What is a particle
▶ Transcript
We will start with the most difficult question right at the beginning. It is the one question, we scientists here at CERN are most afraid of because it is so difficult to give a correct answer to it. The question is, "What is a particle?". This is really a tough one, because, to be honest, we do not know. Do not get me wrong – we do have a good idea about what we mean by describing something as a particle. But what a particle really "is", we do not know.
If you were to ask different physicists from around the world, they would all give you a definition of what they think a particle is. You would probably hear a variety of different models and descriptions of particles. But nobody would be able to tell you what a particle "is"! Indeed, this is the first important message of this chapter. We do not know what a particle is.
And to make it even more confusing, in science, the term "particle" is used to describe completely different things. For example, physicists use it to describe fundamental objects, such as the electron or quarks. But we call more complex objects, such as the proton and the neutron, particles as well. And if we zoom out a little bit more, we can look at atoms and even molecules and call them particles as well. This is usually done when discussing different states of matter, such as gases or liquids. So, this is the second important message of this chapter. The term "particle" can have different definitions that depend on the context.
That is why we want to discuss what we mean by calling something a particle. We will still not be able to tell you what a particle "is", but we will provide you with a general definition of a particle, that is commonly used in high-energy physics, and that we will be using throughout this entire course.
Elementary particles
We will start with the tiniest objects in physics, the so-called elementary particles. To our current knowledge, these are the fundamental objects of our Universe, and we consider them to be indivisible. But of course, this is by no means final. As science progresses, we might find even smaller, more fundamental particles. But for now, we can make incredibly precise calculations and predictions with the particles that we call elementary particles. There are quite a few of them and not only do they have quite strange names, but they can also be sorted in different categories and so-called generations. It gets confusing quite quickly. That is why we will not go through all of them right now. We will have plenty of time throughout this course, and at the end, you will have a super nice overview of all elementary particles and how they interact with each other.
For now, we will only focus on two elementary particles to get going. These are the up quark and the down quark. These are two of my most favourite particles. Both of them are elementary particles. They have different properties that we can measure and as far as we know they are indivisible. But please do not ask me how they look like! We have no idea! We cannot even see them. We only know they exist because they interact with our research instruments, for example, with our amazing particle detectors.
Since we do not know how they look like or what these quarks "are", we will not even bother thinking about it. We know these elementary particles exist, and we can work very precisely with them. This is the third important message of this chapter. There are elementary particles that are indivisible, and that can be measured, but we have no idea how they look like.
Composite particle systems
Now you are probably wondering why the up quark and the down quark are two of my most favourite particles. Well, these two particles are incredibly important for our Universe. Let me explain what I mean.
The up quark and the down quark are elementary particles, but we never observe them on their own. Instead, we always find them confined in different configurations. For example, if two up quarks and one down quark are bound together, we call this composite particle system a proton. Similarly, two down quarks and one up quark bound together make up a different composite particle system, namely the neutron.
You have most likely heard of the proton and the neutron before. These are quite massive objects that make up all the matter of our world. And yes, they are usually called particles as well. This can be confusing as they are made of smaller particles. That is why we will as of now use the term "particle" exclusively to describe elementary particles. This will help to distinguish them from composite particle systems, and we will avoid a lot of confusion later on in the course. This is the fourth important message of this chapter. Elementary particles can combine to make up composite particle systems that have particle-like properties.
Now I want to highlight a super easy trick how you can memorise what a proton and a neutron are made of. Let us look at the proton and the neutron one more time in detail. We see that the symbol of the proton is a "p". If we flip this symbol around, we see a "d". So, it is easy to remember that the proton is made of one down quark and two up quarks. But it gets even better! This trick works for the neutron as well! Let us have a look. We see that the symbol of the neutron is an "n". If we flip this symbol around, we see a "u". So, it is easy to remember that the neutron is made of one up quark and two down quarks. Do you need to know that? Of course not. But why not memorise it if it is that easy. And who knows, you might be able to impress the next particle physicist you meet with this trick! :)
Observing particle tracks
Ok, but how do we know that particles exist? I mean, we keep on repeating that we do not know what particles look like, so why are we so sure they are even real? Well, because we can observe them. Ok, not them. We can observe the signals they leave when they interact with our particle detectors. Let us have a look at this particle detector, for example. This is quite an old one. It is a so-called cloud chamber. It is one of the oldest particle detectors but probably one of the most impressive ones.
Now, does that mean we can see particles with our eyes? No. We cannot see the particles themselves. We can only observe the signals they leave when they interact with a particle detector. In this case, particles leave these white tracks when they interact with a cloud chamber. In a way, this is similar to observing tracks of animals. If we see a series of footprints, we assume that this track was caused by an animal crossing a patch of sand.
Let us look at the cloud chamber one more time. If you look closely, you might notice that you can also observe different types of tracks in this particle detector. For example, you can see short and fat tracks from time to time. And you can also see long and thin tracks. And sometimes you can see short and curly tracks. These different types of tracks are caused by different types of particles with different properties.
Ok, let us come to an end and summarise the key messages of this first chapter. First of all, we do not know what a particle is. Moreover, the term "particle" can have different definitions that depend on the context. For example, there are elementary particles that are indivisible, and that can be measured. But we have no idea how they look like. These elementary particles can combine to make up composite particle systems that have particle-like properties.