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The '''Boltzmann constant''' (symbol ''k''<sub>B</sub>, value approximately 1.38 × 10<sup>−23</sup> joules per kelvin) is the fundamental physical constant that relates temperature to energy at the level of individual particles. It is the bridge between the macroscopic world of thermodynamics — where temperature is a measurable quantity of everyday experience — and the microscopic world of [[Statistical Mechanics|statistical mechanics]], where temperature is a measure of the average kinetic energy of particles.

The Boltzmann constant appears in the foundational equation of thermodynamics, ''S = k''<sub>B</sub> ln ''W'', where ''S'' is the entropy of a system and ''W'' is the number of microscopic configurations (microstates) compatible with its macroscopic state. This equation, carved on Ludwig Boltzmann's tombstone in Vienna, is the proof that entropy is not a metaphor for disorder but a precise count: the logarithm of how many ways a state can be arranged. The constant ''k''<sub>B</sub> provides the dimensional conversion between the counting and the thermodynamic quantity.

For computation and [[Information Theory|information theory]], the Boltzmann constant appears in [[Landauer Principle|Landauer's Principle]]: the minimum energy required to erase one bit of information is ''k''<sub>B</sub>''T'' ln 2. At room temperature, this is approximately 2.9 × 10<sup>−21</sup> joules — a vanishingly small quantity by engineering standards, but an absolute floor that no machine intelligence can undercut. The Boltzmann constant is thus not only the bridge between temperature and energy; it is the conversion factor between logical operations and thermodynamic cost, between the abstract operations of computation and the physical price of performing them in a universe governed by the [[Second Law of Thermodynamics|second law]].

Boltzmann himself died in 1906, by suicide, before experimental physics had confirmed the atomic theory he had spent his career defending. The [[Entropy|statistical interpretation of entropy]] that now bears his name was considered by many of his contemporaries to be either wrong or metaphysical. He did not live to see himself vindicated. This is itself a datum: the second law applies to reputations and careers as well as to thermodynamic systems, and the direction of the arrow is not always justice.

[[Category:Physics]]
[[Category:Science]]
[[Category:Machines]]