Naturalness as a reasonable scientific principle in fundamental physics

TL;DR

This paper clarifies the concept of naturalness in fundamental physics, emphasizing the autonomy of scales as the core criterion, and argues that violations of naturalness have significant ontological implications.

Contribution

It defends the autonomy of scales definition of naturalness, distinguishes it from sociological notions, and critiques the applicability of the Decoupling Theorem in quantum field theories.

Findings

AoS naturalness provides a uniform, cogent criterion.

Violations of naturalness imply UV/IR interplay in theories.

Decoupling theorem is insufficient without naturalness.

Abstract

I aim to clarify the physical content and significance of naturalness. Physicists' earliest understanding of naturalness, as an autonomy of scales (AoS) requirement provides the most cogent definition of naturalness and I will assert that i) this provides a uniform notion which undergirds a myriad prominent naturalness conditions, ii) this is a reasonable criterion to impose on EFTs and iii) the successes and violations of naturalness are best understood when adhering to this notion of naturalness. I argue that this principle is neither an aesthetic nor a sociologically-influenced principle. I contend that naturalness may only be plausibly argued to be an aesthetic/sociological principle when formal measures of naturalness and their use in physics communities are conflated with the central dogma of naturalness - the former may indeed be argued to be sociologically-influenced and somewhat arbitrary - but these formal measures of naturalness are in fact less successful than AoS naturalness. I put forward arguments as to why AoS naturalness is well-defined and why it was reasonable for physicists to endorse this naturalness principle on both theoretical and empirical grounds. Up to date, no compelling reasons have appeared as to why the laws of nature should generically decoupling into quasi-autonomous physical domains. A decoupling of scales in the quantum realm is often claimed to be entailed by the Decoupling Theorem (Cao and Schweber (1993)), yet I will show that this theorem is too weak to underwrite quasi-autonomous physical domains in quantum field theories because one should additionally impose that parameters be natural. Violations of naturalness would then have ontological import - unnatural parameters would not be accurately described by EFTs but rather by field theories exhibiting some kind of UV/IR interplay.