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In physics, the Planck length, denoted , is a unit of length, equal to . It is a base unit in the system of Planck units, developed by physicist Max Planck. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, the Planck constant, and the gravitational constant. According to the generalized uncertainty principle (a concept from speculative models of quantum gravity), the Planck length is, in principle, within a factor of 10, the shortest measurable length – and no theoretically known improvement in measurement instruments could change that. By contrast, the largest measurable thing is the observable universe. The proper distance – the distance as would be measured at a specific time, including the present – between Earth and the edge of the observable universe is , making the diameter of the observable universe about .