Talk:De Sitter invariant special relativity

DRV
It looks worth keeping to me ... 46 citations Abtract (talk) 19:21, 4 December 2008 (UTC)

Caveat: I edited this
It should never have been nominated for deletion in the first place. This stuff is by now so old and well known that it is in it's second and third generation literature. I don't think it's correct as a physical principle, but it's definitely an interesting alternate view.Likebox (talk) 19:35, 4 December 2008 (UTC)


 * Likebox, do you believe that de Sitter general relativity is a coherent idea? If so, can you explain to me what it means? I really don't get it. If not, can we delete all mentions of it?


 * One of the assumptions of "de Sitter general relativity" seems to be that spacetime in general relativity has a Poincaré symmetry that one can replace with de Sitter symmetry. But that's not true. Some spacetimes are Poincaré symmetric, most aren't. The spacetime of the real world isn't. The FLRW spacetime is more symmetric than the real world, but its symmetry group is still just the symmetry group of flat/elliptical/hyperbolic three space, except in a few boundary cases that don't describe our world. The only thing you could possibly call Poincaré symmetric in general relativity is the tangent spaces, and those have nothing to do with the physics, they're just a convenient way of doing the math. You can replace them with de Sitter spaces if you want to, but it doesn't mean anything. The spacetime wasn't Poincaré symmetric before and it isn't de Sitter symmetric after. There's no physics here. You can't get any predictions about gamma rays or dark energy. Do you agree?


 * Of course doing physics on de Sitter space (ignoring gravity) is a perfectly reasonable idea that's been around for ages. But, as I've been saying over and over in the deletion discussion, that's not what this article was about. -- BenRG (talk) 02:17, 5 December 2008 (UTC)


 * I don't know if de Sitter General Relativity is all that coherent, because I don't understand it very well, but it might be. But first, you misunderstand deSitter special relativity--- it is not necessarily exactly the same as doing physics in deSitter space, because it is based on the commutation relations of deSitter group only, not on the points in de Sitter space.


 * de Sitter space in General Relativity is a collection of points and a metric whose symmetries are the de Sitter group. The idea of de Sitter special relativity is to start with the commutation relations of the de Sitter group only, just the group, not the points, and to demand that the physical objects form representations of this group. This might end up having an interpretation as local fields on a collection of points de Sitter space, but if it does, it ends up being boring. If this idea is at all going to give something new, it had better do away with the notion of geometric point entirely. I heard this is called "pointless geometry" sometimes, but that might be an outdated phrase.


 * Since that the deSitter group includes translations in a nontrivial way, a representation of the group will necessarily include some aspect of positional information in the representation. But the notion of position might not end up defining points individually, and leave the whole description a little nonlocal when everything is said and done. Remember, the deSitter group is a lot like the rotation group--- positions in deSitter space would be like the polar angles theta and phi for an electron in a hydrogen orbital--- they would end up being described by spherical harmonics. If you truncate the Harmonic series you get a nonlocal description consistent with deSitter symmetry.


 * deSitter General Relativity, presumably, would then be the idea of incorporating metric fluctuations around the deSitter background in a reasonable pointless way. You would need to define a spin-2 representation, which corresponds to a spin-2 field in the Poincare limit, and then hopefully there will be some nonlocal representation which would be the physical metric in a deSitter background.


 * There is no good way of doing quantum gravity on deSitter backgrounds right now, and even the semiclassical physics is mysterious (what does the horizon-area entropy mean?), this approach might end up giving some insight eventually. But be that as it may, other people wrote papers about this idea, and they probably have much better ideas about this than I do. The first I heard of it was when I ran across the deletion discussion.Likebox (talk) 04:58, 5 December 2008 (UTC)


 * I can't be certain, but if as Ignazio Licata and Leonardo Chiatti say, Fantappié-Arcidiacono theory of relativity is the same thing as the recent work on de Sitter relativity, then de Sitter general relativity should be the same as Fantappié-Arcidiacono projective general relativity. Delaszk (talk) 14:51, 5 December 2008 (UTC)


 * Likebox, that sounds like doubly special relativity, which already has an article. How is de Sitter SR different from doubly special relativity? (I realize the article has a section on this—does the section make sense to you?) I think you're reading into this work far more than is there. The authors of these papers appear to be embarrassingly ignorant of classical cosmology, never mind quantum gravity or quantum anything. Look at section 4 of this paper, where Aldrovandi and Pereira imply that they think that Ωv ~ 1 in all eras in the ΛCDM model. You understand this subject better than they do, and what you're describing is the research you would do, not the "research" they actually did.
 * Delaszk, after briefly looking online for information about Fantappié-Arcidiacono projective relativity, I agree that it appears to be the same thing as de Sitter SR/GR, and it appears to be content-free for the same reason. That would, I hope you'll agree, explain why mainstream cosmologists have completely ignored it. -- BenRG (talk) 16:39, 7 December 2008 (UTC)


 * To BenRG: this is not doubly-special relativity. DSR is a even more speculative proposal that the group of relativity should be altered at high energies to allow for an invariant very small length scale (Planck length). The invariance group of DSR is different than the Poincare group and the DeSitter group both. It is incompatible with ordinary special and General Relativity, and has very serious problems reproducing ordinary physics.


 * DeSitter relativity is a completely different modification, at low energies, which is compatible with ordinary general relativity, and has a very large length scale (the maximum radius of the universe). Because it includes the case of ordinary General Relativity with a small cosmological constant, it could be thought of as "content free", as I said above, but that's the wrong way of looking at it.


 * While you are right that I am describing the research I would do if I was working on it, the only reason is because I don't know exactly what's in the latest papers. I only know the old stuff. But the new stuff is keeping this cluster of ideas alive, and that's important the way I see it. Maybe there are less than stellar papers in this field--- but isn't that true of everything? Don't delete things because you don't like the people or the papers, somebody could get inspired by an incompletely developed or wrongly developed idea to do something interersting and new.


 * Mainstream cosmologists have ignored this theory not because it is so terrible, but because they have other things to do. Not every cosmologist is aware of every idea, and ideas do not get attention according to degree of interest. This is a speculative theory, and it is not being pushed by any powerful people, so it is not very well known. That's precisely why we have Wikipedia, so stuff like this doesn't stay obscure.Likebox (talk) 19:28, 7 December 2008 (UTC)

Dyson's article
I think it should be made clearer how Freemnan Dyson's article relates with the topic of this article. As far as I can see, Dyson does not refer to De Sitter anywhere in his article. In what I think to be the relevant section, he talks about the six axioms of Haag and Kastler, which seem rather different from the De Sitter group. -- Jitse Niesen (talk) 11:56, 10 December 2008 (UTC)


 * You looked at the wrong article. The article is "Missed opportunities", it is referenced here, and it doesn't talk about Haag Kastler axioms at all anywhere, just about this stuff. Two sections of the paper are devoted to explaining this idea in detail.Likebox (talk) 19:22, 10 December 2008 (UTC)

(deindent --- here are the starting paragraphs of the relevant section Dyson '72 from the article)
 * Kinematical groups: The story of the Maxwell equations has a postscript, in which the pure mathematicians again missed an opportunity, though not one of such major importance as that which they missed in 1873. Nobody noticed that Minkowski in his 1908 lecture failed to carry his argument to its logical conclusion. Minkowski did not mention the fact that the Maxwell equations are invariant under the trivial Abelian group 7i of translations of the space-time coordinates. The natural invariance group of the Maxwell theory is not the six-dimensional Lorentz group Gc but the ten-dimensional Poincaré group P which is a semi direct product of Gc with T4. Similarly, the invariance group of Newtonian mechanics is not the six-dimensional G^ but the ten-dimensional Galilei group G which is a semidirect product of G^ with T4.


 * Neither P nor G is a semisimple group. With hindsight it is easy to see that Minkowski's logic ought to have given somebody the idea that there exists a simple group D of which the nonsemisimple group P is a degenerate limit, in exactly the same way as the semisimple Gc has the nonsemisimple G^ as a degenerate limit. This D is the DeSitter group, a real noncompact form of the simple Lie algebra J32- Following Minkowski's argument, a pure mathematician might easily have conjectured in 1908 that the true invariance group of the universe should be D rather than P.Likebox (talk) 19:47, 10 December 2008 (UTC)


 * You are both talking about different sections of the same paper. Section 4 discusses the de Sitter group. Sections 6 and 7 discuss Haag Kastler axioms. Delaszk (talk) 22:15, 10 December 2008 (UTC)


 * Serves me right for not reading to the end!Likebox (talk) 23:12, 10 December 2008 (UTC)

Credit
When discussing an academic idea, it is both traditional and important to give credit to the person who published first, regardless of whether anybody else paid attention.Likebox (talk) 23:37, 11 December 2008 (UTC)

Confusion with DSR?
As a consequence of de Sitter relativity, this makes no sense to me:


 * When applied to the propagation of ultra–high energy photons, some claim that the theory explains a controversial time delay possibly observed in extragalactic gamma ray flares. More precisely, there are claims, not yet accepted by the mainstream physics community, that very–high energy extragalactic gamma–ray flares travel slower than lower energy ones . If this is confirmed by future experiments, it will constitute a clear violation of special relativity.

This looks like a confusion with DSR (as discussed by others above). It also didn't fit in the flow of exposition (mathematical theory -> above stuff -> continuation of mathematical discussion). 66.127.53.120 (talk) 05:03, 8 September 2010 (UTC)

Is "group extension" the correct term?
This edit (and the previous) imply that group extension is the converse of group contraction. Doesn't a group embed in its extension? A group does not in general embed (contain) its contraction. (I don't follow the article Lie algebra extension, so I need to ask.) —Quondum 01:07, 18 May 2015 (UTC)


 * Thanks, now fixed. Lorentz extends to either Poincare, or deSitter; deSitter contracts to Poincare, which expands to de Sitter, in turn. I appreciate the reader would not learn about expansion in the extension article, directly. Cuzkatzimhut (talk) 11:21, 18 May 2015 (UTC)


 * That seems to make sense. (Aside: Here you mention the concept of a group expansion, the converse of group contraction, which might be interesting to document, if it is dealt with in the literature.)  We still have the term "expands" in the lead being used apparently incorrectly, as well as the term "generalizes" being used incorrectly (the latter problem was there before).  I still need to look a little more closely.  —Quondum 13:37, 18 May 2015 (UTC)


 * Yes, "expansion" is covered by Gilmore's book, but is not represented in WP except as the self-evident converse of group contraction. Both a group and its expansion (or contraction) can be thought of as group extensions of a subgroup of both... Because "generalization" was so vague and ambiguous, I thought I'd supplant it, but I've left it there as before, to avoid this ambiguity. I am confused about the lead... You mean "expansion of the universe" could possibly get confused with group issues? Cuzkatzimhut (talk) 18:05, 18 May 2015 (UTC)


 * I am not concerned about "expansion of the universe" being confused.
 * Looking at the text changed in this edit, both the terms 'generalizes' and 'extends' seem to me to be incorrect, so this should be cleaned up. Surely this is exactly where 'expands' would apply, since the Poincaré group is a contraction of the de Sitter group? If you have the energy, adding a mention of group expansion to Group contraction, with Gilmore as reference, would be nice.  —Quondum 18:25, 18 May 2015 (UTC)


 * Oh, OK, not in the lead... I excised the link to extension but reverted to "generalizes", avoiding the technical "expands", which I obviously prefer. I would assume that all the talk of contractions in the article would steer the reader to group contraction and the Gilmore book, which spends the better part of a chapter on expansions, III EXPANSIONS, pp 477—492. I think their logical place would be in Lie algebra extension, as alternate extensions of a subalgebra can lead to the expanded and contracted members of a pair, but this might have to wait. Physicists intuitively grasp the standard examples of special relativity, QM, and GR/BH to sort out the algebras... Given some time, a few short, informative statements could be added, but perhaps not now. Cuzkatzimhut (talk) 18:44, 18 May 2015 (UTC)


 * You replaced "generalizes" with "extends" as opposed to "expands", which I felt was wrong. If you had replaced it with "expands", it would have made sense, even if the linked article does not (yet) have a definition of the term.  An alternative would be to substitute "contracts to".  But, like you, I do not like "generalizes".  —Quondum 20:31, 18 May 2015 (UTC)  But based on your comments, expansion and extension may be more intertwined than I realize, so maybe my comment is off-beam. —Quondum 20:34, 18 May 2015 (UTC)

OK, I straightened out the grammar and parallel structure, I hope, and squelched some "generalization"s in favor of some contractions in the reverse direction. The contraction is actually detailed adequately in the Galilean transformation article now linked. Cuzkatzimhut (talk) 22:38, 18 May 2015 (UTC)


 * I like it – it makes good sense like that. There is still once instance of "generalizes" in this abuse in the introduction: "Minkowski's unification of space and time within special relativity generalizes the Galilean group of Newtonian mechanics with the Lorentz group."  Here we might want to just use the word "replaces", because it is so early and throwing such a deep concept in here might make it more difficult to follow. —Quondum 23:29, 18 May 2015 (UTC)

Sure. Cuzkatzimhut (talk) 23:48, 18 May 2015 (UTC)

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