What Happens at Absolute Zero? Why −273.15 °C Can’t Be Reached, and What Gets Strange on the Way

Absolute zero, 0 K or −273.15 °C, is the lowest temperature there can be. It is often described as the point where “all motion stops.” That description is wrong in an interesting way, and the real story is better: you can never reach absolute zero, atoms keep a minimum jiggle even in principle, and on the way down matter does things that no room-temperature intuition prepares you for. Our Absolute Zero tee is the mascot for this one.

What is absolute zero?

Temperature measures how thermal energy is shared among the microscopic parts of a system. Since 2019, the kelvin itself has been defined by fixing the Boltzmann constant at exactly k = 1.380 649 × 10⁻²³ J/K; one kelvin is the temperature change that changes the thermal energy kT by that amount (BIPM^[1]). Absolute zero is the bottom of that scale: the limit in which a system is in its lowest possible energy state.

Does everything stop moving at absolute zero?

No. Quantum mechanics forbids a particle from having both a perfectly sharp position and a perfectly sharp momentum (the uncertainty principle), so even in the lowest-energy state there is leftover motion, called zero-point energy (Britannica^[2]). The most famous consequence is helium. At ordinary pressure, helium never freezes, even as the temperature approaches absolute zero; you have to squeeze it to make it solid (Britannica^[3]). Its zero-point motion is too large for the weak attraction between helium atoms to lock them into a crystal.

Why can’t we reach absolute zero?

This is the third law of thermodynamics, in the form physicists call the unattainability principle: no process can reach absolute zero in a finite number of steps or in a finite time (Masanes & Oppenheim^[4]). Walther Nernst proposed it in the early twentieth century. For a long time it was argued over rather than proven in general. In 2017 Lluís Masanes and Jonathan Oppenheim published a derivation in Nature Communications that applies to arbitrary cooling processes, “even those exploiting the laws of quantum mechanics or involving an infinite-dimensional reservoir,” and quantified the resources needed to cool a system to any given temperature (Nature Communications^[4]).

The intuition: every cooling step removes some fraction of the remaining heat, but the closer you get to zero, the less heat there is to remove and the harder each step becomes. You can approach as closely as your time and resources allow; you never arrive.

How cold have we actually gotten?

Laboratories now work routinely at small fractions of a kelvin. The point is not the record itself but what appears at these temperatures: quantum effects that are normally hidden by thermal jitter start to dominate whole objects, as the rest of this post shows.

What is a superfluid?

Below about 2.17 K, liquid helium-4 becomes a superfluid that flows with no measurable viscosity. Pyotr Kapitsa in Moscow and, independently, John Allen and Don Misener in Cambridge reported the effect in back-to-back papers in Nature in 1938 (Allen & Misener, Nature^[5]; NobelPrize.org^[6]). Kapitsa received the 1978 Nobel Prize in Physics “for his basic inventions and discoveries in the area of low-temperature physics” (NobelPrize.org^[6]).

Why does it happen? Feynman’s lectures give a clear sketch: a helium-4 atom behaves as a boson, and at low enough temperature the atoms all tend to fall into the same state, so the liquid moves together and resists being broken up into the random motions that cause viscosity. Helium-3, whose atoms are fermions, does not do this in the same way (Feynman Lectures III-4^[7]). That fermion/boson split is exactly the subject of our pillar post Why Can’t Two Electrons Share a State? Our Superfluid tee and the older post The Cool Science of Superfluidity cover the creeping, frictionless flow.

What is the Meissner effect?

Some metals lose all electrical resistance when cooled below a critical temperature: they become superconductors. In 1933 Walther Meissner and Robert Ochsenfeld showed something more: a superconductor actively expels a magnetic field from its interior as it cools through the transition (Britannica^[8]). That expulsion is why a magnet can float above a superconductor. It shows superconductivity is a genuine new state of matter, not just “very good conductivity.” Our sticker Expulsieur is a bouncer for magnetic field lines.

And the quantum Hall effect?

At low temperatures and in strong magnetic fields, a thin layer of electrons shows a Hall resistance that locks onto exact steps of h/e² divided by a whole number. Klaus von Klitzing won the 1985 Nobel Prize in Physics for the discovery (NobelPrize.org^[9]), and the effect is now used to realise the ohm. That is Stairohm, our staircase character.

What is a Bose–Einstein condensate?

If you cool a dilute gas of bosonic atoms far enough, a large fraction of them falls into the single lowest-energy state and behaves as one quantum object. Einstein predicted this in the 1920s, building on Satyendra Nath Bose’s statistics (Britannica^[10]). It was first achieved in 1995 (Britannica^[10]), and Eric Cornell, Wolfgang Ketterle and Carl Wieman shared the 2001 Nobel Prize “for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms” (NobelPrize.org^[11]). See our Bose sticker.

Can anything be colder than absolute zero?

Not colder, but there is a twist. Temperature, in the Boltzmann sense, describes how particles are spread across energy levels. If a system has a maximum energy, you can arrange for more particles to sit in high-energy states than low ones. The Boltzmann formula then gives a negative absolute temperature. Edward Purcell and Robert Pound did this with nuclear spins in 1951 (Phys. Rev.^[12]). In 2013 Simon Braun and colleagues in Munich created a negative-temperature state for the motion of ultracold bosonic atoms in an optical lattice (Braun et al., arXiv^[13]).

Such a system is not below zero; it is, in a precise sense, hotter than any positive temperature, because heat would flow out of it into anything it touched. Whether “negative temperature” is the correct description, or an artefact of using Boltzmann’s rather than Gibbs’s definition of entropy, is debated (Dunkel & Hilbert^[14]; Frenkel & Warren^[15]). Our sticker Invertia carries that label openly. The entropy side of this story is in Why Does Time Only Go Forward?

Why do materials crack in the cold?

Low temperatures do not only produce quantum effects. Some materials that bend when warm can snap when cold, a very classical failure called brittle fracture, which is why cryogenic engineering cares so much about material choice. Our Cryo Fault tee is about that failure mode.

Common misconceptions

• “At absolute zero, atoms stop.” Zero-point motion remains (Britannica^[2]).
• “Absolute zero has been reached.” It cannot be, in a finite number of steps or finite time (Nature Communications^[4]).
• “Negative temperature means colder than zero.” It means hotter than any positive temperature, for systems with an energy ceiling, and the interpretation is debated.

Key terms in plain English

• Kelvin (K): the SI unit of temperature, defined since 2019 by fixing Boltzmann’s constant (BIPM^[1]). A change of 1 K equals a change of 1 °C.
• Ground state: the lowest-energy state a system can have. Absolute zero is the limit in which a system sits there.
• Zero-point energy: the energy left in the ground state, required by quantum mechanics (Britannica^[2]).
• Superfluid: a liquid that flows with no measurable viscosity, first seen in helium-4 (NobelPrize.org^[6]).
• Superconductor: a material with zero electrical resistance below a critical temperature, which also expels magnetic fields (Britannica^[8]).
• Bose–Einstein condensate: a gas of bosons cooled until many atoms share the lowest state and act together (Britannica^[10]).

Why does cold reveal quantum behaviour?

At room temperature, every atom is constantly jostled by thermal energy, of order kT. Quantum effects are still there, but they are scrambled: atoms occupy so many different states that their wave-like character averages out. As temperature falls, kT shrinks until it is smaller than the energy gaps between quantum states. Then particles start to settle into the lowest available states, and whether they are fermions or bosons begins to matter for the whole sample. Feynman uses exactly this argument for liquid helium: at high temperature there is enough thermal energy to put atoms into many different states, while at low temperature they all try to go into the same one (Feynman Lectures^[7]). That is the common thread linking superfluids, superconductors and Bose–Einstein condensates.

What is the third law useful for?

The unattainability version of the third law, as Masanes and Oppenheim show, puts a quantitative price on cooling: the colder you want to go, the more time or resources you must spend (Nature Communications^[4]). For quantum computers and sensors that need very low temperatures, that price is a real engineering constraint.

A short timeline of the race toward absolute zero

• Early 1900s: Walther Nernst formulates the heat theorem and the unattainability principle that became the third law (Nature Communications^[4]).
• 1933: Meissner and Ochsenfeld show that superconductors expel magnetic fields (Britannica^[8]).
• 1934: Pyotr Kapitsa develops a method for producing liquid helium in large quantities (NobelPrize.org^[6]).
• 1937–38: superfluidity of helium-4 is discovered by Kapitsa and, independently, by Allen and Misener (Nature^[5]).
• 1951: Purcell and Pound create a nuclear-spin system at negative temperature (Phys. Rev.^[12]).
• 1995: the first Bose–Einstein condensates are made in dilute atomic gases (Britannica^[10]).
• 2013: negative temperature is achieved for the motion of ultracold atoms (Braun et al.^[13]).
• 2017: a general derivation of the third law’s unattainability principle is published (Nature Communications^[4]).
• 2019: the kelvin is redefined by fixing Boltzmann’s constant (BIPM^[1]).

The bottom line

Absolute zero is a limit, not a destination. You can approach it as closely as your time and resources allow, but never reach it, and even in the limit quantum mechanics leaves a residual jiggle. The reward for trying is a set of states of matter that exist nowhere else: frictionless liquids, perfect conductors and atoms that march in step.

FAQ

What is absolute zero in Celsius and Fahrenheit?

0 K is −273.15 °C, which is −459.67 °F.

Why does helium stay liquid near absolute zero?

Its zero-point motion is too large for its weak interatomic attraction to form a solid at ordinary pressure (Britannica^[3]).

Who proved the third law of thermodynamics?

Nernst proposed it; a general derivation of the unattainability form was published by Masanes and Oppenheim in 2017 (Nature Communications^[4]).

What happens to electricity near absolute zero?

In superconductors, resistance vanishes below a critical temperature and magnetic fields are expelled (the Meissner effect) (Britannica^[8]).

Browse the cold end of physics in our Thermodynamics collection and the Allowed Values Only sticker sheet.

References

1. BIPM, “Kelvin” (SI base unit definition). https://www.bipm.org/en/si-base-units/kelvin
2. Encyclopaedia Britannica, “Zero-point energy”. https://www.britannica.com/science/zero-point-energy
3. Encyclopaedia Britannica, “Helium”. https://www.britannica.com/science/helium-chemical-element
4. Masanes & Oppenheim (2017), “A general derivation and quantification of the third law of thermodynamics,” Nat. Commun. 8, 14538. https://www.nature.com/articles/ncomms14538
5. J. F. Allen & A. D. Misener (1938), “Flow of Liquid Helium II,” Nature 141, 75. https://doi.org/10.1038/141075a0
6. NobelPrize.org, Pyotr Kapitsa, Physics 1978, facts. https://www.nobelprize.org/prizes/physics/1978/kapitsa/facts/
7. Feynman Lectures on Physics, Vol. III, Ch. 4: Identical Particles. https://www.feynmanlectures.caltech.edu/III_04.html
8. Encyclopaedia Britannica, “Meissner effect”. https://www.britannica.com/science/Meissner-effect
9. NobelPrize.org, The Nobel Prize in Physics 1985 (quantized Hall effect). https://www.nobelprize.org/prizes/physics/1985/summary/
10. Encyclopaedia Britannica, “Bose-Einstein condensate”. https://www.britannica.com/science/Bose-Einstein-condensate
11. NobelPrize.org, The Nobel Prize in Physics 2001 (Bose–Einstein condensation). https://www.nobelprize.org/prizes/physics/2001/summary/
12. Purcell & Pound (1951) A Nuclear Spin System at Negative Temperature, Phys. Rev. 81, 279 (reversed state decayed with the 5-min time constant). https://doi.org/10.1103/PhysRev.81.279
13. Braun et al. (2013), “Negative absolute temperature for motional degrees of freedom,” Science 339, 52. https://arxiv.org/abs/1211.0545
14. Dunkel & Hilbert (2014) Consistent thermostatistics forbids negative absolute temperatures, Nature Phys. 10, 67. https://doi.org/10.1038/nphys2815
15. Frenkel & Warren (2015) Gibbs, Boltzmann, and negative temperatures, Am. J. Phys. 83, 163. https://doi.org/10.1119/1.4895828

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.