# It’s cold out there

Having lived in Waikato for five years now, I’d forgotten what cold temperatures feel like. I’ve just got back from Christmas in the UK (explains lack of blog entries) where I was reminded what winter is like: dark and cold. Minus 9 feels pretty nasty, but, how cold is that really?

Anyone from, for example, Moscow will tell me that minus 9 is positively tropical compared to what they would experience in winter, where minus 20 is quite achievable. But for a physicist to call something cold we need to go pretty low indeed.

For example, at minus 196 Celcius, nitrogen will liquify. This temperature is fairly easily achievable and means that liquid nitrogen is commonly used in laboratories, allowing those fun demonstrations of superconductors floating on top of magnets. But that’s not really all that cold. Getting down to minus 269 Celcius, we have helium liquifying. Physicists get excited here, because helium as a liquid can do some really strange things. But we can get colder still. Deep space is about minus 270 Celcius, or about 3 degrees about absolute zero, which is the coldest possible temperature.

What does absolute zero mean? Temperature is related to the idea of thermal energy – an object can have energy which, broadly speaking,  manifests itself as vibrations of atoms – in other words its atoms do not sit still. The more energy the atoms have, the more they vibrate, and the hotter we say the object is. Leaving quantum mechanics aside, at absolute zero (minus 273.15 Celcius), an object has no thermal energy at all in it. It is not possible to get it colder, because  there is simply no energy left to remove from it. That  really is cold. Absolute zero is, unfortunately, unachievable in practice.