What is the reasoning behind MOND's modification of gravity?

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Discussion Overview

The discussion revolves around the Modified Newtonian Dynamics (MOND) theory, which seeks to explain the rotation curves of galaxies without invoking dark matter. Participants explore the reasoning behind MOND's modification of gravitational behavior, particularly its claim of a different distance dependence at large scales.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that MOND suggests gravity behaves as 1/r at large distances instead of the traditional 1/r^2, seeking reasoning behind this modification.
  • Another participant argues that MOND is merely a modification to fit observational data, lacking deeper theoretical justification.
  • A participant challenges the claim about MOND's distance dependence, asserting that it does not state gravity behaves as 1/r.
  • Links to external sources are provided to support the claim that MOND does imply a 1/r dependence, though this is contested by others.
  • A participant presents a formula indicating that under certain assumptions, acceleration behaves as a ~ 1/r at large distances, suggesting this supports the initial claim.
  • Another participant counters that MOND modifies Newton's Laws to introduce a minimum acceleration, emphasizing that simply changing the distance dependence is insufficient to achieve MOND-like behavior.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of MOND's implications about gravitational behavior. There is no consensus on whether MOND correctly describes gravity as 1/r at large distances, and the discussion remains unresolved.

Contextual Notes

Some participants highlight the need to consider the full derivation of MOND's principles, indicating that assumptions and definitions play a crucial role in understanding its claims.

PhDnotForMe
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I've been reading a lot about MOND, an alternate theory attempting to explain the rotation of galaxies without using dark matter. It claims that at large distances gravity is proportional to 1/r rather than 1/r^2. In the published article, there are countless experiments done providing evidence that the idea holds possible. However, I can't seem to find any reasoning behind why gravity may behave this way. Is there any?
 
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No. MOND is just that - modifying the equation to fit the rotation curve. It's a toy model, with no deeper reasoning attached.
 
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PhDnotForMe said:
It claims that at large distances gravity is proportional to 1/r rather than 1/r^2.

It most certainly does not.
 
PhDnotForMe said:
According to the links above, it does.

Untrue. You are misrepresenting what they are saying, and that's not a very nice thing to do. The first link does not say it, and the second discusses a gravitational law of that form to say it's not MOND.
 
With these assumptions, the weak acceleration limit of gravity is:
a = sqrt(GMa_0)/r
with dependence 1/r on distance r from the body of mass M generating the field.

To me that is pretty explicitly saying a ~ 1/r at large distances, so if it's wrong it's not just him...
 
You need to read the entire derivation paragraph. MOND changes Newton's Laws so that there is a minimum acceleration, a0. It has been known for a long time (some of which is referenced in the 2nd paper) that you can't make this work by changing only the distance dependence of the gravitational force. The closest you can come to that statement is that in a single system, if I have two points at r1 and r2, the difference between the accelerations at those points is proportional to (1/r1 - 1/r2). But even that is misleading - if you want MONDy behavior, you have to change the acceleration, not the distance dependence.
 

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