PHYSICAL CONSTANT

any of a set of fundamental, invariant quantities observed in nature and appearing in the basic theoretical equations of physics, mostly relating to the fundamental particles of which all matter is constituted. Accurate evaluation of these constants is essential in order to check the correctness of the theories and to allow useful predictions to be made on the basis of those theories. The speed of light in a vacuum, c, appears in electromagnetic theory and in relativity theory; in the latter it relates energy to mass through the equation E = mc2. Its value does not depend on any particular experimental conditions such as would be required for the determination of the speed of a sound wave in air (for which air temperature, density, and chemical composition would also have to be specified) but depends solely on an intrinsic property of the photon. The charge on the electron, e, is a fundamental property of a physical particle; it is the smallest unit of electric charge. Knowledge of its numerical value is required in many areas of physics and chemistrye.g., in calculating the mass of an element liberated by passage of current through an electrochemical cell. Planck's constant, h, is not itself a property of a fundamental particle but is a constant appearing in the equations of quantum theory. It relates the energy of a photon (a quantum of electromagnetic radiation) to its frequency through the equation E = hn. The universal gravitational constant, G, relates the magnitude of the gravitational attractive force between two bodies to their masses and the distance between them. Its value is extremely difficult to measure experimentally. It has been suggested that G has varied with time throughout the history of the universe and that it is scale-dependent. If such is the case, values determined in the laboratory would not be appropriate for terrestrial or astronomical problems. Some physical constants can be expressed in terms of other constants; e.g., the fine-structure constant (a)which appears in the quantum-mechanical explanation of why certain atoms emit radiation of several closely spaced wavelengths rather than a single wavelengthcan be expressed in terms of the permeability of free space (m0), the speed of light, the charge on the electron, and Planck's constant. Numerical values of physical constants are determined at various laboratories throughout the world, such as the U.S. National Bureau of Standards, and are refined as experimental methods and techniques are improved. Definitive values are arrived at by international agreement.