Quantum Matter Makes Lightcones Quantum

• URL: http://arxiv.org/abs/2606.03671v1
• Date: Tue, 02 Jun 2026 13:55:33 GMT
• Title: Quantum Matter Makes Lightcones Quantum
• Abstract summary: In gravitational physics, matter does not merely move within spacetime; it also determines the light cones that define causal relations.<n>We show that the light cones are affected by an operator- singularity Shapiro delay, with the three consequences.<n>We conclude that quantum matter does not merely fluctuate within spacetime; it makes the causal structure itself quantum, even without quantized gravitons.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• 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 quantized gravitons.

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