Quantum superposition of spacetimes obeys Einstein's Equivalence Principle

Understand

We challenge the view that there is a basic conflict between the fundamental principles of Quantum Theory and General Relativity, and in particular the fact that a superposition of massive bodies would lead to a violation of the Equivalence Principle.

• It has been argued that this violation implies that such a superposition must inevitably spontaneously collapse (like in the Di\'osi-Penrose model).
• We identify the origin of such an assertion in the impossibility of finding a local, classical reference frame in which Einstein's Equivalence Principle would hold.
• In contrast, we argue that the formulation of the Equivalence Principle can be generalised so that it holds for reference frames that are associated to quantum systems in a superposition of spacetimes.