Study Challenges Assumption That Fundamental Quantum Field Theory Couplings Should Be Order Unity
A new paper on arXiv argues that the common expectation in quantum field theory — that all dimensionless couplings in a Lagrangian should be roughly of order one — deserves critical scrutiny and formal quantification. The authors introduce a measure called the 'spread' (the ratio of the largest to smallest coupling magnitudes) and show analytically that large ratios between couplings are statistically far more probable than intuition suggests. This matters because the 'naturalness' or 'order unity' assumption underpins much of how physicists judge whether a theory is fine-tuned or fundamental.
A preprint submitted to arXiv by Ben Allanach and collaborators challenges a foundational heuristic in high-energy physics: that a truly fundamental quantum field theory should have all dimensionless Lagrangian couplings of order one. The paper introduces a quantitative measure called the 'spread' — defined as the ratio of the largest to the smallest coupling magnitude — to assess how well any given theory adheres to this expectation. Using independent identically distributed (IID) couplings as a model for uncertainty over 'order unity' values, the authors derive closed-form results showing that ratios between couplings can be dramatically larger than naive intuition implies. For instance, in a theory with 20 IID unit-normal couplings, there is a 29% probability that the magnitude of the ratio of any two couplings exceeds 100. Crucially, even when the IID couplings have exponentially suppressed tails — meaning extreme values are rare individually — the distribution of their ratios develops fat, power-law tails that grow with the number of couplings. The work spans high-energy phenomenology, theory, and statistical physics, and has implications for how physicists interpret apparent fine-tuning in models beyond the Standard Model.
What's missing
The paper is a preprint and has not yet undergone peer review. The authors' proposed 'spread' measure is new and untested against community consensus; it is unclear whether it will be adopted as a standard diagnostic.
What different sources said
- arXiv physicsCenter
The Fundaments of Unity: ${\mathcal O}(1)$ Couplings in Quantum Field Theories
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