Is spacetime quantum?

TL;DR

This paper presents a theorem suggesting that spacetime exhibits quantum features if certain assumptions about quantum measurement and relativity hold, challenging the classical view of spacetime.

Contribution

It introduces a novel theorem linking quantum measurement collapse and Bell inequality violations to the quantum nature of spacetime, without requiring gravitational wave detection.

Findings

Spacetime and quantum systems violate Bell inequalities under specified assumptions.

The theorem does not depend on gravitational wave manipulation or conservation laws.

Supports the view that spacetime cannot be purely classical if assumptions hold.

Abstract

Although the standard viewpoint in theoretical physics is that the unification of quantum theory and general relativity requires the quantization of gravity and spacetime, there is not consensus about whether spacetime must fundamentally have any quantum features. Here we show a theorem stating that spacetime degrees of freedom and a quantum system violate a Bell inequality in a background Minkowski spacetime if a few properties of general relativity and quantum theory have a broad range of validity, and if the quantum state reduction upon measurement is a real physical process that is completed superluminally when acting on distant quantum particles in a quantum entangled state. We argue that this implies that spacetime cannot be sensibly called classical if the assumptions in our theorem hold. In contrast to the Eppley-Hannah argument for the necessity of quantizing the gravitational field, we discuss the validity of our assumptions, our thought experiment does not require to manipulate or detect gravitational waves, and our theorem does not rely on the conservation of momentum or on the uncertainty principle.