Temperature is really just how hard the tiny particles inside something are jiggling about. Warm things jiggle fast; cold things jiggle slowly. So what happens if you keep cooling and cooling? The particles slow down more and more, until they reach the slowest they can possibly go. That point is the coldest anything can ever get — about −273 °C, which scientists call 0 on the kelvin scale, or absolute zero. You can't go any colder, because there's simply no jiggling left to remove. Drag the slider down in the simulator and watch the whole box of particles freeze into stillness.
Most people think you could always make something a bit colder. In fact temperature is just how hard particles jiggle, and at absolute zero, −273.15 °C, that motion hits its minimum; you cannot go colder because there is no jiggling left to remove.
What's actually happening
The key idea is that heat is not a glowing fluid hiding inside warm things — it's motion. Every particle in any object is constantly jiggling, and temperature is simply the average of how vigorously they do it. A hot cup of coffee is full of molecules barrelling about; a cold one is full of the same molecules moving sluggishly. Once you see temperature as jiggle-speed, the question 'how cold can it get?' turns into a much sharper one: how slow can the jiggling go?
There's an obvious answer. The jiggling can slow right down until there is essentially none left, and at that point you have run out of motion to take away. That bottom of the scale is absolute zero: −273.15 °C, which is why scientists invented the kelvin scale that simply starts counting from there, so absolute zero is 0 K, ice melts at 273 K, and room temperature is about 300 K. You can cool something toward that floor, stripping out jiggle, but you cannot push it below, because below would mean less than no motion — and there is no such thing.
The strange twist is that you can approach absolute zero but never actually arrive. Cooling works by carrying heat away into something colder, and as your sample gets achingly close to the floor, each step removes a tinier and tinier slice of the little energy that's left, like trying to reach a wall by always halving the remaining distance. Physicists have chilled atoms to within a few billionths of a kelvin, colder than anywhere in deep space, and found bizarre new behaviour there, with matter forming strange shared states. But the floor itself stays just out of reach, an exact temperature that nature lets you approach forever and never touch.
Absolute zero is a floor you can approach forever but never touch, because each cooling step removes a smaller slice of the energy that remains.
- 1Put a few drops of food colouring into a glass of hot water and an identical glass of cold water at the same moment.
- 2Watch: the colour spreads fast and wildly through the hot glass and creeps slowly through the cold one — you are literally seeing the difference in how hard the water molecules are jiggling.
- 3Now imagine cooling the cold glass further and further until the spreading stops entirely. That standstill is what absolute zero would be.
Common questions
Cooling works by carrying heat into something colder, and each step removes a smaller and smaller fraction of the energy that remains. Labs have chilled atoms to within billionths of a kelvin, but the floor stays just out of reach forever.
It begins counting from the coldest possible point rather than from freezing water, so 0 K is absolute zero, ice melts at 273 K and a warm room is about 300 K. That way no negative temperatures are ever needed.
Yes. The cosmic background sits around 2.7 K, but laboratory fridges have chilled atoms to billionths of a kelvin, making those experiments the coldest known spots in the entire universe.