 In this video, we're going to ask, what is temperature? Now, we all know what temperature is from everyday experience. We know that when you're lying on a tropical beach, there's lots of temperature. It's very hot, and sometimes it's not so hot. But what actually is this? Philosophers and scientists have debated this for over a thousand years. For a long time, they believed there was some sort of mysterious substance, which was called calorific, or something like that. And when there was lots of this invisible fluid in something, it was a hot something, and when lots of this invisible fluid had gone away, it was a cold something. But no one had ever seen this fluid. It seemed a bit strange. And in the end, when it was discovered that matter was made out of atoms, an easier explanation came to mind. Let us zoom in at the atoms, let's say, in the back of your hand. In the computer graphic simulation, I've shown the atoms as blue spheres, and the chemical bonds between them as orange springs. Now, chemical bonds are not, of course, orange springs, but they do actually behave a lot like springs, so this gives us a fair picture of what things look like on the atomic scales. And what you can see is the atoms in the back of your hand, or in your desk, or in your computer monitor are not stationary. They are moving around at random. Some are moving up, some down, some forwards, and some backwards. And this is what we now think temperature is. The faster the atoms are moving, the higher the temperature. So let's say we take your hand and we plug it into liquid nitrogen. Don't try this at home. Then it will be much colder. The atoms will still be jiggling about, but as you can see in the simulation, they're jiggling about very slowly. You'd have trouble even noticing. If, on the other hand, we take your hand and put it into boiling water. Again, don't try this at home. The atoms will now be moving like crazy, jiggling all over the place, up, down, left, right, in and out. And that's what high temperature looks like. That's in a solid where there are chemical bonds between everything. If we look at a gas, like the air that you're currently breathing, this is what it's doing. The air molecules are bouncing around, up, down, left, right, bouncing off each other all the time. In this particular simulation we've put a small number of air molecules inside a box and we've coloured one of them in yellow, and the yellow one is leaving a trail behind it. And this shows you the sort of path that the molecules are doing. They're bouncing around, constantly colliding with other things, changing their speed and direction. That'd be hot gas. If it was very, very cold gas, it would look more like this very slow and very sedate. Now this actually means that really there is no such thing as temperature and heat. Temperature and heat is just the words we use to describe random motions of large numbers of atoms. We could dispense of them and just calculate the motion of every atom if we liked. We could take each atom and calculate all the forces acting on it and the accelerations as it moves around. And then if we had a really good supercomputer, we could do it for all the atoms. Then we'd have no need of the concept of temperature. Unfortunately, even the head of a pin has more than a billion billion atoms in it, which is well beyond the capacity of the world's fastest supercomputers to simulate. So in practice, calculate where every atom goes. People can do it if they're trying to study, for example, a molecular cluster or maybe even a very small protein, but for anything realistic you can't do it. So how do you calculate situations when you just have so many atoms you can't follow them all one at a time?