  The Schr\"odinger equation predicts the validity of the superposition
principle at any scale, yet we do not experience cats being in a superposition
of "dead" and "alive" in our everyday lives. Modifications to quantum theory at
the fundamental level may be responsible for the objective collapse of the wave
function above a critical mass, thereby breaking down the superposition
principle and restoring classical behavior at the macroscopic scale. One
possibility is that these modifications are related to gravity, as described by
the Di\'osi-Penrose wavefunction collapse model. Here, we investigate this
model using experimental measurements on the decoherence of a Schr\"odinger cat
state of a mechanical resonator with an effective mass of 16 micrograms.
