Talk:Modified Newtonian dynamics
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I removed the following part:
edit- Spiral galaxies offer compelling evidence that this is more than an observational artifact. M51, for example, has two main arms (see fig. 1), each of which has an exterior end approximately 180° behind the interior end connected to the bulge. It thus appears that the edge completes an orbit in almost the same time as the interior. However, if Newton's universal law of gravitation holds for galaxies (as it should), stars at the edge should move much slower, and the spiral arms should be stretched around the bulge a hundred times, which would make the two arms completely indistinguishable.
(Image Removed)
The existence of spiral galaxies alone does not provide compelling evidence for the flattening of the rotation curve, since the galaxy's arms do not consist of stars; they are pressure waves rotating around the galaxy's center independently of the matter comprising the galaxy. AxelBoldt 11:20 Aug 15, 2002 (PDT)
For comparison purpose, the same curve for the Solar system -- (properly scaled) -- is provided (curve C in fig. 2).
- Am I missing something? I see the letter C but no associated curve. --Nate 21:14 Nov 27, 2002 (UTC)
I can't see curve C in Fig. 2 either. [Feb 5, 2004 Wes Hughes]
Galaxy rotation problem
editDoes it make sense to essentially copy all of Galaxy rotation problem into this article?
Aragorn2 15:07, 25 Sep 2003 (UTC)
- I just ran into this article, the first paragraph is indeed copied (but it is allowed under Wiki GNU Free Documentation License. Anyway, the majority of the article is dedicated to the solution of the problem by Milgrom's MOND. A descripition of the problem is neccesary. MathKnight 22:21, 28 Aug 2004 (UTC)
Rotation curves are not evidence
edit"Since MOND was specifically designed to produce flat rotation curves, these do not constitute evidence for the theory."
I don't agree with this formulation. Any theory aims to match its predictions with already known observations. By the same principle the standard model of particle physics would be useless, because it "depends on 19 parameters, whose numerical values are established by experiment".[1]
Only data that has been used to produce should be dismissed as evidence.
Section with original research
editThe article currently contains these paragraphs that lack citations and seem to be original research. Calling the original contributor User:Greg L who wrote most of this, do you have sources for this?
User:Banedon could I bother you for a yay or nay about removing/drastically trimming this? In my opinion the article would be better suited by a diagram showing that the accelerations in the solar system are much higher than a0. That's available in the literature so citations wouldn't be an issue. Thoughts in general about the changes I've made so far would also be appreciated!
Applications
Deep-MOND gravitational effects, which explain galactic-scale observations, have not been detected on Earth or in the solar system because the local gravitational field is many orders of magnitude larger than a0. Even at the edge of the Solar System, where the Sun's gravity drops below a0 the total gravitational field is still above a0 due to the gravitational fields of the rest of the galaxy. On Earth's surface—and in national laboratories when performing ultra-precise gravimetry—the a0 value is equal to 0.012 microgal (μGal), which is only twelve-trillionths the strength of Earth's gravity. A change in the laws of gravity below this acceleration is far too small to be resolved with even the most sensitive free-fall-style absolute gravimeters available to national labs, like the FG5-X, which is accurate to just ±2 μGal. When considering why MOND's effects aren't detectable with precision gravimetry on Earth, it is important to remember that a0 doesn't represent a spurious force; it is the gravitational strength at which MOND is theorized to significantly begin departing from the Newtonian dynamic. Moreover, the a0 strength is equivalent to the change in Earth's gravity brought about by an elevation difference of 0.04 mm—the width of a fine human hair. Such subtle gravitational details, besides being unresolvable with current gravimeters, are overwhelmed by twice-daily distortions in Earth's shape due to lunar gravitational tides, which can cause local elevation changes nearly 10,000 times greater than 0.04 mm. Such disturbances in local gravity due to tidal distortions are even detectable as variations in the rate of a Shortt double-pendulum clock, which was a national timekeeping standard in the late 1920s.
To give a sense of scale to a0, a free-floating mass in space that was exposed for one hour to 1.2 × 10−10 m/s2 would "fall" by just 0.8 millimeter—roughly the thickness of a credit card. An interplanetary spacecraft on a free-flying inertial path well above the Solar System's ecliptic plane (where it is isolated from the gravitational influence of individual planets) would, when at the same distance from the Sun as Neptune, experience a classic Newtonian gravitational strength that is 55,000 times stronger than a0. For small Solar System asteroids, gravitational effects in the realm of a0 are comparable in magnitude to the Yarkovsky effect, which subtly perturbs their orbits over long periods due to momentum transfer from the non-symmetric emission of thermal photons. The Sun's contribution to interstellar galactic gravity doesn't decline to the a0 threshold at which MOND's effects predominate until objects are 41 light-days from the Sun; this is 53 times further away from the Sun than Voyager 2 was in November 2022, which has been in the interstellar medium since 2012.
Despite its vanishingly small and undetectable effects on bodies that are on Earth, within the Solar System, and even in proximity to the Solar System and other planetary systems, MOND successfully explains observed stellar and gas rotation velocities without invoking the existence of as-yet undetected dark matter particles lying outside of the highly successful Standard Model of particle physics.
ScienceDawns (talk) 13:24, 28 February 2025 (UTC)
- @GregL: Fixing the ping. I don't have time to look at this right now, possibly later. Banedon (talk) 14:22, 28 February 2025 (UTC)
- @Greg L: Sorry, really fixing the ping now. Banedon (talk) 14:25, 28 February 2025 (UTC)
Ok, I think the section is includable in different form - under a section heading "MOND on solar system scales" or something like that. It would need sources though, since it makes claims that are not obvious. Without sources, I would say delete the setion. Banedon (talk) 01:15, 2 March 2025 (UTC)
- I've added a well referenced subsection on solar system tests in the "outstanding problems" section that should serve to replace this material. After a fair bit of searching I can't find any sources to back up the claims in the section I've quoted above. So I've gone and deleted the material from the article. We can leave it here in case sources do become available and we can restore some of it then. ScienceDawns (talk) 05:31, 2 March 2025 (UTC)