Daniel and Kelly’s Extraordinary Universe - Does the weak force attract or repel?

Episode Date: September 21, 2021

Daniel and Jorge break down this question that stumped Daniel at his public office hours! Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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Listen to Culture raises us on the iHeart radio app, Apple Podcasts, or wherever you get your podcasts. Hey, Jorge, do the mysteries of physics pull you in, or do they make you runaway screaming? You mean, does physics attract me or repel me? That's right, you know, if you want to think about it physically. Now, are we talking about physics or physicists? No, the whole shebang. You can't separate the physics from the physicists.

Well, you know, I kind of like the big questions that physics ask. They're pretty exciting. But, you know, sometimes the math and all the jargon, it's kind of a little bit hard to put up with. It's a little bit of a push and a pull. That's right. Every time I think I'm free of physics, it just pulls me back in. We're like the mob. You're the Al Pacino of physics.

And you are the pun father. Maybe you're more like the Don Coriolis of physics. Hi, I'm Jorge. I'm a cartoonist and the creator of PhD comics. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm still laughing about that Don Coriolis joke. Do you need a break here? No, but I do love the idea of somebody who's like a master of physics, you know, like the godfather of physics.

But, you know, we just lost somebody who many people consider the godfather of the weak force and the standard model. Stephen Weinberg, a professor at U.T. Austin, who won the Nobel Prize 40 years ago, just passed away last week. So rest in peace to one of the giants of the 20th century. Oh, man, rest in peace, yeah. So who's going to take the mantle of the godfather now of physics? I don't know, because the stakes are high, right? You've got to win the Oscar for the sequel also.

And then not flub it on the third movie, on the third Nobel Prize. You know, it's got to be as good as the rest. But welcome to our podcast, Daniel and Jorge Explain the Universe, a production of our heart radio in which we try to make you the godfather of everything in the universe we try to give you an understanding of how things work at the very smallest scale we share with you what humanity has accomplished in terms of understanding the true nature of reality around us the context of our lives the long cosmic stretch of history the crazy violent events that have led to our existence and everything in between we ask questions about the very nature of time and

and space, and we do our best to answer them. That's right, because it is a pretty wonderful universe full of amazing and incredible forces and energies and places to explore and particles to discover, full of intriguing mysteries, and sometimes weak questions. That's right. It really is incredible to me how varied and complex and gorgeous and strange the universe is. You know, it's filled with bananas and movies and all sorts of silly things. things. And then when you take it apart, you find that at its microscopic level, its nature is

fundamentally totally different, which means that everything we know about the universe is not fundamental. It's instead it's emergent. It's just like an accident of how things come together. And so the real majesty of the universe, the real like nature of the reality around us is embedded in how the basic bits are put together. And so the rules that govern how those bits come together and how they interact or how they don't interact is really what leads. to things like ice cream and movies and bananas. And so it's fascinating to me how the world exists at these different scales and we can somehow pull them apart and understand them at all different levels.

Yeah, I guess that, you know, the universe is pretty majestic at like the galaxy level, at the nebula level. It's pretty amazing. And it's also pretty fascinating at like the smallest, tiniest levels that you can possibly get, right, at the quantum level. Yeah, and that's sort of a philosophical question. Like, why do we find the universe majestic and beautiful? Could it have been that we like look at galaxies and we're like, eh, kind of an ugly smear?

But instead, you know, we look at them, we're like, wow. Maybe evolution filtered for optimists or our internal physicists, you know, somehow made us survive. Yeah, because we also find a lot of the earth beautiful. You know, we see vistas and we're like, wow, we're lucky to live here. Could we have evolved on a planet where we're like, man, this place is a dump? It makes me wonder sometimes. Right? Or maybe like there are aliens who grew up in a totally different planet who think our,

you know, beautiful mountains and trees and rivers are ugly or boring. Yeah, exactly. The aliens who grew up in Irvine and they prefer beige. They don't like color or liveliness. Oh, man, that would be terrible if we met aliens and they're like, wow, your suburbs are nice, but these forests, ugh. Make it all like Irvine. Make it all like Irvine.

What if the aliens come and they tear down everything beautiful and they just build subdivisions? Well, then we should move to their planet because then their planet must be gorgeous. That sounds good. But rolling back up to what we were talking about, I think it's amazing how many incredible things there are in our universe and how it all just comes out of how the little bits interact. You know, the nature of your body is mostly determined by the strength of the electromagnetic force. The structure of the atom is mostly due to a combination of the strong force and electromagnetism. The structure of the galaxy comes from the nature of gravity. It's really the forces that shape the very fabric of reality as we experience it.

Yeah, the forces are pretty good because without. Without forces, the universe would be pretty boring, right? I mean, the forces in the universe are what makes things stick together and do things, right? And move. Otherwise, it'd be a pretty static universe. Yeah, you could say they force it to be interesting. That was a bit of a force joke there, then. Don't force my hand or I'll make some more.

Yeah, we like to talk about the giant things in the universe. But we also like to talk about the little things, like the forces that keep our particles together or repel them or make them do interesting things. And so today we'll be talking about one specific question about, one of the fundamental forces. That's right. And this is a super fun question for me because it came from a listener who attended one of my public office hours. Would I just hang out on Zoom and answer physics questions from all comers?

And this one came from an organic chemistry professor who asked me a pretty tough question that I didn't know the answer to. Wow. Was that scary? Did you freeze? Did you panic? No, those public office hours are pretty low key. And I often get asked questions.

I don't know the answer to. And I just try to figure them out on the fly, hoping the people appreciate seeing a physical this, like make mistakes and back up and hopefully figure it out. All right. Well, let's get to the question that stumped Daniel at his office hours. So today on the program, we'll be asking the question. Does the weak force attract or repel? Now, Daniel, is that repel or rappel?

How exciting is the week? The weak force likes to go climbing. It likes to go bouldering. I don't know if it likes to repel down cliffs also. It's too weak to do it. It just lays in bed and complains until somebody brings it lunch. Well, that's a pretty interesting question. Does the weak force attract or repel?

I guess are there only two options? Can a force only attract or repel? Or can it do something else? Like, I don't know, push you sideways or disinterest you. That's an awesome question. And no, forces can do other things than just attract or repel. We'll dig into it in the podcast. Some forces can do things like change the nature of a particle, you know, change you from an electron to a neutrino, for example.

So, you know, you might end up in a day. different direction and be a different particle. So it's more complex than just getting pushed or pulled. And the force of magnetism can do things like turn you, doesn't change your overall speed, but it bends your path. So there's all sorts of complicated, amazing things that forces can do. But I think the fundamental thing they do seems to be either pull things together and hold them tight so they can build up and make complex things like bananas and ice cream or push things apart, keep them from forming complex structure. And this is a pretty fundamental question because the week force is one of the four fundamental forces. Like there are only four forces in the entire universe, right?

That four forces that make everything work, and this is one of them. Yeah, although Steve Weinberg, the Nobel Prize laureate we just talked about, he's the guy who brought together the weak force with electromagnetism. He showed that really it's one force, which we now call electro-week. So depending on how you count, there are either three fundamental forces or four fundamental forces. In the case where it's three, there's gravity, the strong force, and the electro-weak force. In the case where there's four, you break up electro-week into the weak force and electricity and magnetism. All right, well, we'll get into that.

But first, we were wondering how many people out there knew whether the weak force attracts or repel or had even thought about this question. So Daniel went out there into the internet to ask people, does the weak force attract or repel? So thank you, everybody out there in the internet who was willing to play this silly physics game. And if you would like to be asked questions as stumped physics professor and hear your answers on the podcast, please write to us to questions at Daniel and Jorge.com. Think about it for a second. What would you answer? Here's what people had to say. I can't remember exactly what the weak force is all about. So I'm going to say maybe neither or both.

On this one, I'm going to have to say, I'm going to guess that the weak force repels. And I'm going to say that because, um, things in the universe seem to be, for the most part, stuck together. And I have kind of thought that the weak force and the strong force are working against each other. So if everything's stuck together, I'm going to say that the strong force overpowers that weak force. And that's why, you know, we are in clumps of atoms instead of just spread out everywhere as gas all the time. I guess I think the strong force attracts so that maybe the weak force repels. I genuinely don't know.

The weak force repeller attracts. I want to say maybe both, because although it holds part of this together into what would be structures, I guess it would also have to repel stuff that it doesn't want to react with and allow in. I don't know. I feel like I should know, but I just don't know. I've actually never understood if the weak force attracts a repel. What I know is that the force is responsible for sometimes a rather active decay, like the beta decay, but of forming some particles into other particles, so like electrons to neutrino and vice versa, if I remember well.

The gravitational force is the only one that attracts only. the rest of them attract and repose so the weak force attract and repels not sure if it's the same like an electromagnetic force

but I know that it attracts and repels I got to say I was pretty relieved that these folks also didn't know the answer if they had would you feel inadequate as a physicist I would have to give up my professorship I would have lost it in physics combat

Oh, man. I got to get like a Nobel Prize winner to be one of your respondents just to sabotage you. You got to get a ringer in there just to set me up. Nice. But so this surprised you and this question came from an organic chemistry professor. That's pretty cool. Are you going to take revenge now and go to his office hours and give him a really tough organic chemistry question? I don't even understand organic chemistry well enough to ask a question, not to mention find one that's tricky. But I totally respect this question because it's so simple. It's so basic, right? It's like, Well, we know forces push and pull. So what about the weak force? Where does it fit in? And it's such a simple and basic question, but one I had never thought of before. Because I tend to think about the weak force only in terms of like particle interactions, like particles colliding and annihilating because it doesn't really play a tactile role in our lives, right?

You don't feel the weak force pushing against you or pulling things together. And so I'd never really thought about it in this context. It was really a fascinating question. You just like to break things. You don't worry about putting things together. It's kind of my job to break things at high speed. It's like the top gear of particle physics. And you get to survive most of the time.

So that's good. Most of the time, exactly. Well, let's warm up to the answer here. And maybe let's start at the basics here. So let's talk about what is the weak force and what does it mean in general to attract or repel something. So step us through then. Yeah. So we have a few ways that particles can interact with each other.

And that's really what forces are. They are particles feeling. each other. And when we first discovered that particles can do this, it was sort of a question of like, wait, how does this happen? How do particles like push and pull on each other without actually touching, right? How do they like feel each other? Two electrons, for example, can push against each other. It doesn't mean that the surfaces of the electrons have come into contact like little tiny balls. And the way they do this is with the forces between them. So that each one has a field, right? Each electron has an electric field around it. And with that electric field, really,

is, is a way for the electron to apply a force to other things. And so that's for electricity. And this is a similar field for magnetism. And electricity and magnetism were then shown to be actually two sides of the same coin. It made more sense to think about them together. Some phenomena seem like electricity if you're at rest and they seem like magnetism if you're moving. And so it's pretty clear that it's really just sort of one thing that we were seeing two sides of. And the weak force is in the category of electromagnetism. It's a force that. that particles can use to operate on each other. And so it's a little weirder than the other forces

because it's not something you experience day to day because it's so weak. And the reason that it's so weak and so weird is that the particles that communicate those forces, the fields that it uses, have a lot of mass. They're very, very heavy. And so they don't last for very long. They don't make it very far.

And that mass really weakens their impact. So electromagnetism and the weak force are very closely related, But the big difference is that these particles that communicate the weak force are very heavy. And so they really weaken the force. So maybe let's step back a little bit here because you just went through a lot there. So I think what most people are probably familiar with is the electro-magnetic force. And we know that like, you know, if I have two magnets, they repel or two magnets they can attract each other and push and pull.

And it's all because of the electromagnetic force, right? That's the one that people are most familiar with. And people are, I think, also familiar with gravity, which kind of attracts things together. So that's where I think maybe people expect that all forces attract or repel. Yeah. And it's really interesting because electromagnetism can do both of those things, right? Electromagnetism can attract or repel with the same force. And that's because in electromagnetism, the force depends on the charge of the particle.

Right. So every particle we add this label, like a minus or a plus, right? And each one we call the electric charge. And there's two possible values. There's plus and there's minus. And if two particles have the same charge, like plus plus, then they repel each other. They have opposite charges, they attract each other. It's important to understand, though, that this charge we're talking about what it means is that the particles feel the field.

Like a particle that has no charge is a particle that ignores the electromagnetic field. A particle that has a positive or negative charge is a particle that does feel the field. We discover that there are two of these different kinds of charges, and if you combine them in one way, they repel. and if you combine them in another way, they attract. So that's a really fascinating thing about electromagnetism, is that it can do both. Well, maybe step us through an example. So let's say I have two electrons and they're like sitting close to each other, right?

So they both have negative charge. And then you talked about there being a field and this force being going through the field. Can you step us through what's happening? Like if I have an electron here and an electron there and then they're both the same charge, they're going to push each other away, right? That's right. That's exactly what's going to happen. So you have electron one and it's just hanging out, but it also fills the space around it

with an electromagnetic field. And so now you put another particle in that field, a second electron, and it's going to feel that field. That field exists just to apply forces to other charged particles. And the direction of the force depends on the charge of the two particles. And why it happens, why there's a push or a pull depends on the potential energy of the field. Things like to basically roll downhill. to low potential energy.

And these fields, they have a potential energy which varies with distance. And the potential energy tends to be slanted so the particles like roll down the potential towards each other, then the force attracts. Sort of like if you put two rocks in a valley, they would roll towards each other. The gravitational potential minimizes when they're closer.

Or if the shape of the potential energy is such that it minimizes when the particles are far apart, like you put two rocks on the top of a mountain that tend to roll away from each other, then the force will push the particles apart. And so when you have two charges, the shape of this potential, which determines whether it's pushing or pulling, depends on the charges of the particles. If the charges are the same, then they repel. If the charges are opposite, then they attract.

And it all has to do with the shape of this potential energy created by the field of one of the particles. Right. But maybe I think what might be confusing to some people is you mentioned it's like an electric field. Now, is that the same as a quantum field, right? Because so the electrons are, we know there's sort of like fluctuations in the quantum field of electrons. Are you saying there's like a separate field that's quantum or is this more of a mathematical term when you say field? That's the same field we've been talking about forever. It's the electromagnetic field. You can actually talk about

electricity and magnetism without quantum mechanics. It was developed before quantum mechanics. So we have a classical theory of electricity and magnetism that explains how electrons pull in each other. That was just before we understood that this field was quantum. What that just means is that it just can't have any arbitrary value, but it's like chopped up into discrete bits. It's like if you are eating a cake, it's like you have to have one piece or two pieces or three pieces. You can't just nibble on the cake all day long. And so before quantum mechanics, we thought that these fields were classical fields that could just have any value. And then we discovered that they have a minimum value and they have steps to them. So it is a quantum field. This is a quantum field we're talking about. And so

ripples in this electromagnetic field are, for example, the photon, which is one quantum of the electromagnetic field. So the electron is a ripple in the electromagnetic field, or it's a ripple in its own quantum field? But they're sitting in another field, which is the electromagnetic force field. Right. The electron is a ripple in the electron field, right? And it generates ripples in the electromagnetic field, which is the field of the photon.

And that's because these two particles talk to each other, electrons and photons. talk to each other. Or in the language of fields, you could say these two fields couple. You could have a universe in which you have a particle and then another field and they don't interact at all. For example, like neutrinos are wiggles in the neutrino field, but they don't affect the electromagnetic field at all. They have no charge. And so electrons are wiggles in the electron field. That's a quantum field. But they also create wiggles in the electromagnetic field because they have charge. It's a charge that connects these two fields. like how much these two fields are connected for that one particle.

Yeah, and interestingly, for electromagnetism, there's a sign to it. There's two different kinds of charges, and that determines the shape of the potential from the field and whether they attract or repel. All right, let's field more questions about this. Main question of, does the weak force attract or repel? And so let's get into the weak force itself when we come back from the break. But first, let's take a quick break. Ah, come on.

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That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Well, wait a minute, Sam, maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't.

don't trust her now he's insisting we get to know each other but i just want her gone now hold up isn't that against school policy that sounds totally inappropriate well according to this person this is her boyfriend's former professor and they're the same age it's even more likely that they're cheating he insists there's nothing between them i mean do you believe him well he's certainly trying to get this person to believe him because he now wants them both to meet so do we find out if this person's boyfriend really cheated with his professor or not to hear the explosive finale listen to the okay Storytime Podcasts on the iHeartRadio app, Apple Podcasts, or wherever you get your podcast. A foot washed up a shoe with some bones in it. They had no idea who it was.

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Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're talking about the weak force and whether or not it's attractive or repulsive. I guess Daniel is that the same as asking whether it's hotter or not. Is it appealing or not appealing? Well, I find it mysterious, which makes me. we want to know more. And so I think the weak force is definitely pretty hot. But it's pretty weak.

It can't heat anything up. So it leaves the universe kind of cold. We were talking about the first, the electromagnetic force and how the force of it is actually the interaction of two particles in the electromagnetic field. And that happens when they interchange photons. And it's all because of the charge of the particles. But I guess, you know, that's one way to look at things. But another way to look at things might be the same way that we look at,

gravity, right? Gravity we look at it in a totally different way right now. Yeah, gravity actually is really interesting and you can look at it using the same sort of set of concepts. You can say like, well, what does gravity do? Gravity only attracts and you can think about it like call the mass of an object to be like the version of electric charge for gravity. And because objects only have positive mass, you only have one kind of charge for gravity. So when we say charge, we mean like a more general concept, not just for electromagnetism. We mean like for any kind of force, does the particle feel that field? It feels the field if it has the right charge. And so for electromagnetism, you say, well, electric charge tells you whether it feels the force or not. For gravity,

it's mass. Mass is the charge for gravity. And this sort of blew my mind when I realized this years ago. And it's also different from electromagnetism because in electromagnetism, if you have the same charge, like two electrons, they repel each other. But in gravity, if you have the same charge, two objects with positive mass have the same charge, they're both positive, then they attract each other rather than repel. I see. So there are two main forces that people are most familiar with, gravity and electromagnetism. One of them pushes and pulls, that's electromagnetism, and gravity only pulls things together. So that's kind of weird, right? It is weird. And it makes me wonder like what would happen if we had negative mass right if we had negative mass with two negative

masses attract each other would a positive and negative mass repel each other that would be super weird and fun yeah I think a lot of people would love to have negative mass these days after they have to stay at homes for so long eating things out of the fridge but just to summarize so the electromagnetic force attracts and repels gravity attracts so now now the question is what does the weak force do? Does it also attract and repel or one of the two? Yeah, it's really fascinating and kind of complicated. So the weak force is kind of a big mess. It's not as simple as electromagnetism or gravity. And one reason is it has three particles that mediated. So electromagnetism has the photon. This is everything for electromagnetism. Gravity, we don't know. Maybe it has the graviton. We're not sure

if it's quantized. But it's pretty straightforward from that perspective. The weak force has three of these things. That's three separate particles or three separate fields that mediate its forces. You have the W plus, the W minus, and the Z. So these are like three different weird, heavy photons that communicate it. So it's much more complicated than any of the other forces. Interesting. So it's like when two particles that feel the weak force talk to each other, they do it through three different ways you're saying. Like there's three different particles that can communicate this force. Yeah, exactly. this so the weak force can do sort of three different things, and they're all part of the same force. I mean, you might imagine like a different history of science where we had discovered

these different parts of the weak force separately and called them different things. And then some other version of Steve Weinberg had realized, oh, this makes more sense, we should click them all together. And so it's sort of like three forces fit together into like the ultimate transformer force that makes more sense. And then, you know, you plug in the photon, you get all the beautiful symmetry of having those four particles altogether. But these three, the w plus, the w minus, and the z, we call these together the weak force.

Now, are those separate three different quantum fields, or is it one quantum field, but three different ways to kind of activate it or to wiggle it? There are three quantum fields. There's a field for the w plus, for the w minus, and for the z. The amazing thing is that they work together. There's like symmetries that they respect, like something can move from one to the other, but it conserves this number, which is like the charge for the weak force. So it's definitely clear that they are three separate things, but they work together as part of a whole.

Interesting. Yeah, I guess that question would be, why not call it three different forces? You're saying it's like three quantum fields, but they're all depending on each other so that they actually act as one. Yeah, it's like, why don't you call the heads and the tails two different coins? Well, it makes much more sense to call it one thing because, for example, a coin is either heads up or tails up. It's never both, right? So the two are definitely connected. They're synced.

they fit together. These three fields are the same way. They connect together into one thing which is much more simple mathematically than any of the individual parts. All right. So the weak force talks through three different quantum fields

through three different particles, w plus, w minus, and the z. Now is there the equivalent of a charge in the weak force? Like we have electric charge or mass? There is, yeah. It's got its own kind of charge. It has a really weird name.

The name for it is weak, isospin. And every particle that feels the weak force has a value for weak isospin, which is either up or down. So just like the electromagnetic force can push or pull on everything with an electric charge, which is either plus or minus, the weak force can tug or push on things that have either up or down values of the weak isospin. I guess why call it the weak isospin? Why not call it the weak charge? Yeah, that's a long story that comes from history. People try to create this idea of isospin, which is like a generalized version of spin for the strong force, and it works really well there. And then when people were studying the weak force, they thought,

hmm, maybe there's something similar over here. So they created a version of that. It doesn't really work, but the name stuck. So yeah, it would be better if we called like the weak charge or something, but the weak isospin is the name we got. And these are actually numbers, but instead of just plus or minus, you say up or down, right? These are just numbers that the particles have, just like the electron just happens to have negative charge, some particles have up quantum weak isospin. Yeah, exactly. And so we call them up or down sort of by convention because, for example, of quarks have up weak isospin and down corks have down weak isospin.

And then the electron has down weak isospin and neutrinos have up. You could also map this if you're more comfortable with numbers to the number line, in which case we say, for example, that neutrinos have positive a half weak isospin and electrons have minus a half, up quarks also have positive a half, and down quarks have minus a half. But this is like a different sort of dimension for every particle than electric charge. Like there's no relationship between the electric charge and the weak isospin. For example, neutrinos have no electric charge, but they do have weak isospin.

Interesting. So like an up quark is just a cork that has up. Weak isospin? Exactly. And that's why we call it up because it sits at the top of weak isospin. And that's why we call it down quarks down. And that's why top corks are called top corks

because they have up weak isospin. So when we lay out these like generation of particles, we group them into these pairs that are like up and down, charm and strange, top and bottom. The top row there, up charm, top, they all have up type weak isospin. And the bottom row all have down type. All right.

Well, how would you even define this weak isospin? Is it just sort of like how much a particle interacts with the weak fields? Is that the equivalent of like electric charge? Yeah, exactly. It's the thing that tells you whether a particle feels the weak force, whether it interacts with the weak force. And in analogy with the other forces, if you have no electric charge, you don't feel electric forces.

Like a neutrino can fly right through the most powerful electric fields and totally shrug it off because it doesn't have. charge. And so weak isospin is the thing that tells you whether a particle feels the force. But it's something a little bit more than that, you know, because the fascinating thing is that there's a symmetry, right? These forces, the w plus, the w minus, and the z, they have this really fascinating symmetry to them. It's not a physical symmetry. It's not like, you know, if you rotate it like a sphere, it looks exactly the same. It's an internal symmetry. These forces all have like angles to them. And if you rotate the particles, they rotate together and they work together.

And the upshot is that they end up conserving this thing called weak isospin. So these particles and their interactions, they conserve weak isospin the way the photon, for example, conserves electric charge. You can't just like create electric charge willy-nilly in the same way. The weak force preserves weak isospin. If you have it, it's going to stick around. If you don't have it, you can't create it. Interesting. It's sort of like a universal rule. Yeah.

And it's connected to this concept. We've talked about a few times in the podcast, Nuthos theorem that tells you that any symmetry in physics leads to a conservation law. So there's some like internal symmetry between these particles, the W plus, the W minus, and the Z. Where like if you rotate them internally, they turn into each other. But they do it in this way that preserves a certain number. And that leads directly to conservation. of weak isospin, the same way, for example, the photon is responsible for conserving

electric charge. So it's really like a deep thing in physics when you discover this number which doesn't change. It tells you you've like found something about the universe which feels fundamental or important because it, in its bookkeeping, it makes sure that this number doesn't get changed. The universe has an account and it has a special account for weak isospin is what you're saying. Exactly. And again, these are just sort of random numbers, right? Like, We don't know why, for example, some particles have, you know, up half isospin or negative half isospin, right? These are just sort of arbitrary almost. Yeah, well, we don't know if there's rhyme or reason to it, but you're right.

Our level of understanding is we're just sort of like writing these things down in our lab notebook. We don't understand why particles have different values. We're just sort of tabulating it and looking for patterns. And, you know, the numbers are pretty weird. Like weak isospin. It's actually pretty straightforward. It's either plus a half or minus a half. If you look at like electric charge, it's weird.

You know, like neutrinos have zero. Electrons have minus one. Up quarks have plus two thirds. Down quarks have minus a third. It's kind of crazy. And we don't understand why those numbers are what they are. I mean, in some sense, they're arbitrary. Like you could make them twice what they are or half what they are.

And then you could take that number and like, you know, weaken or strengthen the feeling of the force. But the relationships between them are not arbitrary. Like, that's just what we see in nature. And then also the name weak force, you said, comes from this. It's not that the weak force is weak. It's just that it doesn't have long range or something, right? It's due to these particles, these force particles having mass. Yeah, these particles, the W plus, the W minus, and the Z, they have a lot of mass.

And that's actually really fascinating because it's what makes the weak force different from electricity and magnetism. We think back in the very early days of the universe before the Higgs field, that all these particles were massless and they were flying all the way around the unit. And then the Higgs field came and it made three of these particles have mass. The W plus, the W minus, and the Z got a huge amount of mass from the Higgs field. And now they are very heavy. And as you say, that makes them weak. It means that it's much less likely for this interaction to happen. Like if you shoot two neutrinos at each other,

they're much less likely to interact than if you shoot two electrons because the weak force is weaker than electromagnetism. It's also shorter range, as you say, because the particles can't fly as far. but it's also weaker. It's less likely to sort of like come into play when you shoot two particles at each other. I guess the question is, how does having mass make it less likely to interact

or harder to interact and also a shorter range? You can think of them as connected. Like imagine shooting two particles together and the probability of them interacting sort of depends on their relative distance. Like if you shoot them in the same direction but they're like a meter apart, there's going to be no chance to interact.

As they get closer and closer together, then there's a larger and larger chance that they interact. And we talk about the chance that interaction is being like the relative cross-section. Like physical analogies, you throw like two balls at each other. The chance that they're going to hit each other depends on their cross-section. Like how big does one appear to the other? And so we have sort of a quantum analog of that,

which we call, again, the cross-section. And the cross-section is smaller if the particle that mediate it has a lot of mass. The fact that it's a shorter range force means it has a smaller cross-section when the particles coming the other direction. So you have to sort of like get it right on the nose in order to have that interaction happen. Whereas if you're using photons, they can fly everywhere. You don't have to like shoot electrons to be as close to each other for them to feel each other. Yeah, but I guess, you know, what does it mean to have short range?

Like what happens to the particle? Like it disappears or breaks up into other particles because it has mass, which means it has energy? Yeah, exactly. If you shoot a Z particle, for example, it can't just fly across the universe forever the way a photon can. like a photon made billions of years ago can still just fly through the universe towards you. If you make a Z particle billions of years ago, it will decay

pretty quickly into something else because it's so heavy. It'll turn into a pair of electrons or a pair of neutrinos or something. So they just don't last very long. Right, because it can, right? Because it has this sort of internal energy from the mass and so it tends to like disperse kind of. Exactly. Whereas photons are stable

and they can propagate forever. But these dissipate and turn into other fields. It's like throwing a snowball in a windstorm or something like it just breaks up. Yeah or like throwing a snowball in Death Valley right? It's just it's probably going to melt before it gets to your friend's face which sounds like a fun activity

but I guess maybe a question is is the week four is you know weak in itself like if it could travel further would it still be weak or would it have an inherently lower kind of interactive value or impact? Like is it as strong as the other forces

but because it decays and it has a short range it's you call it weak because of that. Yeah, I would say so. If, for example, we lived in a universe where it didn't get mass, right, where the Z and the W propagated the same way as the photon, it would be as strong as electromagnetism. All right. I think we covered the basics. And now let's get into the main question, which is, does the weak force attract or repel? But first, let's take another quick break. Ah, come on. Why is this taking so long? This thing is ancient. Still using yesterday's tech, upgrade to the ThinkPad X1 Carbon,

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Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone. a hold up, isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor and they're the same age. It's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet.

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Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, does the weak force attract or repel? And Daniel, we talked about it being weak because it doesn't have a very long range. The particles that transmit this force decay before they get very far. But I guess the question is, if it does reach their target, like if two particles do interact with the W plus or W minus or Z, force particle, are they going to make each other come closer or further apart? Yeah, so because the weak force is a lot like electricity and magnetism

and because actually they're just part of a larger force, in general it follows the same rules as electricity and magnetism, which means if you have the same weak charge, right, weak isospin, then particles will repel each other. If you have the opposite weak isospin, then particles will attract each other. So it makes it similar to electromagnetism and different from gravity. All right. Well, then we answered the question. It does attract and repel. It does attract and repel. And there's some interesting wrinkles to it, right? For example, you asked earlier, like, what can a force do other than attract and repel? And the W particle is an example of that, right? The W particle doesn't just attract or repel. In fact, you can't even really think of it as attracting or repelling because the W particle transforms a particle. It doesn't just like push it or pull it. It changes it into something else because the W particle itself.

for example, has electric charge. Wait, what? So, like, let's say I have two particles, you know, two electrons in the electron field and they interact through the weak force. You're saying, like, they exchange a W particle and then that doesn't move them, but it just changes one of them. It both moves them and changes both of them. So, for example, if an electron interacts with a W particle, what, how does it do that?

Well, to give off a W particle, it has to lose its electric charge because the W particle, itself has electric charge. Like an electron can radiate a photon, photon is charge zero, doesn't change the electron, it's still an electron. It's just an electron with less energy now or something. But the electron radiates a W, the electric charge has to be conserved. And so that electric charge goes with the W. And the electron is no longer an electron.

It turns into a neutrino. So for an electron to radiate a W, it has to become a neutrino. And for a neutrino to feel a W, it has to turn into an electron. So the W particles here do much more than just push or pull. They transform these particles. Interesting. But does that mean that an electron can talk to another electron through the W or it cannot? Absolutely, it can.

It can read a W and that W can interact with an electron because the W feels a charge and the electron feels a charge. And so then that W and the electron can like attract or repel each other based on their relative charges. So absolutely. And the electron can also interact with other electrons using the Z particle. So it's kind of like the electromagnetic force, but it's just like it has added complications because these force particles have, you know, more to them. They have energy and electric charge, whereas the photon doesn't. Exactly.

So it's sort of this big, complicated mess. It's got two of these charged bosons, the W plus and W minus, which can do these crazy things, you know, like change the charge of the particle that was radiating it. Wait, wait, wait. That's what the plus means. The plus means it has positive electric charge. Yeah. for once the plus actually means something reasonable.

It's actually makes sense. Yeah, good job. All this time you thought the W plus would just like, well, it can't actually mean positive charge because that would make sense. I don't know. It seems like you guys use it kind of willy-neely, right? Because even the up and that you use plus and minus for it, right?

Yeah, that's right. But no, the W plus has a plus one electric charge and the W-minus has a minus one electric charge. That's exactly what it means. And the Z is neutral. The Z has no electric charge. So like if two electric, electrons exchange a Z, then they don't change, or they still change, or they push each other?

What happens when they exchange a Z? Yeah, so if you have electrons and positrons, they can exchange photons and they can exchange Z particles. And now if they exchange photons, you know what the rules are, right? If the charges are the same, they repel each other. If the charges are the opposite, they attract each other. The same rule applies for the Z, but the charge is different because the charge for the Z is the weak isospin.

And so you can then ask that. the question like, do two electrons attract or repel each other using the Z? And the answer is that the electrons have the same weak iso spin because it's just two electrons and so they repel each other. The same way like two positrons will repel each other. They have the same weak ISO spin so they have to repel. But I guess maybe a difference is that the Z force particle doesn't go at the speed of light, right? Because it has mass. So it's like a slower force too? Yeah. And so the Z is weaker. And so usually if two electrons are interacting, it's just dominated by the photon interaction because that's much stronger and longer range.

The Z contributes, but it's like almost ignorable, which is why in chemistry, for example, you're mostly thinking about like the electromagnetic interactions between electrons in atoms and electrons in neighboring atoms. There are some weak force interactions there, but they're mostly ignorable because the weak force is so much weaker than electromagnetism. Oh, interesting. It's like it's always happening there just like electromagnetic forces, but they're just, you know, so weak that nobody cares.

But it's always happening. Like, even within the particles in my body, there's a whole bunch of, you know, weak stuff going on, but it's sort of negligible. Yeah, exactly. It's mostly just washed out by electromagnetism. And it's interesting to think about, like, is it always pushing and pulling in the same direction

or is it sometimes, like, opposing electromagnetism? And the answer is that it's always pushing and pulling in the same direction. And the reason is that it follows the same direction. basic rule that like charges repel and opposite charges attract. And it just so happens that like electrons and neutrinos have opposite charges and upcorks and downcorks have opposite electric charges and opposite weak isospin charges. So, you know, for example, take an upcork and a downcork, for example, an upcork and a downcork have opposite sign electric charges. One of them is plus two thirds. The other one is minus one third. So they attract these.

other. They also have opposite weak isospin. One is plus a half. One is minus a half. So they attract each other. So in that case, the up cork and the down cork, the photon is pulling them together and the z is pulling them together. You mean like plus electric charge is always associated with up kind of or one of those two. Like you never mix and match him that much. Plus electric charge is always associated with up weak isospin and minus electric charge is always associated with minus weak isospin. Exactly. Exactly. But there's one weird case, right? There's always a weird case. And the weird case is the neutrino. The neutrino has no electric charge, right? So the photon doesn't push or pull. So in the case of the neutrino, the Z is the only thing of pushing and pulling. And so there are no other weird cases where, like, you know, it has a plus electric charge, but like a down. That never happens. It's always sort of a line, these two forces.

Exactly. Because the antiparticles have the opposite values, but they flip together. So, for example, an upcork has positive electric charge and positive weak isospin, and an anti-upcork has negative both values, negative electric charge and negative weak isospin. So they're always pointing in the same direction, and they flip together. But theoretically, there could be particles maybe that have a weird combination. We just don't see them. Yeah, absolutely. And the interesting thing about the weak force is everything feels it. Like the other forces, there's always some particle that ignores it.

You know, the strong force? Electrons don't feel the strong force. Totally ignore it. Electromagnetism, neutrinos, totally ignore it. Nothing ignores the weak force. The weak force feels everything. Wait, what do you mean it feels everything? Like every particle in the universe feels the weak force.

There is no particle out there that doesn't interact with the weak force. There are particles that don't interact with electricity and magnetism, and there are particles that don't interact with the strong force, but there's no particle we've discovered that doesn't interact with the weak force. Sort of like gravity. right? Like we don't know of any particle that doesn't interact with gravity. Yeah. Although we don't really understand, you know, how particles and gravity interact, you know, what's going on there on the quantum level. It's not something we can really say

specifically how that works. But you're right that like everything we know moves through space and the curvature of that space depends on the mass of stuff around it. But again, we don't understand it at the quantum level. But I think it's fun to think about like two neutrinos. Like you shoot two neutrinos at each other. They actually do repel or a attract each other based on their weak isospin. Like two neutrinos will repel each other. And a neutrino and an antineutrino will attract each other because they have opposite weak isospin.

Yeah, just like two electrons will repel each other. Now, you said that the weak force is sort of like part of the electromagnetic force or they're all part of the same force? What does that mean? Like they're sort of coupled. They depend on each other. They're not independent. What does that mean?

It means that it makes more sense mathematically when you plug them together. And in fact, we don't think about weak isospin on its own, usually. What we do is we think about electricity and magnetism together, and we make this new number, which is a combination of electric charge and weak isospin. And we call it weak hypercharge. And then we have these two numbers, weak isospin and weak hypercharge. And together, these four particles conserve these numbers. Like these four particles respect these values.

They do what they can to make sure that in the bookkeeping of the, universe, these two quantities weak hypercharge and weak isospin are conserved. And so we see them all working together to keep this symmetry in line, which is why we think they're all part of the same thing. Oh, I see. You notice that they're sort of part of the same team, or they work for the same company, which has the same accounting, you know, ledger. And so you sort of group them together, unlike, say, like the strong force or maybe gravity, like gravity and the strong force don't care about the conservation of this hypercharge. That's exactly right.

And we tend to think about things that way. We're like, if things transform together, if when you rotate the world, things move together, we think that there's part of the same thing. Just like if you pick something up and you turn it around and it holds itself together and you look at it from different directions and it feels like it keeps the same shape, you think it's, oh, it's one thing.

Whereas if you pick it up and it falls into pieces, you're like, oh, it's a bunch of different stuff. It just happened to be near each other. And in that same way, the photon, like, fits in with these other three particles to make one cohesive mathematical object, which if you rotate in this, like, mathematical way, keeps its same shape. It conserves these numbers. All right. Well, I think that answer is a question. Does the weak force attract or repel?

And the answer is yes. But it's a little bit more complicated, right? Because the force that transmits it also has charge and mass. And so it gets complicated. It's not a straight up relationship. It does get complicated, but I think it's pretty cool that we can use sort of the similar ideas we use to think about electricity and magnetism, charges and potentials, to think about whether these things push and pull. And it's even much richer than we got to talk about on the podcast because there's two kinds of particles. There are left-handed particles and right-handed particles, and only the left-handed particles actually do this kind of interaction.

The right-handed particles don't do it. So it's even more complicated than we talked about. the weak force is like a huge mathematical headache slash delicious puzzle slash weak strong problem kind of sense yeah it's sort of like attracts and repels physicists all right well um i guess maybe one last question is why did you find this question so hard like you know we broke it down and it seemed like something that you know two things have the same weak eyes you spend the repeller or or and if they don't they attract what was the thing that was stomping you I think it's just because I'd never thought about the weak force in terms of its potential.

You mean to push and pull? Like its ability to push or pull thing just because it's so weak. Yeah, just because it's so weak. And also because, frankly, there's some layers of mathematical complexity. I'm hiding from you here. Like, there's actually two different potentials that the Z has. It has an axial potential and an axial vector potential. And so I had to actually work through whether they're always working together

or sometimes working apart from each other. And so whether or not we need to dig into that. So one issue was like how deep to go. into this. We could do like 10 podcast episodes of quantum field theory and gauge symmetry before we get to the answer. All right. Let's do it, Daniel. I got 10 hours. All right. Well, thank you to the person who asked this question, the organic chemistry professor. I think you should watch out because Daniel might come to your office hours and lay down some tough organic chemistry. That sounds like a threat, man. An academic threat. He's going to be nervous every time he has office hours from now

on. Oh, man. He's going to be looking out for you. All right. Well, we hope you enjoyed that. made you think a little bit about all the things that are happening inside of your body right now. There's a lot of stuff going on between your particles, not just electromagnetic forces, but this really strange and complicated thing called the weak force. And all these forces working together are what create the fabric of reality that we experience around us. The world wouldn't be the same without just one of these forces. It would feel palpably different. It wouldn't be nearly as delicious.

We might be those aliens that look at this landscape and go, If this had a weak force, it would be much cooler. That's right. It's too weak right now. It's a weak sauce. We should have called the weak force the spicy force to give it some pride, you know? Yeah, there you go. Because it's sort of not weak, right?

It's sort of strong in its mathematical complexity. Yeah, it's doing a lot, man. Yeah, maybe weak is in you strong. And isospin is a new plus or minus. I don't know. Anyways, thanks for joining us. See you next time. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio.

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In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System. on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her.

Now he's insisting we get to know each other, but I just want her gone. Oh, hold up. Isn't that against school? policy that seems inappropriate maybe find out how it ends by listening to the okay storytime podcast and the iHeart radio app apple podcast or wherever you get your podcasts this is an iHeart podcast