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It can be contrasted with a mathematical constant, which is a fixed value that does not directly involve a physical measurement.

There are many physical constants in science, some of the most famous being Planck's constant, the gravitational constant, and Avogadro's number. Constants can take many forms: the Planck length represents a fundamental physical distance; the speed of light in a vacuum signifies a maximum speed limit of the universe; and the fine-structure constant, which characterizes the interaction between electrons and photons, is dimensionless.

Beginning with Paul Dirac in 1937, some scientists have speculated that physical constants may actually decrease in proportion to the age of the universe. Scientific experiments have not yet pinpointed any definite evidence that this is the case, although they have placed upper bounds on the maximum possible relative change per year at very small amounts (roughly 10^-5 per year for the fine structure constant and 10^-11 for the gravitational constant ).

Some "constants" are really artifacts of the unit system used, such as SI or cgs. In natural units, some of these supposedly physical constants turn out to be conversion factors.

Constants that are independent of systems of units are typically dimensionless numberIn the physical sciences, a dimensionless number (or more precisely, a number with the dimensions of 1 is a quantity which describes a certain physical system and which is a pure number without any physical units. Such a number is typically defined as a ps, and are known as fundamental physical constantIn physics, fundamental physical constants are physical constants that are independent of systems of units and are in general dimensionless numbers. Physicists have long tried to make their theories as simple and elegant as possible by reducing the numbers.

Some believe that if the physical constants had slightly different values, our universe would be so different that intelligent life would probably not have emerged, and that our universe seems to be fine-tunedOur universe seems to be fine-tuned (for life), because any small changes in the 20 or so physical constants would make it very different, and presumably not hospitable to life. For example, stars would not be able to fuse hydrogen and helium if the elect for intelligent life.

1 See also

• scientific constants named after peopleThis is a list of physical and mathematical constants named after people . For other lists of eponyms (names derived from people) see eponym. For a list of eponyms sorted by name see List of eponyms. Avogadro's number Amedeo Avogadro Bohr magneton Niels B
• fine-tuned universeOur universe seems to be fine-tuned (for life), because any small changes in the 20 or so physical constants would make it very different, and presumably not hospitable to life. For example, stars would not be able to fuse hydrogen and helium if the elect
• physical lawA physical law or a law of nature is a scientific generalization based on empirical observations. Laws of nature are conclusions drawn from, or hypotheses confirmed by, scientific experiments. The production of a summary description of nature in the form