Talk:Loop quantum gravity/Archive 2

message: it seems to me this article doesnt explain what loop quantum gravity is to a layman.
  how about some more intuitive examples or analogies to explain say what a loop is?
               the dogman
=== Wilson loops and spin networks ===
: The development of a quantum field theory of a force invariably results in infinite (and therefore useless) answers. Physicists have developed mathematical techniques (renormalization) to eliminate these infinities which work for the electromagnetic, strong nuclear and weak nuclear forces, but not gravity. 
: The most obvious ways of combining the two (such as treating gravity as simply another particle field) run quickly into what is known as the renormalization problem. Gravity particles would attract each other and if you add together all of the interactions you end up with many infinite results which can not easily be cancelled out.  This is in contrast with quantum electrodynamics where the interactions do result in some infinite results, but those are few enough in number to be removable via renormalization.
: Thus the development of a quantum theory of gravity must come about by different means than were used for the other forces.
: In LQG, the fabric of spacetime is a foamy network of interacting loops mathematically described by spin networks. These loops are about 10-35 meters in size, called the Planck scale. The loops knot together forming edges, surfaces, and vertices, much as do soap bubbles joined together. In other words, spacetime itself is quantized. Any attempt to divide a loop would, if successful, cause it to divide into two loops each with the original size. In LQG, spin networks represent the quantum states of the geometry of relative spacetime.  Looked at another way, Einstein's theory of general relativity is (as Einstein predicted) a classical approximation of a quantized geometry.
in my opinion this section is relevant to the subsection "space atoms", and "space atoms" is a relevant implication of LQG, but if you disagree miguel, and you decide to remove it, i'll respect your decision and won't reverse it. 
:This should actually be part of the general quantum gravity article. — Miguel 16:40, 2004 Nov 22 (UTC)
=== Special relativity and Lorentz invariance ===
I have replaced the following
:In order to account for the structure of space and time at the Planck scale, LQG breaks Lorentz invariance and posits that certain well known effects of special relativity such as length contraction and time dilation cannot occur below the threshold of the Planck scale. It also predicts that the speed of photon propagation in vacuum may be dependent on the photon's wavelength, and not constant as demanded by special relativity (where it is denoted by c). It is not clear whether an approximate Lorentz invariance can be recovered in LQG at long distances and whether LQG can explain the plethora of successful experimental tests of special theory of relativity. LQG proposes a privileged reference frame associated with the spin foam, and therefore it is natural to expect that it may suffer from the usual problems of the old-fashioned theories of luminiferous aether. 
by an entirely new account. Comments are welcome! At the very least, the readability of the section needs to be improved. — Miguel 21:21, 25 Jul 2004 (UTC)
thread_title:  intuitive understanding 

message: I have replaced the following
:In order to account for the structure of space and time at the Planck scale, LQG breaks Lorentz invariance and posits that certain well known effects of special relativity such as length contraction and time dilation cannot occur below the threshold of the Planck scale. It also predicts that the speed of photon propagation in vacuum may be dependent on the photon's wavelength, and not constant as demanded by special relativity (where it is denoted by c). It is not clear whether an approximate Lorentz invariance can be recovered in LQG at long distances and whether LQG can explain the plethora of successful experimental tests of special theory of relativity. LQG proposes a privileged reference frame associated with the spin foam, and therefore it is natural to expect that it may suffer from the usual problems of the old-fashioned theories of luminiferous aether. 
by an entirely new account. Comments are welcome! At the very least, the readability of the section needs to be improved. — Miguel 21:21, 25 Jul 2004 (UTC)
thread_title:  Special relativity and Lorentz invariance 

message: 1. The introduction. "String theory claims to be a theory of everything". First, string theory is not "claiming" anything: it is *people* who might make such claims. Second, is there any citation for a leading figure in string theory actually saying or writing "String theory is a theory of everything"? I don't think anyone has seriously believed this since 1985 or so. The sentence could be rephrased as follows: "while LQG is intended to be only a quantum theory of gravity, string theory can accomodate many other phenomena including matter and gauge (vector) particles; this fact has led to attempts to model not only gravity but also elementary particle theory with strings.
The sentence about "mutually exclusive/incompatible models" with "6,7, or 22 extra dimensions" is somewhat misleading, because since about 1995, it is known that different types of superstring theory are interrelated through dualities, so that what were previously apparently "mutually exclusive" solutions are now seen as different points in a single parameter space or space of vacua. For example it is thought that there is a smooth transition from IIA strings in 10D to 11D supergravity. So, it would be more correct to say that at most one point in the (apparently very large) space of possible vacua could describe our patch of the Universe.
Most obviously, the reason why there are so many more strings papers is the fact that they *do* have matter and gauge particles, so there are many more opportunities to compare models with data. The number of particle theory papers *without* strings is similarly large. If you don't concern yourself with particle physics, then obviously you have a lot less to write about.
:IMHO, there are too many mentions to String Theory in this article. There was an old version where I wrote something to the effect that "no attempt will be made to describe String theory in more detail here since it wouldn't be possible to do it justice" and left it at that. This page is slowly morphing into a debate of the relative merits of String Theory and LQG as theories of quantum gravity, which belongs neither in string theory nor here, but under quantum gravity. 
::Yes, I quite agree. I will continue to edit, reminding myself not to mention string theory unless necessary.
:I think your criticism is well-places and I encourage you to act on this criticism by editing the article yourself. There are a lot of argumentative statements in the article that really get in the way of presenting LQG and, as you point out, cause internal consistency problems.
:Miguel 02:38, 10 Jul 2004 (UTC)
replies: text: In the same way that the fine structure of the Hydrogen atom is an improvement over the Balmer series and not a way to argue that Pauli's theory of electron spin must be wrong. You just can't use an empirically untested semiclassical result as a benchmark for a purported nonperturbative quantization. Miguel 02:38, 10 Jul 2004 (UTC)
replies: text: I'm still puzzled. Do you mean improvement in the 'technical' sense of a more accurate calculation within a given theory, e.g. the scalar potential "improved" with radiative corrections?
5. Quantum field theory. It should be mentioned that string theory also addresses the "ultraviolet catastrophe" (a.k.a. problem of divergences). String theory allows the calculation of graviton scattering to one-loop, which is finite, and in principle to arbitrary loop order, although this presents considerable technical difficulty.
6. Graviton. Similar remarks apply here. While of course it does not have a real background-independent formulation, string theory has a graviton.
7. Theory of Everything. TOE is not a theory, as yet, since no-one knows if such a thing exists. Best to say "hoped-for theory" or similar. Is string theory "intended" to be a unified TOE? As far as I know it was invented as a theory of the strong interactions. Does string theory "have the capacity" to describe all matter and interactions? No-one knows, and I don't know of any leading string theorist who makes precisely such "grandiose" claims. Reference please? 
There is a common error in stating that the electromagnetic and weak forces "become indistinguishable" at the electroweak breaking scale. Not so, they just get mixed up into two other *distinguishable* gauge groups SU(2)xU(1). The statement "unification can no longer be expected from low-energy phenomenology" is extremely dubious. It depends very much on the details, for example threshold corrections. Unified models exist that still work perfectly well, although they are increasingly tightly constrained. The sentence implies that unification is ruled out by data, which is not true. Unification is not just a matter of philosophy, it is a matter of physics, since it implies constraints and predictions, and the more constraints and predictions, the better the theory (ceteris paribus).
8. Experimental tests. In fact, people such as Kostelecky have done work on spontaneous breaking of Lorentz symmetry in string theory. Hence it is incorrect to say that 'string theory predicts exact Lorentz symmetry at all scales'. To be sure, spontaneous violation of Lorentz symmetry would be an unexpected result, but it would not precipitate a crisis.
9. Sociology and politics. I'm not sure what this section, which consists mainly of opinion and gossip, is doing in an encyclopedia. It seems to have a (completely unwarranted) assumption that LQG and string theory should have "equal rights" and thus any inequality ("imbalance"!) between them is remarkable and worthy of discussion. Since LQG deals with only quantum gravity, whereas strings deal with both gravity and particle physics, I don't see any reason why they should be treated equally. (Besides which, there is the great difference in the number of things that can be calculated in each theory.) The apparent complaint that string theory funding is not fungible into LQG funding is particularly bizarre. If a department has a professor who is expert in string theory, they are hardly going to accept a grad student to study LQG (or vice versa). The remark "it is psychologically difficult for anyone to accept he or she may have dedicated their graduate studies and academic careers on the wrong theory" is tangential and (at present) irrelevant, since we have no idea what is "the wrong theory".
I don't know why the fact that Motl and Baez and Carlip used to argue on s.p.r. (etc.) is important enough to mention here.
--Tdent 15:39, 9 Jul 2004 (UTC)
thread_title:  Criticisms of the article 

message: I assume something is missing here, which the contributors of the article may find self evident:
I read, that there is a string community and a loop community. And the 
Differences between LQG and string/M-theory may come from the fact, that
String theory and LQG are the products of different communities. Namely
String theory emerged from the particle physics community.
O.K. now the $1M question which isn't addressed: Where and in which community did LQG emerge?
Or can all this community-talk be moved that a sub-article?
Pjacobi 17:21, 11 Jul 2004 (UTC)
thread_title:  On "communities" 

message: The article is rather long (42k) and it contains sections, which don't give core information but are meta to the topic. Also the extensive lists in Research in LQG and related areas detract from the core message. IMHO, and believe I don't want to start editing the page, the lists should go to Research in LQG and LQG and the sociology and politics of science, Differences between LQG and string/M-theory, and Philosophy around LQG should go to LQG in context (not that brilliant a title). Pjacobi 22:09, 24 Jul 2004 (UTC)
thread_title:  Content rearrangement