Physical constant
A physical constant is a physical quantity that is believed to be unchanging over time, either because it represents a fundamental property of the universe (e.g. the electron mass) or because of its relation to a given system of units (e.g. the Avogadro constant). The number of possible physical constants is infinite, although some are considered much more important (or "fundamental") than others.[1][2]
The value of a physical constant must usually be determined by measurement, although some constants are used as bases for the International System of Units (e.g. the speed of light, the magnetic constant) and so have defined values in SI units:[1][3] nevertheless, even physical constants with defined values in SI units could in principle be measured in a different system of units. This distinguishes physical constants from mathematical constants such as π and e, which are defined by mathematical expressions.
Physical constants may be divided into those of dimension one ("dimensionless"), such as the fine structure constant, and those with other dimensions, such as the electric constant. Dimensionless constants are often considered to be particularly fundamental, as their value does not depend on the system of units used to measure them.[2] The numerical value of constants with dimensions other than one will depend on the system of units used to express them: indeed, such constants may be used as measurement units themselves, such as when a speed is expressed as a fraction of the speed of light in relativity.[2]
References
- ↑ 1.0 1.1 Mohr, Peter J.; Taylor, Barry N.; Newell, David B. CODATA Recommended Values of the Fundamental Physical Constants: 2006. Rev. Mod. Phys. 2008, 80 (2), 633–730. doi:10.1103/RevModPhys.80.633, <http://physics.nist.gov/cuu/Constants/codata.pdf>.
- ↑ 2.0 2.1 2.2 Duff, Michael J.; Okun, Lev B.; Veneziano, Gabriele Trialogue on the number of fundamental constants. J. High Energy Phys. 2002 (3), 023. DOI: 10.1088/1126-6708/2002/03/023.
- ↑ The International System of Units (SI), 8th ed.; International Bureau of Weights and Measures: Sèvres, France, 2006. ISBN 92-822-2213-6, <http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf>.
Further reading
- Barrow, John D. The Constants of Nature; From Alpha to Omega; Pantheon, 2002. ISBN 0-375-42221-8.
External links
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