- Physical constant
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**physical**is aconstant physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with amathematical constant , which is a fixed numerical value but does not directly involve any physical measurement.There are many physical constants in science, some of the most widely recognized being the rationalized

Planck's constant "h", thegravitational constant "G", thespeed of light in vacuum "c", theelectric constant ε_{0}, and theelementary charge "e". Physical constants can take many dimensional forms: the speed of light signifies a maximumspeed limit of theuniverse and is expressed dimensionally aslength divided bytime ; while thefine-structure constant α, which characterizes the strength of the electromagnetic interaction, isdimensionless .**Dimensionful and dimensionless physical constants**Whereas the values of physical constants do not depend on the unit system used, the numerical values of dimensionful physical constants do depend on the unit used. Therefore, these numerical values (such as 299,792,458 for the constant

speed of light "c" expressed in units of meters per second) are not values that a theory of physics can be expected to predict.Ratios of like-dimensioned physical constants do not depend on unit systems in this way (the units cancel), so they are pure (dimensionless) numbers whose values a future theory of physics could conceivably hope to predict. Additionally, all equations describing laws of physics can be expressed without dimensional physical constants via a process known as

nondimensionalisation , but the dimensionless constants will remain. Thus, theoretical physicists tend to regard these dimensionless quantities asfundamental physical constant s.However, the phrase "fundamental physical constant" is also used in other ways. For example, the National Institute of Standards and Technology [

*[*] uses it to refer to any universal physical quantity believed to be constant, such as the speed of light, "c", and the*http://physics.nist.gov/cuu/Constants/ Latest (2006) Values of the Constants*] ; NIST, 2006.gravitational constant "G".The

fine-structure constant α is probably the best knowndimensionless fundamental physical constant. Many attempts have been made to derive its value (currently measured at about 1/137.035999) from theory, but so far none have succeeded. The same holds for the dimensionless ratios of masses offundamental particles (the most apparent is "m_{p}"/"m_{e}", approximately 1836.152673). With the development of quantum chemistry in the 20th century, however, a vast number of previously inexplicable dimensionless physical constants "were" successfully computed from theory. As such, some theoretical physicists still hope for continued progress in explaining the values of dimensionless physical constants.It is known that the universe would be very different if these constants took values significantly different from those we observe. For example, a few percent change in the value of the fine structure constant would be enough to eliminate stars like our Sun. This has prompted attempts at anthropic explanations of the dimensionless physical constants.

**How constant are the physical constants?**Beginning with

Paul Dirac in1937 , some scientists have speculated that physical constants may actually decrease in proportion to the age of the universe. Scientific experiments have not yet pinpointed any definite evidence that this is the case, although they have placed upper bounds on the maximum possible relative change per year at very small amounts (roughly 10^{−5}per year for the fine structure constant α and 10^{−11}for the gravitational constant "G").It is currently disputed [

*Duff, Michael J. " [*] [*http://www.arxiv.org/abs/hep-th/0208093 Comment on time-variation of fundamental constants*] ." High Energy Physics - Theory, 2004.*Duff, M. J.; Okun, L. B.; Veneziano, G. " [*] that any changes in "dimensional" physical constants such as "G", "c", "ħ", or ε*http://www.arxiv.org/abs/physics/0110060 Trialogue on the number of fundamental constants*] ." Classical Physics, 2002._{0}are operationally meaningful; [*"c", and ε*] however, a sufficient change in a dimensionless constant such as α is generally agreed to be something that would definitely be noticed. If a measurement indicated that a dimensional physical constant had changed, this would be the result or "interpretation" of a more fundamental dimensionless constant changing, which is the salient metric. From_{0}now are "defined" numerical values, independent of experiment, so observations now are trained elsewhere, for example, upon a changing value of the meter.John D. Barrow 2002::" [An] important lesson we learn from the way that pure numbers like α define the world is what it really means for worlds to be different. The pure number we call the fine structure constant and denote by α is a combination of the electron charge, "e", the speed of light, "c", and Planck's constant, "h". At first we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake. If "c", "h", and "e" were all changed so that the values they have in metric (or any other) units were different when we looked them up in our tables of physical constants, but the value of α remained the same, this new world would be "observationally indistinguishable" from our world. The only thing that counts in the definition of worlds are the values of the dimensionless constants of Nature. If all masses were doubled in value you cannot tell because all the pure numbers defined by the ratios of any pair of masses are unchanged."

**Anthropic principle**Some physicists have explored the notion that if the (dimensionless)

fundamental physical constants had sufficiently different values, our universe would be so radically different that intelligent life would probably not have emerged, and that our universe therefore seems to be fine-tuned for intelligent life. TheStrong anthropic principle states that it must be because these fundamental constants acquired their respective values that there was sufficient order in the Universe and richness in elemental diversity for life to have formed, which subsequently evolved the necessary intelligence toward observing that these constants have taken on the values they have, which then allowed for our privileged perspective from theWeak anthropic principle standpoint.**Table of universal constants****Table of physico-chemical constants****Table of adopted values****Notes****References*** [

*http://physics.nist.gov/cuu/Constants CODATA Recommendations*] - 2006 CODATA Internationally recommended values of the Fundamental Physical Constants

*Barrow, John D., "The Constants of Nature; From Alpha to Omega - The Numbers that Encode the Deepest Secrets of the Universe". Pantheon Books, 2002. ISBN 0-375-42221-8.

* Mohr, Peter J., Taylor, Barry N., Newell, David B., [*http://arxiv.org/abs/0801.0028 CODATA Recommended Values of the Fundamental Physical Constants: 2006*]

* Michael Sheppard, [*http://www.msu.edu/~sheppa28/constants/constants.html Systematic Search for Expressions of Dimensionless Constants using the NIST database of Physical Constants*] , 2007

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