 So temperature tells us how much the atoms in some substance are jiggling around at random. But how can we actually put a number to it? Well, you can measure a temperature in any way you like, but the way it's used by physicists is the Kelvin scale. Now to have a nice scale that's useful for physicists, you'd like two things. One is that you'd really like the temperature to be zero when the atoms are not jiggling at all, when it's so cold that all the motion has stopped. And experimentally that happens at a temperature called absolute zero. And absolute zero is minus 273.15 degrees centigrade Celsius. So the Kelvin scale starts there. Zero Kelvin is minus 273.15 Celsius, and zero Celsius is plus 273.15 Kelvin. So the temperature as measured in Kelvin equals the temperature as measured in Celsius plus 273.15. So Kelvin has that nice property that when the Kelvin temperature is zero, the atoms are not moving. You can never actually reach that, as we'll see later, but you can get pretty close. And now we want what is actually happening when the temperature increases. Temperature could be a measure of the average speed, but in practice what we use is that temperature is a measure of the average kinetic energy of the particle. Particularly, you can define temperature by the equation that the kinetic energy, this is the average kinetic energy of all an atom. Some atoms are going fast and slow at any particular moment, but the average kinetic energy is equal to 3 halves, a constant K, a Boltzmann constant, times the temperature as measured in Kelvin. So K is the Boltzmann constant. It has a value of 1.38 by 10 to the minus 23 joules per Kelvin. So with that equation, if you know the temperature, you can look at the average kinetic energy of a particle. Now that's not all the energy that is in a hot object. A hot object might also have some energy stored in the bonds. It might also have some energy stored in the rotation and vibration of molecules. But the kinetic energy alone is given by that. How cold can you actually get something? When it turns out the answer to that is in a lab just down the corridor, I'm recording this. Let's have a look. Hello, I'm Kyle Hardman and we are in the Quantum Sensors Lab at the Australian National University. Inside this machine here, we have the coldest place in Canberra and possibly the coldest place in the universe. So in this apparatus, we cool down a gas of rubidium atoms and we're able to cool them down to about a billionth of a degree above absolute zero. At this point in time, we actually have go through a phase transition in which we make a new state of matter, an exotic state of matter called the Bose-Einstein condensate. How do you get it this cold? So we have two methods of making the atoms very cold. The first is we slow down the kinetic energy of the atoms through collisions and interactions with photons, which are light particles. The other method is we use evaporative cooling, which is similar to how a pot of coffee cools. Essentially, you let the hot atoms leave the collection and the rest re-thermalize at a colder temperature. And what do you actually use this for? Why do you do this experiment? The cold atoms in this experiment are used for the precision measurement of gravitational acceleration. So we actually make the cold atoms, then we drop them and we measure their position as they're falling. For measuring the position, we can find the exact value of the gravitational acceleration at this point in space.