How did we discover the weak nuclear force? Why is it so strange compared to the other forces? What do mirrors have to do with all of this? I discuss these questions and more in today’s Ask a Spaceman!

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Hosted by Paul M. Sutter.

 

EPISODE TRANSCRIPT (AUTO-GENERATED)

One of my favorite stories about Enrico Fermi, of which there are many to choose from, is what he did during the test of the first nuclear weapon, the Trinity Test, the culmination of the Manhattan Project on July 16th, 1945. Robert Oppenheimer, the leader of the Manhattan Project, would later say that during the test and the witnessing of the fireball that, quote, a few people laughed, a few people cried, most people were silent. Well, he left out what Enrico Fermi was up to. Enrico Fermi had a bunch of bits of paper, some confetti in his hand. And when the fireball came and as the shockwave was approaching their position, their observation position, Fermi let the confetti drop from his hand. And he watched as the blast wave came by, the wind blew and blew the confetti down some distance away before it hit the ground. Now, why was he doing this? He was not throwing a party. He was making a measurement.

He was estimating the energy released during the first nuclear test explosion by calculating how far the shock wave, miles away, would displace some bits of paper. Remember at the time it was all theoretical, we didn't know exactly how well the weapon would work, we didn't know exactly how powerful it would be. Fermi estimated that it was roughly equivalent to around 10,000 tons of TNT. The actual number is somewhat higher, 18 or so kilotons of TNT. But that's not far off for a calculation involving tiny bits of paper. And this is just how Fermi worked. He just was able to come up with ingenious solutions, think through things, a brilliant theorist and experimentalist, all-around awesome guy. He's kind of a big deal. He certainly deserves a BioPod episode all of his own, so feel free to ask about the life and times of Enrico Fermi. Fascinating character. We've met him before with his famous Fermi's paradox about alien life.

We've met him before with his coining of the word neutrino, which is Italian-ish for little neutral one. But what I haven't told you yet is two things. A, he is one of the few humans ever to live on the planet to discover a new force of nature. And B, he was called the Pope of Physics. Now we're going to spend plenty of time today on part A, But can we just talk about part B for a second, the Pope of Physics thing? Seriously, Pope of Physics, he got the nickname in his homeland, Italy, because he was essentially infallible. When he said something, he was just right all the time. And I gotta say, Pope of Physics does roll off the tongue better than Nobel Laureate, which he also was, because of course he was. But I don't know if I won the Nobel Prize, but I also had the nickname of Pope of Physics. I'm sticking with that. Hello everyone, introducing our guest speaker today, Paul Sutter. He is the Pope of Physics. Okay, okay, side fantasy over.

Let's talk about the whole discovering a new force of nature part. Back in the 1930s, physicists were doing experiments involving what they called beta decay. Okay, we still also call it beta decay, hasn't changed the name, but it was just, it's called beta decay. It's where randomly an element just sitting around minding its own business would suddenly spit out a fast moving electron. And once it was done doing that, the element would be different. Sometimes it'd be a different isotope of the same element. This is where you have the same number of protons, but you change the number of neutrons. And sometimes you'd have a different element altogether. So the question on everybody's mind was, How? How exactly did this decay process unfold? How can I have an element sitting here, and then out of nowhere, it spits out an electron, the electron has tons of energy, and now I have a different element? That's weird. Fermi had an answer. A new force of nature. And a new particle.

He would call the new particle the neutrino. but with the new force of nature involved. He realized that whatever is happening inside of an atomic nucleus, and at this time, we didn't know exactly what was going on inside of atomic nuclei. We knew that atomic nuclei were things. We knew that there were something vaguely resembling protons and neutrons. We had not learned about quarks yet. We had not learned about the strong nuclear force yet. But what he realized is that this process of beta decay. At the time, we only knew of two forces of nature. We knew of gravity and electromagnetism. That was it. If you wanted to do something in the universe, you had two choices. You could use gravity. You could use electromagnetism. I suppose there's a third where you could use both at the same time, but you get the idea. And Fermi's like, well, this beta decay process where an element changes to a different element by spitting out an electron, I think gravity ain't going to cut it.

I think electromagnetism isn't going to cut it. I think we need a new force of nature. Fermi hypothesized that some new force could change a proton into a neutron or vice versa, and in the process release an electron in a neutrino. that there was some force of nature that could change the properties of protons and neutrons, could change how atomic nuclei work. It was just a guess, but Fermi, it was just a guess, but Fermi had a habit, he had a knack for this, Pope of Physics over here, of being right when he guessed. According to physics lore, we're not exactly sure if this is true, he submitted this paper with a tentative proposal, the tentativo he called it, to the journal Nature, the most prestigious journal in the world. Nature rejected it because it contained speculations too remote from reality to be of interest to the reader. He was then able to publish in Italian and German journals, and his work wouldn't get an English translation until 1968, decades later.

Apparently, he was so discouraged by the rejection that he took time off from theoretical work to focus on nuclear experimentation, a little side quest that would lead to the birth of the atom bomb and a Nobel prize, but he proposed this new force in this new particle. Others would pick up on the pieces that Fermi laid down, and eventually everyone would recognize that he was right. There is a new force of nature. This is our first new discovered force of nature. We've kind of always had gravity and electromagnetism. We've always known about it. Gravity's always been the thing. It was the first force to be described mathematically by Newton. And we've always had electricity and magnetism. You know, there's the great James Clerk Maxwell sitting there in the mid-1800s who realized that electricity and magnetism are the same thing, labeled under one thing, the electromagnetic force. But they've always been there. We've always had lightning bolts. We've always had stuff falling down.

But the weak force was new, and Rico Fermi was the first person to identify a new force of nature. We call it the weak nuclear force, and right from the beginning, the weak force stood apart. It was unexpected. Nobody asked for it. We knew about gravity. We knew about electromagnetism. Even the strong force, which would come later, we had hints of because once we started digging into atoms, we're like, oh, well, you know, I think there needs to be a strong force of nature. No one expected the Spanish Inquisition. No one expected the weak nuclear force. Experiments forced us to acknowledge it with no theory or observational reality of a millennia of human experience to prepare us for what it does. And so maybe because of that, because we're so used to how gravity operates, intuitively, we know how gravity operates. And it's been the job of physics to explain that mathematically. Intuitively, we know how electromagnetism works. We have a feeling for it.

We don't have a feel for the weak nuclear force. And so it just, it ends up when this episode, I'm just going to list the weird stuff that the weak nuclear force does that no other force does. And that's just the way it is. And we're just left here scratching our heads thinking, why is the weak nuclear force so weird? I don't know. It just is. Why does the weak force have the properties that it does? I don't know. It just does. Now, we know we've advanced since the days of Noriko Fermi. He thought that maybe this strange force of nature had the power, had the ability to change protons into neutrons, neutrons into protons. That is how you are able to get one element changing into another. You gotta change the protons, you gotta change the neutrons, that's how you get different isotopes, that's how you get different elements. Now we know that protons and neutrons themselves are composite particles, they're bags of smaller things called quarks.

But now we know the weak force operates on quarks. It can take one kind of quark and change it into another kind of quark. You change the quark, you change a proton into a neutron, or vice versa. You change one kind of quark into another. A proton is two up quarks and a down quark. You flip one of those and you say, you're not going to be a top anymore. You're going to be a, you're not going to be an up anymore. You're going to be a down. Now you got two downs and an up. Now you're a neutron. That's what the week four does. It flips quarks back and forth. which is weird. We're used to forces changing properties of objects. I push on something, it moves. I let go of something, gravity pulls it down. I stick something in the microwave, the radiation heats it up. Forces do things. They push, they pull, they energize, they contribute to Patreon. That's patreon.com slash PM Sutter. Sometimes if you just feel alone and weird and like you don't fit in, go to patreon.

com, contribute to the show, and you have a new best friend. Me, kinda sorta yes, but more importantly, the weak nuclear force. The weak nuclear force is that kid in the back of the room that doesn't quite fit in. And if you're feeling like that, you can commiserate with the weak nuclear force. You're not alone in the universe. Patreon.com slash PM Sutter. We're used to forces changing properties of objects. That's kinda what forces do. But now we have a force, the weak nuclear force. That's not changing. It looks like it's not changing a property of a quark, but the quark itself. How do you change an up quark into a down quark? We're talking about a real fundamental particle here. The quark is just there. I can't hand you a single up quark. It's complicated. They always bind together. But imagine I did. I'm like, here's a gift. Here's an up quark. Then the weak force can just come on in and say, magic now it's a down quark. We're used to forces changing properties of objects.

We're not used to forces changing objects themselves. Well how do we think about this? How can we explain this? Well hold on to your popats here. Maybe what we call different quarks are really just different properties of quarks. Hear me out, hear me out. There are six kinds of quarks. Up, down, top, bottom, strange, and charm. Why six? I don't know. Different episode. Go ahead and ask. There are six quarks, but instead of thinking of these six quarks as different particles, I want you to think of them as the same kind of particle, but with a different flavor. And yes, flavor is the real physics word we use, no joke. Instead of six quarks and you hold in your head, well, there are six quarks. There are six particles that we call quarks. I want you to think of, no, there's just quark There's one kind of particle called quark, but it can come in six different flavors. Like, like ice cream. Mint chocolate chip and vanilla are different flavors of ice cream.

We understand them to fundamentally be ice cream, just different types. If you go to the ice cream store and you're looking at the, you know, the array of flavors and you're taking the samples and maybe you're on your fourth sample and somehow you can't quite decide when you're sampling butter pecan. You don't think of it as its own entity of I am now sampling butter pecan. No, you're thinking of it as a flavor of ice cream. You go over there and there's, I don't know, strawberry rhubarb. Is that a flavor of ice cream? It should be, because strawberry rhubarb pie is amazing. Anyway, it's a real risk for me to record these episodes before lunch, by the way. All the food metaphors come out. When you encounter strawberry rhubarb ice cream, you're not in the ice cream store, you're not like, what is strawberry rhubarb doing here? You're thinking, no, this is a, it doesn't belong here, strawberry, no, it's a flavor of ice cream.

It still contains the essence of ice cream, deliciousness, creaminess, et cetera, but there are different variations. And you can imagine changing one flavor of ice cream into another. You can add and remove ingredients. and still keep it ice cream. You're not changing the ice cream, you're changing the flavor of ice cream. You can go in there and pluck out every chocolate chip and then you can toss in some cookie dough bites. You still have ice cream, you're just changing the flavor. Quarks are like ice cream. I know when I talked about neutrinos a bunch of episodes ago, I also used an ice cream analogy, and you're lucky I just didn't choose a cheese analogy here, but it still works because when we talked about neutrinos and flavor, how they're different neutrino flavors, it's the same as quarks. They're different quark flavors. There is just quark, and quarks come in different flavors.

They aren't necessarily different particles, or the best way to think about this isn't exactly to think about them as different particles, but different flavors of the same kind of particle. So now we can put the weak force in the proper context. The weak force is not changing one particle into another, it's changing a property of a particle. It's not changing its momentum or its energy level, it's changing the flavor. The weak force has the power unlike any other force of nature. This is what the weak force can do. The weak force doesn't push, it doesn't pull, it doesn't repel, it changes flavor. It looks at a quark, it can change the flavor of a quark. It looks at a neutrino, it can change the flavor of a neutrino. That's what the weak force does. It's all good, right? The weak force totally makes sense and isn't confusing at all as long as we use the right magic words to describe the math. That's not going to last long.

So the weak force has this power that no other force of nature has. It has the power to change flavors. And if you're a kind of particle like a quark that exists in multiple different kinds of flavors, boom, it could wave its magic wand and turn you from mint chocolate chip over to Neapolitan. I guess that's three flavors the analogy does work great just turned you into strawberry let's keep it simpler that's cool it's also appropriately named the weak force is weak it has uh to describe how weak it is we use something called the coupling constant which is a fancy word to describe how rare an interaction can happen with a particular force like if i put to electrons nearby each other, and I want to estimate the chances that they will interact through, say, the electromagnetic force, then I can assign a number to that. And then if I put them together and I want to estimate the chances that they will interact through the weak nuclear force, I assign a number to that.

That number that I assigned is called the coupling constant. The weak nuclear force's coupling constant is 100,000 times smaller than the electromagnetic force's coupling constant. That means if I put two electrons together, then 99,999 times they will interact through the electromagnetic force, and once they'll do it through the weak nuclear force. That's how weak it is. It doesn't turn on often. It doesn't do its thing very often. It also has incredibly short range. If I want particles to interact through the weak nuclear force, they have to get around 10 to the minus 18 meters. That's a thousand times smaller than an atomic nucleus. At that range, the weak force at 10 to the minus 18 meters, the weak force has the same strength as the electromagnetic force, but just an order of magnitude larger, like 10 to the minus 17 meters, already the weak force is 10,000 times weaker than electromagnetism. The weak force is weak.

It doesn't have very big range, and its interactions are incredibly rare. And this incredible weakness of the weak nuclear force comes from another property of the weak force that is totally unlike the other forces, and that has to do with what carries the weak force. Just like we're used to forces changing properties of objects, we're used to forces being carried by something, especially in the language of quantum mechanics and quantum field theory. If I want to interact with you, I have to exchange something. We have to share something between us. If I want to interact with you electromagnetically, I have to use photons. If I want to interact with you with a strong nuclear force, I have to use gluons. If we want to become friends, we need cheese. If gravity, well we don't really talk about gravity, we suspect that there are things called gravitons, the carriers of the gravitational force.

But all these force carriers, the gluons, the gravitons, the photons, the carriers of the other forces, they have something in common, they're massless. Photons and gluons have no mass. But the carriers of the weak force, they have mass. And there are three of them, by the way. That's weird. There's only one graviton. There's only one photon. There are nine gluons, but that's a different topic. There are three carriers. There are three ways that the weak nuclear force can interact with the rest of the universe. And the three carriers, the three particles, quantum fields, whatever you want to call them, they have mass, big mass. They're called the W and Z bosons. W for weak Z, because there's another letter. These force carriers are heavier than a proton. Let that sink in. If I want to interact with you via the weak force, I have to exchange particles with you that are heavier than the heavy particles that make up matter.

When one quark changes into another flavor of quark through the weak nuclear force, it involves an interaction of a particle that travels all of 10 to the minus 18 meters. But while it's traveling all of 10 to the minus 18 meters, while it is existing, while it is carrying back and forth, it is heavier than a proton. It is heavier than the entire particle that the gluon is inside of. You can imagine when electrons interact with each other. They like shoot lasers at each other. pew pew pew, little laser pointers at each other, they're exchanging photons back and forth to do their little electromagnetic dance. But when the weak nuclear force is involved, it's like throwing bowling balls at each other. This was such a big surprise in the 1940s and 50s that it demanded an explanation, like how in the world do you get a weak force with short range with these bowling ball particles that have the ability to change the flavor of other particles? How in the world?

Did the weak force end up with such massive force carriers? The answer would come from Peter Higgs. And the whole reason for the existence of the Higgs boson was to explain why the weak force is the way it is, with the side benefit of creating mass for the other particles. That was a side benefit of the Higgs boson. So the weak force doesn't play by the normal rules. And in fact, it breaks one of the biggest rules of all. Prior to the weak force. we knew of two forces of nature, gravity and electromagnetism. These forces share a lot in common. They have infinite range, one over r squared dependence, a singular force carrier. And we thought that these forces would set the template for everything else that we could possibly discover. Yeah, for a long time, we didn't even think that we would have to need a weak and strong nuclear forces.

But then once it became apparent, once we started sticking our noses inside of atomic nuclei, we realized that we needed some new forces of nature to explain what was going on. We thought it set the template. based on what we knew from gravity and electromagnetism. In using these two forces, and then even once we discovered the strong nuclear force later, once we had three other forces to compare against, we thought, we developed what we thought were permanent, unalterable, fundamental, basic bedrock rules of the universe, the rules of symmetry. And there are three big ones, charge, time, and parity. If I take some interaction involving any of the forces of nature, and I look at it at a particle by particle accounting, and I see the particles that come into a scenario, they interact with each other through the forces of nature, and then they come out. Maybe they come out different. Maybe they come out with their flavors change. Maybe they come out with different momenta.

But the point of interacting through a force is to change what's going on. And there's a list of things that could change whether, you know, depending on the force. But we thought there were some rules. governing how these interactions unfold. We call these rules symmetry. Like if you take all the particles and you flip the charges, okay, look at you. If you've got a negative charge now, you're a positive charge. If you're a positive charge now, you're going to be a negative charge. And if you're a neutral, you stay chill. Then you run the experiment again, you should get the exact result. That's called a symmetry of charge. There's also a time reversal symmetry. If you have particles going into an interaction, then coming out, you should be able to flip that around and get the exact same result. If you run the interaction past to future and future to past, you get the exact same result. Yes, this opens up a giant question of how exactly do we get a flow of time? Different episode.

But this is what we saw in particle collider experiments and all the theory and all the math that would come out like, oh, if you all these fundamental subatomic particle interactions can run back and forth in time and it doesn't change. And then there's the question of parity, which is if I run a particle experiment and then I look at that experiment in the mirror, I should get the same answer. This started to break down with an experiment run by Chenxin Wu, the first lady of nuclear physics. Madame Wu is her name. She left her family in China behind in the 1930s to come study in the United States. And two decades later, she performed an experiment that allowed her to become the first person in history to witness the violation of one of these bedrock principles of physics. And that was the symmetry of parody.

That was our first crack that this, these laws, these symmetry, these things that we thought were guiding principles behind every interaction in the universe turned out to be not so accurate. And the first one to fall was parody. The symmetry of parity is that a mirror universe should operate the same, but what she was finding was that some radioactive decay involving beta decay, which means it's evolving the weak nuclear force, was preferring one direction over another. Which means if you look at the experiment in the mirror, if everything's going in one direction, you look in the mirror, all of a sudden everything's going in the opposite direction, the mirror does not look like our universe at a fundamental particle level. And this is a problem. We're now able to explain her results in the language of the weak nuclear force, and that's through a property of particles that we call its handedness. So every particle has spin.

Electrons have spin, neutrinos have spin, quarks have spin, everyone's got spin. And when it's moving, when a particle is moving, which they tend to do, this spin will spiral around. You can imagine, you can imagine like throwing a baseball. you throw that baseball and the baseball is moving away from you and if you give it like a little twist then that baseball will be spinning as it's moving forward and if it's spinning clockwise from your perspective then we call that a right-handed particle and if the and if the baseball is spinning counterclockwise we call that a left-handed particle. So it's this combination. In the phrase we use here, we call it helicity because why not? But it's spin in motion. It's things can be spinning clockwise or things can be spinning counterclockwise as they move. And this applies to baseballs. This applies to fundamental particles. And all particles come in both left and right handed varieties.

I have some interaction And a proton comes out, an electron comes out. You know, it might be spinning clockwise, it might be spinning counterclockwise. It doesn't change any of the interaction. It doesn't change any of the end result. It doesn't change any of the fundamental interactions. Everything's chill. And so if I look at some interaction, some fundamental particle interaction, some explosion, some particle collider experiment, you know, something really digging down deep into subatomic physics. And all these particles come flying out. Some of them are spinning right hand, some of them are spinning left hand, some are clockwise, some are counterclockwise as they move out. I look in the mirror, everything's fine. The parity symmetry is preserved. Why? Because the handedness of the particles doesn't matter. It doesn't change anything. Oh, I get an electron over here and it happens to be left-handed. And in the mirror universe, I look at something spinning.

If I throw a baseball and it's spinning counterclockwise, I look over in the mirror and I look in the mirror and it looks like it's spinning clockwise. I have the mirror version of it, but it doesn't change the physics. It's all fine. Handedness doesn't matter, so why am I talking about handedness? Because surprise! Weak nuclear force, it matters. The weak nuclear force is responsible for creating particles called neutrinos. No other particle gets to create a neutrino. The weak nuclear force does. It can change flavors and it makes neutrinos. Every single neutrino in the universe is left-handed. I'll say that again because that's kind of weird. Every single neutrino in the entire universe is always and forever left-handed. All neutrinos. Electrons, half of them are left-handed, half of them are right-handed, it doesn't change any of the physics, mirror universe is fine. Top quark. Half of them are left-handed, half of them are right-handed. Doesn't change any of the physics.

Everything is fine. Neutrino? They're only ever left-handed. There's no such thing as a right-handed neutrino in the universe. Now what does this mean for the symmetry of parity? It means when I throw a baseball, it doesn't matter if it's left-handed or right-handed. On a fundamental subatomic physics basis, it's fine. Mirror universe doesn't matter. If I throw a neutrino, I can only ever throw left-handed neutrinos, neutrinos that spin counterclockwise as I throw them. I look in the mirror at that neutrino. What do I see? I see a right-handed neutrino. But right-handed neutrinos don't exist. My mirror universe does not look like my universe. And the symmetry of parity is broken through the weak nuclear force. This manifests in the experiments of Matt and Wu where certain beta decays only happen in one direction because the handedness of the neutrinos comes into play, manifests as, yeah, I'm only going to beta decay over here in this one direction.

I'm not going to beta decay in the other direction. Symmetry is broken. So the weak force, the force that nobody asked for, is the only force that can change particle flavors, it's the only force with massive force carriers, and it's the only force to violate parity symmetry. And for all that, what do we get, huh? What has the weak force ever done for us? Well, if you want to fuse two hydrogen atoms together, you can't just do that because they repel each other. So you need to do a little dance. You need to change one of the protons into a neutron. And then the proton and neutron combine together, creating something called deuteron. And then those deuterons go on to fuse to become helium and energy is released. And what converts a proton into a neutron? That's right, the weak nuclear force. So in addition to changing particle flavors, using massive force carriers, and making the universe left-handed. Congratulations lefties, by the way, you rule the universe. You won the battle.

In addition to all of that, the weak nuclear force allows the sun to shine. Not a bad trade-off for such a weird force. Thanks to Graeme D., Colin S., and Evan T. for the questions that led to today's episode. Keep those questions coming, please. That's askaspaceman at gmail.com, or go to the website, askaspaceman.com. And thank you to everyone who contributed to Patreon. That's patreon.com slash pmsutter, P-M-S-U-T-T-E-R, is how you keep this show going. And I'd like to thank my top Patreon contributors this month. Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, Joshua, Scott M, Scott M, Louis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Eileen G, Steven W, Deb A, Michael J, Phillip L, and Steven B. That's patreon.com slash PM Sutter. Keep those questions coming. Keep the reviews coming on your favorite podcasting platform. It really helps to show visibility.

And please just keep asking questions. That's the most important thing you can possibly do. Thank you so much, everyone. And I'll see you next time for more complete knowledge of time and space. you

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