REVIEW 4 major objections 6 minor 1 cited by
Asymmetry of the tidal tails of open star clusters in direct N-body integrations in Milgrom-law dynamics
T0 review · 4 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read Applying Milgrom's law to discrete stars, this paper finds that low-mass open clusters develop leading tidal tails with up to twice as many stars as the trailing tail and dissolve about 25% faster than Newtonian clusters.
desk verdict A useful first MLD N-body tool with sensible sanity checks, but the headline tail asymmetry and dissolution numbers rest on an ad hoc softening that does not actually suppress Newtonisation of close pairs. 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 object is the vectorial discrete Milgrom law, Eq. (10): $$\mu\left(\frac{|\mathbf{a}_i|}{a_0}\right)\!\mathbf{a}_i = G\sum_{j\neq i} \frac{m_j\,(\mathbf{r}_j-\mathbf{r}_i)}{|\mathbf{r}_j-\mathbf{r}_i|^3}.$$ This replaces the AQUAL/QUMOND field equations, which need smooth densities and become impractical below about 5000 $M_\odot$. The integration is a standard Hermite predictor-corrector scheme extended to supply both the MONDian acceleration and its jerk, with the standard interpolation function $\mu(x)=x/\sqrt{1+x^2}$. A softening length $\varepsilon=0.001$ pc ($\approx 206$ AU) is introduced both to regularize close encounters and, physically, to suppress the Newtonisation of compact subsystems, so the clusters stay internally in the MOND regime. For the isolated two-body deep-MOND case the paper derives an explicit Lagrangian whose conserved MLD momentum and centre of mass carry the conservation-law analysis.
What would settle it
Measure the 50\,--\,200 pc leading-to-trailing member ratio, $q_{50-200}$, and the member-loss fraction for a sample of nearby open clusters near 200 $M_\odot$ with reliable astrometric membership; MLD predicts a mean $q_{50-200}$ between 1.5 and 2 and roughly 25% faster evaporation than Newtonian N-body models, so a sample that sits at $q\approx 1$ with Newtonian-loss rates would refute the central claim.
Extended reading notes
Core claim
The central claim is that in Milgrom-law dynamics a low-mass open star cluster embedded in a Galactic disk develops a persistent leading-tail excess: in the 50\,--\,200 pc annulus the leading arm holds 1.5 to 2 times as many stars as the trailing arm, and the cluster loses its members about 25% faster than in Newtonian dynamics, with a mean retained fraction of 0.26 after 1 Gyr versus 0.41 in the Newtonian models. The same qualitative asymmetry had been found in QUMOND simulations of heavier clusters, and the paper argues this convergence means the asymmetry belongs to MOND generally, not to a particular equation. On the dynamical side, MLD does not conserve the Newtonian linear momentum, angular momentum, or Hamiltonian; for an isolated binary in the deep-MOND limit an alternative MLD centre of mass moves uniformly while the Newtonian centre of mass wobbles around it, and the MLD equations follow from a Lagrangian with a logarithmic potential and square-root masses.
Load-bearing premise
The load-bearing assumption is that Milgrom's law, applied star by star as a vector relation with a fixed 0.001 pc softening, is a valid approximation to real MOND for low-mass open clusters; if a full MOND field theory would not reduce to this vector law for point masses, the simulated tail asymmetry and the 25% faster dissolution would not describe actual clusters.
Editorial extensions
If this is right
- A 200 $M_\odot$, 400-particle open cluster on a circular 8.3 kpc orbit retains 26% of its stars after 1 Gyr in MLD, versus 41% in Newtonian gravity.
- The $q$-parameter $N_{\rm lead}/N_{\rm trail}$ in the 50\,--\,200 pc annulus settles between 1.5 and 2 in MLD, while Newtonian models stay near 1.
- The leading-tail excess appears in both QUMOND field-theory simulations and discrete MLD simulations, which the paper reads as evidence that asymmetric tails are inherent to MOND-like dynamics.
- Newtonian conservation laws fail in MLD, so quantities like the Newtonian centre of mass and angular momentum drift or oscillate instead of staying fixed.
- Nearby open clusters, with their tails resolved by astrometry, become direct test beds for whether gravity below $a_0$ is Newtonian or MONDian.
Reading between the lines
- If MLD is a fair stand-in for MOND, the observed Hyades tail asymmetry, cited as a 6.7$\sigma$ outlier in Newtonian stochastic models, would be the expected MOND signature rather than a rare fluctuation; this is a consequence the paper points toward but does not itself simulate for the Hyades.
- The 0.001 pc softening is doing physical work, effectively assuming no internally Newtonian binaries exist in the cluster; varying $\varepsilon$ or seeding the cluster with hard binaries would be a direct numerical test of how the tail asymmetry and evaporation rate depend on that assumption.
- Because the external field effect enters through the factor $\mu(a_{\rm ext}/a_0)$, the predicted asymmetry should depend on the cluster's galactic radius and orbital speed; comparing clusters at different radii in the same survey could separate MOND's tail asymmetry from bar- or spiral-arm-induced perturbations.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper postulates a vectorial Milgrom-law dynamics (MLD) for discrete N-body systems (Eq. 10), extends the standard Hermite integration scheme to compute MONDian accelerations and jerks, and uses it to simulate the tidal-tail evolution of a 400-particle, 200 Msun open cluster on a circular orbit at 8300 pc in a flat rotation curve. The central results are that in MLD the leading tidal tail is more populated than the trailing tail (q50-200 between 1.5 and 2) and that the cluster dissolves about 25% faster than in a Newtonian control run. The paper also presents analytic/numerical tests of an isolated binary, binaries in an external field, a hierarchical triple, and an isolated Plummer sphere, and compares the deep-MOND two-body force with Milgrom's field-theoretic expression.
Significance. If correct, the paper provides the first collisional direct N-body treatment of open clusters in a MOND-type dynamics, and it makes a falsifiable prediction that low-mass open clusters in the Solar neighborhood should show leading-tail excesses and enhanced dissolution relative to Newtonian expectations. The analytic derivation of the MONDian jerk (Eqs. 26-28) and the binary/triple sanity checks are internally consistent and clearly presented. The paper is also explicit about many caveats, including the lack of a variational principle for MLD and the approximate nature of the softening procedure. However, the significance is diminished by the ad hoc status of the central equation of motion and by the softening-scale inconsistency discussed below, because the quantitative claims may describe a hybrid Newtonian/MOND hybrid system rather than pure MOND dynamics.
major comments (4)
- [Sec. 3.5 and Sec. 5.1, Eqs. (32)-(33), Fig. 13] The claim that softening with epsilon=0.001 pc suppresses the Newtonisation of compact subsystems is internally inconsistent. For a 0.5 Msun pair, Gm/r^2 = a0 at r = sqrt(Gm/a0) ≈ 0.024 pc, which is 24 times larger than epsilon. The softened force (Eq. 33) differs appreciably from the unsoftened force only for r < epsilon; in the range 0.001 < r < 0.024 pc the force is nearly Newtonian and exceeds a0. Thus the simulation contains internally Newtonian binaries, and Sec. 4.2 demonstrates that such binaries follow Newtonian rather than MONDian Galactic orbits. The paper provides no analysis of how often such binaries form in the cluster or how the tail asymmetry and dissolution rate depend on epsilon. The central quantitative result (q50-200 ≈ 1.5-2, 25% faster dissolution) could therefore be caused by these Newtonian binaries or by the recoil they induce (Sec. 5.2), rather than by the 'general MONDian dynamical concept' claimed in Sec. 6.
- [Sec. 2, Eq. (10), and Sec. 6] The equation of motion (Eq. 10) is a postulate that is not derived from AQUAL or QUMOND, and the paper correctly states that no variational principle is known. Given this, the conclusion in Sec. 6 that the tail asymmetry is 'a property of the general MONDian dynamical concept' because it appears in both QUMOND and MLD is too strong. MLD is an ad hoc prescription with an additional softening parameter, and the QUMOND simulations cited operate at different cluster masses (≳5000 Msun). At minimum, the abstract and conclusions should restrict the claim to 'MLD with the chosen softening' and state explicitly that it remains to be shown that actual MOND field theories produce the same effect for 200 Msun clusters.
- [Sec. 5.4 and Sec. 5.5, Figs. 19-21] The headline numbers are based on only five realizations per model. Fig. 20 shows large run-to-run scatter in q50-200, yet no uncertainties or significance tests are reported for the quoted range q50-200 = 1.5-2. Similarly, the remaining fractions after 1 Gyr are given as 0.26 ± 0.07 (MLD) and 0.41 ± 0.06 (Newtonian); the 1-sigma bands are separated by only about 1.5 sigma, making the claim of a ~25% faster dissolution statistically fragile. The paper should provide per-realization values, standard errors on the mean, and a test (e.g., a t-test or bootstrap) before drawing quantitative conclusions.
- [Sec. 3.1-3.2 and Sec. 5.2] The Hermite scheme is used for a non-conservative, non-Hamiltonian system, but the paper gives no numerical convergence tests (e.g., energy or momentum diagnostics, or runs with shorter time steps) to verify that the integration error is controlled over 1 Gyr. Since the MLD equations of motion do not conserve the Newtonian integrals, the usual error checks based on energy conservation are unavailable, and the paper's Fig. 11/Fig. 21 results could depend on the integration accuracy. A convergence study for at least one MLD cluster run is needed to support the quantitative dissolution and asymmetry claims.
minor comments (6)
- [Sec. 3.3] The units of a0 in the Newtonian limit are written as '10^-20 Myr/pc^2'; they should be pc/Myr^2 (or equivalent) to match the text and equations.
- [Sec. 4.1] The sentence 'the masses of the particles in the kinetic potential turn into their square roots' should read 'kinetic term' rather than 'kinetic potential'.
- [Fig. 13] Figure 13 would be much more informative if it included a vertical line at r ≈ 0.024 pc, the radius where a = a0 for a 0.5 Msun particle, because that is the scale at which the MOND/Newtonian transition occurs for the particles used in the cluster simulations.
- [Sec. 4.5] The citation 'Milgrom (2014, E.q 23)' contains a typo; it should be 'Eq. (23)'.
- [Sec. 4.4] In the phrase 'In lack of a known conserved quantity in MLD', 'lack' should be 'the absence'.
- [Sec. 5.1] The reference to 'solar neighborhood as used in related studies' should be followed by a period and likely the appropriate citation (e.g., Jerabkova et al. 2021).
Circularity Check
No circularity: the tail asymmetry and dissolution rates are direct outputs of an explicitly postulated MLD N-body dynamics, with no fitted parameter renamed as a prediction.
full rationale
The paper's central quantitative claims (q50-200 between 1.5 and 2, and retained fractions 0.26 vs 0.41 after 1 Gyr) are direct outputs of integrating the postulated MLD equations of motion, Eq. (10)/(11), with fixed parameters chosen before the runs: a0 = 3.8 pc/Myr^2, vc = 230 pc/Myr, M_tot = 200 Msun, Plummer b = 3.1 pc, and epsilon = 0.001 pc. No subset of the asymmetry or dissolution data is used to calibrate any parameter, and the external Galactic field is not fitted: Eq. (36) is the Milgrom-law condition for a circular orbit in the adopted flat rotation curve, derived from the same interpolation function used in the N-body forces. The Section 5.5 external-field-effect estimate (T_diss ratio 0.93) is a post-hoc interpretation and actually disagrees mildly with the simulated ratio 0.8, which is evidence that the simulation was not constructed to reproduce it. The self-citations (Pflamm-Altenburg et al. 2023 for the tail-membership criterion and q-parameter definition; Kroupa et al. 2022 for a QUMOND comparison) are background, definitional, or corroborative rather than load-bearing; the MLD tail asymmetry is computed from this paper's own equations, and the QUMOND comparison is an independent externally published simulation that does not force the MLD result. The softening epsilon = 0.001 pc is an acknowledged approximation, and whether it adequately suppresses Newtonisation of compact subsystems is a correctness/robustness concern, not a circularity. No circular step is present.
Assumptions & free parameters
free parameters (3)
- Softening length epsilon =
0.001 pc (~206 AU)
- Plummer parameter b =
3.1 pc
- Particle number N =
400
assumptions (5)
- ad hoc to paper Milgrom's law in vectorial form holds for discrete point-mass systems (Eq. 10).
- domain assumption The external galactic field is a fixed, analytic Newtonian potential whose circular acceleration matches the assumed flat rotation curve (Eqs. 35-38).
- ad hoc to paper Softening with epsilon=0.001 pc prevents the Newtonisation of compact subsystems, keeping the cluster in the MOND regime.
- standard math The Hermite predictor-corrector scheme remains valid for the MLD equations of motion.
- domain assumption A Plummer model with n=400 equal-mass stars represents a low-mass open cluster.
Cite this review
Pith. "Pith review of Asymmetry of the tidal tails of open star clusters in direct N-body integrations in Milgrom-law dynamics." pith.science (2026). https://pith.science/paper/JSOUDHIZ
@misc{pith2026241113675,
author = {Pith},
title = {Pith review of: Asymmetry of the tidal tails of open star clusters in direct N-body integrations in Milgrom-law dynamics},
year = {2026},
howpublished = {\url{https://pith.science/paper/JSOUDHIZ}},
note = {Machine review of arXiv:2411.13675}
}
read the original abstract
Numerical QUMOND-simulations of star clusters orbiting in a Galactic disk potential show that the leading tidal arm of open star clusters contains tendentially more members than the trailing arm. However, these type of simulations are performed by solving the field-equations of QUMOND and already become non-practical for star cluster masses at around 5000 Msun. Nearby star clusters have masses of 1000 Msun or ~1000 particles and less/fewer and can currently not be simulated reliably in field-theoretical formulations of MOND. In order to handle particle numbers below the QUMOND-limit the star cluster is simulated in Milgrom-law dynamics (MLD): Milgrom's law is postulated to be valid for discrete systems in vectorial form. In order to suppress the Newtonisation of compact subsystems in the star cluster the gravitational force is softened below particle distances of 0.001 pc ~206 AU. Thus, MLD can only be considered as an approximation of a full MOND-theoretical description of discrete systems which are internally in the MOND regime. The MLD equations of motion are integrated by the standard Hermite scheme generally applied to Newtonian N-body systems, which is extended to solve for the accelerations and jerks associated with Milgrom's law. It is found that the tidal tails of a low-mass star cluster are populated asymmetrically in the MLD-treatment, very similar to the QUMOND simulations of the higher-mass star clusters. In the MLD-simulations the leading tail hosts up to twice as many members than the trailing arm and the low-mass open star cluster dissolves approximately 25% faster than in the respective Newtonian case.
Figures
Figures from the paper (13 more)
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Reference graph
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Reviewed August 12, 2026 · model on record in the stance chip above.
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