Spectroscopy as a Probe of Spacetime Foam
Abstract
Quantum-gravitational spacetime foam varying in space and in time imprints, respectively, a transverse phase decoherence and an achromatic, stochastic arrival-time jitter on light from distant sources. We show that the jitter modifies the first-order temporal coherence g(1)(τ) and hence the observed spectrum, providing a spectroscopic probe of the temporal variation, complementary to existing imaging-channel constraints on the spatial variation. In the dynamical-foam limit, where the foam decorrelates within the light's coherence time, the spectrum splits into a coherent line core suppressed by e−σϕ2, with σϕ2 the variance of the foam-induced phase jitter, and an incoherent pedestal, yielding a spectral shape-distortion bound on the foam exponent α. Three probes follow: the outright survival of broadband flux, the fractional width of a spectral line, and a chromatic deficit across a broadband continuum. Applied to the MeV emission lines of GRB\,221009A we obtain α≳ 0.63, a weak, lever-arm-dominated floor. For the broadband CMB the foam induces an e−σϕ2(ν) deficit (σϕ2 ∝ ν2); a fit to the COBE/FIRAS monopole spectrum reaches α≳ 0.52 (95% CL), and next-generation spectrometers reach only α≳ 0.54--$0.55$ (forecast), limited by the foam--temperature degeneracy. The pedestal-independent line-width and CMB bounds are floors below the holographic value α= 2/3, consistent with and excluding no part of the imaging window reported for GRB\,221009A; only the conditional TeV survival bound (α≳0.74), which holds only for near-Planck-refresh foam (τfoam≲4×10−28\,s), would exclude it --- and, with it, the window.
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BibTeX
@misc{kim2026spectroscopy,
title = {{Spectroscopy as a Probe of Spacetime Foam}},
author = {A. G. Kim and P. E. Nugent and L. Wang},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.26766},
eprint = {2610.26766},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.26766},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Spectroscopy as a Probe of Spacetime Foam
AU - A. G. Kim
AU - P. E. Nugent
AU - L. Wang
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2610.26766
UR - https://arxiv.org/abs/2610.26766
AB - Quantum-gravitational spacetime foam varying in space and in time imprints, respectively, a transverse phase decoherence and an achromatic, stochastic arrival-time jitter on light from distant sources. We show that the jitter modifies the first-order temporal coherence g^(1)(τ) and hence the observed spectrum, providing a spectroscopic probe of the temporal variation, complementary to existing imaging-channel constraints on the spatial variation. In the dynamical-foam limit, where the foam decorrelates within the light's coherence time, the spectrum splits into a coherent line core suppressed by e^−σ_ϕ^2, with σ_ϕ^2 the variance of the foam-induced phase jitter, and an incoherent pedestal, yielding a spectral shape-distortion bound on the foam exponent α. Three probes follow: the outright survival of broadband flux, the fractional width of a spectral line, and a chromatic deficit across a broadband continuum. Applied to the MeV emission lines of GRB\,221009A we obtain α≳ 0.63, a weak, lever-arm-dominated floor. For the broadband CMB the foam induces an e^−σ_ϕ^2(ν) deficit (σ_ϕ^2 ∝ ν^2); a fit to the COBE/FIRAS monopole spectrum reaches α≳ 0.52 (95% CL), and next-generation spectrometers reach only α≳ 0.54--$0.55$ (forecast), limited by the foam--temperature degeneracy. The pedestal-independent line-width and CMB bounds are floors below the holographic value α= 2/3, consistent with and excluding no part of the imaging window reported for GRB\,221009A; only the conditional TeV survival bound (α≳0.74), which holds only for near-Planck-refresh foam (τ_foam≲4×10^−28\,s), would exclude it --- and, with it, the window.
ER -