  Physical interactions between particles are the result of the exchange of
gauge bosons. Human interactions are mediated by the exchange of messages,
goods, money, promises, hostilities, etc. While in the physical world
interactions and their associated forces have immediate dynamical consequences
(Newton's law) the situation is not clear for human interactions. Here we study
the acceleration between humans who interact through the exchange of messages,
goods and hostilities in a massive multiplayer online game. For this game we
have complete information about all interactions (exchange events) between
about 1/2 million players, and about their trajectories (positions) in a metric
space of the game universe at any point in time. We derive the interaction
potentials for communication, trade and attacks and show that they are harmonic
in nature. Individuals who exchange messages and trade goods generally attract
each other and start to separate immediately after exchange events stop. The
interaction potential for attacks mirrors the usual "hit-and-run" tactics of
aggressive players. By measuring interaction intensities as a function of
distance, velocity and acceleration, we show that "forces" between players are
directly related to the number of exchange events. The power-law of the
likelihood for interactions vs. distance is in accordance with previous real
world empirical work. We show that the obtained potentials can be understood
with a simple model assuming an exchange-driven force in combination with a
distance dependent exchange rate.
