physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...

the Stoney units form a

system of units A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement i ...

named after the Irish physicist

George Johnstone Stoney George Johnstone Stoney FRS (15 February 1826 – 5 July 1911) was an Irish physicist. He is most famous for introducing the term ''electron'' as the "fundamental unit quantity of electricity". He had introduced the concept, though not the wor ...

, who first proposed them in 1881. They are the earliest example of

natural units In physics, natural units are physical units of measurement in which only universal physical constants are used as defining constants, such that each of these constants acts as a coherent unit of a quantity. For example, the elementary charge ma ...

, i.e., a coherent set of units of measurement designed so that chosen

physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is contrasted with a mathematical constant ...

s fully define and are included in the set.

Units

The constants that Stoney used to define his set of units is the following: * ''c'', the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...

in a vacuum, * ''G'', the

gravitational constant The gravitational constant (also known as the universal gravitational constant, the Newtonian constant of gravitation, or the Cavendish gravitational constant), denoted by the capital letter , is an empirical physical constant involved in ...

, * ''k''_e = 1/(4''πε''₀), the

Coulomb constant The Coulomb constant, the electric force constant, or the electrostatic constant (denoted , or ) is a proportionality constant in electrostatics equations. In SI base units it is equal to .Derived from ''k''e = 1/(4''πε''0) – It was named ...

, * ''e'', the

elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundam ...

. This means that the numerical values of all these constants, when expressed in coherent Stoney units, is equal one: : ''c'' = 1 ''l''_S⋅''t''_S⁻¹ :''G'' = 1 ''l''_S³⋅''t''_S⁻²⋅''m''_S⁻¹ :''k''_e = 1 ''l''_S³⋅''t''_S⁻²⋅''m''_S⋅''q''_S⁻² :''e'' = 1 ''q''_S In Stoney units, the numerical value of the reduced Planck constant is :

\hbar = \frac ~l_\text^2t_\text^m_\text \approx 137.036 ~l_\text^2t_\text^m_\text^,

where ''α'' is the

fine-structure constant In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by (the Greek letter ''alpha''), is a fundamental physical constant which quantifies the strength of the electromagnetic interaction between el ...

History

George Stoney was one of the first scientists to understand that electric charge was quantized; from this quantization and three other constants that he perceived as being universal (a speed from electromagnetism, and the coefficients in the electrostatic and gravitational force equations) he derived the units that are now named after him. Stoney's derived estimate of the ''unit of charge'', 10⁻²⁰ ampere-second, was of the modern value of the charge of the electron J.G. O'Hara (1993). George Johnstone Stoney and the Conceptual Discovery of the Electron, Occasional Papers in Science and Technology, Royal Dublin Society 8, 5–28. due to Stoney using the approximated value of 10¹⁸ for the number of molecules presented in one cubic millimetre of gas at

standard temperature and pressure Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...

. Using the modern values for the

Avogadro constant The Avogadro constant, commonly denoted or , is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. It is an SI defining ...

and for the volume of a gram-molecule under these conditions of , the modern value is , instead of Stoney's 10¹⁸.

Stoney units and Planck units

Stoney's set of base units is similar to the one used in Planck units, proposed independently by Planck thirty years later, in which Planck normalized the

Planck constant The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivale ...

in place of the elementary charge.Space: in science, art and society, by Penz, Radick, Howell, p191
/ref> Planck units are more commonly used than Stoney units in modern physics, especially for

quantum gravity Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vi ...

(including

string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...

). Rarely, Planck units are referred to as Planck–Stoney units. The Stoney length and the Stoney energy, collectively called the ''Stoney scale'', are not far from the Planck length and the Planck energy, the ''Planck scale''. The Stoney scale and the Planck scale are the length and energy scales at which quantum processes and gravity occur together. At these scales, a unified theory of physics is thus required. The only notable attempt to construct such a theory from the Stoney scale was that of H. Weyl, who associated a gravitational unit of charge with the Stoney length K. Tomilin, "Natural System of Units", Proc. of the XX11 International Workshop on High Energy Physics and Field theory, (2 000) 289.H. Weyl, "Gravitation und Elekrizitaet", Koniglich Preussische Akademie der Wissenschaften (1918) 465–78 H. Weyl, "Eine Neue Erweiterung der Relativitaetstheorie", Annalen der Physik 59 (1919) 101–3. and who appears to have inspired Dirac's fascination with the large numbers hypothesis. G. Gorelik, "Hermann Weyl and Large Numbers in Relativistic Cosmology", Einstein Studies in Russia, (ed. Y. Balashov and V. Vizgin), Birkhaeuser. (2002). Since then, the Stoney scale has been largely neglected in the development of modern physics, although it is still occasionally discussed. M. Castans and J. Belinchon(1998). "Enlargement of Planck's System of Absolute Units", preprint: physics/9811018 The ratio of Stoney units to Planck units of length, time and mass is

\sqrt

, where

\alpha

Units

History

Stoney units and Planck units

See also

Notes

References

External links