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Comment by ollysb | Hacker News Reader

> A side effect of this is that the motions of objects would depend not just on their own mass, but all other masses in their neighborhood.

As I understand gravity warps spacetime which seems to imply that the masses of all objects in the neighbourhood would be taken into account because they all exert a warping effect on the same spacetime. Could somebody clarify what the difference is that they’re referring to? (or possibly the error in my understanding)

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MOND is a modification of Newtonian gravity and thus doesn't (yet?) take relativistic corrections into a account (see the last paragraph in [0]). So it's hard to predict what the fully relativistic picture is going to look like. I'm not a MOND theorist, though, so I might be wrong. (There are some relativistic generalizations[1] of MOND but it doesn't appear the MOND community has settled/agreed on one approach yet.)

[0]: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ou...

[1]: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Re...

There's conformal gravity, which replaces the Einstein field equations with ones derived from a conformally invariant action. Like MOND, this allows you to fit rotation curves nicely without invoking dark matter, see eg [1][2].

[1] https://arxiv.org/abs/1211.0188 (2012)

[2] https://arxiv.org/abs/1812.03152v3 (2018)

I think what it is saying is that the gravitational effect of a mass depends on the gravitational environment in which it exists. So a given sample of mass warps space more or less depending on whether it is near other masses or not.

I’m not a MOND theorist though. That’s just my generous interpretation of that statement.

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