REVIEW 3 major objections 6 minor 2 cited by
Simple Perturbatively Traversable Wormholes from Bulk Fermions
T0 review · 3 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Bulk spinors alone can render a wormhole traversable at any mass, with the time advance diverging toward the extremal limit.
desk verdict A careful and mostly convincing fermion extension of the scalar wormhole computation, with a real sign-change result, but the zero-mode normalization used for the extremal extrapolation has a typo that needs fixing. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing device is the method of images for fermions on a $\mathbb{Z}_2$ quotient, with the isometry $J_3$ extended to spinors by a vielbein-preserving lift whose spinorial action is $\tilde j = i\gamma^1\gamma^2$. This lift makes the spinor field on the quotient $\psi(x) = (\tilde\psi(x) + [\hat J_3\tilde\psi](x))/\sqrt{2}$, so the horizon stress-energy reduces to cross-correlators between $x$ and $J_3 x$. The spinor propagator in AdS$_3$ takes the maximally symmetric form $S(x,x') = [\alpha(s) + \not n\,\beta(s)]\Lambda(x,x')$, and combining it with the Belinfante stress-energy tensor and the back-reaction Green's function $H(\varphi-\varphi')$ of Eq. (2.20), whose zero mode diverges as $r_+ \to r_-$, converts the computed null energy into the time shift $\langle\Delta V\rangle$. The central identity is Eq. (3.13)/(B.32): $\langle T_{kk}\rangle = -4(k^\mu n_\mu)(k^\mu \phi_\mu)\big(\frac{d}{ds} - \frac{1}{2\ell}\coth\frac{s}{2\ell}\big)\beta(s)$. This identity is what fixes the sign and magnitude of the null stress-energy on the horizon.
What would settle it
Compute the second-order or full back-reaction for $r_+$ close to $r_-$: if the negative time shift predicted by the divergent zero mode is corrected or cancelled so that $\langle\Delta V\rangle$ no longer stays negative and grows as $1/T$, the claim of eternal traversability in the extremal limit fails. Alternatively, a numerical evaluation of the integrated null energy for periodic Weyl fermions on KKZBO at any mass and rotation that yields a positive average would directly contradict the paper's central result.
Extended reading notes
Core claim
The central claim is that on the KKZBO spacetime, the orientable $\mathbb{Z}_2$ quotient of rotating BTZ $\times S^1$, a bulk Weyl fermion of any mass with periodic boundary conditions under the extended isometry $J_3$ yields a negative integrated null-null stress-energy on the horizon, so first-order back-reaction gives $\langle \Delta V\rangle < 0$ and the wormhole becomes traversable. The computation is exact: the fermion propagator in AdS$_3$ is written as $S(x,x') = [\alpha(s) + \not n\,\beta(s)]\Lambda(x,x')$, and the method of images with the spinor extension $\tilde j = i\gamma^1\gamma^2$ gives Eq. (3.13)/(B.32), $\langle T_{kk}\rangle = -4(k^\mu n_\mu)(k^\mu \phi_\mu)\big(\frac{d}{ds} - \frac{1}{2\ell}\coth\frac{s}{2\ell}\big)\beta(s)$. Because the factor $k^\mu\phi_\mu$ is odd in $\varphi$ for non-rotating BTZ, the spinor $\langle T_{kk}\rangle$ is odd in $\varphi$ and its full $\varphi$-average vanishes without rotation; rotation breaks this symmetry and makes the average negative. Numerically, $T\langle\Delta V\rangle_{\rm average}$ is independent of temperature, so $\langle\Delta V\rangle$ diverges as $T \to 0$, suggesting that an eternally traversable wormhole forms in the extremal limit.
Load-bearing premise
The conclusion relies on the linearized back-reaction formula (2.20), whose Green's function is taken to describe how any horizon stress-energy source shifts the null time, even though its zero mode diverges at extremality and the formula is only derived in first-order perturbation theory.
Editorial extensions
If this is right
- A free bulk fermion, with no non-local boundary coupling, is enough to make the wormhole traversable at every mass once the correct periodicity is chosen.
- The average time advance grows as $1/T$ as the rotating black hole approaches extremality, so in that limit the linearized computation points toward an eternally traversable wormhole, subject to a non-perturbative analysis.
- Spinor back-reaction is generically smaller than scalar back-reaction with the same number of degrees of freedom, because the sign of the integrated null energy varies with $\varphi$ and partial cancellations occur.
- In the non-rotating case the full $\varphi$-average of the spinor null energy vanishes, so traversability is restricted to one half of the throat circle.
- Supersymmetric models do not cancel the effect: even with matching boson and fermion masses, the KKZBO back-reaction from spinors does not vanish, since the quotient breaks the Killing symmetry that is part of the supersymmetry algebra.
Reading between the lines
- If the direction-dependent sign is physical, the same geometry could act as a half-open throat: signals entering through one half of the circle experience advance while those entering through the other half experience delay; this could be probed by scattering experiments in a holographic dual.
- The method-of-images construction for spinors should extend to vector, gravitino, and graviton fields in AdS$_3$ using known propagators, which would let the same calculation measure whether linearized graviton back-reaction enhances or opposes the fermion effect.
- A sharp testable extension is that, at fixed non-zero rotation, the averaged time shift should depend on the fermion mass only through the effective three-dimensional Kaluza-Klein mass, so results at different masses and $S^1$ radii should collapse onto one curve when plotted against that effective mass.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript studies a massive Weyl fermion on the KKZBO spacetime, a Z2 quotient of rotating BTZ × S1, and computes the null-null component of the renormalized stress-energy tensor by a method-of-images construction that requires a careful lift of the J3 isometry to spinors. The main results are an explicit image-sum formula for ⟨Tkk⟩ (Eq. (3.13) and Appendix B) and numerical evidence that for periodic spinors the integrated null energy is negative on average for rotating backgrounds, so the first-order back-reaction shifts null geodesics by a negative ⟨ΔV⟩ and renders the wormhole traversable. The paper further suggests that T⟨ΔV⟩ remains finite and nonzero at extremality, so that the wormhole could become eternally traversable in the T → 0 limit.
Significance. The perturbative spinor computation is a genuine technical advance: it provides one of the first exact higher-spin stress-energy calculations in the Z2-wormhole setting, and it identifies a kinematic feature absent for scalars, namely a factor of the φ-odd vector φμkμ that makes the sign of the integrated null energy vary around the throat and reduces the angular average. The derivation is mostly self-contained, with explicit gamma-matrix conventions, a clear treatment of the image-sum divergences, and a transparent statement of the boundary-condition choices controlling the sign of the back-reaction. If the time-delay conversion is corrected, the paper will be a useful reference for fermionic and higher-spin effects in traversable-wormhole models.
major comments (3)
- [2.4, Eqs. (2.21)-(2.23)] The identity in Eq. (2.22), T = (r_+^2 - r_-^2)/(2π r_+ ℓ^2) = 2 r_+ H_0, is not correct with H_0 from Eq. (2.21). Substituting H_0 = ℓ^2 r_+^2/[π(r_+^2 - r_-^2)] gives 2 r_+ H_0 = 2ℓ^2 r_+^3/[π(r_+^2 - r_-^2)], whereas T = (r_+^2 - r_-^2)/(2π r_+ ℓ^2); equality would require (r_+^2 - r_-^2)^2 = 4ℓ^4 r_+^4, which already fails in the non-rotating case r_- = 0. The actual product is T H_0 = r_+/(2π^2), not 1/(2r_+), so Eq. (2.23) is not the angular average of Eq. (2.20) as stated. Since the r_+-independence of the integral in Fig. 5 is interpreted through Eq. (2.23) as evidence that ⟨ΔV⟩_avg ∼ 1/T as T → 0, the eternal-traversability suggestion in the abstract currently rests on an algebraic inconsistency. Please correct the prefactor, recalculate the plotted quantity, and re-evaluate the conclusion.
- [3.4, Fig. 5] Even after the prefactor in (2.23) is corrected, the step from the numerical constancy of the plotted double integral to the claim ⟨ΔV⟩_avg ∼ 1/T requires that the corrected T⟨ΔV⟩_avg be nonzero at T = 0. The manuscript does not quantify the range of r_+ - r_- in which the linearized Green's function relation (2.20) is reliable, and the small-r_+ behaviour in Fig. 5 shows visible finite-N drift for m = 0. Please provide a quantitative discussion of the perturbative regime and of how the cutoff and finite-N uncertainties affect the extremal extrapolation; otherwise the eternal-traversability suggestion should be presented as an uncontrolled speculation.
- [Abstract and Sec. 4] The claim that perturbative back-reaction renders the wormhole traversable 'at any m' is stronger than what the displayed analysis establishes. Equation (3.13) is analytic in the effective mass, but the sign of the angular average of ⟨Tkk⟩ is checked numerically for representative masses and fixed ℓ/RS1, and the mass dependence enters through the hypergeometric function β(s) in Appendix B whose sign is not analyzed. Either supply an analytic sign argument valid for all m, or restrict the abstract and conclusions to the mass range actually investigated.
minor comments (6)
- [2.4, Eq. (2.23)] There is a typo in the text near Eq. (2.23): 'symemtries' should read 'symmetries'.
- [3.4] The sentence 'We choose our spinors to be periodic under J3 such that we get a positive overall contribution to the stress-energy tensor' appears to conflict with the later statement that the sign must be chosen so that the average is negative for traversability. Please clarify the sign convention or correct the wording.
- [3.4, Eq. (3.21)] The description of the regularization term in Eq. (3.21) is unclear: the text says the extra term is added 'so as to sum over an even number of terms, N of which have an additional sign change', but the displayed expression involves N + 1 terms. Please make the counting precise.
- [3.2, Eq. (3.10)] The notation 'ΣA,B' is used without definition; presumably it denotes the sum over the two three-dimensional spinor representations, but this should be stated explicitly at first use.
- [3.3, Eq. (3.16)] The notation m3(p) is introduced in Eq. (3.16) but is not defined at that point; please connect it explicitly to the Kaluza-Klein effective mass in Eq. (2.12).
- [Figure 5] The axis label 'ℓ∫r+⟨Tkk⟩ψ' is ambiguous: the placement of r+ inside the integral sign makes it look like an integration variable, while it is presumably an overall factor. Please rewrite the label unambiguously.
Circularity Check
No significant circularity: the fermion stress-energy computation is self-contained; the only imported back-reaction formula is a cited prior result, not an input repackaged as a prediction.
full rationale
The paper's derivation is not circular. The central new object, the Weyl-fermion null-null stress-energy (Eqs. (3.13) and (B.32)), is obtained from the standard AdS3 spinor propagator and a method-of-images construction; no term in that calculation is fitted to the time delay or to the final traversability conclusion. The conversion from T_kk to Delta V uses Eq. (2.20), quoted from the authors' prior work [23]; although this is a self-citation, it is an externally stated, parameter-free linearized back-reaction formula, and not an equation whose output is re-imported into the fermion computation. The boundary-condition dependence is explicit ('We choose our spinors to be periodic under J3...'), so the abstract's 'appropriate boundary conditions' is a conditional choice rather than a hidden prediction supplied by the computation. The eternal-traversability suggestion is explicitly flagged as outside the perturbative regime ('this invalidates our perturbative analysis for r_+ very close to r_-'), making it a non-perturbative conjecture rather than a claimed derived result. The zero-mode normalization concern raised in review, concerning the relation between H0 and T in Eqs. (2.21)-(2.23), is a potential correctness issue affecting the prefactor of T times Delta V, but it does not make the fermion stress-energy an input to itself.
Assumptions & free parameters
assumptions (5)
- domain assumption The Hartle-Hawking state on rBTZ x S1 has vanishing null-null stress-energy on the horizon for any field, by the Killing symmetry; coincident-point terms in the image sum therefore vanish.
- domain assumption The isometry J3 can be lifted to a vielbein-preserving action J3 = i gamma1 gamma2 on spinors, and the quotient is orientable so a single Weyl fermion is consistent.
- standard math The spinor propagator in AdS3 has the maximally symmetric form S = (alpha(s) + /n beta(s)) Lambda, with Lambda trivial in adapted coordinates.
- domain assumption The linearized back-reaction of the stress-energy source is governed by Eq. (2.20) with Green's function H from [23], valid for rotating BTZ and for sources that break rotational symmetry.
- domain assumption The Kaluza-Klein sums and BTZ image sums can be regulated: non-integrable singularities cancel and results are stable under changes of cutoffs.
Cite this review
Pith. "Pith review of Simple Perturbatively Traversable Wormholes from Bulk Fermions." pith.science (2026). https://pith.science/paper/6QX47CFG
@misc{pith2026190803998,
author = {Pith},
title = {Pith review of: Simple Perturbatively Traversable Wormholes from Bulk Fermions},
year = {2026},
howpublished = {\url{https://pith.science/paper/6QX47CFG}},
note = {Machine review of arXiv:1908.03998}
}
abstract
A new class of traversable wormholes was recently constructed which relies only on local bulk dynamics rather than an explicit coupling between distinct boundaries. Here we begin with a four-dimensional Weyl fermion field of any mass $m$ propagating on a classical background defined by a ${\mathbb Z}_2$ quotient of (rotating) BTZ $\times \, S^1$. This setup allows one to compute the fermion stress-energy tensor exactly. For appropriate boundary conditions around a non-contractible curve, perturbative back-reaction at any $m$ renders the associated wormhole traversable and suggests it can become eternally traversable at the limit where the background becomes extremal. A key technical step is the proper formulation of the method of images for fermions in curved spacetime. We find the stress-energy of spinor fields to have important kinematic differences from that of scalar fields, typically causing the sign of the integrated null stress-energy (and thus in many cases the sign of the time delay/advance) to vary around the throat of the wormhole. Similar effects may arise for higher-spin fields.
Forward citations
Cited by 2 Pith papers
-
The Speed of Quantum Information Spreading in Chaotic Systems
For chaotic systems with initial entanglement fraction f, quantum information spreads at speed v_E(f)/(1-f), interpolating between the entanglement speed at f=0 and the butterfly speed at f=1.
-
Traversable Asymptotically Flat Wormholes with Short Transit Times
Quantum back-reaction from cosmic-string fluctuations can make a non-extremal charged wormhole traversable for early signals, with transit time d plus logarithmic terms, approaching the minimum allowed in higher dimensions.
Reference graph
Works this paper leans on
-
[23]
A perturbative perspective on self-supporting wormholes,
Z. Fu, B. Grado-White, and D. Marolf, “A perturbative perspective on self-supporting wormholes,” arXiv:1807.07917 [hep-th]
-
[1]
The Particle Problem in the General Theory of Relativity,
A. Einstein and N. Rosen, “The Particle Problem in the General Theory of Relativity,” Phys. Rev. 48 (Jul, 1935) 73–77
work page 1935
-
[2]
Causality and Multiply Connected Space-Time,
R. W. Fuller and J. A. Wheeler, “Causality and Multiply Connected Space-Time,” Phys. Rev. 128 (Oct, 1962) 919–929
work page 1962
-
[3]
Ether flow through a drainhole: A particle model in general relativity,
H. G. Ellis, “Ether flow through a drainhole: A particle model in general relativity,” Journal of Mathematical Physics 14 no. 1, (1973) 104–118
work page 1973
-
[4]
M. S. Morris and K. S. Thorne, “Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity,” American Journal of Physics 56 no. 5, (1988) 395–412
work page 1988
-
[5]
Wormholes, Time Machines, and the Weak Energy Condition,
M. S. Morris, K. S. Thorne, and U. Yurtsever, “Wormholes, Time Machines, and the Weak Energy Condition,” Phys. Rev. Lett. 61 (Sep, 1988) 1446–1449
work page 1988
-
[6]
Visualizing Interstellar’s Wormhole,
O. James, E. von Tunzelmann, P. Franklin, and K. S. Thorne, “Visualizing Interstellar’s Wormhole,” Am. J. Phys. 83 (2015) 486, arXiv:1502.03809 [gr-qc]
arXiv 2015
-
[7]
J. L. Friedman, K. Schleich, and D. M. Witt, “Topological censorship,” Phys. Rev. Lett. 71 (1993) 1486–1489, arXiv:gr-qc/9305017 [gr-qc]
arXiv 1993
Show all 39 references
-
[8]
Topological censorship and higher genus black holes,
G. J. Galloway, K. Schleich, D. M. Witt, and E. Woolgar, “Topological censorship and higher genus black holes,” Phys. Rev. D60 (1999) 104039, arXiv:gr-qc/9902061 [gr-qc]
1999 arXiv
-
[9]
Analytic self-gravitating Skyrmions, cosmological bounces and AdS wormholes,
E. Ayon-Beato, F. Canfora, and J. Zanelli, “Analytic self-gravitating Skyrmions, cosmological bounces and AdS wormholes,” Phys. Lett. B752 (2016) 201–205, arXiv:1509.02659 [gr-qc]
2016 arXiv
-
[10]
Topologically nontrivial configurations in the 4d Einstein-nonlinear σ-model system,
F. Canfora, N. Dimakis, and A. Paliathanasis, “Topologically nontrivial configurations in the 4d Einstein-nonlinear σ-model system,” Phys. Rev. D 96 (Jul, 2017) 025021. https://link.aps.org/doi/10.1103/PhysRevD.96.025021
2017 doi
-
[11]
Achronal averaged null energy condition,
N. Graham and K. D. Olum, “Achronal averaged null energy condition,” Phys. Rev. D76 (2007) 064001, arXiv:0705.3193 [gr-qc]
2007 arXiv
-
[12]
Proving the Achronal Averaged Null Energy Condition from the Generalized Second Law,
A. C. Wall, “Proving the Achronal Averaged Null Energy Condition from the Generalized Second Law,” Phys. Rev. D81 (2010) 024038, arXiv:0910.5751 [gr-qc]
2010 arXiv
-
[13]
The Large N Limit of Superconformal Field Theories and Supergravity,
J. M. Maldacena, “The Large N Limit of Superconformal Field Theories and Supergravity,” http://arxiv.org/abs/hep-th/9711200v3
-
[14]
Traversable Wormholes via a Double Trace Deformation,
P. Gao, D. L. Jafferis, and A. Wall, “Traversable Wormholes via a Double Trace Deformation,” JHEP 12 (2017) 151, arXiv:1608.05687 [hep-th]
2017 arXiv
-
[15]
Diving into traversable wormholes,
J. Maldacena, D. Stanford, and Z. Yang, “Diving into traversable wormholes,” Fortsch. Phys. 65 no. 5, (2017) 1700034, arXiv:1704.05333 [hep-th]
2017 arXiv
-
[16]
Solving the Schwarzian via the Conformal Bootstrap,
T. G. Mertens, G. J. Turiaci, and H. L. Verlinde, “Solving the Schwarzian via the Conformal Bootstrap,” JHEP 08 (2017) 136, arXiv:1705.08408 [hep-th]
2017 arXiv
-
[17]
Eternal traversable wormhole,
J. Maldacena and X.-L. Qi, “Eternal traversable wormhole,” arXiv:1804.00491 [hep-th]
-
[18]
Rotating traversable wormholes in AdS,
E. Caceres, A. S. Misobuchi, and M.-L. Xiao, “Rotating traversable wormholes in AdS,” http://arxiv.org/abs/1807.07239v2
-
[19]
Cool horizons for entangled black holes,
J. Maldacena and L. Susskind, “Cool horizons for entangled black holes,” Fortsch. Phys. 61 (2013) 781–811, arXiv:1306.0533 [hep-th] . – 29 –
2013 arXiv
-
[20]
Teleportation through the wormhole,
L. Susskind and Y. Zhao, “Teleportation through the wormhole,” Phys. Rev. D98 no. 4, (2018) 046016, arXiv:1707.04354 [hep-th]
2018 arXiv
- [21]
-
[22]
Traversable wormholes in four dimensions,
J. Maldacena, A. Milekhin, and F. Popov, “Traversable wormholes in four dimensions,” arXiv:1807.04726 [hep-th]
-
[24]
Traversable Asymptotically Flat Wormholes with Short Transit Times,
Z. Fu, B. Grado-White, and D. Marolf, “Traversable Asymptotically Flat Wormholes with Short Transit Times,” arXiv:1908.03273 [hep-th]
1908 arXiv
-
[25]
The Black hole in three-dimensional space-time,
M. Banados, C. Teitelboim, and J. Zanelli, “The Black hole in three-dimensional space-time,” Phys. Rev. Lett. 69 (1992) 1849–1851, arXiv:hep-th/9204099 [hep-th]
1992 arXiv
-
[26]
Geometry of the (2+1) black hole,
M. Banados, M. Henneaux, C. Teitelboim, and J. Zanelli, “Geometry of the (2+1) black hole,” Phys. Rev. D48 (1993) 1506–1525, arXiv:gr-qc/9302012 [gr-qc] . [Erratum: Phys. Rev.D88,069902(2013)]
1993 arXiv
-
[27]
Single exterior black holes and the AdS / CFT conjecture,
J. Louko and D. Marolf, “Single exterior black holes and the AdS / CFT conjecture,” Phys. Rev. D59 (1999) 066002, arXiv:hep-th/9808081 [hep-th]
1999 arXiv
-
[28]
Entropies of scalar fields on three-dimensional black holes,
I. Ichinose and Y. Satoh, “Entropies of scalar fields on three-dimensional black holes,” Nucl. Phys. B447 (1995) 340–372, arXiv:hep-th/9412144 [hep-th]
1995 arXiv
-
[29]
Large N field theories, string theory and gravity,
O. Aharony, S. S. Gubser, J. M. Maldacena, H. Ooguri, and Y. Oz, “Large N field theories, string theory and gravity,” Phys. Rept. 323 (2000) 183–386, arXiv:hep-th/9905111 [hep-th]
2000 arXiv
-
[30]
Polchinski, String theory
J. Polchinski, String theory. Vol. 2: Superstring theory and beyond . Cambridge Monographs on Mathematical Physics. Cambridge University Press, 2007
2007
-
[31]
Fermions in odd space-time dimensions: Back to basics,
M. de Jesus Anguiano Galicia and A. Bashir, “Fermions in odd space-time dimensions: Back to basics,” Few Body Syst. 37 (2005) 71–78, arXiv:hep-ph/0502089 [hep-ph]
2005 arXiv
-
[32]
Freedman and A
D. Freedman and A. Van Proeyen, Supergravity. Cambridge University Press, 2012
2012
-
[33]
Di Francesco, P
P. Di Francesco, P. Mathieu, and D. Senechal, Conformal Field Theory . Graduate Texts in Contemporary Physics. Springer-Verlag, New York, 1997
1997
-
[34]
Spinor parallel propagator and Green’s function in maximally symmetric spaces,
W. M¨ uck, “Spinor parallel propagator and Green’s function in maximally symmetric spaces,” J. Phys. A33 (2000) 3021–3026, arXiv:hep-th/9912059 [hep-th]
2000 arXiv
-
[35]
Vector two-point functions in maximally symmetric spaces,
B. Allen and T. Jacobson, “Vector two-point functions in maximally symmetric spaces,” Comm. Math. Phys. 103 no. 4, (1986) 669–692
1986
-
[36]
Information Transfer and Black Hole Evaporation via Traversable BTZ Wormholes,
S. Hirano, Y. Lei, and S. van Leuven, “Information Transfer and Black Hole Evaporation via Traversable BTZ Wormholes,” arXiv:1906.10715 [hep-th]
1906 arXiv
-
[37]
Traversable wormholes in AdS and bounds on information transfer,
B. Freivogel, D. A. Galante, D. Nikolakopoulou, and A. Rotundo, “Traversable wormholes in AdS and bounds on information transfer,” arXiv:1907.13140 [hep-th]
1907 arXiv
-
[38]
Nastase, Introduction to the ADS/CFT Correspondence
H. Nastase, Introduction to the ADS/CFT Correspondence . Cambridge University Press, 2015
2015
-
[39]
V. E. Ambru¸ s,Dirac fermions on rotating space-times . PhD thesis, University of Sheffield, 2014. – 30 –
2014
Reviewed August 14, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.