  Attempting to reconcile general relativity with quantum mechanics is one of
the great undertakings of contemporary physics. Here we present how the
incompatibility between the two theories arises in the simple thought
experiment of preparing a heavy object in a quantum superposition. Following
Penrose's analysis of the problem, we determine the requirements on physical
parameters to perform experiments where both theories potentially interplay. We
use these requirements to compare different systems, focusing on mechanical
oscillators which can be coupled to superconducting circuits.
