Changing Tables & Units

In physics, Planck units are physical units of measurement defined exclusively in terms of the five universal physical constants shown in the table below in such a manner that all of these physical constants take on the numerical value of one when expressed in terms of these units. more...

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Planck units elegantly simplify particular algebraic expressions appearing in physical law. Originally proposed by Max Planck, these units are also known as natural units because the origin of their definition comes only from properties of nature and not from any human construct. Planck units are only one system of natural units among other systems, but might be considered unique in that these units are not based on properties of any prototype, object, or particle but are based only on properties of free space.

The Planck units are often semi-humorously referred to by physicists as "God's units". They eliminate anthropocentric arbitrariness from the system of units: some physicists believe that an extra-terrestrial intelligence might be expected to use the same system of units.

Natural units can help physicists reframe questions. Frank Wilczek once described it as:

...We see that the question is not, "Why is gravity so feeble?" but rather, "Why is the proton's mass so small?" For in Natural (Planck) Units, the strength of gravity simply is what it is, a primary quantity, while the proton's mass is the tiny number ...

—June 2001 Physics Today

The strength of gravity is simply what it is and the strength of the electromagnetic force simply is what it is. The electromagnetic force operates on a different physical quantity (electric charge) than gravity (mass) so it cannot be compared directly to gravity. To note that gravity is an extremely weak force is, from the point-of-view of Planck units, like comparing apples to oranges. It is true that the electrostatic repulsive force between two protons (alone in free space) greatly exceeds the gravitational attractive force between the same two protons, and that is because the charge on the protons are approximately the Planck unit of charge but the mass of the protons are far, far less than the Planck mass.

Base Planck units

Constraining the numerical values of the above five fundamental constants to be 1 defines units of length, mass, time, charge, and temperature, called "base units." As is the case with other systems of units (e.g., SI units and cgs units), a host of other "derived" units can be defined in terms of these base units.

Derived Planck units

As in other systems of units, the following physical units are derived from the base units.

Nondimensionalization of fundamental physical equations

By setting the numerical values of the five fundamental constants to unity, natural units simplify many equations of physical law; examples are shown below. The simplifications Planck units afford make them common in quantum gravity research.

Read more at Wikipedia.org