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Smaller than an atom, but majorly important: introducing the quark! Quarks helped make sense of particle physics, and we'll tell you all about it in this new episode of SciShow!

Hosted by: Stefan Chin

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Physicists strive for simple explanations for complicated phenomena. It’s kind of their thing.

But sixty years ago, particle physics was not simple. There were dozens of known subatomic particles that all seemed to follow different rules at different times, and nobody knew what to make of all of them. Then, a few physicists found the simplicity hiding behind the chaos: quarks.

And fifty years after their discovery, the universe just doesn’t seem to make sense without them. Quarks originally helped explain a torrent of particle physics discoveries that happened after World War II. But our knowledge of particles goes back further than that.

Before the war, physicists knew about familiar subatomic building blocks like protons and electrons. And they knew that particles sometimes decayed:. Some of those building blocks could transform into other particles, although the rules were still being worked out.

In general, though, it seemed like there weren’t that many fundamental pieces of matter. The world was pretty simple. Then, experiments got better at crashing those pieces into each other and seeing what happened.

By 1950, the list of known particles included pions and kaons and sigmas with new particles being found all the time. Soon, there were dozens of types of them. And no one knew why they were showing up or how they might be related to more familiar particles.

But throughout the fifties there were hints at some sort of underlying order. Physicists quickly realized that the new particles were much more like protons and neutrons than electrons or photons. They also noticed that the particles didn’t come randomly.

There were family relationships. For example, after the pion was discovered they found a particle that acted a lot like the pion, but had the opposite electric charge. Then, there was a third particle that acted like both of those, but had no charge at all.

So eventually, physicists just grouped them together as positive, negative, and neutral pions. Meanwhile, some particles, like the kaon, took strange-looking paths through particle detectors. So scientists said that the kaon had -- and I’m not making this up -- a quality called “strangeness”.

And they grouped together particles with strangeness and particles without it. There were lots of other groupings floating around, too, with different people thinking different ones were more or less important. But in 1961, a physicist named Murray Gell-Mann took a huge step forward.

He found ways of grouping particles together that naturally included lots of the other ways people had proposed over the previous decade. His groups came in a few different sizes, but since they most commonly contained eight particles,. Gell-Mann called his idea “The Eightfold Way”, after the Buddhist path to enlightenment.

Some of his groups were incomplete, though. For example, the Eightfold Way said that if you grouped certain particles by strangeness, the group should have ten members. But physicists didn’t find the tenth member until 1964 -- a few years after Gell-Mann predicted it.

Still, that was a good sign. Once they found that Omega-Minus Baryon, as it’s now known, everyone knew Gell-Mann was on the right track. Then, around 1964, Gell-Mann and two other physicists named George Zweig and André Petermann all independently came up with the same explanation for the Eightfold Way’s success:.

Each of the particles was itself made of a few tiny, indivisible pieces. Petermann wanted to call the pieces “spinors” and Zweig wanted to call them “aces”. But Gell-Mann had a way with naming things, and his name for them stuck: quarks.

By combining just three types of quarks -- called “up”, “down”, and “strange” quarks -- into collections of two or three, you could produce all the particles physicists had seen over the previous few decades. Except for things in the same family as electrons. But those have their own models.

And there were only a couple of those at the time, anyway. Admittedly, though, the quark model did have some weird features. It said that, unlike every particle ever observed, quarks didn’t have an electric charge that was a whole number times the charge of an electron.

Instead, they had fractional charges: either negative one-third or positive two-thirds. And that struck everyone as very weird. It was unlike every particle they’d ever seen.

But there was a reason they’d never seen fractional charges: Over the next decade, physicists working with the model realized that you could never see an isolated quark on its own. Quarks could only ever be in groups of two or more, because there was just too much energy sitting in the space between groups to ever tear them fully apart. Again, it’s very weird.

But just like the Eightfold Way, the quark model caught on because it worked:. It was a simple explanation for complex mysteries. It explained why neutrons — which are, like the name says, electrically neutral — act like they have electric charges in them in specific cases:.

They’re made up of two down quarks and an up quark, which each have charges. And it naturally explained strangeness, too:. Particles with strangeness were particles with strange quarks in them.

The real kickers came in 1968 and 1974, when two different experiments showed conclusive evidence that the quark model was right. In 1968, experiments at the Stanford Linear Accelerator fired electrons into protons. They found that they bounced off in ways that only made sense if the protons were made of individual pieces like quarks, instead of being solid balls of charge.

Then in 1974, experiments at two different labs revealed a new particle that couldn’t exist in the original three-quark model. Which would have been a disaster -- if physicists hadn’t already extended the original model to predict a fourth quark, called the “charm”. Today, we know of only two more quarks: The bottom quark and the top quark.

Which were originally called “truth” and “beauty”, but apparently that was just too much. And there are good reasons to think that that’s it: Our universe has six total types of quarks, and no more. With just those quarks, plus the forces between them and a couple other particles that are cousins of the electron, we can figure out a ton.

We can understand the behavior of the hundreds or even thousands of different types of particles that are created all the time in particle accelerators. And that’s the kind of simplicity physicists strive for. Ah, Physics.

It’s so beautiful. Just like a good spreadsheet or really flexible Trello board. I co-host SciShow with Hank, Olivia, and Michael, but I’m also the producer for the channel, so I spend a lot of my creative energy and time organizing the workflow of the team.

People always ask us how we put out more than a video per day with such a small team. And it’s because we’re very organized. We have multiple spreadsheets, docs, calendars, Trello boards, and slack channels to manage it all.

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And, if you want a trademark Stefan Chin productivity tip, watch it at 2x speed. [ ♪ Output ].