What's meant by- background independence/ dependence-landscape

[Ratzinger]

What's meant by
- background independence/ dependence
-landscape ?

thank you

 

[marcus]

What's meant by
- background independence/ dependence
-landscape ?

thank you

hi Ratzinger,

"Landscape" in its theoretical physics sense was introduced by Lee Smolin
in his popular book 1997 "The Life of the Cosmos". In 2003 the word was adopted by string theorists.

Intuitively the idea is that the Standard Model of particle physics depends on a couple of dozen or so parameters---numbers you have to plug in, like the masses of the electron and the quarks and the strengths of various interactions.

If these numbers were different the world would look different, with different chemical elements and different chemical and nuclear reactions. If they were TOO different there might be no stable atoms at all, just a soup of particles. And gravity could be stronger or weaker compared with the other forces, and so on. So by varying the numbers in the SM you get infinitely many VERSIONS OF PHYSICS.

Up into 1990s the dream of physicists was that a comprehensive theory, say with only ONE free parameter or none at all, would explain why the two dozen numbers had to be what they were. Then there would be no range of possibility, or at most a very trivial choice represented by the possible values of that single parameter input. String theorists in the 1990s shared this common aspiration, and hoped that string theory, when perfected, might uniquely determine the SM parameters. So they did not need the Landscape concept to describe the range of possible versions of physics, having other things to think about.

In 2003 a famous paper by four string theorists (Kachru, Kallosh, Linde, Trivedi) suggested that this goal of string theory would never be attained and that there would always be a huge range of possibility in string theory. The theory could not distinguish among something on the order of 10¹⁰⁰ different ground states, or "vacuums". Later people estimated more, like 10⁵⁰⁰ or an infinite number of string vacua.
To many people there seemed no hope of finding a physical principle that would pick out the right one. At that point string theorists adopted the word "Landscape".

The word had already been given its physics-context definition in Smolin's popular 1997 book. Smolin says he chose the word "Landscape", for the range of possible versions of physics, to make the analogy with evolutionary biology, where the concept of a "Fitness Landscape" was well established. Think of all the different versions of an organism you can get by varying its genes (like the parameters of the SM in physics). Picture these possibilities laid out like some terrain and HEIGHT given by reproductive success, or fitness. Then natural selection will drive that species of organism to explore the landscape and find one or more "high-points" or fitness maxima.

In his 1994 paper, and later in the book, Smolin was investigating some possible mechanism by which the parameters of the Standard Model could be determined by a process analogous to EVOLUTION.

Some related discussion is in http://arxiv.org/hep-th/0407213 , "Scientific Alternatives to the Anthropic Principle"

---quotes from "Scientific Alternatives...---
Page 12 Footnote: It is perhaps worth mentioning that the word “landscape” was chosen in [8, 9] to make the transition to the concept of fitness landscape, well known in evolutionary theory, more transparent.

[8] L. Smolin, On the fate of black hole singularities and the parameters of the standard model submitted to Physical Review D. gr-qc/9404011, CGPG-94/3-5 ;
Cosmology as a problem in critical phenomena in the proceedings of the Guanajuato Conference on Complex systems and binary networks, (Springer,1995), eds. R. Lopez-Pena, R. Capovilla, R. Garcia-Pelayo, H. Waalebroeck and F. Zertuche. gr-qc/9505022;
Experimental Signatures of Quantum Gravity in the Proceedings of the Fourth Drexel Conference on Quantum Nonintegrability, International Press, to appear, gr-qc/9503027.

[9] L. Smolin The Life of the Cosmos, 1997 from Oxford University Press (in the USA), Weidenfeld and Nicolson (in the United Kingdom) and Einaudi Editorici (in Italy.)
---end quote---

 

[Ratzinger]

You are the man marcus...that was fast and crystal-clear...you need to write a book or something. Sorry for asking about background, after searching this section I found plenty explanations and noticed how central it is.

Watched this Toronto conference. Even I didn't understand close to nothing, it was pure excitement to hear these bright people talk about the (possible) laws of our universe.

 

[marcus]

What's meant by
- background independence/ dependence
-landscape ?

thank you

Oops, crossed post! I replied before I realized that you already had answred it for yourself. But I will leave this post around anyway because
someone else might be interested in background independence. Glad you found the other one useful!

-----earlier post before i saw yours----

I think the best discussion of background independence and dependence is in this paper by Smolin. He does by far the best job of discussing it.

http://arxiv.org/hep-th/0507235

The Case for Background Independence

It is an accident that I should have to refer to Smolin's writings in both cases---for both of these currently important basic ideas: landscape and background.

In most physical theory, before you can define anything (a wave, a string, a field, whatever) you have to have a space for it to live in, and in most theories that space cannot be a limp shapeless space--it has to have coordinates, and it has to have distances defined between its points by a metric---it has to have some prior fixed GEOMETRY.

Otherwise things like strings can't function in the space. Most physics stuff cannot ripple or vibrate or propagate without some prior geometry.

this is usually equated with the "background metric".

there are ways to get around the need for a prior choice of geometry
and theories that do that are often called background independent.

Einstein 1915 Gen Rel is a good example. In Gen Rel there is no prior chosen metric, the metric (spacetime geometry) is purely dynamic, it arises as a solution to the equations.

But you should really look at Smolin's paper "The Case for B.I." because it is much more sophisticated than other discussions of B.I.
He points out that B.I. is not an ALL OR NOTHING black-white thing. there are DEGREES of independence. there is always SOME prior choice, some initial assumptions you can't get away from. yes Einstein avoided having to choose a prior metric, but he didnt get rid of the coordinates...and so on.

Smolin gives instructive examples of how in the history of physics some steps of progress have been made by gradually REDUCING the prior assumptions and gradually increasing the amount of independence.

He makes what I think is a rather convincing argument that for String the way out of the trouble that String is now in would be for theorists to focus on reformulating it in a more background independent way.

He says if they would get rid of some of their prior assumptions then the theory would start to make unique predictions
Because it is HARDER to build a theory when you assume less, so it narrows down the possibilities of how the theory can turn out.
So it would give them a way out of the Landscape (which some now call the Swamp) of too many possible vacua or too great a richness of possibility.
It is an interesting argument, you should read it for yourself.
Again the link is
http://arxiv.org/hep-th/0507235
The Case for Background Independence

 

[Ratzinger]

man, you are fast... hope someone is paying you for this

again, thanks for the effort, that was very enlightening

 

[selfAdjoint]

Marcus, this new paper by Smolin is not just an explanation/defense of background independence, but an interpretation of it in terms of a relational view of physics, which is a much more controversial topic than background independence in itself .

 

[marcus]

Marcus, this new paper by Smolin is not just an explanation/defense of background independence, but an interpretation of it in terms of a relational view of physics, which is a much more controversial topic than background independence in itself .

good warning to have on the label
conventional background independence means just get rid of the metric
(so you still can start with a smooth manifold, with coordinate functions and a uniform dimensionality at all scales and that good stuff, you just don't have a prior geometry)

and Smolin makes a case for bringing that degree of B. indep. into string theory.
but he also extends the idea (in more "philosophical" parts of the paper)
so that from this paper you can get ideas of how the idea of B. indep. can be carried farther and involve getting rid of more prior assumptions.

I don't think it is necessarily bad to be exposed to an extension of the idea, where you put the conventional idea on a scale and visualize going beyond it. but it is definitely good to be warned.
In this paper he is not just giving an exposition of the usual idea
but he is developing it further.

however, his suggestion to string theorists, I believe, is simply that they reformulate String so it is background independent in the usual sense of not needing a prior metric. Or do you see more in it?