Difference between revisions of "Physical constant"
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| − | 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 [[Measurement unit|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.<ref name="CODATA06">{{CODATA 2006}}.</ref><ref name="trialogue">{{citation | title = Trialogue on the number of fundamental constants | first1 = Michael J. | last1 = Duff | first2 = Lev B. | last2 = Okun | first3 = Gabriele | last3 = Veneziano | doi = 10.1088/1126-6708/2002/03/023 | journal = J. High Energy Phys. | year = 2002 | issue = 3 | pages = 023}}.</ref> | + | 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 [[Measurement unit|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.<ref name="CODATA06">{{CODATA 2006}}.</ref><ref name="trialogue">{{citation | title = Trialogue on the number of fundamental constants | first1 = Michael J. | last1 = Duff | first2 = Lev B. | last2 = Okun | first3 = Gabriele | last3 = Veneziano | doi = 10.1088/1126-6708/2002/03/023 | journal = J. High Energy Phys. | year = 2002 | issue = 3 | pages = 023 | arxiv = physics/0110060v3}}.</ref> |
| − | 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 unit]]s: nevertheless, even constants with defined values in SI units could in principle be measured in a different system of units. This distinguishes physical constants from [[mathematical constant]]s such as [[Pi|π]] and [[e (mathematical constant)|e]], which are defined by mathematical expressions. | + | 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 unit]]s:<ref name="CODATA06"/><ref>{{SIbrochure8th}}.</ref> 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 constant]]s such as [[Pi|π]] and [[e (mathematical constant)|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]]. | + | 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.<ref name="trialogue"/> 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]].<ref name="trialogue"/> |
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==References== | ==References== | ||
{{reflist}} | {{reflist}} | ||
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| + | ==Further reading== | ||
| + | *{{citation | last = Barrow | first = John D. | title = The Constants of Nature; From Alpha to Omega | publisher = Pantheon | year = 2002 | isbn = 0-375-42221-8}}. | ||
==External links== | ==External links== | ||
Latest revision as of 17:51, 22 July 2010
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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See also the corresponding article on Wikipedia. |
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