The coldest place in the universe

This is the inside of a fridge at Lancaster University that is sometimes the coldest place in the universe when it is working. The very cold bit is actually a small pot at the bottom of the structure hanging from the ceiling. The temperature is a small fraction of a degree above absolute zero, zero Kelvin. This is -273 degrees Celsius. I explained absolute zero to my class using this model

It has been branded the "Wobble Cube"! I use it as a more accurate than normal representation of a solid: atoms are joined and in a regular pattern but are free to move. Temperature is a measure of the amount of "atom wobble": the more they are wobbling, the higher the temperature. Absolute zero is when they stop wobbling altogether. You can't get "negative wobble" so that's the coldest possible temperature. Celsius is a wonderfully human temperature scale: I experience ice and boiling water every day. But it's limitation is the need for negative temperatures. -273 degrees Celsius exists but -274 degrees Celsius is as real as a unicorn! You can say it but that doesn't make it real.