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Portrait of Absolute zero

Science · Physics

Absolute zero

1848

Physics — thermodynamics

The lowest possible temperature, at which a system has the minimum energy thermodynamics allows: 0 kelvin, −273.15 °C or −459.67 °F. William Thomson, later Lord Kelvin, proposed the absolute scale in 1848, building on Guillaume Amontons's observation that gas pressure falls steadily as a gas cools. The third law of thermodynamics says it can be approached but never actually reached; laboratories have come within a few tens of trillionths of a degree of it, and near that limit matter behaves strangely, forming superfluids and Bose–Einstein condensates.

How to spot them

  • ▸−273.15 °C, the temperature at which an ideal gas would exert no pressure
  • ▸The zero point of the scale named after a Glasgow professor ennobled after the transatlantic telegraph cable
  • ▸Unreachable by the third law of thermodynamics, though laboratories have come within picokelvins
  • ▸Approach it and helium flows without friction and atoms collapse into a single quantum state

Key facts

−273.15 °C, or −459.67 °F

The defining value on the Celsius and Fahrenheit scales

Proposed in 1848 by the Glasgow physicist later ennobled as Lord Kelvin (1848)

William Thomson's 'absolute thermometric scale', with degrees the size of Celsius degrees

The third law of thermodynamics says it cannot be reached in a finite number of steps

Walther Nernst's heat theorem of 1906

The state of matter predicted by Bose and Einstein in the 1920s and first made in 1995 (1995)

The Bose–Einstein condensate, produced at 170 nanokelvin in Colorado

The cosmic microwave background sits just 2.7 degrees above it

The natural temperature of empty space today

A record 38 trillionths of a degree above it, set in a German drop tower in 2021 (2021)

38 picokelvin, reached by letting a condensate fall freely for seconds

Key ideas

  • Temperature measures the energy of random motion; at the bottom of the scale that motion is at its quantum minimum, though never truly zero.
  • Cooling means removing entropy, and every step removes less than the last, so no finite process can take a system all the way down.

Quick-fire facts

  • •0 K = −273.15 °C = −459.67 °F; the kelvin is written without a degree sign
  • •William Thomson, later Lord Kelvin, proposed the absolute scale in 1848, aged 24, as professor at Glasgow
  • •Guillaume Amontons had extrapolated a limiting cold from falling gas pressure as early as 1702
  • •Since the 2019 SI redefinition the kelvin is fixed by the Boltzmann constant, 1.380649 × 10⁻²³ J/K, rather than by water's triple point
  • •The third law of thermodynamics, from Nernst's 1906 heat theorem, forbids reaching it
  • •Atoms are not motionless there: quantum zero-point energy persists, and helium stays liquid at 0 K under its own vapour pressure
  • •The 1997 physics Nobel went to Chu, Cohen-Tannoudji and Phillips for laser cooling; the 2001 prize to Cornell, Ketterle and Wieman for the first Bose–Einstein condensates
  • •The coldest known natural place is the Boomerang Nebula, about 1 K — colder than the 2.7 K cosmic microwave background
  • •The laboratory record is 38 picokelvin, set in 2021 at the Bremen drop tower
  • •Negative absolute temperatures do exist in special systems, and are hotter than any positive temperature, not colder

Drill it — 8 cards

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Sources