Fundamental physical constants

In physics, the term constant is used with different meanings:

• the first level corresponds to a physical quantity that has the same value in all circumstances, for example, Planck's constant, gravitational constant; in this case, it is generally referred to as the universal constant;

• the second level is that of a physical quantity which, for a particular entity or for a given phenomenon, has the same value in all circumstances, for example, the decay constant of a particular nuclide, or the time constant of a quantity which decays exponentially over time;

• the term constant is also used for physical quantities that only take on the same value in particular circumstances; this is the case, for example, of the equilibrium constant for a chemical reaction, which varies with temperature; although the use of the term has become widespread, these are not strictly speaking constants; it is therefore preferable to reserve the term for the first two meanings and not to use it to form names of physical quantities that vary as a function of other quantities;

• Finally, the term also covers reference quantities whose value has been conventionally chosen, for example, normal atmospheric pressure or normal acceleration due to gravity.

The constants presented in this article correspond essentially to the first two levels. A special paragraph is devoted to various constants, reference quantities or certain units (the liter, for example) with historical peculiarities that we thought it would be useful to point out.