perusvakio

Perusvakio, or fundamental physical constant, is a physical quantity whose value is universal and independent of the observer’s chosen system of units. A perusvakio serves as a fixed parameter in the laws of physics and appears in the core equations that describe natural phenomena. They can be dimensionless, such as the fine-structure constant, or dimensional, such as the speed of light in vacuum or the Planck constant. Dimensionless constants are particularly unit‑independent, while dimensional constants relate quantities with units and often gain exact values when units are defined accordingly.

Common examples include the speed of light in vacuum (c), the Planck constant (h), the elementary charge (e), the Boltzmann constant (kB), and the Avogadro constant (NA). These constants provide links between different areas of physics, chemistry and thermodynamics and enable conversions between disparate physical quantities. They are determined experimentally and carry uncertainties that reflect measurement precision.

In metrology, several perusvakio have been fixed to exact values to define the SI units. In the 2019 redefinition of the SI, the values of c, h, e, kB, NA and the luminous efficacy Kcd were fixed, making them exact within the SI framework. This shift enhances the universality and stability of measurements by anchoring units to invariant constants of nature.