fundamentalconstant

Fundamental constants are quantities that appear in the laws of physics with values that are not determined by any theory but are measured experimentally. They set the scale of physical processes and, in a given theory, their numerical values are the parameters that must be established by observation. Constants can be categorized as dimensionful, which depend on the unit system (for example, the speed of light c, Newton's gravitational constant G, Planck constant h, elementary charge e, and Boltzmann constant kB), or dimensionless, which have no units (for example the fine-structure constant α ≈ 1/137, the proton-to-electron mass ratio mp/me).

Dimensionful constants are often tied to the unit system. For example, the speed of light in vacuum is defined to be exactly 299,792,458 meters per second, and the kilogram is defined by fixing the Planck constant h to an exact value. In this sense, their numerical values can reflect unit definitions as well as experimental uncertainties, rather than intrinsic physics alone.

Dimensionless constants carry universal significance because their values do not depend on the unit system. They encode the strength of interactions or the ratios of fundamental quantities, and variations would imply new physics. Researchers study whether any fundamental constants may vary over time or space, a topic with implications for cosmology and theories beyond the Standard Model.

The term fundamental constant is used to refer to constants that appear in the fundamental laws of nature, including both dimensionful and dimensionless members. The set of such constants is theory-dependent and subject to revision as physical theories evolve.