Kelvin to Reaumur

In 2021, researchers at the University of Bremen cooled rubidium atoms to 38 picokelvin (38 trillionths of a kelvin above absolute zero) inside a drop tower — the coldest temperature ever achieved. At these extremes, atoms form a Bose–Einstein condensate where quantum effects become visible at macroscopic scales. True 0 K is impossible to reach (the third law of thermodynamics forbids it), but each new record pushes closer, enabling research into superfluidity, quantum computing, and exotic states of matter.

Scientific calculations in thermodynamics, astrophysics, and chemistry require an absolute scale with no negative values. Many physical laws — the ideal gas law (PV = nRT), Wien's displacement law, Stefan–Boltzmann law — only work correctly with absolute temperature. Using Celsius would require constant offsets of 273.15.

Kelvin and Celsius have identical degree sizes, so a temperature difference of 5 K equals a difference of 5°C. The only difference is the zero point: 0 K = −273.15°C. To convert: K = °C + 273.15. Kelvin has no negative values; Celsius does.

Kelvin is rarely used in everyday life but appears in lighting (color temperature — a warm white bulb is around 3,000 K, daylight is 6,500 K), industrial processes, cryogenics (liquid helium: 4.2 K; liquid nitrogen: 77 K), and the specification of laser wavelengths and scientific instruments.

The Boomerang Nebula, a protoplanetary nebula about 5,000 light-years away, has a measured temperature of approximately 1 K (−272°C), making it the coldest known naturally occurring place in the universe — colder than the cosmic microwave background (2.725 K).

The Réaumur scale was proposed by René Antoine Ferchault de Réaumur in 1730. Réaumur was a French scientist primarily known for his contributions to natural history, particularly entomology. His thermometer used dilute alcohol and was calibrated between the freezing and boiling points of water, with the scale divided according to the actual expansion of the alcohol.

Réaumur calibrated his scale based on the physical expansion of his thermometric fluid — a specific dilute alcohol mixture. Between water's freezing and boiling points, the fluid expanded by exactly 80 parts in 1000 of its volume at freezing. He used this empirical measurement directly as the degree count, making 80°Ré the boiling point. The number 80 was not chosen arbitrarily but measured.

The French Revolution's embrace of the metric system in the 1790s demanded decimal-friendly units. Celsius's 0-to-100 scale fit the decimal philosophy perfectly; Réaumur's 0-to-80 did not. The French Academy of Sciences standardized Celsius for scientific work, and it spread through Napoleonic Europe. Réaumur lingered in German-speaking regions and Russia into the mid-19th century but eventually yielded everywhere. The irony is that Réaumur was French — his own country was the first to abandon his scale.

Multiply the Réaumur value by 5/4 (or 1.25). For example, 16°Ré × 1.25 = 20°C. To convert Celsius to Réaumur, multiply by 4/5 (or 0.8). The two scales share the same zero (0°C = 0°Ré); they differ only in degree size, since 100°C = 80°Ré.

Classic French confectionery texts list sugar syrup stages in Réaumur: "thread" stage at 80°Ré (100°C), "soft ball" at 94°Ré (117°C), "hard crack" at 124°Ré (155°C). Pastry chefs memorized these benchmarks and tested with a thermometer or by dripping syrup into cold water. Some vintage French and Swiss recipe books still reference Réaumur temperatures — modern reprints add Celsius equivalents in brackets. It is the last practical domain where Réaumur had a lingering foothold.