|
In physics, a dimensionless physical constant, sometimes called a fundamental physical constant, is a physical constant that is dimensionless. It has no units attached, but has a numerical value that is independent of the system of units used. Perhaps the best-known example is the fine-structure constant α, which has approximate value The term ''fundamental physical constant'' has also been used to refer to universal but dimensioned physical constants such as the speed of light ''c'', vacuum permittivity ''ε''0, Planck's constant ''h'', and the gravitational constant ''G''.〔http://physics.nist.gov/cuu/Constants/ NIST〕 However, the numerical values of these constants are not fundamental, since they depend on the units used to express them. Increasingly, physicists reserve the use of the term ''fundamental physical constant'' for dimensionless physical constants that cannot be derived from any other source. ==Introduction== The numerical values of dimensional physical constants depend on the units used to express them. So one can define a basis set of units so that selected dimensional physical constants have numerical value 1. The basis set may consist of units of time, length, mass, charge, and temperature, or an equivalent set. A choice of units is called a system of units. For example, the International System of Units (SI) is the most generally used system of units today. The numerical values of dimensional physical constants are arbitrary and have no natural significance. (An exception is the vacuum permeability constant µ0, whose numerical value of 4π×10−7 is a mathematical constant determined by the definition of the ampere in the SI system.) The Planck units constitute another system of units. It is a system of natural units chosen so that the numerical values of the vacuum speed of light, the universal gravitational constant, and the constants of Planck, Coulomb, and Boltzmann are unity. These five dimensional physical constants then disappear from equations of physical laws, but as this occurs merely from the choice of units, these constants are considered not fundamental in an operationally distinguishable sense.〔Michael Duff (2002) "(Comment on time-variation of fundamental constants )"〕〔Michael Duff, O. Okun and Gabriele Veneziano (2002) "(Trialogue on the number of fundamental constants, )" ''Journal of High Energy Physics'' 3: 023.〕 In contrast, the numerical values of dimensionless physical constants are independent of the units used. These constants cannot be eliminated by any choice of a system of units. Such constants include: *α, the fine structure constant, the coupling constant for the electromagnetic interaction (≈1/137.036). Also the square of the electron charge, expressed in Planck units, which defines the scale of charge of elementary particles with charge. *μ or β, the proton-to-electron mass ratio, the rest mass of the proton divided by that of the electron (≈1836.15). More generally, the ratio of the rest masses of any pair of elementary particles. *αs, the coupling constant for the strong force (≈1) *αG, the gravitational coupling constant (≈10−45) which is the square of the electron mass, expressed in Planck units. This defines the scale of the masses of elementary particles and has also been used to express the relative strength of gravitation. Unlike mathematical constants, the values of the dimensionless fundamental physical constants cannot be calculated; they are determined only by physical measurement. This is one of the unsolved problems of physics. One of the dimensionless fundamental constants is the fine structure constant: : where ''e'' is the elementary charge, ''ħ'' is the reduced Planck's constant, ''c'' is the speed of light in a vacuum, and ''ε''0 is the permittivity of free space. The fine structure constant is fixed to the strength of the electromagnetic force. At low energies, α ≈ 1/137, whereas at the scale of the Z boson, about 90 GeV, one measures α ≈ 1/127. There is no accepted theory explaining the value of α; Richard Feynman elaborates: The analog of the fine structure constant for gravitation is the gravitational coupling constant. This constant requires the arbitrary choice of a pair of objects having mass. The electron and proton are natural choices because they are stable, and their properties are well measured and well understood. If αG is calculated from two protons, its value is ≈10−38. The list of dimensionless physical constants increases in length whenever experiments measure new relationships between physical phenomena. The list of fundamental dimensionless constants, however, decreases when advances in physics show how some previously known constant can be computed in terms of others. A long-sought goal of theoretical physics is to find first principles from which all of the fundamental dimensionless constants can be calculated and compared to the measured values. A successful "Theory of Everything" would allow such a calculation, but so far, this goal has remained elusive. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dimensionless physical constant」の詳細全文を読む スポンサード リンク
|