What’s MOND really mean? Here’s the Wikipedia entry https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
His arguments are very convincing and relatively clear. I am not an astrophysicist but I have two degrees in physics and have always found the dark matter theory to be lacking -- in absence of any evidence of causation whatsoever, dark matter can only be described trivially as "where we would put matter if we could to make our theory of gravity make sense," which is totally backwards from a basic scientific perspective.
Predictions based on modern MOND postulates are shown to be more and more accurate as our observational instruments continue to improve in sensitivity.
Dark matter - so far - isn't.
I think a better analogy would be "that approach is exactly how we explain failing to find planets like Vulcan; we hypothesize that they are made of as-yet-unknown stuff that you can't see, touch, hear, smell, or in fact detect at all. But we know they're there because our calculations say they are."
Have we found a way to verify the presence of dark matter yet? Or is it still an untestable hypothesis sprinkled around distant galaxies so their acceleration curves look right?
This is not right, because if we have a situation where our theories and observations don't cohere, it's not given whether the theory requires modification or we're missing something in our observations (or both). A classical illustration is the orbit of Uranus being observed in the nineteenth century to be contrary to the predictions of Newtonian theory. Calculations were made assuming the truth of the Newtonian theory and that we were missing something in our observations - the position of Neptune was predicted and it was subsequently discovered.
On the other hand, the orbit of Mercury diverged from the prediction of Newton's theory. Again, a previously unobserved planet closer to the sun was postulated as being responsible, but in this case it really did require a modification to the theory of gravity: general relativity, which accurately predicted the 43 arcseconds per century of perihelion precession by which Mercury's orbit diverges from Newtonian predicitions.
GR has obviously made many other predictions, such as the gravitational bending of light, black holes, and gravitational waves, which have been vindicated.
So there's obviously a problem of the theory and observations not cohering, but whether the solution is a modification of the theory or a new form of matter is not clear in advance, and the latter is not unreasonable and certainly it's not unscientific to make as a hypothesis, to see where it leads.
The difficulty is in coming up with a theoretical framework that retains all the successful predictions of GR while also accounting for the galactic rotation curves.
Another comment linked to https://tritonstation.com/new-blog-page/, which is an excellent read. It makes the case that GR has never been tested at low accelerations, that is might be wrong. But we know for a fact MOND is wrong at high accelerations. Unless your theory can cover both, I don't see how it can be pitched as an improvement to GR.
Edit: this sounds a bit hostile. to be clear, I think modified gravity is absolutely worth researching. but it isn't a silver bullet
Dark matter behaves in a fundamentally different way from baryonic matter. We can constrain the total amount of matter in the universe (both dark and baryonic) from the observed abundances of baryogenesis. But dark matter has a different effect on the relative amplitudes of peaks in the CMB.
As far as I can tell, MOND has never really had any success outside of modeling galaxy rotation curves.
The skepticism I've seen towards dark matter vs. MOND has always been strange to me. Dark matter doesn't really require much in the way of new physics --- there's just a new particle to add to the standard model. But most MOND theories violate Lorentz invariance which is a vastly more radical departure from standard physics. (And in my mind, the more sophisticated MOND theories that maintain Lorentz invariance like TeVeS are really a theory of dark matter dressed up in the language of MOND.)
There are relativistic versions of MOND, for example, TeVeS [1], but they all still have some problems.
[1] https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%...
Once we've arrived at this point, we can compare the two theoretical re-workings on their own terms: one is that we're glossing over some important detail of how gravitational relations in spacetime work, and the other is that we're failing to observe some new class of matter. I mean, right? There's no way this conundrum will be solved by "whoops turns out there was more plain ol' dust than we thought" at this point, right?
In those terms, I feel parsimony clearly favors one possibility over the other. Every hypothesis is worth exploring (I mean, QM and GR are dumb as hell, yet nonetheless turned out to be correct), but when funding is on the line it's also not out of line to favor one explanation explicitly. That's already happening anyway, just in the other direction.
But also I'm just some kid who's awed and grateful to be living in times of such profound mystery and discovery. Could be totally off base -- I barely passed physics I!
By contrast something like baryon acoustic oscillations are very simple to model, so you can be quite confident that you've incorporated all the relevant processes. And in that regime LCDM performs beautifully and MOND completely fails. So it's reasonable to suspect that in more complicated environments the problem is that we're not modeling the systems correctly rather than that there's new physics going on.
And yeah, that seems like pretty terrible cheating. It's one thing to hang a big theory on a single conjecture, but you still need to be trying to prove the conjecture.
What we have learned so far is that our theories and models are only correct up to our ability to precisely observe and measure.
In that sense, Newtonian physics is still very much correct under a very wide set of circumstances, and as such amazingly useful.
GR improves on that (adds precision) on what would be extreme cases for NP, but it is likely as correct as Newtonian laws are: up to a point.
All this to say that "correct" is not the right term to use: many of the theories are simultaneously "correct" with sufficient constraints and a particular error range. What matters more is if they are useful in predicting behaviour, and that's where I like using "correct" instead (as above).
https://ieeexplore.ieee.org/document/8193356
And, of course, it predicted that the early universe would have bigger and more structured galaxies (which is what the posted article is about).
Dark matter has a slew of problems of its own; it's not the case that LCDM is problem free, despite good success in some areas.
In other words, reasonable minds do disagree. AFAIU as an amateur.
[citation needed]
The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places.
Emphasized part added because people in the replies thought that I literally think that General Relativity is somehow wrong. Don't be dense. All I'm saying is that gravity at galactic scales works as Newton described it. General Relativity has extremely tiny effect at those scales.
What MOND has going for it is that galactic rotation curves are readily consumed by popsci readers and the story of the "little guy" vs the scientific establishment is an easily available frame story popsci authors can sell clicks for.
The proportion of lay people who think MOND could be true greatly outnumbers the proportion of MOND researchers and doesn't reflect the veracity of the theory.
It annoys me but I suppose every theory has to do that now, "the mouse trap must go to market now" and all.
Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.
Also Newton's laws famously could not account for Mercury's orbit. Mercury is just an ordinary planet orbiting an ordinary star. Nothing extreme is involved. He knew his laws were incomplete. But they were so dead-on in basically every other scenario that could be physically observed at the time that he figured there was some small tweak missing (or maybe another planetary body that hadn't been spotted yet).
But ok, let me put it this way: outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe.
If we are asking whether MOND is useful, then the answer is probably yes. You might use it for simulations of galaxy formation where Newtonian gravity is considered a reasonable approximation today. But MOND is not a correct model of the universe. There is no place in the universe that Newtonian gravity applies, only places where the error is an acceptable trade-off for simpler calculation.
For something more technical, this article just came out as an overview of the evidence for dark matter: https://arxiv.org/abs/2411.05062
All I'm saying is that the effect of general relativity at galactic scales is so minuscule, that galactic dynamics is - for all intents and purposes - governed by the Newtonian limit of gravity.
If you propose that gravity doesn't behave like the Newtonian limit at those scales, then you're contradicting general relativity as well, since the far-field limit of the Schwartzschild metric is literally Newton's inverse square law.
In layman terms, modified Newtonian gravity, that the article talks about, is an attempt to explain why galaxies don't rotate the way they should according to Newton (and Einstein, because at those distances the two are the same!!!).
But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.
It's just a tweak to Newtonian gravity, which surprisingly matches observation very well, and has accurately predicted quite a few things in the regime it operates in, before they were observed.
The fact it works so well in the areas it does apply to is the reason that science hasn't given up on it yet (regardless of what pop science or lay people think).
Every theory of dark matter is based exclusively on light-emitting objects. There is no "contrast" between JWST's methods and those of others. Casting aspersions on JWST because it can only see light is like casting aspersions on Galileo because he could only build telescopes. If we could teleport to the things we study and get more information that way, it would be nice, but we live in reality and must bend to its rules.
> highly sensitive to the galaxy formation model that's adopted
I should only need to remind the reader of the classic idiom "cart before the horse" to remind them that this line of reasoning is invalid.
The article is about modified Newtonian dynamics (MOND), which is a theory that modifies Newtonian gravitation to fix some observed differences in galaxies' motion, without invoking dark matter. The original commenter then proclaims "haha, MOND cannot be right, because we know that Newtonian gravity is incorrect". Yeah, no shit Sherlock; it is "incorrect" because it is just a limiting case of general relativity. But that's completely besides the whole point of MOND, which tries to "fix" gravity at galactic scales, which is a Newtonian regime even with general relativity. MOND is trying to tweak the Newtonian formula at those extreme distances, and if it works, then maybe it can be worked out to be a limiting case of a "modified general relativity", just as Newtonian gravity is a limiting case of GR. Got it?
Maybe if you're being very broad in definitions then some class of proposals describable as "dark matter" might be unfalsifiable, but to be taken seriously as a scientific proposal I think it should be specific, concrete, and indeed testable, and there are a few of these within the "dark matter" class.
Again, we're in the perhaps unsatisfying position of having observations which don't cohere with our current theoretical understanding. What's the solution? It's not easy...
To be fair, that is absolutely not the way ΛCDM would have been described to someone in the pre-Webb days. It was a well-regarded theory and the hope was (a-la the Higgs detection) that new data would just better constrain the edges and get us on to the next phase of the problem.
But instead it's a wreck, and we didn't see what we were expecting at all, and so now we're retreating to "Well, ΛCDM wasn't exactly proven wrong, was it?!"
That doesn't mean it's wrong either, and it for sure doesn't mean MOND is right. But equally for sure this is a Kuhnian paradigm shift moment and I think it's important for the community to be willing to step back and entertain broader ideas.
For precession of perihelion of Mercury we mostly noticed because any error is cumulative over time and we could integrate over an arbitrarily wide timebase. The relativistic effects are <10^-8 of the total, around 1/10th of the change imparted by Newtonian gravity of planets much, much further away. The BepiColombo orbiter should allow us to correct for the relativistic effects of other planets' pull on Mercury, but it's expected to be a change of <10^-12.
So I guess "many, many decimal places" is in the ballpark of 6-12.
Fwiw, we know for a fact also that for edge cases GR is wrong because it doesn't agree with quantum mechanics (unless QM is wrong), so it's maybe not right to take GR as gospel, especially for a theory that only seems to also change GR in edge cases, and the only reason why "it doesn't agree" might amount to "the math is hard and the physicists haven't put enough work in yet"
To wit, accepting a mond-ified GR is probably not going to change how GPS works so the claim that "GR has withstood the test of time and engineering" is not a totally solid refutation of MOND
It seems at the moment that the minimal and most elegant adjustment to the worldview required is to postulate the new form of matter. But I think it's safe to say it's a genuine problem in our knowledge: we don't know how to solve it
There's zero mention of MOND being a rejection of general relativity.
OF COURSE, any tweaking of Newton's formula at galactic scales will necessarily invalidate general relativity, since general relativity predicts Newton's formula at those scales! But MOND tries to work backwards: they propose a modification of the far-field Newtonian formula, and the belief is that it can eventually be worked out to be a limiting case of a "modified general relativity", for lack of a better name. Just how Newtonian gravity was eventually worked out to be a limiting case of a theory called general relativity.
If someone emerges with a proof that the two systems are irreconcilable then yeah you have an argument that it's "shitting on GR"
It absolutely does not. Newtonian gravity occurs instantly. It has no notion of information taking time to propagate. But we know gravitational waves happen, so Newtonian gravity is wrong _at even very large scales_. If the sun disappeared Newton tells us we'd know immediately. In GR we'd know about 8 min later.
The bigger problem is not that the quantitative effect is large, but that the _qualitative_ difference of going from the instantaneous effect to one that needs to propagate is enormous. It's the whole point of relativity as a concept.
Even going to GEM as a true, non-singular linear approximation of GR would be a step up from Newton's laws, at least there we can have gravitational waves and causal flow of information.
There is plenty of evidence that either dark matter or an alternative is needed and CDM is just the most popular take.
MOND is just some wild idea, but a little thought should convince every physicist of its uselessness. It has major issues both in explaining experimental data and in its theoretical consistency. It justifiably receives next to no attention from the vast majority of (astro)physicists.
In popular science the idea however does not seem to want to die, perhaps because it is so easily explained to a layperson. Of course this is a little frustrating for the community, but perhaps we should look at the upsides: more attention for science is probably a good thing, and explaining to people why MOND is so useless provides a good opportunity to discuss some proper physics.
Mass of Sun: Ms = 1.99e30 kg
Distance to Mercury from Sun: Rm = 5.83e10 m
Mass of Milky Way galaxy: Mg = 6e42 kg
Q: At what distance R from the Milky Way would something have to be to experience the same gravitational field strength from the Milky Way that Mercury feels from the Sun?
A: We want R such that Ms/Rm^2 = Mg/R^2 or R = Rm sqrt(Mg/Ms) = 1.0e17 m.
Let's convert that to lightyears. There are 9.46e15 m/ly. The final result is 10.75 ly. Note that everyplace that close to the center of mass of the Milky Way is inside the galaxy. Anything actually outside the galaxy would be at least 5000 ly away and feel a gravity field at most 1/200000th as strong as what Mercury feels.
For Earth use the same calculation from above but replace Mg with the mass of the Earth, 5.97e24 kg. That gives that the distance from Earth where something would feel the same field strength from Earth that Mercury feels from the Sun is 1.0e9 m. That's a little over 4x the radius of the orbits of GPS satellites, so GPS satellites are feeling a little under 16x the field strength from Earth that Mercury feels from the Sun.
I don't think it doesn't agree. It's just that we never managed to neither formulate quantum mechanics on 4 dimensional space time nor quantize gravitational force. So we simply have no idea what happens in small scale in significant gravitational fieldd.
Anyway, a bit clueless about this, just curious what gravitons are supposed to mean for either theory (MOND, LCDM, etc.).
The fact that MOND fits a lot of the data troubled cosmologists, because they know that a General Relativistic theory is needed to explain pretty much the rest of gravity.
TeVeS is an extension to General Relativity that reduces to MOND in the non-relativistic limit. For comparison, General Relativity reduces to Newtonian gravity in the non-relativistic limit. The non-relativistic limit is when speeds and spacetime curvature are small.
What "edge cases" are you talking about? I agree that GR is not a quantum theory, but it's not established that that has to be a problem, nor is it a matter of "edge cases".
https://physicsworld.com/a/cosmic-combat-delving-into-the-ba...
Similarly there are papers that tries to explain the effects attributed to dark matter on the scale of tenths and hundreds megaparsecs using just proper accounting of GR effects. They are rather speculative, but still they show that even on very huge distances Newtonian approximation may not be valid.
The word "mass" is used in physics in three different general contexts. First, we have mass in mass-energy, as in "how much energy can I get for trading in this mass?" Mass-energy is the coin paid as the price of existence. If it exists, it has mass-energy. Mostly mass for us. Mostly. We can skip that one for now.
The second context of mass is inertial. Mass has the property of inertia, of resisting a change in its direction or speed. It resists stopping if it is motion, and if it is stopped, it resists moving. The degree of the resistance is also called mass. Put a pin in this one.
The third context of mass is gravitational. Two masses, attracting one another because a force between them, a force which is not based on charge or the relatively nearby exchange of some more exotic bosons, no, just attraction based on how much mass is present. Nothing more special.
Now, curiously, values of each one of these seem to agree!
Einstein's absolute core concept in general relativity, the idea from which all else is built, is that inertial mass is identical to gravitational mass, not merely in number, but so fundamentally intertwined that there is no real difference between them, other than being two faces of the same coin. Now, that does not sound like much, but it gives birth to experiments such as an elevator which is falling toward versus an elevator floating far from gravitational sources, and that they are, from the inside of the elevator, impossible to differentiate.
Einstein then constructs general relativity from this, that the "m" in "F = ma" is identical to the first m in "F = -G m1 * m2 / r^2"
In MOND, the two ms are not identical, they only appear close most places, and so you cannot construct general relativity atop it. You will get most correct approximations but you're missing out in some cases.
https://www.youtube.com/watch?v=n33aurhg788
Is this typical behaviour for physicsts? Extremely strong opinions expressed in an abrasive way, out of proportion to the available evidence?
Sometimes you gotta be wrong before you get it right.
I mean, Newtonian mechanics are "wrong" but served us well at some scales for a while, and that it observationally failed in others led us to relativity. Even "relativity" took iterative steps, from Poincaré's Lorentz invariant theory (or even earlier with Galilean relativity) all the way to GR via special/restricted relativity, the latter name having been retconned because it's only valid in restricted special cases and fails to unify generally. And we know GR fails to unify with quantum mechanics, so one of them (or both) gotta give.
So even if something as MOND were "wrong" and known to be wrong (definitely so), there's still value in experimenting with it to get a better understanding of things. That's just how things work.
https://www.preposterousuniverse.com/blog/2011/02/26/dark-ma...
I find this treatment more compelling.
It is the entirely general point that all we can observe is the light, and we have to infer what that means. Maybe things are bright because there's a lot of stars. Maybe there aren't but there is not much dust. Maybe there aren't so many stars but they are bigger and brighter. There is room to fit many different models on the basis of the light that is observed.
Do you have some references handy for this? Or are you talking about the work of Deur?
I meant it in the sense that "most of the cosmos runs on Newtonian gravity, therefore we can ignore GR" is similar to "most of the visible matter in the cosmos is hydrogen/helium, so we can ignore chemistry".
The interesting part is in the 0.0000001% that isn't like the others.
You refer to a non-scientific article and to a youtube video, but any vaccine sceptic can probably easily find exactly the same kind of material to support their view. That would almost certainly include a video by a "professional doctor".
You might call me abrasive, but I am really just trying to be as clear as possible: this is the consensus in the field.
And before you continue this discussion it might be worth pondering the following questions. How do you think doctors should convince vaccine skeptics that vaccines work? And how big a percentage of their weekend do you think they should spend engaging on the details with anti-vaxxers? (And, in this forum, how many downvotes from obvious non-experts should they be willing to accept?)
In other words, what could I do to convince you in a reasonable amount of time?
I disagree: some experiments are just not worth our time. I wrote about such a situation three years ago:
https://news.ycombinator.com/item?id=26656206
My view is that it applies here as well.
At the very least the term Modified Gravity or MOG should be used instead of MOND to avoid a lot of pointless back and forth about MOND.
No, many of LCDM's successes were not predictions but post-hoc adjustments, where MOND had many successful predictions, even though we had no expectation for it to work:
From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity, https://arxiv.org/abs/2110.06936
Yours is an opinion shared by particle physicists because they focus on particles, but astronomers are more neutral on MOND. It almost always just works (it's an "effective theory"), even though we don't know why.
Yes... we can claim that the gravitational effects are what let us 'observe' it, but this is like the former view of geocentrism and then using various orbital corrections to make things work. That is to say, one can choose almost any axiom and then fit predictive models to work around it, but it doesn't mean that the axiom itself is more accurate, and indeed we should always be looking to vet our axioms anyway.
I think this is the root of the problem, because most 'vaccine skeptics' don't actually claim that vaccines don't work. I say this as someone who is not skeptical of vaccines at all. But when I read doctors defending vaccines it comes across as so out of touch with what the 'skeptics' are concerned about.
> In other words, what could I do to convince you in a reasonable amount of time?
For me at least, you don't need to convince me. It's clear that there are a lot of issues with all current formulations of gravitation. It's a pick your poison deal. You say MOND is wrong due to overwhelming evidence. I say the dark matter theories are wrong due to overwhelming lack of evidence that the stuff that is purported to exist even exists. Both wrong... It's hardly a bad thing to be labeled wrong when no one is right.
In general, if you're not right, then I don't see the point in dissing on those you consider wrong
It’s not really about one model being correct. GR is not a perfect model because its predictions don’t match what’s observed on the scale where QM gives predictions which do.
"Wrong" is overall a poor way of thinking about models. People would like a model which is both general and elegant, not simply a model which is "right". A large and very general model with a lot of parameters which can be well tuned to fit all the observations we have would be "correct" but I am not convinced it would be very useful.
you know what N in MOND stands for, right?
huh?!? there are GR corrections to Newton which include terms like 1/r^3 iirc
I see the same simplification in the most advanced writings. Namely 1) matter out to a radius can be treated as a point mass in the center and 2) we can ignore gravity from mass outside a radius because it all cancels.
These simplifications work for spherical shells or solids of uniform density. They do not apply to disks or rings (galaxies). Period.
More precisely, GR allows spacetime solutions which are geodesically incomplete.
> It's definitely wrong in those regimes.
No, that's too strong a claim. Most physicists believe that the solutions that are geodesically incomplete will turn out not to be valid in the regimes close enough to the endpoints of the incomplete geodesics. But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically.
Dark matter generally is less a theory and more a question: Where is all this mass? Does it really exist? What can explain it? What is missing from or wrong with our understanding of physics that explains our observations?
If you want to complain about a specific theory of dark matter like lambda-CDM or challenge our understanding of gravity or whatever, it'd be more correct to name the actual theory.
It feels like I read claims that dark matter is both a "given fact" and a "placeholder abstraction" -- from the same person, or at least perspective. They just choose to shift betweeen the two based on what serves their upper hand in a discussion better.
Or reading someone mention that "there is no fundamental difference between mass and energy" while simultaneously defending an entirely gravity-based cosmology that depends on the mass of particles... as if simple energy could not also contribute the same impact of said particles.
I think in general there is a feeling that any theory or speculation which is not dubbed into the dignity of mainstream, accepted dogma needs to be kicked out of the discourse. The fact that we are ultimately inferring in all cases is left out of the discussion and that seems to flatten all "outside" perspectives into a single umbrella of pseudoscience, despite this label accurately fitting -- under the above conditions -- onto heliocentrism, germ theory, chromosonal genetics, plate tectonics, the physical existence of Troy, and just about every paradigm that has resulted in scientific progress in the past.
I'm absolutely not claiming that all things currently labelled psuedoscientific are built the same. I only mean to highlight that science highlights "this is all inference" when it suits it best but otherwise -- in my experience -- discourages such frank reference to its own fallibility when confronted with alternative inferences.
The References in your "speculative paper" include at least five citations of the same author's previous work, at least one of which didn't even find its way into SCIRP's OALJ, and does not cite the Ludwig paper.
The full text also has such writing and editing gems, in the published version, as "the disk, the bugle and the halo of dark matter" immediately before eqn 23.
The paper's central argument is not obviously worth untangling, because the decomposition into the g and k fields (eqn 10) isn't Lorentz-invariant which raises questions about higher speed observables like cosmic rays, lensed background, "kicked" post-merger BHs, and even stars flung out of globular star clusters. There is no general transform avaiable in his equations of motion between two subsystems (e.g., outer stars and inner stars) related by a Lorentz boost. As far as I can tell the notational approach (and even the expression "gravitic field" to stand for the the gravitomagnetic field B_g) is unique to the author. It's so atypical (for quite ordinary equations) that I'd be surprised if there was any sort of reviewer or editor at all.
The author <https://people.epfl.ch/stephane.lecorre/> is a computer engineer in the university's architecture department, and claims a master's degree in theoretical physics <https://www.researchgate.net/profile/Stephane-Le-Corre-2>. I admire his continuing interest in and even investigations of "Astrophsics" (sic), but would not point to him as a persuasive expert as you have.
The Ludwig paper (EPJC 2021) is by comparison cited by 60, only a couple of which are self-cites. Whatever take one might have on Springer's approaches to open access journals, EPJC has an IF of almost 5.
Ludwig is an electrical engineer and plasma physicist. With the many cites on his set of related papers, it's clear he was not ignored by virtue of not being an astrophysicist or relativist. So we can't blame Le Corre's background for the lack of published engagement with his no-dark-matter-needed papers.
I don't think that Ludwig's gravitomagnetic vortex model is particuarly interesting in galaxy rotation curves because the fall-off off of the Lorentz force pulling outer margins of the galaxy inwards must have some arbitrary per-galaxy cutoff that also suppresses wild lensing effects at the cutoff point; we're interested mainly in doppler corrections on the HII spectrum rather than luminous stars (we don't necessarily need DM to explain flat rotations for the outer stars - we do need DM for rotating HI gas well beyond those outer stars) so the cutoff point is beyond the optical limb (meaning we should see wild lensing even in HST/WFC3); the gravitomagnetic effects must be smaller than the gravitoelectric effects (and capturing that somewhat in (v/c)^2 terms corrections to Newtonian/Keplerian orbits (v ~ 0.001 c in Andromeda-like galaxies) should be on the order of 10^-6 whereas in this approach we'd need corrections on the order of 10^-5 and higher for lower-mass lower-v dwarfs); and because the formulation does not work well with elliptical and irregular galaxies (both of which can have low circumferential rotational support - blobs of gas move radially in and out) without treating them differently from discoids (and when you do that in this approach you get divergences at galactic cores); and even for discoids there must be a minimum rotational support. More prosaically, the problem with the model is to avoid having to stabilize satellite dwarfs around a galaxy: you have to make the attractive Lorentz force not pull them right into the parent's middle and you have to avoid having satellites tear the crap out of the outer orbits of the parent galaxy's HI gas.
The paper's central idea certainly does not succeed as a general theory for flat rotation curves of HI dust as opposed to stars in circular orbits in a thin-disc plane.
However Ludwig's wasn't an obviously misguided idea, the paper's arguments are pretty clear, he's done follow-on work that is interesting, and the academic dialogue it produced is well deserved. But to say that anyone could use this paper to point to which mathematical object in GR (or which physical aspect of GR) stabilizes the relevant HI and dwarf orbits is, I wager, a huuuuge stretch.
Finally, quoting you:
> For example, properly accounting for GR effects is enough to explain the observed rotational curve for our Galaxy without the need for any dark matter hypothesis
This is not at all borne out by your choice of papers. Ludwig's text doesn't even mention the Milky Way.
Anyways, to claim that failing equivalence principle is disqualifying is begging the question since support of the equivalence principle depends on the observations... And already we observe the rotation curves are "messed up". If that means EP is violated, so be it?
You wouldn't argue against a symmetry violation like CP because "it makes the cute rule fail"
Every physical theory with singularities has has broken down in that regime. It's not even clear what it would mean for reality to permit singularities. That's a bit more than just a belief.
Can you give some examples? Note that GR has not even been tested anywhere close to the regime you are talking about.
> It's not even clear what it would mean for reality to permit singularities.
GR doesn't "permit singularities" in the sense I think you are using that phrase. "Singularity" in GR actually does not mean what I suspect you think it means, that things like spacetime curvature "become infinite". Notice that in my previous post I was careful to use the term "geodesic incompleteness", since that's what "singularity" actually means in the GR literature. And even in particular cases where there are invariants that increase without bound along incomplete geodesics, the limit points, such as r = 0 in Schwarzschild spacetime, are not actually part of the spacetime in GR. All invariants are finite at every point in the actual spacetime.
See Baez's paper, Struggles with the Continuum, https://arxiv.org/abs/1609.01421
The UV catastrophe is probably the most well known.
Most physicists believe that our best current theories, GR and quantum field theory, are approximations anyway ("effective theories" is the term often used in the literature), so that in itself is not a new idea. Baez's paper points at one fairly common hypothesis for why they are approximations and what the underlying theory they are approximations to might look like. I don't have an issue with that as a hypothesis; it's just something we aren't going to be able to test by experiment any time soon, since the most likely scale for where the approximation will break down, the Planck scale, is some twenty orders of magnitude away from the scales we can currently probe with experiments.
I don't think MOND dosen't agree with GR. It's just that we never managed to formulate MOND in a spacetime compatible with GR. So we simply have no idea what happens on the galactic scale in a significant gravitational field.
Proposing detectors for particles that no one is even sure can exist is like setting up traps for Bigfoot...