow! Every  time you watch one of our videos, it helps them get out to even more people.

So thank you for supporting  the channel with your time. And if that made you feel all warm  and fuzzy, you’ll love this next part. We have even more ways to support  the videos you like to watch.

Coming November 3 — the second annual Complexly Learnathon is a celebration of learning and  the free content we make here at SciShow, Crash Course, and all of Complexly! We’re doing activities, videos,  and livestreams all month. And we have lots of opportunities  for you to support our shows, including some very cool new merch.

Mark your calendars and go  to complexlylearnathon.com to check out the schedule of events! The following year, in 1957, Wu’s colleagues who worked on  the theories of parity violation won the Nobel Prize in Physics, while she was left out. Even as they accepted their prizes, they acknowledged how critical her contributions were to the success of their theory.

Systemic injustice aside, Wu’s work made lots of new things possible. Including our riddle of the left-handed aliens. Once we teach them the experiment, then we’ll share a common  definition of left versus right.

Just like we’d be able to use experiments  to agree on the speed of light, or the definition of a hydrogen atom. So when we finally encounter the aliens in person, and their ambassador reaches out a left  hand, one of two things must be true: Either something was lost in translation, or the aliens come from the “mirror  world” where right and left are reversed. Now, from a distance a “mirror world”  would look just the same as ours.

Atoms, stars, galaxies, and even life could exist, just with the basic definitions  of right and left flipped. But there is one catch about this  hypothetical “mirror world”... The Wu Experiment showed that the  only way for “right” and “left” to be flipped is if the charge of  every particle is also flipped.

The atoms in the “mirror world” must be made of positively charged antielectrons and negatively charged antiprotons. In other words, a “mirror world”  is also an antimatter world. And when matter and antimatter meet, the result is a burst of energy as  the particles annihilate each other. there’s a lot of matter and antimatter combining … let’s just say you don’t want to be nearby.

So if the alien ambassador  offers you their left hand, either they got really nervous and forgot  which hand we prefer to shake with … or they’ve done the Wu  experiment in a “mirror world”. And if they’re from a “mirror world”,  they must be made of antimatter. In which case you certainly should not shake hands unless you want to be destroyed  in a blast that releases 150,000 times as much energy  as the Trinity nuclear bomb.

This isn’t just a silly,  hypothetical thought experiment. Okay well, it mostly is. But this riddle also points to one of the  biggest unresolved questions in physics:   where is all the antimatter, anyway??

Antimatter exists, and it’s possible  for mirror worlds to form from it … so where is all of it? In our neighborhood of the Universe, there’s a lot more matter than antimatter … after all, we’re here! We weren’t annihilated before we could form because  there wasn’t enough antimatter around to oppose all our normal matter.

And that isn’t something that  our current theories can explain. Physicists are still searching  for clues about antimatter, and it’s tied up in this unexpected  violation of parity symmetry. For all we know, there might be a “mirror world” on the other side of the Universe  asking these exact same questions.

Maybe in the distant future, the answer will come down  to a near-miss handshake. [ OUTRO ]