A unit of time in an absolute system of units devised by Max Planck, about 5.391 247 × 10⁻⁴⁴ second¹. Symbol, tP. Algebraically, it is
where c = the speed of light, G = Newtonian gravitational constant, and ħ = the reduced Planck's constant.
From the equations, you can see that a velocity of 1 Planck length per Planck time is the speed of light.
1. This is the 2018 CODATA value. For the most up-to-date value, see NIST's CODATA pages: https://www.physics.nist.gov/cgi-bin/cuu/Value?plkt
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Last revised: 28 September 2011.