REVIEW 4 major objections 5 minor 1 cited by
Renormalization in Minkowski space-time
T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Renormalization group flows computed directly in Minkowski space-time become complex, acquire new crossover scales, and cannot be recovered by Wick rotation from Euclidean flows.
desk verdict Worth a serious look for real-time FRG, but the headline claim that quasi-particle poles make Wick rotation singular is not established—it rests on a scheme-dependent pole in a sharp-cutoff beta function. 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 central object is the functional renormalization-group evolution equation (25), $\partial_\tau S = -(ik/2)\mathrm{Tr}\ln(\delta^2 S/\delta\varphi\delta\varphi)$, derived by blocking from cutoff $k$ to $k-\Delta k$ with the saddle point of the blocked field set to zero. The paper projects this equation onto a second-order gradient-expansion ansatz for the blocked action, uses a sharp non-relativistic cutoff $P_k = \{(p_0, p) : |p_0| < \Omega_k,\ |p| < k\}$, and extracts running parameters by evaluating the blocked action at a subtraction point built from a homogeneous field plus an on-shell plane wave, with $\omega_s$ chosen just above the quasi-particle mass. The mechanism that carries the argument is the mass-shell singularity: each loop integral acquires an imaginary part when an internal line can go on shell, making the $\beta$-function coefficients complex and generating new crossover scales.
What would settle it
Compute the same functional renormalization group flow in a less restrictive truncation, or keep the non-trivial saddle points in the blocking step: if a trajectory then extends to $k \to \infty$ without flipping the sign of $\mathrm{Im}\,g_2$ or $\mathrm{Im}\,g_4$, or if the two crossover scales in $\mathrm{Im}\,g_4$ disappear, the paper's central conclusions would be artifacts of the approximation. A cleaner kinematic check is to place the subtraction point just below versus just above the two-particle threshold and verify that the $\beta$ functions stay real below it and become complex only above it, as the mass-shell mechanism predicts.
Extended reading notes
Core claim
The paper's central discovery is that renormalization-group trajectories in Minkowski space-time are intrinsically complex and are not analytically continuable from Euclidean ones. In the multiplicative scheme, $\beta$ functions pick up imaginary parts as soon as loop-internal propagators can go on shell: the coefficient of $g^2$ in the $\beta$ function of $g$ develops an imaginary part for $\omega_s > 2m$, and the mass and wavefunction renormalization become complex for $\omega_s > 3m$. In the functional scheme, integrating the one-loop exact evolution equation in a second-order gradient expansion gives complex running couplings $g_2$, $g_4$, and $z_t$, with a fast change of $\mathrm{Im}\,g_4$ between two characteristic scales that the authors associate with the decreasing lifetime of two-particle resonances. No trajectory was found that extends to $k \to \infty$ without the imaginary part of $g_2$ or $g_4$ flipping sign, which would make the path integral diverge. The authors conclude that the quasi-particle poles make the Wick rotation singular, so real-time physics is inaccessible from Euclidean space-time by analytic continuation.
Load-bearing premise
The derivation of the functional flow equation assumes that in each blocking step the dominant field configuration is the trivial zero configuration; if a non-zero configuration is important, the numerically found crossover scales and the absence of infinite-cutoff trajectories could be artifacts of that assumption.
Editorial extensions
If this is right
- Euclidean functional renormalization group studies cannot be continued to real-time quantities by a simple Wick rotation once the subtraction point sits in the quasi-particle domain.
- Minkowski-space running parameters must be treated as complex, doubling the number of real couplings and allowing non-monotonic trajectories and extra crossover scales absent in Euclidean theories.
- The rapid change of $\mathrm{Im}\,g_4$ between two characteristic scales points to a regime of fast-decreasing two-particle resonance lifetime, a genuinely real-time feature.
- Within the studied truncation, no flow reaches $k \to \infty$ without flipping the sign of $\mathrm{Im}\,g_2$ or $\mathrm{Im}\,g_4$, so the theory may require a finite cutoff or a more complete real-time construction such as the Closed Time Path formalism.
- The non-relativistic cutoff needed for practical calculations breaks boost invariance explicitly, making the boost-symmetry problem an unavoidable part of non-perturbative Minkowski-space renormalization group schemes.
Reading between the lines
- Editorial inference: the same mass-shell mechanism should make complex flows appear in other theories with long-lived particles, such as Yukawa or gauge theories, with similar extra crossover scales.
- Editorial inference: a testable extension is to extract spectral functions from the complex Minkowski propagators; the imaginary parts of running masses and couplings should appear as resonance widths, and the two crossover scales should be visible as two-particle thresholds.
- Editorial inference: analytic continuation around, rather than through, the quasi-particle poles may recover some Euclidean predictions, but cannot reproduce the full real-time flow; this could be checked by constructing such contours explicitly.
- Editorial inference: the absence of infinite-cutoff trajectories may be a truncation artifact; retaining non-local saddle-point contributions could either restore such trajectories or strengthen the cutoff dependence, and the distinction is testable numerically.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper develops renormalization group schemes for a four-dimensional scalar φ^4 theory directly in Minkowski space-time. Using multiplicative renormalization with renormalized and bare perturbation expansions, it shows that the running parameters become complex when the subtraction point lies in the quasi-particle (on-shell) regime, with imaginary parts arising from mass-shell contributions. It then constructs a functional RG flow using a sharp non-relativistic momentum-space cutoff and a gradient expansion truncated to second order, and presents numerical trajectories. The authors claim that the quasi-particle poles make the Wick rotation singular, that the choice of subtraction point is more important in Minkowski than in Euclidean space, and that the complexified flow generates new crossover scales. They also report that within their truncation no trajectory extends to arbitrarily high cutoff without a sign flip in g2 or g4.
Significance. If substantiated, the central claim would be significant: it would show that Minkowski-space RG flows are not simply Wick-rotated Euclidean flows and that real-time methods are necessary for on-shell physics. The paper's treatment of the complex running parameters is supported by explicit two-loop integrals and the optical theorem, and the authors are commendably explicit about the limitations of their truncations. However, the strongest version of the claim—the inaccessibility by Wick rotation—is not proven, and several load-bearing statements rest on unshown equations or unvalidated extrapolations. The paper is a useful exploratory contribution, but it requires substantial revision before it can be accepted.
major comments (4)
- [Section III.C and Eq. (32)] The claim, repeated in the abstract and in Section V, that the quasi-particle poles make the Wick rotation singular and make the results inaccessible from Euclidean space-time by Wick rotation is not established by the evidence presented. The finite imaginary parts in the two-loop beta functions and the singular term 1/[2(ω_s^2 − ω_k^2)] in Eq. (32) are computed with the sharp non-relativistic cutoff P_k and with a subtraction point on the quasi-particle shell. Threshold branch cuts in Minkowski Green functions are, in the standard analytic structure, boundary values of Euclidean functions, and the iε prescription is designed to navigate around them. The paper demonstrates that this particular sharp-cutoff beta function has a pole when the subtraction energy equals the cutoff-shell energy, but it does not show that a Euclidean RG flow cannot be continued around the threshold. Because beta functions and threshold singularities are scheme-dependent, the 'inaccessible by Wick rotation' conclusion requires either a smooth/covariant regulator or a direct analytic-continuation comparison; neither is provided.
- [Section IV.B and IV.C] The numerical flows in Figs. 4 and 5 are the main functional-RG results, but the full beta functions from which they are obtained are not given; after Eq. (31) the authors state they are 'too lengthy to record here.' This makes the results irreproducible and prevents a full assessment of truncation errors. Moreover, Eq. (25) is derived under the assumption φ0 = 0, and the paper itself notes in Section IV.A(a) that non-trivial saddle points generate non-local contributions omitted by the gradient expansion. The conclusion in Section IV.C that no trajectory extends to k → ∞ without flipping the sign of g2,i or g4,i could therefore be an artifact of the vanishing-saddle-point assumption. Please provide the explicit projected flow equations, or at least an independent check of the numerical integration, and a discussion of the sensitivity to non-trivial saddle points.
- [Section III.B and III.D] The statement that the theory is asymptotically free because complex beta-function coefficients turn the Landau pole into a Landau peak is not supported. The one-loop beta functions (12)-(13) are derived in the renormalized perturbation expansion, whose validity is explicitly limited to g_B(Λ0) Λ^2 ≪ m_B^2(Λ0) in the text following Eq. (15). Equation (14) is then extrapolated to arbitrarily large Λ to infer the absence of a Landau pole and the UV decay of |g_B|. This extrapolation requires control over the scheme dependence of the complex beta-function coefficients and over higher-order terms, neither of which is provided. The claim should either be derived within a controlled approximation (e.g., a resummation or a proper functional-RG analysis) or removed.
- [Section IV.A(g) and V] The sharp energy-momentum cutoff P_k = { (p0,p) : |p0| < Ω_k, |p| < k } is not boost invariant, and the paper's remedy is to project the blocked action back to the relativistic form (26) after each step. The paper acknowledges that this is an open problem. Since the numerical flows in Section IV.C are obtained with this cutoff, the claimed crossover scales and the sign-flip barriers could be artifacts of the non-relativistic regulator. Please provide evidence of insensitivity to the shape of the cutoff and to the choice of Ω_k, or at least quantify the symmetry-breaking effects.
minor comments (5)
- [Abstract and Introduction] There are several typos, including 'renormalization gro up' in the abstract and 'phyicis' in the Introduction; please proofread the manuscript carefully.
- [Eq. (31)] The expression 'Θ( k − |p| < k )' appears garbled; the theta function should presumably enforce the momentum-shell condition, but the current notation is not well-defined.
- [Section IV.C] The phrase 'The importance of gt, 2,r < 0' appears to contain a typo; the symbol should likely be zt,2,r, based on the surrounding discussion of the wave-function renormalization constant.
- [Figure 1 and its caption] The caption describes dashed lines along the imaginary and real axes, but the figure itself is not displayed clearly in the text; please ensure the figure actually shows the claimed behavior and that the axes are labeled unambiguously.
- [References] Reference [44] is incomplete as printed ('JHEP 05, 021 (2012)'), and reference [48] lacks the year; please complete all bibliographic entries.
Circularity Check
No significant circularity: the Minkowski RG results are computed from explicit loop integrals and initial conditions, with only non-load-bearing self-citations.
full rationale
I find no circular step that makes a claimed prediction equivalent to an input by construction. The running parameters are computed from the explicitly displayed loop integrals (8)-(9) and the functional evolution equation (25), with stated initial conditions; no parameter is fitted to external data and then relabeled as a prediction. The complex character of m^2 and g in Minkowski space follows from the i factors and the on-shell intermediate-state cuts in the standard loop integrals, not from assuming the desired conclusion. The self-citations ([48], [49], and [52], [53]) concern the choice of a non-relativistic sharp cutoff and spinodal saddle-point background; they are technical scheme choices and do not carry the central claim that the quasi-particle subtraction point produces complex flows. The potentially singular denominator in Eq. (32) is tied to the subtraction-point projector (33), with c_s=3 chosen away from the pole; this is openly discussed scheme dependence, not a hidden fit. Whether the Wick-rotation statement is ultimately correct is a physics-correctness question, not a circularity.
Assumptions & free parameters
free parameters (3)
- cs =
3
- Omega_k / omega_s ratio =
10^4
- initial Im g2 =
-0.001
assumptions (5)
- domain assumption Optical theorem and unitarity make on-shell intermediate states produce complex vertex functions.
- domain assumption The one-loop evolution equation (25) with saddle point phi0 = 0 is exact for an infinitesimal blocking step.
- domain assumption The gradient expansion truncated to O(delta^2) with a polynomial potential of degree 4 adequately represents the blocked action.
- ad hoc to paper The path integral converges when Im g4 < 0, used as the only stability restriction.
- standard math Standard perturbative loop integrals and renormalization conditions in Minkowski space with sharp momentum cutoff and epsilon prescription are valid.
Cite this review
Pith. "Pith review of Renormalization in Minkowski space-time." pith.science (2026). https://pith.science/paper/ZNWE4Q3D
@misc{pith2026190811311,
author = {Pith},
title = {Pith review of: Renormalization in Minkowski space-time},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZNWE4Q3D}},
note = {Machine review of arXiv:1908.11311}
}
read the original abstract
The multiplicative and the functional renormalization group methods are applied for the four dimensional scalar theory in Minkowski space-time. It is argued that the appropriate choice of the subtraction point is more important in Minkowski than in Euclidean space-time. The parameters of the cutoff theory, defined by a subtraction point in the quasi-particle domain, are complex due to the mass-shell contributions and the renormalization group flow becomes much more involved than its Euclidean counterpart.
Figures
Forward citations
Cited by 1 Pith paper
-
Wilsonian renormalisation group and thermal field theory in the Schwinger-Keldysh closed-time-path formalism
A one-loop Schwinger-Keldysh Wilsonian RG calculation generates a dissipative cross-coupling between time branches and predicts two reduced-space fixed points in d=4, related to the Gaussian and Wilson-Fisher fixed points.
Reference graph
Works this paper leans on
-
[1]
× 10-5 τ -Im(g˜ 4) -3 -2 -1 0 1 2
-
[2]
4: The trajectories of the local potential
× 10-5 τ -Im(g˜ 4) (a) (b) FIG. 4: The trajectories of the local potential. function renormalization constant remains unchanged, Zs = 1, when it is field independent in the initial condition. The thin line corresponds to the local potentia l approximation (LPA) where the running action is truncated to Zt = Zs = 1. The integration of the full evolution equa...
-
[3]
N. N. Bogoliubov, D. V. Shirkov, Introduction to the Theory of Quantum Fields , Inerscience, New York (1958)
work page 1958
-
[4]
J. D. Bjorken, S. D. Drell, Relativistic Quantum Fields , McGraw-Hill, New York (1965)
work page 1965
-
[5]
C. Itzykson, J. B. Zuber, Quantum Field Theory , McGraw-Hill, New York (1980)
work page 1980
- [6]
-
[7]
N. N. Bogoliubov, O. Parasiuk, Acta Math. 97, 227 (1957)
work page 1957
- [8]
Show all 55 references
-
[9]
Zimmermann, Comm
W. Zimmermann, Comm. Math. Phys. 15, 208 (1969)
1969
-
[10]
’tHooft, M
G. ’tHooft, M. Veltman, Nucl. Phys. B44, 189 (1972)
1972
-
[11]
K. G. Wilson, J. Kogut, Phys. Rep. 12, 75 (1974)
1974
-
[12]
D. J. Amit, Field Theory, the Renormalization group, and Critical Phenom ena, World Scien- tific, Singapore (1978)
1978
-
[13]
F. J. Wegner, A. Houghton, Phys. Rev. A8, 401 (1973)
1973
-
[14]
J. F. Nicoll, T. S. Chang, Phys. Lett. A62, 287 (1977)
1977
-
[15]
Polchinski, Nucl
J. Polchinski, Nucl. Phys. B231, 269 (1984)
1984
-
[16]
Wetterich, Nucl
C. Wetterich, Nucl. Phys. B352, 529 (1991)
1991
-
[17]
Ellwanger, Phys
U. Ellwanger, Phys. Lett. B335, 364 (1994)
1994
-
[18]
T. R. Morris, Phys. Lett. B329, 241 (1994)
1994
-
[19]
Berges, N
J. Berges, N. Tetradis, C. Wetterich, Phys. Rep. 363, 223 (2002). 24
2002
-
[20]
Lombardo, F
F. Lombardo, F. D. Mazzitelli, Phys. Rev. D53, 2001 (1996)
1996
-
[21]
D. A. R. Dalvit, F. D. Mazzitelli, Phys. Rev. D54, 6338 (1996)
1996
-
[22]
Anastopoulos, Phys
C. Anastopoulos, Phys. Rev. D56, 1009 (1997)
1997
-
[23]
Gezzi, Th
R. Gezzi, Th. Pruschke, V. Meden, Phys. Rev. B75, 045324 (2007)
2007
-
[24]
Mitra, S
A. Mitra, S. Pakei, Y. B. Kim, A. J. Millis, Phys. Rev. Let t. 97, 236808 (2006)
2006
-
[25]
S. G. Jacobs, V. Meden, H. Schoeller, Phys. Rev. Lett. 99, 150603 (2007)
2007
-
[26]
Zanella, E
J. Zanella, E. Calzetta, Renormalization group study of damping in nonequilibrium fi eld theory, arXiv:hep-th/0611222
-
[27]
Zanella, E
J. Zanella, E. Calzetta, J. Phys. A40, 7037 (2007)
2007
-
[28]
E. A. Calzetta, B. L. Hu, F. D. Mazzitelli, Phys. Rep. 352, 459 (2001)
2001
-
[29]
Canet, H
L. Canet, H. Chat´ e, J. Phys. A40, 1937 (2007)
2007
-
[30]
Mesterh´ azy, J
D. Mesterh´ azy, J. H. Stckemer, L. F. Palhares, J. Berge s, Phys. Rev. B88, 174301 (2013)
2013
-
[31]
L. M. Sieberer, S. D. Huber, E. Altman, S. Diehl, Phys. Re v. Lett. 110, 195301 (2013)
2013
-
[32]
Huelsmann, S
S. Huelsmann, S. Schlichting, P. Scior, Phys. Rev. D102, 096004 (2020)
2020
-
[33]
Zanella, E
J. Zanella, E. Calzetta, Phys. Rev. E66, 036134 (2002)
2002
-
[34]
Berges, G
J. Berges, G. Hoffmeister, Nucl. Phys. B813, 383 (2009)
2009
-
[35]
Gasenzer, J
T. Gasenzer, J. Pawlowski, Phys. Lett. B670, 135 (2008); T. Gasenzer, S. Kessler, J. Pawlowski, Eur. Phys. J. C70, 423 (2010)
2008
-
[36]
K. I. Aoki, A. Horikoshi, Phys. Rev. A66, 042105 (2002)
2002
-
[37]
Kov´ acs, B
J. Kov´ acs, B. Fazekas, S. Nagy, K. Sailer, Ann. Phys. 376, 372 (2017)
2017
-
[38]
Avinash, C. Jana, R. Loganayagam, A. Rudra, Renormalization in Open Quantum Field Theory I: Scalar field theory , arXiv:1704.08335; arXiv:1906.10180
1906 arXiv
-
[39]
Manrique, S
E. Manrique, S. Rechenberger, F. Saueressig, Phys. Rev . Lett. 106, 251302 (2011)
2011
-
[40]
Strodthoff, B
N. Strodthoff, B. J. Schaefer, L. Smekal, Phys. Rev. D85, 074007 (2012); N. Strodthoff, L. Smekal, J. Wambach Phys. Lett. B718, 1044 (2013)
2012
-
[41]
Kamikado, N
K. Kamikado, N. Strodthoff, L. Smekal, J. Wambach, Phys. L ett. B718, 1044 (2013)
2013
-
[42]
R. A. Tripolt, N. Strodthoff, L. Smekal, J. Wambach, Phys. Rev. D89, 034010 (2014)
2014
-
[43]
Kamikado, N
K. Kamikado, N. Strodthoff, L. Smekal, J. Wambach, Eur. J. Phys. C74, 2806 (2014)
2014
-
[44]
Strodthoff, Phys
N. Strodthoff, Phys. Rev. D95, 076002 (2017)
2017
-
[45]
Wambach, R
J. Wambach, R. A. Tripolt, N. Strodthoff, L. Smekal, Nucl. Phys. A928, 156 (2014)
2014
-
[46]
Floerchinger, JHEP 05, 021 (2012)
S. Floerchinger, JHEP 05, 021 (2012). 25
2012
-
[47]
Pawlowski, N
J. Pawlowski, N. Strodthoff, Phys. Rev. D92, 094009 (2015)
2015
-
[48]
Georgi, H
H. Georgi, H. D. Politzer, Phys. Rev. D14, 1829 (1976)
1976
-
[49]
L. P. Kadanoff, Physics 2, 263 (1966)
1966
-
[50]
Polonyi, Int
J. Polonyi, Int. J. Mod. Phys. A34, 1950017 (2019)
2019
-
[51]
Polonyi, Ann
J. Polonyi, Ann. Phys. (NY) 252, 300 (1996)
1996
-
[52]
R. B. Israel, in Random Fields , J. Fritz, J. L. Lebowitz, D. Szasz eds. (North-Holland, Ams - terdam, 1981)
1981
-
[53]
A. C. D. van Enter, R. Fernandez, A. Sokal, J. Stat. Phys. 72, 879 (1993)
1993
-
[54]
Alexandre, V
J. Alexandre, V. Branchina, J. Polonyi, Phys. Lett. B445, 351 (1999)
1999
-
[55]
Pangon, S
V. Pangon, S. Nagy, J. Polonyi, K. Sailer, Int. J. Mod. Ph ys. A26, 1327 (2011)
2011
Reviewed August 14, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.