Quantum Matter Makes Lightcones Quantum

[2606.03671] Quantum Matter Makes Lightcones Quantum Skip to main content Search Submit Donate Log in General Relativity and Quantum Cosmology arXiv:2606.03671 (gr-qc) [Submitted on 2 Jun 2026 ( v1 ), last revised 25 Aug 2026 (this version, v2)] Title: Quantum Matter Makes Lightcones Quantum Authors: Tomohiro Fujita , Misao Sasaki View PDF HTML (experimental) Abstract: In gravitational physics, matter does not merely move within spacetime; it also determines the light cones that define causal relations. What happens when the matter that determines these light cones is itself in a quantum state? We address this question in a controlled low-energy setting: a massless scalar field propagating in the spacetime with the Newtonian gravitational potential sourced by a non-relativistic quantum particle. We show that the light cones are affected by an operator-valued Shapiro delay, with the three consequences: (i) causal-boundary shifts are promoted to noncommuting observables, giving the causal structure an irreducible quantum uncertainty; (ii) the causal relation between two fixed spacetime points can become a superposition of timelike and spacelike configurations; and (iii) tracing out the source smears the Wightman light-cone singularity, producing an effective UV cutoff. Thus, quantum matter does not merely fluctuate within spacetime; it makes the causal structure itself quantum, even without including propagating graviton modes. Comments: 16 pages, 4 figures, matched to published version Subjects: General Relativity and Quantum Cosmology (gr-qc) ; High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph) Report number: YITP-26-57 Cite as: arXiv:2606.03671 [gr-qc] (or arXiv:2606.03671v2 [gr-qc] for this version) https://doi.org/10.48550/arXiv.2606.03671 Focus to learn more arXiv-issued DOI via DataCite Journal reference: Phys.Rev.D 114 (2026) 4, 044047 Related DOI : https://doi.org/10.1103/rg3n-98lr Focus to learn more DOI(s) linking to related resources Submission history From: Tomohiro Fujita [ view email ] [v1] Tue, 2 Jun 2026 13:55:33 UTC (294 KB) [v2] Tue, 25 Aug 2026 09:09:25 UTC (294 KB) Full-text links: Access Paper: View PDF HTML (experimental) TeX Source view license Current browse context: gr-qc < prev | next > new | recent | 2026-06 Change to browse by: hep-th quant-ph References & Citations INSPIRE HEP SciX Google Scholar Semantic Scholar export BibTeX citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer ( What is the Explorer? ) Connected Papers Toggle Connected Papers ( What is Connected Papers? ) Litmaps Toggle Litmaps ( What is Litmaps? ) scite.ai Toggle scite Smart Citations ( What are Smart Citations? ) Code, Data, Media Code, Data and Media Associated with this Article alphaXiv Toggle alphaXiv ( What is alphaXiv? ) Links to Code Toggle CatalyzeX Code Finder for Papers ( What is CatalyzeX? ) DagsHub Toggle DagsHub ( What is DagsHub? ) GotitPub Toggle Gotit.pub ( What is GotitPub? ) Huggingface Toggle Hugging Face ( What is Huggingface? ) ScienceCast Toggle ScienceCast ( What is ScienceCast? ) Demos Demos Replicate Toggle Replicate ( What is Replicate? ) Spaces Toggle Hugging Face Spaces ( What is Spaces? ) Spaces Toggle TXYZ.AI ( What is TXYZ.AI? ) Related Papers Recommenders and Search Tools Link to Influence Flower Influence Flower ( What are Influence Flowers? ) Core recommender toggle CORE Recommender ( What is CORE? ) IArxiv recommender toggle IArxiv Recommender ( What is IArxiv? ) Author Venue Institution Topic About arXivLabs arXivLabs: experimental projects with community collaborators arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them. Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs . Which authors of this paper are endorsers? | Disable MathJax ( What is MathJax? ) We gratefully acknowledge support from our major funders , member institutions , and all contributors. About · Help · Contact · Subscribe · Copyright · Privacy · Accessibility · Operational Status (opens in new tab)

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