Talk:Background independence

message: I have ever seen so much bullshitting on what concerns "active" and "passive" covariance. This article is simply false.
"As both metric functions have the same functional form but belong to different coordinate systems, they impose different spacetime geometries. Thus we have generated a second distinct solution!" It is just exactly the same in a different coordinate system.... You can count known solutions of Einstein equations in the vacuum on your fingers. Too bad physicists were so stupid during one century they didn't used "active" covariance to find infinitely many different solutions !
pyc 25/04/2011  —Preceding unsigned comment added by 85.218.96.37 (talk) 16:12, 25 April 2011 (UTC)
thread_title: Bullshit

message: Article claims, "Below is given an easy argument which uses only the very basics of GR making it accessible to anyone..."
Come on.  I mean, really.
:Yes, it's incomprehensible. --88.74.131.82 (talk) 19:55, 26 October 2009 (UTC)
thread_title: Untitled

message: The wording of the article "independent of the actual shape of the spacetime and the value of various fields within the spacetime, and in particular to not refer to a specific coordinate system or metric" seems to me careful and important.  I contrast it with writing that seems to suggest that the idea of time is absent from the theory.
For example, in the glossary of his book "The Fabric of the Cosmos," Brain Greene gives this entry for background independence: "Property of a physical theory in which space and time emerge from a more fundamental concept, rather than being inserted axiomatically."  In the text (p.487) he writes, in relation to applying this concept to string theory, "In this proposal, concepts of space and time fail to have meaning until innumerable strings weave together to produce them."
What Greene writes seems very problematic. How can we conceptualize anything to "emerge" from something else without some time-like dimension along which change takes place?   How can we conceptualize the difference between before and after the "weaving together of innumerable strings" without that difference taking place over something like a dimension of time?  It seems to me that such questions signify that the idea of a time-like dimension is contained in the theory ab initio, but not a specific metric. And it seems to me that the formulation in the present article takes this position.  Doe this reading have merit?
jjb 00:09, 11 February 2007 (UTC)
thread_title:  Does emergence itself signify a concept of time 

message: One could imagine a model of “reality” in which some sufficiently rich vector space could contain definitions of every possible (and impossible) state and state transition, and that this vector space, itself, would be background independent (have no attributes of space or time). For example, a complete description of the number pi requires infinite information. Yet, pi “exists” everywhere, at all times. Pi is background independent.
In such a model, one might posit that every system that can be defined and that contains an observer performing an observation “exists”. In a sense, these observers and events would spontaneously “find themselves” within this background independent vector space.
In such a model, spacetime would be an emergent property of a system that possessed no space or time (background independent).
Interestingly, Einstein's spacetime is a fixed and unchangeable entity; that which happens tomorrow is exactly as fixed and immutable as that which happened yesterday or even that which happens now. Each observer in spacetime follows a different worldline with its own, unique view of the same spacetime. Straight worldlines are inertial reference frames and curved worldlines are accelerated reference frames, embedded within the same fixed spacetime.
 — Michael McGinnis (talk) 22:18, 8 February 2010 (UTC)
thread_title: Space and Time as Emergent Properties of a Background Independent System

message: I didn't add that tag, but I agree that there is a major problem with that section as written.  I also agree with the comment in the section labelled "bullshit" above.  Einstein's hole argument says exactly the opposite of what is asserted in that section - it says that solutions that differ by diffeomorphisms are NOT distinct, but rather are identical.  Diffeos are the gauge transformations of GR, and gauge transformations are not real symmetries - they are redundancies in the description, and performing one does NOT lead to a new, distinct solution.
There is such a thing as manifest background independence - it's just a formulation of the theory that never refers to any specific background, for instance a path integral over metrics - but I think the section as written doesn't describe and is factually wrong on a number of counts.  I suggest deleting it entirely, making the article much shorter, more correct, and more comprehensible.  But I'll leave this comment for a while first to see if anyone disagrees.  Waleswatcher (talk) 18:09, 11 January 2012 (UTC)
replies: text: I just read (part of) the paper of Rovelli and Gaul.  It does not support what is written in that section.  Specifically, it asserts that an "active" diffeo on a metric produces a distinct metric on the SAME spacetime manifold.  That's of course correct (although the distinction between active and passive is meaningless).  Instead, the article asserts that "active" diffeos produce "different spacetime geometries" and "distinct solutions", which is completely wrong.  I'll wait a few days and then edit, drastically. Waleswatcher (talk) 18:21, 11 January 2012 (UTC)
thread_title:  Neutrality disputed? 

message: After some thought, I don't see any way to salvage most of the material in the article as it was prior to my edits.  The introductory paragraph had nothing to do with background independence as the term is used in physics, or in any of the references.  The "Principle of universality" section was pretty much nonsense - for instance, the idea that the fine-structure constant cannot change in a BI theory is absurd and totally unsupported.  Again, totally uncited.  Then, as discussed elsewhere on the talk page, the diffeo invariance section was simply wrong (and contradicted by its own references).  
So I just reverted to the last sensible version, and then added back some links and references.  I'll add a comment about AdS/CFT too. Waleswatcher (talk) 02:16, 14 January 2012 (UTC)
thread_title:  Article reverted to old version 

message: In the section "Loop quantum gravity" the article states that "any consistent quantum theory of gravity should include topology change as a dynamical process." Is there any simple argument showing that, or could someone add references that back this statement? --bajo (talk) 14:25, 28 February 2012 (UTC)
replies: text: Well, a background independent theory can't be restricted to a unique topology any more than it can be restricted to a unique geometry (at most, it could depend on some kind of boundary conditions).  When you integrate over metrics, that automatically includes more than one topology.  So that's really part of the definition of background independence.  With that said, I agree it needs a reference - the entire article is inadequately sourced.  Waleswatcher (talk) 14:38, 28 February 2012 (UTC)
replies: text: Thank you, but I see two separate issues here. One is whether "background independence" can be defined as to include only one topology, and while this might be difficult/impossible, it is not what the article states. The other is whether a quantum theory of gravity can be consistent if restricted to a single topological sector. I am not sure if there is an argument showing that in a path-integral formulation one is forced to include all the topological sectors. For example, in the case of YM theories, one can show that you need to include all the distinct instanton sectors using the cluster decomposition principle; maybe a similar argument can be used in the case of gravity, but I am not sure. Also, even if such an argument could be found, I would be a bit wary to generalize it to "any consistent theory of quantum gravity", since the path-integral is most certainly ill-defined in the case of gravity, and it is not difficult to imagine that different formulations could accomodate a single topology in a consistent fashion. As it is stated now, the sentence under discussion seems to imply that LQG is not a consistent theory of quantum gravity because it requires a fixed topology. --bajo (talk) 10:55, 29 February 2012 (UTC)
replies: text: I think it should simply state that this makes LQG not explicitly BG independent - or at best, BG independent only in a weak sense.  Asserting or implying that it makes LQG inconsistent is too strong. Waleswatcher (talk) 12:17, 29 February 2012 (UTC)
thread_title:  Topology change 

message: Background independence is not a Quantum Gravity specific issue. It means something largely unambiguous in the case of General Relativity, and is uncontroversial there as well. In Quantum gravity however, it's subject to a lot of debate, with differing views about what counts as background independence, what it means, whether or not one should have it, etc. In my opinion, this page seems to be the result of these arguments, where people are trying to extol the virtues of their favorite QG theory, and so all that remains on this page is a list of features, loosely related to background independence, of the two main QG theories. This articles doesn't even explain what background independence actually is! Clearly, the contributers have lost sight of what is important.
I propose, and I have started working on this, that we focus the article mostly on classical background independence, where things are much better understood and less speculative/"political", and leave the QG subtleties to a minor mention at the end. DimReg (talk) 07:39, 30 April 2013 (UTC)
thread_title:  "Depoliticizing" the article 

message: 70.247.175.236 has proposed this article to merge with "General covariance." But "General covariance" is of the view point of orthogonal coordinates, i.e. General relativity. I think "background independence" is rather of the coordinate itself.--Enyokoyama (talk) 11:12, 31 December 2013 (UTC)
thread_title:  about merging with general covariance 

message: Parts of the article are far from being clear, or sometimes incomprehensible. Sure that background independence at least means that it must be possible to not refer to a specific coordinate system., but what is the meaning of the defining equations of a theory to be independent of the actual shape of the spacetime? So in a 5-dimensional spacetime we should have the same (number of) Einstein equations?
Sure, Background-independence is a loosely defined property of a theory of physics, but this should appear at the beginning. 
Next, the main trouble arises in the section "Manifest background-independence". Background-independence is loosely defined, so we know what it means in particular instances only; anyway, whatever background-independence means, a theory is background-independent exactly in case it (admits a formalism which) is manifest background-independence. Thus I do not understand the section at all: making a property manifest is not only aesthetic, or a useful tool: it is the only way to check that some theory is actually background-independent! PS: Of course, maybe one does not care to check that a theory is background independent, that's fine, but if someone wants to check, the only way is finding manifest independence! — Preceding unsigned comment added by 78.15.203.91 (talk) 19:05, 19 September 2014 (UTC)
thread_title:  Not clear 

message: near the emptyness of intension or sth. of good & bad shapes?
But, by the way, what is spacetime, if it has different shapes? 
Why not different flavors, colours or versions?
If the world of talking would be not limited (actually by the time humans are able to think about), mankind would talk and talk about that without coming to an end. The grandsons of the grandsons of Lee could find a newly flavored style of spacetime that currently is unknown. What I want to point out is: 
If someone is not able to conclude from his assumptions that his proposal is sound and complete, we should wait until they are und just avoid to waste life time.  — Preceding unsigned comment added by 84.59.227.47 (talk) 14:34, 19 December 2015 (UTC)
thread_title:  What is as "shape of the spacetime"? 

message: In the quantum gravity section...
"in 3D, quantum gravity has no local degrees of freedom"
I don't see how that is intuitively obvious.  Any chance further explanation could be included?  — Preceding unsigned comment added by DarkSky7 (talk • contribs) 01:15, 18 May 2022 (UTC)
thread_title:  quantum gravity has no local degrees of freedom in 3D - why?