The strange physics of absolute zero and what it takes to get there

Weird things happen down at -273°C, the coldest possible temperature. Now we’re building quantum fridges, which could make things even weirder

By Karmela Padavic-Callaghan

TO IMAGINE working your way down the temperature scale into the realms of extreme cold, you might start with the inside of an industrial freezer. At about -18°C (0°F), it is uncomfortable, but bearable with some warm clothes. Now, turn your mind to the -60°C (-76°F) or so that explorers experience during the Antarctic winter, a temperature so severe it can ruin human flesh. And then, for the ultimate in cold, think of space itself at -270°C.

Except, it might surprise you to hear that the coldest places in the universe aren’t in space, but in the physics departments of many universities. Here, over the past few decades, researchers have been contriving ways to reach ever closer to the coldest possible temperature, absolute zero. In the process, we have entered a new realm where bizarre states of matter can flow uphill, chemical reactions can be paused and designer materials can be assembled.

Now, though, the conquest of cold is entering a new phase as we build the ultimate cooling technology: the quantum fridge. Power up one of these ultimate cooling machines, and things get stranger still, with heat flowing backwards and temperature itself ceasing to have any meaning.

The question of what it means for something to cool is thousands of years old. In 450 BC, the Greek philosopher Parmenides thought up frigidum primum, a hypothetical substance that was as cold as possible and could imbue other objects with coldness. In 1664, after …