Topics Physics and everyday science

What is absolute zero?

Absolute zero is the lowest possible temperature: zero on the kelvin scale, which is about minus 273 degrees Celsius or minus 459 degrees Fahrenheit. Temperature reflects the average motion of the particles in something, and absolute zero is the floor of that scale, the point where no more energy can be removed from a system. As far as physics says, nothing can be cooled all the way there.

What makes it interesting is that the floor is not simply stillness. Quantum physics says particles keep a small leftover motion, called zero-point energy, even at the lowest energy state. Getting close is also hard, because each step colder takes more effort. Laboratories have cooled small samples to a tiny fraction of a degree above zero, where matter does strange things, like flowing without friction.

An episode would start with what temperature actually measures, then walk through the kelvin scale, why the floor exists, and why we can approach it but not arrive. bre's hosts are AI, so they can be wrong, and you can press Talk to ask a question mid-episode.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. What temperature really measuresTemperature reflects the average energy of motion of atoms and molecules, which is why hot things have jostling particles and cold things have slower ones.
  2. The kelvin scale and its zeroKelvin starts at the floor instead of at an arbitrary point like the freezing of water, so zero kelvin is about minus 273 degrees Celsius.
  3. Why there is a floorOnce a system is in its lowest energy state, there is no further energy to remove. Physics also gives strong reasons why that state can never be fully reached.
  4. Zero-point energyQuantum physics says particles retain a small residual motion even in the lowest state, so absolute zero is not the same as everything standing perfectly still.
  5. How labs get closeTechniques such as laser cooling and magnetic cooling bring small samples to a tiny fraction of a degree above zero, though never exactly to it.
  6. Strange matter near the floorSuperconductors and superfluids appear at very low temperatures, and they are used in scanners, sensors and quantum research.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, picture a cup of coffee. The heat in it is just a huge number of molecules bouncing around. Temperature is basically how much they're bouncing, on average.

  2. CalAI host

    So cold means slower. I get that. But then my brain goes, how slow? Can you just keep slowing things down forever?

  3. breAI host

    That's the question. The answer is no. The scale has a floor, a point where there's no more energy left to take out. That's absolute zero.

  4. CalAI host

    Hold on, how does that actually work? Why can't you just keep pulling heat out? Like, I can always put my drink in a colder freezer, right?

  5. breAI host

    You can, but each step gets harder, and at the floor there's nothing left to remove. And as far as physics says, nobody can quite reach it. People get astonishingly close.

  6. CalAI host

    And it's not even perfectly still down there, is it? I read something about atoms still wiggling a little. Which feels like cheating.

  7. breAI host

    It's quantum physics, so it's a little cheating. Yes, a small leftover motion stays. Okay, here's the part nobody tells you: the floor isn't a stop, it's the lowest state.

  8. CalAI host

    So zero kelvin is less 'everything freezes solid' and more 'the lowest the energy can go.' Let's start with the scale itself.

Questions people also ask

What is absolute zero in Celsius and Fahrenheit?
Absolute zero is zero kelvin, which is about minus 273 degrees Celsius and about minus 459 degrees Fahrenheit. The kelvin scale starts at that floor, so it has no negative numbers, and one kelvin step is the same size as one Celsius degree.
Can anything reach absolute zero?
As far as physics says, no. Cooling gets harder as you approach the floor, and the laws of thermodynamics are usually understood to forbid reaching it in a finite number of steps. Laboratories have cooled small samples to a tiny fraction of a degree above it.
Do atoms stop moving at absolute zero?
Not completely. Quantum physics says particles keep a small residual motion, called zero-point energy, even in their lowest energy state. So absolute zero means the lowest energy a system can have, not perfect stillness.
Is there anything colder than outer space?
Yes. Deep space is about 2.7 kelvin, because it is filled with leftover radiation from the early universe called the cosmic microwave background. Laboratory samples have been cooled far below that, to a tiny fraction of a kelvin.

Related topics

More: all 300 topics, physics and everyday science, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.