What is a model?
▶ Transcript
In this chapter, we will focus on a very important aspect of science: modelling. No, not this kind of modelling, but the use of scientific models. We will discuss what we mean by calling something a model and why models are so important for science in general, but especially in particle physics.
Let us start again by recalling what we discussed during the first chapter of this course. You have heard me stating that "we do not know what a particle is" over and over again. And we do not really have a problem with that. Of course, it would be fantastic to know exactly how our Universe functions and how everything looks like on a fundamental level. But this is not how science works. We can only observe our Universe and the objects we interact with as precisely as possible and then try our best to describe these observations as accurately as possible. For these descriptions, we use models – scientific models, to be precise. Indeed, this is the first important message of this chapter. We use scientific models to describe and explain observations of our Universe.
There are plenty of different scientific models. The up quark and the down quark, for example, are part of a scientific model. And the same is true for the proton and the neutron. They are all part of our particle model. But sometimes we use different models to describe the same object or the same phenomenon. For instance, let us look at another elementary particle, the electron. It is probably the most famous elementary particle. However, you might have heard that depending on the situation, we sometimes describe particles like the electron with a wave model, instead of a particle model. This is especially true in quantum physics when we consider low energetic objects. This can be quite confusing. I mean, a particle and a wave seem to be completely different models, so why do we use both of them? Well, every scientific model allows us to describe and explain phenomena only to some extent. Depending on the situation, one scientific model can be better suited than another one. This is the second important message of this chapter. Different scientific models can describe the same object or phenomenon.
So, what are the features of a good scientific model? Well, first of all, it needs to be simple. A good scientific model is a simplified version of an object or phenomenon that helps us to accurately describe and explain processes in our Universe. But a scientific model also enables us to make predictions about the outcomes of new phenomena. Indeed, we can use scientific models in two ways. Either to describe and explain observations or to predict the outcome of new observations. This is the third important message of this chapter. Every scientific model allows us to make predictions.
But of course, every scientific model has its limitations. After all, we never really know how Nature "really" looks like. Depending on the situation, we use the most suitable scientific model to describe our research results. This also means that scientific models evolve over time. Every new prediction about our World can be put to the test through experiments that lead to new observations. As long as they agree with the scientific model, we keep it. 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. This is the fourth important message of this chapter. Every scientific model has limitations.
The Standard Model of particle physics
So, why is this important for particle physics? Well, everything we discuss in the world of particle physics relies on scientific models. We need them to describe and communicate our theories and our experimental results. Indeed, everything we will discuss during this particle physics course will involve scientific models. And here, the probably most famous and most important model is the so-called Standard Model of particle physics. It is an absolutely brilliant model. Actually, the name "Standard Model" is quite misleading. We should really call it the Awesome Model of particle physics!
You have probably heard about the Standard Model of particle physics already. To date, this is the best and most successful model in particle physics. It is usually described as a classification of all known elementary particles. And while this is true, the Standard Model of particle physics is so much more! Most importantly, it accurately describes and explains the fundamental interactions between elementary particles. And it even allows us to make assumptions about hypothetical new particles and interactions that we might discover in the future. We will discuss the Standard Model of particle physics a little bit later in this course, but I wanted to mention it early on as an example of a highly successful scientific model.
Mystery boxes
Now, let us take a closer look at how we use models in science. For this, we can use so-called mystery boxes. We made these ourselves with the help of one of our 3D printers, but they can easily be built out of cardboard or other materials as well. This box now represents an object that we would like to investigate. We do not know how it looks like on the inside or if it has an inner structure at all. And unfortunately, we cannot open the box. It is just not possible. So, we have to make observations and try to come up with a model that describes the inner layout of this box as accurately as possible.
Ok, if I shake the box, we can hear a sound. Apparently, there is something inside that can move freely. Now let us turn the box carefully and listen. We can hear a clear sound from time to time, so probably this box is not completely empty. It might have an inner structure, and based on our first observations, we can make assumptions about it. For me, this is still quite difficult, but I will give it a try. So, I think I can come up with at least three possible models to describe the inner structure of this mystery box. I will call them the circle model, the triangle model, and the square model. Do you think that these are useful models? What do you think is the most accurate model out of the three to describe the inner structure of this mystery box?
To be honest, I do not know myself how this box looks like on the inside. From my initial observations, just based on the clicking sound inside, I think I prefer model number three, the square model. But how can I be sure? Well, as we discussed earlier in this chapter, we can never be entirely sure. Because that is not how science works. We can only try to test our models as precisely as possible and adjust our scientific models based on the results of our experiments.
So, let us try to gather more information about this mystery box. For this, we will have to upgrade our research instruments. This is precisely the same approach as in real research projects. We always try to upgrade and perfect our research instruments to gather more and new data. And this is exactly what I want to do now. Instead of just shaking the box and listening in, I want to investigate the box a little bit more systematically. For this, I will see if I can use this strong magnet to make new observations. Maybe this thing inside is made out of metal, in which case I might be able to move it around with this magnet. Let us give it a try.
Nice! It looks like I can really move this thing inside around with the help of this magnet. Well, this should make things a little bit easier. So, if I carefully move this thing around, I should be able to trace out the inner structure of this mystery box. Yes, it appears that there is one corner here, and a second corner here, and a third one over here. I do not think I missed a fourth corner. No, I think this is quite an accurate description. So, I think I have to reject two of my initial models. With model number one, the circle model, I can definitely not describe the observations I just made with the help of the magnet. And model number three, the square model, looked promising at first, but I do not think that it is the most accurate description of my observations. After all, I only managed to trace out three corners of the inner structure. So, I think model number two, the triangle model, is the best description of the inner structure of this mystery box.
Alright, let us summarise what we just discussed. Same as with this mystery box, we do not know how our Universe "really looks like". This is what science is all about. We perform experiments that lead to observations, and we then try our best to describe these results with a scientific model. In this example, we started by just shaking the box, which allowed us to come up with some initial models. However, once we upgraded our research methods by using this magnet, we could make more precise observations about the mystery box. We were actually quite lucky that this thing inside the box is apparently made out of a type of metal that can be attracted by a magnet. Otherwise, this research approach would not have worked at all. This nicely shows that we also need some luck from time to time. Ultimately, this led us to the triangle model, that we can now use to describe the inner structure of this mystery box.
But can we be sure about this? Well, no, not completely – but based on the observations we just made, we can say with high confidence that the triangle model is a very accurate description of the inside of this mystery box. However, if we perform more experiments, maybe with even more advanced research methods, like an x-ray machine, we might make additional observations that contradict the triangle model. In this case, we would then have to adapt the triangle model or replace it with an even better one that fits all experimental results. This is what we like to call the scientific process, and we will hear more about it later on during this course.