Talk:Quadratic gravity

Not a quantum theory

Quadratic gravity is a thing. Quantum quadratic gravity is some speculation. The conclusion of

• Alvarez‐Gaume, L., Kehagias, A., Kounnas, C., Lüst, D., & Riotto, A. (2016). Aspects of quadratic gravity. Fortschritte der Physik, 64(2-3), 176-189.
• We have studied here quadratic curvature gravity theories. Such theories have attracted considerable attention, among others things, due to their renormalization properties. In particular, they offer the interesting possibility of yielding a well defined quantum theory, which in addition, reduces to general relativity at long distances.

The introduction to

• Salvio, Alberto (2018). "Quadratic Gravity". Frontiers in Physics. 6 (77) 77. arXiv:1804.09944. Bibcode:2018FrP.....6...77S. doi:10.3389/fphy.2018.00077.
• What happens if the quantum dynamics of the gravitational field is taken into account in QG?
• Donoghue, J. F., & Menezes, G. (2021). On quadratic gravity. arXiv preprint arXiv:2112.01974.
• While some phenomenology has been explored [19, 20], the emphasis here will be on whether Quadratic Gravity works as a viable quantum field theory, and what its novel features are.

Johnjbarton (talk) 03:21, 22 November 2025 (UTC)

Thanks for the fixes, it seems like the main interest in this theory is its quantized version though? https://journals.aps.org/prd/abstract/10.1103/PhysRevD.106.126015 mentions QQG by name so I put back QQG in the article lede, partly to assist search queries. I hope that's ok. I wish I understood this stuff better so I could tell what it means for a classical theory to be renormalizeable. I thought that renormalizeability was purely a QFT thing, whose purpose is to get rid of singularities arising from quantization. But, I haven't yet found an explanation of what it is or how it works. Maybe someday. Would it be more precise to say QG's quantization is renormalizable, rather than QG is renormalizable? QG should also probably be returned somehow to the quantum gravity template, though not in the string theory section. ~2025-35499-09 (talk) 07:34, 22 November 2025 (UTC)

Renormalization is a math technique for dealing with divergence. One source that discusses its application to classical theory is

• Effective Field Theories Alexey A Petrov (Wayne State University, USA) and Andrew E Blechman (Wayne State University, USA) January 2016 https://doi-org.wikipedialibrary.idm.oclc.org/10.1142/8619

This source is especially useful in this context because it discusses many of the issues related to the article. On page 234 for example it points out that quantum gravity effects cannot be verified by any known technique: its only a theoretical curiosity. Johnjbarton (talk) 17:33, 22 November 2025 (UTC)

This looks good too: https://arxiv.org/abs/2404.08034 it discusses the quantization procedure explicitly. ~2025-35499-09 (talk) 07:39, 22 November 2025 (UTC)

There are dozens of extensions of general relativity, each which sound amazing. None individually meet the notability criteria for articles, including this one. A dozen primary papers by advocates makes the topic of interest for a session at a physics conference, but the criteria for wikipedia is secondary sources. I think we could get consensus to merge this in to f(r) gravity.

The quantum version is below that bar. As far as wikipedia is concerned, there is no "quantum quadratic gravity" until it appears more broadly or in reviews.

If you don't agree, ask for more input on eg WT:Physics. Johnjbarton (talk) 17:18, 22 November 2025 (UTC)

Oh come on, this isn't a medical article. Most of the citations for this article are arxiv preprints and that is fine. Fwiw the Scholarpedia article on f(r) gravity specifically calls out the quadratic case as being of particular relevance to cosomlogy. And the quadratic case is certainly a topic in quantum gravity given its focus on renormalizeability. B

The Quanta magazine article on quadratic gravity, plus the amount of research literature visible from web search, are good enough for me as far as notability is concerned. My question about renormalizeability is whether the concept even applies to non-quantized classical theories, since they don't diverge at the Planck scale, they're just nonphysical there. ~2025-35499-09 (talk) 00:45, 23 November 2025 (UTC)

Merging to f(r) gravity could be ok but I'd want to keep most of the contents/citations in the merge, and add the Salvio 2024 one which isn't there now. Note that f(r) gravity already discusses R² in the section on Starobinsky inflation and links to a separate article about that. I just wouldn't worry about this organizational stuff too much. If there were a lot more duplicated info between the 3 articles it would be worth some effort to clean up, but right now it is ok imho. ~2025-35499-09 (talk) 01:41, 23 November 2025 (UTC)

Also looks interesting: https://www.aanda.org/articles/aa/full_html/2023/05/aa44080-22/aa44080-22.html about testing some quadratic gravity predictions using Event Horizon Telescope data. I haven't tried reading it yet. ~2025-35812-73 (talk) 09:31, 23 November 2025 (UTC)

I looked at the Petrov & Blechman book but I'm afraid it's too advanced for me. I don't know any QFT or basically any QM beyond the popularization level. I do appreciate the recommendation. I may try to work through an introductory QM book, but it won't talk about renormalization, I'm pretty sure. Maybe there's a mathematical treatment somewhere that doesn't rely on as much physics background. ~2025-35812-73 (talk) 10:11, 23 November 2025 (UTC)

Please stop removing page numbers.

@BradNorton1979 I replaced the page number once and I reverted your change once. If you have some reason you need to give in this Talk page. Otherwise stop removing it. Johnjbarton (talk) 20:09, 8 March 2026 (UTC)