  Nanoswimmers are ubiquitous in bio- and nano-technology but are extremely
challenging to measure due to their minute size and driving forces. A simple
method is proposed for detecting the elusive physical features of nanoswimmers
by observing how they affect the motion of much larger, easily traceable
particles. Modeling the swimmers as hydrodynamic force dipoles, we find direct,
easy-to-calibrate relations between the observable power spectrum and
diffusivity of the tracers and the dynamic characteristics of the swimmers:
their force dipole moment and correlation times.
