The length scale at which a classical description of gravity ceases to be valid, and quantum mechanics must be taken into account. It is given by LP=√(Gћ/c3), where G is the gravitational constant, ћ is the rationalized Planck constant, and c is the speed of light. The value of the Planck length is of order 10−35 m (twenty orders of magnitude smaller than the size of a proton, 10−15 m).