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Comment by i_no_can_eat | Hacker News Reader

Exactly. And the well-posed gravitational theory that has MOND as a limit has not yet been found. So why is MOND considered as anything but an effective model?

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Wouldn't Bekenstein's TeVeS [2004] fit the bill?

Or that doesn't count as MOND because it effectively adds another source of gravity and thus dark matter in disguise?

TeVeS theories have many problems. Among others, they have been ruled out by observational data [1,2].

[1] https://journals.aps.org/prx/pdf/10.1103/PhysRevX.11.041050

[2] https://arxiv.org/abs/1710.06168

Why is fluid dynamics considered to be an effective model when we haven't even proven the soundness of the navier-stokes equations?

For that matter, how is QM considered to be an effective model when we can't even reconcile it with gravity?

Fluid dynamics is not a fundamental theory, but an effective model, as you said. The point here is that MOND can't explain something that GR has been able to explain for 100 years (precession of Mercury). So any serious gravitational theory needs to at least that for serious consideration.

If you're claiming that dark matter is superior because it is a fundamental theory, I fail to see why a fundamental theory that's empirically wrong is supposed to be better than an effective model. Rather than propping up dark matter, astrophysicists should be focusing on novel ideas that reproduce MOND, like superfluid dark matter, or at GR modifications in the MOND regime.

I never mentioned dark matter. My only point is that any modern gravitational theory needs to do at least as well as general relativity. That means (among others): explaining the precession or Mercury; describe black holes; predict gravitational waves. So if MOND is to be taken seriously, it needs to do all of that. That's all, really.

That's a really bad bar, beacuse it creates a bootstrapping problem: There's a lot of very difficult math that you have to do to get to that point.

And if you say "don't take it seriously until it explains all those things" then why should anyone bother to do the heavy lifting for all that math?

Let me make a different suggestion: As soon as you have one or two very interesting observations that modern GR can't explain that are apparently gravitational, then you should start encouraging people to take it seriously and start working on the hard math.

> I never mentioned dark matter. My only point is that any modern gravitational theory needs to do at least as well as general relativity.

It's a misconception that MOND involves abandoning GR:

https://tritonstation.com/2023/02/27/take-it-where/

There is no misconception. The only class of theories that I'm aware of that have MOND as their non-relativistic limit are TeVeS, and these have been ruled out by experiments. If you have something else in mind, please provide a paper reference.

> Why is fluid dynamics considered to be an effective model when we haven't even proven the soundness of the navier-stokes equations?

Fluid dynamics is a field of study, not any kind of model. Navier-Stokes is just what you get when you apply F=ma to Newtonian fluids.

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