The kelvin is the scientist's temperature scale. Unlike Celsius, which sets its zero at the freezing point of water, or Fahrenheit, which uses an arbitrary brine mixture, the kelvin scale starts at the most fundamental point possible: absolute zero, where a system has its minimum thermodynamic energy. One kelvin is exactly the same size as one degree Celsius — they differ only in their starting point. To convert, simply add 273.15: water freezes at 273.15 K and boils at 373.15 K. Note that we say "273 kelvin," not "273 degrees kelvin" — since 1967, the kelvin stands alone as a unit without the degree symbol, a subtle distinction that marks it as an absolute scale.
The scale was named after William Thomson, 1st Baron Kelvin, the Belfast-born physicist who proposed the absolute temperature concept in 1848. In 2019, the definition of the kelvin was revolutionized: instead of being tied to the triple point of water (273.16 K), it was redefined by fixing the value of the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K. This means the kelvin is now defined in terms of a fundamental constant of nature, not a physical substance — making it more precise and universally reproducible. This was part of the same SI overhaul that redefined the kilogram, ampere, and mole.
Scientists prefer kelvin because many physical laws only work correctly with absolute temperatures. The ideal gas law (PV = nRT) requires temperature in kelvin — plug in Celsius and you get nonsensical results when T goes negative. Wien's displacement law, which tells you the color of a star from its temperature, and the Stefan-Boltzmann law, which calculates radiated power, both demand kelvin. In astrophysics, temperatures range from the cosmic microwave background at 2.725 K to the cores of massive stars at over 10⁹ K. In cryogenics, scientists work in millikelvin and microkelvin. The kelvin scale elegantly spans this entire range without ever hitting zero, because reaching 0 K is physically impossible.