I've heard that planck length is the smallest length ever! but if something that his length is equal to planck length and moving by speed dv which is infinitesimal change in speed or higher than that , then according to special relativity his length must be equal to L'=L \sqrt1-v^2/c^2 which is violate that planck length is the smallest length. so I considered two soultions to this dilemma, first maybe I misunderstood and this is the smallest length possible when object at rest. maybe when object get to planck length he can't ever move agian! so what is the solutions? and other thing since string length is equal to planck length? how they propagate in spacetime?