Context and objective
The study examines the Boltzmann constant, which connects thermodynamic temperature with microscopic energy, in the context of redefining the kelvin through a fundamental constant. It brings together the physical principles and experimental methods used to determine that constant.
Method
Two metrological approaches are detailed. Spectroscopy measures Doppler broadening of a low-pressure ammonia absorption line near 10 µm and the triple point of water; the width of the Maxwell–Boltzmann profile depends on kT. Acoustic thermometry derives k from the speed of sound in a rare gas inside a quasi-spherical resonator, using acoustic and electromagnetic resonances.
Implementation
The study describes laser frequency and intensity control, gas temperature stabilization and line-shape analysis for the spectroscopic method. For the acoustic route, it examines gas purity, pressure, temperature, cavity geometry and fluid–structure coupling disturbances that affect measured frequencies.
Results and deliverables
The deliverable is a 33-page synthesis report bringing together definitions, physical relationships, experimental apparatus, published results and the revision of the International System of Units. It notably presents the exact value fixed since 2019, 1.380 649 × 10⁻²³ J·K⁻¹, and the contribution of spectroscopic and acoustic work to the redefinition.
Lessons learned
The project shows how a fundamental constant connects statistical models, macroscopic measurements and unit definitions. It also highlights the need for an uncertainty budget: thermal control, line shape, gas composition, geometry and coupling effects become decisive in reference metrology.