Could someone explain Brans-Dicke theory?

In summary, the scalar-tensor Brans-Dicke theory is an alternative to general relativity. It is very similar to GR, except it has an extra scalar field in it, which essentially plays the role of a variable gravitational "constant". It's a fairly tunable theory, so it's hard to distinguish its predictions experimentally from the predictions of GR; as far as existing experiments are concerned, either theory could be right (but Brans-Dicke is more complicated, and the scalar field needed to comply with experiment seems kind of artificial).
  • #1
alanzhu
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0
Is it right?Is it the result of Mach's principle in GR?
 
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  • #2
Originally posted by alanzhu
Is it right?Is it the result of Mach's principle in GR?

The scalar-tensor Brans-Dicke theory is an alternative to general relativity. It is very similar to GR, except it has an extra scalar field in it, which essentially plays the role of a variable gravitational "constant". It's a fairly tunable theory, so it's hard to distinguish its predictions experimentally from the predictions of GR; as far as existing experiments are concerned, either theory could be right (but Brans-Dicke is more complicated, and the scalar field needed to comply with experiment seems kind of artificial). See also:

http://nedwww.ipac.caltech.edu/level5/Glossary/Essay_bekenstein.html

As for Mach's principle, there are many principles floating around to which people have attached Mach's name:

http://arXiv.org/abs/gr-qc/9607009

Brans-Dicke theory is "Machian" in the "Mach1" sense of that paper: Newton's gravitational constant is not really constant, but is a dynamical field. That means that the gravitational mass of a body depends on all the matter in the rest of the universe. Personally, I don't think that was exactly what Mach had in mind, but some people call it a Machian idea.
 
  • #3


Originally posted by Ambitwistor
the scalar field needed to comply with experiment seems kind of artificial)

Not from the perspective of string theory.
 
  • #4


I'm not talking about the existence of extra couplings; it's easy to cook up theories that have them (e.g. dilatons in string theory, Kaluza-Klein theory, etc.). The question is whether there is a natural mechanism to drive the field to a stable value compatible with observation.
 
  • #5


Originally posted by Ambitwistor
I'm not talking about the existence of extra couplings; it's easy to cook up theories that have them (e.g. dilatons in string theory, Kaluza-Klein theory, etc.). The question is whether there is a natural mechanism to drive the field to a stable value compatible with observation.

I realize this. I was referring to mechanisms that have been proposed in the context of string cosmology.
 
  • #6
Could this theory explain dark matter? I mean is it possible that there is no dark matter definitely.Because the gravitation constant is not a real constant , the prediction made by GR may be totally wrong.
 
  • #7
Originally posted by alanzhu
Could this theory explain dark matter? I mean is it possible that there is no dark matter definitely. Because the gravitation constant is not a real constant, the prediction made by GR may be totally wrong.

Many attempts have been made to replace dark matter by instead changing the laws of gravity, but none of them have succeeded.

Dark matter explains several phenomena: galactic rotation curves, the expansion history of the universe, early structure formation, etc. It's very difficult to produce an alternate explanation that correctly explains all of these different phenomena at the same time. In particular, I believe that scalar-tensor theories can account for the expansion of the universe without resorting to dark matter, but they run into problems with at least one of the other phenomena.
 
  • #8
Originally posted by Ambitwistor
...scalar-tensor theories can account for the expansion of the universe without resorting to dark matter...

Sorry, but are you referring to brans-dicke scalar-driven inflation (whose relation to this thread is unclear to me), or scalar-tensor theories of gravity producing cosmological features similar to those arising from presence of dark matter?
 
  • #9
Originally posted by jeff
Sorry, but are you referring to brans-dicke scalar-driven inflation (whose relation to this thread is unclear to me), or scalar-tensor theories of gravity producing cosmological features similar to those arising from presence of dark matter?

The latter. Although I don't know how dark energy changes things; I've only heard of attempts to use Brans-Dicke cosmology to get rid of dark matter in the absence of a cosmological constant.
 
  • #10
Some 2006 references for Brans-Dicke theory of gravitation

Looks like someone was asking for an explanation of the best known scalar tensor theory of gravitation, the Brans-Dicke theory, way back in 2003. Just thought I'd point out that http://www.arxiv.org/abs/gr-qc/0506063 will be of interest to anyone curious about this. I'd also recommend the discussion in Weinberg, Gravitation and Cosmology. My own provisional attempt to explain some of the basic ideas can be found in the article titled "Brans-Dicke theory of gravitation", archived at http://en.wikipedia.org/wiki/User:Hillman/Archive but I know enough to realize that this cries out for expansion since it completely omits several important topics.

Chris Hillman
 
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  • #12
Talk by Carroll

Hi, Max,

MaxPolun said:
I recently ran across a talk on this very subject (well... more generally can modified gravity eliminate dark matter): http://online.itp.ucsb.edu/online/lens06/carroll/

I just glanced at a few of the slides and didn't see any mention of Brans-Dicke theory!

BTW, due to inexperience in this forum, I posted in a thread from 2003; this caused confusion in another thread where I did the same thing, so I apologize in advance for any confusion I might have caused here. In future, I think I'll try to start new threads should I notice other long past questions here which I wish to comment on.

Chris Hillman
 
  • #13
Chris Hillman said:
Hi, Max,



I just glanced at a few of the slides and didn't see any mention of Brans-Dicke theory!

BTW, due to inexperience in this forum, I posted in a thread from 2003; this caused confusion in another thread where I did the same thing, so I apologize in advance for any confusion I might have caused here. In future, I think I'll try to start new threads should I notice other long past questions here which I wish to comment on.

Chris Hillman


A good way is to start the new thread with a post that indicates the old one with a link, so those who remember, or want to research, can do so, but the forum ins't clutted with old threads.

BTW I was thinking this morning. Is the WWW and the link philosophy the first time we have given brute matter the power of recursiveness? The writings of Homer, and Shakespeare, and us are reduced to boxes with arrows pointing to them - and then you get duals (who links to me?) and think about products, and voila! Hopf Algebras! All done with things.
 

1. What is Brans-Dicke theory?

Brans-Dicke theory is an alternative theory of gravity that was developed in the 1960s by physicists Carl Brans and Robert Dicke. It proposes a different mathematical framework for understanding gravity, in which the strength of the gravitational force is not a fixed constant but is instead determined by a field called the "scalar field". This theory is an alternative to Einstein's theory of general relativity.

2. How does Brans-Dicke theory differ from general relativity?

Brans-Dicke theory differs from general relativity in several key ways. Firstly, in Brans-Dicke theory, the gravitational constant is not a fixed number but is instead determined by the scalar field. Secondly, this theory predicts that the gravitational force becomes stronger at very large distances, while general relativity predicts that it becomes weaker. Finally, Brans-Dicke theory introduces a new term in the equations of motion that allows for the possibility of a non-zero cosmological constant, which is not present in general relativity.

3. What evidence supports Brans-Dicke theory?

There is currently no experimental evidence that definitively supports Brans-Dicke theory over general relativity. However, there are some observations that this theory can potentially explain better than general relativity, such as the accelerated expansion of the universe and the existence of dark matter. Additionally, some scientists argue that Brans-Dicke theory is a more elegant and mathematically consistent theory than general relativity.

4. What are some potential implications of Brans-Dicke theory?

If Brans-Dicke theory is proven to be a more accurate description of gravity than general relativity, it would have significant implications for our understanding of the universe. It could potentially change our understanding of the origin and evolution of the universe, as well as the behavior of objects in extreme gravitational environments, such as black holes. It could also have practical applications, such as improving our ability to predict and control the motion of objects in space.

5. Are there any limitations or challenges to Brans-Dicke theory?

Like any scientific theory, Brans-Dicke theory has its limitations and challenges. One major limitation is that it has not yet been experimentally confirmed, so it remains a theoretical possibility rather than an established fact. Additionally, this theory has many free parameters that can be adjusted to fit different observations, which some scientists view as a weakness. Finally, there are some unresolved mathematical and conceptual issues within Brans-Dicke theory, such as the "strong singularity problem", that continue to be the subject of ongoing research and debate.

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