<p>Yes, this is pretty much exactly how <a href="https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics#:%7E:text=Modified%20Newtonian%20dynamics%20(MOND)%20is,currently%20understood%20laws%20of%20physics." rel="nofollow noreferrer">Modified Newtonian Dynamics (MOND)</a> works. It is observed that Newtonian mechanics works very well for large accelerations, but an <em>ad hoc</em> correction function to Newtonian gravity is proposed at small accelerations. This function can be tuned such that the rotation curves of galaxies can be explained without the need for dark matter.</p>
<p>Edit in response to edited question:</p>
<p>Note that explaining rotation curves is <em>of course</em> only one empirical constraint. And MOND does poorly at matching others.</p>
<p>If you are looking for some universal gravitational law that explains all possible empirical observations of stellar and galaxy motions (which is only one of the constraints any theory must satisfy), without dark matter, then I'm afraid there isn't one (afaik).</p>
<p>Any approach to a universal theory of how gravity works must <em>of course</em> be identical (or at least similar to a very great extent) to inverse-square Newtonian gravity outside the regime of very strong or very weak accelerations (as indeed General Relativity is), because there is a wealth of observational evidence that shows simple Newtonian gravity works very well in those cases.</p>

CC BY-SA 4.0
2021-06-04T15:15:10.627Z
I edited my question to clarify why MOND is not what I was asking for - thanks :)
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-04T20:17:52.380Z
@JacekKołodziejek but it is what you are asking about. Repulsive gravity is not required
ProfRob
CC BY-SA 4.0
2021-06-04T20:28:05.770Z
Well, MOND does not explain several things Dark Matter does from what I heard.
Obviously I'm too dumb (and don't have the time) to read proper scientific papers but I do listen to scientific videos/podcasts (while I work) and e.g. based on this video https://www.youtube.com/watch?v=dtfEzDAlL5k (9:23-12:20) it seems that MOND simply replaces one issue with another that is better solved by Dark Matter. So it doesn't get it all right. Also, it is another theory based on something, instead of a clean slate.
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-04T12:58:12.220Z
2022-07-14T14:45:18.197Z
2022-07-14T14:45:18.197Z
ProfRob
<p>In the emergent gravity theories, dark matter is thought to be inherent to normal matter and spacetime. It's a consequence of the interplay between conformal fields in 5d anti-de Sitter spacetime and the 4d-spacetime (ours) it encloses. This theory (contrary to MOND) is there even if (conjectured) dark matter-induced motion wasn't observed. In this respect, it is preferable to dark matter theories, which indeed cause Vulcan-like situations. See for example <a href="https://arxiv.org/abs/1611.02269" rel="nofollow noreferrer">this</a> article. Even normal gravity itself is explained in this way.</p>

CC BY-SA 4.0
2021-06-05T22:27:54.297Z
Emergent Gravity seems very appealing to me and I very much like the section 8.1 of the paper you linked to: "the evidence in favour of dark matter is just as much evidence
for the possible breakdown of the currently known laws of gravity". Spot on.
However, this paper conjures a massive underlying theory to come up with a set of equations, whereas I asked if there was an attempt at just getting equations out of the data and seeing if it's at all possible. EG also seems to be strongly disputed, though I'm unqualified to opine on the arguments (see EG page on wiki).
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-04T13:50:37.527Z
2021-06-04T13:50:37.527Z
Deschele Schilder
<p>I'm not sure if it's appropriate to add another answer but as the question has been edit to make its intention more clear, I suppose it is.</p>
<p>Let me explain why this is not the way it's done. To simplify the situation, consider the motion of a particle that falls down on Earth. One can take a number of these particles, with various masses and do measurements on these.</p>
<p>Somewhat like Galileo once did in his tower experiment. Galileo, who said that the only way to read the universe was to learn the language of mathematics, is thought to have thrown two massive spheres from Pisa's tower and thereby showed that all masses fall in the same way. The spheres reached the ground in the same time, hence the conclusion. The experiment falsified Aristotles' conjecture that the falling of objects depend on their mass. If two objects are dropped the heavier would reach the floor earlier as the light one, according to him. Now this is indeed the case for many different objects when you let them fall on the ground. A feather will end up with less velocity than a small lead ball with the same mass. Heavy objects, compared to light ones of comparable size, take generally less time to reach the Earth than light ones (because of air resistance). Aristotle didn't know anything about air resistance (like we don't know a dark matter) and because of this he thought gravity acting differently for different masses.</p>
<p>The (alleged) measurements of falling times done by Galileo were done though after the workings of gravity were already anticipated. The measurements were an experiment to confirm (or contradict) these.</p>
<p>Now he could have done a lot of experiments on a lot of falling objects. He could have used different masses and different shapes, different initial velocities and positions, and performed the experiment at different locations and different circumstances. This would have resulted in a huge amount of data: rows of initial positions, initial velocities, rows of masses and shapes, rows of times, velocities, and positions, rows of circumstances, and rows of locations. If the data were ordered in the right way (for which no theoretical assumption about gravity has to be made), and if he had quantum super computer like capabilities, he could search for a connection between the data. How to find the algorithm? Well, he could have made a visualization of the data. If he had the mathematical knowledge of today (differentiable functions only saw the daylight after the birth of Newton) he could use his knowledge of the graphic form of functions to see if any of them (or a function of a function, say a sum) matched the data. If he had found any functional correspondence he could have started theorizing about gravity (or the circumstances of the experiment).</p>
<p>I doubt that this is done in practice. Now this is exactly your question, but I merely answer to state that this way of proceeding isn't followed in practice. Pure naked observation cannot be detached from theory. That is, empiricism and theory cannot be separated. Even in the case of measuring times, distances, positions, velocities, or masses, you already make a theoretical assumption (for example, that all velocities refer to displacement in space, while it could be that space itself expands).</p>
<p>Now, as seen, it <em>could</em> be done in this way. That is, measuring masses, velocities, and positions (and distances) of different celestial objects, after which you look for a functional relationship between them, but mostly it's done the other way round. On top of that, it would be very time-consuming, although a computer could do the job. So in the case of dark matter, the computer will spit out a functional relation between the data (the one that fits these data best). This relationship will be undisputable (though this is disputable!). To find out how these functional relationships between the data comes about a theory is needed. This can change the very Nature of the quantities that you measured (like mass, velocity, and space), thereby changing the Nature of your assumptions made in the measuring (like space time being Euclid).</p>
<p>So again, it <em>could</em> be done, but to separate empiricism completely from theory is impossible.</p>

CC BY-SA 4.0
2021-06-05T00:03:46.587Z
You can use a blank line to separate paragraphs, This would make your answer easier to read.
James K
CC BY-SA 4.0
2021-06-05T00:08:51.367Z
@JamesK You edit at the same time I did! Thanks.
Deschele Schilder
CC BY-SA 4.0
2021-06-05T07:17:15.327Z
So in short, the reason this is not done is because our measurements are affected by our understanding of the subject matter, i.e. how we calculate the apparent gravitational pull to feed into the approximation algorithm will be affected by e.g. General Relativity.
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-05T07:26:01.233Z
continued cause I missed the edit window :P: 

That's a fair point, but not a reason not to try it. Applying a function on the measurements does not remove the relation. For example, if you pick a number of pairs (X,Y) where X=Y, relationship will approximate to linear. If you square all Ys and approximate again, the relationship will be the square function. If the underlying data is in a strict relationship and affected uniformly by some continuous function, the modified data is still in a strict relationship, just modified.
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-05T07:26:45.960Z
@JacekKołodziejek Yes, exactly. you can make a list of data (as you have done in your question), but the interpretation of these data involves theoretical assumptions about the data. The data cannot exist on their own. You *can* let a computer do the job of finding data (by using a telescope connected to a computer) and finding the appropriate algorithm (function) relating the data. But the interpretation involves a theory of ours (maybe the computer can find a theory too, but then it has to be very advanced!). : D
Deschele Schilder
CC BY-SA 4.0
2021-06-05T07:30:59.150Z
@JacekKołodziejek Isn't this massive data gathering done? with computers? I'm not sure if trying to find an algorithm is done.
Deschele Schilder
CC BY-SA 4.0
2021-06-05T08:34:18.783Z
@Barbierium - I'm not exactly sure what you're trying to say, sorry. The process I would expect is: gather data, perform an unrestricted approximation of the relationship, hypothesize on the theory behind the relationship. If some preexisting theory skews the data, then it is fair to assume that it is in the form of some more or less regular function that would just compound on top of the resulting relationship and which can be untangled later. And even if it can't and it all fails - I just want to know if it has been tried. One paper that says "we tried - it failed" is all I need.
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-05T08:51:15.770Z
@JacekKołodziejek As I wrote, why should you do it like this if it's easier to find a theory by other means? If it's a computer that gives you the algorithm, that is, the function that makes the connection between the data (the numbers) , then it would still be us who endow the data with theory. The real way doesn't correspond to a 1-1 relationship between data and theory. By which I mean that in the real world observation and theory "alternate".  You *could* observe falling bodies and write down all positions, velocities, times, but this is simply not how a theory of falling bodies is found.
Deschele Schilder
CC BY-SA 4.0
2021-06-05T20:07:03.867Z
Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/126122/discussion-between-jacek-kolodziejek-and-barbierium).
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-04T23:51:50.870Z
2021-06-05T00:07:35.500Z
2021-06-05T00:07:35.500Z
Deschele Schilder
<p>Being a mathematician I wonder, <strong>has there been an attempt to blindly approximate a formula for gravitational attraction that would match the data</strong>, based on &quot;normal&quot; matter we can see plus that which we can't, but can  reasonably expect, e.g. black holes, dust etc? I'm talking about an unrestrained approximation, one that can take a negative value, regardless of how preposterous that might seem, just to see if we can find one that fits and try to make sense of it.</p>
<p>It seems to my uneducated brain that Dark Matter hypothesis is essentially identical to the planet Vulcan hypothesis, i.e. we assume we understand gravity so we explain weird behavior with mass we didn't yet detect (only this time we cheat by hypothesizing the mass is undetectable). Also, we know of forces that don't abide by inverse square laws and can either effectively pull or push depending on the distance (nuclear forces). So we have a precedent of us being wrong about gravity (despite prior experimental success e.g. finding of Neptune) and examples of forces that behave differently to gravity... so maybe let's just assume gravity grossly defies our intuition, try to match a formula to observations and see if we can figure it out from there?</p>
<p>Sorry if it's not the space for such amateur questions - please let me know where I could go with that. Thanks!</p>
<h1>EDIT</h1>
<p>Just to clarify, I am specifically asking about <strong>approaches that do not derive equations out of logical interpretations of the data, but out of the data directly</strong>. As such, for example, MOND does not qualify because it derives from Newtonian interpretations and just adds an extra piece on top, inheriting all of the limitations of the Newtonian formulae (like the idea of gravity being always attracting, never repelling).</p>
<p>I am literally asking if somebody (<strong>recently, as new measurements are made</strong>) tried to feed bulk data like below to an array of approximation algorithms to see if any of them produces something that fits the data:</p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th>Object position</th>
<th>Object mass</th>
<th>Apparent G vector</th>
</tr>
</thead>
<tbody>
<tr>
<td><span class="math-container">$p_0$</span></td>
<td><span class="math-container">$M_0$</span></td>
<td><span class="math-container">$\vec v_0$</span></td>
</tr>
<tr>
<td><span class="math-container">$p_0$</span></td>
<td><span class="math-container">$M_1$</span></td>
<td><span class="math-container">$\vec v_1$</span></td>
</tr>
</tbody>
</table>
</div>
<p>... where position and acceleration vector are in any coordinate system that makes calculations easier and objects are aggregated as needed (e.g. instead of 400 billion stars per galaxy, put clusters of stars as singular objects instead).</p>

CC BY-SA 4.0
2021-06-03T15:00:33.000Z
Have you ever read into the history of the theoretical development of gravity? Literally so many people developed empirical relationships relating forces and distances (a similar history for Coulomb's law). Hooke was such a contemporary to Newton, but Newton had the mathematical know-how to formalize it with symbolic rigor. Your questions might be addressed better in the history of S&M stack exchange, for examples: https://hsm.stackexchange.com/questions/8094/how-close-was-hooke-to-developing-a-comprehensive-system-of-gravity
Daddy Kropotkin
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2021-06-03T15:01:27.943Z
and https://hsm.stackexchange.com/questions/12915/did-einstein-know-about-laplaces-query-into-whether-gravity-is-instantaneous
Daddy Kropotkin
CC BY-SA 4.0
2021-06-03T15:04:38.457Z
Also, people have done what you suggest for the dark matter hypothesis. Many lines of research explore this, in much more sophisticated ways than you've proposed, e.g. see this old review https://www.cambridge.org/core/journals/european-astronomical-society-publications-series/article/abs/review-of-observational-evidence-for-dark-matter-in-the-universe-and-in-upcoming-searches-for-dark-stars/843789157604D5627BAB6119C5084333 I'll lastly add that the model known as MOND is another way of attempting this by modifying Newton's law. Dark matter is more of a paradigm than a theory, per say.
Daddy Kropotkin
CC BY-SA 4.0
2021-06-03T17:17:28.640Z
Already Einstein added the cosmological constant... which does a similar thing at large distances (but does not solve all gravity-related problems either)... yet much more rigor and approaches have been applied to this issue in quite open-minded ways than you seem to assume. The problem is that a theory has to explain quite a lot, sometimes even so far contradictory observations. It's not just finding the right parameters for a known equation type.
https://en.wikipedia.org/wiki/Cosmological_constant
planetmaker
CC BY-SA 4.0
2021-06-03T17:18:37.100Z
In order to solve this mystery or problem it might even require to come up with a theory of quantum gravitation - a subject where everyone so far came up with a blank.
planetmaker
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2021-06-03T19:08:22.277Z
Relevant (and obligatory)  xkcd  https://xkcd.com/1758/
James K
CC BY-SA 4.0
2021-06-04T17:56:07.877Z
What does "approximation algorithm" mean? Are you saying, "pretend gravity can be represented by a really massive polynomial equation"? In which case, sure, with enough terms you can probably fit anything, but it may be terrible at predicting new data. And most people won't be happy with the idea that each galaxy (or galaxy group or galaxy cluster) operates according to its own "equation", which won't be applicable to cosmoloty...
Peter Erwin
CC BY-SA 4.0
2021-06-04T20:36:39.620Z
@DaddyKropotkin - I'm not asking about historical attempts, but recent ones, ones that came after we started finding issues with Dark Matter.
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-04T20:36:51.697Z
@planetmaker - Cosmological Constant was a modification of an existing set of formulae. That's what I know was being attempted a lot and that's not what I am asking about. If there were "open-minded" approaches that failed, please link, that's what I'm asking for.
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-04T20:37:28.623Z
@JamesK - beautiful picture, but seriously - I'd love to see a paper that says "we tried to plot these values and come up with a dumb estimate of the function with no restrictions and here's the issues we found...". This is literally what I'm asking for.
Jacek Kołodziejek
CC BY-SA 4.0
2021-06-04T20:49:21.380Z
@PeterErwin - If the "massive polynomial" converges to a compound of well known power series, then we could compose the formula from what it seems to converge to and check if that's what works. If it truly turns out that every galaxy has its own function then this will be evidence for Dark Matter, i.e. that variables are needed beyond the masses we can see. That's my question: did we try letting masses we see explain everything we see, but without any assumptions on the form of the formulae whatsoever, i.e. not starting off from Newton's equations or any other logical background.
Jacek Kołodziejek
CC BY-SA 4.0
2022-07-14T14:27:51.043Z
As I point out in my answer - this is exactly how MOND is formulated, by empirically matching the "gravity" required to explain the observations of the motions of objects in the small acceleration regime, No formula for gravity will disagree with Newton's laws (because they work) other than in the very strong or very weak gravitational regimes, so all simple formulations *will* be "based on" inverse-square Newtonian gravity outside of those regimes.
ProfRob
CC BY-SA 4.0
2022-07-15T15:22:51.710Z
@ProfRob Not "exactly". MOND, based on the first couple of paragraphs of its entry on Wikipedia, is an attempt to fit a formula to a mechanistical hypothesis: "(...) Milgrom noted that this discrepancy could be resolved if the gravitational force experienced by a star (...) was proportional to the square of its centripetal acceleration (...) or (...) if gravitational force came to vary inversely linearly with radius (...)". In other words, it seems that a mechanism was proposed first, then a formula that reflects the mechanism and then the search for parameters begins....
Jacek Kołodziejek
CC BY-SA 4.0
2022-07-15T15:30:37.667Z
@ProfRob ... but what I'm asking is: "have we tried to calculate the apparent gravitational acceleration affecting stellar objects based on how much mass we are able to see in their vicinity, WITHOUT presupposing a mechanism that would dictate the formula". The way I see it is that Newton nailed the formulae that works in some spectrum of conditions, Einstein seems to have had a good guess at the mechanism behind gravity that gave him somewhat better formulae, but perhaps we can find even better formulae by just looking at data, without even trying to guess the mechanism.
Jacek Kołodziejek
<gravity><general-relativity><dark-matter>
Was there a formula-first hypothesis for gravity?
Jacek Kołodziejek
# Was there a formula-first hypothesis for gravity?

(Asked by: username_0 on 2021-06-03)

Being a mathematician I wonder, **has there been an attempt to blindly approximate a formula for gravitational attraction that would match the data**, based on "normal" matter we can see plus that which we can't, but can reasonably expect, e.g. black holes, dust etc? I'm talking about an unrestrained approximation, one that can take a negative value, regardless of how preposterous that might seem, just to see if we can find one that fits and try to make sense of it.

It seems to my uneducated brain that Dark Matter hypothesis is essentially identical to the planet Vulcan hypothesis, i.e. we assume we understand gravity so we explain weird behavior with mass we didn't yet detect (only this time we cheat by hypothesizing the mass is undetectable). Also, we know of forces that don't abide by inverse square laws and can either effectively pull or push depending on the distance (nuclear forces). So we have a precedent of us being wrong about gravity (despite prior experimental success e.g. finding of Neptune) and examples of forces that behave differently to gravity... so maybe let's just assume gravity grossly defies our intuition, try to match a formula to observations and see if we can figure it out from there?

Sorry if it's not the space for such amateur questions - please let me know where I could go with that. Thanks!

# EDIT

Just to clarify, I am specifically asking about **approaches that do not derive equations out of logical interpretations of the data, but out of the data directly**. As such, for example, MOND does not qualify because it derives from Newtonian interpretations and just adds an extra piece on top, inheriting all of the limitations of the Newtonian formulae (like the idea of gravity being always attracting, never repelling).

I am literally asking if somebody (**recently, as new measurements are made**) tried to feed bulk data like below to an array of approximation algorithms to see if any of them produces something that fits the data:

| Object position | Object mass | Apparent G vector |
| --- | --- | --- |
| $p\_0$ | $M\_0$ | $\vec v\_0$ |
| $p\_0$ | $M\_1$ | $\vec v\_1$ |

... where position and acceleration vector are in any coordinate system that makes calculations easier and objects are aggregated as needed (e.g. instead of 400 billion stars per galaxy, put clusters of stars as singular objects instead).

**Tags:** gravity, general-relativity, dark-matter

> **Comments:**
> **username_1**: Have you ever read into the history of the theoretical development of gravity? Literally so many people developed empirical relationships relating forces and distances (a similar history for Coulomb's law). Hooke was such a contemporary to Newton, but Newton had the mathematical know-how to formalize it with symbolic rigor. Your questions might be addressed better in the history of S&M stack exchange, for examples: https://hsm.stackexchange.com/questions/8094/how-close-was-hooke-to-developing-a-comprehensive-system-of-gravity  
> **username_1**: and https://hsm.stackexchange.com/questions/12915/did-einstein-know-about-laplaces-query-into-whether-gravity-is-instantaneous  
> **username_1**: Also, people have done what you suggest for the dark matter hypothesis. Many lines of research explore this, in much more sophisticated ways than you've proposed, e.g. see this old review https://www.cambridge.org/core/journals/european-astronomical-society-publications-series/article/abs/review-of-observational-evidence-for-dark-matter-in-the-universe-and-in-upcoming-searches-for-dark-stars/843789157604D5627BAB6119C5084333 I'll lastly add that the model known as MOND is another way of attempting this by modifying Newton's law. Dark matter is more of a paradigm than a theory, per say.  
> **username_2**: Already Einstein added the cosmological constant... which does a similar thing at large distances (but does not solve all gravity-related problems either)... yet much more rigor and approaches have been applied to this issue in quite open-minded ways than you seem to assume. The problem is that a theory has to explain quite a lot, sometimes even so far contradictory observations. It's not just finding the right parameters for a known equation type. https://en.wikipedia.org/wiki/Cosmological_constant  
> **username_2**: In order to solve this mystery or problem it might even require to come up with a theory of quantum gravitation - a subject where everyone so far came up with a blank.  
> **username_3**: Relevant (and obligatory)  xkcd  https://xkcd.com/1758/  
> **username_4**: What does "approximation algorithm" mean? Are you saying, "pretend gravity can be represented by a really massive polynomial equation"? In which case, sure, with enough terms you can probably fit anything, but it may be terrible at predicting new data. And most people won't be happy with the idea that each galaxy (or galaxy group or galaxy cluster) operates according to its own "equation", which won't be applicable to cosmoloty...  
> **username_0**: @username_1 - I'm not asking about historical attempts, but recent ones, ones that came after we started finding issues with Dark Matter.  
> **username_0**: @username_2 - Cosmological Constant was a modification of an existing set of formulae. That's what I know was being attempted a lot and that's not what I am asking about. If there were "open-minded" approaches that failed, please link, that's what I'm asking for.  
> **username_0**: @username_3 - beautiful picture, but seriously - I'd love to see a paper that says "we tried to plot these values and come up with a dumb estimate of the function with no restrictions and here's the issues we found...". This is literally what I'm asking for.  
> **username_0**: @username_4 - If the "massive polynomial" converges to a compound of well known power series, then we could compose the formula from what it seems to converge to and check if that's what works. If it truly turns out that every galaxy has its own function then this will be evidence for Dark Matter, i.e. that variables are needed beyond the masses we can see. That's my question: did we try letting masses we see explain everything we see, but without any assumptions on the form of the formulae whatsoever, i.e. not starting off from Newton's equations or any other logical background.  
> **username_5**: As I point out in my answer - this is exactly how MOND is formulated, by empirically matching the "gravity" required to explain the observations of the motions of objects in the small acceleration regime, No formula for gravity will disagree with Newton's laws (because they work) other than in the very strong or very weak gravitational regimes, so all simple formulations *will* be "based on" inverse-square Newtonian gravity outside of those regimes.  
> **username_0**: @username_5 Not "exactly". MOND, based on the first couple of paragraphs of its entry on Wikipedia, is an attempt to fit a formula to a mechanistical hypothesis: "(...) Milgrom noted that this discrepancy could be resolved if the gravitational force experienced by a star (...) was proportional to the square of its centripetal acceleration (...) or (...) if gravitational force came to vary inversely linearly with radius (...)". In other words, it seems that a mechanism was proposed first, then a formula that reflects the mechanism and then the search for parameters begins....  
> **username_0**: @username_5 ... but what I'm asking is: "have we tried to calculate the apparent gravitational acceleration affecting stellar objects based on how much mass we are able to see in their vicinity, WITHOUT presupposing a mechanism that would dictate the formula". The way I see it is that Newton nailed the formulae that works in some spectrum of conditions, Einstein seems to have had a good guess at the mechanism behind gravity that gave him somewhat better formulae, but perhaps we can find even better formulae by just looking at data, without even trying to guess the mechanism.  

---

## 3 answers

### Answer by username_5 on 2021-06-04. Score: 3

Yes, this is pretty much exactly how [Modified Newtonian Dynamics (MOND)](https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics#:%7E:text=Modified%20Newtonian%20dynamics%20(MOND)%20is,currently%20understood%20laws%20of%20physics.) works. It is observed that Newtonian mechanics works very well for large accelerations, but an *ad hoc* correction function to Newtonian gravity is proposed at small accelerations. This function can be tuned such that the rotation curves of galaxies can be explained without the need for dark matter.

Edit in response to edited question:

Note that explaining rotation curves is *of course* only one empirical constraint. And MOND does poorly at matching others.

If you are looking for some universal gravitational law that explains all possible empirical observations of stellar and galaxy motions (which is only one of the constraints any theory must satisfy), without dark matter, then I'm afraid there isn't one (afaik).

Any approach to a universal theory of how gravity works must *of course* be identical (or at least similar to a very great extent) to inverse-square Newtonian gravity outside the regime of very strong or very weak accelerations (as indeed General Relativity is), because there is a wealth of observational evidence that shows simple Newtonian gravity works very well in those cases.

> **Comments:**
> **username_0**: I edited my question to clarify why MOND is not what I was asking for - thanks :)  
> **username_5**: @username_0 but it is what you are asking about. Repulsive gravity is not required  
> **username_0**: Well, MOND does not explain several things Dark Matter does from what I heard. Obviously I'm too dumb (and don't have the time) to read proper scientific papers but I do listen to scientific videos/podcasts (while I work) and e.g. based on this video https://www.youtube.com/watch?v=dtfEzDAlL5k (9:23-12:20) it seems that MOND simply replaces one issue with another that is better solved by Dark Matter. So it doesn't get it all right. Also, it is another theory based on something, instead of a clean slate.  

---

### Answer by username_6 on 2021-06-04. Score: 1

In the emergent gravity theories, dark matter is thought to be inherent to normal matter and spacetime. It's a consequence of the interplay between conformal fields in 5d anti-de Sitter spacetime and the 4d-spacetime (ours) it encloses. This theory (contrary to MOND) is there even if (conjectured) dark matter-induced motion wasn't observed. In this respect, it is preferable to dark matter theories, which indeed cause Vulcan-like situations. See for example [this](https://arxiv.org/abs/1611.02269) article. Even normal gravity itself is explained in this way.

> **Comments:**
> **username_0**: Emergent Gravity seems very appealing to me and I very much like the section 8.1 of the paper you linked to: "the evidence in favour of dark matter is just as much evidence for the possible breakdown of the currently known laws of gravity". Spot on. However, this paper conjures a massive underlying theory to come up with a set of equations, whereas I asked if there was an attempt at just getting equations out of the data and seeing if it's at all possible. EG also seems to be strongly disputed, though I'm unqualified to opine on the arguments (see EG page on wiki).  

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### Answer by username_6 on 2021-06-04. Score: 0

I'm not sure if it's appropriate to add another answer but as the question has been edit to make its intention more clear, I suppose it is.

Let me explain why this is not the way it's done. To simplify the situation, consider the motion of a particle that falls down on Earth. One can take a number of these particles, with various masses and do measurements on these.

Somewhat like Galileo once did in his tower experiment. Galileo, who said that the only way to read the universe was to learn the language of mathematics, is thought to have thrown two massive spheres from Pisa's tower and thereby showed that all masses fall in the same way. The spheres reached the ground in the same time, hence the conclusion. The experiment falsified Aristotles' conjecture that the falling of objects depend on their mass. If two objects are dropped the heavier would reach the floor earlier as the light one, according to him. Now this is indeed the case for many different objects when you let them fall on the ground. A feather will end up with less velocity than a small lead ball with the same mass. Heavy objects, compared to light ones of comparable size, take generally less time to reach the Earth than light ones (because of air resistance). Aristotle didn't know anything about air resistance (like we don't know a dark matter) and because of this he thought gravity acting differently for different masses.

The (alleged) measurements of falling times done by Galileo were done though after the workings of gravity were already anticipated. The measurements were an experiment to confirm (or contradict) these.

Now he could have done a lot of experiments on a lot of falling objects. He could have used different masses and different shapes, different initial velocities and positions, and performed the experiment at different locations and different circumstances. This would have resulted in a huge amount of data: rows of initial positions, initial velocities, rows of masses and shapes, rows of times, velocities, and positions, rows of circumstances, and rows of locations. If the data were ordered in the right way (for which no theoretical assumption about gravity has to be made), and if he had quantum super computer like capabilities, he could search for a connection between the data. How to find the algorithm? Well, he could have made a visualization of the data. If he had the mathematical knowledge of today (differentiable functions only saw the daylight after the birth of Newton) he could use his knowledge of the graphic form of functions to see if any of them (or a function of a function, say a sum) matched the data. If he had found any functional correspondence he could have started theorizing about gravity (or the circumstances of the experiment).

I doubt that this is done in practice. Now this is exactly your question, but I merely answer to state that this way of proceeding isn't followed in practice. Pure naked observation cannot be detached from theory. That is, empiricism and theory cannot be separated. Even in the case of measuring times, distances, positions, velocities, or masses, you already make a theoretical assumption (for example, that all velocities refer to displacement in space, while it could be that space itself expands).

Now, as seen, it *could* be done in this way. That is, measuring masses, velocities, and positions (and distances) of different celestial objects, after which you look for a functional relationship between them, but mostly it's done the other way round. On top of that, it would be very time-consuming, although a computer could do the job. So in the case of dark matter, the computer will spit out a functional relation between the data (the one that fits these data best). This relationship will be undisputable (though this is disputable!). To find out how these functional relationships between the data comes about a theory is needed. This can change the very Nature of the quantities that you measured (like mass, velocity, and space), thereby changing the Nature of your assumptions made in the measuring (like space time being Euclid).

So again, it *could* be done, but to separate empiricism completely from theory is impossible.

> **Comments:**
> **username_3**: You can use a blank line to separate paragraphs, This would make your answer easier to read.  
> **username_6**: @username_3 You edit at the same time I did! Thanks.  
> **username_0**: So in short, the reason this is not done is because our measurements are affected by our understanding of the subject matter, i.e. how we calculate the apparent gravitational pull to feed into the approximation algorithm will be affected by e.g. General Relativity.  
> **username_0**: continued cause I missed the edit window :P:   That's a fair point, but not a reason not to try it. Applying a function on the measurements does not remove the relation. For example, if you pick a number of pairs (X,Y) where X=Y, relationship will approximate to linear. If you square all Ys and approximate again, the relationship will be the square function. If the underlying data is in a strict relationship and affected uniformly by some continuous function, the modified data is still in a strict relationship, just modified.  
> **username_6**: @username_0 Yes, exactly. you can make a list of data (as you have done in your question), but the interpretation of these data involves theoretical assumptions about the data. The data cannot exist on their own. You *can* let a computer do the job of finding data (by using a telescope connected to a computer) and finding the appropriate algorithm (function) relating the data. But the interpretation involves a theory of ours (maybe the computer can find a theory too, but then it has to be very advanced!). : D  
> **username_6**: @username_0 Isn't this massive data gathering done? with computers? I'm not sure if trying to find an algorithm is done.  
> **username_0**: @Barbierium - I'm not exactly sure what you're trying to say, sorry. The process I would expect is: gather data, perform an unrestricted approximation of the relationship, hypothesize on the theory behind the relationship. If some preexisting theory skews the data, then it is fair to assume that it is in the form of some more or less regular function that would just compound on top of the resulting relationship and which can be untangled later. And even if it can't and it all fails - I just want to know if it has been tried. One paper that says "we tried - it failed" is all I need.  
> **username_6**: @username_0 As I wrote, why should you do it like this if it's easier to find a theory by other means? If it's a computer that gives you the algorithm, that is, the function that makes the connection between the data (the numbers) , then it would still be us who endow the data with theory. The real way doesn't correspond to a 1-1 relationship between data and theory. By which I mean that in the real world observation and theory "alternate".  You *could* observe falling bodies and write down all positions, velocities, times, but this is simply not how a theory of falling bodies is found.  
> **username_0**: Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/126122/discussion-between-username_0-kolodziejek-and-barbierium).  

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