What is temperature and how do we measure it? In the early 18th century, a German scientist named Fahrenheit defined 0⁰F as the lowest temperature he could achieve with a mixed solution of water, ice, and salt, and 96⁰F as the temperature of a healthy adult. This definition is not precise and the math is unnecessarily cumbersome, which is why most of the world eventually adopted the Celsius scale. The Swedish scientist Celsius in 1742 defined 0⁰C as the boiling point of water and 100⁰C as the freezing point of water (The scale was reversed the next year). This definition is easier to replicate but is not complete. It’s only true under normal atmospheric pressure. Scientists don’t like ambiguity, so in 1954 the anchor points of the Celsius scale were changed. On one side is absolute zero, a theoretical value reached by extrapolating the ideal gas law. On the other side is the triple point of water: The single combination of pressure and temperature at which pure water, pure ice, and pure water vapor can coexist in a stable equilibrium. The problem with this definition is that it’s difficult to precisely and consistently measured in labs.. In 2019, the International System of units underwent its biggest revolution since the meter–kilogram–second (MKS) system was established with the 1875 Convention du mètre. Instead of depending on human artifacts or measurements, the SI became wholly derivable from unchanging natural phenomena based on fundamental physical constants. Specifically, the new definition of Kelvin is calculated from the energy equivalent as given by the Boltzmann's equation. Here is the exact mathematical formula:

$$1 Kelvin = {1.380649 \times 10^{-23} \over (6.62607015 \times 10^{-34})(9192631779)} {h\space \Delta V_{cs} \over k}$$

• k: the Boltzmann constant
• h: the Plank constant
• $\Delta V_{cs}$: the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom

This brings us to what temperature is. At a microscopic level, all atoms and molecules are moving all the time (except at absolute zero, which the third law of thermodynamics tells us is unreachable). The Austrian physicist Boltzmann invented the science of statistical mechanics, which models a system in terms of the average behavior of the particles making up the system. Temperature is the macro indication of the micro-movements of atoms and molecules. Mathematically, temperature is proportional to the square of the average speed of particles. These days this shouldn’t be too much of a stretch of the imagination, but Boltzmann's work was under constant attack in his lifetime. He became depressed and committed suicide just before experiments verified his theories.