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In particle physics and physical cosmology, Planck units are a set of units of measurement defined exclusively in terms of four universal physical constants, in such a manner that these physical constants take on the numerical value of 1 when expressed in terms of these units. Originally proposed in 1899 by German physicist Max Planck, these units are a system of natural units because their definition is based on properties of nature, more specifically the properties of free space, rather than a choice of prototype object. They are relevant in research on unified theories such as quantum gravity. The term Planck scale refers to quantities of space, time, energy and other units that are similar in magnitude to corresponding Planck units. This region may be characterized by particle energies of around e19GeV or e9J, time intervals of around e−43s and lengths of around e-35m (approximately the energy-equivalent of the Planck mass, the Planck time and the Planck length, respectively). At the Planck scale, the predictions of the Standard Model, quantum field theory and general relativity are not expected to apply, and quantum effects of gravity are expected to dominate. The best-known example is represented by the conditions in the first 10−43 seconds of our universe after the Big Bang, approximately 13.8 billion years ago. The four universal constants that, by definition, have a numeric value 1 when expressed in these units are: the speed of light in vacuum, c, the gravitational constant, G, the reduced Planck constant, ħ, and the Boltzmann constant, kB. Planck units do not incorporate an electromagnetic dimension. Some authors choose to extend the system to electromagnetism by, for example, adding either the Coulomb constant (k_e = 1/4πε_0) or the electric constant (ε_0) to this list. Similarly, authors choose to use variants of the system that give other numeric values to one or more of the four constants above. Any system of measurement may be assigned a mutually independent set of base quantities and associated base units, from which all other quantities and units may be derived.

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