Physical constant

A physical constant is a non-varying quantity that appears in a theory or model of some physical phenomena. Some physical constants appear in models of fundamental phenomena and impact many parts of physics. These constants include the speed of light in vacuum c, the gravitational constant G, the Planck constant h, the electric constant ε₀, and the elementary charge e for examples. Tables of the numerical value of these fundamental physical constants or universal constants are carefully maintained and widely consulted. Some physical constants are coefficients or parameters assumed to be constant in specialized models without being universal constants. Examples include the characteristic time, characteristic length, or characteristic number (dimensionless) of a given system, or material constants (e.g., Madelung constant, electrical resistivity, and heat capacity) of a particular material or substance. Physical constants cannot be explained by the theory that incorporates them but their values may be determined by other, more fundamental theories.

Dimensioned constants include the values set by the International System of Units convention in the defining constants and the derived units built upon those constants, as well as many nonstandard constants still in widespread use as units. Dimensionless constants are characteristics of natural phenomena and their values must be measured experimentally. Dimensionless constants may be fundamental, like the fine-structure constant α, which characterizes the strength of the electromagnetic interaction, or they may arise from empirical dimensional analysis.