The Science of Everything Podcast - Episode 163: The Standard Model of Particle Physics

Episode Date: August 8, 2026

An introduction to the particles and forces that constitute the standard model of particle physics. After some historical background, we introduce each of the three fundamental forces that comprise the standard model - electromagnetism, the strong nuclear force, and the weak nuclear force. We then compare and contrast the different types of particles these forces interact with, including the difference between fermions and bosons, and between quarks and leptons. We consider the types of interactions experienced by these particles and the three generations of matter. Recommended pre-listening is Episodes 158 & 159: Quantum Electrodynamics Parts 1 and 2. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

Hello, you're listening to The Science of Everything podcast, episode 163, the standard model of particle physics. I'm your host, James Fodor. So this episode, we're going to talk about the different particles and their interactions that exist for all of the other forces beyond just electromagnetism, which is what we discussed in the previous episode in this series, quantum electrodynamics. So this is an extension of that episode. particularly we're going to be talking about the weak and strong nuclear forces and how they can be described using the formalism of quantum field theory and then we'll put everything together and talk about the different categories of particles leptons quarks fermions bosons and so forth so we'll discuss each of these particles how they relate to the three different fundamental

forces and how all of these different things are organized within the context of the standard model. Recommended pre-listening, as I've indicated, is episodes 158 and 159 on quantum electrodynamics, and those episodes in turn have a number of prerequisites. So I'm going to be assuming that you know something about quantum field theory and quantum electrodynamics. So if not, I recommend listening to those episodes in advance. Although I won't go into quite as much detail about the formalism as we did in the quantum electrodynamics episode, but we will discuss things like Legrangians, equations of motion and things like that. So I'm assuming some amount of familiarity with those.

So let's begin with a bit of introduction and historical background to understand what the standard model is and how it relates to other theories in particle physics and quantum field theory. So the standard model describes the interactions of three of the four fundamental forces that exist in nature. Describes electromagnetism and then the weak and strong nuclear forces or just weak and strong forces is there often called. The standard model does not describe the interactions of gravity.

So gravity is the fourth force, and that is described using general relativity, which I've discussed in a previous episode. One of the major outstanding problems in physics is to integrate these theories of general relativity for gravity on one hand, and then the standard model for the other three forces on the other hand, and that is still very much an open problem. So I'm not going to be talking about gravity. So quantum mechanics sort of originated early 20th century, particularly the 1920s and 30s

was when a lot of the initial work was completed. Those theories mostly describe non-relativistic quantum mechanics of single particles. And so that is what you would likely study in introductory courses to quantum mechanics. And it's what I discussed in the first episodes that I did as well on quantum mechanics. For example, episode 14 principles of quantum mechanics. So there I talked about non-relativistic quantum mechanics and the Schrodinger equation and so forth. In the 1940s through 1960s, non-relativistic quantum mechanics was developed into a more comprehensive and broader theory called quantum electrodynamics, which is able to describe quantum systems in which

there are variable numbers of particles and at relativistic velocities. Those are connected, obviously, because when you have very high energies of relativistic velocities, then you can also have interactions that change the numbers of particles. So that is what I discussed in the two episodes on quantum electrodynamics that I talked about, episodes 158-159, and also the earlier more general theories, more general theory of quantum field theory, episode 85. However, as I described at the time, quantum electrodynamics only describes one of the four forces, which is the electromagnetic force.

It does not apply to any of the other forces of nature. And so once essentially this theory of relativistic, multi-particle or variable particle, quantum mechanics, quantum electrodynamics, was developed by the late 1960s, there was a desire to expand that theory to account for other forces of nature. So let's then just take a step back. When did we find out about these other forces of nature? Obviously, scientists have known about gravity and electromagnetism for a very long time. I mean, Maxwell's equations are developed in the 19th century,

and prior to that, of course, people knew about electric charges and things since antiquity. But the two other fundamental forces, the weak and the strong nuclear forces, are much more recent discoveries. Now, I'm not exactly sure when it was sort of regarded as discovered that the weak and strong forces existed. Definitely, it was known that some additional force must exist to bind the nucleus together since the early 20th century. I think around the 1910s or so, the nucleus was discovered to be the consolidated, very small

relative to the size of the entire atom location where all of the positive charge in an atom was located. So it was clear from that point that there must be some other force that's not electromagnetism that keeps the nucleus together. And so you can call that the origins of sort of the discovery of the strong force, or that they didn't know that that's what it was at the time. The weak force is mostly associated with radioactivity. and so early work in discovery and exploration of radioactivity in the early 20th century also fairly quickly gave rise to the notion that there needed to be another force to account for this.

So certainly by the mid-20th century, it was clear that there were two additional forces that needed to be accounted for the weak and the strong force, although initially not that much was known about them. Major theoretical developments in describing these two theories occurred from the 1950s through the late 1970s, which, which established their quantum mechanical nature as the weak and the strong forces. So most of these theories about the weak force and the strong force were developed after the development of quantum electrodynamics.

So most of what we're talking about here, the standard model was sort of all in place by about 1980, although in some cases experimental verification of some of the particles took decades later. Some of the quarks were only discovered in the 80s and 90s, and then the last to be discovered was the Higgs boson in 2012. The point of that is just to establish some sort of historical timeline and to understand how the different, I guess, fields or subjects in quantum mechanics and slash particle physics relate to each other. So essentially, we can think of it as initially was non-relativistic single particle quantum mechanics. That's Schrodinger's equation. That describes things like single

atoms or molecules. That was early 20th century. And then mid-20th century was the development of quantum electrodynamics, which describes relativistic particles and systems where there's changes in particles, like interactions in a particle accelerator, for example, but only quantum electrodynamics, that's the electromagnetic force. And then moving into the later 20th century, that was extended to describe, or new theories were developed using quantum electrodynamics as a template to describe the weak and strong nuclear forces. So then by about 1980, we had more or less the standard model in place. And more recent theoretical work has focused on trying to integrate gravity into the standard model and that that relates to string theory. But there haven't been any

experimental verifications of string theory. So that's not regarded as sort of an established scientific theory at this point. And I may discuss string theory in a future episode. Okay, so that gives some historical background and contexts as to how the different sort of theories relate to each other. Let's now talk about the four fundamental forces in a bit more detail. As I said, there's gravity, which is described by general relativity. Gravity is a very weak force, and that's why it's really only noticeable at very large scales, planets and stars and so forth. Gravity also seems to be only attractive, as opposed to the other three forces which can be attractive or repulsive, so that's another difference. And there's various reasons as to why gravity has been particularly

hard to incorporate into a quantum framework. Again, I'll discuss those in the a future episode on string theory or quantum gravity more generally. So I mention it because I often talk about the four forces, but really we're only talking about three of them, but just bear in mind that gravity is also there as well, but it's not described by the standard model. So let's then move on to the three forces that are explained under the standard model. So the most familiar will be electromagnetism. Electromagnetism affects electrically charged particles. So this includes electrons, for example, but also quarks, which are the constituent particles that make up protons and neutrons. We'll talk more about those in a minute. So those have positive and negative charges as well.

So anything that has an electrical charge interacts with is affected by the electromagnetic force. The electromagnetic force is mediated by a particle called the photon, which you'd be familiar with. It forms, the electromagnetic force allows for or facilitates the formation of bound states, which are sort of like stable states where the particles stay in a consistent relationship with each other over time, a sort of like a local low energy state, such as atoms and molecules. So those are obviously important for most of the rest of the chemistry of the periodic table and so forth, those bound states. So electromagnetism is a very important force. Electromagnetism has an infinite range, and so even very distant charged objects will feel attractive or repulsive forces from other charged objects.

Let's then move to the strong nuclear force. So the strong nuclear force affects particles that have what's called color charge. This is an important concept to get clear. So just as we have electromagnetic charge for, say, electrons or protons and the quarks that make them up, quarks also have another type of charge or another quantum number, if you prefer to phrase it that way, which is called color charge. So anything that has a color charge feels the effect of the strong nuclear force. Unlike the electric charge, which only has positive and negative versions, so there's like two versions of the electric charge.

Color charge has three variants. The colors are called red, green, and blue. And then there's also a corresponding anti-color version, anti-red, anti-green, and anti-blue. So in some sense, there's six different colors, charges consisting of one color and then it's anti-color, just like we have sort of positive and negative charge in electromagnetism. Now it's important to understand that while this term color has historical origins, and it sort of roughly stands in analogy to how the primary colors all combine to give something that's colorless, that's white. So red, green and blue, you combine those together, they give white.

Likewise, if you combine a red quark, a green quark, and a blue quark, you get a composite particle, which is colorless. So they kind of all cancel each other out, if you like. You know, just like if you have a positive and a negative electric charge, they form something that's electrically neutral. Likewise, you need to have one of each color to form something that is color neutral or colorless. So that's where the name originates from. It's the sort of analogy with you combine the three colors together and you get something that's white or colorless. But bear in mind that quarks are too small to be visible and they don't individually scatter light of any particular frequency. So they don't have a color in the way that we think about it.

it's not a physical visible color. It has nothing to do with the color spectrum or anything like that. It's just a name that is a loose analogy to the primary colors combining to give something that's colorless. So quarks, as I was saying, are the major type of particles that are affected by the strong nuclear force because they have a color charge. Quarks are the type of particle that make up protons and neutrons. So as I said earlier, once it was discovered that all of the positive charge of an atom is concentrated in the small nucleus at the center, it was pretty clear that there must be some new force that keeps those positive charges together, because the electromagnetic force would tend to push them apart. And I've also talked about, when I've discussed radioactivity, that nuclei become unstable when you have certain combinations of protons and neutrons.

Like, you need more neutrons to stabilize a large nucleus because essentially, neutrons help to increase the attractive forces to keep the nucleus together, whereas protons also, I mean, they can add attractive force as well, but they also add repulsive forces because the protons will push each other apart because of their, they're all electrically positive charge, so they repel each other. Now, those attractive forces between protons and neutrons that keep nuclei together are precisely due to the strong nuclear force. They're the attractive forces that are felt by any particles that have color charge. Just like the electromagnetic force is mediated by photons. Likewise, the strong nuclear force is mediated by another type of particle

called the gluon. So gluons are directly analogous to photons, although they do differ in important ways which you'll get to. Basically, in loose terms, you can think about a nucleus as consisting of different types of quarks that are all constantly interacting and sort of dancing around together in a sea of gluons, these particles that mediate the strong nuclear force. The strong nuclear force the most complicated force and it's the one that we have greatest difficulty with performing accurate computations particularly with regard to describing the nucleus so i'm not going to get into details of that here i'll save that for a future episode where we talk about the chromodynamics of the nucleus so chromodynamics is the term that's used to describe the uh study of the strong

force just like we have electrodynamics the electromagnetism chromodynamics with the strong force because of the color charge uh one important thing to know about gluons is that gluons have color charge themselves. So that means that they also experience interactions with the strong nuclear force. That's different from photons, which are electrically neutral. So electrons don't directly interact with each other. They can indirectly interact with each other through like scattering off charged particles. But they don't directly interact with each other because they are electrically neutral. So they don't themselves feel the electromagnetic magnetic force in that sense. Whereas gluons have a color charge and therefore they do direct.

directly interact with each other. So that makes the force much more complicated. The strong force also has a very short range. This is related to a concept called color confinement. The strength of the strong force differs depending on the distance. I mean, that's true for electromagnetic force, but in the case of electromagnetic force and gravity, there's an inverse square law. So basically just the further away you go, the further away are two charged particles. The less is the force they experience between each other, and it decreases by a factor of one over R squared. which is sort of a very simple relationship. For the case of the strong nuclear force, there is no simple relationship.

The nature of the force depends on the distance. Beyond a certain point, if you imagine taking two quarks that are bound with the strong force and then move them further and further apart, at some point, the strength of the force between them actually starts to increase with distance, which is very strange if you think about in terms of electromagnetism. We never see that in electromagnetism. The force always decreases with the distance. But in the strong nuclear force, beyond a certain point, it actually increases with the distance.

And this means effectively that beyond a certain point, it's just impossible to pull them further apart because you need forces that are too large. You'll actually just create new quarks using all the energy that you're adding in. So this gives rise to color confinement, which means that quarks can never really get very far apart from each other when they're bound together. And we don't see long distance interactions between color charged particles. They're always bound together. Generally, in groups of two or three or five, there's different possible combinations, but they always have to be colour neutral. We never see isolated quarks that just have a single colour.

As far as we know that colour charged particles are always bound up together in colour neutral combination. So this can either be three different quarks or three different anti-quarks, so red, green and blue combined together to give something that's colour neutral or anti-green, anti-red, anti-blue. The other possibility is that you can have two quark combinations with, say, anti-blue and blue, or red and anti-red and so forth. So that's the other combination that can give rise to an overall charge-neutral object.

Composite particles made up of these of three quarks are called handrons, and these are the bound states that are formed by the strong force, just like we have atoms that are formed by electromagnetism. Hadrons, so protons and neutrons, are the bound states that are formed by the strong force, and they always consist of three quarks together, which, when combined, have a overall colour neutral charge. So they need one of each colour charge. Hadrons can actually have more than three quarks.

They can have five or even theoretically more than that. But usually they have three and they form protons and neutrons. So we'll just talk about it in those terms for simplicity. Composite particles that are comprised of a quark and an anti-quark are called mesons. And they have very different properties to hadrons. And they're not stable for a long period of time. The strong nuclear force is so-called because it's quite strong. So the strength is hard to measure because it depends on the distance.

But at the scale of quarks, comparing it to electromagnetism, it's about 60 times stronger. And that's one of the reasons why the protons are held together in the nucleus, because although they're being pushed apart by the electromagnetic force, the repulsion of all those positive charges, the force holding them together, the strong nuclear force is stronger. So it overcomes that. it depends on the size of the nucleus and how many positive charges you have. But broadly speaking, the strength of the strong force is one of the things that allows nucleinide to be stable.

So one additional wrinkle to mention at this point, I've talked about quarks in general terms and talked about how they have a color charge, you know, red, green, and blue, and they need to combine together to give a color neutral composite particle. But I haven't told you about the different types of quarks. The quarks that make up protons and neutrons, at least in most cases, are called up and down quarks. So there's two types. Up quarks have a positive fractional charge, so positive two-thirds, and down quarks have a negative fractional charge of negative one-third. So that gives up rise to the difference between protons and neutrons.

If you have one up quark and two down quarks, then those charges cancel out, and you have a neutral composite particle. So that is a neutron. On the other hand, if you have two up quarks and one down quark, you have a total charge of positive one, and that is a proton. that's essentially just all there is to the difference between protons and neutrons, it's the combination of clarks they have. Two ups and one down or one up and two downs.

All right. So now let's move on to the weak nuclear force, which is the third of our forces that we're going to be talking about today. The weak nuclear force is probably the least well known, and in some ways the hardest to, it's not the most complicated because the strong is sort of harder to compute, but the weak can be harder to initially understand because it behaves differently in certain ways.

So the weak nuclear force affects all particles that have a special quantum number called weak isospin. And I wouldn't try to explain intuitively what this is. It's just another quantum number. Don't get it confused with what we ordinarily call just spin. All fermions have half integer spin. But there's another different quantum number called weak iso spin, which varies between different types of particles. Most of the fermions either have plus half or minus half. But for our purposes, the most important thing to understand is just that any particle that has a non-zero-week iso spin will be affected by the weak nuclear force.

So that includes the electron, quarks, and also neutrinos, which are another important type of particle that we'll talk a bit more about in a moment. So what does the weak force do? We know what the electromagnetic force does, and we know that the strong force keeps nuclei together. what does the weak force do? Well, it's a bit harder to explain because the weak force does not form any bound states. So remember a bound state is sort of like a stable state with multiple particles joined together in a form that's sort of stable over long periods of time, like atoms and molecules for the electromagnetic force or hadrons and nuclei for the strong force. The weak force doesn't have any bound states. And so that means that it's a little bit more difficult to sort of

intuitively understand what it's doing because it doesn't maintain any bound states in the way that the others do. The simplest way to describe what the weak force does is that it mediates certain forms of radioactive decay. So it is responsible for processes, for example, that can turn a proton into a neutron or a neutron into a proton in an atomic nucleus. We talked about those types of physical changes that occur in radioactive decay in the episodes that I did on that. I didn't really talk about the underlying nuclear physics though, and one of the reasons for that is just because it's very complicated. But what actually happens is that the weak nuclear force mediates changes between different types of quarks. So, for example, you might start with a neutron that has one up

and two down quarks, but if one of those down quarks changes into an up quark, then that neutron will turn into a proton. And that process is mediated by the weak nuclear force and it involves either emission or absorption of neutrinos, depending on other details. So neutrinos are important because they are involved in these radioactive decay processes. Now, just as the electromagnetic force is mediated by the photon and the strong force is mediated by gluons, the weak nuclear force is also mediated by particular particles. They're sometimes referred to collectively as the intermediate vector bosons,

but that's kind of a mouthful and not very informative. So often they're just called by their individual names, the W and Z bosons. So that's the letter W and the letter Z bosons. or Z for our American listeners. The W bosons are often written as W plus and W minus because they have an electric charge. One is plus and one is minus. And the Z boson, or the neutral boson, it's also called, has no electric charge.

And so it's sometimes written as Z zero. So three different intermediate vector bosons which mediate the weak force. Another aspect of the weak force, which is interesting but makes it more complicated, is that the W plus and W minus bosons also themselves have non-zero weak iso spin, which means they themselves experience the weak nuclear force and interact with it. That means they interact with themselves. So that leads to self-interactions, which is very different to how we think about the photon, which doesn't directly interact with itself because it doesn't have a charge.

So just to review the different types of force-carrying bosons, the photon does not have any type of charge. It doesn't have electric charge. It doesn't have a color charge, and it doesn't have the weak ISO spin. So that means it doesn't interact directly with any of the fundamental forces. All it does is mediate the force between electrically charged particles. Now, if we move to the strong nuclear force, gluons have color charge, which means that they do interact with other gluons and with quarks.

so they, as I mentioned before, leads to complicated sort of self-interactions. But blue ones do not have an electric charge, so they don't interact with electromagnetism, and they don't have weak isospin, which means they don't interact with the weak nuclear force. The intermediate vector bosons, the W-1-scent-bozons. Of those, the W-bosons, W-plus and W-W-minus, do have weak isosin, so they do interact, as I said, with other W-bosons and with the weak nuclear force. They also have electric charge, so they interact with the electromagnetic force, but they don't have color charge, so they don't interact with the strong force. Interestingly, the Z boson, the neutral one, does not have color charge, just like the other bosons, but it also does not have electric charge because it's electrically neutral, and it does not have weak iso spin, which means it's quite similar to the photon in many ways, that it doesn't interact with anything else.

All it does is mediate certain types of weak interactions between particles that have weak isospin, but it itself does not directly feel those interactions. It doesn't interact with itself. So in many ways, it's sort of more analogous to the photon, whereas the W plus and minus bosons, as well as the gluons, each have self-interactions. The weak force is also another short-range force. So that's one of the reasons why we don't see its effects very directly in everyday life, because it only affects over very, very short ranges, even shorter than the strong force.

Although its potential behavior is simpler than the strong force, it doesn't have this same sort of confinement property where it gets stronger as you move further apart. The weak force gets weaker as you move further apart, just very, very quickly, exponentially quickly so that it only affects tiny, tiny distances like atomic nucleus scale, or even smaller than the nucleus, actually. The weak force is so named because over the relevant scales, it's weaker than either the electromagnetic or the strong force. At the scale of quarks, it's about 1,10,000th of the strength of the electromagnetic force. Remember that strong force at that scale was about 60 times. So the order

is strong, then electromagnetic, and then weak at that scale. Gravity, by the way, is way down. If electromagnetic has a strength of one, then weak has a strength of 10 to the minus 4. Gravity has a strength of 10 to the minus 41 estimated. So it's absurdly weak compared to the other forces. And why that is is one of the open questions of, well, of all of physics, really, as to why gravity is so different. Okay, so that's an introduction to the three main forces that we're going to be discussing, electromagnetism, the strong nuclear force, and the weak nuclear force. Now, we're going to try to try to systematize this a bit and try to bring a bit of order to all the particles we've been talking about by discussing kind of like the periodic

table of fundamental particles. It's not called that, but that's sort of what it is to help conceptualize what we're talking about. There's a particular way of visualizing and arranging the particles to emphasize their commonalities and differences. And this is one way that if you like Google's standard model, you often see this diagram as an illustration of what the standard model is. It's sort of this description, systematic description of the different particles. So to start with, let's make a distinction between fermions and bosons. This is the most important distinction in the standard model and underpins kind of most of the rest of what we're going to be talking about. So a fermion is a particle that has half integer spin. So spin half. Again, technically it can be like spin

three three over two or five over two as well, but I'll just say spin half is simplicity because the fundamental particles all have spin half. So we know that electrons have spin half and so they are fermions. Quarks also have spin half, so quarks are also fermions. And as it turns out, so do neutrinos, so they are spin half. So fermions are not just fundamental particles. There are composite particles that are fermions as well. We're just going to be talking about fundamental fermions. And So some of the things that I say will only apply to that. We talked about spin before. Spin essentially describes an intrinsic angular momentum.

So we can imagine like every particle is a tiny top that's spinning about its central axis. And the angular momentum that it has as a result of that is described by spin. Spin half actually refers to a wave that has to undergo two full oscillations to return to the same state. So normally if you think about rotating an object, if you rotate at 360 degrees, it'll be back to the same orientation. But that's not necessarily true for arbitrary objects. And there are objects such as a standing wave on a Mobius strip. A Mobius strip is like you cut a piece of paper and then you sort of rotate it about a bend and then glue the two ends together. And you've got a piece of paper that only has one side. If you're not familiar, look up Mobia strip and you'll see

what I'm talking about. If you imagine a standing wave, just a water wave or something similar that was on a Mobia strip. That wave would actually have to go through two full oscillations to return to the same state, because it goes around the Mobi strip once. When it does that, it's actually pointing down. If it starts pointing up, it goes around once and now it's pointing down. It has to go around again to come back to being point up, to pointing up. So that's one sort of loose way of thinking about what's intrinsic spin is,

intrinsic angular momentum. If you imagine, and again, this is by analogy, but if you imagine each fundamental particle as this tiny Mobia strip on a standing wave in kind of a Mobia strip arrangement, then spin half means that that standing wave has to go around two revolutions in order to return to the initial state. Spin one would mean that it only has to go around one rotation to get back to the initial state. Spin two would mean it only has to go around half a rotation to come back to the initial state because it's sort of like symmetric on both sides. So different spins relate directly to sort of how many rotations you have to go around to get back to the initial state. Now, I'm not saying that fundamental particles literally are a standing wave on a Mobius strip.

I'm saying that they have the same properties in this instance as one, and that's a useful way to visualize what's happening. But the point is that fermions all have spin half, and that makes their physics and mathematics a bit more complicated, as we talked about in the quantum electrodynamics episode, when we had to go from a Klein Gordon equation, which describes spin zero particles, to the Dirac equation, which describes spin half particles, and the mathematics becomes much more complicated because of the needed, need to describe spin and how it transforms. So all of the quarks and electron and neutrinos all have spin half. So that's a bit about fermions. Now I said that there's two types of particles, fermions and bosons. Bosons are defined as particles that have integer spin, and they, in the case of the standard model,

the fundamental bosons are mostly these force-mediating particles. So the photon is a boson. Likewise are gluons and also the W-N-Z boson. So that's the term that I use there. It's a bit weird that we use the term W-N-Z boson just to describe the weak force, whereas you could call the photon a boson as well. It's just normally you give it its name. The W-N-Z particles don't really have their own names. They're just written as W-N-Z boson, or just W-N-Z. So don't get confused there. They're all bosons. It's just for some reason we use the term for the weak force ones. So these particles that mediate a force that are responsible for sort of particles feeling the attraction or repulsion of other forces, of other particles because of that force,

they're all bosons. And specifically, they're called gauge bosons. I'll explain later why they're called gauge bosons, but that relates to something called gauge theory. Now, there is one other type of boson in the standard model, and that's the Higgs boson, which I mentioned earlier. The Higgs boson is a bit different to the others. It is a boson because it has integer spin, but otherwise it's really quite different to the others. So generally when I'm talking about bosons, I'm talking about the gauge bosons. We'll put the Higgs aside for a moment and come back to that later. It was the last of it discovered and it has a sort of a very distinct role. So there are four types of gauge bosons, luans, photons, Z bosons and W bosons, but you should probably just

grouped them into three based on the forces that they mediate. Electromagnetism, that's the photon. strong nuclear force glue on weak nuclear force w and z bosons there is a hypothesized boson that mediates gravity called the graviton but that's purely theoretical and we don't really have a description of that so again i'm just mentioning that for completeness but it's not really part of the standard model all of these gauge bosons have a spin of one so that means you know to go back to our moby's strip analogy that they'd have to be rotated a full 360 to get back to their initial states it's kind of more intuitive way in some sense the higgs boson has a spin of zero, so it's different in that respect. So we have this broad division between fermions and

bosons. Again, fermions and bosons literally just refers to the spin. So there's many other types of composite particles that are fermions and bosons. But here I'm restricting it. When I say things about fermions and bosons here, the two, I'm talking about just in the standard model, so just the fundamental particles that are fermions and bosons. So don't get confused there. In that context, there is another sort of way to distinguish between the fermions and bosons, in that fermions, are matter particles. They're sort of stuff. They have rest mass and act like matter. Whereas the gauge bosons can be thought of as energy. They mediate interactions between particles. Although confusing, W and Z bosons also have mass. So that's not exactly as cleanly

a distinction as we would like, but we're trying to help you to understand the distinction. So loosely you can think of fermions are matter and gauge bosons are energy. Although, again, some of them also have mass. But the photons obviously are massless, as are gluons, so they don't have mass either. W and Z bosons do have mass. And that difference actually turns out to be quite important. We'll come back to you later. All of the fermions have mass.

It was historically not clear whether neutrinos had mass. It seems increasingly clear that they do have mass. And indeed, to explain neutrino oscillation, they must have mass. So I'm just going to talk about it as if they do have mass. Some presentations of the standard model will still talk about neutrinos as being mass. That's just because we haven't finished off that theory yet of neutrino mass. So different sources will say slightly different things about this, but it's generally agreed that neutrinas do have mass.

So at this point I've introduced to you the fermions and the bosons. We've talked about the three different classes of bosons. You gluons, your photons and your WZ bosons, one class for each of the three forces. And then we've talked about quarks, which make up protons and neutrons in the nuclear. We've talked about electrons. know a lot of electrons already, and we talk about neutrinos, which are mostly involved in, like, radioactive decay. So at this point, I'm going to introduce another distinction between quarks and leptons. So quarks, I've already talked about how they are involved in the strong nuclear force.

Leptons, we've already met the leptons, that's just I'm introducing a new name for them. Leptons are electrons and neutrinos. They are grouped together as leptons. Why are they grouped together? Well, there's deeper reasons for this. The simplest way to understand the difference is that quarks feel the strong force, whereas leptons do not. So quarks have a color charge, which means that they experience the strong force, whereas leptons do not have a color charge, so they do not experience the strong force.

Pretty simple. Okay, so that's another distinction. Let me review where we are at this point. At this point, I've introduced to you four types of matter particles and three types of energy particles. The matter particles are fermions. They have spin half. They're divided into two.

There's your quarks and your leptons. Two types of quarks, up and down quarks. They make up protons and neutrons. And then your two types of electrons, you've got your electrons, which, well, we know they make up atoms and give rise to electricity and all that good stuff. And then there's you neutrinos, which are mostly involved in radioactive decay. Electrinos are electrically neutral, so they don't interact very often. The up and down quarks have a color charge.

They feel the strong force. Electrons and neutrinos do not have a color charge. They do not feel the strong force. Electrons and quarks all have an electric charge, so they all feel the electromagnetic. force and they all have weak isospin so they all feel the weak force. All right, well that's all nice and neat and tidy, but those of you who are familiar with this will know that there's an important aspect that I've left off at this point for simplicity, but we now need to introduce.

In particular, I've talked about just up and down quarks and electrons as if there's only one type of those, but actually there's not. There's an up and a down quark and then electron and an electron neutrino, actually those four particles form just one of what are called generations of matter or generations of fermions. And there are three of these generations. What does that mean? Well, it means that just like there's an up and a down quark and an electron and electron neutrino, there's also another lot of four of these.

There's a charm quark, a strange quark, a muon, and a muon neutrino. And then there's a third. There's the top and bottom quark, and then a tau and a tau neutrality. So people are probably familiar with the name Electron and up and down quark, you know, at least that's sort of a simple name. It doesn't really mean anything in terms of direction. That's just the names that they have. However, the other names that are introducing here are probably sounding a bit weird.

So like charm and strange, top and bottom, like what do these mean? The names don't really mean anything. They're historical reasons why they have these names, but we could just as well call them like quark one and quark two and quark three, quark four, quark five, quark six might actually be more useful because they wouldn't be as confusing. I know that these names were a bit weird to me for a while. Try not to read too much into the names. Just like color charge doesn't mean visible color. Charm and strange. There's nothing particularly strange about strange quarks. They're not really strange than any other type of quark or like top and bottom. Top and bottom quarks are heavier

than the other quarks, but that's the only real sense in which their names are meaningful. And that's for Mewan and Tao, well, they're just names. They're not really of any particular significance. So now what are the differences between these three generations? of Fermions, apart from the fact that they have weird names. What the main difference lies in their mass. So essentially the first generation, up and down quarks, electron, electron neutrino, have the lightest mass. If you go to the second generation, Charm and Strange Quarks, Muon and Muon Neutrino, they all

have a higher mass. It's not like a strict multiple though. There's no simple relationship between the masses of the first and second generations, or the second and the third, but they are all bigger in the second generation. And then when you move to the third generation, top and bottom quarks, and then the tail and the town neutrino, they all have the higher mass again. Ordinary matter is made up pretty much entirely of up and down quarks and electrons, and then decays mediated by the electron neutrino. The other two generations are, to my knowledge, not really found at all in ordinary matter because the particles are all unstable. They have a very short half-life.

And so they're only really formed in rare interactions. Like, for example, cosmic ray particles from outside of the solar system that hit the Earth's atmosphere have extremely high energy and result in scatterings that form these higher generations of matter. But they're unstable, and so they quickly decay down into the first generation. So we don't normally see these higher generations in ordinary matter. And so for many purposes, we can kind of ignore them. However, when we're talking about the standard model, you can't ignore them because they actually form essential. They play essential roles, which will get to you later. That's actually partly how they discovered because it was sort of determined that we need these high generations in order to account for certain behaviors.

But most certainly ordinary chemistry and most physics that we're familiar with involves only the first generation. Apart from the differences in half-life, like stability and the differences in mass, otherwise the generations all have essentially the same properties. So, for example, a muon, other than its half-life and higher mass, behaves like an electron. on. A charm quark behaves like an up quark and so forth. Again, there are certain differences in certain contexts which you'll get to, but broadly speaking, that they have the same properties. So, for example, a charm quark has the same spin as an up quark. I mean, they all have spin half, so that's the same. It has the same charge as an up quark, so positive two over three. There are properties in terms of color charge are also the same. So charm quarks can have any of the three colors and then

they're corresponding anti-colors, just like an up quark can. So there's no different. between them in most respects. There are only very certain respects in which they differ, which we'll get to in a little bit. Now there's one further complication to the standard model, which is that each of the particles that I've talked about also has its corresponding antiparticle. So you've probably heard of antimatter. The most commonly known example of antimatter is the anti-electron, which is also called a positron. Antimatter has exactly the same properties as regular matter, except it has the opposite electric charge. So a positron is the same as an electron except it has a charge of positive one instead of minus one. Every other particle

also has its corresponding antiparticle. So just like there's an upquark, there's an anti-upquark, a down quark, an anti-down quark, and so forth. And in fact, because antimatter has the same properties as regular matter, apart from the electric charge, you can actually form antiprotons, anti-neutrons which are just formed of their regular up-and-down quark constituents except replaced with the anti-up and anti-quark versions you can even form an anti-hydrogen atom which has an anti-proton and an anti-electron orbiting it. It's even been hypothesized that you could theoretically have entire planets made up of antimatter with exactly the same chemistry as our planet just with the antimatter versions although as far as we know no such things exist because the very

early universe seemed to have a huge asymmetry between matter and antimatter, and almost all of the antimatter was eliminated. When an electron and a positron come close enough together, they'll eliminate, they'll cancel each other out effectively and produce a photon. The same thing happens with other types of matter and antimatter as well. So early in the universe, it's thought that there was at some point equal matter and antimatter, but most of the antimatter was eliminated, leaving a small residual left over, which is the matter. But we don't know where this initial asymmetry came from. That's still an open question. So theoretically from the standard model, there could just be as much matter and antimatter around as each other. They could be equal because all of the forces work the same

as far as you know with matter and antimatter. But as it turns out, the universe currently is filled almost entirely with matter and we don't entirely know why this is the case. So at this point, I've introduced all of the particles that are in the standard model. So there are 24 different fermions. That's a spin half particle and think matter particles. So there are six. Six quarks, six leptoms, and then the corresponding anti-quarks and anti-leptons. There are then three different sort of families of gauge bosons, each corresponding to a different fundamental force. There's the gluons for the strong nuclear force, photon for the electromagnetic force, and the W and Z bosons for the weak nuclear force.

Bosons have a spin of, or these bosons, I should say have a spin of one, and they can be thought of as energy as opposed to the matter that constitutes the fermions. So that concludes the introduction to the standard model that I wanted to talk about today. In the next episode, we're going to get into more detail into particularly gauge theory, which I've mentioned earlier. And that's really the heart of the standard model is understanding these symmetry relationships within each of the three fundamental forces and then how they kind of relate to each other and the mathematics group theory behind that.

So we'll talk about gauge theory. and then I'll also discuss the Higgs boson and the Higgs mechanism more generally in the origin of mass and spontaneous symmetry breaking. So be on the lookout for that. If you enjoyed this episode, consider giving the podcast a favorable rating or review on the aggregator of your choice. Another way you can support the podcast is to go onto YouTube, where we have a lot of the episodes, a lot of the past episodes, have been now uploaded with visual accompaniments. So if you would like to give favorable ratings or leave comments on those videos, I would greatly appreciate. that. That's a good way to support the show, help bring it to a new audience. If you'd like to make a one-off donation through PayPal to my email, Fods12 at gmail.com, or you can also become a

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