The Planck length is the length at which classical ideas about gravity and space-time cease to be valid , and quantum effects dominate . Because the Planck length is the only length that can be formed from the constants c , G , and Ä , dimensional analysis suggests that lengths of special significance in quantum gravity are likely to be small multiples of the Planck length . In some theories or forms of quantum gravity , it is the length scale at which the structure of spacetime becomes dominated by quantum effects , giving it a discrete or foamy structure , but other theories of quantum gravity predict no such effects .