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355 points jchanimal | 1 comments | | HN request time: 0s | source
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samsartor ◴[] No.42158987[source]
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

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meindnoch ◴[] No.42159582[source]
>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.

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EPWN3D ◴[] No.42159764[source]
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).

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meindnoch ◴[] No.42160083[source]
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!!!).

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fpoling ◴[] No.42162780[source]
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.

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magicalhippo ◴[] No.42163277[source]
> 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?

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fpoling ◴[] No.42165916[source]
See references in https://en.m.wikipedia.org/wiki/Galaxy_rotation_curve right before See also section. For more speculative paper about behavior of galaxy clusters see https://www.scirp.org/journal/paperinformation?paperid=12663...
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1. raattgift ◴[] No.42169158{3}[source]
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 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.