Daniel and Jorge Explain the UniverseDaniel and Jorge break down this question that stumped Daniel at his public office hours!
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Hey, Jorge, do the mysteries of physics pull you in? Or do they make you run away? 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 as they're pretty exciting, but you know, it's Alice, the math and all the jargon. It's kind of a little bit hard to put up with.
So 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 Pacina of physics.
And you are the pun Father.
No, 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 U c Irvine. And I'm still laughing about that Don Coriolist 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 fource and the standard model. Stephen Weinberg, a professor at UT Austin who won the Nobel Prize forty years ago, just passed away last week. So rest in peace to one of the giants of the twentieth 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 got to win the Oscar for the sequel also.
And then not flub it on the third movie, on the third novel price, 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 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 we 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. 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, 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.
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 are internal physicists you know, somehow made us survive.
Yeah, because we also find a lot of the Earth beautiful. You know, we see vistas, we're like, wow, we're lucky you'll 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 turned planet who think, are 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 meet aliens and they're like, Wow, your suburbs are nice, but these forests.
Ugh 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 through their planet because then they're 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 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.
It was a bit of a forced joke there with.
Any 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 or 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 when 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 that people appreciate seeing a physicist like make mistakes and back up and hopefully figure it out.
All right, Well, let's get to the question that stump 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 repel? How exciting is the week?
The week force likes to go climbing. It likes to go 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 it 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 new treno for example, so you know you might end up in a 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 weak 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 Laurier 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 electroweak 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 two questions at Danielandjorge dot com.
Think about it for a second. What would you answer us 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 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 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 then maybe the weak force repels.
I genuinely don't know.
That's the weak force repeller attract.
I want to say maybe both, because although it hold part together into what would the structures, I guess it would also.
Have to repel stuff that it doesn't want to react to with allowing I don't know.
I feel like I should know, but I just don't know.
I've actually never understood if the week fourth attractor repel. But I know if that the fourth if recomfible. For sometimes you're rather active the key like the bit of the kyming from particles into other particles, or like electronicstrematrino and befaa. If I remember.
Well, the gravitational force is the only one that attracts. Only the rest of them attract and repel. So the weak force attract n repels. Not sure or if it's the same like an electromagnetic force, but I know that it attracts and rebels.
I got to say, I was pretty relieved that these folks also didn't know.
The answer they have. Would you feel inadequate as a physicist?
Yeah, 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 Price winner to be one of your respondents, just to sabotash he.
You got to get a ringer in there, just to set me up nice.
So this surprice 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 pulls, 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 there 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 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 you know, 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 to 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. There 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 with 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. Where 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 it's 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'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 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 experienced day to day, because it's so weird. 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 electromagnetic force. And we know that, like you know, if I have two magnets, they repel or two max they can attract each other and push and pull, and it's all because of the eleochromagnetic force. Right, that's the one that people are most familiar with. And 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 their 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, this plus and this 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 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 it tip us through through an example. So let's say 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, an electron there, and then they're both the same charge, they're gonna push each other away.
Right, That's right, that's exactly what's gonna 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 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. 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 quantumar or is this more of a mathematical term when you say.
Field, that's the same field we've been talking about forever, 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 pulling 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. Right, 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. That's a charge that connects these two fields.
Charge is 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 attractor 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.
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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 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 inter exchange 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 it. 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 is what 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 in 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, with 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 that, 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 not. 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 that has three particles that mediate it. 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. It has 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 week 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 and you get all the beautiful symmetry of having those four particles all together. But these three the W pls us 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 fields 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 syncd 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 or 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. 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 tried to create this idea of isospin, which is like a generalized version of spin for the wrong force, and it works really well there, and then when people were studying the weak force, they thought 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 it like the week charge or something. But the week isospin is the name we got.
And these are actually numbers, but instead of just plus or mind as 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 week isospin.
Yeah, exactly, and so we call them up or down so to by convention, because for example, of quarks have up week isospin and down quarks have down week isospin, and then the electron has down week 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 week isospin, and electrons have 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 upquark is just a quark 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 quarks are called top quarks 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 charming 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 week 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 spear, 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 in their interact as 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.
Mmm.
Interesting. It's sort of like a universal or rule.
Yeah, And it's connected to this concept we've talked about a few times in the podcast. Northo's theorem that tells you that any symmetry in physics leads to a conservation law. So there's some like internal symmetry between these particles that W plus w mis and the Z where like if you rotate them internally, they turned 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 in it's book keeping, it makes sure that this number doesn't yet changed.
The universe has an accountant, and it has a special account for the 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 have 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 is 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 their 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, weakend or strengthen the feeling of the force. But the relationships between them and our arbitrary. Like that's just what we see in nature.
And then also the name weak force you said, comes from this, not that 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 universe. 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 masks 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 you know 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 a 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 and of that which we call again the cross section, And the cross section is smaller if the particle that mediates 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 wherever 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.
Like throwing a snowball in a windstorm or something like the 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 weak force, 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 with 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.
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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 you if two particles do interact with a W plus or W minus or Z force particle, are they gonna make each other come closer or further apart?
Yes? So, because the week is a lot like electricity, 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. Via 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 and 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 have two particles you know, to electrons in the electron field, and they interact through the weak fours. 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, 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 the 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 are have you know, more to them. They have energy and electric charge, whereas the photon.
Doesn't exactly, So it's out of this big complicated mess. It's got two of these charged bosons, a W plus and W minus, which can do these crazy things, you know, like change the charge of the particle that was radiating it.
Oh 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 it's like, well, it can't actually mean positive charge, because that would make sense.
It seems like you guys use it kind of willy nieleak, right, because even the up and down 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 electrons exchange a Z, then they don't change. 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. The charges are the opposite, they attract each other. The same rule applies for the Z, but if the charge is different, because the charge for the Z is the weak isospin. And so you can then ask the question like do two electrons attract or repel each other using the Z And the answer is that the electrons have the same we isospin because it's just two electrons, and so they repel each other the same way like two positrons will repel each other. They're the same weak isospin, 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. They're 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 basic rule that like charges repel and opposite charges attract, And it just so happens that like electrons and neutrinos have opposite charges, and upquarks and down corks have opposite electric charges and opposite weak isospin charges. So you know, for example, take an upcork and a down cork. For example, an upcork and a down cork have opposite sign electric charges. One of them is plus two thirds, the other one is minus one third, so they attract each other. They also have opposite week isospin. One is plus a half, one is minus a half, so they attract each other. In that case, the upcork and the down cork, the photon is pulling them together and the z is pulling them together.
I 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. 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 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 of aligned 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 with 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, 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 have any particle that doesn't interact with gravity.
Yeah, although we don't really understand, you know, how particles in gravity interact, you know, what's going on there on the quantum line. 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 attract each other based on their weak isospin, Like two neutrinos will repel each other, and a neutrino and an anti neutrino 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 couple. 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 week isospin, and we call it week hypercharge. And then we have these two numbers week isospin and week hypercharge, and together these four particles can serve 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 week hypercharge and week 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.
Well, I see you noticed that they're sort of part of the same team, or they weren't 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 forces don't care about the conservation of this hypercharge.
That's exactly right, And you know, we tend to think about things that way, where like if things transform together, if when you rotate the world, things move together, we think that there's part of the 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 like falls into pieces, 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 answers the 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 problems kind of.
Yeah, it's sort of like attracts and repels physicists.
All right, Well, I guess maybe one last question is why did you find this question so hard? Like we broke it down and it seemed like something that you know, if two things have the same week, I just spend day repel or and if they don't, they attract. What was the thing that was stumping in.
I think it's just because I never thought about the weak force in terms of its potential.
You mean to push and pull, like its ability to push or pool things, 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, that has an axl 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, as so whether or not we need to dig into that. So one issue was like how deep to go into this? You know, we could do like ten podcast episodes of quantum field theory engage symmetry before we get to the answer.
All right, let's do it, Daniel, I got ten hours?
All right?
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 and 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 eh, if this had a weak force, it would be much cooler.
That's right, it's two 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 not sort of not weak, right, it's sort of strong and it's a mathematical complexity.
Yeah, it's doing a lot.
Man.
Yeah, maybe week is a new strong and isospin is a ne 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 iHeart Radio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is US dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit usdairy dot COM's Last Sustainability to learn more.
Hi, I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I mean neuroscientists at Stanford and I've spent my career exploring the three pound universe in our heads.
Join me weekly to explore the relationship.
Between your brain and your life, Because the more we know about what's running under the hood bet or we can steer our lives. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.
I'm doctor Laurie Santos, host of the Happiness Lab podcast. Is the US elections approach? It can feel like we're angrier and more divided than ever, But in a new copule season of my podcast, I'll share with the Science, it really shows that we're surprisingly more united than most people think.
We all know something is wrong in our culture and our politics, and that we need to do better and that we can be better.
Listen on the iHeartRadio app, Apple podcasts, or wherever you listen to podcasts.
