Harmonic oscillators belong to the most fundamental concepts in physics and
are central to many current research fields such as circuit QED, cavity
optomechanics and photon-pressure systems. Here, we engineer a microwave mode
in a superconducting LC circuit that mimics the dynamics of a negative mass
oscillator, and couple it via photon-pressure to a second low-frequency
circuit. We demonstrate that the effective negative mass dynamics lead to an
inversion of dynamical backaction and to sideband-cooling of the low-frequency
circuit by a blue-detuned pump field, which can be intuitively understood by
the inverted energy ladder of a negative mass oscillator