 Hi, I'm Zor. Welcome to Unizor Education. Today we will talk about strong forces, the forces which are inside the nucleus of the atom, and they hold basically the nucleus together. Now, this lecture is part of the course called Physics for Teens, presented on Unizor.com. The course presented there, well, it's a course which means there is a manual, there is a sequence of lectures which logically follow one from another, and I do suggest you to basically take the whole course on the website rather than just individual lectures which you can find on YouTube or somewhere else. Now, the website contains also a prerequisite course called Math for Teens. Knowledge of mathematics is mandatory to study physics, so I do suggest you to at least get acquainted with this course as well. Well, maybe you know the math, so everything is fine then. For each lecture there are notes which basically are arranged like a textbook. For each lecture there is a corresponding chapter, if you wish, of the textbook. The website is totally free, there are no advertisements, no financial strings attached, even signing on is not necessary. I mean, you can do it, but it's not necessary. Well, okay, that's it. Let's get to strong forces. What I'm supposed to talk about today is a contemporary view, and I have simplified it in many aspects. I avoid completely all the calculation part of it. So, talking about a general understanding of what strong forces actually are and how they act inside the nucleus with certain details, obviously, but not the nitty gritty of quantum theory. Absolutely not. It's beyond the level of this course. So, and again, there are certain things which are like theories. Some of them are not necessarily firmly confirmed by experiment. But nevertheless, again, it's introduction into a theory of strong forces. It's not really a complete explanation of how everything is working. So, let's start from the concept which we used to deal with before. It's a concept of field. And examples are gravitational field and electromagnetic field. Now, what's important about the field as we know it? Well, number one, we need something which we call a charge, right? Well, the charge is the word which we have borrowed from the electromagnetic field. Electric charge, basically. Positive or negative. In the world of gravitation, the equivalent of the charge is mass. But we will use the word charge equally applicable to electromagnetic field as basically their charge and the gravitational field, which basically is something which we use to call mass. But we will use the word charge generally for the field. So, if there is a field, it means somebody should produce this field. And what exactly produces the field? A charge. Okay. So, field needs the charge. Also the field needs. Field needs something which acts and the force, basically. What is the field? Field is something where we feel the force of something. Well, the force of this charge, for example. So, if you are, let's say, positive charge and there is another positive charge if you are repelling force, right? Now, here we have, let's talk about electromagnetic field. Here we have duality between the wave theory and corpuscle theory. And we have basically agreed that electromagnetic field has basically dual properties. And there is something which we called photon, if you remember. Photon is something which is basically a carrier of the energy. And you can say that exchange of photons, which basically means exchange of quantum, of energy, is what makes actually the force. So, if there is a field, there is a charge, and there is a force which is supposed to be connected to something which basically activates that force. It's a carrier, if you wish, of energy of the field. So, the photon is a carrier of the energy of electromagnetic field. Now, speaking about gravitational field. Well, there is a theory that there is something which is called graviton. And, again, there are some experiments which were suggesting that they exist. But in any case, as a theory, we do kind of accept, or we don't find that this is completely outrageous, that there is something which is called graviton, which is, again, a quantum of energy of gravitational field, which basically carries this energy. And manifests, existence of these manifests as a force. Now, why do I talk about this? Because strong nuclear force is something which we definitely observe. Let's talk about the nucleus. Nucleus contains protons and neutrons. Protons are positively charged, which means they all repel each other. We need force which keeps them together. It's not gravitational force, because it's much weaker than even electromagnetic field. I mean, like, millions of times weaker. So, there is another force. So, this is called a strong force, which people basically have to have. I mean, I'm not really saying we observe it, but the manifestation of existence of this force is that our, well, universe exists. And the atoms exist, and the nucleus is not really breaking apart because of repelling electric forces. Now, if there is a force, it means there is a force field, because it's not really acting like pushing each other. No, it's on a slight distance from each other, right? So, if there is a force, there is a field, if there is a field, there is a charge, and there is a carrier. Okay, so we have come to a conclusion that there is something which is called a strong nuclear force field, that it needs a source of this field, something which is called basically a charge, and we need carriers, agents which carry the energy of the strong nuclear force from one participating particle to another. Okay, so that's kind of a logical explanation of what will be next, because the next will be basically an explanation of what is the charge and what are the carriers of these charges, carriers of energy of these charges in the strong nuclear force field. Now, I'm not talking about why, I'm talking about how. So, I will explain how people are, how physicists are actually thinking about what this particular force field is and what are the carriers, etc. I cannot answer the question why it exists this way. I don't know why two positive, electric positive protons are repelling each other, or positive and negative are attracting to each other. But I can explain what exactly is happening. So, the same thing with this nuclear force field. Let's not talk about why it exists and basically some kind of a reasoning or maybe axiomatic derivation from something else. Well, it exists, we witnessed to this, we experimented with this and there is a theory of what exactly are the charges which produce this field and what exactly are particles which carry the energy of this field, the quantum of energy of this field. So, that's what I'm going to talk about, not why, but exactly how it's done according to contemporary view and it was different like 50 years ago when I was in high school, I was not really learning anything like that because it did not exist. And in the whole revolution about this started in mid-60s of last century. About 70 years ago or whatever, 60 years ago. Okay, so charges and particles that carry the energy of strong nuclear force field. Now, first of all we know about quarks. We did mention that protons and neutrons and some other complex particles contains quarks. Now, there are different quarks. I did mention that something like up quark, down quark, top quark, bottom quark, strange quark and something else I don't remember. So there are six, I think, types of quarks and corresponding anti-quarks. Now, since quarks are inside the proton and neutron and some other particles, like mesons for instance, they have to be kept together. And this is the strong force basically. Something which maintains the integrity of protons and neutrons and some other complex particles, that's a strong force. Then there is another manifestation of that strong force. How these particles are held together, proton and proton, or neutron and neutron, or proton and neutron. How these strong forces hold particles together. So it's two different questions. Number one, how quarks are held together to form a heavy particle inside the nucleus. It's called nucleon, by the way. The general term about proton and neutron is nucleon because they are in the nucleus. So how nucleon is held together as one particle. And the second, how different particles of a nucleus, different nucleons, are held together to form a nucleus. So these two things we will consider separately. So first of all, let's talk about strong forces inside a nuclear, inside a proton or a neutron. Okay, now we have talked about the theory and again I'm not talking about why. I'm just talking about how it's basically done according to contemporary view. Proton, proton basically is a combination of three quarks. Up quarks, up quarks and down quarks. Each one of them has, now, up quark has plus two-third electric charge and down has minus one-third. So two-third plus two-third is four-third minus one-third, the charge will be one. So that's basically what it is. It's one minimal unit of charge, same as electron has only one but minus one. Proton has plus one. Now the neutron has one up and two downs. So it's plus two-third, minus one-third, minus one-third. It's zero, so the neutron is electrically neutral. Okay, so now the next thing, speaking about charges. And again, I'm not talking about why, I'm talking about how it's basically, how it's viewed. Now according to contemporary theory, there are not two like in electric field charges, positive and negative. Not one type of charge like in gravitational field. Mass is always positive, but three. So this is a theory which was suggested again in the middle of 1960s. And according to this theory, there are three different types of charges. Combined together, they make it neutral. And as positive and negative together in equal proportions will give you zero charge. Like neutron for instance, positive and negative gives you zero. Same thing in case of strong forces, but there are three different types. Again, not one like gravitation, not two like electromagnetic, but three. Three different charges make up basically a strong field. Now we have to call these charges somehow, right? Now in the world of electricity, we call it positive and negative. Now why do we call it this way? Are they really like negative numbers and positive numbers? No, but we can measure these electric forces with some clever devices. And the way how we measure it, we basically use some kind of a unit of charge and we convert the charge into a unit. And it happens to be that the positive charge we call positive because we convert it into a positive real number. And negative charge we convert into negative real number. That's how our devices were basically done. Now we use basically these types of charges in electromagnetic field. How it is convenient basically to operate with these. Now we have three for strong force. So we have to somehow name them to conveniently manipulate with them. And what's interesting is the computer technology related to colors is built on RGB principle red, green and blue. The combination of red, green and blue in equal proportions gives you basically no color at all. Either zero for white and actually... I don't want to call it zero. If you combine them together in equal proportion you will have the white color, right? Also you can combine them in any other proportion and get any other color. But that's beside the point. In equal proportion RGB gives you white. And the absence of RGB gives you black obviously. Now if you take an opposite to these... If you take for instance opposite to R, so what is opposite? Opposite is something which added to real gives you let's say white. So what's opposite to R? Well obviously G to B, right? Since G and B and R, blue, green and R gives you white. It means that blue and green is opposite to R. And similarly opposite to green is what B plus R and opposite to blue is R plus G. So what are these colors? This is magenta I think. This is green and blue is cyan, right? And this is what R and G yellow. So opposite to R, to red is cyan, opposite to green is magenta, opposite to blue is yellow. Alright, now we know this from the colors. And considering that we have this arithmetic of colors people have decided to call three different types of strong nuclear field charges. Colors, red, green and blue. And then there are quarks and anti-quarks. Anti-quarks have anti-colors which means cyan, magenta and yellow. So that's basically the names. They have nothing to do with visible light and its colors as we perceive it. Everything is happening inside the nucleus. But we call them this way because it's convenient to do arithmetic. Same way as we call electric charges positive and negative not because they are real numbers, positive and negative but because it's convenient to operate with them. So these are three colors which basically are names for three different types of charges. So I hope it doesn't really sound strange that we call a charge red or green or blue. That's the name of it. We call it positive and negative for electricity and we call it red, green and blue for strong forces. Now what's interesting is that to maintain neutrality of the color let's use the color analogy. To maintain neutrality of the color we need three colors together combined in the same kind of quantity if you wish. But there is no question about quantity right now because we are talking about quarks. That's the smallest thing. So that's the smallest amount of quantity and it's the same quantity no matter what quark we take. So if we take one particular quark it has certain charge. It might be either red or green or blue. But we need three quarks to maintain stability. Now what does it mean to maintain stability? Look at for instance atom of hydrogen. It has positive proton, one proton and one negative neutron. They are together, they hold together or they hold on together. If you take a separate electron it will go and go and it will look for wherever the positive charge is to stick to it. So it's kind of active until it gets the pair. So same thing here. If you would like stability, neutrality and by the way you have to observe the particle, right? So if you would like it to be observed it should be somehow stable. So you can see this in some kind of device or have a trace of it in some kind of device. So we need three colors. So we need three charges, three colors, three quarks. So these three quarks will have to have different colors to maintain neutrality of proton or a neutron. Now they can be different. It can be RGB, it can be GB, R or RBG or whatever. As long as there are three different colors. So three different colors means three different charge types of three different quarks which make up a particle inside a nucleus, a nucleus, a proton or a neutron. That's what's needed. Okay, so we have three charges. That's good. Now there are some other by the way particles like mesomes for instance, mesome which are a combination of two quarks. But again we need neutrality which means it can be something R and let's say R opposite which is like red and cyan or it can be blue and whatever is yellow or something. And that can be different quarks. For instance this can be U and this can be anti-quark, the lower case. Up quark and anti-down quark which should have anti-color to this one. So the combination of these two colors gives you neutrality and the combination of quark and anti-quark gives you mesome. So these are not parts of the nucleus. They're just flying somewhere. Primarily they were discovered in cosmic radiation and obviously we can artificially produce in some smart devices like cyclotrons, whatever. Okay, so we have come to understanding of what exactly charges which produce the strong nuclear field are. Okay, so charges are fine but now we need agents which basically deliver these charges like photons for electromagnetic field or gravitons for gravitational field. We need some agents which deliver the force which maintain the glue between three different quarks to make a proton or a neutron. Okay, so we have charges, now we need agents, we need particles which deliver the energy of these fields. Okay, and this is a particle which called glue on and obviously it's related to glue which glues together different quarks. Okay, so what's interesting about glue on? Well, the way how it's explained is glue on carries, it's not a quark, it's a completely different particle. So quark has one single charge which is either red or green or blue, I mean that's the names which we antique quarks have, cyan, magenta and yellow for instance. Now, the glue on has two colors, it has two charges together. It's a theory, again. I'm not sure how much experimental it was really confirmed but it's a theory and it gives the right results of experiment as far as we can predict certain things and then these things happen which means our prediction is based on the theory which seems to be right at the moment. So, at the moment we are thinking about particles which are for strong field equivalent to let's say photon or for electromagnetic field. It's the particle which carries a quantum of energy. We call it glue on and we are saying that, and this is the difference, photon doesn't carry electric charge within itself but glue on does but what's interesting is it carries two different charges at the same time. Don't ask me why. Telling how we view this thing. So, it carries the color, some color and some antique color let's say blue or something. So, the combination of these two colors is inside this glue on and then, and this is again similar to photon and two different charged particles exchange photons to basically that's how the energy is transferred and the force is manifested. Same thing here. Now, this is a particle which quarks exchange between themselves and that's what establishes the force which holds them together. Now, let's talk about how it's done as far as mechanism of exchanging gluons. Let's say quark which has a color green emits a glue on which has a combination of two colors G and R and anti-R. So, we're talking about glue on as having always two different types of charges. Some charge from the main colors and another charge from anti-color, anti-charge. So, this is green, this is cyan. So, what happens? Well, what happens is the following. With this quark, there is an arithmetic. That's the original charge, right? Green. Now, we emit this glue on which means we subtract it. So, we subtract G and we subtract anti-R which is equal to quark. Well, G minus G is canceled. What's minus R? It's a negation of cyan which is red, right? Negation of anti-red is red. So, what happens is the G quark is converted into R quark. So, the charge is transformed from green to red. Now, obviously this guy, this glue on should be consumed by another quark. Now, there is a quark G in the proton or a neutron so there must be quark R and another quark blue, right? So, let's talk about the quark R. Now, this quark consumes this glue on. What happens? Well, it means that we have a new quark. I mean, the same quark but with different charges. R plus G plus anti-R which is equal to, obviously it will be equal to G because R and anti-R would cancel each other, will give you white color basically which means no color and gives you G. So, this converts from red to green. So, what happens when glue on of this type is exchanged between green and R. Green converts into R, R converts into green so they exchange colors. And this exchange of the colors actually is a manifestation of the force between these two quarks using the exchange of gluons. And what happens in contemporary understanding of this mechanism is that these quarks inside the proton or any other particle like meson, for instance, they are exchanging these gluons like crazy all the time. So, the colors are always shifting, always exchange of colors. Red to green, green to red. Blue to green, green to blue. And this exchange basically holds the nucleon or any other particle which consists of quarks together. Well, that's as much as I can say basically about what's inside the nucleon, inside the proton or neutron and how the quarks are actually held together using this exchange of gluons constant with a huge speed obviously. Well, that's what it is. Now, the last thing which I wanted to talk about is how different protons and neutrons are held together inside the nucleus. And again, as I was saying, it's a result of the same exchange of gluons, but there is a twist in this because now we are going outside of the boundary of one particular nucleon, outside of proton or outside of neutron and we have to have a force between two different nucleons like proton to proton or neutron to proton. How is that done? Okay, now it's a mechanism. It's called residual strong force. It's not exactly the same as this one, but again I will just give you an explanation how physicists understand it right now. And again, there are some indirect confirmation experimentally, but anyway. Here is what happens. Let's consider we have two particles. One particle is called proton and another is called neutron. So index is particle number. One is proton, another is neutron. So what happens? Now, let's say proton. It contains up, up and down quarks, right? And now the quarks inside the proton are exchanging the gluons like crazy all the time. Now what happens is for some reason, and again, don't ask me why, this proton emits a pair of down and anti-down quarks. Now since it's down and anti-down, it doesn't really change the electric component. It's still positive. So everything seems to be more or less intact. Why it happens? Again, as I told, I don't answer the questions why. But let's consider it does happen. Now, when it happens, it emits these quarks. But again, the quarks are exchanging gluons all the time. So the colors are changing. So there is always some kind of a very transitive environment. And what happens is that this D is substitutes one of these quarks and this one is getting released. So instead of P1, we have U, D and D quarks plus U plus D. But now this is a configuration not of a proton. This is a configuration of neutron. So the proton becomes neutron. The first index is one, you see? So that's what happens. And this together is a combination of quark and anti-quark. And it's called pi meson or pion. Now, this thing goes to this one. And two. Now, its combination is up, down and down since it's a neutron, right? Now, plus U, D, what happens? Well, the same thing happens basically. U replaces D. D goes out, so it's N2, U, U and D plus D plus D entity. And they somehow annihilate each other since it's too different. So it's the same quark and anti-quark. So they annihilate each other. And what happens after this? Our first particle, which used to be proton, becomes neutron. And this, as you see, I put N2, but now it's actually P2, right? It's a proton because it's up, up and down. And the neutron becomes proton. So this proton and neutron, this and this, basically exchange their type. This becomes N1 and this becomes P2. Isn't that interesting? I mean, it's kind of wonderful thing. So that's what actually, this conversion, inside the nucleon we exchange colors and that's what holds together. Here we exchange type between protons and neutrons. Proton becomes neutron, neutron becomes proton. Well, and that's establishing this conversion, this transitive movement is establishing this link between them and that's what holds them together. It's called residual strong force. Okay, and now as a consequence of this, you see why in the nucleus the number of protons and neutrons, well, at least in light elements it's the same, basically, because they're always converging one into another. In heavier elements, neutrons, the number of neutrons exceeds the number of protons. So protons must be exchanging with somebody always because that's what holds protons from repelling each other. So that's why we have a number of neutrons at least as much or greater than the number of protons. Neutrons do not repel each other, but protons do. Protons needs this force to be held together. Neutrons do not. So in some cases protons exchange their color with some other neutron but not necessarily all neutrons have this connection. Or maybe they have, you know, once proton exchanges with this neutron and another time with that neutron and back or something. I don't know. That's it. That's all I wanted to talk about strong nuclear forces. It's a lot. And I suggest you to read the notes for this particular lecture. You go to Unisor.com. It's a physics 14 course. Next topic is atoms and then elementary particles. And this lecture is about one of the elementary particles chapters. That's it. Thank you very much and good luck.