Does LQG Do Nothing Cool Even If It Is Right?

[julcab12]

do I understand correctly you are drawing them to fit the math, an artists conception, as it were? Now I have to thin

I tried lol. The original picture involves gluing and slicing in the Fotini's dual spin network diagram-- where the faces are labeled as spins while the tetra are intertwiners. http://inspirehep.net/record/922593/plots.

BTW. This fundamental thingy is still new to me. I've been working on fluid dynamics and fractals in the past but haven't skimmed on the basic for a while. I'll try to animate the standard picture soon 'gluing and slicing'.

Do I understand correctly that the stick diagrams inside the tets are the dual spin network representation

The joint -- where the lines meet (the animation is still a work in progress^^) are the spin network vertex – the so called building block of space. The animation(4) is shown in reversed. It is a 5-n move where the resulting triangulation is the new state and the initial triangulation is the past set. The 5-n are the future sets. If you notice, The splitting creates trajectories and rigid solid. The original picture involves slicing erasing those tetra and replaces with a complement tretra while keeping it intact.

Copy from Group Field Theory.

 

[julcab12]

Here is a remake of the animation(4). Spin network (I was unable to edit my old post... )

and the drawing stays geometrically consistent with that - is the result significantly different from an algorithm that would start from the math? I guess it might be hard to tell. Do I understand correctly that the stick diagrams inside the tets are the dual spin network representation?

...Quite a tall order. It would be cool if it does. The construction is quite neat to some extent. IMHO. I don't even know if it can be subject to testing. But, what's remarkable about it is that we have some fundamental actions that somehow relates to geometry.

. Do I understand correctly that the stick diagrams inside the tets are the dual spin network representation?

Yes. I'll create the slicing/gluing action later on. ^^

 

[Jimster41]

Fotini's dual spin network diagram-- where the faces are labeled as spins while the tetra are intertwiners. http://inspirehep.net/record/922593/plots.

This site is really helpful. Took me a minute to realize the figures are all duplicated! I thought I was really not getting it.

I'm still a bit confused about the slicing, which seems intuitive to you. So far I have been picturing a thing like Marcus' Coral Reef where Tets that were glued together to form the present 2d surface are still under there (back there?). When you say "sliced off" I interpret that to mean that the present always slices it's way forward via one transaction that conserves a tet and so must leave the past tet behind. I also get how, when viewed from the top, the addition of a new point (gluing a tet) "slices" the underlying state into three new states. Is that consistent with your view?

Why does that sound a lot like Entropy?

Your most recent animation is perfectly aligned with the movie I have been seeing since Marcus' tutorial, and I can now really see the relationship to the dual. I probably think those things are even cooler than you do! I'd love to be able to play with them and have a sense that what we see is something consistent with the Cortes, Smolin, Wieland SF-ECS model. But yeah that is a tall order and I'm not suggesting I wish you would do that. I appreciate your visualization helping me understand this stuff to the extent I do.

I'm now stuck on trying to picture the difference (in the 2d+1 case) between a transaction tet for a photon and one for a neutron. Or one for a neutron at rest vs. a neutron accelerated. I get that ECS theory tets preserve the "geometricity" between the two cases, making space-time invariant under Lorentz transformations(?). But a dynamically disrupted foam must not look like it's made of equilateral tets?

I want to picture the (run of the mill) photon tet as a big flat almost-triangle, compared to chuncky (run of the mill) neutron tet.

 

[julcab12]

I'm still a bit confused about the slicing, which seems intuitive to you. So far I have been picturing a thing like Marcus' Coral Reef where Tets that were glued together to form the present 2d surface are still under there (back there?). When you say "sliced off" I interpret that to mean that the present always slices it's way forward via one transaction that conserves a tet and so must leave the past tet behind. I also get how, when viewed from the top, the addition of a new point (gluing a tet) "slices" the underlying state into three new states. Is that consistent with your view?

Forgive me for my poor wording and hope i can demonstrate the diagram more.^^. Intuitive in sense that shapes has that property.

I haven't shown the 1-3 in a 2+1d pachner (trivalent) move yet. According to causal set spin foam models. 1 node becomes 3 nodes through move/slicing. Each node has trivalent spin network. The area or the face is the intertwiner and the edges of the triangle are spins. ( http://arxiv.org/pdf/gr-qc/9704013v1.pdf see figure 1:). We have triangle with a spin network content. In my animation. I added a dynamic of splitting. The 'Splitting' of 1 vertex (where 2 edges meet) can have the same geometry as dual triangulation in casual spin foam model --- 1 triangle becomes 3 triangles. The splitting produces a transition between trivalent SN to four-valent spin network shown in animation(5). Ops. Had to go. I'll be back later..

 

[Jimster41]

Thanks Julcab,

I just scanned the first couple of pages of the paper you reference. It looks like a good one (lots of pictures and words) and I'm looking forward to beating my head against it.

Currently, I have very little intuitive feel for how spin valences varying across the causal chain relate to the "shape of space time", whereas for momentum, mass, energy, I at least have an illusion of "feel".

 

[julcab12]

In continuation #64. Let's step back a bit and create a slide diagram of the animation(4). In the LCG diagram.

http://www.einstein-online.info/spotlights/spin_networks#section-0

"The arrows give each line a direction (in mathematics-speak, an orientation). Each line is labelled with a half-integral number. The mathematical background of this number is the same as that of spin numbers inparticle physics, a type of number used to describe a basic property of elementary particles. Consequently, this number is called a spin label. (In fact, there are also labels associated with each node, but their meaning is more complicated mathematically, and we will ignore them in this simplified account.) The result is what is called a spin network, and it is taken to represent the quantum state of space at a certain point in time.

The lines do not have an a priori length - after all, there is no background geometry from which to derive lengths. Instead, we are dealing with a quantum system: In order to to talk about geometric entities like lengths or areas one must give a prescription how to measure these entities. (In technical terms, one must construct suitable operators with respect to which the spin network states are eigenstates.) In order to visualize this, it is convenient to look at graphs like the one sketched above in a different way, in what mathematicians call a dual description."

---- Now we have the basic diagram. Let's incorporate a geometry(solidity) on the evolving spin network. In my animation(4). We have 2dual evolutionary representation of the solidity. (1) integration of shape evolution (splitting of points creating edges-triangle-tetra and so on ) and (2) evolving orientation of spin network. We have now the basic dynamic-structure of the foam.

"Here is an artist's conception of the dual of a spin network state, this problem is solved by expressing a face's spin label (and thus its area value) by a colour. Thus, somewhat counterintuitively, a face can have more or less area (determined by the label) than another face even if both faces appear to have the same extension and shape in the illustration. More specifically, red means a smaller area and violet means large area. Notice that what appears as an empty region in the picture is actually no space at all: Space is only existent wherever there are faces that are excited (coloured). If we would add matter to this model, it could only exist on coloured faces (equivalently: on the lines of the spin-network)."

http://www.nature.com/news/theoretical-physics-the-origins-of-space-and-time-1.13613#reality-- LQG section:Over time, lines of the spin network can disapppear or new lines can be created, and the value of the different spin labels can increase or decrease. One therefore could call the resulting theory Quantum Spin Dynamics (QSD) in analogy to the theory of the strong nuclear force, quantum chromodynamics. (Quantum chromodynamics defines a very similar dynamics of what is called the colour quantum numbers of elementary particles like quarks and gluons.) The details of quantum spin dynamics are such that new faces are always created or annihilated with lowest possible area (in the illustration: the red faces). The animation that can be downloaded from the following links shows an artist's impression of this. Notice that, in the animation, the colour on the faces of the whole complex changes discontinuously while we fly through the empty space of the complex... "

 

[julcab12]

Thanks Julcab,

I just scanned the first couple of pages of the paper you reference. It looks like a good one (lots of pictures and words) and I'm looking forward to beating my head against it.

Currently, I have very little intuitive feel for how spin valences varying across the causal chain relate to the "shape of space time", whereas for momentum, mass, energy, I at least have an illusion of "feel".

Your welcome men. TBH. I'm not even qualified to answer such questions lol. I hope i represented the idea well. I was hoping that someone would add or correct me. I've been working on shape dynamics my entire career but i never imagined this form of fascinating relational oddity. This development is intriguing to me and been following it ever since with a very little physics( I'm still working on that) in my arsenal. To me. The transition from geometry to particle physics is still a long shot(possibility?) but an exciting development nonetheless.

 

[Jimster41]

That was extremely helpful. And that movie is... well it woke me up before eve one sip of coffee
Mostly what I got from that was wonderful clarity.

But,

Notice how the artist in the animation is conflicted in that he is using the color as a proxy for structure "foam geometricity", but then he still feels the need to reduce color value (grayscale) to signify receding distance. At that point the movie seemed to me like an afterthought, meant to dazzle,not conceived as a practical tool.

but then I still may be missing something big

Why can't we practically depict an evolving 2d+1 causal spin foam in a 3d+1 media player or simulator without breaking the key feature of into color. The key feature to me is "the way 3d shape evolves from 3d geometricity accreted over steps.

I keep picturing the triangular surface Marcus portrayed, in lovely black lines against a white backdrop, but as if I'm hovering over it (the lie that is required to create a visualization). I have a menu of pre-defined tets at the top of my viewer ("photon at rest", "neutron at rest", "Plain Tet"). I click one and a Tet appears in view, it is given gray scale shading from a stage right light source, but just enough gray to expose it's shape and orientation in the volume of the view, but that is all. I pull it down to the surface and the triangles light up to show me where I can attach it. Once attached I can go to a menu and configure or edit it's momenta. Boom. Next one I do, say I attach it to the first one. Now the lines of the first one fade, to show deprecation w/ respect to the surface of the present. Also I can turn the dual on!

Okay, yeah that's a neat fantasy, implying about 900 man hours of work, even in a moderately sportive platform...

I had thought that to ask questions, like "What would an approximation of a Two-Slit problem look like" or "what would an EBR event horizon look like" you wouldn't need to account for all the permutations of momenta conservation that could solve the network, just someone who could stack tets in a way consistent with an approximation of the ECS rules, maybe constraining tet shape and connection to a subset sufficient for a human being to build a movie-like depiction of relationships of mass difference and boost. The addition of enforced assembly rules could be solved as part of the user's interaction with the assembly, "if you stick this Tet here it can only take on this shape or that one, due to momenta conservation, and causality. I realize I may be giving interest in particle types and interactions short shrift here compared to jest ions about puzzles of gravitation, mass and spacetime curvature here. I guess I could imagine breaking the problem down so you aren't necessarily trying to see both networks at once, but could work on how one, once built, might inform or constrain the other.

I'm a bit confused as to whether or not an ECS-SF simulator in 1d+1 is interesting? Clearly not as cool too look at, but the graph in the article you referenced does convey kinematic to some degree.

Yeah, I've had enough coffe...

Where are you from by the way? I get the feeling we are in different time zones.

 

[Jimster41]

Holy moly! That's what I'm talkin about. It looks fractal to me...

From the link you provided up top
http://www.nature.com/news/theoretical-physics-the-origins-of-space-and-time-1.13613#reality--

I'm not having any luck figuring exactly how this was made, what tool was used, who?

 

[julcab12]

Why can't we practically depict an evolving 2d+1 causal spin foam in a 3d+1 media player or simulator without breaking the key feature of into color. The key feature to me is "the way 3d shape evolves from 3d geometricity accreted over steps.

Actually they can or at least they are trying. http://www.nbi.dk/~budd/.

https://www.learner.org/courses/physics/visual/visual.html?shortname=causal_dynamical_triangulation

"Causal Dynamical Triangulation is an attempt to build four-dimensional spacetime from two-dimensional triangular regions that, when glued together, represent a possible configuration of quantum spacetime fluctuations. Each configuration of triangles, like the one shown above, obeys the postulates of special relativity. The sum of all possible configurations is a spacetime with the same four dimensions in which we live".

I had thought that to ask questions, like "What would an approximation of a Two-Slit problem look like" or "what would an EBR event horizon look like" you wouldn't need to account for all the permutations of momenta conservation that could solve the network, just someone who could stack tets in a way consistent with an approximation of the ECS rules, maybe constraining tet shape and connection to a subset sufficient for a human being to build a movie-like depiction of relationships of mass difference and boost

.

Hmm. That's a neat idea. I have a good picture now in my head. I'll try to break that idea into moves soon.

Where are you from by the way? I get the feeling we are in different time zones

Yep. Here in Dubai lol.

Holy moly! That's what I'm talkin about. It looks fractal to me...

Eerily familiar to julia sets in fractal.

Spatial topology change and tree bijections in 2d
"In causal dynamical triangulations the spatial topology of the universe is not allowed to change in time. However, at least in two dimensions, it is possible to incorporate sporadic topology changes while maintaining a sensible continuum limit, leading to so-called generalized CDT. We demonstrate how one can study this model by taking the continuum limit of random quadrangulations. The analysis relies heavily on bijections between quadrangulations and labeled trees."

OK.. now. They stretching this a bit..

http://www.nbi.dk/~budd/docs/slidesnijmegen.pdf

They already have the fundamental picture in 2d and 3d animation, Even the interactive part. Now it makes sense(well, geometrically speaking) especially when you look at "Geodesic distance in quantum Liouville gravity and Zooming in on 2D Quantum Gravity" . 0_0

http://www.nbi.dk/~budd/#slides1 --- scroll down on videos

 

[Jimster41]

Nice find, thanks. His research is really cool. I need to spend some time with his slides etc. I'm definitely behind in my reading now (I am going to try to catch up though, so If I don't reply for awhile it's because I'm studying to learn how to say less goofy things)

Seems clear he's using Mathematica. Among other things he's trying to prove out, most of which I can't follow, is how Brownian Motion is emergent, which to my mind is sort of what Louiville's theorem says. Maybe somewhere in his presentation, which I just haven't seen yet, he describes using purely evolutionary approach to solution discovery, maybe for the fractal looking ones? To me that is what causal chain or network must be, pure stochastic iteration under rules.

I keep thinking there needs to be a problem setup consistent as pure stochastic evolution, which is why the manually controlled simulator has appeal, starting with just state n, a coherent description of the last event (surface) just before the photon hits the screen, and then see if there is anything anomalous, weird or cool, in what the rules would permit or require of state n+1. Or maybe working backwards from state n+1 requirement of interferene to see if any legal shape of an ECS space-time surface at state n is possible. I know that the ECS theory says "always causal" and "in one order", "momenta conserved" so - "everywhere local" but somehow that can't be the case for both those events (n, n+1). Unless I am missing some way of looking at the two slit experiment that makes it seem... more mundane.

Dubai. That is so cool. My wife is studying Sustainability Management at Harvard (she is the clever one of the family). She talks about Dubai often (she's into "Smart Cities") and I have heard enough and seen enough pictures that - having a beer in a lounge at the top of one of those skyscrapers, looking out over the 22nd century, is on my bucket list.

 

[marcus]

Jimster and Julcab (neat about Dubai!) I wanted to share this with you, a new paper NOT, first of all, to READ, but simply to KNOW EXISTS. It has to do with the emerging interest in (and possibly importance of) pachner moves in quantum geometry/gravity.

It seems to dovetail with the lines of research pursued by Bianca Dittrich and by both Wolfgang Wieland and Cortes Smolin. The idea of a geometric process based on Pachner moves.
http://arxiv.org/abs/1411.5672
Canonical linearized Regge Calculus: counting lattice gravitons with Pachner moves
Philipp A. Hoehn
(Submitted on 20 Nov 2014)
We afford a systematic and comprehensive account of the canonical dynamics of 4D Regge Calculus perturbatively expanded to linear order around a flat background. To this end, we consider the Pachner moves which generate the most basic and general simplicial evolution scheme. The linearized regime features a vertex displacement (`diffeomorphism') symmetry for which we derive an abelian constraint algebra. This permits to identify gauge invariant `lattice gravitons' as propagating curvature degrees of freedom. The Pachner moves admit a simple method to explicitly count the gauge and `graviton' degrees of freedom on an evolving triangulated hypersurface and we clarify the distinct role of each move in the dynamics. It is shown that the 1-4 move generates four `lapse and shift' variables and four conjugate vertex displacement generators; the 2-3 move generates a `graviton'; the 3-2 move removes one `graviton' and produces the only non-trivial equation of motion; and the 4-1 move removes four `lapse and shift' variables and trivializes the four conjugate symmetry generators. It is further shown that the Pachner moves preserve the vertex displacement generators. These results may provide new impetus for exploring `graviton dynamics' in discrete quantum gravity models.
26+12 pages, 2 appendices, many figures. This article is fairly self-contained

I think part of the mathematical appeal or convenience is that Pachner moves offer the possibility of a discrete finite vocabulary of geometric changes. So you can model geometric process in a discrete way. To me it seems great that Höhn has taken a LINEARIZED approach reminiscent of PERTURBATION theory, where you fix on a (usually flat) background geometry and study small "ripples" on that background. That's when the graviton concept really comes into its own, when instead of the whole dynamic geometry you can focus on small quanta of geometric disturbance on a fixed background. It seems great that he is able to represent that with the simple vocabulary of pachner moves.

It has some diagrams of the moves, and it has an idea of how the past is covered over with a layer of change (the present) which then becomes part of the past---and ready for a new layer. And this layer of change is made of pachner moves.

I'm not suggesting anybody read this particular article, just that it's one to know about---part, I think, of a trend on the QG front. And maybe one of the "cool things" that we take notice of in this thread : ^)

 

[Jimster41]

Thanks Marcus. I appreciate your synopsis for the layman. It enables me to read (re-read) and actually somewhat interpret the abstract at least! I know enough about the Perturbation approaches to have at least an image of the scheme.

Without looking at the article yet, I'm curious whether they are on the ECS train and just looking for clever ways to model stochastically evolving geometry, or whether they are still holding out for a fundamental continuum that propagates discretized waves of geometric information? I can imagine their scheme as you outline it being a useful tool with which to scale ECS up to semi-classical domains - where continuity and smoothness can be expected due to Gaussian expectations over lots of foamy quantum events (Brownian motion, Louiville's theorem?) I think I got something like that from the researcher Julcab referenced also - his analysis of distributions of space-time events in 2d over regions.

It hadn't occurred to me that any setup of the Two-slit experiment has to combine classical and quantum domains? Semi-classical? Sorry to keep coming back to the damn Two slit experiment. It just itches. (Later) after saying that I think that classical-quantum combination is consistent with a smooth-ish geometric surface with a rare hole or some kind of relatively extreme or non-Gaussian distortion (the tangled photons?)

 

[marcus]

Without looking at the article yet, I'm curious whether they are on the ECS train and just looking for clever ways to model stochastically evolving geometry, or whether they are still holding out for a fundamental continuum that propagates discretized waves of geometric information? ...

Hi Jimster, I think you are referring to Philipp Höhn's paper and to tell the truth I don't know what "ontology" or idea of reality he has in mind. Maybe no one in particular.
There are mathematical advances that provide technical grasp and utility that different people can use in different ways. Maybe in some sense they are the best kind, the more different uses the better! I see a potential for this to help the ECS program. But Philipp has collaborated a lot with Bianca Dittrich and I feel confident it could also be used in her approach to completing straight LQG/Spinfoams, and deriving their continuum limit. Her direction is more, to adapt your way of putting it, to model evolving quantum geometry in a clever way and show it behaves right in the appropriate limit.

I'm delighted when I see people with different goals trying to accomplish different things getting thrown together on convergent paths.

This convergence phenomenon was noted in a video seminar talk by Sylvain Carrozza that was just posted at PIRSA (perimeter institute recorded seminar archive). It was in an entirely different context! He was working on an approach to QG called "Group Field Theory" and, in particular, trying to see how renormalization could be done. And basically he commented "hey, this is looking more and more like LQG!" Don't let this distract you, its hard to stay focused with the variety of research paths being pursued. But just to make sure we know of Carrozza and that GFT exists here is his talk:
http://pirsa.org/14110129/
UV completion in Group Field Theories
Speaker(s): Sylvain Carrozza
Abstract: I will review recent work on tensorial group field theories (TGFTs). The renormalization methods being developed in this context provide more and more control over their field-theoretic structures, and for models which increasingly resemble loop quantum gravity. Perhaps surprisingly, some of these models are asymptotically free and can therefore be made sense of at arbitrary values of the (abstract) scale with respect to which they are organized. They define in this sense UV complete quantum field theories. I will focus on TGFTs with gauge invariance condition, which allow for such a behavior but also for more complicated scenarii involving non-trivial fixed points of the renormalization group flow. I will finally comment on the physical relevance of this notion of UV completion.
Date: 20/11/2014 - 2:30 pm
Series: Quantum Gravity

 

[Jimster41]

Thanks Marcus. Really appreciated your perspective, and all the work you do here keeping track of what is going on out there.

 

[marcus]

No problem, I feel tugged in two ways about this thread, one is your particular interest in ECS (especially the ontology or world picture latent in the Cortes Smolin work) and the other is the main topic of the thread which is the "cool things" coming out of current Loop gravity research. I'm always wobbling back and forth trying to do both, which is all right for me but may not work for you.

On the ECS side, we should note that Smolin's new book, with Roberto Unger, is scheduled to come out within a week---30 November. That the nominal date it goes on sale, that the Amazon web page gives:
https://www.amazon.com/dp/1107074061/?tag=pfamazon01-20
The web page let's you "Look inside". It allows some browsing.

It is always possible they won't make the published release date and the book will be delayed---that's happened before. But my sense is that it will be on the market very soon.

People will want to take issue with the book's assertion of the reality of time. I think that arose because the idea of the universe running on arbitrary fixed and eternal laws for which there is no explanation is philosophically unsatisfying. the idea is very appealing that the laws of physics evolved . But then one asks "evolved in what? don't you need a global time for them to evolve in?"

One of the foremost takeaway lessons of GR is that there is no global time. Change occurs at rates that vary all over the place depending very much on the observer's perspective. Moreover the original Causal Sets people like Sorkin Dowker Rideout and others took extreme care to avoid being forced into the trap of a global time. In stuff we've read in this thread we've seen this over and over. They manage to get a representation of the present moment and of happening without accepting a preferred global time.

How do they do this? By giving non-chronological causal structure priority over time. Metaphors we used were a family tree of events (where incest and inbreeding are possible) or the growth of a coral reef. in either case, birth order is not recorded. the growth does not occur in time, it occurs simply in the causal structure.

So the question naturally arises: "why do Smolin and Unger have to declare "the reality of time"? Why can't they just have physical laws evolve in the causal structure?

To me this seems more satisfactory, partly because I take the momentary signature change of the "silent bounce" seriously---that business proposed by Barrau Linsefors Mielczarek. It is an extremely bold proposal, but I have not seen it shot down yet. Essentially interaction between all microscopic degrees of freedom is SUPPRESSED at extreme density. A brief Euclidean phase, of total isolation and silence, at the bounce. In which the speed of light is effectively zero, light cones close and then, as density lessens, reopen.
The causal structure is momentarily trivial and I think time is impossible because no oscillator or cyclic process of any kind is possible, nor any counting of cycles. Operationally speaking, time cannot be defined or measured under such a clockless condition, so time is in some sense paralyzed, or it "crashes" and must be "rebooted" at the bounce. Degrees of freedom must be able to interact in order for there to be any sort of repetitive cycle. There is inevitably a kind of "round robin" in any oscillator. And I suspect that an even higher degree of interactive connectivity is needed to have a counter that is accumulating information about the causal past. So I can imagine a world in which causality functions (the coral reef grows) but interaction is as yet too primitive and limited for time to apply. Nothing there to "keep" it.

The "Silent Initial Conditions" paper is the fourth item on the fourth quarter MIP poll. Here's the poll link if you want to check out the abstracts of this and other interesting recent papers:
https://www.physicsforums.com/threads/first-part-of-fourth-quarter-2014-mip-poll.782086/

Probably one can get the Mielczarek et al paper just by googling "silent initial conditions". Yes! it is the first hit.
http://arxiv.org/abs/1411.0272
Silent initial conditions for cosmological perturbations with a change of space-time signature
Jakub Mielczarek, Linda Linsefors, Aurelien Barrau
(Submitted on 2 Nov 2014)

 

[Jimster41]

Marcus, thanks, I have been thinking about the important tension between mathematical "technology" as it were, and ontology. Clearly it's complicated but as you can probably guess I wish there was more out there for those who are not fluent in mathematics (compared to the practitioners) but who want to follow the way mathematical techniques are used in pursuit of ontology, because we understand that is "seeing the ontology for oneself" at some level, and somewhat irreducible. This Pachner moves tutorial, though way too time consuming (for you and Julcab) and too dialectic to be scale-able or sustainable is a great example of it - continued education for the non-practitioner, for which I am thankful.

To wish out loud for something more incrementally practical I wish someone would create an equation translator/annotator, even if all it did at first was list a glossary of all symbols used in whatever calculation was selected below the calc... or in call-out bubble (the live text model is appealing here because you can layer things in such away - to be accessed interactively). Mathematical software languages are an interesting hybrid, more instructive semantically, but I think not specific to the task, which is stay as close to canonical symbolism, but also explicate. I can read, and with my finger on the glossary I can often times track the technical description at the fine grain scale of ontology, often times. But it's logistically, a nightmare just to get them together.

Anyway, all that wishing aside

I do love when you get ontological...

To me this seems more satisfactory, partly because I take the momentary signature change of the "silent bounce" seriously---that business proposed by Barrau Linsefors Mielczarek. It is an extremely bold proposal, but I have not seen it shot down yet. Essentially interaction between all microscopic degrees of freedom is SUPPRESSED at extreme density. A brief Euclidean phase, of total isolation and silence, at the bounce. In which the speed of light is effectively zero, light cones close and then, as density lessens, reopen.
The causal structure is momentarily trivial and I think time is impossible because no oscillator or cyclic process of any kind is possible, nor any counting of cycles. Operationally speaking, time cannot be defined or measured under such a clockless condition, so time is in some sense "reset" or "rebooted" at the bounce. Degrees of freedom must be able to interact in order for there to be an kind of repetitive cycle. There is inevitably a kind of "round robin" in any oscillator. And I suspect that an even higher degree of interactive connectivity is needed to have a counter that is accumulating information about the causal past. So I can imagine a world in which causality functions (the coral reef grows) but interaction is as yet too primitive and limited for time to apply. Nothing there to "keep" it.

Clearly you understand this stuff at a level from which you can teach (is that what you do?)
I want to understand more about the bounce. I printed the paper you reference about the inflationary period implications of LQG but it was a bit referential and not enough context for me to get a lot from it. I got stuck on the ontology of the Friedman Equation - and the assumption of mass and energy defining the changing size of the universe, the shape of which seems to define them...? That paper though is a perfect example of one I think I could get better with an in-situ math glossary, because it is talking about physics or "specific ontology" - not just pure mathematics.

But my cartoon is that this is a new description of the "singularity" we found at the "going back down space-time" limit, the one that has been giving fits. With this one, instead of an infinity we get something that remains coherent, if zero is more coherent than infinity, which I think it is. I get why it could be consistent with a notion of cyclic time (I'm in the middle of Penrose "Cycles of Time"). I'm also curious about the "how would be know we were on the other side of the signularity?" question. In other words how do we know it's not a bounce as much as it is a passage - through the boundary-singularity, to the voluminous other-side...where things are different. I'd like to have a better picture of all this.

I am pretty enamored with this ECS notion to be sure, exactly because it is free of global time. To my mind this is an unavoidable requirement given the EPR paradox (or at least my cartoon of it). Clearly a simple table-top experiment with fairly common-place light and two slits, poses a staggering puzzle to ontology. How can there be such accessible domains that are... without time? How can temporally separated "events" interact as though happening at once? Eliminating Global-time seems the only option. I get that there are rules regarding that interaction, moving information through the time-less domain is a fail, mass energy velocity affect our visibility onto that domain, etc - so it's more subtle than sci-fi, but the mother of all puzzles is there. If anything the rules only make is even more tantalizing. I say it's more subtle than sci-fi but then I haven't seen "Interstellar" yet.

Causality itself is still a problem though IMHO when considering ECS vis-a-vis EPR, and I read a researcher dismissing ECS saying that it does no better than continuum approaches at the limit because of this implied background "ordinal" that separates events.

Maybe it doesn't describe the ultimate container-less set, But I don't agree it does no better, rather I take from it a picture composed of a universe of pure black unknown, except we know it can (and apparently must) contain no less than two "causal sets", each with different space-time, but intersecting - or more properly "inter-acting".

And I can't help but think of Entropy (the leaking in/out of total set configuration space) as the only veil we have, that outlines the collision. This is why Verlinde got my attention when he suggested that space-time is emergent from Entropy. I would be fascinated to see Entropy described in the SF causal set model - some uniform curvature convolved with the foam?

Anyway, I haven't forgotten about the Smolin et al. book. I am definitely getting it.
Thanks again..

 

[marcus]

ECS, and its connection with Causal Spin Foams via Wieland's work is definitely one of the cool things happening at present.
Something else just came up that should also be on our cool things list: Work involving curved-simplex simplicial complexes, incorporating the cosmological curvature constant Λ.
This has not appeared yet on arxiv. It is in progress, co-authored by Haggard, Han, Kaminski, and Riello.
Haggard and Riello just gave an hour talk on it at the ILQGS (international LQG seminar).

You know the so-called EPRL model is the standard spin foam model used in LQG and it is based on the idea of 4d simplicial complex, and its dual cell complex which carries certain information and constitutes the spin foam. Well, the 4d simplicial complex has always consisted of FLAT simplices, pents
, tets , triangles etc. But a nonzero Λ curvature constant says that there is a residual curvature that never goes away and is independent of the disposition of matter and energy. So why not incorporate curvature right at the basic simplex level, in the 3 simplices (tets ) and 4 simplices (pents ) themselves?
This they seem to be doing.

they call their new version of spin foam LQG by the name ΛEPRL ( "LambdaEPRL") this is work in progress that has clearly started to pick up momentum

 

[Jimster41]

Thanks Marcus. I look forward to hearing more. Just got a bit further through Penrose "Cycles Of Times" while traveling today - very helpful, and interesting.The research sound really intriguing. I am curious to know, If one applied a "distributable" curvature requirement to stochastic causal-set evolution (the spin foam), might we see emergent "massive particles". To me this is Verlinde. Mass doesn't curve space-time, probabilistically curved space-time causes massive Events?

 

[marcus]

... I am curious to know, If one applied a "distributable" curvature requirement to stochastic causal-set evolution (the spin foam), might we see emergent "massive particles"...

That is a really good general idea. in a general sense both GEOMETRY and MATTER must arise from the same thing, or be different aspects of the same thing, because they are so closely entwined with each other. geometry guides matter, which, in turn, curves geometry. they interact so constantly and profoundly that it seems intuitive they should be essentially the same being, at root. So you would like some theoretician to come up with a mathematical model of this their essential unity. I would too. Theorists have to work on problems that are ripe, problems that are ready for solution given the human concepts, the mathematics, the knowledge and language of the times. All of that is evolving, which means that problems are ripening and getting ready to be productively addressed. But its hard to know which are. I wonder if this problem (the common ground of geometry and matter) is actually ripe now, and i simply don't know that it is, and won't know, until some theorist springs a good idea on us.

Back in the mid Naughties, in 05 or 06, I think some theorist proposed that matter particles were the "topological defects" in geometry. Maybe it was Louis Crane (Kansas) or Laurent Freidel (Perimeter). Matter was perhaps imagined to be "conical singularities" The points of cones, where normal curvature would not be mathematically definable.
"Kinks" in the curvature, so to speak. I can't remember who it was who was briefly exploring this idea---several papers appeared, as I recall.

There is probably a lot more history I am not aware of. maybe the idea goes back to Einstein, or Lucretius, or Thales of Miletus. Or Adam :w
But to my limited and non expert knowledge the problem is not yet ripe.

Lee Smolin definitely WANTS matter to arise in some (perhaps still uninvented) type of energetic causal set, doesn't he? Making geometry arise in some sort of ecs is just the beginning, for him :)
Your idea could resemble something he tries along the way.

 

[inflector]

Lee Smolin definitely WANTS matter to arise in some (perhaps still uninvented) type of energetic causal set, doesn't he? Making geometry arise in some sort of ecs is just the beginning, for him :)
Your idea could resemble something he tries along the way.

Seems like in the Smolin world, matter could come from repeating causal loops where the energy of the causation is stored in the loop. By loop here, I mean loop in the algorithmic sense not in the geometric circle sense. More like the repeating oscillators in cellular automata like Conway's Game of Life.

The one problem I see in approaches so far, and I could easily be missing something obvious, is that the spacetime quanta approach, i.e. the building up from units of spacetime like the pentachorons still presents boundaries across which the very essence of matterness may be transferred in ways the simulations using quantized bits of spacetime (i.e. the pents) won't capture. It may be that there needs to be some torsional (curvature?) transfer between pents to represent matter. Another way of saying this is that perhaps simple causality is insufficient to represent matter. There may need to be some bending moment transfer across causal boundaries.

Curved spacetime units might be a step in this direction. However, since from GR we know that curvature differs locally from region to region depending on how curved adjacent spacetime is and how much matter is present; so it seems likely that an approach the allowed for varying degrees of curvature / torsion in the spacetime causality flow would be more likely to produce matter emergence than one that had uniform curvature and no allowance for much higher local curvature.

Perhaps that's all matter is, spacetime that's so locally curved as to maintain that curvature absent external inputs for at least a little time.

Anyway, I sure hope someone does take up your idea marcus.

 

[atyy]

Perhaps that's all matter is, spacetime that's so locally curved as to maintain that curvature absent external inputs for at least a little time.

The way Rovelli thinks about LQG, from its canonical roots to EPRL/FK is that it is a theory only of gravity, which can be coupled to matter as needed. However, there have been some ideas that the spin foam formalism secretly contains matter, eg.
http://arxiv.org/abs/hep-th/0512113
http://arxiv.org/abs/gr-qc/0702125
http://arxiv.org/abs/1001.2702

 

[Demystifier]

Atyy, do you perhaps think that you know more about physics than about biology? ;)
In any case, it seems to me that you know about physics more than many physicists.

 

[inflector]

The way Rovelli thinks about LQG, from its canonical roots to EPRL/FK is that it is a theory only of gravity, which can be coupled to matter as needed.

I've always interpreted Rovelli's hedging as him being fairly conservative in his claims, not that he didn't necessarily expect more over time.

What marcus said rings true:

In a general sense both GEOMETRY and MATTER must arise from the same thing, or be different aspects of the same thing, because they are so closely entwined with each other. geometry guides matter, which, in turn, curves geometry. they interact so constantly and profoundly that it seems intuitive they should be essentially the same being, at root.

So I'm encouraged by approaches that move in this direction.

 

[Jimster41]

Seems like in the Smolin world, matter could come from repeating causal loops where the energy of the causation is stored in the loop. By loop here, I mean loop in the algorithmic sense not in the geometric circle sense. More like the repeating oscillators in cellular automata like Conway's Game of Life.

Curved spacetime units might be a step in this direction. However, since from GR we know that curvature differs locally from region to region depending on how curved adjacent spacetime is and how much matter is present; so it seems likely that an approach the allowed for varying degrees of curvature / torsion in the spacetime causality flow would be more likely to produce matter emergence than one that had uniform curvature and no allowance for much higher local curvature.

Assuming for a second that some distributable (uneven) geometric curvature causes mass, one question I keep getting stuck on is what type of distribution evolution could explain such ubiquitous periodicity of mass (and energy). I can imagine a stochastic evolution that shows some kindof discrete scale invariance, a zoo of periodically similar stable curvature "attractors" but I also keep thinking that wave interference could be... In there

 

[atyy]

Atyy, do you perhaps think that you know more about physics than about biology? ;)
In any case, it seems to me that you know about physics more than many physicists.

Ah ha ha, thanks! I really am just an innocent bystander and could hardly calculate my way out of a paper bag. But maybe I am getting too much physics. Just the other day a colleague and I were discussing to what extent it is advantageous for the brain to represent the external world accurately. I thought it didn't have to be so accurate, just good enough to survive. Then he said that the counterargument was that there is a real world out there, and real photons coming out from objects. "Right?" he asked. I hesitated because I was trying to figure out whether photons are real. He sensed my uncertainty, and burst out laughing, "What, you don't believe in reality?" :D

 

[Paulibus]

Curved spacetime units might be a step in this direction...(and) I sure
hope someone does take up (this sort of?) idea

Assuming for a second that some distributable (uneven) geometric
curvature causes mass, one question I keep getting stuck on is what type of distribution evolution
could explain such ubiquitous periodicity of mass (and energy). I can imagine a stochastic
evolution that shows some kind of discrete scale invariance, a zoo of periodically similar stable
curvature "attractors" but I also keep thinking that wave interference could be... In there

It sounds to me as if some sort of wheel is waiting to be to be re-invented here, as the
attention of particle-physicists seems now to be turning towards patterns of distortion in
spacetime, with localised curvature, and variations in curvature, somehow endowing
mass/energy with a ‘gravitational’ interaction.

Are such folk aware that describing the behavior of patterns of the localised distortions that
disturb the translational symmetry in crystals is a well-developed cottage industry? Which might
serve as a template for understanding symmetry-disturbing ‘defects’ of the ‘vacuum’? For
crystal physicists linear ‘dislocations’, together with areal and point defects are quite ‘real’
interacting entities, although they are in essence just local symmetry-distorting patterns in an
underlying quite-symmetric substrate. Perhaps mass and energy masquerade similarly on the
symmetric stage we call the ‘vacuum’?

Squalid-state physics to the rescue?

 

[Paulibus]

The central thesis of LQG, namely that space is discrete rather than continuous, and the question:
how many angels can dance on the head of a pin? both seem to me to skate rather close to the
thin ice of sophistry. If space is intrinsically discrete, then I’d expect measurements of localised
‘position’ to yield discrete results, when a fine-enough probe was used. Reminiscent of
measurements of localised ‘energy’ in an atom revealing quantised energy levels, perhaps.

I then wonder what happens to a cornerstone of dynamics; namely the concept of conserved
momentum.

If a moving particle, say an electron or neutrino, has to be imagined as somehow jumping jerkily
from one ‘place’ to another; sans intervening ‘motion’, as it were: how is momentum then
conserved ‘during’ such a jump? Or is momentum conservation really just a convenient non-local
chimera that we use when describing the dynamics of macroscopic material objects, akin to a
macroscopic variable such as ‘position’ in discrete space? Are both concept of ‘place’ and
momentum only apparently continuous concepts?

What observable physics, if any, could be expected if this were so?

 

[marcus]

The central thesis of LQG, namely that space is discrete rather than continuous, ..
...
If a moving particle, say an electron or neutrino, has to be imagined as somehow jumping jerkily...

Hi Paulibus, this opens a rather deep question, when you say space, or geometry, or matter IS discrete it verges on ontology, and things like strict realism versus relational or interactive realism. I'm not well enough versed to do more than just tell you my own personal point of view. I'll try to do that, though.

I think the central stance of Lqg and indeed a lot of quantum thinking is that matter and geometry are discrete as you interact with them, their interactions are discrete, but they can be continuous all they want in private, on their own time.

Like an photon wave that undulates around all over the place but is only detected in a certain spot by a certain event in a certain detector.
Or an electron wave that does not have a continuous trajectory but does not "jump" either---it appears to have swum thru various slits on its way from emitter to detector so we know a DISCRETE sequence of locations corresponding to the event of passing thru one or more slits but we cannot say that a continuous trajectory even exists.

Such is the world. Information, interactions, measurements are finite and discrete, but what it IS we cannot say so definitely, it is rather more vague, like a vector in somebody's Hilbert space. Indeed we take the trouble to employ Hilbert spaces in part because they capture the right amount of vagueness, they and C* algebras. Or so I suspect.

I don't think of space as a thing or fabric or material. I think of geometry as a thing, that matter interacts with, that matter curves, and is guided by. And geometry is a thing you interact with by measuring the angles of triangles, and the relations between radiuses of round objects and their areas, or their volumes. So I think that measurements of areas, and angles, ought properly to be DISCRETE, like the measurement of the energy of an electron ought to be discrete. Because it is an interaction. Even though on its own time, in the privacy of its own parlor, the electron can have all different energies and spread out in a completely relaxed way. So likewise geometry should be allowed to be continuous as chocolate pudding or yoghurt, except when you measure something or when it is interacting with some matter in an in principle measurable way.

I admit that this attitude is subject to paradoxes and puzzles. It's hard to imagine how either matter or geometry could be like this! I also suspect that matter and geometry are fundamentally the same thing. This is why they can be so successfully enigmatic :D

 

[julcab12]

It's hard to imagine how either matter or geometry could be like this! I also suspect that matter and geometry are fundamentally the same thing. This is why they can be so successfully enigmatic

You are not alone Marcus! I used to think this picture of -- matter/energy instructing space how to curve and then space tells matter how to move. But 'what if' it's a masquerade of the same thing like what happened to the early fields.