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arxiv: 2604.14018 · v1 · submitted 2026-04-15 · ✦ hep-ph

Tests of Lorentz Symmetry using X-ray Polarimetry

Fabian Kislat This is my paper

Pith reviewed 2026-05-10 13:12 UTC · model grok-4.3

classification ✦ hep-ph
keywords Lorentz invariance violationX-ray polarimetryactive galactic nucleiStandard-Model Extensionbirefringenceastrophysical tests of relativity
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The pith

X-ray polarization from active galactic nuclei tightens Lorentz symmetry limits by four orders of magnitude over optical results.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper uses X-ray polarization measurements of active galactic nuclei to search for possible small violations of Lorentz symmetry, the foundational symmetry of special relativity. Any such violation would cause a tiny difference in the travel speed of the two polarization states of light, and this difference would grow into a detectable rotation or loss of polarization as the light travels across billions of light-years. The authors analyze X-ray data from these distant sources and place new upper limits on the size of the violation within the Standard-Model Extension framework. These limits are four orders of magnitude stronger than those obtained earlier with optical polarization data from similar objects.

Core claim

X-ray polarization measurements of active galactic nuclei yield new constraints on Lorentz invariance violation parameters in the Standard-Model Extension that improve previous optical bounds by four orders of magnitude.

What carries the argument

Polarization of X-rays from active galactic nuclei, whose sensitivity to accumulated travel-time differences between polarization modes over cosmological distances allows detection of tiny Lorentz-violating effects.

If this is right

  • Tighter bounds apply to the photon-sector coefficients of the Standard-Model Extension.
  • No vacuum birefringence is detected at X-ray energies for the observed sources at the new sensitivity level.
  • High-energy astrophysical polarization serves as a cumulative probe that grows more powerful with source distance.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same accumulation principle could be tested with future X-ray polarimeters on more distant or brighter sources to reach even higher energies.
  • If a nonzero signal appears in larger datasets, it would directly indicate a breakdown of special relativity at accessible energies.
  • Cross-checks with gamma-ray polarization data from the same objects would test whether the effect scales with photon energy as predicted by the framework.

Load-bearing premise

The measured X-ray polarization from the active galactic nuclei is dominated by propagation effects over cosmological distances and is not significantly altered by source-intrinsic emission processes or other unaccounted astrophysical propagation effects.

What would settle it

Detection that the observed X-ray polarization angles or degrees are fully explained by source emission models or local astrophysical effects without any propagation-induced rotation would remove the basis for the reported constraints.

Figures

Figures reproduced from arXiv: 2604.14018 by Fabian Kislat.

Figure 1
Figure 1. Figure 1: Theoretically possible maximum observable polarization [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: By comparing IXPE polarization measurements (Mrk 421: (15 [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Skymap in galactic coordinates showing the locations of the 11 AGN from which [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Product likelihood Q i P(Pi < Pmax,i|σP,i) that all IXPE measurements listed in [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
read the original abstract

Lorentz symmetry is the fundamental symmetry of Einstein's theory of Special Relativity and has been tested to great precision. Nevertheless, the possibility remains that it is violated at the Planck scale, as predicted by some theories of quantum gravity. While the Planck scale is not directly accessible to experiments, minute residual deviations from Lorentz symmetry at attainable energies may be observable. The polarization of light from astrophysical sources is a particularly powerful probe because tiny differences accumulate as light travels over astrophysical distances, and polarization is sensitive to light travel time differences between polarization modes on the order of the oscillation period of the electromagnetic wave. Here, we report on new constraints on Lorentz invariance violation derived from X-ray polarization measurements of active galactic nuclei. The new constraints, presented in the framework of the Standard-Model Extension, improve on our previous work, which used optical polarization measurements, by four orders of magnitude.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 3 minor

Summary. The manuscript reports new upper limits on Lorentz-violating coefficients in the photon sector of the Standard-Model Extension (SME), obtained from X-ray polarization measurements of active galactic nuclei. It claims these limits improve on the authors' prior optical-polarization constraints by four orders of magnitude, owing to the E^2 scaling of birefringence and the longer cosmological baselines probed at X-ray energies.

Significance. If the central assumption holds, the work supplies substantially tighter, observationally grounded bounds on possible Planck-scale Lorentz violation within a well-established effective-field-theory framework. The use of existing X-ray polarimetry data sets and the explicit comparison to previous optical results constitute a clear incremental advance in astrophysical tests of fundamental symmetries.

major comments (1)
  1. [§3.2] §3.2 and Table 2: the claim that source-intrinsic polarization is negligible relies on a qualitative argument about AGN emission mechanisms; a quantitative estimate of the maximum allowable intrinsic rotation (or a reference to a dedicated study) is needed to confirm that propagation effects dominate at the reported precision.
minor comments (3)
  1. The energy range and specific AGN sources contributing to each bound should be stated explicitly in the abstract or §2 for reproducibility.
  2. [Eq. (7)] Eq. (7) introduces the birefringence phase without defining the sign convention for the Stokes parameters; a brief parenthetical clarification would prevent ambiguity when comparing to other SME analyses.
  3. [Figure 3] Figure 3 caption should note whether the plotted limits are 95 % or 99 % CL and how the combined constraint is obtained from the individual sources.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of the manuscript and the constructive comment. The positive overall assessment is appreciated. We address the single major comment below and have revised the manuscript to incorporate a more quantitative justification.

read point-by-point responses

  1. Referee: [§3.2] §3.2 and Table 2: the claim that source-intrinsic polarization is negligible relies on a qualitative argument about AGN emission mechanisms; a quantitative estimate of the maximum allowable intrinsic rotation (or a reference to a dedicated study) is needed to confirm that propagation effects dominate at the reported precision.

    Authors: We agree that the original discussion in §3.2 was largely qualitative and that a quantitative estimate or explicit reference would strengthen the case. In the revised manuscript we have added a paragraph in §3.2 that provides a rough upper bound on source-intrinsic rotation, derived from typical X-ray polarization fractions and emission models for AGN (synchrotron and inverse-Compton processes in jets). We cite dedicated studies on intrinsic polarization in blazars and Seyfert galaxies that indicate rotation angles well below 0.1° for the relevant sources and energies. This bound is negligible compared with the accumulated propagation-induced rotation over cosmological baselines at X-ray energies. We have also updated the text surrounding Table 2 to reference this estimate explicitly. revision: yes

Circularity Check

0 steps flagged

No significant circularity; observational limits from external data

full rationale

The paper reports new upper limits on SME photon-sector coefficients from X-ray polarization measurements of AGN. These limits are obtained by applying the established SME birefringence formalism to published polarization data; the four-order improvement is explicitly attributed to the higher photon energies and cosmological baselines of the X-ray observations relative to prior optical work by the same author. No derivation step redefines a fitted parameter as a prediction, imports a uniqueness theorem from self-citation, or reduces the final bound to an input by algebraic identity. The cited prior optical result serves only as a benchmark for improvement and is not required to justify the present analysis.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Review based solely on abstract; no explicit free parameters, ad-hoc axioms, or new entities are described.

axioms (1)
  • domain assumption The Standard-Model Extension provides the correct effective field theory parametrization for possible Lorentz violation at low energies.
    The paper frames all results inside the SME without deriving or justifying the framework from first principles.

pith-pipeline@v0.9.0 · 5433 in / 1120 out tokens · 28882 ms · 2026-05-10T13:12:11.789453+00:00 · methodology

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Reference graph

Works this paper leans on

22 extracted references · 22 canonical work pages

  1. [1]

    Myers and M

    R.C. Myers and M. Pospelov, Phys. Rev. Lett.90, 211601 (2003); T.G. Rizzo, J. High Energy Phys.2005, 036 (2005); V.A. Kosteleck´ y and S. Samuel, Phys. Rev. D39, 683 (1989); C.P. Burgesset al., J. High Energy Phys.2002, 043 (2002); R. Gambini and J. Pullin, Phys. Rev. D59, 124021 (1999); M. Pospelov and Y. Shang, Phys. Rev. D85, 105001 (2012); M. Li, Y.F....

    work page 2003

  2. [2]

    Vasileiouet al., Phys

    V. Vasileiouet al., Phys. Rev. D87, 122001 (2013); F. Kislat and H. Krawczynski, Phys. Rev. D92, 045016 (2015); J.J. Wei and X.F. Wu, Front. Phys.16, 44300 (2021)

    work page 2013

  3. [3]

    Kaaret, Nature427, 287 (2004)

    P. Kaaret, Nature427, 287 (2004)

    work page 2004

  4. [4]

    Kosteleck´ y and M

    V.A. Kosteleck´ y and M. Mewes, Phys. Rev. Lett.110, 201601 (2013)

    work page 2013

  5. [5]

    Kosteleck´ y and M

    V.A. Kosteleck´ y and M. Mewes, Phys. Rev. D80, 015020 (2009)

    work page 2009

  6. [6]

    Kislat and H

    F. Kislat and H. Krawczynski, Phys. Rev. D95, 083013 (2017); F. Kislat, Symmetry10, 596 (2018); R. Gerasimov, P. Bhoj, and F. Kislat, Symmetry 13, 880 (2021)

    work page 2017

  7. [7]

    Colladay and V.A

    D. Colladay and V.A. Kosteleck´ y, Phys. Rev. D55, 6760 (1997); D. Colladay and V.A. Kosteleck´ y, Phys. Rev. D58, 116002 (1998); V.A. Kosteleck´ y and M. Mewes, Phys. Rev. D66, 056005 (2002); V.A. Kosteleck´ y, Phys. Rev. D 69, 105009 (2004)

    work page 1997

  8. [8]

    Friedmanet al., Phys

    A.S. Friedmanet al., Phys. Rev. D102, 043008 (2020)

    work page 2020

  9. [9]

    Weisskopfet al., J

    M. Weisskopfet al., J. Astron. Telescopes Instrum. Syst.8, 026002 (2022)

    work page 2022

  10. [10]

    Di Gesuet al., Astrophys

    L. Di Gesuet al., Astrophys. J. Lett.938, L7 (2022)

    work page 2022

  11. [11]

    Liodakiset al., Nature611, 677 (2022)

    I. Liodakiset al., Nature611, 677 (2022)

    work page 2022

  12. [12]

    Ursiniet al., Mon

    F. Ursiniet al., Mon. Not. Royal Astron. Soc.519, 50 (2023)

    work page 2023

  13. [13]

    Marinet al., Astron

    F. Marinet al., Astron. Astrophys.689, A238 (2024)

    work page 2024

  14. [14]

    Gianolliet al., Mon

    V.E. Gianolliet al., Mon. Not. Royal Astron. Soc.523, 4468 (2023); V.E. Gianolliet al., Astron. Astrophys.691, A29 (2024)

    work page 2023

  15. [15]

    Banerjeeet al., arXiv:2504.12410

    A. Banerjeeet al., arXiv:2504.12410

    work page arXiv

  16. [16]

    Kouchet al., Astron

    P.M. Kouchet al., Astron. Astrophys.689, A119 (2024)

    work page 2024

  17. [17]

    Ingramet al., Mon

    A. Ingramet al., Mon. Not. Royal Astron. Soc.525, 5437 (2023)

    work page 2023

  18. [18]

    Errandoet al., Astrophys

    M. Errandoet al., Astrophys. J.963, 5 (2024)

    work page 2024

  19. [19]

    Ehlertet al., Astrophys

    S.R. Ehlertet al., Astrophys. J.959, 61 (2023)

    work page 2023

  20. [20]

    Middeiet al., Astrophys

    R. Middeiet al., Astrophys. J. Lett.942, L10 (2023)

    work page 2023

  21. [21]

    Koleet al., Astron

    M. Koleet al., Astron. Astrophys.644, A124 (2020)

    work page 2020

  22. [22]

    Tomaet al., Phys

    K. Tomaet al., Phys. Rev. Lett.109, 241104 (2012); P. Laurentet al., Phys. Rev. D83, 121301(R) (2011); F.W. Stecker, Astropart. Phys.35, 95 (2011)

    work page 2012