  The intrinsic factor that drives the human movement remains unclear for
decades. While our observations from intra-urban and inter-urban trips both
demonstrate a universal law in human mobility. Be specific, the probability
from one location to another is inversely proportional to the number of
population living in locations which are closer than the destination. A simple
rank-based model is then presented, which is parameterless but predicts human
flows with a convincing fidelity. Besides, comparison with other models shows
that our model is more stable and fundamental at different spatial scales by
implying the strong correlation between human mobility and social relationship.
