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In physics, the Planck length, denoted , is a unit of length in the system of Planck units that was originally proposed by physicist Max Planck, equal to The Planck length can be defined from three fundamental physical constants: the speed of light, the Planck constant, and the gravitational constant. It is also the reduced Compton wavelength of a particle with Planck mass. Regardless of whether it represents some fundamental limit to the universe, it is a useful unit in theoretical physics.