A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value, but does not directly involve any physical measurement.
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, Planck's constant h, the electric constant ε0, and the elementary charge e. Physical constants can take many dimensional forms: the speed of light signifies a maximum speed limit of the Universe and is expressed dimensionally as length divided by time; while the fine-structure constant α, which characterizes the strength of the electromagnetic interaction, is dimensionless.
Read more about Physical Constant: Dimensional and Dimensionless Physical Constants, How Constant Are The Physical Constants?, Anthropic Principle, Table of Universal Constants, Table of Electromagnetic Constants, Table of Atomic and Nuclear Constants, Table of Physico-chemical Constants, Table of Adopted Values, Natural Units
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