In my opinion, this article is misleading at best. "...scans of ancient galaxies gathered by the JWST seem to contradict the commonly accepted predictions of the most widely accepted Cold Dark Matter theory, Lambda-CDM." --> LCDM doesn't predict what galaxies should look like, it simply predicts how much mass is in collapsed structures and that dark matter haloes grow hierarchically. In contrast, with JWST we see light and need to infer what the underlying properties of the system are. It was shown very early on that the theoretical upper limit (i.e. taking all of the gas that is available in collapsed structures and turning it into stars) predicts a luminosity function (i.e. number of galaxies per unit luminosity) that is orders of above what JWST has observed (e.g. https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstra...). This means that there is plenty of space within the context of LCDM to have bright and seemingly large and massive galaxies early on. Based on current JWST data at these early epochs, there are really no convincing arguments for or against LCDM because it's highly sensitive to the galaxy formation model that's adopted.
ajross 4 days ago [-]
> there are really no convincing arguments for or against LCDM because it's highly sensitive to the galaxy formation model that's adopted.
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.
astroH 4 days ago [-]
Again, LCDM and galaxy formation are two different things. "...and we didn't see what we were expecting at all..." It depends on who you ask. There were many pre-JWST models that did well in this regard. A particularly interesting one is this from 2018 (https://ui.adsabs.harvard.edu/abs/2018MNRAS.474.2352C/abstra...). That group even had to write another paper reminding everyone of what they predicted (https://ui.adsabs.harvard.edu/abs/2024arXiv240602672L/abstra...). Another example is here (https://ui.adsabs.harvard.edu/abs/2023OJAp....6E..47M/abstra...) which shows results from a simulation from ~2014. I can provide numerous other examples of this. My point isn't which theory is or isn't wrong, my point is that what is presented in this particular article is not a constraint on any realistic theory of gravity as the sensitivity of these particular observations to galaxy formation modeling is so strong.
Davidzheng 4 days ago [-]
Absolutely not in the field, so if you are please completely disregard. But from conversations with physicists (not cosmologists) I always thought people thought a lot of evidence for ΛCDM was dubious at best.
liquidpele 4 days ago [-]
Dubious at best, lol, Isn’t that half of all of cosmology? Lots of pure math these days and little science it seems.
haccount 4 days ago [-]
[dead]
uoaei 4 days ago [-]
> with JWST we see light and need to infer what the underlying properties of the system are
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.
astroH 4 days ago [-]
This is a misrepresentation of what I am saying. By no means am I casting an aspersion on JWST. I am casting an aspersion on this particular observation as a test of MOND and LCDM. Also I highly disagree about your comments on my line of reasoning. The fact that you can obtain a huge range of possible galaxy properties in the context of LCDM indicates that in general, tests of LCDM and MOND that rely on galaxy formation model are in usually not strong tests. This is the key issue with using the abundance of high-z galaxies (or even their masses -- despite the fact that these aren't measured) as a test. In the context of LCDM, you need haloes to form galaxies but it has been shown many times that there are enough haloes to solve the problem (see the paper linked) by a huge amount.
uoaei 4 days ago [-]
The skepticism you display in this comment is completely absent when you reference lambda-CDM elsewhere. Consistency invites zero criticism :)
astroH 4 days ago [-]
And so you have proved my point. The observations presented in this article can be made consistent with both...as such one should think about stronger tests of both LCDM and MOND.
uoaei 4 days ago [-]
Your point was orthogonal to the point of epistemology. This isn't Reddit, we respect actual arguments here.
MattPalmer1086 4 days ago [-]
You are missing the point. JWST is not being singled out as different here, and no aspersions are being cast.
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.
ab5tract 4 days ago [-]
I like this point a lot. But then I have to ask... why do I continuously encounter arguments that claim that the fundamental science of these scales is simply settled?
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.
uoaei 3 days ago [-]
In my experience this mostly occurs when people don't recognize names like Popper and Feierabend, and consider science and humanities to be mutually exclusive. Likely borne of a neurotic desire for certainty in all things. At least, that's my best guess having been steeped in the cultures of academia and industry for so long.
samsartor 5 days ago [-]
My hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gravity based on an inverse law is simply wrong.
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
MattPalmer1086 5 days ago [-]
MOND isn't pitched as an improvement to GR. It was always a Newtonian theory - it's in its name!
There are relativistic versions of MOND, for example, TeVeS [1], but they all still have some problems.
TeVeS is definitely interesting, but it still has problems like you said. AFAICT gravitational wave observations are particularly bad for TeVeS theories. TeVeS isn't dead, but if dark matter theories are criticized for being patched up post-hoc, that standard should also apply to modified gravity.
gliptic 5 days ago [-]
The weirdest thing about TeVeS IMO is that it adds additional fields that warp spacetime, so how is it not a dark matter theory?
MathMonkeyMan 5 days ago [-]
For the fields to be considered particles, they have to be freely propagating in space. TeVeS adds a vector field, a scalar field, and some lagrange fields that are part of their coupling. The degrees of freedom aren't consistent with one or more particles.
throwawaymaths 4 days ago [-]
> My hangup with MOND is still general relativity.
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
mort96 4 days ago [-]
Well this doesn't seem like such a conundrum. We know for sure that ND is wrong because it predict things incorrectly which GR predicts correctly. We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct. Essentially, GR and ND are both wrong, but ND is more wrong than GR.
RandomThoughts3 4 days ago [-]
> We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct.
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.
codethief 4 days ago [-]
> we know some form of QM is more or less correct
This is a wild take, given all the issues QM and QFT have.
mort96 4 days ago [-]
Sure, a better way of saying it is tat we observe natural phenomena which are explained by something like QM and for which we have no other explanation. I think people got the idea.
scotty79 4 days ago [-]
> because it doesn't agree with quantum mechanics
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.
throwawaymaths 3 days ago [-]
> 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.
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.
naasking 4 days ago [-]
We absolutely know GR is wrong, at the very least because of its singularities.
scotty79 4 days ago [-]
All we can tell from singularities is that GR might have a realm of applicability ... like every other theory ever. Not that its wrong.
anon84873628 4 days ago [-]
"Has a realm of applicability" is what people mean by "wrong" here. As in, the equations we have don't fully generalize and explain all regimes. The "right" model will cover all regimes and still reduce to match what we already have.
scotty79 4 days ago [-]
Then every physical (and nature) law is wrong. Only math is right. There's not a single law that covers full regime for at least one definition of "full".
naasking 3 days ago [-]
Possibly, but we're converging on something correct as we cover more regimes in this inductive process called science. That's the best we can do.
pdonis 4 days ago [-]
> for edge cases GR is wrong because it doesn't agree with quantum mechanics
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".
naasking 4 days ago [-]
GR has singularities. It's definitely wrong in those regimes.
pdonis 4 days ago [-]
> GR has singularities.
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.
naasking 3 days ago [-]
> But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically.
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.
pdonis 3 days ago [-]
> Every physical theory with singularities has has broken down in that regime.
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.
The UV catastrophe is probably the most well known.
pdonis 3 days ago [-]
The issue in that paper isn't really limited to "singularities". The basic issue is that we use the continuum in our physical models, and "reality" might not actually be a continuum, so the continuum math we use is just an approximation. But if "reality" isn't a continuum, it isn't a continuum everywhere, not just near "singularities", so the continuum is an approximation everywhere, not just near "singularities". The approximation would just become unworkable near "singularities", while remaining workable in other regimes.
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.
naasking 13 hours ago [-]
Yes, there are many issues in our theories and even our formalisms. All I was trying to is point out, in the simplest way possible, was at least one way we know that GR is "wrong" (incomplete), which is its singularities.
pdonis 7 hours ago [-]
> we know
No, we don't know. Most physicists believe it, but that's not the same as knowing. We won't know unless and until we are able to actually do experiments in the relevant regime.
meindnoch 5 days ago [-]
>We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold
[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.
EPWN3D 5 days ago [-]
You're simply wrong. There's no other way to put it. The GPS system would have been simply impossible to deploy without the general theory of relativity. There's no extreme energy or mass involved, just precision requirements that are influenced by the minuscule differences in time experienced by the surface of the earth and orbiting satellites.
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).
ahazred8ta 4 days ago [-]
We know that spacetime is einsteinian, not euclidean, yes. But that's not what's being discussed here. The issue is whether the force of gravity deviates from the expected 1/r^2 value. Experiments, measurements and observations within the solar system have not revealed any deviation. The precession of mercury is not due to a deviation from 1/r^2; it is due to space near the sun being bent instead of flat. Ditto GPS; we have to adjust for time dilation and curved space, but not for any deviation from 1/r^2. MOND theories predict that MOND gravity is indistinguishable from normal at short ranges less than several light years; the MOND effects only show up at distances of many light years.
anon84873628 4 days ago [-]
You seem to just be arguing about the definition of "gravity" now.
mafuyu 4 days ago [-]
That’s kinda the whole point, isn’t it? I’m just a layman, but my understanding is that the incompatibilities of GR and QM point to a need for a proper theory of gravity. Looking at the dark matter problem from a purely GR-perspective will miss that context.
tzs 4 days ago [-]
Compared to the gravitational fields galaxies orbiting other galaxies deal with Mercury orbiting the Sun is extreme. So are GPS satellites orbiting Earth.
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.
meindnoch 4 days ago [-]
Easy there champ. Noone is shitting on general relativity.
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!!!).
fpoling 4 days ago [-]
We already know that one must not use Newtonian gravity on the galaxy scale. 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.
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.
magicalhippo 4 days ago [-]
> 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.
Do you have some references handy for this? Or are you talking about the work of Deur?
Do yourself a favour and check your "speculative paper" in google scholar, look at who cites this and the author's related papers, and notice that it's exclusively self-citations. Do yourself another favour and absorb: "Scientific Research Publishing (SCIRP) is a predatory[1][2][3] academic publisher of open-access electronic journals, conference proceedings, and scientific anthologies that are considered to be of questionable quality.[4][5][6]...In 2021 Cabells' Predatory Reports described SCIRP as a "well-known predatory publisher".[2] In the Norwegian Scientific Index the publisher and all of its journals have a rating of 0 (non-academic).[18]". SCIRP itself wildly claims an OALibJ impact factor of 1.18.
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 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.
4 days ago [-]
jfengel 4 days ago [-]
I had the impression that "shitting on general relativity" was exactly what MOND was about. That is, it starts from the position that Einstein is wrong, and searches for ways to support that.
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.
Iwan-Zotow 4 days ago [-]
> There's zero mention of MOND being a rejection of general relativity.
you know what N in MOND stands for, right?
throwawaymaths 4 days ago [-]
Can you explain how MOND shits on GR? My understanding is it's more like. "GR is mostly right but...". As for MOND being exclusively Newtonian, yeah. In terms of solving the math, you gotta crawl, walk, run. Let's not kid ourselves, GR invokes way harder math than algebra and simple integral calculus. TeVeS Is a first attempt at "walk", let's say, but even it might not be correct even if adjusting gravity may be correct.
If someone emerges with a proof that the two systems are irreconcilable then yeah you have an argument that it's "shitting on GR"
at_a_remove 4 days ago [-]
Hi! Physics BS, but they let me take some grad courses, including a Spacetime and Relativity class. I can help.
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.
throwawaymaths 3 days ago [-]
Why would they not be identical? You'd change either the Fg function or adjust F=ma (more common). The weak equivalence principle holds in MOND IIUC. You can't make a statement about the strong equivalence principle until the resolution of MOND with GR is well-defined, in which case the strong equivalence principle may still hold.
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"
auntienomen 5 days ago [-]
Citation needed? That's ridiculous. The empirical evidence is well over century old at this point. Start with the anomalous precession of Mercury's perihelion. That already can't be accounted for by Newtonian gravity.
bobmcnamara 4 days ago [-]
I don't think they're saying the relativistic effects don't exist, just that they're still largely unimportant compared to Newtonian effects.
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.
ahazred8ta 4 days ago [-]
Samsartor seems to think that the inverse square law does not hold at short distances (e.g. between the sun and mercury). Meindnoch agrees with mainstream physics that the inverse square law does indeed hold at short distances. You're confusing newtonian physics (busted) with the inverse square strength of gravity (still strongly supported); those are two different things. GR says gravity should be strictly 1/r^2, and this is what we observe in the solar system.
Iwan-Zotow 4 days ago [-]
"GR says gravity should be strictly 1/r^2, and this is what we observe in the solar system"
huh?!? there are GR corrections to Newton which include terms like 1/r^3 iirc
ahazred8ta 3 days ago [-]
There are (ȓ/r^3) terms involving unit vectors, but that works out to 1/r^2 in practice. There are cubed terms in string theory and Quantum General Relativity (QGR) / Loop Quantum Gravity, but these do not apply at macroscopic distances. If you know of a url link to a non-theoretical inverse-cube effect which has actually been confirmed in lab experiments or actual observation, please post it.
Iwan-Zotow 3 days ago [-]
Huh!?! Classic GR test of Mercury periphelon precession is mainly due to inverse cube correction to Newton
nimish 4 days ago [-]
>The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places.
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.
the__alchemist 4 days ago [-]
Thanks for bringing this up; this is the central reason why I'm skeptical of Newtonian models that predict dark matter, and why I don't think the term MOND makes sense as the simplest alternative.
Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.
meindnoch 4 days ago [-]
Nobody said that general relativity is "switched on" around black holes.
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.
samsartor 4 days ago [-]
Not for all intents and purposes.
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.
meindnoch 4 days ago [-]
By the same logic, there's no place in the universe that general relativity applies either, since it breaks down at the quantum level. There's no place in the universe where any theory other than the one true grand unified theory applies, because everything else is just an approximation. At which point we're just arguing about semantics, and I don't see a reason for continuing it on my part.
radishingr 4 days ago [-]
There are vastly different scales where the approximation is correct for newton vs general relativity. Perhaps you can define the scales that you are calling relevant so we understand what you mean.
meindnoch 4 days ago [-]
The scale of galaxies? Which the original article is about? I feel like I need to spell out everything, but ok:
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?
DiogenesKynikos 4 days ago [-]
The inaccuracy of the Newtonian theory of gravity is large enough that it was already noticed by astronomers in the mid-1800s.
bobmcnamara 4 days ago [-]
My first thought was that we only know Cavendish's constant to a little over 4 significant figures, so how could this be right? The relativistic effects at Earth's surface would change this by only ~10^-8, so I think the challenge in refining the Cavendish gravitic constant lie elsewhere.
exe34 4 days ago [-]
that's like saying the visible mass of the universe is 99% hydrogen and helium, so we don't need to learn about chemistry.
meindnoch 4 days ago [-]
So you're saying we should model galaxies down to the level of individual protons? Lol.
Galactic dynamics is governed by gravity, which is Newtonian at those scales.
exe34 4 days ago [-]
No I did not say that.
meindnoch 4 days ago [-]
Ok, then how does your chemistry comment have anything to do with the motion of galaxies? Reminder: you're commenting on an article about MOND, which is a theory that stems from trying to explain the motion of galaxies.
exe34 4 days ago [-]
> 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.
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.
radishingr 4 days ago [-]
So spacetime (interactions between mass, space, and time) are required for any sort of precision explanation. If "extreme" means planet size masses, I guess, but I generally consider our solar system pretty normal. However we cannot explain the planetary motion of mercury without relativity, so define your extreme.
But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.
hobs 5 days ago [-]
When you say "outside of" - that's the thing where it doesn't hold.
It's interesting and not even wrong to say "these rules work in these contexts" but as far as I can tell we're looking for the scenario invariant rules.
superjan 5 days ago [-]
These extremes exist, and GR predictions are better than Newton’s in those cases. Closest to home is mercury’s perihelion drift. We have observed black hole mergers, gravitational lensing, and GR is also an essential component in understanding the universe’s expansion(that we know from redshift and the CMB). Likely MOND will address these, but Newtonian mechanics will not get you there.
wbl 5 days ago [-]
We can see gravitational redshift on Harvard's campus thanks to gamma ray Mossbauer spectroscopy.
phkahler 4 days ago [-]
IANA physicist but everywhere I look I see the same mistake being made. The shell theorem does not apply to disks or galaxies.
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.
ajross 5 days ago [-]
To be fair, there are relativistic generalizations of MOND, in the sense of relativistic theories that simplify to MOND dynamics in the low energy case. My understanding (this not being my field) is that they're sort of kludgey and non-calculable and that no one takes them very seriously. All the "real work" on MOND is just done using the classical stuff.
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.
twothreeone 4 days ago [-]
GR says spacetime is curved by mass, right. So what's the basis for explaining the curvature of space (which can be measured, e.g., LIGO) in MOND?
MathMonkeyMan 4 days ago [-]
MOND has nothing to say about the curvature of spacetime, since MOND is Newtonian (MOdified Newtonian Dynamics). It goes back to "F=ma and gravity is a force" and modifies the rules so that gravity grows weaker faster at a certain scale.
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.
Gooblebrai 4 days ago [-]
How does MOND deal with the effects of time dilation and length contraction? Do we have to go back to Newton's time where there's a universal time?
MathMonkeyMan 3 days ago [-]
I don't know if Newtonian gravity can be reconciled with Special Relativity. First thought is "no, that's why Einstein arrived at General Relativity." But I'm not in the field, so I don't know.
naasking 4 days ago [-]
MOND is an effective theory, it only describes observations and doesn't put forward any explanation of what's really going on.
oneshtein 4 days ago [-]
> GR says spacetime is curved by mass, right.
Wrong. GR says that gravitation can be modeled as acceleration.
mog_dev 4 days ago [-]
General Relativity states that mass-energy curves spacetime, and objects follow the straightest possible paths (geodesics) through this curved geometry. The equivalence principle relates gravity and acceleration, but it's not the main description of gravity in GR.
oneshtein 4 days ago [-]
Spacetime is model.
haccount 5 days ago [-]
[dead]
Bengalilol 5 days ago [-]
« Stunning evidence »
… then later on:
« Instead, the readings _seem_ to support a basis for MOND, which _would_ force astronomers and cosmologists to reconsider this alternative and long-controversial theory of gravity. »
What’s conditional evidence? I may be missing the overall picture, but I view such writing as non precise at its best.
MattPalmer1086 5 days ago [-]
It's just typical pop sci journalism, with a click baity headline. Read the paper instead.
Not entirely typical. MOND proponents seem to be trying more and more sell their approach to the public.
It annoys me but I suppose every theory has to do that now, "the mouse trap must go to market now" and all.
akvadrako 4 days ago [-]
Well you have to convince somebody to pay researchers for their time, which ultimately means selling your idea to non-experts.
bbor 5 days ago [-]
Well, it’s evidence that a) must be verified on a mathematical and empirical level, and b) (arguably) fits better with a currently unpopular theory than the dominant one. There’s so many unknowns in physics that opponents can easily reply “well your theory doesn’t explain XYZ yet, so we likely just need to tweak our theory”.
In other words, reasonable minds do disagree. AFAIU as an amateur.
5 days ago [-]
yieldcrv 5 days ago [-]
There is no consensus yet, there is no repeatable metric
It is perfectly valid to say “hey look over there for further review”
verzali 4 days ago [-]
Why why why do people share articles with sensational headlines like this? Its no wonder science journalism gets a bad rap. This kind of thing really undermines all the people who are actually trying to communicate science properly.
muglug 4 days ago [-]
Without this article and HN discussion I’d never have known about MOND, which is (at the very least) a fun theory.
verzali 4 days ago [-]
There are much better articles on MOND that don't make misleading claims that the James Webb has proven it. This one, for example:
Personally, I think it would be better that way. Science works in pursuit of truth, not towards the obfuscation of it for personal and selfish financial gain. That should hopefully explain the outage that scientists have towards articles like this one. In place of relying on articles like this, you might try searching scholarly articles or subscribing to them.
prof-dr-ir 4 days ago [-]
The trouble is that MOND is just not worth your time. In fact, I would even object to calling it a 'theory' in the first place.
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.
ogogmad 4 days ago [-]
This is a weirdly arrogant comment given both TFA and the fact that professional physicists have worked on MOND and disagree with everything you've said.
Is this typical behaviour for physicsts? Extremely strong opinions expressed in an abrasive way, out of proportion to the available evidence?
anon291 4 days ago [-]
In general, scientists (and academics more generally) suffer from some of the most dogmatic thinking on the planet. It's no surprise that many of them find themselves in institutions that were once known for their theology departments.
prof-dr-ir 4 days ago [-]
I just want to convey the following point: for the vast, vast majority of physicists the status of MOND is akin to what doctors think of the anti-vaccine theories. The evidence in the opposite direction is simply overwhelming.
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?
anon291 4 days ago [-]
> . How do you think doctors should convince vaccine skeptics that vaccines work
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
naasking 4 days ago [-]
> The evidence in the opposite direction is simply overwhelming.
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.
haccount 4 days ago [-]
[dead]
4 days ago [-]
naasking 4 days ago [-]
> I would even object to calling it a 'theory' in the first place.
That's why we have the term "effective theory".
lloeki 4 days ago [-]
> The trouble is that MOND is just not worth your time. [...] MOND is just some wild idea
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.
prof-dr-ir 4 days ago [-]
> there's still value in experimenting with it
I disagree: some experiments are just not worth our time. I wrote about such a situation three years ago:
Because every incentive tells everyone along the chain to do so, and then rewards them with money, views, influence, notoriety, points, and/or attention. It’s like anything, to get people to stop you must remove the incentives.
uoaei 5 days ago [-]
I follow the lead author, Stacy McGaugh, via his blog where he posts discussions and musings about the latest research into the dark matter vs MOND debate: https://tritonstation.com/new-blog-page/
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.
griffzhowl 5 days ago [-]
> which is totally backwards from a basic scientific perspective
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.
njtransit 5 days ago [-]
One difference between dark matter and Neptune is that the existence of Neptune is falsifiable. The formulation of dark matter inherently is not. Falsifiable hypotheses is the cornerstone of science.
LegionMammal978 5 days ago [-]
Is the existence of a planet so easily falsifiable? It hasn't been so long since the Planet Nine hypothesis started going around, and while we've observationally ruled out a big chunk of the original parameter space, there's still lots of room for a big dark dwarf planet to be floating around out there. It doesn't seem so different from how we've gradually been ruling out the parameter space for dark-matter observations.
uoaei 4 days ago [-]
Planets that reflect light are easy to detect.
renewiltord 5 days ago [-]
Surely the idea of it being a new kind of matter that interacts gravitationally but not electromagnetically yields some testable result? Does it actually yield nothing testable with today’s experimental methods?
MattPalmer1086 5 days ago [-]
There is a lot of indirect evidence for dark matter. All the direct tests for dark matter particles we have performed have found nothing so far - but since we have no idea what it might be, there's a lot of possibilities to test.
uoaei 4 days ago [-]
"Evidence" in heavy scare quotes, considering, again, the tautological nature of the claims around the existence of dark matter. "Something must be here that we are missing" is, frankly, a bullshit hypothesis that need not be entertained unless researchers can actually prove there is some worthiness to the claim. Anything stronger than "maybe our theory is wrong" would suffice!
mannykannot 4 days ago [-]
It is tendentious to point out only the difficulties in finding affirmative evidence for dark matter when MOND is doing no better in that regard. If, by that standard, dark matter is bullshit, then, mutatis mutandis, so is every other hypothesis that has been presented so far - but the observations that prompted them in the first place are not going away. It is inconsistent to call just one of them bullshit, and pointless to call them all that.
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...
uoaei 4 days ago [-]
Have you ever encountered the phrase "grasping at straws"? The pursuit of explaining dark matter has gone through many waves of "we just need to invent detectors for this particle that has never been observed" and is littered with the wreckage.
4 days ago [-]
griffzhowl 3 days ago [-]
Most of the history of physics involves making detectors for things that weren't previously observed... Consider: either most researchers in the field are stupid, for still pursuing an idea which you've apparently ruled out by simple reasoning, or your simple reasoning is fallacious
uoaei 3 days ago [-]
Almost: most of the history of physics is based on detectors being sensitive to things people didn't even know existed. Fits and starts based on happy accidents. The teleology of scientific progress is a myth. The most famous example is of course the Galilean moons.
Proposing detectors for particles that no one is even sure can exist is like setting up traps for Bigfoot...
pixl97 5 days ago [-]
I mean, dark matter may be discoverable, we just don't know how if it exists. There was time between the irregularities that were noticed in the orbit and the discovery of a new planet.
uoaei 4 days ago [-]
By that extremely simplistic logic, so is literally any other theory of gravity. This is not an argument, this is a flailing and empty justification.
bbor 5 days ago [-]
Well put, thanks for sharing! Never saw it phrased in such a clear narrative. As a novice, it seems like there's one big difference between those anecdotes and the current situation, though: sample size. Sure, if we were observing Andromeda spinning too slowly I'd be open to our instruments not capturing some massive objects/clouds, but we're actively observing, what, ~1E5-6 galaxies? In the case of a missing planet there were accidents of history/solar system makeup that led to our otherwise solid frameworks missing a key piece of information. But that clearly couldn't happen millions of times; whatever explains the inconsistencies we're seeing has to be a fundamental misunderstanding.
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!
necovek 5 days ago [-]
> ...turned out to be correct
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).
griffzhowl 4 days ago [-]
Thanks. I'm also no expert - I'm just learning general relativity - but that's also my rough understanding: either there needs to be a modification of the theory, or there's a new form of matter. It might seem more parsimonious to modify the theory, but then how do you do that in a way that retains all the successful predictions of GR while explaining the recalcitrant observations? That's the hard part.
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
antognini 5 days ago [-]
> where we would put matter if we could to make our theory of gravity make sense
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.)
MattPalmer1086 5 days ago [-]
There are more successful predictions than just rotation curves. For example, see:
These successful predictions are all generally variants on modeling galactic dynamics, though. The trouble is that galaxies and galaxy clusters are very messy places, so it's hard to make sure you've incorporated all the relevant physics.
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.
MattPalmer1086 5 days ago [-]
There are other predictions MOND makes. For example, it predicts higher collision velocities than LCDM, for example, see:
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.
kelseyfrog 5 days ago [-]
MOND doesn't cover the existence of CBM, distribution of galaxies, non-metallic abundance - things all covered by LCDM.
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.
MattPalmer1086 4 days ago [-]
MOND is not a cosmological theory unlike LCDM, and it isn't relativistic. So we should not expect it to cover the range of things that LCDM tries to.
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).
gus_massa 5 days ago [-]
Very interesting. Do you know an article that ELI25 this?
For something more technical, this article just came out as an overview of the evidence for dark matter: https://arxiv.org/abs/2411.05062
russdill 5 days ago [-]
The mond theories that add a factor that behaves like dark matter do a rather good job of matching observational data.
simonh 5 days ago [-]
I don’t think that’s quite fair. That approach is exactly how we find planets. Here’s an unexpected variance in the motion of a planet or star. It could be explained by a planet over there. Oh look, there’s a planet over there.
solid_fuel 5 days ago [-]
Hypothesizing that a planet might be over there is a testable hypothesis.
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?
mr_mitm 5 days ago [-]
Dark matter predicted lensing effect which were successfully tested. Same for the baryonic acoustic oscillations in the CMB.
MattPalmer1086 5 days ago [-]
That's not quite true. General relativity predicts gravitational lensing, not dark matter. Lensing has been used as an experimental probe for the presence of dark matter.
elashri 5 days ago [-]
MOND is an alternative theory of gravity competing with GR. People usually forget that while MOND started to present a different explanation for Dark Matter, it is a theory of gravity. Dark Matter is not a theory of gravity and is compatible with GR.
zeroonetwothree 5 days ago [-]
Dark matter isn’t much of a theory in the first place.
mr_mitm 4 days ago [-]
Well of course. What I clearly meant was that DM predicts lensing effects in a magnitude that cannot be explained with ordinary matter. See bullet cluster or weak lensing observations.
User23 5 days ago [-]
I’m particularly amused by the hypothesis that spacetime can be bent without the presence of matter. We can’t detect dark matter because there’s no such thing, it’s just a brute topological fact.
MarkusQ 5 days ago [-]
Right, which is why it quickly led to the detection of dark matter...hmm.
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."
TheOtherHobbes 5 days ago [-]
Planets are visible when you look for them.
Dark matter - so far - isn't.
drdeca 4 days ago [-]
What do you mean by “visible when you look for them”? Like, with light?
Does gravitational lensing count as “visible” to you?
russdill 5 days ago [-]
It's actually a better example than you think. This exact theory led to long and protracted searches for the planet Vulcan, which would explain Mercury's strange behavior.
halgir 5 days ago [-]
I usually understand "dark matter" to be shorthand for the discrepancy between theory and observation. The explanation might indeed be matter that is dark, or it might be solved by entirely unexpected observations and/or changes to theory.
mr_mitm 5 days ago [-]
Not really. You might think this after watching Angela Coulliers video, but when you read something like "25% of the universe's energy content is made of dark matter", they do not mean changes to some theory. They literally mean non-baryonic matter.
zeroonetwothree 4 days ago [-]
Energy content not only comes from matter but also from fields.
OutOfHere 5 days ago [-]
Nope. It can mean change to some theory, without a need for matter. It is the difference between relativistic gravity and the corresponding observed mass.
nathan_compton 4 days ago [-]
I wish science reporters would stop using MOND to stand in for all theories for which MOND is the low curvature limit. MOND itself is not covariant and has a lot of other really well known issues which make it obviously a non-starter and more sophisticated theories in the family of General Relativity reproduce MOND like behavior but are better behaved and more plausible.
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.
Waiting for Angela Collier to make a video on this, I'm sure many people will forward her this article. MOND is actually a niche in cosmology despite its PR.
Glyptodon 4 days ago [-]
Is there a quantized version of MOND where the increased acceleration is because a quantized unit of gravity will exert force across distances that would otherwise suggest that that the force would be less than a "g quanta" or because maybe quantization "ceilings" more than floors at very large distances? If gravity does have some kind of particle or fundamental quantization like a photon, and basically still exerts at huge or "infinite" distance, does it make sense that it's more likely there's some kind quantization floor or maybe quantization bands or something? Or is it thought that quantization of gravity imposes a limit on distance for the exertion of gravitational attraction? (Or is it thought that that with quantized gravity that what's happening is a decreased rate of "gravitons" interacting between the objects?)
Anyway, a bit clueless about this, just curious what gravitons are supposed to mean for either theory (MOND, LCDM, etc.).
mgraczyk 4 days ago [-]
Are any of the MOND theories consistent with this new data also consistent with recent gravitational wave observations? My understanding is that gravitational wave detectors have recently ruled out most plausible MOND theories. The linked paper doesn't seem to discuss this.
It shows early galaxies forming way faster and bigger than expected, which kind of shakes up the whole dark matter idea. Seems like it supports the MOND theory—that gravity might not work the way we think. Pretty wild, but it’s still up for debate.
steve_adams_86 4 days ago [-]
That would be a fun surprise to me as a lay person who doesn’t actually understand these things, because I see a lot of disparagement towards the MOND theory.
I_am_tiberius 4 days ago [-]
What I find implausible about MOND is the constant a0 (~1.2). Why stick with a measurement based constant instead of exploring a parameter that varies with distance?
docflabby 4 days ago [-]
Dark matter is just made up bs if you replace "magic" for dark whenever its mentioned its the same difference - theres no tangable evidence it exists at all.
akvadrako 4 days ago [-]
Dark energy is literally this - it just means something is different than predicted by current leading theories.
There is plenty of evidence that either dark matter or an alternative is needed and CDM is just the most popular take.
smolder 4 days ago [-]
It represents a discrepancy between our models and observations of the universe, which imply there is a lot of unaccounted-for non-interactive mass out there.
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.
XCSme 4 days ago [-]
There is no evidence that anything exists...
anon291 4 days ago [-]
One can use multiple different instruments to corroborate our own senses that matter does indeed exist, whereas the explicitly inferred properties of dark matter / energy make it impossible to detect. That honestly seems more religious than anything.
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.
XCSme 4 days ago [-]
> own senses that matter does indeed exist
What about people with schizofrenia? They can also use their senses and say something exists, when it actually doesn't.
Rendered at 10:11:01 GMT+0000 (Coordinated Universal Time) with Vercel.
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.
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.
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.
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.
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
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%...
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’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.
This is a wild take, given all the issues QM and QFT have.
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.
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.
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".
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.
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.
No, we don't know. Most physicists believe it, but that's not the same as knowing. We won't know unless and until we are able to actually do experiments in the relevant regime.
[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.
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).
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.
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!!!).
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.
Do you have some references handy for this? Or are you talking about the work of Deur?
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.
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.
you know what N in MOND stands for, right?
If someone emerges with a proof that the two systems are irreconcilable then yeah you have an argument that it's "shitting on GR"
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.
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"
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.
huh?!? there are GR corrections to Newton which include terms like 1/r^3 iirc
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.
Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.
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.
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?
Galactic dynamics is governed by gravity, which is Newtonian at those scales.
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.
But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.
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.
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.
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.
Wrong. GR says that gravitation can be modeled as acceleration.
It annoys me but I suppose every theory has to do that now, "the mouse trap must go to market now" and all.
In other words, reasonable minds do disagree. AFAIU as an amateur.
It is perfectly valid to say “hey look over there for further review”
https://physicsworld.com/a/cosmic-combat-delving-into-the-ba...
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.
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?
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 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
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.
That's why we have the term "effective theory".
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.
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.
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.
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.
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...
Proposing detectors for particles that no one is even sure can exist is like setting up traps for Bigfoot...
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!
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).
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
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.)
http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html
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.
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.
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'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).
For something more technical, this article just came out as an overview of the evidence for dark matter: https://arxiv.org/abs/2411.05062
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?
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."
Dark matter - so far - isn't.
Does gravitational lensing count as “visible” to you?
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.
https://www.preposterousuniverse.com/blog/2011/02/26/dark-ma...
I find this treatment more compelling.
Anyway, a bit clueless about this, just curious what gravitons are supposed to mean for either theory (MOND, LCDM, etc.).
There is plenty of evidence that either dark matter or an alternative is needed and CDM is just the most popular take.
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.
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.
What about people with schizofrenia? They can also use their senses and say something exists, when it actually doesn't.