  In the absence of directional motion it is often hard to recognize athermal
fluctuations. Probability currents provide such a measure in terms of the rate
at which they enclose area in the reduced phase space. We measure this area
enclosing rate for trapped colloidal particles, where only one particle is
driven. By combining experiment, theory, and simulation, we single out the
effect of the different time scales in the system on the measured probability
currents. In this controlled experimental setup, particles interact
hydrodynamically. These interactions lead to a strong spatial dependence of the
probability currents and to a local influence of athermal agitation. In a
multiple-particle system, we show that even when the driving acts only on one
particle, probability currents occur between other, non-driven particles. This
may have significant implications for the interpretation of fluctuations in
biological systems containing elastic networks in addition to a suspending
fluid.
