Talk:Alternatives to general relativity

new Wikipedia page

Welcome to a new Wikipedia page. I started it after finding that Wikipedia articles on gravity and alternatives to General Relativity were a complete mess, with several excellent articles (eg. on Brans-Dicke), many substandard entries, and nothing like a proper index.

As I write this the page is still under construction. I've still to add the rest of the equations, a big table of PPN results, add footnotes, add all the cross-references within, from and to the article, and fix the glaringly obvious faults.

So please don't edit the page until 2 Sept 2006. After that, go for it! Mollwollfumble 01:09, 25 August 2006 (UTC)

My advice is to keep plugging away on this article. In both content and title it is much better than classical theories of gravitation. --EMS | Talk 03:12, 31 August 2006 (UTC)

Suggestions for improvement

Hi there. I would really like to see this article improve, as it has the potential to be a very good article indeed. That's not to say it's not a good artile at the moment. I would say that, overall, most (all ?) of the important information is already present in the article (good work Mollwollfumble!). The only question is one of making the article much better (by Wikipedia's rigorous standards). I would like to suggest the following improvements/points for discussion:

• The 'Classification of theories' (section 4.1) should come before the 'Early theories' section (number 3).

• Having thus classified the theories, is it better to group the theories in chronological order or by type (bimetric theories, scalar theories etc...) ? My preference is by type; this should be discussed though.

• The 'Testing of alternatives to GR' section could be split of into another article. Thoughts ?

• There should perhaps be main article links to the articles that describe the theories in more detail (as there is for the 'Testing of alternatives to GR' section) - this is mainly for better and more consistent presentation.

These are just a few ideas off the top of my head that I felt should be worth discussing to improve the article. Mollwollfumble, you have done some very good work in presenting details of Lagrangians etc... and finding all those references. I hope more editors will be encouraged to help improve this article. Thanks for reading this. MP (talk) 20:00, 1 April 2007 (UTC)

This appears under "Cosmological Tests": With GR, the combination of baryonic matter, dark matter and dark energy add up to make the universe exactly flat. " I'm not an expert, but last time I looked, a positive cosmological constant was confirmed with high confidence. —Preceding unsigned comment added by Fairandbalanced (talk • contribs) 04:57, 30 July 2008 (UTC)

Process Physics

The section for Process Physics, under "Other Modern Theories", seems to have been removed for the reason that it has not been published in well-known notable journals. May I suggest you read Process Physics, including it's talk section, and reconsider whether it belongs here or not. There are several scientific articles referenced on that page which would definitely be worth a look.

I personally think it at least needs to be mentioned as an alternative theory, with a link to the main article. I'll leave it up to you as to what you want to do with this. Bill Killed The Unicorns

As far as I see, process physics is in the category "fringe physics" and the "Fringe subjects without critical scientific evaluation". Ok, I'm also interested in alternative theories, but what do you think is happening if every non-mainstream theory, which is not published in reputable sources, will be included in the article? For example, look at Heim theory and so on. --D.H 09:19, 4 May 2007 (UTC)

You make a good point, but since the article is "Alternatives to general relativity" it would be a bit biased to exclude some just becuase they don't have as much evidence behind them as some of the others mentioned here. Perhaps a better alternative would be to add a section entitled "other theories" or something like that, and then list there links to all other alternative theories, including those that have too little evidence behind them to merit a detailed explanation in this article. And then if one particular theory gets some better evidence to support it, it will be a reasonably simple process to add a new section to this article where it does get a detailed explanation. Bill Killed The Unicorns 02:50, 11 May 2007 (UTC)

Transported text

I've transferred the following text from Talk:General relativity/WIP#Alternatives to general relativity; that section has been considerably shortened, but the removed info. (i.e. the following text) may still be useful for this article:

• Nordström's theory of gravitation (1913) was one of the earliest metric theories (an aspect brought out by Einstein and Fokker in 1914). Nordström soon abandoned his theory in favor of general relativity on theoretical grounds, but this theory, which is a scalar theory, and which features a notion of prior geometry, does not predict any light bending, so it is solidly incompatible with observation.
• Alfred North Whitehead formulated an alternative theory of gravity that was regarded as a viable contender for several decades, until Clifford Will noticed in 1971 that it predicts grossly incorrect behavior for the ocean tides.
• George David Birkhoff's (1943) yields the same predictions for the classical four solar system tests as general relativity, but unfortunately requires sound waves to travel at the speed of light. Thus, like Whitehead's theory, it was never a viable theory after all, despite making an initially good impression on many experts.
• Like Nordström's theory, the gravitation theory of Wei-Tou Ni (1971) features a notion of prior geometry, but Will soon showed that it is not fully compatible with observation and experiment.
• The Brans-Dicke theory and the Rosen bimetric theory are two alternatives to general relativity which have been around for a very long time and which have also withstood many tests. However, they are less elegant and more complicated than general relativity, in several senses.
• There have been many attempts to formulate consistent theories which combine gravity and electromagnetism. The first of these, Weyl's gauge theory of gravitation, was immediately shot down (on a postcard) by Einstein himself, who pointed out to Hermann Weyl that in his theory, hydrogen atoms would have variable size, which they do not. Another early attempt, the original Kaluza-Klein theory, at first seemed to unify general relativity with classical electromagnetism, but is no longer regarded as successful for that purpose. Both these theories have turned out to be historically important for other reasons: Weyl's idea of gauge invariance survived and in fact is omnipresent in modern physics, while Kaluza's idea of compact extra dimensions has been resurrected in the modern notion of a braneworld.
• The Fierz-Pauli spin-two theory was an optimistic attempt to quantize general relativity, but it turned out to be internally inconsistent. Pascual Jordan's work toward fixing these problems eventually motivated the Brans-Dicke theory, and also influenced Richard Feynman's unsuccessful attempts to quantize gravity.
• Einstein-Cartan theory includes torsion terms, so it is not a metric theory in the strict sense.
• Teleparallel gravity goes further and replaces connections with nonzero curvature (but vanishing torsion) by ones with nonzero torsion (but vanishing curvature).
• The Nonsymmetric Gravitational Theory (NGT) of John W. Moffat is a dark horse in the race.

MP ^{(talk•contribs)} 20:58, 29 November 2007 (UTC)

How about just tweaking the dependence of the action on the Riemann–Christoffel tensor?

In Einstein–Hilbert action, we see that the Einstein field equations follow from using a constant times the Scalar curvature as the gravitational Lagrangian. Suppose we just add a little bit of the square of the Riemann–Christoffel tensor, $R_{\beta \gamma \delta }^{\alpha }R_{\nu \rho \sigma }^{\mu }g_{\alpha \mu }g^{\beta \nu }g^{\gamma \rho }g^{\delta \sigma }\,$? Have any alternative theories like this been proposed or evaluated? JRSpriggs (talk) 18:22, 13 June 2008 (UTC)

I should have said something about why one would want to do this. I was wondering how we could modify general relativity to eliminate the problem of singularities. If intense gravitational fields created anti-gravity (similar to the cosmological constant), then that might do it. Adding a term like the one I mentioned (if one gets the sign right) might do it, I hope. JRSpriggs (talk) 14:55, 15 June 2008 (UTC)

I recently saw a mention of Einstein-Gauss-Bonnet gravity which apparently adds a small contribution for the square of the Riemann-Christoffel tensor and the square of the Ricci curvature and the square of the scalar curvature. I think that this article should cover EGB! JRSpriggs (talk) 06:42, 20 July 2008 (UTC)

Throwmeaway (talk · contribs) just created an article on Gauss-Bonnet gravity. JRSpriggs (talk) 07:16, 4 July 2010 (UTC)

New alternative gravity theory

There is a new gravity theory that has been published. It passes all experimental tests of GR, and in addition a corporation is using the new theory as the basis for their alternative energy tests. This theory should really be listed on this page... even if it is just a link to the theory. The theory is called Gravity Theory Based on Mass-Energy Equivalence. Gravityforce (talk) 22:54, 20 November 2008 (UTC)

J. W. Moffat?

I can't find the article for J. W. Moffat. Anybody know what it is listed under?

Moffat is of no help. FX (talk) 20:02, 4 February 2009 (UTC)

Is John Moffat (physicist) the right one? - Eldereft (cont.) 20:24, 4 February 2009 (UTC)

Yes! Thanks. FX (talk) 03:58, 5 February 2009 (UTC)

Nasa

Which formula does Nasa find most accurate when calculating orbits etc? —Preceding unsigned comment added by 86.135.98.182 (talk) 01:27, 9 April 2009 (UTC)

I do not know what they do, but I would guess that they use Parameterized post-Newtonian formalism with parameters which match General Relativity itself. But the general relativistic corrections are probably small compared to the corrections needed to deal with the Sun, Moon, and planets, and mass concentrations in the Earth (e.g. mountains), and special relativity. JRSpriggs (talk) 04:07, 9 April 2009 (UTC)

Why is this article such a mess?

I thought I'd fix the blatantly incorrect capitals in section headings. But then I saw tons of cases of "displayed" TeX with no identation. Then I found a sentence that said "I'm not worrying about $\lambda$, it's discussed [later in the article]". Sigh........ Can someone be capable of understanding physics but at the same time think a sentence like that can belong in a Wikipedia article? How did such a long article manage to avoid the attention of anyone familiar with the basics of WP:MOS and WP:MOSMATH? I've put a "cleanup" tag on it. More work is obviously needed. Michael Hardy (talk) 22:27, 16 October 2009 (UTC)

Just one example

I found this:

${\begin{matrix}S_{S}=-\int d^{4}x\,{\sqrt {-g}}({1 \over G^{3}}(\textstyle {\frac {1}{2}}g^{\mu \nu }\nabla _{\mu }G\nabla _{\nu }G-V(G))+{1 \over G}(\textstyle {\frac {1}{2}}g^{\mu \nu }\nabla _{\mu }\omega \nabla _{\nu }\omega -V(\omega ))\\\ +{1 \over \mu ^{2}G}(\textstyle {\frac {1}{2}}g^{\mu \nu }\nabla _{\mu }\mu \nabla _{\nu }\mu -V(mu)))\end{matrix}}$

I changed it to this:

${\begin{aligned}S_{S}&=-\int d^{4}x\,{\sqrt {-g}}{1 \over G^{3}}\left({\frac {1}{2}}g^{\mu \nu }\,\nabla _{\mu }G\,\nabla _{\nu }G-V(G)\right)\\&{}\qquad \qquad +{1 \over G}\left({\frac {1}{2}}g^{\mu \nu }\nabla _{\mu }\omega \,\nabla _{\nu }\omega -V(\omega )\right)+{1 \over \mu ^{2}G}\left({\frac {1}{2}}g^{\mu \nu }\,\nabla _{\mu }\mu \,\nabla _{\nu }\mu -V(\mu )\right)\end{aligned}}$

Does Wikipedia's physics community regard things like this as not worth bothering with? In physics articles generally, there seems to be less respect for conventions such as WP:MOS and WP:MOSMATH than there is in Wikipedia generally. Michael Hardy (talk) 20:10, 18 October 2009 (UTC)

I think it's a case of "wanting to spend time on formatting" and "wanting to spend time on physics" and "knowing LaTeX" being three uncorrelated qualities. This isn't a problem if there are enough editors that cleanup eventually happens, but we're a bit understaffed :). By all means make an editing pass if you like, and by all means ask people at WT:PHYS to vet it afterwards if you feel it would be useful for them to do so. --Christopher Thomas (talk) 21:32, 18 October 2009 (UTC)

In the formula: removing the m in mu at the end: the mu was an (non style related) error? Mpm (talk) 23:39, 14 February 2022 (UTC)

I moved your comment to the bottom. See Help:Using talk pages.

@Mpm: Note that there was not really an m removed in mu at the end. The mu was replaced with the Greek letter $\mu$. If that looks like a u, you might need to enlarge your font and/or set your Preferences, Appearance, Math to MathML- DVdm (talk) 09:37, 15 February 2022 (UTC)

Bitensor are more complex than GR

In the section Alternatives to general relativity#Classification of theories, the various types of metric theories are supposed to be listed from simplest to most complex. However, bitensor (η,g) theories are listed before tensor (g) theories (including GR) even though they are more complex than GR. JRSpriggs (talk) 13:49, 14 March 2011 (UTC)

Minor error

Text uses eta, table xi, for "check for preferred location effects". At least I think that's the problem. Too stupid to try to change it. 200.104.177.67 (talk) 20:05, 19 January 2013 (UTC)

If you look at Parameterized post-Newtonian formalism, you will see that there are two different sets of parameters used. One of them includes η and the other ξ. Does that answer your question? JRSpriggs (talk) 22:26, 19 January 2013 (UTC)

Why are quantum gravity and TOE's part of the lead?

They should not be. General relativity is understood to be theory that only works at distances large compared to the Planck length. To talk about general relativity or its alternatives is already to imply that we're talking about macroscopic distances, so an article about "alternatives to GR" should clearly mean theories that lead to different experimental consequences at observable scales. Quantum gravity theories/TOEs aren't "alternatives" to GR because GR doesn't make predictions for Planck-scale experiments. The rest of the text seems more or less wise to this fact, since I think they're only mentioned one other brief time. Im guessing this part of the lead should be rewrote to say something along the lines of what I just wrote. Isocliff (talk) 23:17, 26 April 2013 (UTC)

The lead merely mentions them to make it clear that this article does not cover them, and provide links to articles that do. Is that not just what you are asking for? Perhaps you should read the whole lead more carefully. JRSpriggs (talk) 06:44, 27 April 2013 (UTC)

Merger proposal

It seems to me that the article on Modified models of gravity should be merged into this article. The "Modified models of gravity" article is very short, doesn't really contain any information that's not found in "Alternatives to general relativity", and is not in fact a different topic. The worst part of having two separate articles is that readers of one aren't necessarily aware of the other's existence -- looking for the article on Modified Gravity I came first upon "Modified models of gravity" and have only recently discovered that this much more detailed article exists. HFD90 (talk) 02:13, 24 October 2014 (UTC)

Yes, I agree and will do this soon if no one disagrees. Absolutelypuremilk (talk) 14:35, 6 December 2015 (UTC)

I have done this now Absolutelypuremilk (talk) 14:18, 14 December 2015 (UTC)

Merger proposal with Jambalaya

It seems like a joke at first, but in fact this article is an abstract form of a Jambalaya of fringe items, incorrect items, unsourced meaningless statements, etc. So half seriously, should be done, and 80% of the material here removed. I am not really dead (talk) 19:01, 14 February 2016 (UTC)

It might be more helpful if you identified some of the incorrect items William M. Connolley (talk) 19:55, 14 February 2016 (UTC)

Yes, it would be helpful, but would be a very very long list. As a token, look at "Theories from 1917 to the 1980s" as a section. It is insane to lump over 70 years of rapidly changing physics together. Reflects a lack of understanding of the concepts. Unfortunately I do not have the time to do this, partly because I am not sure if it will survive the edits of some undergrad in 6 months. I will just leave it for whoever wants to spend the time. Sorry, I might be dead by the time I finish cleaning up the physics in Wikipedia.

By the way, a bigger problem is this one [1]. Misses the really key items. I am not really dead (talk) 21:24, 14 February 2016 (UTC)

Rename

I'd like to recommend that this article be renamed. These theories aren't necessarily defined by General Relativity. My feeling is that they should be described as Contemporary Gravitational Theories, or something similar. That would do more to establish them as potentially viable, and make it clearer that these are gravitational theories. That also would allow the article to focus on the motivations for alternatives, rather than being list-like in nature. In other words, it would have a more coherent theme, and would give the article more direction. — Preceding unsigned comment added by 70.247.163.128 (talk) 21:05, 30 August 2016 (UTC)

I disagree - GR is pretty widely accepted and most theories are examined as ways to explain the few things that GR does not. However, it could be good to include gravity in the title, what about "Alternative gravitational theories to General Relativity"? Bit clunky unfortunately though! Absolutelypuremilk (talk) 21:49, 30 August 2016 (UTC)

Indeed, the current title does not suggest that these theories are necessarily defined by general relativity. On the contrary.

I don't think the addition of "gravitational" would be needed, as gravitation is automatically associated with general relativity. And clucky indeed . - DVdm (talk) 06:36, 31 August 2016 (UTC)

I agree with this rename. GR contradicts observation. Unless "dark matter" turns out to be real. Jikybebna (talk) 13:02, 17 April 2022 (UTC)

This article should be called Alternative Theories of Gravity. A casual reader might not even know that GR is a theory of gravity. The oracle 2015 (talk) 14:09, 14 December 2023 (UTC)

Alternative Theories of Gravity The oracle 2015 (talk) 14:11, 14 December 2023 (UTC)

Article split

The article tries to cover two topics simultaneously: the historical alternatives to general relativity, and modern day modifications of general relativity. That's why the article's such a mess. Plus, since the article is defined itself so broadly, it lets people try and sneak in fringe theories which reject general relativity altogether. The article should be cleaned up and split into two new articles, Historical alternatives to general relativity, and Theories modifying general relativity. cnte ^(talk) 17:33, 22 March 2018 (UTC)

Well, it is really a list of notable theories of gravity. It shows what the competition is or has been for GTR. I do not see the benefit of narrowing the focus as you are effectively suggesting. There will always be people trying to sneak in fringe ideas. Better here than in some other places. JRSpriggs (talk) 01:27, 23 March 2018 (UTC)

It's not really a narrowing of focus. There's two different topics, the history of general relativity and the theories which people developed against it, and modern theories which attempt to modify general relativity. One's a topic about history, the other's a topic about modern theoretical physics, so they require different approaches and treatments. My suggestion *would* eliminate theories which outright contradict general relativity, but that's ridiculous. Wikipedia isn't the place for fringe theories, unless they're notable for some other reason, and they can always be included in a subsection. cnte ^(talk) 21:00, 23 March 2018 (UTC)

Virtually all theories of gravity other than general relativity itself and those theories which include it as a special case (for certain values of their parameters) have already been disproven either by experiment or by showing that they are self-contradictory. So this article is really about those "fringe" theories. If you take them out, there is nothing left. And they should be covered by Wikipedia, so that readers can know what they are and why they were rejected. JRSpriggs (talk) 21:31, 23 March 2018 (UTC)

Huh? As far as I know, theories which modify GR (to e.g., explain the dark force) still exist and are debated by physicists, not to mention string theory and loop gravity. They are not yet disproven, and these are qualitatively different from fringe theories which outright ignore parts of general relativity. And this is missing the point anyway, because the article still covers two different topics. Do historical alternatives to GR and modern modifications of GR really need to be crammed into the same article? On some level, they might be same thing, but from an encyclopedic point of view, they deserve separate treatment. cnte ^(talk) 08:53, 24 March 2018 (UTC)

I think the article is fine as it is. It would be pretty difficult to decide on any definitive way to split up between "Historical theories" and current theories. Absolutelypuremilk (talk) 21:03, 24 March 2018 (UTC)

Correct me if I am wrong, but no one knows how to calculate the perihelion precession of Mercury or the deflection of light by the Sun from either string theory or loop gravity. If the theory cannot make a unique prediction which can be tested, then that amounts to failing the test. Maybe the theories can be patched up someday, but that has not happened yet. JRSpriggs (talk) 06:48, 25 March 2018 (UTC)

String theory, LQG and whatever else is discussed have to reproduce general relativity in the limit of large distances. They all agree with the GR calculation for the perihelion precession of Mercury and similar tests of relativity. The differences appear where quantum mechanics becomes relevant (together with gravity). It is unclear how this region can be tested, but that doesn't mean it would be impossible. --mfb (talk) 08:42, 25 March 2018 (UTC)

Their proponents want them to approach general relativity as $\hbar \to 0$, but can they prove that they do? JRSpriggs (talk) 01:08, 26 March 2018 (UTC)

I would leave the article exactly as it is. 47.201.190.53 (talk) 13:12, 16 October 2018 (UTC)

LQM?

As far as I understand, loop quantum gravity seeks to merge relativistic effects with quantum mechanics, creating a theory that satisfies both. LQM doesn't compete with relativity; it accepts it as correct and synthesizes it with quantum mechanics. With this in mind, I don't quite see its relevance to this article, which concerns alternatives to the theory of relativity. — Preceding unsigned comment added by Micah von stauffenberg (talk • contribs) 22:29, 12 December 2018 (UTC)

Just like special relativity has classical mechanics as a limit LQG should have general relativity as a limit, but that is true for all alternatives discussed here: They all have to reproduce the predictions of general relativity where they have been tested. It is still a separate theory that tries to be more general. --mfb (talk) 15:05, 13 December 2018 (UTC)

Logunov's theory??

I could not find any reference to Logunov's theory of relativistic gravity (RTG).

162.211.38.110 (talk) 16:51, 28 July 2020 (UTC)

If you have any independent reliable sources describing Anatoly Logunov's theory and taking it seriously, please add a section to the article based on those sources. JRSpriggs (talk) 14:34, 29 July 2020 (UTC)

PPN parameters for Newton's gravity?

For completeness the table of PPN parameters should have the parameters for Newton's theory at the top before the parameters for Einstein's theory. I believe that they would be all zeros. Does anyone have a reliable source to back that up? JRSpriggs (talk) 16:21, 4 September 2020 (UTC)

My suggestion

Please see User:JRSpriggs/MOND. JRSpriggs (talk) 21:49, 19 March 2023 (UTC)

Effective Mass Theory of Gravity

[2]https://zenodo.org/records/10035511

Significant enough for inclusion in this article?

$Mss1={\frac {2\pi {R1}^{3}/3(1-cos(sin{^{-}}{^{1}}(R2/D)))}{\pi {R1}^{3}/3\cdot 4}}M1$

$Mss2={\frac {2\pi {R2}^{3}/3(1-cos(sin{^{-}}{^{1}}(R1/D)))}{\pi {R2}^{3}/3\cdot 4}}M2$

Then:

$F=(G\cdot {Mss1})\cdot (G\cdot {Mss2})$

Where:

G = 1.13832865e−9
F = Gravitational force (N)
M1 = Mass of body 1(kg)
M2 = Mass of body 2(kg)
Mss1 = Effective mass of body 1 (kg)
Mss2 = Effective mass of body 2 (kg)
D = Distance between bodies (m)
R1 = Radius of body 1 (m)
R2 = Radius of body 2 (m)

The theory basically dispenses with dark matter, general relativity, Newton’s law on gravity and the use of the inverse square. It also explains gravitational waves classically.

The oracle 2015 (talk) 23:17, 13 December 2023 (UTC)

Newer MOND theories

Copied out of the article

• the section doesn't include any of the newer relativistic MOND theories like bimetric MOND
  • Milgrom, Mordehai (10 October 2022). "Broader view of bimetric MOND". Physical Review D. 106 (8). doi:10.1103/PhysRevLett.127.161302.
• or Aether-scalar-tensor theory
  • Skordis, Constantinos; Zlosnik, Tom (18 November 2022). "Aether scalar tensor theory: Linear stability on Minkowski space". Physical Review D. 108 (10). doi:10.1103/PhysRevD.106.104041.

---

These are relatively new primary publications with few citations. We need to find a secondary source for these additions. Johnjbarton (talk) 19:51, 26 August 2024 (UTC)

See Wikipedia_talk:WikiProject_Physics#Physics_Essays. DVdm (talk) 20:54, 26 August 2024 (UTC)

WP:UNDUE addition General relativity self-interaction (GRSI)

@Klbrain The decision to merge the article General relativity self-interaction (GRSI) was reached because it was not considered independently notable. Consequently is it not appropriate to add almost all of the separate content into this article. Rather than reverting your addition I request you cut the content down as you seem to be interested in it being presented clearly. Johnjbarton (talk) 18:16, 11 September 2024 (UTC)

A selective merge was not in the summary at Wikipedia:Articles for deletion/GRSI model; full-content merge is also the standard for merge proposals (see WP:FMERGE). Indeed, full-content merges are cleanest for continuity of attribution. Note also that the concept of notability is different from the reliably sourced; just because the topic isn't independently notable doesn't mean that the content isn't reliable and therefore appropriate for a full-content merge. However, any editor is very welcome to delete material that is inappropriate or not properly sourced. Klbrain (talk) 19:19, 11 September 2024 (UTC)

String theory has lost popularity?

From the "Motivations" section:

Further, shortly after that, theorists switched to string theory which was starting to look promising, but has since lost popularity.

I didn't think string theory had lost popularity -- in fact just the opposite.

(As an aside: a "motivations" section seems like something you see in research papers, not wikipedia articles. Is this potentially original research or a copyvio?)

-- Avocado (talk) 21:16, 14 October 2024 (UTC)

There is nothing wrong with a Motivations section. However this one has no sources so I deleted it. As motivation implies issues with mainstream theory, such a section needs references. Johnjbarton (talk) 22:23, 14 October 2024 (UTC)

Dark matter and dark energy.

In a recent edit a lot of new content was added to the intro by @ScienceDawns. The gist of the addition is in this sentence:

• However on galactic and cosmological scales tests show that discrepancies exist between the expectation from general relativity based on the visible matter present on the one hand and the observed behaviour of matter and light on the other.

The theory of general relativity has no connection to "visible matter" and consequently cannot have a discrepancy. General relativity only relates to mass-energy density.

This sentence should have a specific reference supporting its claim. The two sources given are actually contrary to the sentence. They describe mainstream physics derived by assuming general relativity. Johnjbarton (talk) 16:40, 11 March 2025 (UTC)

Hi! @Johnjbarton Thanks for opening a discussion and not starting an edit war over this! That should definitely be worded differently. I don't think what I was trying to say is controversial. In a way you are actually making the same point I am I think. I'll lay out my reasoning and propose a new phrasing.

• "Visible matter" is too colloquial and vague (of course GR only relates curvature to the stress-energy tensor of which the mass-energy density is the most important component). I was referring to the mass-energy density of all particles from the standard model of particle physics, that is, all mass-energy excluding that of dark matter and dark energy. Unfortunately there is no overarching word to describe that. The astrophysics community often uses "baryonic" as a stand-in since the mass of leptons is negligible compared to that of baryons and the mass-energy density of neutrino's and photons is negligible after 50 ka following the Big Bang. I didn't want to use baryonic in that particular sentence because big bang nucleosynthesis (BBN) also points to a mass-discrepancy if one only considers known particles from the standard model of particle physics and BBN occured in an era when baryons were a subdominant component of the mass-energy density. Settling for "visible matter" was a poor choice since it still excludes photons and not all baryonic matter is practically visible (neutral atomic hydrogen, planets and brown dwarfs barely emit radiation are often hard to observe).
• Two pillars of modern physics. There currently are two major theories underpinning all of well-established physics: General relativity (GR) and the Standard model of particle physics (SMPP). The former governs our best understanding of gravity and the latter our best understanding of the electromagnetic, strong and weak forces. Basically all of what we teach undergrads and most of what we teach master students is part of these two theories. Assuming you have some sort of physics background I think this is self-evident. Hypotheses, models and theories that go beyond these two frameworks are to varying degrees less well established.
• Discrepancies There is a truly vast amount of evidence that merely applying GR and SMPP leads to discrepancies between theory and observation. Baryons+GR is not enough. A selection covering all size scales is summarized below (sorry not all graphs are particularly beautiful, it's just what I have on hand right now that doesn't violate Wikipedia's free-content policies). Something more than these two theories is needed:

The missing mass problem (or missing gravity problem according to some)

On cosmological scales Figure 1: The shape and amplitude of the CMB temperature, E & B polarization power spectra show a discrepancy between GR+Standard Model only and observation. Baryons are just 2.24% of the mass-energy density, the remainder is assumed to be DM and DE. Use this tool to make your own power spectrum by tweaking the baryons, DM and DE. Graph by Planck 2018. Figure 2: The matter power spectrum describes the density contrast of the universe (the difference between the local density and the mean density) as a function of scale. Compare this to theoretical models in figure 3. Graph by Planck 2018. Figure 3: Theoretical models of the matter power spectrum at different baryon densities. Any reasonable model has a baryon density that is only a small part of the total mass-energy density of the universe. Figure 4: The cosmic distance ladder can be used to create a Hubble diagram which absolutely requires the need for both DM and DE if one assumes GR is correct. The baryons are insufficient by a factor of 1:44.

On galaxy scales Figure 5: Galaxy clusters. The x-axis shows the baryonic gravitational acceleration. That is, the Newtonian gravitational acceleration given the baryonic mass distribution of the cluster and its associated brightest central galaxy (BCG). On the y-axis is the kinematic acceleration. If the data had been on the line of unity there would not have been a need for dark matter or modified gravity because GR + the standard model would have been enough. Data from the following sources. Figure 6: Elliptical galaxies and galaxy groups show a clear discrepancy between the baryonic acceleration and the kinematic acceleration. These data were obtained using X-ray hydrostatics, weak lensing and rotation curves of subdominant disks. There is a slight offset between elliptical galaxies and spiral galaxies if one uses the same mass-to-light ratio's. It is unknown at this time if this is due to a slightly different stellar population distribution or some other reason. Data from the following sources. Figure 7: Spiral galaxies show a very tight correlation between the baryonic acceleration and the kinematic acceleration. If there were no DM all data would lie on the line of unity. The tightness of this particular correlation is the motivation behind all MOND-like theories of gravity. Data from the following sources.

On subgalactic scales Figure 8: Molecular clouds also show a discrepancy in the baryonic-kinematic acceleration plane. According to LCDM these discrepancies are not due to DM because DM is uniformly distributed on parsec scales. In LCDM these structures are assumed to be out of equilibrium. Data from Miville-Deschênes et al. (2017). Figure 9: Open clusters like molecular clouds show a discrepancy in the baryonic-kinematic acceleration plane. Also assumed to be out of equilibrium. Data from Hunt & Reffert (2024). Figure 10: On even smaller scales there are claims that binary stars with very wide separations also show mass-discrepancies. Though this claim is disputed.

• Two solutions In order to resolve these discrepancies there are two main logical possibilities. Either GR is wrong/incomplete or the Standard Model of particle physics is (of course both could be wrong but that is unlikely given Occam's Razor).

1: If one assumes GR is correct and complete then dark matter and dark energy are needed and the SMPP is wrong or incomplete. This assumption is what you are pointing out in your initial comment above.

2: If one assumes the standard model is correct and complete then modified gravity is needed and GR is wrong or incomplete. This is what this wiki article covers.

Dark matter and modified gravity are what Merritt calls "auxiliary hypotheses". Necessary additions to a theory without which it is outright falsified. Which of these two auxiliary hypotheses and which specific flavour (WIMPs, axions, strangelets, some MOND-like theory) will ultimately resolve the discrepancy is an area of active research.

• LCDM the majority position The first of these two assumptions is chosen by the majority of astrophysicists. But until such time as the many billion dollar dark matter detection experiments actually find something all we'll have is indirect evidence. And thus the possibility of some alternative to GR being the right way forward remains open. This is highly non-trivial however. It is hard to out-Einstein Einstein.
• Modified gravity fails (so far) To date there is no relativistic theory of gravity that gets better results than GR does. Attempts have been made at explaining these mass-acceleration discrepancies by modifying gravity but so far no dice.

So given all of that I propose the following change (emphasized):

• Current: However on galactic and cosmological scales tests show that discrepancies exist between the expectation from general relativity based on the visible matter present on the one hand and the observed behaviour of matter and light on the other.
• Suggested: However test on galactic and cosmological scales show that discrepancies exist between the expectation from general relativity based on the mass-energy density of known particles from the standard model of particle physics present on the one hand and the observed behaviour of matter and light on the other.

That's more wordy but should satisfy your concerns I think. I hope it's now clear what I meant there.

References

ScienceDawns (talk) 11:02, 16 March 2025 (UTC)

Wow. I appreciate your effort and tone in your reply!

As your discussion here highlights, the role of the introduction to "Alternatives to general relativity" is to explain why such a thing is legitimate scientific subject while also not as successful as Lambda-CDM. You put this clearly in your paragraph above with "LCDM the majority position".

The current introduction almost accomplishes this goal. In my opinion the added sentence doesn't bring us closer to that goal. I think it is important to state the mainstream position first, say that it requires hypothetical particles, then that another approach is to consider alternatives to GR using only SMPP. The sentence we discuss here, coming in the intro before LCDM, creates the appearance of a close race. As far as I am aware, no "alternative-to-GR + SMPP" is close when all the data are compared.

The first half of your sentence, However test on galactic and cosmological scales show that discrepancies exist between the expectation from general relativity based on the mass-energy density of known particles from the standard model of particle physics present on the one hand..., is a non-existent alternative. It implies that Big Bang cosmology invented dark matter/energy. A review like

• Turner, M. S. (2022). The road to precision cosmology. Annual Review of Nuclear and Particle Science, 72(1), 1-35.

takes quite a different point of view. Multiple lines of evidence point to the mainstream choice, LCDM.

I don't understand the second half of your sentence ...and the observed behaviour of matter and light on the other.. To me it says GR+SMPP, which has already been condemned in the sentence, also disagrees with electromagnetism.

(In terms of WP:Intro we are also failing our mission: the content we are discussing should be in a new section, "Motivation" say, not in the intro directly. With a small section we could say bit more and use your sentence to try to give a very short description of LCDM. The Intro would then just have a single sentence summary). Johnjbarton (talk) 17:19, 16 March 2025 (UTC)

> the role of the introduction to "Alternatives to general relativity" is to explain why such a thing is legitimate scientific subject while also not as successful as Lambda-CDM.

> In terms of WP:Intro we are also failing our mission: the content we are discussing should be in a new section, "Motivation" say, not in the intro directly

Agreed! And a "Motivation"-section is a good idea. There is a motivation subsection under "Modern theories 1980s to present"-section but it is full of errors and should be at the top of the article since it applies to the previous time period too. Some common motivations I've seen are:

• Removing the need for dark matter and dark energy
• Removing singularities from the theory
• Quantizing general relativity
• Unifying general relativity with the other forces

Any others that should be included?

> I don't understand the second half of your sentence ...and the observed behaviour of matter and light on the other.. To me it says GR+SMPP, which has already been condemned in the sentence, also disagrees with electromagnetism.

Ah no I'm still not getting my point across despite my previous comment. Let's look at the example of a rotation curve because the math is straightforward. It does not have to do with electromagnetic forces.

In the solar system the planets go around the Sun. The Sun's gravitational field declines as GM/R^2 the further you get from the Sun. This inward gravitational acceleration is balanced by the centripetal acceleration V^2/R. If gravity were stronger the planet would spiral into the Sun, if gravity were weaker the planet would spiral away from the solar system into the void.

This first term is known as the baryonic acceleration:

g_bar = GM/R^2

The second terms is known as the kinematic acceleration:

g_kin = V^2/R

If the system is in equilibrium we must have g_bar=g_kin. If you plot these orbital velocities we notice that they decrease the further you get from the Sun. This is called Keplerian decline. The declining gravitational field strength is matched by a declining circular velocity.

In a galaxy the mass is more spread out than in the solar system where basically all the mass is in the Sun. So a rotation curve which plots velocity against radius will initially increase as more mass is included and the gravitational field becomes stronger the further out we go. Then at some point most of the mass is within the radius being considered and the Keplerian decline should set in. Except that is not what we observe. In galaxies we observe g_bar by measuring the photometry of a Galaxy (the light emitted by stars and gas) and g_kin by measuring peculiar velocities of stars or other tracers of the rotation (which gets converted to circular motion by assuming an inclination). What we observed is that the baryonic acceleration g_bar is consistenly smaller than the kinematic acceleration g_kin. See figures 6 and 7 in my previous comment. This means that objects in galaxies are moving way too fast and should be flinging themselves out of galaxies. Yet we observe galaxies to be stable. It is this discrepancy between g_bar and g_kin that my sentence is trying to convey.

We can solve this discrepancy by assuming there is dark matter in a galaxy so that instead of just g_bar, the gravitational field is given by gN=g_bar+g_dm, where g_dm is just enough so that gN=g_kin. Or we change the law of gravity such that g_bar has a different value which equals g_kin. This is what Milgrom's law in MOND does for example.

This discrepancy is broader than the Newtonian example I've given above. As you can see in figures 6 and 7 gravitational lensing shows the same discrepancy. Instead of using Newton's law to calculate g_bar you use the metric from the observed SMPP mass-energy density and solve for the Christofel symbols. This gives you the second term of the geodesic equation. The first term is calculated from the observed bending of the light ray. Just like g_bar=gkin, the first and second term of the geodesic equation should be equal (with a minus sign for the second term) otherwise the geodesic equation is not zero. If the geodesic equation is not zero we are dealing with an accelerating object or light ray that is not just freely falling. Just like with gbar and gkin, we observe that the two terms of the geodesic equation do not cancel and we have to assume there is dark matter there to make up the difference or assume GR is wrong. For cosmological tests (i.e. on scales much larger than a galaxy cluster) the reasoning is similar.

This measured discrepancy between g_bar and g_kin is why I have such a problem with the phrase "General relativity is consistent with all tests" which is what the article used to say until I changed it with the sentence in question. General relativity can be made to be consistent with the tests I've graphed in my previous comment. But only if you assume there is dark matter and dark energy. And that is a very very big assumption (think 97.5% of the mass-energy density of the universe big).

This measured discrepancy between g_bar and g_kin is what I was trying to convey with my sentence marked in green. The first part describes g_bar and the second g_kin. But I appreciate that that is not obvious if you do not already know what it is trying to say.

> The sentence we discuss here, coming in the intro before LCDM, creates the appearance of a close race. As far as I am aware, no "alternative-to-GR + SMPP" is close when all the data are compared.

Hmm I don't think it does. Just because two theoretical possibilities exist doesn't automatically mean they are equally likely or popular. I agree with the second sentence there. The alternative to GR that comes closest is MOND and that clearly fails in clusters (compare the black line in figure 5 to the data) and can't do cosmology (figures 1 through 4). LCDM is currently the only framework that can plausibly fit all the data, though it has some theoretical problems that to me show it ultimately can't be the whole picture in galaxies and miiiight even be plain wrong (high surface brightness disks for example where the discrepancy only starts at a considerable radius implying a dark matter shell instead of a halo which shouldn't be possible). At the very least LCDM is extremely succesful in fitting and predicting cosmological data.

ScienceDawns (talk) 21:39, 18 March 2025 (UTC)

• "It is this discrepancy between g_bar and g_kin that my sentence is trying to convey."

I understand your paragraph, but I can't see any reasonable way that this condenses into that sentence, sorry.

• "General relativity can be made to be consistent with the tests I've graphed in my previous comment. But only if you assume there is dark matter and dark energy."

You can make the same point by saying that when general relativity is compared to astrophysical observations it predicts dark matter and dark energy. The difficulty of observing dark matter leaves open the possibility of alternative explanations. This way of phrasing the status is consistent with mainstream opinions. The "only if you assume" is a statement of doubt not shared by mainstream cosmologists.

I think the article should have the g_bar / g_kin discussion and when that is set we can summarize it in the intro. We can't have the full argument in the intro and I'm not convinced a single sentence will suffice or be appropriate. Johnjbarton (talk) 22:32, 18 March 2025 (UTC)

> We can't have the full argument in the intro and I'm not convinced a single sentence will suffice or be appropriate

Jup that makes sense. Sorry for trying to condense everything above into a single sentence. Reading it back you're right that it doesn't convey what I want it too. I guess it only makes sense if you already know what it is trying to say which is obviously not how communication is supposed to work..

> You can make the same point by saying that when general relativity is compared to astrophysical observations it predicts dark matter and dark energy.

That depends. If by "dark matter" you are referring to dark matter particles, you are quite correct. GR does predict that. And if they are ever found it will be a massive victory for GR. If by "dark matter" you are referring to the gbar-gkin discrepancy in the cmb, clusters, galaxes and so on however, then no. That fact is a postdiction made using GR not a prediction. The observations of the discrepancy came first and then they were interpreted as dark matter particles, not the other way around. That distinction may seem minor but it is also important not to overstate the case for dark matter particles by using the stronger term prediction where that is not warranted.

The gbar-gkin discrepancy (or dark matter problem, whatever you want to call it) is maybe the biggest unsolved problem in all of physics. So naturally people from every corner develop their own pet theories (sterile neutrino's, MACHO's, axions, WIMPS, superfluid dark matter, dipolar dark matter, MOND, NGT, etc. etc.) which they zealously believe is the only idea that can be right while all others are wrong. What follows are statements like:

• So far, all tests of general relativity have been shown to be in agreement with the theory.
• Numerous astrophysical tests falsify general relativity and show the need for modified gravity.
• Dark matter and dark energy were predicted by general relativity.
• Modified gravity was predicted by the standard model of particle physics.

All of which will piss off one physicist or another. In my opinion a more neutral phrasing would be:

• So far, all tests confirm general relativity or can be accommodated in it with the presence of dark matter and dark energy as supported by a large majority of physicists. A minority of physicists see eliminating the need for dark matter and dark energy as motivation to work on alternatives of general relativity.

ScienceDawns (talk) 07:10, 24 March 2025 (UTC)

I agree that physicist won't agree ;-) But claims of majority/minority are almost never backed by sources which have done scientific or even trivial polls.

I think the previous intro had all the ingredients you are asking for, though maybe not in the order you like. I rewrote the intro a bit more along the lines we discuss here and added broad review refs from cosmology and particle physics. Johnjbarton (talk) 16:16, 24 March 2025 (UTC)

I'm happy with that wording :) ScienceDawns (talk) 18:18, 24 March 2025 (UTC)

I did change it from plural to singular since there is only one concordance model of cosmology (Ωλ=0.7, Ωm=0.3, Ωk=0). I particularly like that you mentioned the high precision predictions. It might have seemed from my previous message that I was not happy with the word prediction there but LCDM has made a large number of true a-priori predictions (E-mode & B-mode polarization, all CMB temperature peaks higher than the third one, the BAO peaks in the matter power spectrum and more) so the way it is worded now is quite accurate. Especially because it doesn't claim all of the gbar-gkin/missing mass problem as a prediction. ScienceDawns (talk) 18:28, 24 March 2025 (UTC)

Deleting source in intro

This sentence does not verify with the source given.

• These observations exclusively exist in systems where the spacetime curvature is very small.

For example from the source However, there is no a priori reason why this transition from GR to modified gravity should happen at a particular energy scale. Other physical parameters could instead provide the trigger.

This discussion belongs in article and maybe in the intro afterwards.

References

Johnjbarton (talk) 22:36, 18 March 2025 (UTC)

All phenomena mentioned that require dark matter and dark energy to fit (the surface of last scattering, the CMB peaks, P(k), galaxies, clusters) are at curvatures smaller than 10^-46 cm^2. That is backed up by figure 1 from that article. The article is indeed agnostic whether the discrepancies which require dark matter and dark energy are possibly due to a modification at an energy scale, acceleration scale or some other physical parameter. Where the discrepancies in parameter space occur and which parameter would exactly be used by a successful alternative to GR are two separate questions though isn't it? You could also use an acceleration scale to separate all observations requiring dark matter and dark energy from ones that don't. I don't think that counts as original research. It's just reading the graph and putting what it says into words.

I'll get back to the other reply later. ScienceDawns (talk) 09:34, 19 March 2025 (UTC)

As I noted above, this topic and source would fit in the article especially if there are more sources. Think the sentence is worthless as it was because it implies that the "exclusively exist" is somehow a negative without giving any reasoning. Johnjbarton (talk) 15:18, 24 March 2025 (UTC)

It did not include relativistic effects such as gravitational lensing

In the section on Relativistic MOND, a list includes "It did not include relativistic effects such as gravitational lensing". The Felten source cited says nothing about lensing and to me gravitational lensing is an experimental observation. It is only a relativitivistic effect if your theory needs relativity to explain it.

In my opinion this section is simply off base. The purpose of this article is it outline theories such as MOND and perhaps give a status update. Instead this section attempts a half-baked historical review of MOND. Johnjbarton (talk) 21:10, 24 March 2025 (UTC)

Yeah the length of it bothered me too. I corrected the factual errors that were there but I'd be fine with removing the entire exposition on MOND and AQUAL until it starts talking about actually relativistic theories (RAQUAL, TeVeS, etc.) or I should say attempts at theories since they all basically crashed and burned :P The main page template link there should suffice.

As for the lensing, you're right that that isn't inherently relativistic. A Newtonian argument can be made for it even though it's off by 2x or something iirc. Only in the last decade have we gotten the data to test MOND to that precision on lensing. So I doubt there's a source to be found to back up that particular claim the article makes that lensing was a problem in 1983. ScienceDawns (talk) 23:35, 24 March 2025 (UTC)

I trimmed it down a bunch. Should be good now. ScienceDawns (talk) 07:57, 25 March 2025 (UTC)