  The social phenomenon of familiar strangers was identified by Stanley Milgram
in 1972 with a small-scale experiment. However, there has been limited research
focusing on uncovering the phenomenon at a societal scale and simultaneously
investigating the social relationships between familiar strangers. With the
help of the large-scale mobile phone records, we empirically show the existence
of the relationship in the country of Andorra. Built upon the temporal and
spatial distributions, we investigate the mechanisms, especially collective
temporal regularity and spatial structure that trigger this phenomenon.
Moreover, we explore the relationship between social distances on the
communication network and the number of encounters and show that larger number
of encounters indicates shorter social distances in a social network. The
understanding of the physical encounter network could have important
implications to understand the phenomena such as epidemics spreading and
information diffusion.
