# Physical Constants

## Physical Constants

(also fundamental constants), numerical factors that appear in the equations describing physical laws and, in many cases, are scale characteristics of physical processes and microparticles. Physical constants include the speed of light, Planck’s constant, the electron charge, the fine structure constant, Avogadro’s number, and the Rydberg constant.

Physical constants encompass both independent constants and combinations of independent constants, such as the fine structure constant α = e2/ℏc, where e is the electron charge, ℏ is Planck’s constant, and c is the speed of light. The numerical values of physical constants or combinations of such constants are determined on the basis of experimental measurements and are expressed in the units of some established system of units. The determination of the most accurate and reliable values for an entire set of physical constants from data obtained by measurements is called the adjustment of the constants. Adjustment includes the analysis of errors of measurement, the determination of the reliability of the measurements, and the calculation of the adjusted values of the constants by the method of least squares.

The physical constants are more precisely defined with the continuing development of physical experimentation, since new experimental and theoretical possibilities become available for the determination of the constants. For example, the discovery of the Josephson effect made it possible to measure the ratio e/h with a high degree of accuracy and to define many physical constants with substantially more precision. The recommended adjusted values of physical constants as of 1976 are presented in Table 1.

The values of physical constants must be defined more precisely in order to verify physical theories, that is, to compare theoretical predictions with experimental data.

Many measurements in present-day physics and engineering also require knowledge of exact values of physical constants. For example, the exact value of the speed of light must be used in radar measurements.

In metrology, exact values of physical constants are required for the development of reproducible standards for the units in which physical quantities are expressed.

### REFERENCES

Taylor, B., W. Parker, and D. Langenberg. Fundamental’nye konstanty i kvantovaia elektrodinamika. Moscow, 1972. (Translated from English.)
“Rekomenduemye soglasovannye znacheniia fundamental’nykh fizicheskikh postoiannykh—1973.” Uspekhi fizicheskikh nauk, 1975, vol. 115, issue 4.
Tabl. standartnykh spravochnykh dannykh: Fundamental’nye fizicheskie konstanty. Moscow, 1976.

L. G. ASLAMAZOV

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