Was ist ein/eine Kelvin (K)?
Formal Definition
The kelvin (symbol: K) is the SI base unit of thermodynamic temperature. Since 20 May 2019, it is defined by fixing the numerical value of the Boltzmann constant k at exactly 1.380649 × 10⁻²³ joules per kelvin (J/K). This definition ties the kelvin to the fundamental relationship between temperature and the average kinetic energy of particles: kT represents the thermal energy per degree of freedom per particle.
The kelvin is an absolute temperature scale — its zero point (0 K) corresponds to absolute zero, the lowest possible temperature, at which classical thermal motion ceases. There are no negative temperatures on the kelvin scale in classical thermodynamics. One kelvin is equal in magnitude to one degree Celsius: a temperature change of 1 K is the same as a change of 1 °C. The kelvin scale is offset from the Celsius scale by exactly 273.15: K = °C + 273.15.
Note on Terminology
The kelvin is written without the degree symbol — it is "kelvin" not "degree kelvin" and "K" not "°K." This convention was adopted by the 13th General Conference on Weights and Measures in 1967 to emphasize that the kelvin is an absolute unit, not a relative scale like Celsius or Fahrenheit. The unit is named after William Thomson, 1st Baron Kelvin (1824-1907), the Irish-Scottish physicist who first proposed the concept of an absolute temperature scale.
Etymology
William Thomson, Lord Kelvin
The kelvin is named after William Thomson, 1st Baron Kelvin (1824-1907), one of the most important physicists of the 19th century. Born in Belfast, Ireland, Thomson became a professor of natural philosophy at the University of Glasgow at the age of 22, a position he held for 53 years. He was elevated to the peerage in 1892 as Baron Kelvin of Largs, taking his title from the River Kelvin that flows past the University of Glasgow.
Thomson proposed the concept of an absolute temperature scale in 1848, reasoning from the principles of Carnot's heat engine theory that there must exist a natural zero point of temperature at which the efficiency of a heat engine would reach its theoretical maximum. He published his proposal in the paper "On an Absolute Thermometric Scale" in the Cambridge Philosophical Society's journal.
From "Degree Kelvin" to "Kelvin"
Originally, the unit was called the "degree Kelvin" (symbol: °K). In 1967, the 13th CGPM dropped the word "degree" and the symbol became simply "K." This change was made to distinguish the kelvin from the Celsius and Fahrenheit scales, which measure temperature relative to arbitrary reference points. The kelvin, as an absolute unit, was deemed to deserve its own unadorned symbol, parallel to other SI base units like the meter, kilogram, and second.
Precise Definition
The Boltzmann Constant Definition
Since 20 May 2019, the kelvin is defined by fixing the Boltzmann constant at exactly k = 1.380649 × 10⁻²³ J/K = 1.380649 × 10⁻²³ kg·m²·s⁻²·K⁻¹. This means that one kelvin corresponds to a change in thermal energy kT of exactly 1.380649 × 10⁻²³ joules per particle per degree of freedom.
Previous Definition
Before 2019, the kelvin was defined by fixing the triple point of water at exactly 273.16 K (0.01 °C). Under this definition, 1 kelvin was exactly 1/273.16 of the thermodynamic temperature of the triple point of water. This definition was problematic because the triple point of water depends on the isotopic composition of the water — standard measurements used Vienna Standard Mean Ocean Water (VSMOW) with a specified isotopic composition.
Practical Realization
The kelvin is realized experimentally through primary thermometry methods: acoustic gas thermometry (measuring the speed of sound in a gas), Johnson noise thermometry (measuring the electrical noise produced by thermal fluctuations in a resistor), and dielectric constant gas thermometry. For practical calibration, the International Temperature Scale of 1990 (ITS-90) defines fixed points from 0.65 K to 1357.77 K (the freezing point of copper), with specified interpolation instruments and methods between these points.
Geschichte
The Concept of Absolute Zero
The idea that temperature has a natural lower limit emerged gradually in the 18th and 19th centuries. In 1702, Guillaume Amontons observed that air pressure in a constant-volume gas thermometer decreased linearly with temperature and extrapolated that pressure would reach zero at approximately -240 °C (his estimate was inaccurate but the concept was sound). In the 1780s, Johann Heinrich Lambert refined this estimate. By the 1840s, several scientists had estimated absolute zero at approximately -273 °C.
William Thomson's Proposal
In 1848, William Thomson (later Lord Kelvin) placed the concept of absolute temperature on a rigorous theoretical foundation. Drawing on Sadi Carnot's theory of heat engines, Thomson showed that the efficiency of a perfect heat engine depends only on the ratio of the temperatures of its hot and cold reservoirs. This relationship required a temperature scale with a true zero point — a scale on which temperature ratios are physically meaningful.
Thomson's original absolute scale was based on the Celsius degree, with absolute zero at -273 °C (later refined to -273.15 °C). He defined 0 on his scale as absolute zero and used the same degree size as Celsius, creating a scale where water freezes at approximately 273 K and boils at approximately 373 K.
International Adoption
In 1954, the 10th CGPM formally defined the kelvin by setting the triple point of water at exactly 273.16 K and absolute zero at 0 K. This made the kelvin independent of the Celsius scale (instead, Celsius became defined through the kelvin). In 1967, the 13th CGPM renamed the unit from "degree Kelvin" (°K) to simply "kelvin" (K).
The 2019 Redefinition
The 2019 SI redefinition replaced the water-based definition with one based on the Boltzmann constant. This change freed the kelvin from dependence on a specific substance (water of a particular isotopic composition) and instead linked it to a fundamental constant of nature. The redefinition was part of the broader overhaul of the SI that also redefined the kilogram, ampere, and mole. The practical impact on temperature measurement was negligible — the change was within measurement uncertainties — but the conceptual shift was profound.
Aktuelle Verwendung
In Physics and Chemistry
The kelvin is the standard temperature unit in physics and chemistry worldwide. Thermodynamic equations — the ideal gas law (PV = nRT), the Stefan-Boltzmann law (P = σT⁴), the Boltzmann distribution, and the Planck radiation law — all require absolute temperature in kelvins. Using Celsius or Fahrenheit in these equations would produce incorrect results because these scales have arbitrary zero points.
In Astronomy and Astrophysics
In astronomy, the kelvin is the standard for expressing stellar temperatures, planetary temperatures, and cosmic background radiation. The Sun's surface temperature is approximately 5,778 K. The cosmic microwave background radiation has a temperature of 2.725 K. The cores of massive stars reach temperatures of billions of kelvins. These vast ranges would be awkward to express in Celsius (which would require negative values only slightly below kelvin) but are natural in kelvin.
In Color Temperature
The kelvin is used to describe the color temperature of light sources. A warm incandescent bulb has a color temperature of approximately 2,700 K. Daylight ranges from 5,000 to 6,500 K. A clear blue sky can reach 10,000-15,000 K. This usage appears in photography, cinematography, display technology, and lighting design. Consumers may encounter kelvin values on light bulb packaging and display settings.
In Cryogenics and Superconductivity
In cryogenic science, temperatures near absolute zero are expressed in kelvins (or millikelvins and microkelvins). Liquid nitrogen boils at 77 K. Liquid helium boils at 4.2 K. High-temperature superconductors operate below approximately 93 K (YBCO), while conventional superconductors require temperatures below about 10 K. Dilution refrigerators for quantum computing reach millikelvin temperatures.
Everyday Use
Not a Daily Life Unit
The kelvin is rarely used in everyday life by non-scientists. Ordinary people think of temperature in Celsius (most of the world) or Fahrenheit (the US). The kelvin appears in everyday contexts only through color temperature — for example, when choosing light bulbs (2700 K = warm white, 4000 K = neutral white, 5000 K+ = daylight) or adjusting white balance in photography.
Light Bulb Labels
The most common everyday encounter with kelvins is on light bulb packaging. LED and CFL bulbs are labeled with their color temperature in kelvins: 2700 K produces a warm, yellowish light similar to traditional incandescent bulbs; 3000 K is slightly cooler; 4000 K is neutral white; 5000-6500 K approximates daylight. Consumers shopping for light bulbs encounter kelvin values regularly, even if they don't fully understand the physics behind the scale.
Photography and Videography
Photographers and videographers routinely work with kelvin values for white balance settings. Camera white balance can be set manually in kelvins: 3200 K for tungsten lighting, 5500 K for daylight, 7000 K or higher for shade or overcast conditions. Professional photographers learn to think of lighting conditions in terms of kelvin values.
Weather Reports of Extreme Cold
In extreme cold conditions — reports from Antarctica, space weather, or cryogenic research — temperatures may be reported in kelvins in science news. The public occasionally encounters kelvin values in news stories about scientific achievements, such as cooling atoms to near absolute zero or measuring the cosmic microwave background.
In Science & Industry
Thermodynamics
The kelvin is fundamental to thermodynamics. The laws of thermodynamics are formulated using absolute temperature. The second law, entropy, and the concept of thermodynamic equilibrium all require temperature measured from absolute zero. The Carnot efficiency η = 1 - T_cold/T_hot requires kelvins. The entropy formula S = k ln W uses temperature in kelvins through the Boltzmann constant.
Quantum Mechanics
In quantum mechanics, temperature appears through the Boltzmann factor e^(-E/kT), which describes the probability of a system being in a state of energy E at temperature T. The thermal de Broglie wavelength, Bose-Einstein condensation temperature, and Fermi temperature are all expressed in kelvins. Quantum computing operates at millikelvin temperatures (typically 10-20 mK in dilution refrigerators).
Cosmology
The temperature of the cosmic microwave background radiation — 2.725 K — is one of the most precisely measured quantities in cosmology. The temperature of the universe as a function of time after the Big Bang is a fundamental parameter in cosmological models. At approximately 3,000 K, the primordial plasma cooled enough for atoms to form (recombination epoch), making the universe transparent to photons.
Materials Science
Materials science uses the kelvin for phase transition temperatures, thermal properties, and material characterization. Superconducting critical temperatures (Tc), Curie temperatures for magnetic transitions, and glass transition temperatures are reported in kelvins in scientific literature. The melting point of tungsten (3,695 K) is the highest of any element.
Metrology
In metrology, the kelvin defines the scale against which all temperature measurements are traceable. National metrology institutes maintain primary temperature standards using acoustic gas thermometry, Johnson noise thermometry, and other primary methods. The International Temperature Scale of 1990 (ITS-90) provides practical fixed points from 0.65 K (the vapor-pressure point of helium-3) to 1,357.77 K (the freezing point of copper).
Interesting Facts
Absolute zero (0 K) is the lowest possible temperature, where all classical thermal motion ceases. Scientists have cooled matter to within billionths of a kelvin of absolute zero, but the third law of thermodynamics states that reaching exactly 0 K is physically impossible.
The cosmic microwave background radiation — the afterglow of the Big Bang — has a temperature of 2.725 K, making it the most precisely measured blackbody radiation in the universe.
The kelvin was originally called the 'degree Kelvin' (°K) until 1967, when the CGPM dropped the 'degree' to emphasize that the kelvin is an absolute unit, not a relative scale. The correct notation is 'K' without a degree symbol.
William Thomson (Lord Kelvin) became a professor at the University of Glasgow at age 22 and held the position for 53 years. He took his peerage title from the River Kelvin that flows past the university, not from the temperature unit.
The surface of the Sun has a temperature of approximately 5,778 K, while its core reaches about 15 million K. The cores of the most massive stars can exceed 3 billion K.
Quantum computers operate at temperatures of about 10-20 millikelvins (0.010-0.020 K), colder than outer space. Dilution refrigerators achieve these extreme temperatures by mixing helium-3 and helium-4 isotopes.
Color temperature in kelvins describes the hue of light: 2700 K is warm (yellowish), 5500 K is daylight (white), and 10000+ K is cool (bluish). This is the most common everyday encounter with kelvin values — on light bulb labels.
The Planck temperature — approximately 1.416 × 10³² K — is the theoretical maximum temperature in physics, corresponding to conditions in the first 10⁻⁴³ seconds after the Big Bang. Beyond this temperature, known physics breaks down.
Liquid nitrogen boils at 77 K (-196 °C), and liquid helium boils at 4.2 K (-269 °C). Helium-3, a rare isotope, boils at 3.2 K. These cryogenic liquids are essential tools in low-temperature physics.
The 2019 redefinition of the kelvin through the Boltzmann constant means that temperature is now defined by the energy of thermal motion — arguably the most physically intuitive definition possible.
Regional Variations
Universal Scientific Use
The kelvin is used uniformly worldwide in all scientific contexts. There are no regional variations in its definition, symbol, or application. Every country's scientific community uses the kelvin as the SI base unit of temperature.
Everyday Temperature Scales
While the kelvin is the official SI temperature unit, everyday temperature communication differs by region. Most of the world uses Celsius for daily life; the United States uses Fahrenheit. The kelvin is universally understood by scientists but rarely used by the general public in any country.
Color Temperature Standards
Color temperature in kelvins is used worldwide in lighting and photography without regional variation. A "2700 K" bulb is the same color temperature everywhere. This is one area where consumers globally encounter kelvin values directly.
Cryogenic Research
Cryogenic research facilities around the world — CERN in Switzerland, Fermilab in the US, KEK in Japan, JINR in Russia — all use kelvins and millikelvins. The International Temperature Scale of 1990 (ITS-90) is the same worldwide, ensuring that a temperature measurement in one laboratory is directly comparable to a measurement in any other.