Talk:Brans–Dicke theory

message: I find your statement that GR lacks for tunable parameters to be odd.  Technically, the EFE are G_{\mu\nu} = \kappa T_{\mu\nu}, where \kappa = 8 \pi G/c^4.  G is a value that is very much tuned to fit observation.
I agree with your overall point, but I see GR as having only one tunable parameter, where as Brans-Dicke theory has two.
--EMS | Talk 14:53, 9 Jun 2005 (UTC)
replies: text: G is dimensionful and is thus a mere irrelevant conversion factor which can be set equal to 1, 1/(8 pi) or whatever value one finds convenient. See here. Count Iblis 22:10, 7 February 2006 (UTC)

text: Joke, I didn't see your comments below! I certainly agree with your point. Ultimately, only dimensionless quantities matter. By changing G you just redefine the Plank mass in terms of SI units. Then, if you keep the proton mass fixed in terms of SI units you do make a relavant change, because the ratio of the proton mass to the Planck mass changes.Count Iblis 22:57, 8 February 2006 (UTC)
thread_title:  GR does not have parameters?! 

message: I don't think this article is unsatisfactory as it stands, because it fails to explain the historical origins of Brans/Dicke theory or to place it in the context of other topics which have articles like the Dirac large numbers hypothesis.  The paper by Brans which I cited is a good source for this background, but I haven't found time to incorporate that material into a new section on Historical Origins, which should probably be placed between Introduction and Comparision with General Relativity.  Anyone else wnat to have a go?
---CH —Preceding undated comment added 03:54, 25 June 2005 (UTC)
thread_title: Historical Origins

message: "The Brans/Dicke theory is "less stringent" than general relativity in another sense: it admits more solutions. That is, some of the spacetimes which are exact solutions to the Einstein field equation of general relativity are also admissible in Brans/Dicke theory, but the converse is false": the statement in bold is equivalent to its converse, and both are equivalent to the statement that there exist spacetimes admissible in both theories (correct me if I'm wrong). Therefore, the part of the article quoted is incorrect, because inconsistent. I imagine "some" should read "all", but I don't know, so I'm not changing it. Comments?
Buster79 13:57, 17 September 2005 (UTC)
replies: text: Hi, Buster, yeah this needs to be more clearly expressed, and more examples would probably help too.  The confusion is due to the fact that in Brans-Dicke, to specify a solution you must give a Lorentzian manifold together with a scalar field on that manifold.  IIRC, the correct statement is that every vacuum solution of the EFE can be made into a vacuum Brans-Dicke solution by choosing a trivial scalar field, but various non-solutions to the vacuum EFE can also be made into a vacuum Brans-Dicke solution by choosing an appropriate scalar field.  When I have time, I'll try to remember to check this and fix the article.  It would be nice to generalize this to non-vacuum solutions such as electrovacuums, but AFAIK this has not be carefully investigated.---CH  (talk) 00:07, 18 September 2005 (UTC)

text: The author was me.  Looking at the Lagrangian, I guess your point is pretty obvious :-/  I just modified the offending paragraph to restate the relationship in a way which is not definitive (since I don't say much about nonvacuum equations) but should be correct as far as it goes.---CH  (talk) 22:13, 21 September 2005 (UTC)
thread_title: Logical Inconsistency?

message: I won't be able to get to this for an unknown amount of time, but I have been planning improvements to this article for some time:
*better explanation of motivation as described in cited Brans article,
*Jordan frame versus Einstein frame,
*discussion of weak-field limit, gtr limit,
*discussion of how gravitational waves differ from gtr,
*compare static spherically symmetric solutions with gtr.
A critical and subtle point to explain here is why not all interpretations which attempt to answer the question, "will the physical metric please stand up?", are equally justifiable.  I plan to add a few more references when I introduce this material.---CH 23:35, 8 January 2006 (UTC)
replies: text: I have copied these plans to the todo list I made above. ---CH 22:11, 3 February 2006 (UTC)
thread_title: Planned Improvements

message: The article contains
:At present, both general relativity and the Brans-Dicke theory are compatible with all well accepted observational and experimental evidence.
While strictly true, it is meaningless since GR is a limit of Brans-Dicke theory. The latest constraint on the Brans-Dicke parameter is from Cassini-Huygens, and specifies ω > 40,000, which is pretty severe if you ask me. I'm being a jerk and not reading the whole discussion above, but you can think of both Brans-Dicke theory and GR as having exactly one tunable parameter. In the case of Brans-Dicke theory it is obviously ω. In GR, it is not G, since that is a dimensionful quantity, but rather the ratio of G to some relevant mass scale (say, G mp2, where mp is the proton mass scale, say, or the Higgs mass, or the QCD scale ΛQCD, and G is Newton's constant, which is the inverse Planck mass squared). This is something Frank Wilczek has been talking about recently, although in a slightly different context. –Joke 22:05, 3 February 2006 (UTC)
replies: text: Hi, Joke, this is an important point and when I get a chance to rewrite the article, I think I will be able to convince you that my statement was not so trivial after all!   It turns out that saying that gtr is a limiting case of Brans-Dicke involves nontrivial assumptions related to Jordan versus Einstein "frames", and these assumptions turn out to be dubious! ---CH 22:13, 3 February 2006 (UTC)

text: P.S.: even if one accepts G/(fundamental length) scale as a "tunable parameter" for gtr, Brans-Dicke has one more tunable parameter.  ---CH 22:18, 3 February 2006 (UTC)

text: BTW, this project is currently rather high on my list. ---CH 22:22, 3 February 2006 (UTC)

text: I have abandoned my initial attempt (before I learned of a technical issue involving article namespace) to promulgate using terms like "von Neumann/Bessel-Hagen/de Sitter Theorem" for reasons which should be obvious from this example.  (Real ones are easy enough to find.) ---CH 01:38, 4 February 2006 (UTC)
thread_title:  Compatible with evidence? 

message: DogsBreakfast changed a well-known competitor of to an alternative theory to.  Brans-Dicke and other theories are often referred to in the research literature as either competitors or alternatives, but BD is often cited as the best known competitor of gtr.  Therefore, I think the original phrase is a more suitable description for the introduction of this article, so I reverted the change.  DB, if you disagree, can you say why?  TIA ---CH 01:21, 23 February 2006 (UTC)
replies: text: I'm ok with returning to the original wording as in this case it seems it might be justified, however in general I think claims to being the 'best known' are hard to verify, and as such I'd prefer to see the verifiable facts speak for themselves.  If it really is the best-known, then can't we show this in some way, rather than just tell it, e.g. number of citations, or references in more popular publications, perhaps? --DogsBreakfast 13:13, 23 February 2006 (UTC)
replies: text:  Clifford Will's "popular" 1986 book on general relativity and testing, "Was Einstein Right?" devotes a chapter to Brans-Dicke. It doesn't give that honour to any other GR-competitor. ErkDemon 17:53, 29 April 2007 (UTC)
thread_title: "Alternative to" versus "competitor of"

message: As a courtesy, I have removed the todo list.  I am leaving WP and doubt anyone else will know how to implement the suggested improvements since this was mostly a note to myself.  Sadly, I am now abandoning this article to its fate.  See User:Hillman/Archive for the last version I edited. ---CH 22:06, 30 June 2006 (UTC)
thread_title: To do list

message: since my english is rudimentary i would apreciate collaboration to define in layman terms the meaning of the brans-dicke and Smolin/zee theories, please do not erase, this articles needs a full explanation of its implications as most
serious relativists consider today GR a specific case of the Higgs/brans field.  —Preceding unsigned comment added by 76.89.242.7 (talk) 10:42, 23 February 2008 (UTC)
thread_title: you need indeedd a specialist

message: Next formula fails to display on my browser, Firefox 3.5.3, on Windows XP. The browser displays its source text without any rendering. What's wrong ? If I remove the \Box it renders all right, ditto if I insert "666" (with or without a space) before the "3".
:\Box\phi = \frac{8\pi}{3+2\omega}T
I'm using the classic skin, and math option "HTML if very simple of else PNG" (but what I see is bare LaTeX source code, not HTML rendering of it).
--FvdP (talk) 19:38, 14 October 2009 (UTC)
replies: text: Update: well, it works now, both here and in the article. Don't know what happened... --FvdP (talk) 17:24, 26 October 2009 (UTC)
thread_title: Strange LaTeX behaviour

message: What is the point of Brans-Dicke theory?  What was the motivation of Brans and Dicke for devising a theory more complex than GR that doesn't fit the data any better?  I don't know the answers to these questions, but it seems an article like this should answer them.  Probably Brans and Dicke had some reason for disquiet with GR.  What were those reasons?  What physical insights did they have? 190.46.5.229 (talk) 08:43, 6 November 2011 (UTC)
replies: text: The motivation largely lies in Mach's principle. I agree the article should introduce that. Currently it is only in the "See also" section. Jason Quinn (talk) 03:08, 24 August 2012 (UTC)
thread_title:  Why? 

message: when comparing the action of the brans-dicke theory with the action of GR, the gravitational constant G pops up (so here one uses non-planck units) whereas in comparing the field equations of both theories G does not appear (planck units). So for consitency one should check weather the article uses planck units (G=1) or not. Since phi is a function of G, I would propose to not use planck units, since then the differences of the equations are more obvious.  — Preceding unsigned comment added by 178.25.164.122 (talk) 21:57, 8 December 2011 (UTC)
thread_title:  comparison with general relativity (GR) 

message: Tagging per the RFC on Talk:Self-creation cosmology. I am not the proposer of the RFC. Please discuss there. Jerod Lycett (talk) 00:17, 6 October 2015 (UTC)
thread_title:  Proposed merge with [[Self-creation cosmology]] 

message: The Lagrangian according to Brans/Dicke is given in the article as 
:(I) S=\frac{1}{16 \pi}\int d^4x\sqrt{-g} \; 
\left(\phi R - \frac{\omega}{\phi}\partial_a\phi\partial^a\phi + \mathcal{L}_\mathrm{M}\right)
In general relativity it is given in the article as
:(II) S=\int d^4x\sqrt{-g} \; \left(\frac{R }{16\pi G} + \mathcal{L}_\mathrm{M}\right)
This is very starange, as for \phi = 1 (I) does not simplify to (II), because the right most term in (I) is
:(I') \frac{1}{16 \pi}\int d^4x\sqrt{-g} \;\mathcal{L}_\mathrm{M},
but in (II) it is
:(II') \int d^4x\sqrt{-g} \; \mathcal{L}_\mathrm{M}
So we have a discrepancy of a factor 16 \pi.
In order to make it even more clear we may have a look at German Wikipedia. The Brans/Dicke Lagrangian (German: Wirkung) is given as
:(III) S = \frac{1}{16 \, \pi}\int \mathrm{d}^4x \sqrt{-g} \left( \phi \, R - \omega \, \phi^{-1}\partial_{\mu}\phi \, \partial^{\mu}\phi \right) + S_\mathrm{M}
and the value in general relativity (German: ART) is given as
:(IV) S = \frac{1}{16 \, \pi}\int \mathrm{d}^4x \, \sqrt{-g} \, R + S_\mathrm{M}
Comparing (I) with (III) we conclude
:S_\mathrm{M} = \frac{1}{16 \pi}\int d^4x\sqrt{-g} \;\mathcal{L}_\mathrm{M},
but comparing (II) with (IV) it must be
:S_\mathrm{M} = \int d^4x\sqrt{-g} \;\mathcal{L}_\mathrm{M},
which almoust obviously is a contradiction.
Unfortunaltely there are no term specific online sources provided, so I am not the specialist to identify the incorrect term and correct it. But who knows what is right? --Ernsts (talk) 22:45, 11 October 2018 (UTC)
replies: text: Found the following article  and equation (2.9) shows how the term (I) has to be corrected. Will do so. --Ernsts (talk) 07:44, 12 October 2018 (UTC)
thread_title:  Matter Term 

message: I've seen it stated a few times (e.g. https://medium.com/starts-with-a-bang/this-is-why-physicists-think-string-theory-might-be-our-theory-of-everything-d4a653e15bf9 ) that string theory predicts Brans-Dicke.  Should this be mentioned in the article?  Seems notable and newsworthy to me. ~~  cheers, Michael C. Price talk 11:12, 24 April 2023 (UTC)
thread_title:  String theory