  Models of social mobility inspired by the Newton's law of gravity have been
used for several decades to describe migrations of people, goods, and
information. Despite an eminent reference and widespread use, these models lack
the background theory, being often viewed as a collection of empirical recipes
which rely on adjustable parameters. Here we propose a tractable and
fundamental mechanism of social mobility, which explains distance scaling of
migration flows and predicts the value of the scaling exponent. The mechanism
reveals two key aspects framing social flows, which have direct analogy in
physics: symmetry and dissipation. In particular, we identify the conditions
for the social gravity scaling, when the power law exponent equals 2, and
explain deviations from this behaviour, including saturation transitions. The
resulting flow distribution is determined by the spatial structure of the
underlying social network, rather than by distance explicitly. The theory is
verified for residential migration in suburb networks of major Australian
cities with diverse structure, population and size. The mechanism is directly
translatable to other social contexts, such as flows of goods or information,
and provides a universal understanding of how dynamic patterns of social
mobility emerge from structural network properties.
