Talk:Modified Newtonian dynamics

I removed the following part:

Spiral galaxies offer compelling evidence that this is more than an observational artifact. M51, for example, has two main arms (see fig. 1), each of which has an exterior end approximately 180° behind the interior end connected to the bulge. It thus appears that the edge completes an orbit in almost the same time as the interior. However, if Newton's universal law of gravitation holds for galaxies (as it should), stars at the edge should move much slower, and the spiral arms should be stretched around the bulge a hundred times, which would make the two arms completely indistinguishable.

(Image Removed)

The existence of spiral galaxies alone does not provide compelling evidence for the flattening of the rotation curve, since the galaxy's arms do not consist of stars; they are pressure waves rotating around the galaxy's center independently of the matter comprising the galaxy. AxelBoldt 11:20 Aug 15, 2002 (PDT)

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For comparison purpose, the same curve for the Solar system -- (properly scaled) -- is provided (curve C in fig. 2).

Am I missing something? I see the letter C but no associated curve. --Nate 21:14 Nov 27, 2002 (UTC)

I can't see curve C in Fig. 2 either. [Feb 5, 2004 Wes Hughes]

Galaxy rotation problem

Does it make sense to essentially copy all of Galaxy rotation problem into this article?

Aragorn2 15:07, 25 Sep 2003 (UTC)

I just ran into this article, the first paragraph is indeed copied (but it is allowed under Wiki GNU Free Documentation License. Anyway, the majority of the article is dedicated to the solution of the problem by Milgrom's MOND. A descripition of the problem is neccesary. MathKnight 22:21, 28 Aug 2004 (UTC)

Rotation curves are not evidence

"Since MOND was specifically designed to produce flat rotation curves, these do not constitute evidence for the theory."

I don't agree with this formulation. Any theory aims to match its predictions with already known observations. By the same principle the standard model of particle physics would be useless, because it "depends on 19 parameters, whose numerical values are established by experiment".

Only data that has been used to produce $a_{0}$ should be dismissed as evidence.

References

On James Webb Space Telescope & MOND

Here are two lay articles, perhaps somewhat sensationalized, & the scholarly article that they cite.

• Randall, Ian (2024-11-12). "NASA Webb revelations challenge traditional theory of gravity". Newsweek. Retrieved 2024-11-17.
• Plain, Christopher; Whalen, Ryan (2024-11-15). "James Webb Space Telescope Finds Stunning Evidence for Alternate Theory of Gravity". The Debrief. Retrieved 2024-11-17.
• McGaugh, Stacy S.; Schombert, James M.; Lelli, Federico; Franck, Jay (2024-11-01). "Accelerated Structure Formation: The Early Emergence of Massive Galaxies and Clusters of Galaxies". The Astrophysical Journal. 976 (1): 13. doi:10.3847/1538-4357/ad834d. ISSN 0004-637X.

Peaceray (talk) 23:21, 17 November 2024 (UTC)

The fact that MOND supporters keep publishing papers supporting MOND is totally unsuprising. It also doesn't mean that there has been a paradigm shift where MOND has now suddently become the scientific consensus or on equal footing to LambdaCDM. Lots of papers are published in cosmological physics, and unless this paper is widely considered significant in future publications I think its undue to include. Hemiauchenia (talk) 23:32, 17 November 2024 (UTC)

I've added this paper to the article. One can certainly argue it's undue, but then one would have to cleanse the relevant section. Banedon (talk) 07:33, 18 November 2024 (UTC)

MOND FAQ

After attending the MOND 40 conference in St. Andrews in 2023 I realized there are a lot of misunderstandings about MOND floating around. To clear these up I've been writing a FAQ on my webpage with the help of several authors in the field. There is a lot of material there, far more than could possibly be included in this article without creating several spin off articles. But I'm posting the link here anyone wants to read up on MOND and include material in the wiki article.

The MOND FAQ

I have also added this link to the External links section because I think the engaged reader may benefit from the additional material. Once the FAQ is finished I'll see about merging the content onto the wiki. ScienceDawns (talk) 20:38, 4 February 2025 (UTC)

I would like to see some astrophysics expertise, particularly in modified gravity, for this kind of link. Banedon (talk) 01:43, 5 February 2025 (UTC)

Concur with Banedon. We don't need FAQs from people with no expertise in the topic. I would highly suggest not merging the content from the FAQ onto Wikipedia. Wikipedia exists to summarise existing literature, which considers MOND to be a minority theory. It does not exist to provide defenses for MONDs validity against academic criticisms. Hemiauchenia (talk) 02:39, 5 February 2025 (UTC)

Sure I get that. Authority is all you have to go on to trust my FAQ summary. I can't expect you to read the entire FAQ, read the literature to then add the citations and rewrite it in encyclopedic style before it is fit for inclusion, just to make sure what I've said there is correct. I do find it curious you think it defends MOND against academic criticisms? There are plenty of fatal failures to the theory (dwarf galaxies, no relativity so no lensing, the whole thing about clusters, the ad hoc interpolation function which is not properly known at high accelerations and of course literally all of cosmology because again no relativity). To the extent the FAQ has been completed it explicitly points out such failures. Perhaps the somewhat clickbaity titles bother you, which is reasonable. And yes, it does debunk a number of misconceptions about MOND in the popular media (ok that have on rare occasions also popped up in careless literature). But I don't think that counts as "defending MOND against academic criticism". That's just science communication. You won't hear ridiculous and baseless claims out of me like that "LCDM has been ruled out with infinity sigma" (actual quote from Pavel Kroupa). I care about facts. MOND is a good theory for explaining galaxy dynamics and several related phenomena that tells us how dark matter particles or whatever makes up Ωm should behave in galactic environments. And it deserves to be properly understood because of that.

Why this article needs improvement

As for this wiki article, there are numerous errors present (edit: this is a to-do list now):

• The argument of the interpolation functions given are inverted on the right hand sides (should be a/a0 not a0/a). The arguments are given correctly on the LHS.
• The article says: "[gravity's] rate of change—including the curvature of spacetime—increases with the square root of mass (rather than linearly as per Newtonian law) and decreases linearly with distance (rather than distance squared)". This is incorrect because curvature is the second derivative of the potential. Only the first derivative of the potential (the gravitational field/acceleration) decays with these factors.
• It also says: "For example, it has been claimed that MOND offers a poor fit to the velocity dispersion profile of globular clusters and the temperature profile of galaxy clusters,[74][75]" Source 74 is an old popular science publication that's not publically available but I've managed to track it down. It says nothing about globular clusters. Neither does Shayae (the other source). GCs actually do have velocity dispersions that flatten out. See Hernandez et al. (2012).
• The article states that the discrepancy was first identified for clusters by Swiss astronomer Fritz Zwicky in 1933. While technically true as stated, the dark matter problem had been discovered already well before that (see the history section on the dark matter article).
• The article states that "MOND-compliant gravity, which explains galactic-scale observations, was not previously detected closer to Earth". This is not correct. MOND contains Newtonian gravity as a limit. All measurements on Earth that conform to Newtonian dynamics such as the laboratory tests by (Gundlach et al., 2007) satisfy MOND. All relativistic experiments falsify MOND however.
• This bit: "It is important to note that the Newtonian component of MOND's dynamics remains active at accelerations well below the a0 value of 1.2 × 10−10 m/s2; the equations of MOND assert no minimum acceleration for the Newtonian component. However, because the residual Newtonian-like dynamics of MOND continue to decline as the inverse square of distance below a0—just as they do above—they comparatively vanish as they become overwhelmed by the stronger “deep-MOND” linear dynamics of the theory." is incorrect. You can't just add the Deep-MOND and Newtonian limits together and get MOND. You need to use the interpolation function as this same wiki article explains. If you just add the limits you double the expected kinematic acceleration at a0 which is not what MOND predicts and is strongly ruled out by data from rotation curves and elliptical galaxies (see Lelli et al, 2017) and strong lensing (Tian & Ko, 2017).
• The following is just wrong: "Within the Solar System, the v 4 = GMa0 equation makes the effect of the a0 term virtually nonexistent; it is overwhelmed by the enormous—and highly Newtonian—gravitational influence of the Sun as well as the variability of Earth's surface gravity." The baryonic Tully-Fisher does not apply in the solar system. The interpolation function applies. Same mistake as the previous point.
• Milgrom did not determine a0 to be 1.2*10^-10m/s^2. Begeman et al. (1991) did that. Milgrom's own estimate was about 70% too high at 2*10^-10m/s^2. Subsequent measurements have supported the value by Begeman et al.
• The article talks about a 1-to-1 correspondence in the high acceleration regime and then states MOND and Newton diverge rapidly. That's imprecise. In the Deep-MOND regime there is still a bijective relation between the baryonic and kinematic accelerations. For every one baryonic acceleration there is only one kinematic acceleration, it's just that the two aren't equal anymore.
• The article says "The external field effect implies a fundamental break of the strong equivalance principle (but not necessarily the weak equivalence principle)". This is inaccurate. Both the AQUAL and QUMOND lagrangian *require* the WEP to be satisfied. The way it says it here implies that the WEP could be broken.

The overview section also lacks citations and contains a number of paragraphs that read like original research. The description of MOND as modified inertia or modified gravity is messy. The lede describes MOND as modified inertia even though the vast majority of the literature uses MOND as modified gravity (either QUMOND or AQUAL). In fact the article itself does tell you this much later on. Throughout the article switches several times between modified inertia, modified gravity or what Milgrom calls "pristine" MOND which is essentially agnostic on the issue. It would be better to introduce MOND in the lede as Milgrom's forumula as it applies to modified gravity, mention that a modified inertia version exists and relegate all discussion of that to its own section. That would give it the weight it is due without giving the reader the wrong impression of what MOND is about in practice.

The article also lacks up to date information. Topics that are lacking are for example:

• That Cassini rules out MOND from solar system dynamics (Hees et al., 2014)
• That long period comets rule out MOND (Vokrouhlický et al., 2024)
• That LLR rules out the simple interpolation function (Exirifard, 2013)
• The radial acceleration relation (RAR) & its interpolation function (Lelli et al., 2017)
• The weak lensing RAR (Brouwer et al., 2021 & Mistele et al. 2024)
• The strong lensing RAR (Tian & Ko, 2017)
• The work on bar rotation speeds (Roshan et al., 2021)
• The work on satelite galaxy planes (Pawlowski et al. 2013 & Müller et al. 2018)
• The QUMOND lagrangian is missing, though QUMOND is mentioned (Milgrom, 2010)
• The laboratory tests of small accelerations (Gundlach et al., 2007)

There are several more minor wording issues I'd want to tweak but I think I've made my point.

References:

Begeman, K. G., Broeils, A. H., & Sanders, R. H. (1991). Extended rotation curves of spiral galaxies: dark haloes and modified dynamics. Monthly Notices of the Royal Astronomical Society, 249, 523–537. https://doi.org/10.1093/mnras/249.3.523

Brouwer, M. M., Oman, K. A., Valentijn, E. A., Bilicki, M., Heymans, C., Hoekstra, H., Napolitano, N. R., Roy, N., Tortora, C., Wright, A. H., Asgari, M., van den Busch, J. L., Dvornik, A., Erben, T., Giblin, B., Graham, A. W., Hildebrandt, H., Hopkins, A. M., Kannawadi, A., … Visser, M. (2021). The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000. Astronomy and Astrophysics, 650. https://doi.org/10.1051/0004-6361/202040108

Exirifard, Q. (2013). Lunar system constraints on the modified theories of gravity. International Journal of Modern Physics D, 22(9). https://doi.org/10.1142/S0218271813500648

Gundlach, J. H., Schlamminger, S., Spitzer, C. D., Choi, K. Y., Woodahl, B. A., Coy, J. J., & Fischbach, E. (2007). Laboratory test of Newton’s second law for small accelerations. Physical Review Letters, 98(15). https://doi.org/10.1103/PhysRevLett.98.150801

Hees, A., Folkner, W. M., Jacobson, R. A., & Park, R. S. (2014). Constraints on modified Newtonian dynamics theories from radio tracking data of the Cassini spacecraft. Physical Review D - Particles, Fields, Gravitation and Cosmology, 89(10). https://doi.org/10.1103/PhysRevD.89.102002

Hernandez, X., Jiménez, M. A., & Allen, C. (2013). Flattened velocity dispersion profiles in globular clusters: Newtonian tides or modified gravity? Monthly Notices of the Royal Astronomical Society, 428(4), 3196–3205. https://doi.org/10.1093/mnras/sts263

Lelli, F., McGaugh, S. S., Schombert, J. M., & Pawlowski, M. S. (2017). One Law to Rule Them All: The Radial Acceleration Relation of Galaxies. The Astrophysical Journal, 836(2), 152. https://doi.org/10.3847/1538-4357/836/2/152

Milgrom, M. (2010). Quasi-linear formulation of MOND. Monthly Notices of the Royal Astronomical Society, 403(2), 886–895. https://doi.org/10.1111/j.1365-2966.2009.16184.x

Mistele, T., McGaugh, S., Lelli, F., Schombert, J., & Li, P. (2024). Radial acceleration relation of galaxies with joint kinematic and weak-lensing data. Journal of Cosmology and Astroparticle Physics, 2024(4). https://doi.org/10.1088/1475-7516/2024/04/020

Müller, O., Pawlowski, M. S., Jerjen, H., & Lelli, F. (2018). A whirling plane of satellite galaxies around Centaurus A challenges cold dark matter cosmology. Science, 359(6375), 534–537. https://doi.org/10.1126/science.aao1858

Pawlowski, M. S., Kroupa, P., & Jerjen, H. (2013). Dwarf galaxy planes: The discovery of symmetric structures in the local group. Monthly Notices of the Royal Astronomical Society, 435(3), 1928–1957. https://doi.org/10.1093/mnras/stt1384

Roshan, M., Ghafourian, N., Kashfi, T., Banik, I., Haslbauer, M., Cuomo, V., Famaey, B., & Kroupa, P. (2021). Fast galaxy bars continue to challenge standard cosmology. Monthly Notices of the Royal Astronomical Society, 508(1), 926–939. https://doi.org/10.1093/mnras/stab2553

Tian, Y., & Ko, C. M. (2017). Mass discrepancy-acceleration relation in Einstein rings. Monthly Notices of the Royal Astronomical Society, 472(1), 765–771. https://doi.org/10.1093/MNRAS/STX2056

Vokrouhlický, D., Nesvorný, D., & Tremaine, S. (2024). Testing MOND on Small Bodies in the Remote Solar System. The Astrophysical Journal, 968(1), 47. https://doi.org/10.3847/1538-4357/ad40a3

ScienceDawns (talk) 10:08, 22 February 2025 (UTC)

I think you should directly make the changes to the article, instead of linking a FAQ. Banedon (talk) 11:19, 22 February 2025 (UTC)

Will do :) ScienceDawns (talk) 02:17, 24 February 2025 (UTC)

Section with original research

The article currently contains these paragraphs that lack citations and seem to be original research. Calling the original contributor User:Greg L who wrote most of this, do you have sources for this?

User:Banedon could I bother you for a yay or nay about removing/drastically trimming this? In my opinion the article would be better suited by a diagram showing that the accelerations in the solar system are much higher than a0. That's available in the literature so citations wouldn't be an issue. Thoughts in general about the changes I've made so far would also be appreciated!

Applications

Deep-MOND gravitational effects, which explain galactic-scale observations, have not been detected on Earth or in the solar system because the local gravitational field is many orders of magnitude larger than a₀. Even at the edge of the Solar System, where the Sun's gravity drops below a₀ the total gravitational field is still above a₀ due to the gravitational fields of the rest of the galaxy. On Earth's surface—and in national laboratories when performing ultra-precise gravimetry—the a₀ value is equal to 0.012 microgal (μGal), which is only twelve-trillionths the strength of Earth's gravity. A change in the laws of gravity below this acceleration is far too small to be resolved with even the most sensitive free-fall-style absolute gravimeters available to national labs, like the FG5-X, which is accurate to just ±2 μGal. When considering why MOND's effects aren't detectable with precision gravimetry on Earth, it is important to remember that a₀ doesn't represent a spurious force; it is the gravitational strength at which MOND is theorized to significantly begin departing from the Newtonian dynamic. Moreover, the a₀ strength is equivalent to the change in Earth's gravity brought about by an elevation difference of 0.04 mm—the width of a fine human hair. Such subtle gravitational details, besides being unresolvable with current gravimeters, are overwhelmed by twice-daily distortions in Earth's shape due to lunar gravitational tides, which can cause local elevation changes nearly 10,000 times greater than 0.04 mm. Such disturbances in local gravity due to tidal distortions are even detectable as variations in the rate of a Shortt double-pendulum clock, which was a national timekeeping standard in the late 1920s.

To give a sense of scale to a₀, a free-floating mass in space that was exposed for one hour to 1.2 × 10⁻¹⁰ m/s² would "fall" by just 0.8 millimeter—roughly the thickness of a credit card. An interplanetary spacecraft on a free-flying inertial path well above the Solar System's ecliptic plane (where it is isolated from the gravitational influence of individual planets) would, when at the same distance from the Sun as Neptune, experience a classic Newtonian gravitational strength that is 55,000 times stronger than a₀. For small Solar System asteroids, gravitational effects in the realm of a₀ are comparable in magnitude to the Yarkovsky effect, which subtly perturbs their orbits over long periods due to momentum transfer from the non-symmetric emission of thermal photons. The Sun's contribution to interstellar galactic gravity doesn't decline to the a₀ threshold at which MOND's effects predominate until objects are 41 light-days from the Sun; this is 53 times further away from the Sun than Voyager 2 was in November 2022, which has been in the interstellar medium since 2012.

Despite its vanishingly small and undetectable effects on bodies that are on Earth, within the Solar System, and even in proximity to the Solar System and other planetary systems, MOND successfully explains observed stellar and gas rotation velocities without invoking the existence of as-yet undetected dark matter particles lying outside of the highly successful Standard Model of particle physics.

ScienceDawns (talk) 13:24, 28 February 2025 (UTC)

@GregL: Fixing the ping. I don't have time to look at this right now, possibly later. Banedon (talk) 14:22, 28 February 2025 (UTC)

@Greg L: Sorry, really fixing the ping now. Banedon (talk) 14:25, 28 February 2025 (UTC)

Ok, I think the section is includable in different form - under a section heading "MOND on solar system scales" or something like that. It would need sources though, since it makes claims that are not obvious. Without sources, I would say delete the setion. Banedon (talk) 01:15, 2 March 2025 (UTC)

I've added a well referenced subsection on solar system tests in the "outstanding problems" section that should serve to replace this material. After a fair bit of searching I can't find any sources to back up the claims in the section I've quoted above. So I've gone and deleted the material from the article. We can leave it here in case sources do become available and we can restore some of it then. ScienceDawns (talk) 05:31, 2 March 2025 (UTC)