# Physical constant

constantconstantsfundamental constantsuniversal constantfundamental physical constantsphysicalphysical constantsconstants of physicsfundamental constantfundamental 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.wikipedia
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### Speed of light

clight speedspeed of light in vacuum
There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constant G, the Planck constant h, the electric constant ε 0, and the elementary charge e. For example, the speed of light is defined as having the numerical value of 299792458 when expressed in the SI unit metres per second, and as having the numerical value of 1 when expressed in the natural units Planck length per Planck time.
The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics.

### Planck constant

Planck's constantreduced Planck constantreduced Planck's constant
There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constant G, the Planck constant h, the electric constant ε 0, and the elementary charge e.
The Planck constant, or Planck's constant, denoted h, is a physical constant that is the quantum of electromagnetic action, which relates the energy carried by a photon to its frequency.

### Gravitational constant

Newton's constantGuniversal gravitational constant
There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constant G, the Planck constant h, the electric constant ε 0, and the elementary charge e.
, is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general theory of relativity.

### Elementary charge

eelectron chargecharge
There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constant G, the Planck constant h, the electric constant ε 0, and the elementary charge e. It is the value of the elementary charge squared expressed in Planck units.
The elementary charge, usually denoted by e or sometimes q e, is the electric charge carried by a single proton or, equivalently, the magnitude of the electric charge carried by a single electron, which has charge −1 e. This elementary charge is a fundamental physical constant.

### Constant (mathematics)

constantconstantsConstant functions
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.
The term mathematical constant (and also physical constant) is sometimes used to distinguish this meaning from the other one.

### Dimensionless physical constant

fundamental physical constantfundamental physical constantsfundamental constants
Increasingly, however, physicists reserve the use of the term fundamental physical constant for dimensionless physical constants, such as the fine-structure constant α.
In physics, a dimensionless physical constant, is a physical constant that is dimensionless, i.e. a pure number having no units attached and having a numerical value that is independent of whatever system of units may be used.

### List of materials properties

mechanical propertiesmaterial propertiesMaterials property
Physical constant in the sense under discussion in this article should not be confused with other quantities called "constants" that are assumed to be constant in a given context without the implication that they are fundamental, such as the "time constant" characteristic of a given system, or material constants, such as the Madelung constant, electrical resistivity, and heat capacity.
A property may be a constant or may be a function of one or more independent variables, such as temperature.

### Fine-structure constant

fine structure constant137coupling constant
Physical constants can take many dimensional forms: the speed of light signifies a maximum speed for any object and its dimension is length divided by time; while the fine-structure constant α, which characterizes the strength of the electromagnetic interaction, is dimensionless.
in terms of other fundamental physical constants are:

### Boltzmann constant

Boltzmann's constantkthermal voltage
The International Bureau of Weights and Measures decided to redefine several SI base units as from 20 May 2019 by fixing the SI value of several physical constants, including the Planck constant, h, the elementary charge, e, the Boltzmann constant, k B, and the Avogadro constant, N A.
or k), named after its discoverer, Ludwig Boltzmann, is a physical constant that relates the average relative kinetic energy of particles in a gas with the temperature of the gas.

### 2019 redefinition of the SI base units

2019 redefinition of SI base unitsredefinitionredefinition of the SI base units
The International Bureau of Weights and Measures decided to redefine several SI base units as from 20 May 2019 by fixing the SI value of several physical constants, including the Planck constant, h, the elementary charge, e, the Boltzmann constant, k B, and the Avogadro constant, N A.
The second, metre, and candela were already defined by physical constants and were subject to correction to their definitions.

### Planck length

Planck volumelengthPlanck area
The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, the Planck constant, and the gravitational constant.

### Dimensionless quantity

dimensionlessdimensionless numberdimensionless quantities
Physical constants can take many dimensional forms: the speed of light signifies a maximum speed for any object and its dimension is length divided by time; while the fine-structure constant α, which characterizes the strength of the electromagnetic interaction, is dimensionless.

### Mathematical constant

constantconstantsMathematical constants
It is contrasted with a mathematical constant, which has a fixed numerical value, but does not directly involve any physical measurement.

### Natural units

natural unitGeometric variableshas been set to one
For example, the speed of light is defined as having the numerical value of 299792458 when expressed in the SI unit metres per second, and as having the numerical value of 1 when expressed in the natural units Planck length per Planck time.
Out of the many physical constants, the designer of a system of natural unit systems must choose a few of these constants to normalize (set equal to 1).

### Planck mass

Planck scalemassPlanck-scale
In this approach, one starts with the three physical constants ħ, c, and G, and attempts to combine them to get a quantity whose dimension is mass. The formula sought is of the form

### Variable speed of light

varying speed of lightchange" in the speed of lightdifferent speeds
For example, a "change" in the speed of light c would be meaningless if accompanied by a corresponding change in the elementary charge e so that the ratio
The speed of light in vacuum instead is considered a constant, and defined by the SI as 299792458 m/s.

### International System of Units

SISI unitsSI unit
For example, the speed of light is defined as having the numerical value of 299792458 when expressed in the SI unit metres per second, and as having the numerical value of 1 when expressed in the natural units Planck length per Planck time.
The reliability of the SI depends not only on the precise measurement of standards for the base units in terms of various physical constants of nature, but also on precise definition of those constants.

### Measurement

measuremeasuringmeasurements
By definition, fundamental physical constants are subject to measurement, so that their being constant (independent on both the time and position of the performance of the measurement) is necessarily an experimental result and subject to verification.
Artifact-free definitions fix measurements at an exact value related to a physical constant or other invariable phenomena in nature, in contrast to standard artifacts which are subject to deterioration or destruction.

### Multiverse

parallel universesparallel universemultiversal
There are a variety of interpretations of the constants' values, including that of a divine creator (the apparent fine-tuning is actual and intentional), or that ours is one universe of many in a multiverse (e.g. the many-worlds interpretation of quantum mechanics), or even that, if information is an innate property of the universe and logically inseparable from consciousness, a universe without the capacity for conscious beings cannot exist.
Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them.

### Vacuum permittivity

permittivity of free spaceelectric constantvacuum electric permittivity
There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constant G, the Planck constant h, the electric constant ε 0, and the elementary charge e.
The physical constant

### Vacuum permeability

magnetic constantpermeability of free spacepermeability of vacuum
The physical constant μ 0, (pronounced "mu nought" or "mu zero"), commonly called the vacuum permeability, permeability of free space, permeability of vacuum, or magnetic constant, is the magnetic permeability in a classical vacuum.

### Planck units

Planck scalePlanck epochPlanck density
It is the value of the elementary charge squared expressed in Planck units.
If a physical constant that is not dimensionless, such as the speed of light, did in fact change, would we be able to notice it or measure it unambiguously?

### Rydberg constant

Rydberg energyRydbergenergy to excite the hydrogen
In spectroscopy, the Rydberg constant, symbol R_\infty for heavy atoms or R_\text{H} for hydrogen, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to the electromagnetic spectra of an atom.

### Gas constant

universal gas constantideal gas constantspecific gas constant
It is a physical constant that is featured in many fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation.

### SI base unit

base unitSI base unitsbase units
Similarly, with effect from May 2019, the Planck constant has a defined value, such that all SI base units are now defined in terms of fundamental physical constants.
It has long been an objective in metrology to define the kilogram in terms of a fundamental constant, in the same way that the metre is now defined in terms of the speed of light.