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>We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold [citation needed] The consensus is that gravity - outside of ex

>We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold [citation needed] The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places. Emphasized part added because people in the replies thought that I literally think that General Relativity is somehow wrong. Don't be dense. All I'm saying is that gravity at galactic scales works as Newton described it. General Relativity has extremely tiny effect at those scales.

hobs 2y ago
When you say "outside of" - that's the thing where it doesn't hold. It's interesting and not even wrong to say "these rules work in these contexts" but as far as I can tell we're looking for the scenario invariant rules.
samsartor 2y ago
https://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury https://en.wikipedia.org/wiki/Tests_of_general_relativity#Pe... is the example I learned in school. You don't need to be around a black hole for GR to suddenly switch on. Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.
- meindnoch 2y ago
  Nobody said that general relativity is "switched on" around black holes. But ok, let me put it this way: outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe.
  - DiogenesKynikos 2y ago
    The inaccuracy of the Newtonian theory of gravity is large enough that it was already noticed by astronomers in the mid-1800s.
  - exe34 2y ago
    that's like saying the visible mass of the universe is 99% hydrogen and helium, so we don't need to learn about chemistry.
    - meindnoch 2y ago
      So you're saying we should model galaxies down to the level of individual protons? Lol. Galactic dynamics is governed by gravity, which is Newtonian at those scales.
      - exe34 2y ago
        No I did not say that.
        meindnoch 2y ago
        Ok, then how does your chemistry comment have anything to do with the motion of galaxies? Reminder: you're commenting on an article about MOND, which is a theory that stems from trying to explain the motion of galaxies.
        exe34 2y ago
        > outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe. I meant it in the sense that "most of the cosmos runs on Newtonian gravity, therefore we can ignore GR" is similar to "most of the visible matter in the cosmos is hydrogen/helium, so we can ignore chemistry". The interesting part is in the 0.0000001% that isn't like the others.
  - samsartor 2y ago
    Not for all intents and purposes. If we are asking whether MOND is useful, then the answer is probably yes. You might use it for simulations of galaxy formation where Newtonian gravity is considered a reasonable approximation today. But MOND is not a correct model of the universe. There is no place in the universe that Newtonian gravity applies, only places where the error is an acceptable trade-off for simpler calculation.
    - meindnoch 2y ago
      By the same logic, there's no place in the universe that general relativity applies either, since it breaks down at the quantum level. There's no place in the universe where any theory other than the one true grand unified theory applies, because everything else is just an approximation. At which point we're just arguing about semantics, and I don't see a reason for continuing it on my part.
      - radishingr 2y ago
        There are vastly different scales where the approximation is correct for newton vs general relativity. Perhaps you can define the scales that you are calling relevant so we understand what you mean.
        meindnoch 2y ago
        The scale of galaxies? Which the original article is about? I feel like I need to spell out everything, but ok: The article is about modified Newtonian dynamics (MOND), which is a theory that modifies Newtonian gravitation to fix some observed differences in galaxies' motion, without invoking dark matter. The original commenter then proclaims "haha, MOND cannot be right, because we know that Newtonian gravity is incorrect". Yeah, no shit Sherlock; it is "incorrect" because it is just a limiting case of general relativity. But that's completely besides the whole point of MOND, which tries to "fix" gravity at galactic scales, which is a Newtonian regime even with general relativity. MOND is trying to tweak the Newtonian formula at those extreme distances, and if it works, then maybe it can be worked out to be a limiting case of a "modified general relativity", just as Newtonian gravity is a limiting case of GR. Got it?
  - radishingr 2y ago
    So spacetime (interactions between mass, space, and time) are required for any sort of precision explanation. If "extreme" means planet size masses, I guess, but I generally consider our solar system pretty normal. However we cannot explain the planetary motion of mercury without relativity, so define your extreme. But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.
  - bobmcnamara 2y ago
    My first thought was that we only know Cavendish's constant to a little over 4 significant figures, so how could this be right? The relativistic effects at Earth's surface would change this by only ~10^-8, so I think the challenge in refining the Cavendish gravitic constant lie elsewhere.
superjan 2y ago
These extremes exist, and GR predictions are better than Newton’s in those cases. Closest to home is mercury’s perihelion drift. We have observed black hole mergers, gravitational lensing, and GR is also an essential component in understanding the universe’s expansion(that we know from redshift and the CMB). Likely MOND will address these, but Newtonian mechanics will not get you there.
auntienomen 2y ago
Citation needed? That's ridiculous. The empirical evidence is well over century old at this point. Start with the anomalous precession of Mercury's perihelion. That already can't be accounted for by Newtonian gravity.
- bobmcnamara 2y ago
  I don't think they're saying the relativistic effects don't exist, just that they're still largely unimportant compared to Newtonian effects. For precession of perihelion of Mercury we mostly noticed because any error is cumulative over time and we could integrate over an arbitrarily wide timebase. The relativistic effects are <10^-8 of the total, around 1/10th of the change imparted by Newtonian gravity of planets much, much further away. The BepiColombo orbiter should allow us to correct for the relativistic effects of other planets' pull on Mercury, but it's expected to be a change of <10^-12. So I guess "many, many decimal places" is in the ballpark of 6-12.
- ahazred8ta 2y ago
  Samsartor seems to think that the inverse square law does not hold at short distances (e.g. between the sun and mercury). Meindnoch agrees with mainstream physics that the inverse square law does indeed hold at short distances. You're confusing newtonian physics (busted) with the inverse square strength of gravity (still strongly supported); those are two different things. GR says gravity should be strictly 1/r^2, and this is what we observe in the solar system.
  - Iwan-Zotow 2y ago
    "GR says gravity should be strictly 1/r^2, and this is what we observe in the solar system" huh?!? there are GR corrections to Newton which include terms like 1/r^3 iirc
    - ahazred8ta 2y ago
      There are (ȓ/r^3) terms involving unit vectors, but that works out to 1/r^2 in practice. There are cubed terms in string theory and Quantum General Relativity (QGR) / Loop Quantum Gravity, but these do not apply at macroscopic distances. If you know of a url link to a non-theoretical inverse-cube effect which has actually been confirmed in lab experiments or actual observation, please post it.
EPWN3D 2y ago
You're simply wrong. There's no other way to put it. The GPS system would have been simply impossible to deploy without the general theory of relativity. There's no extreme energy or mass involved, just precision requirements that are influenced by the minuscule differences in time experienced by the surface of the earth and orbiting satellites. Also Newton's laws famously could not account for Mercury's orbit. Mercury is just an ordinary planet orbiting an ordinary star. Nothing extreme is involved. He knew his laws were incomplete. But they were so dead-on in basically every other scenario that could be physically observed at the time that he figured there was some small tweak missing (or maybe another planetary body that hadn't been spotted yet).
- meindnoch 2y ago
  Easy there champ. Noone is shitting on general relativity. All I'm saying is that the effect of general relativity at galactic scales is so minuscule, that galactic dynamics is - for all intents and purposes - governed by the Newtonian limit of gravity. If you propose that gravity doesn't behave like the Newtonian limit at those scales, then you're contradicting general relativity as well, since the far-field limit of the Schwartzschild metric is literally Newton's inverse square law. In layman terms, modified Newtonian gravity, that the article talks about, is an attempt to explain why galaxies don't rotate the way they should according to Newton (and Einstein, because at those distances the two are the same!!!).
  - jfengel 2y ago
    I had the impression that "shitting on general relativity" was exactly what MOND was about. That is, it starts from the position that Einstein is wrong, and searches for ways to support that.
    - meindnoch 2y ago
      The Wikipedia article on MOND literally starts with galaxy rotation curves: https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics There's zero mention of MOND being a rejection of general relativity. OF COURSE, any tweaking of Newton's formula at galactic scales will necessarily invalidate general relativity, since general relativity predicts Newton's formula at those scales! But MOND tries to work backwards: they propose a modification of the far-field Newtonian formula, and the belief is that it can eventually be worked out to be a limiting case of a "modified general relativity", for lack of a better name. Just how Newtonian gravity was eventually worked out to be a limiting case of a theory called general relativity.
wbl 2y ago
We can see gravitational redshift on Harvard's campus thanks to gamma ray Mossbauer spectroscopy.
nimish 2y ago
>The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places. It absolutely does not. Newtonian gravity occurs instantly. It has no notion of information taking time to propagate. But we know gravitational waves happen, so Newtonian gravity is wrong _at even very large scales_. If the sun disappeared Newton tells us we'd know immediately. In GR we'd know about 8 min later. The bigger problem is not that the quantitative effect is large, but that the _qualitative_ difference of going from the instantaneous effect to one that needs to propagate is enormous. It's the whole point of relativity as a concept. Even going to GEM as a true, non-singular linear approximation of GR would be a step up from Newton's laws, at least there we can have gravitational waves and causal flow of information.
- the__alchemist 2y ago
  Thanks for bringing this up; this is the central reason why I'm skeptical of Newtonian models that predict dark matter, and why I don't think the term MOND makes sense as the simplest alternative.

hobs 2y ago

When you say "outside of" - that's the thing where it doesn't hold. It's interesting and not even wrong to say "these rules work in these contexts" but as far as I can tell we're looking for the scenario invariant rules.

samsartor 2y ago

https://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury https://en.wikipedia.org/wiki/Tests_of_general_relativity#Pe... is the example I learned in school. You don't need to be around a black hole for GR to suddenly switch on. Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.

meindnoch 2y ago
Nobody said that general relativity is "switched on" around black holes. But ok, let me put it this way: outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe.
- DiogenesKynikos 2y ago
  The inaccuracy of the Newtonian theory of gravity is large enough that it was already noticed by astronomers in the mid-1800s.
- exe34 2y ago
  that's like saying the visible mass of the universe is 99% hydrogen and helium, so we don't need to learn about chemistry.
  - meindnoch 2y ago
    So you're saying we should model galaxies down to the level of individual protons? Lol. Galactic dynamics is governed by gravity, which is Newtonian at those scales.
    - exe34 2y ago
      No I did not say that.
      - meindnoch 2y ago
        Ok, then how does your chemistry comment have anything to do with the motion of galaxies? Reminder: you're commenting on an article about MOND, which is a theory that stems from trying to explain the motion of galaxies.
        exe34 2y ago
        > outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe. I meant it in the sense that "most of the cosmos runs on Newtonian gravity, therefore we can ignore GR" is similar to "most of the visible matter in the cosmos is hydrogen/helium, so we can ignore chemistry". The interesting part is in the 0.0000001% that isn't like the others.
- samsartor 2y ago
  Not for all intents and purposes. If we are asking whether MOND is useful, then the answer is probably yes. You might use it for simulations of galaxy formation where Newtonian gravity is considered a reasonable approximation today. But MOND is not a correct model of the universe. There is no place in the universe that Newtonian gravity applies, only places where the error is an acceptable trade-off for simpler calculation.
  - meindnoch 2y ago
    By the same logic, there's no place in the universe that general relativity applies either, since it breaks down at the quantum level. There's no place in the universe where any theory other than the one true grand unified theory applies, because everything else is just an approximation. At which point we're just arguing about semantics, and I don't see a reason for continuing it on my part.
    - radishingr 2y ago
      There are vastly different scales where the approximation is correct for newton vs general relativity. Perhaps you can define the scales that you are calling relevant so we understand what you mean.
      - meindnoch 2y ago
        The scale of galaxies? Which the original article is about? I feel like I need to spell out everything, but ok: The article is about modified Newtonian dynamics (MOND), which is a theory that modifies Newtonian gravitation to fix some observed differences in galaxies' motion, without invoking dark matter. The original commenter then proclaims "haha, MOND cannot be right, because we know that Newtonian gravity is incorrect". Yeah, no shit Sherlock; it is "incorrect" because it is just a limiting case of general relativity. But that's completely besides the whole point of MOND, which tries to "fix" gravity at galactic scales, which is a Newtonian regime even with general relativity. MOND is trying to tweak the Newtonian formula at those extreme distances, and if it works, then maybe it can be worked out to be a limiting case of a "modified general relativity", just as Newtonian gravity is a limiting case of GR. Got it?
- radishingr 2y ago
  So spacetime (interactions between mass, space, and time) are required for any sort of precision explanation. If "extreme" means planet size masses, I guess, but I generally consider our solar system pretty normal. However we cannot explain the planetary motion of mercury without relativity, so define your extreme. But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.
- bobmcnamara 2y ago
  My first thought was that we only know Cavendish's constant to a little over 4 significant figures, so how could this be right? The relativistic effects at Earth's surface would change this by only ~10^-8, so I think the challenge in refining the Cavendish gravitic constant lie elsewhere.

superjan 2y ago

These extremes exist, and GR predictions are better than Newton’s in those cases. Closest to home is mercury’s perihelion drift. We have observed black hole mergers, gravitational lensing, and GR is also an essential component in understanding the universe’s expansion(that we know from redshift and the CMB). Likely MOND will address these, but Newtonian mechanics will not get you there.

auntienomen 2y ago

Citation needed? That's ridiculous. The empirical evidence is well over century old at this point. Start with the anomalous precession of Mercury's perihelion. That already can't be accounted for by Newtonian gravity.

bobmcnamara 2y ago
I don't think they're saying the relativistic effects don't exist, just that they're still largely unimportant compared to Newtonian effects. For precession of perihelion of Mercury we mostly noticed because any error is cumulative over time and we could integrate over an arbitrarily wide timebase. The relativistic effects are <10^-8 of the total, around 1/10th of the change imparted by Newtonian gravity of planets much, much further away. The BepiColombo orbiter should allow us to correct for the relativistic effects of other planets' pull on Mercury, but it's expected to be a change of <10^-12. So I guess "many, many decimal places" is in the ballpark of 6-12.
ahazred8ta 2y ago
Samsartor seems to think that the inverse square law does not hold at short distances (e.g. between the sun and mercury). Meindnoch agrees with mainstream physics that the inverse square law does indeed hold at short distances. You're confusing newtonian physics (busted) with the inverse square strength of gravity (still strongly supported); those are two different things. GR says gravity should be strictly 1/r^2, and this is what we observe in the solar system.
- Iwan-Zotow 2y ago
  "GR says gravity should be strictly 1/r^2, and this is what we observe in the solar system" huh?!? there are GR corrections to Newton which include terms like 1/r^3 iirc
  - ahazred8ta 2y ago
    There are (ȓ/r^3) terms involving unit vectors, but that works out to 1/r^2 in practice. There are cubed terms in string theory and Quantum General Relativity (QGR) / Loop Quantum Gravity, but these do not apply at macroscopic distances. If you know of a url link to a non-theoretical inverse-cube effect which has actually been confirmed in lab experiments or actual observation, please post it.

EPWN3D 2y ago

You're simply wrong. There's no other way to put it. The GPS system would have been simply impossible to deploy without the general theory of relativity. There's no extreme energy or mass involved, just precision requirements that are influenced by the minuscule differences in time experienced by the surface of the earth and orbiting satellites. Also Newton's laws famously could not account for Mercury's orbit. Mercury is just an ordinary planet orbiting an ordinary star. Nothing extreme is involved. He knew his laws were incomplete. But they were so dead-on in basically every other scenario that could be physically observed at the time that he figured there was some small tweak missing (or maybe another planetary body that hadn't been spotted yet).

meindnoch 2y ago
Easy there champ. Noone is shitting on general relativity. All I'm saying is that the effect of general relativity at galactic scales is so minuscule, that galactic dynamics is - for all intents and purposes - governed by the Newtonian limit of gravity. If you propose that gravity doesn't behave like the Newtonian limit at those scales, then you're contradicting general relativity as well, since the far-field limit of the Schwartzschild metric is literally Newton's inverse square law. In layman terms, modified Newtonian gravity, that the article talks about, is an attempt to explain why galaxies don't rotate the way they should according to Newton (and Einstein, because at those distances the two are the same!!!).
- jfengel 2y ago
  I had the impression that "shitting on general relativity" was exactly what MOND was about. That is, it starts from the position that Einstein is wrong, and searches for ways to support that.
  - meindnoch 2y ago
    The Wikipedia article on MOND literally starts with galaxy rotation curves: https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics There's zero mention of MOND being a rejection of general relativity. OF COURSE, any tweaking of Newton's formula at galactic scales will necessarily invalidate general relativity, since general relativity predicts Newton's formula at those scales! But MOND tries to work backwards: they propose a modification of the far-field Newtonian formula, and the belief is that it can eventually be worked out to be a limiting case of a "modified general relativity", for lack of a better name. Just how Newtonian gravity was eventually worked out to be a limiting case of a theory called general relativity.

wbl 2y ago

We can see gravitational redshift on Harvard's campus thanks to gamma ray Mossbauer spectroscopy.

nimish 2y ago

>The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places. It absolutely does not. Newtonian gravity occurs instantly. It has no notion of information taking time to propagate. But we know gravitational waves happen, so Newtonian gravity is wrong _at even very large scales_. If the sun disappeared Newton tells us we'd know immediately. In GR we'd know about 8 min later. The bigger problem is not that the quantitative effect is large, but that the _qualitative_ difference of going from the instantaneous effect to one that needs to propagate is enormous. It's the whole point of relativity as a concept. Even going to GEM as a true, non-singular linear approximation of GR would be a step up from Newton's laws, at least there we can have gravitational waves and causal flow of information.

the__alchemist 2y ago
Thanks for bringing this up; this is the central reason why I'm skeptical of Newtonian models that predict dark matter, and why I don't think the term MOND makes sense as the simplest alternative.