Physics constants and reference tables
Quantitative physics depends on a shared set of constants, units and tabulated material properties. Physical constants are quantities that, as far as can be measured, do not change with time or place. Familiar examples include the speed of light in vacuum, the gravitational constant, the elementary charge, the Planck constant and the Boltzmann constant. Their accepted values are determined by international metrology bodies and revised as measurement precision improves.
Constants are meaningful only alongside a consistent system of units. The International System of Units (SI) defines base units such as the metre, kilogram, second, ampere, kelvin, mole and candela, from which all other units are derived. Reference tables list conversion factors between SI and older or specialised units, and collect the scientific and technical acronyms, symbols and abbreviations used in equations and on instruments.
A large part of physical reference data describes the properties of materials and atoms. Tables of electrical conductivity and resistivity rank materials from good conductors to insulators, while acoustic data give the speed of sound and impedance in different media. Atomic physics contributes atomic weights and spectroscopic data: the characteristic wavelengths of light that each element emits or absorbs, which allow substances to be identified by their spectra. Data on condensed matter cover densities, melting and boiling points, and thermal and mechanical behaviour of solids and liquids. Gathered into handbooks and databases, these values let scientists and engineers carry out calculations, design experiments and compare measurements against established standards.
Frequently asked questions
- What is a physical constant?
- A physical constant is a measured quantity, such as the speed of light or the elementary charge, that is believed to have the same value everywhere and at all times.
- Why are SI units important?
- SI units give scientists a single agreed system for expressing measurements, so that results and constants can be compared and combined consistently worldwide.
- What is atomic spectroscopic data used for?
- It records the specific wavelengths of light that each element emits or absorbs, which lets researchers identify elements and study the structure of atoms.
