Nonlocality in Quantum Gravity and the Breakdown of Effective Field Theory

TL;DR

This paper discusses the inherent nonlocal nature of quantum gravity, highlighting the challenges and potential impossibility of formulating a conventional effective field theory due to gravity's influence on length scales and cutoff definitions.

Contribution

It emphasizes the difficulty of constructing an effective field theory for quantum gravity, especially because gravity alters length scales and redefines cutoffs, leading to a breakdown of EFT applicability.

Findings

Quantum gravity is fundamentally nonlocal.

Constructing an EFT for quantum gravity is likely impossible.

Naive EFT application in gravity can cause conceptual problems, exemplified by black hole issues.

Abstract

We argue that quantum gravity is nonlocal, first by recalling well-known arguments that support this idea and then by focusing on a point not usually emphasized: that making a conventional effective field theory (EFT) for quantum gravity is particularly difficult, and perhaps impossible in principle. This inability to realize an EFT comes down to the fact that gravity itself sets length scales for a problem: when integrating out degrees of freedom above some cutoff, the effective metric one uses will be different, which will itself re-define the cutoff. We also point out that even if the previous problem is fixed, naively applying EFT in gravity can lead to problems - we give a particular example in the case of black holes.