Why specifically −273.15 °C?

It's where you'd extrapolate to if you cooled an ideal gas and watched its pressure fall linearly. The pressure-vs-temperature line crosses zero pressure at −273.15 °C. Below that, you'd need negative pressure, which has no physical meaning.

Has anyone actually reached it?

No. The third law of thermodynamics forbids it: each step of cooling requires more energy than the last, so you can get arbitrarily close but never all the way. The record is currently around 38 picokelvin (38 × 10⁻¹² K) achieved with cold atom experiments.

Would everything stop moving at absolute zero?

Classically yes, but quantum mechanics says no. Even at 0 K, particles retain 'zero-point energy' — irreducible quantum jitter from the uncertainty principle. Helium, for example, never freezes solid at atmospheric pressure because its zero-point motion is enough to keep it liquid all the way down.

What Is Absolute Zero? The Coldest It Can Possibly Get

A temperature floor, not a temperature

If you ask "how cold can it get?" — that turns out to be a real, specific number. Absolute zero: 0 kelvin, equal to −273.15 °C, or −459.67 °F. Below it, the question stops meaning anything.

How do we know there's a floor? The story starts with gases.

How gas pressure points at the answer

If you trap a fixed amount of gas in a sealed container and slowly cool it, the pressure drops. Cool it more, pressure drops more. The relationship turns out to be a clean straight line.

Now extrapolate the line backward: where would the pressure hit zero? For every gas you can do this experiment with — air, helium, nitrogen, argon — the line crosses zero at the same temperature. −273.15 °C. Every time.

Below that, you'd need negative pressure to keep the line going, which is nonsense. The temperature where pressure (and, equivalently, where the molecular jiggling that produces pressure) hits zero is the natural bottom of the temperature scale. That's why physicists use kelvin: it's the Celsius scale shifted so the floor is zero. Water freezes at 273.15 K, boils at 373.15 K, and there's no negative kelvin.

Why "absolute"

The Celsius scale is calibrated to water — 0 for freezing, 100 for boiling at atmospheric pressure. Fahrenheit is calibrated to a brine solution and a horse, depending on which historian you ask. Both are arbitrary; you could shift them by any number.

The kelvin scale isn't arbitrary. It's anchored to the temperature where motion would, classically, stop. There's only one such temperature; it's universal. That's what absolute means here.

What "motion stops" actually means

Temperature, microscopically, is the average kinetic energy of particles. A hot gas is fast-jiggling molecules; a cold gas is slow-jiggling molecules. Cool the gas further and the jiggling slows further. At absolute zero — classically — molecules would have no kinetic energy at all. They'd sit perfectly still.

That's the textbook picture, and it's almost right. Quantum mechanics adds one wrinkle: thanks to the uncertainty principle, a particle can never have exactly zero position and exactly zero momentum. Even at 0 K, every particle retains a small irreducible amount of motion called zero-point energy.

For some materials this is a footnote. For helium, it's the headline. Helium has so little mass and such weak interatomic forces that its zero-point jitter prevents it from freezing at all under atmospheric pressure, no matter how cold you make it. You can chill liquid helium to near 0 K and it stays liquid — the only substance that does this.

Why you can't actually reach it

The third law of thermodynamics, stated roughly: it's impossible to bring any system to absolute zero in a finite number of steps. Each step of cooling requires more effort than the last, so you can get arbitrarily close — picokelvin, femtokelvin — but never touch zero.

The current world record sits around 38 picokelvin (38 × 10⁻¹² K), achieved with laser-cooled atoms in microgravity aboard the Cold Atom Lab on the ISS. That's about 100 billion times colder than the cosmic microwave background, which itself is a brisk 2.725 K.

Why so much effort? Because at those temperatures, very strange things happen. Bose-Einstein condensates form — clouds of atoms that collapse into a single quantum state and behave like one giant atom. Superconductivity, superfluidity, quantized vortices, all show up. The cold isn't the point. The cold is the condition that lets quantum behaviour become visible at the scale of objects you can see.

Outer space is not absolute zero

Common confusion: empty space is cold, but it isn't 0 K. Outer space's "temperature" is dominated by the cosmic microwave background, the leftover heat from the early universe. That's 2.725 K — close to absolute zero, but distinctly above it.

If you put an isolated thermometer in deep space, far from any star, it would equilibrate to roughly 2.725 K. To cool it below that, you'd need active refrigeration. The James Webb Space Telescope's instruments run at around 7 K passively, then go colder using mechanical coolers.

The takeaway

Absolute zero is a real, specific, unambiguous temperature: −273.15 °C. It's where classical motion would stop, where the kelvin scale starts, and where reality refuses to let you go entirely. Every approach to it gets exponentially harder, and along the way, you discover that the universe behaves in ways that completely contradict everyday intuition. The floor isn't dull — it's where most of modern condensed-matter physics lives.