Skip to ContentSkip to Navigation
About usFaculty of PhilosophyEducationThesis summaries

Dijkstra, C.D.

Department of Theoretical Philosophy

Naturalness as a reasonable scientific principle in fundamental physics

A plethora of theories in fundamental physics predicts modifications of well-entrenched quantum field theories, posing an urgent problem of underdetermination by data because these energies cannot be probed in the near future. Several heuristics are often used to non-empirically assess theories of high energy physics, including "naturalness" since the late 1970s. The utility of naturalness has however become progressively more contested, for several reasons. The fact that many notions of the principle have been put forward in the literature (some of which are discordant) has obscured the conceptual foundation of the principle, causing confusion as to what naturalness actually is. Additionally, naturalness has been criticized to be a "sociological instrument"  by Grinbaum (2009), an ill-defined dogma by Hossenfelder (2018) and has an "aesthetic character" according to Donoghue (2007), all of which seem to undermine the validity of the principle.

I aim to clarify the physical content and significance of naturalness in the context of effectice field theories (EFTs) and invalidate said criticisms. Physicists' earliest understanding of naturalness, as an autonomy of scales (AoS) requirement, provides the most cogent definition of naturalness and I will assert that this i) provides a uniform notion which undergirds prominent naturalness conditions, ii) is a reasonable criterion to impose on EFTs and iii) allows us to understand both successes and violations of naturalness. I state that AoS naturalness is neither an aesthetic nor a sociologically-influenced principle - 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. I put forward arguments as to why AoS naturalness is well-defined and why it was reasonable for physicists to endorse this naturalness principle - this is justified on both theoretical and empirical grounds.

Two large violations of naturalness - the Higgs mass and the cosmological constant - have been discovered in EFTs several decades ago. These violations of AoS naturalness led to the widespread belief among physicists that naturalness can be recovered by extending the Standard Model - "If one has to summarise in one word what drove the efforts in physics beyond the Standard Model of the last several decades, the answer is naturalness'' (Giudice 2017). More recently, the surmise that these parameters may actually be unnatural has gained credibility, partially due to anthropic arguments in the context of the multiverse. I will argue that these arguments are sound and entail that several parameters may be unnatural. Although most parameters are natural, no compelling reasons have been put forward as to why parameters should generically be natural. Violations of naturalness would then have ontological implications for quantum field theories. EFTs, while ravishingly successful for natural parameters, cannot accurately describe unnatural parameters. The latter should instead be described by a novel kind of field theory in which high-energy physics is no longer negligible for the description of low-energetic phenomena.

Last modified:24 September 2018 12.44 p.m.