Alright, now that we have discussed the concepts of charges and interactions, let us continue with one of the four fundamental interactions and its associated charge. Specifically, we will discuss the so-called strong interaction. It is not necessarily something we encounter in our everyday life, but you will see, this interaction is incredibly important. To start, we will first refresh our knowledge about the structure of matter and how we can describe it.

There is an easy definition of what matter is. Matter is everything that we can touch. I am made of matter, this wall is made out of matter, even the air is made out of matter. This is the first important message of this chapter. Matter is everything that can be touched, practically or theoretically. Yes, I say practically or theoretically, because this is also true for objects that are difficult to touch. For instance, if you ever encounter a lion in wildlife, you do not have to touch it to see if it is made out of matter. Trust me, it is! :) But the same is also true for objects that are far way, such as, for example, the Moon. So far, only a few human beings were fortunate enough to walk on the surface of the Moon. In doing so, they touched the Moon and proved to us that it is indeed made out of matter.

Ok, but what is matter? What is matter made of? How can we best describe it? These questions have been with us for more than 2500 years. Already in ancient Greece, the philosopher Democritus came up with the best model so far to describe what matter is. According to his model, matter consists of indivisible units, which he called atoms. In Greek, "átomos" means indivisible and that is how Democritus imagined these atoms. His description of matter is that everything consists of tiny, indivisible atoms that can connect with each other to form larger structures. Today, we call these combinations of atoms molecules. This is the second important message of this chapter. Matter is made of atoms that can combine to form molecules.

This model is now very old, but on a general level it has been proven to be very accurate. Yes, it is still used in science. However, we had to update it several times. As we discussed earlier at the beginning of this course, this is in the nature of scientific models. We only keep them as long as they fit our experimental observations. But the moment an experimental result disagrees with the predictions of a scientific model, we need to change the model or even reject it completely.

In the case of atoms, we discovered that they are not indivisible. Instead, we found that an atom is best described by dividing it into two areas: a tiny nucleus space in the centre and a huge orbital space all around. All of it is empty space, but somewhere in the orbital space it is likely to find the electrons of an atom. Indeed, we describe the orbital space as the probability density of the electrons. And in the nucleus space, the protons and the neutrons of an atom are located. The precise number of protons, neutrons, and electrons defines the properties of an atom and which chemical element it can make up.

We already discussed that we describe the electron as an elementary particle. Because, as far as we know, it really is indivisible. The proton and the neutron, however, are composite particle systems that are made of up quarks and down quarks. Do you remember the trick I told you during the first chapter of this course how to memorise their composition? It has been some time, but maybe you still know what a proton and neutron are made of? If you want, you can pause the video and think about it for a couple of seconds.

Alright, did you manage? It is super easy. The proton is represented by a "p" and if we flip this symbol around, we see a "d". So, the proton is made of one down quark and two up quarks. And the neutron is represented by an "n". If we flip this symbol around, we see a "u", which helps us remember that the neutron is made of one up quark and two down quarks.

And now we have completed the overview of the subatomic structure of matter. Atoms, as Democritus had imagined them already more than 2500 years ago, are not indivisible after all. But our current understanding of the atomic model is that atoms are made of indivisible particles. Specifically, they are made of quarks that form the protons and neutrons in the nucleus space and of electrons that can be found somewhere in the orbital space. This is the third important message of this chapter. Atoms are not indivisible, but they are made of elementary particles that are indivisible.

Strong interaction

Ok, but what has the atomic model to do with the fundamental interactions that we discussed earlier? Well, to be honest, I still have not given you the full details about the subatomic structure of matter. Let us have another look at the proton and the neutron, and I will explain what I mean.

So far, we have used these very simple representations of the proton and the neutron. Each of them is made of three quarks. One down quark and two up quarks make up the proton, and one up quark and two down quarks make up the neutron. But this is not yet the full picture. You might remember from the first chapter of this course that we consider quarks to be elementary particles, but we never observe them on their own. Instead, we always find them confined in different configurations. Like in the case of the proton or the neutron, we see three quarks stuck together to make up a composite particle system. And the reason for this confinement is one of the four fundamental interactions, the so-called strong interaction. Indeed, one of the quarks' properties is their strong charge, which allows them to interact via the strong interaction.

So, how do we describe this kind of interaction and how is it mediated? Well, let me introduce you to another one of my favourite particles. You might have already heard of it, we call it the gluon! Yes, like the glue. This is exactly where its name comes from. The interaction particle of the strong interaction is called gluon, because it is responsible for confining quarks together into composite particle systems. You see, we physicists try to make our scientific models as simple as possible. But of course, gluons have nothing to do with glue. I mean, we do not even know what particles are! This is just a simple way to describe our model of quarks and gluons. So, this means that we can now update our representation of the proton and the neutron by adding gluons between the quarks. This is the fourth important message of this chapter. Because of the strong interaction, which is mediated by gluons, quarks are confined in composite particle systems, such as the proton or the neutron.

Colour charge

Now, let us look a little bit closer at our current understanding of the strong interaction. Specifically, I want to focus on the concept of strong charge. This might still sound strange to you, as we usually only talk about one kind of charge – the electric charge. But as we already mentioned, particles can have up to three charges. Or, let me be precise, we know of at least three different charges. The electric charge is quite straight forward. An electrically charged particle can either be positive or negative. But how does this work for the strong charge? I mean, if we look again at the proton and the neutron, we see that this binary model of plus and minus does not work if we have three quarks. This was exactly the problem that a lot of physicists were facing in the 1960s! They needed a model to describe how the sum of three things can equal something neutral. And what they came up with is an analogy to what we already know from optics: combining red light with blue light and green light will result in white light. This is a property of visible light, called additive colour mixing. It has nothing to do with particle physics, but it is a brilliant example of how three things can sum up to equal something neutral. Well, and this is exactly what the physicists used in the 1960s. According to their scientific model, every quark is assigned a colour to represent their strong charge. That is why we describe the strong charge as colour charge. Again, this is really just a model to distinguish quarks from each other. So, if we look at the proton, we see that in this case the down quark is shown with a red charge, and the two up quarks are shown with a blue charge and a green charge. And together their three colour charges add up to white, which means in this model that the proton is colour neutral. But of course, it is also possible to have the following configuration: now the down quark is shown with a blue charge, and the two up quarks are shown with a red charge and a green charge. According to this model, every quark can have any one of the three colour charges. The important aspect is that we have to have three quarks with three different colour charges. It is not possible to combine, for example, one quark with a green charge and two quarks with red charges. This would not add up to white and therefore the model does not allow us to combine them together into one composite particle system. This is the fifth important message of this chapter. The charge associated with the strong interaction is described as colour charge.

Quark puzzle

The concept of colour charge can be nicely illustrated with this quark puzzle. Here, I have different pieces. Each of them represents one quark. This, for example, illustrates an up quark with a blue colour charge. This one is a down quark with a green colour charge. And here is another down quark with a red colour charge. But you see, we have 3D-printed all possible combinations. So, there is a red up quark, a blue up quark, and a green up quark. And the same is true for the down quarks.

Now, this puzzle works as follows: we have to combine three different quarks into a triangle. In this puzzle, a triangle illustrates a composite particle system. For example, we already know that the proton is made of one down quark and two up quarks, that are held together by the strong interaction, which is mediated by gluons. So, if we want to puzzle together a proton, we just have to find a down quark and two up quarks, like this. But we have to make sure that each quark has a different colour charge. Here, I have a red down quark, so now I only need to find two up quarks – one with a green charge and the other one with a blue charge. Alright, and we see, the three puzzle pieces fit together nicely and form a triangle. If I would want to replace one of the two up quarks with a red up quark, the pieces would not fit together, because this puzzle really follows the model of colour charge. Instead, I can replace one of the two up quarks – for example this blue one – with a down quark of the same colour. Now we have three different colours again and the pieces fit together into a triangle. However, this is no longer representing a proton, because it combines only one up quark and two down quarks. So, this represents a neutron instead.

As you can see, we have 3D-printed the pieces ourselves. And of course, all files are freely available online for you to print them yourself as well. The link is down in the video description. But there is also a simpler version of the puzzle pieces available, if you want to try it out at home. If you download the files, you will notice that they not only contain pieces for the up quark and the down quark. You will find several additional pieces of different quarks. Yes, this is correct. According to our current understanding, there exist a number of quarks and all of them can combine together in different configurations to make up composite particle systems. As long as the colour charges of the quarks add up to neutral, various different combinations are possible. Does this sound interesting to you? Well, then you are in luck because a little bit later on we will highlight all quarks and the possible composite particle systems they can combine into. But now we will continue by discussing another essential concept of physics, the fundamental conservation laws.

DIY Experiment

3D-printable Quark Puzzle