Search FQXi

If you have an idea for a blog post or a new forum thread, then please contact us at, with a summary of the topic and its source (e.g., an academic paper, conference talk, external blog post or news item).
Forum Home
Terms of Use

Order posts by:
 chronological order
 most recent first

 all posts
 member posts highlighted
 member posts only

By using the FQXi Forum, you acknowledge reading and agree to abide by the Terms of Use

 RSS feed | RSS help

Peter Jackson: on 9/27/13 at 11:16am UTC, wrote Zeeya, I'd be delighted if Olivier wishes to discuss my points on his...

Zeeya Merali: on 9/26/13 at 12:14pm UTC, wrote Hi everyone, I should have added that I have opened this thread on behalf...

Peter Jackson: on 9/26/13 at 10:49am UTC, wrote Zeeya, Is it more valuable to find gamma as maths or mechanism? Am I alone...

Zeeya Merali: on 9/25/13 at 17:20pm UTC, wrote This is a thread to discuss the new paper by Olivier Minazzoli, which has...

August 2, 2014

CATEGORY: High Energy Physics [back]
TOPIC: γ parameter in Brans-Dicke-like (light-)scalar-tensor theory with a universal scalar-matter coupling [refresh]
Bookmark and Share

FQXi Administrator Zeeya Merali wrote on Sep. 25, 2013 @ 17:20 GMT
This is a thread to discuss the new paper by Olivier Minazzoli, which has just been published in Phys. Rev. D.. The full paper is available here:

From the abstract: "The post-Newtonian parameter gamma resulting from a universal scalar/matter coupling is investigated in Brans-Dicke-like Scalar-Tensor theories where the scalar potential is assumed to be negligible. Conversely to previous studies, we use a perfect fluid formalism in order to get the explicit scalar-field equation. It is shown that the metric can be put in its standard post-Newtonian form. However, it is pointed out that 1-gamma could be either positive, null or negative for finite value of omega_0, depending on the coupling function; while Scalar-Tensor theories without coupling always predict gamma less than 1 for finite value of omega_0."

Thank you Olivier for sharing this with the FQXi community!

Minazzoli also summarises his work here:

"Putting constraints from observations on string theory has become a pressing issue. Many concerns have been raised about the apparent lack of any possible way to constrain the theory. The effective gravitational sector of string theory predicts a violation of the various versions of the Equivalence Principle (EP). Notably, the universality of free-fall is one of the aspect of the EP that is expected to be an approximation of a more complex dynamic. The string theory gravitational phenomenology makes several other predictions, yet these are usually qualitative and not quantitative.

"In a recent work, I found a possible way to put a "ruling-out" constraint on the effective gravitational action of string theory. Indeed, as an approximate scalar-tensor theory, the low energy limit of string theory predicts that the post-Newtonian parameter gamma is different to one, while general relativity predicts gamma=1. Gamma is used, for instance, in solar system experiments in order to check whether the theory of gravitation is general relativity or maybe something else. Until now, it was thought that gamma was less than one in all scalar-tensor theories (including the effective string theory action). However, I found out that, depending on the coupling function between the scalar-field (dilaton) and the material part of the Lagrangian, gamma could also be more than one, or even in a very specific case, equal to one. Furthermore, the tree level limit of string theory, for which the dilaton couples universally to the rest of the Lagrangian, leads to gamma greater than 1; while usual scalar-tensor theories (aka Jordan-Brans-Dicke theories) lead to gamma less than 1.

"Now, if the actual theory of gravitation is indeed a scalar-tensor theory (as many suspect and as string theory predicts), we may be able to measure a possible deviation of gamma from 1 in the near future. Indeed, several projects propose to increase the accuracy of the measurement of gamma. Therefore, whether we measure gamma to be less or more than one will rule out either usual scalar-tensor theories (that predict gamma less than 1), or the tree level limit of string theory (that predict gamma greater than 1), as an effective theory of gravitation. However, the full loop expansion might lead to an effective gravitational theory that leads to gamma less than 1 (as in usual Jordan-Brans-Dicke scalar-tensor theories). But in any case, the sign of 1-gamma will be a strong constraint on the full loop action of string theory: if not right, any effective action of string theory may be ruled out."

this post has been edited by the forum administrator

report post as inappropriate

Peter Jackson wrote on Sep. 26, 2013 @ 10:49 GMT

Is it more valuable to find gamma as maths or mechanism? Am I alone suggesting maths is more useful to maths, but mechanism is more useful for understanding?

If you've read my essays as promised you should have recognised the empirically consistent scalar/matter (plasma) coupling implicit therein via n=~1. (I refer somewhere to Dicke).

I agree Olivier's paper is...

view entire post

report post as inappropriate

FQXi Administrator Zeeya Merali wrote on Sep. 26, 2013 @ 12:14 GMT
Hi everyone,

I should have added that I have opened this thread on behalf of Olivier, so you can address comments to him, if you want to discuss his paper directly, rather than to me.

report post as inappropriate

Peter Jackson wrote on Sep. 27, 2013 @ 11:16 GMT

I'd be delighted if Olivier wishes to discuss my points on his paper. Perhaps better to respond to my questions to you under 'Alternative Models..' where I've just posted a set of postulates to JC, or my essay blog, or direct.

If you think the models re-appraised fundamental propositions are 'crackpot' or flawed please do say so, but some scientific grounds would be really helpful, which nobody (anywhere!) has yet found or offered.

Thanks and best wishes.


report post as inappropriate

Please enter your e-mail address:
Note: Joining the FQXi mailing list does not give you a login account or constitute membership in the organization.