Comment by throwaway4PP | Hacker News Reader

Comment by throwaway4PP | Hacker News Reader

Exciting times! Any physicist here aware of how these two papers would impact the applicability or validity of Modified Newtonian Dynamics (MOND)?

background: my lay understanding of MOND is that it modifies the gravitational interaction parameter over cosmological distances. the force exerted on objects due to gravity is currently accepted to scale with distance in a certain manner (ex. 1/distance^2) while MOND postulates a different scaling relation (ex. 1/distance^3). Those are just examples, not actual values. The currently accepted gravitational interaction force scaling is what gives rise for the need for dark matter, and the corresponding lambda cold dark matter (LCDM) theory. Of course, we have not been able to observe dark matter, which is a problem for a theory. That is what has given rise to MOND, amongst other things. There are prominent, esteemed physicists who have recognized many issues with LCDM, some of which are addressed by MOND (https://astro.uni-bonn.de/~pavel/kroupa_SciLogs.html)

previous HN posts with interesting discussions / links re: MOND

https://news.ycombinator.com/item?id=37012052

https://news.ycombinator.com/item?id=33261981

https://news.ycombinator.com/item?id=23982814

It's of no relevance to MOND, at least none that I can see.

MOND gives a different scaling relation, and is therefore contradicting general relativity. Its goal is to explain the effects we associate with dark matter, without the need for dark matter.

General relativity is (we are pretty sure) inconsistent with quantum field theory. String theory tries to fix the issue by replacing the particles in field theory with strings. Oppenheim is trying to fix it by putting general relativity as a classical phenomenon that lives "outside" of quantum field theory.

They're trying to solve different problems. And, Oppenheim's classical gravity picture could be used just as well with MOND instead of standard general relativity, if that's what you wanted.

MOND is getting less popular every year as evidence for dark matter piles up. The Bullet Cluster is a particular instance where we can actually "see" the dark matter flying around, in a way MOND couldn't hope to explain. LIGO has also given us a lot of confidence we have the right theory of gravity, at least up to the quantum scale.

> The Bullet Cluster is a particular instance where we can actually "see" the dark matter flying around, in a way MOND couldn't hope to explain

This is not correct. In fact, LCDM can't even explain the Bullet Cluster [1]. The evidence is not so favourable to LCDM over MOND [2] when taken as a whole.

More recent observations on wide binary stars disfavour MOND more strongly, but the classic reasons you cite are not valid reasons.

[1] https://arxiv.org/abs/0704.0381

[2] https://arxiv.org/abs/2110.06936

Edit: See for instance what Milgrom said about the Bullet Cluster back in 2006:

http://astroweb.case.edu/ssm/mond/moti_bullet.html

I am pretty sure that MOND has been completely ruled out[0] given some recent evidence. And by more than 5 sigma.

[0]https://bigthink.com/hard-science/dark-matter-alternative-mo...

That article linked does not completely rule out MOND. It is a great article, but it even mentions several difficulties that complicate their observations.

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