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. | + | 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> |
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: 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. | ||
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| + | 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]]. | ||
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| + | ==Dimensions of a physical constant== | ||
==References== | ==References== | ||
{{reflist}} | {{reflist}} | ||
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| + | ==External links== | ||
| + | {{wikipedia|Physical constant}} | ||
| + | *[http://physics.nist.gov/cuu/index.html NIST Reference on Constants, Units, and Uncertainty] | ||
[[Category:Physical constants| ]] | [[Category:Physical constants| ]] | ||
{{CC-BY-3.0}} | {{CC-BY-3.0}} | ||
Revision as of 17:21, 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: 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 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.
Dimensions of a physical constant
References
- ↑ 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>.
- ↑ 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.
External links
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See also the corresponding article on Wikipedia. |
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