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Introduction to the constants for nonexperts

The following material is intended to give nonexperts insight into the general subject of fundamental physical constants. It is adopted from an article written by Barry N. Taylor in about 1971 for the 15th edition of the Encyclopaedia Britannica © 1974 , and is reproduced with permission. Although a considerable portion of the article is now out-of-date, the introductory and historical portions given here remain generally valid. Note, however, that the terms accuracy and uncertainty are used in the text in a manner that is not always consistent with current practice. This also applies to other terms such as parts per million and its symbol ppm, and the now obsolete terms atomic weight and molecular weight. To add a more modern flavor to the material, a fundamental constant experiment of current interest is also described: the determination of the fine-structure constant by means of the quantum Hall effect and calculable cross capacitor.

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Throughout all of the formulations of the basic theories of physics and their application to the real world, there appear again and again certain fundamental invariant quantities. These quantities, called the fundamental physical constants, and which have specific and universally used symbols, are of such importance that they must be known to as high an accuracy as is possible. They include the velocity of light in vacuum (c); the charge of the electron, the absolute value of which is the fundamental unit of electric charge (e); the mass of the electron (me); Planck's constant (h); and the fine-structure constant, symbolized by the Greek letter alpha. These will all be considered in detail below.

There are, of course, many other important quantities that can be measured with high accuracy -- the density of a particular piece of silver, for example, or the lattice spacing (the distance between the planes of atoms) of a particular crystal of silicon, or the distance from the Earth to the Sun. These quantities, however, are generally not considered to be fundamental constants. First, they are not universal invariants because they are too specific, too closely associated with the particular properties of the material or system upon which the measurements are carried out. Second, such quantities lack universality because they do not consistently appear in the basic theoretical equations of physics upon which the entire science rests, nor are they properties of the fundamental particles of physics of which all matter is constituted.

It is important to know the numerical values of the fundamental constants with high accuracy for at least two reasons. First, the quantitative predictions of the basic theories of physics depend on the numerical values of the constants that appear in the theories. An accurate knowledge of their values is therefore essential if man hopes to achieve an accurate quantitative description of the physical universe. Second, and more important, the careful study of the numerical values of these constants, as determined from various experiments in the different fields of physics, can in turn test the overall consistency and correctness of the basic theories of physics themselves.