## Planck Time

# Planck Time

The Planck time is a unit of time that is defined by three of the fundamental constants of nature: Isaac Newton's constant of gravitation, *G* ; the velocity of light in vacuum, *c* ; and Max Planck's constant, *h.* These constants may be combined in one and only one way to give a quantity that has the dimensions of a time:

*t* _{planck} = (*Gh/c* 5)1/2 = 1.3 × 10-43 *s*

This unit of time exists independently of all human standards of time measurement. It is defined by the gravitational, relativistic, and quantum aspects of the universe. The universe can be said to be "old" in the well defined sense that it is about 1060 Planck times in age (about thirteen billion years). The Planck time has cosmological significance. It marks the time before which the entire universe displays wave-particle duality. In order to understand events earlier than the Planck time, quantum cosmology is required. The Planck time was first identified by German theoretical physicist Max Planck (1858–1947) in 1899, although the idea of a natural unit of time based on the fundamental constants *G, c,* and *e* (the charge on the electron) was first presented by Irish physicist George Johnstone Stoney (1826–1911) in 1881.

*See also* Age of the Universe; Cosmology, Physical Aspects; Physics, Quantum

*Bibliography*

barrow, john. the constants of nature. london: jonathan cape, 2002.

john d. barrow

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