### Negative-Mass Hydrodynamics¶

Negative mass is a peculiar concept. Counter to everyday experience, an object with negative effective mass will accelerate backward when pushed forward. This effect is known to play a crucial role in many condensed matter contexts, where a particle's dispersion can have a rather complicated shape as a function of lattice geometry and doping. In our work we show that negative mass hydrodynamics can also be investigated in ultracold atoms in free space and that these systems offer powerful and unique controls.