Planck units

Planck units are natural units built from fundamental constants, including the speed of light, the gravitational constant, and the reduced Planck constant. They express scales at which quantum theory and gravity are expected to intersect.

Speed of light: The speed at which light propagates in a vacuum, denoted c. Setting c to one makes time and distance share the same unit.
Planck length: The length √(ℏG/c³), approximately 1.6 × 10⁻³⁵ metres. It is the basic length scale formed from quantum action, gravity, and light speed.
Max Planck: A German theoretical physicist whose work on blackbody radiation initiated quantum theory. Planck introduced natural units built from universal constants in 1899.
Quantum gravity: A framework or research program for describing gravity using quantum principles. Planck units identify the regime where a quantum description of gravity is expected to be necessary.
String theory: A framework in which fundamental excitations are one-dimensional strings rather than point particles. Its characteristic length is often compared with the Planck length, though the relationship depends on the model.
Newtonian gravitational constant: The constant G that sets the strength of Newtonian gravity and appears in general relativity. Its value helps define the characteristic length and mass scales.
Planck time: The time √(ℏG/c⁵), approximately 5.4 × 10⁻⁴⁴ seconds. It is the light-travel time across one Planck length.
Planck constant: The constant h that relates a photon's energy to its frequency and sets the scale of quantum action. Planck's original natural units used h; modern conventions commonly use ℏ.
Planck epoch: The earliest proposed phase of the universe, when its age was near the Planck time. Its defining timescale is expressed directly by the Planck time.
Loop quantum gravity: A proposed quantum theory of gravity that quantizes geometric observables such as area and volume. Its discrete geometric spectra are commonly expressed using the Planck length.

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