  From Newtons third law, the principle of actio et reactio, we expect the
forces between interacting particles to be equal and opposite. However,
non-reciprocal forces can arise. Specifically, this has recently been shown
theoretically in the interaction between dissimilar optically trapped particles
mediated by an external field. As a result, despite the incident external field
not having a transverse component of momentum, the particle pair experiences a
force in a direction transverse to the light propagation direction. In this
letter, we directly measure the net non-reciprocal force in electrodynamically
interacting nanoparticle dimers illuminated by plane waves and confined to
pseudo one-dimensional geometries. We show by electrodynamic theory and
simulations that inter-particle interactions cause asymmetric scattering from
heterodimers and therefore, the non-reciprocal forces are a consequence of
momentum conservation. Finally, we demonstrate experimentally that
non-reciprocal dynamics occur generally for illuminated asymmetric scatterers.
