I’m trying to understand the External Field Effect (EFE) in Modified Newtonian Dynamics (MOND) and how it is conceptually distinct from GR and Newtonian gravity. More specifically, the descriptions I read about the EFE from press releases confuse me with following phrases as I attempt to discern MOND from everything else. Please bear with me, I am trying to learn and I realize I don’t understand these concepts as well as I would like:
“In Newtonian physics, the inertial mass of an object is an inherent property that exists independent of anything around it. In MoND, the inertial mass depends on the gravitational mass of the object as well as the net gravitational pull from the rest of the universe. In other words, inertial mass is an emergent property rather than an inherent one.” https://www.universetoday.com/149416/new-data-supports-the-modified-gravity-explanation-for-dark-matter-much-to-the-surprise-of-the-researchers/
Inertial mass is considered an emergent property in MOND. I don’t see how the inertial mass in MOND would be different than the inertial mass in Newtonian gravity. For example, in Newtonian gravity, wouldn’t we need to know the masses and positions of everything in the Universe to create a precise gravitational field? For practicality sakes, I know this is impossible and unnecessary. Ie. To first order, everything but the Sun and possible Jupiter can be ignored in our Solar System. I mention this for the sake of argument. Wouldn’t the pull from “everything in the Universe” in the Newtonian sense be the same then as the inertial mass in MOND? On the most precise level, when would we neglect mass from “the rest of the universe” in Newtonian dynamics? Again, assuming we could be this precise in Newtonian dynamics. I hope that makes sense. MOND seems to discuss things as if there is this extra field and I don’t understand what that extra field really is. Shouldn’t all fields be considered? Ie. The outskirts of galaxies experience low acceleration. Why would it be surprising/possibly controversial to consider the influence of external bodies? What am I not understanding here?
“One of the core tenets of general relativity is the strong equivalence principle, which states that the motion of stars in a galaxy should be independent of an external uniform gravitational field.”
More of my confusion: What would this motion be independent? I don't understand how the SEP negates the concern of a gravitational field of other bodies but I also don't know what is meant by "external" here so I guess we can start there.
Other snippets for context:
“The researchers looked at 153 galaxies and measured the speed of stars within them at different distances from the galactic center. They then looked at the acceleration of each galaxy caused by the gravitational fields generated by other galaxies surrounding it. The strongly accelerated galaxies experienced ten times the acceleration of the most weakly accelerated ones."
“They then selected the two galaxies which felt the most gravitational tug from their surroundings and compared their rotational behavior to two galaxies that were isolated. They found that the outer stars of the galaxies in strong gravitational fields orbited slower than predicted by the behavior of the isolated galaxies. They also studied galaxies with intermediate external gravitational fields and found that the data was consistent with the extreme examples, with the rotational characteristics of each galaxy depending on its surroundings. Their data appears to violate the strong equivalence principle.”
I guess my main confusion is as follows: Isn’t there always just one gravitational field at a specific point that is dependent on ALL the masses and positions of everything in the universe (at the most precise, albeit impossible and impractical level). What is all this talk of external fields? I have been trying to parse this through on my own and got lost along the way. I appreciate conceptual explanations.