Physical constant

A physical constant is a measurable quantity assigned a fixed value in a physical theory or law. Constants may be universal, such as the speed of light, or specific to a material or system.

Physical quantity: A property of a system that can be quantified, such as length, energy, or temperature. Constants are quantities, but their role in laws differs from ordinary changing properties.
Dimensional constant: A physical constant with units, such as metres per second or joule-seconds. Its units supply physical scales that equations and measurements require.
CODATA: The Committee on Data of the International Science Council, which periodically recommends values of fundamental physical constants. Its evaluations reconcile measurements and theory into recommended values.
Boltzmann constant: The constant relating temperature to energy per degree of freedom in statistical physics. Its fixed value defines the kelvin and links temperature to microscopic energy.
Variation of fundamental constants: The hypothesis that quantities such as the fine-structure constant change across time or space. Tests constrain whether constants assumed fixed in physics actually vary.
International System of Units: The internationally standardized system of measurement units, based on seven defining constants. Its definitions show how fixed constants can establish measurement units.
Dimensionless constant: A constant expressed as a pure number, independent of the choice of measurement units. Its numerical value cannot be changed merely by choosing different units.
Newton's law of universal gravitation: Newton's law states that two masses attract with force proportional to their product and inversely proportional to distance squared. Its gravitational constant entered physics through the quantitative formulation of gravity.
Avogadro constant: The number of specified elementary entities in one mole, exactly 6.02214076 × 10²³. Its exact value defines the mole and connects counting to amount of substance.
Fine-tuning of the universe: The apparent sensitivity of cosmic structure to the values of physical parameters and initial conditions. It raises questions about why constants fall within ranges compatible with complex structures.

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