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Graviweak unification at ILQGS (online talk by Marciano) 
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#19
Feb2213, 01:51 AM

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It was hard deciding what to say here. My inner Lubos, whose opinions I respect, says that http://arxiv.org/abs/1212.5246 is a bad paper. All their manipulations are either formal or classical, they never construct the quantum theory, so they never face the problems of treating gravity as a gauge theory (lack of a semiclassical limit, abnormal method of quantization, infinite undetermined couplings of spin foams) or the problems of their proposed spinorial unification (how can a boson, the Higgs, be related by parity to a fermion, the sterile neutrino?).
But my inner Lubos doesn't have the knowledge of the real Lubos, and the debates about loop gravity get pretty technical. Loopinspired research is diverse enough that most of it could be wrong and still someone might have a good idea about quantum geometry; and I am engaged elsewhere in "wishful thinking" of my own, i.e. theoretical speculations that I don't even know to be mathematically welldefined; so I could keep my opinions to myself and say, live and let live. However, this paper can be viewed from another angle where I have more of an open mind. This is the possibility that the use of the Plebanski action in loop gravity might have some crossover with new twistor gravity. I mentioned this in the "twistor networks" thread, and it turns out that Lionel Mason suggested this at Zakopane in 2010  see the last slide of his talk. If you look at slide 33, you will see where I think there's an opening for "chiral graviweak unification" to enter  the division of that action into two parts. So for me that's the interesting question now  if you express the graviweak ideas of Nesti & Percacci, etc., in twistorial variables, how closely do they resemble Mason's division of a twistor action into "antiselfdual" and "maximumhelicityviolating" parts? And this is something I have not yet tried to answer, at this point it's pure intuition and could be thoroughly misguided. 


#20
Feb2213, 11:00 PM

Astronomy
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PF Gold
P: 23,235

Since we turned a page I will bring forward the main links to what we're discussing in this thread. There's an online gravity+matter unification talk Tuesday 26 April.
The links to audio and to slides PDF will be posted here: http://relativity.phys.lsu.edu/ilqgs/ The title of the talk is Gravity Electroweak Unification The speaker is Stephon Alexander, a professor of physics at Dartmouth. The paper to read, to prepare for the talk is: http://arxiv.org/abs/1212.5246 Gravitational origin of the weak interaction's chirality Stephon Alexander, Antonino Marciano, Lee Smolin (Submitted on 20 Dec 2012) We present a new unification of the electroweak and gravitational interactions based on the joining the weak SU(2) gauge fields with the left handed part of the spacetime connection, into a single gauge field valued in the complexification of the local Lorentz group. Hence, the weak interactions emerge as the right handed chiral half of the spacetime connection, which explains the chirality of the weak interaction. This is possible, because, as shown by Plebanski, Ashtekar, and others, the other chiral half of the spacetime connection is enough to code the dynamics of the gravitational degrees of freedom. This unification is achieved within an extension of the Plebanski action previously proposed by one of us. The theory has two phases. A parity symmetric phase yields, as shown by Speziale, a bimetric theory with eight degrees of freedom: the massless graviton, a massive spin two field and a scalar ghost. Because of the latter this phase is unstable. Parity is broken in a stable phase where the eight degrees of freedom arrange themselves as the massless graviton coupled to an SU(2) triplet of chirally coupled YangMills fields. It is also shown that under this breaking a Dirac fermion expresses itself as a chiral neutrino paired with a scalar field with the quantum numbers of the Higgs. 21 pages Video of two previous talks on this topic (by Alexander and by Smolin) begins at minute 18 of this PIRSA resource: http://pirsa.org/12100116 Quantum Gravity and the Weak Interactions (Recorded in October 2012) Alexander's talk begins shortly after minute 18. Smolin's talk comes immediately after that and begins around minute 35. Alexander's online ILQGS talk next week will be followed up later this Spring by a second ILQGS talk, by Marciano, on the emergence of graviweak Plebanski models from spinors. The Plebanski action for General Relativity is obviously the basis for this whole development (that plus the Ashtekar connection formalism). So we should know who Jerzy Plebanski (19282005) was. He proposed this action for GR in 1977 [19] J.F. Plebanski. On the separation of einsteinian substructures. J. Math. Phys., 18:2511, 1977. AFAICS the above paper and talks are not directly concerned with quantum gravity. The work here is carried out on a classical level. The Plebanski action is a formulation of classical GR (where the variable is a connection rather than a metric) and the idea here is to extend the Plebanski action to include some types of matter interaction. What mathematicians call a connection seems equivalent (or nearly so) to what physicists call a gauge field. So this line of research seems to be aimed at opening a way to treat both geometry and matter as gauge fields, in a single Plebanskitype action. Later on, if the program is successful, one might consider some backgroundindependent, nonperturbative way of quantizing this combined gauge theory of geometry and matter. But that is not what this paper is about. A Spin Foam quantization of the Plebanski action was given for example by Perez in 2002: http://arxiv.org/abs/grqc/0203058 Spin foam quantization of SO(4) Plebanski's action Alejandro Perez (Submitted on 15 Mar 2002) Adv.Theor.Math.Phys. 5 (2002) 947968 For people unfamiliar with the term, spin foam is a kind of LATTICE GAUGE THEORY, constructed without assuming a prior metric geometry. So if the classical program initiated by Alexander Marciano Smolin were successful, one way to follow up on it would presumably be to quantize it in the context of a lattice gauge theory. I expect all these issues will be clearer in a few days, after Stephon Alexander's Tuesday seminar talk. 


#21
Feb2313, 01:03 AM

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PF Gold
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Hi Mitchell, thanks for your comment #19it's quite interesting how you think about that AMS paper! Some of the points you raise involve what you think are issues with LQG. And you indicate you do not want to get into a discussion of current LQG developments, so out of regard for your wishes I can't respond.
But in any case these points are of limited relevance to the AMS paper! That we are discussing and it is not a LQG paper! The paper seems to me to aim at a classical unification of gravity and weak interactions as connections ("gauge fields") within the extended Plebanski action of classical GR. Lee Smolin does an excellent job of putting this work in context in that 15 minute talk of his. It could open the way to a unified quantization of some typephysicists have had a lot of experience quantizing connections. Anyone interested in this topic should certainly watch the video. http://pirsa.org/12100116 Start around minute 35. (Or start around minute 20 to get Alexander's talk leading in to Smolin's) Both Percacci and Reuter are in the audiencePercacci asks questions and gets into discussion but Reuter just listens. Smolin devotes a substantial part of his 15 minute talk to explaining just what you were asking abouthow various components could be combined in a Dirac spinor, which could break spontaneously into several parts (including bosonic). If you had been at the conference you could have expressed incredulity and challenged him on this, but as far as I could tell no one at the conference did. Maybe there will be some questioning about it at seminar on Tuesday. 


#22
Feb2313, 06:33 AM

P: 756

Quantum issues aside, my feeling for now is that this is probably just based on a coincidence  that both the weak gauge field and the Plebanski spin connection are valued in SU(2)  and that the stuff about breaking a spinor into a Higgs and a neutrino is the sort of line of thought that you end up following, when you start with something wrong but plausible, and try to make it work: you end up considering increasingly implausible ideas. Still, the original idea is so simple, and brings together so many topics, that I expect to be exploring the tangents from it for quite some time.



#23
Feb2313, 09:48 AM

Astronomy
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PF Gold
P: 23,235

I think the (possible coincidence) key to it is that when you put classical GR into a connection form, using Ashtekar method, the SL(2,C) connection splits into two SU(2) parts, and strangely enough only one of them is needed. The other is suppressed or thrown away, or one can try to use it for some purpose. A number of people have noticed this (possible coincidence) and worked on it. Stephon A's first paper on it was scooped, he says by just 3 days, by Percacci and Nesti. Kirill Krasnov has also written some papers about this. The AMS paper mentions a bunch of other people. The (apparent) coincidence has tempted quite a few souls, but no one has yet found a way to use the other half and brought it to fruition. One possible danger signal that Smolin discusses in his 15 minute video (at the Perimeter conference on Experimental Tests of QG) is that it requires the cosmological constant Lambda to be LARGE, and then have most of it canceled off by higherorder corrections. Because astronomically speaking we only observe a SMALL cosmological constant. So the idea depends in an essential way on work that had not been done as of October last year when they spoke at the Experimental QG conference. The reason they were speaking at the Experimental QG conference is of course because their way of unifying gravity with weak interactions generates predictions to test at LHC. It is drastic enough that it must have immediate consequences particlewise. In case new people are reading, the essential stuff we are discussing is in two consecutive 15 minute talks on this conference video: http://pirsa.org/12100116 Start just before minute 20 to get Alexander's talk and around minute 35 for Smolin's. It's an easy way to get familiar with their idea without having to wade through the paper. 


#24
Feb2313, 10:57 AM

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Nesti and Percacci's paper was discussed extensively at Jacques Distler's http://golem.ph.utexas.edu/~distler/...es/002140.html . Nesti and Distler have a very informative conversation.



#25
Feb2313, 11:13 AM

Astronomy
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PF Gold
P: 23,235

http://arxiv.org/abs/0909.4537 


#26
Feb2313, 11:28 AM

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PF Gold
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Maybe people who have more than a a superficial impression of this Graviweak unification gambit and are actually interested in finding out about it will want to look at the 2007 Nesti Percacci paper.
http://arxiv.org/abs/0706.3307 Graviweak Unification F. Nesti, R. Percacci (Submitted on 22 Jun 2007) The coupling of chiral fermions to gravity makes use only of the selfdual SU(2) subalgebra of the (complexified) SO(3,1) algebra. It is possible to identify the antiselfdual subalgebra with the SU(2)_{L} isospin group that appears in the Standard Model, or with its righthanded counterpart SU(2)_{R} that appears in some extensions. Based on this observation, we describe a form of unification of the gravitational and weak interactions. We also discuss models with fermions of both chiralities, the inclusion strong interactions, and the way in which these unified models of gravitational and gauge interactions avoid conflict with the ColemanMandula theorem. 18 pages It's interesting that Roberto Percacci was at last year's Experimental QG conference and responded to Lee's talk. Here is the paper by Alexander which appeared about the same time. http://arxiv.org/abs/0706.4481 Isogravity: Toward an Electroweak and Gravitational Unification Stephon H.S. Alexander (Submitted on 29 Jun 2007) We present a model that unites the electroweak interaction with general relativity without specifying a spacetime metric. This is made possible by embedding the kinetic terms for gravity and electroweak theory using one SL connection variable. The gauge theory is specified without relying on a spacetime metric. We show that once a symmetry breaking mechanism is implemented that selects a global timelike direction, the electroweak theory and general relativity emerge with their associated massless degrees of freedom; the spin 1 vector boson and the spin 2 graviton. 


#27
Feb2313, 11:40 AM

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#28
Feb2313, 11:51 AM

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PF Gold
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Numerous different proposals avoid ColemanMandula, don't let yourself be hypnotized by the words ColemanMandula into thinking they are all the same ideas. I don't see even a superficial resemblance with 2007 N&P "Graviweak". Where is SO(3,11)? 


#29
Feb2313, 12:04 PM

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"Here we discuss the issue of chirality in the context of theories where the Lorentz group, which is gauged in theories of gravity, is uniﬁed with a GUT group in a larger group G. By this we mean that the gravitational connection and the gauge ﬁelds of a GUT are components of a connection for the unifying group G. We will call such a theory a “GraviGUT” (GGUT). Unlike in [5], we do not insist on putting all ﬁelds in a single representation of G: gravitons, gauge ﬁelds, fermions and scalars will belong to diﬀerent multiplets. The general idea for this kind of uniﬁcation has been discussed in [7–9]. ... The main diﬀerence is that the order parameter cannot be a scalar but must include a multiplet of one forms, called the soldering form." When Nesti discusses the ColemanMandula theorem with Haelfix and Distler, he explicitly references the 2007 paper. 


#30
Feb2313, 12:20 PM

Astronomy
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PF Gold
P: 23,235

That's tangential to this discussion however. One way you can tell is that the 2009 GraviGUT work of N&P is not mentioned in either of the Perimeter talks. It is mentioned in passing on page 5 of the paper but it is immediately explained that it is a different idea. A onesentence reason is given. I think you have gotten off on the wrong track and are mixing things up. 


#31
Feb2313, 12:21 PM

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#32
Feb2313, 12:32 PM

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PF Gold
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Clearly different ideas. 2009 starts at 2007 and goes off in another direction. Difference was explained at top of page 5 in 2012 AMS paperpassing reference to 2009. This discussion we are having is too superficial. I suggest you watch the 2012 October video talks. Clear the Distler blog stuff out of your head and take a serious interest in the topic here. 


#33
Feb2313, 12:38 PM

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#34
Feb2313, 12:47 PM

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PF Gold
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#35
Feb2313, 12:57 PM

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According to Alexander, Marciano and Smolin the 2009 paper is an elaboration of the 2007 paper.
"Nesti and Percacci have discussed issues related to the Higgs phenomenon and the electroweak symmetry breaking in [25], and elaborated those topics for the graviGUT uniﬁcation model they have presented in [19]." 


#36
Feb2313, 01:40 PM

Astronomy
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PF Gold
P: 23,235

"The latter work develops a different perspective than the one addressed in [18], in that the graviweak and color gauge sectors have been accounted separately in [18]." The paper goes no further with that offshoot approach [19] they tried in 2009. It makes repeated use of the 2007 paper [18]. Same with the two Perimeter talks. Same root different branches. Not to be confused. We have to get on the road shortly, but I'll try to check in as time permits. I'm glad to see you are reading at least portions of AMS now, but suggest you watch the two brief Perimeter talks. Alexander and Smolin do an excellent job of presenting the ideas to a mixed audience (not all specialized in same area of QG.) 


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