Is the graviton the only thing to find in LQG?

[MTd2]

Has anyone thought if the there are particles to find in LQG, other than gravitons, when going to a "more classical limit"? Or that GR is not the only thing to find at classical limit, but, maybe, something like a GUT, for example, naturally emerging?

 

[marcus]

I think you know that the focus of some LQG research has shifted towards finding OBSERVABLE consequences. Like consequences of the cosmological bounce that might show up in the CMB ancient light. Or the "planck star" type of gamma ray burst.
I'm not sure what you have in mind---maybe it is non-observable things. I doubt gravitons will ever be observed---at realistic frequencies their energies are too low. And a "GUT" if it emerged might be at too *high* an energy to be observed. these are things you are suggesting. So I'm trying to think of more conceptual consequences of LQG.

How about this? A curved conjugate momentum space to geometry, leading to a compact phase space for dynamic geometry. New meaning given to the cosmological constant having to do with a minimum measurable angle, and a smallest step in phases space between distinguishable states of geometry.
http://arxiv.org/abs/1502.00278
Compact phase space, cosmological constant, discrete time
Carlo Rovelli, Francesca Vidotto
(Submitted on 1 Feb 2015)
We study the quantization of geometry in the presence of a cosmological constant, using a discretization with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.
6 pages

If you are asking about something that is conceptually transformative, but (like the graviton) not directly observable, this could be what you are looking for.

 

[MTd2]

Marcus, you almost got it right what I was trying to say. I am asking you if LQG can be a TOE, where particles emerge at low energy limit.

This may look unreasonable. But think about this analogy. What if someone gives you just the SM Lagrangian (suppose you live in a place with senses detached from real world). And asks you to verify what are the physical consequences of these equations. I think it is quite reasonable you'd never hardly know about the existence of protons, its nuclei, and consequently, you'd never know about chemistry. These are complex systems emerging from relatively very simple behaviors.

 

[wabbit]

I think this might be the kind of approach you have in mind :
http://arxiv.org/abs/hep-th/0603022
"Quantum gravity and the standard model"
Sundance O. Bilson-Thompson, Fotini Markopoulou, Lee Smolin

They study how some kinds of knots/braids which might arise in quantum geometry can be connected to SM particles. There are a couple more recent papers in that line, but I don't know how far that programme has gotten.

 

[romsofia]

I was under the impression that there is no prediction of a graviton in LQG because the notion of a particle doesn't really work in curved spacetime. If anyone could correct me, i'd be more than happy as I've only recently been reading into the notion of particles.

 

[marcus]

I was under the impression that there is no prediction of a graviton in LQG because the notion of a particle doesn't really work in curved spacetime. ...

I share your point of view about that. Flatness is a very special case. One can study the graviton propagator in LQG in a given spacetime region with a given boundary by specifying restrictions at the boundary. So one can force the "graviton" to exist just enough so one can study it and check that something like the familiar inverse square law holds. But particles are not in general a feature of the theory, for the reason you mentioned.

 

[marcus]

I think this might be the kind of approach you have in mind :
http://arxiv.org/abs/hep-th/0603022
... There are a couple more recent papers in that line, but I don't know how far that programme has gotten.

I haven't seen much activity. Aside from self-citation, the only 2014 paper I know of that cited the 2006-2007 work was http://arxiv.org/pdf/1402.2274.pdf. It did so in passing, at the conclusions (page 11) in a not-very-encouraging way. There was this sole reference to [63}, the last item in the bibliography:
==quote==
The spatial diffeomorphism constraint Hi of LQG generates diffeomorphisms, which are connected to the identity and one could naturally ask about the behaviour of H_Diffunder large diffeomorhisms of the fixed 3-manifold M. We recall that the quantum states of the geometry of M are given through embedded spin networks.

Through their embedding, spin networks acquire topological degrees of freedom in terms of their knotting behaviour in M and hence, diffeomorphism invariant spin network states fall into different knot classes. A proper and closed physical interpretation of these topological degrees of freedom is not yet available though tentative ideas were put forward in [63]. In quantum theories of gravity, which are based on a spacetime topology R × M , distinct quantum sectors labeled by the inequivalent unitary irreducible representations of the mapping class group of M exist. These inequivalent quantizations, also called θ-sectors, show up if the configuration space of a quantum system has a non-trivial first homotopy group [31, 51]. As pointed out e.g. in [29], in LQG ...
==endquote==
References [31, 51, 29] are to different work.
It was an intriguing and exciting idea when it appeared (Bilson-Thompson was the originator) but I think it proved difficult to develop. Another factor would, I guess, be that recently LQG has been mainly formulated using spin foams and spin networks that are not embedded in any manifold. The manifold and the embeddings may have been "extra baggage" in that respect. As long as that remains the prevailing fashion spin networks will not have the topological degrees of freedom needed for braiding. I don't know enough to judge the odds or make any predictions, though.

 

[marcus]

I suspect that to conceive of the consequences of Loop and such QG one has to think more deeply than what I see in MTd2's opening post.
Just to give an example of what I mean, here is a conceptual exploration: suppose that geometry consists of events--geometric interactions occurring in a present. (see Gambini&Pullin http://arxiv.org/abs/1502.03410 http://arxiv.org/abs/1502.03831 )
Suppose, as Rovelli Vidotto http://arxiv.org/abs/1502.00278 suggested, that the presence of a smallest measurable angle, or equivalently, a cosmological curvature constant Λ,
causes spatial and temporal geometric interactions to be discrete occurring in small quantum hops shakes spasms or repercussions to speak figuratively.
This then affects how we think of spacetime--the habitat of other quantum fields---as a growing heap of events where only the surface is interactive and actual---the past events can no longer be interacted with and so no longer exist. Gambini and Pullin estimate the thickness of the live layer for us.
So then the consequence is simply that we have to rebuild the theory of quantum fields in this new spacetime geometry residence. In terms of interaction events that so to speak inhabit the geometric ones. In this world there are only events and fields describing which are possible. No little pebbles.

Here are the G&P references to help fill in what I'm saying
http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages,

http://arxiv.org/abs/1502.03831
Quantum mechanics, strong emergence and ontological non-reducibility
Rodolfo Gambini, Lucia Lewowicz, Jorge Pullin
(Submitted on 12 Feb 2015)
We show that a new interpretation of quantum mechanics, in which the notion of event is defined without reference to measurement or observers, allows to construct a quantum general ontology based on systems, states and events. Unlike the Copenhagen interpretation, it does not resort to elements of a classical ontology. The quantum ontology in turn allows us to recognize that a typical behavior of quantum systems exhibits strong emergence and ontological non-reducibility. Such phenomena are not exceptional but natural, and are rooted in the basic mathematical structure of quantum mechanics.
8 pages, to appear in Foundations of Chemistry

 

[wabbit]

It was an intriguing and exciting idea when it appeared (Bilson-Thompson was the originator) but I think it proved difficult to develop.

Perhaps a reason for that might be that the relation actually goes the other way and spacetime is an emergent property of matter fields rather than the other way round. The Causal Sets version of this in http://arxiv.org/abs/1209.0881 seems quite interesting in this regard.

 

[MTd2]

I thought you people would remember of Torsten :P

 

[wabbit]

So then the consequence is simply that we have to rebuild the theory of quantum fields in this new spacetime geometry residence. In terms of interaction events that so to speak inhabit the geometric ones. In this world there are only events and fields describing which are possible. No little pebbles.

Thanks for the links. Lots to chew on:) The interpretation and ontology seem compelling, essentially eliminating all the weirdness that arises from remnants of a classical view. The fact that it links Gravity and QM at the fundamental level so that not only is gravity quantized but QM is just as much "gravitized" is very satisfying. All of this by subtly and naturally modifying standard QM...
It still seems somewhat tentative as the fundamental influence of gravity on measurement is qualitatively clear but not completely defined, but hopefully this will be sorted out.

I would not necessarily agree about the formulation "quantum fields in this new residence" which sounds a bit like "spacetime is where things happen" instead of "a relation between things that happen" but I think I get your meaning.

 

[wabbit]

This then affects how we think of spacetime--the habitat of other quantum fields---as a growing heap of events where only the surface is interactive and actual---the past events can no longer be interacted with and so no longer exist. Gambini and Pullin estimate the thickness of the live layer for us.

I couldn't find this (esp. the live layer thickness estimate) in the Gambini & Pulin paper you point to. Did I read it too superficially or is that in another paper by these authors ? Thanks

 

[marcus]

I couldn't find this (esp. the live layer thickness estimate) in the Gambini & Pulin paper you point to. Did I read it too superficially or is that in another paper by these authors ? Thanks

By no means! I am who read it too superficially. Or too speculatively. I'm grateful to you for reading the two G&P papers, it gives me a reason to shift gears and try to explain what I think is the consequence of their way of thinking.

In earlier papers they applied their idea of the gradual loss of unitarity (due to the quantum imprecision of time) to black holes---this let them resolve the BH information loss paradox. they had an upper bound estimate for how long it would take (on the order of the evaporation time for a stellar mass BH). Let's try to discuss this based on the two recent papers. They should be self-contained. IIRC the timetable for loss of unitarity was mass dependent.
But the discussion was specialized to BH. I need to take time to review. It helps to have a critical reader. Have another thing to do right now, but will be back shortly.

The key think with MV idea is enhanced decoherence. Not just environmental decoherence which is a kind of dissipation of indefiniteness, dissipation of the superposedness out into a huge number of little environmental degrees of freedom. Not just environmental decoherence but enhanced by the fact that unitarity itself depends on a mythical ideal time which in reality does not exist. Over long durations clocks themselves decay (real ones), and so must unitarity. I've got to take care of something else but will try to discuss this more later today.

 

[marcus]

Wabbit, I regret to say I can't follow through on the idea I sketched earlier. When I first read the recent G&P papers I thought I could see a way to connect their MV quantum mechanics to a Causal Sets idea of Rafael Sorkin that I like very much---so called "asynchronous becoming".
I thought there would be enough, just in those two G&P papers, to draw the connection.

I have to give up on this at least for now. Having read the papers more carefully and thought about it, I don't see how to proceed. In any case the purpose would be to suggest a possible consequence of Loop-and-related QG, producing another way to think about the world (which might or might not be found to be wrong) rather than, as MTd2 suggested, producing another particle.

What I like about the MV picture is that a myriad of microscopic events can be proliferating autonomously, without the formality of "measurement" by a classical observer-creature. And I think of them as space-like separated so that it is impossible to say which occurred before which. Change occurs without benefit of clock, as a kind of spontaneous mass disorderly conduct. this is what the enhanced decoherence of G&P seems to suggest---it is like environmental decoherence but fundamentally irreversible. unitarity has a shelf-life. I'll get some sleep (it's after 11 here) and try to think a bit more clearly about this in the morning.

 

[wabbit]

In earlier papers they applied their idea of the gradual loss of unitarity (due to the quantum imprecision of time) to black holes---this let them resolve the BH information loss paradox. they had an upper bound estimate for how long it would take (on the order of the evaporation time for a stellar mass BH). Let's try to discuss this based on the two recent papers. They should be self-contained. IIRC the timetable for loss of unitarity was mass dependent.
But the discussion was specialized to BH..

I can't say I understand all their formulas and reasoning, but what I get is that formula (30) in the review gives the decoherence time including mass dependency. That's for a general system interacting with an environment, not specific to BH.
Very interesting how they get there - Schrödinger's equation is replaced by a more general non-unitary one (7) which governs both unitary evolution and decoherence. This comes from an analysis of (mass related) time measurement uncertainty and how this requires revising what "t" is in the evolution - not an absolute time parameter but a physical quantity measured by an observer's clock.

Edit - sorry, repeating what you said in your next paragraph:)

 

[marcus]

I can't say I understand all their formulas and reasoning, but what I get is that formula (30) in the review gives the decoherence time including mass dependency. That's for a general system interacting with an environment, not specific to BH.
Very interesting how they get there - Schrödinger's equation is replaced by a more general non-unitary one (7) which governs both unitary evolution and decoherence. This comes from an analysis of (mass related) time measurement uncertainty and how this requires revising what "t" is in the evolution - not an absolute time parameter but a physical quantity measured by an observer's clock.

Thanks for carrying this forward (even though I'm not holding up my end of the discussion all that well at the moment.) It helps a lot to have someone to talk to about this.
I don't think I can or should go into detail here in MTd2's thread, maybe we could eventually have a discussion thread on this precise topic!
But here, in respect to MTd2's question, I just want to suggest in very general terms that there ARE things we can hope for from QG besides the prediction of yet another type of "particle". ("Is the graviton the only thing?" Probably not )

We can expect that rigorous analysis of quantum spacetime geometry will give us new ways to think about space, and time, and events, the passage of time, the present (which is not mathematically represented in GR), causation, information, unitarity. We can hope, I think, that QG will allow physicists to resolve or push beyond some classical singularities such as BH and BB, and reconcile some apparent contradictions such as information loss. Above all it must be hoped (insisted ) that whatever new ways to think about the world are developed yield testable consequences.

 

[marcus]

I can't say I understand all their formulas and reasoning, but what I get is that formula (30) in the review gives the decoherence time including mass dependency. That's for a general system interacting with an environment, not specific to BH.
Very interesting how they get there - Schrödinger's equation is replaced by a more general non-unitary one (7) which governs both unitary evolution and decoherence. This comes from an analysis of (mass related) time measurement uncertainty and how this requires revising what "t" is in the evolution - not an absolute time parameter but a physical quantity measured by an observer's clock...

Wow! I looked again at formula (30) in http://arxiv.org/abs/1502.03410 and it is neat!
In this particular experiment if the number N of environmental degrees of freedom (spins in this case) is more than ten million then you cannot tell even in principle whether or not collapse has occurred. There is no possibility of "recoherence" ever. What they are describing looks to my non-expert eye like an experiment that could someday be performed. A spin direction is represented by an atom (say) in a chamber and a beam of N particles passes through the chamber in a direction orthogonal to the field. How hard could that be to implement?
They cite two papers [22] and [24] giving additional information:
[22] R. Gambini, L. P. Garcia-Pintos and J. Pullin, Int. J. Mod. Phys. D 20, 909 (2011) [arXiv:1009.3817 [quant-ph]].
[23] C. Brukner, J. Kofler, “Are there fundamental limits for observing quantum phenomena from within quantum theory?”,[arXiv:1009.2654 [quan-ph]].
[24] L. P. G. Pintos, M. Sc. Thesis, Universidad de la Repu ́blica, Montevideo, Uruguay (2011).
====================================
I was wondering if the ILQGS list of online talks would have anything specifically related to the two recent Gambini and Pullin papers
so I checked the schedule: http://relativity.phys.lsu.edu/ilqgs/schedulesp15.html
Mar 24th Information loss Matteo Smerlak Perimeter Institute
Apr 7th Explicit computation of the evaporation of a quantum BH Jorge Pullin LSU
Apr 21st Separability and quantum mechanics Fernando Barbero CSIC, Madrid
No idea how much overlap any of these will have, just have to wait and see.
===
For convenience I'll repeat the links for the two recent G&P papers:
http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
http://arxiv.org/abs/1502.03831
Quantum mechanics, strong emergence and ontological non-reducibility
===

 

[wabbit]

I don't think I can or should go into detail here in MTd2's thread, maybe we could eventually have a discussion thread on this precise topic!

Indeed you're right ! Sorry about that MTd2 I hijacked your thread... It was nice and all so I just made myself at home : )

 

[marcus]

I think it's all right. It's all constructive, and the discourtesy if any is mine. I'm really intrigued by this stuff. How best to proceed? It would be a plus to hear from MTd2. Maybe he LIKES having the topic broadened a bit (to included qg consequences besides particles). Or maybe he'd say to start a different thread. In a sense it is quite a successful thread---isn't even 20 posts long yet and seems to have hit a rich vein of ideas.

Wabbit, I have a vague memory of your mentioning something by Dowker, or by Sorkin. Did you by any chance happen to have a look at http://arxiv.org/abs/1405.3492 ?

 

[wabbit]

But here, in respect to MTd2's question, I just want to suggest in very general terms that there ARE things we can hope for from QG besides the prediction of yet another type of "particle". ("Is the graviton the only thing?" Probably not )

Indeed. And specifically about other particles, actually there is one - or rather, a field: I think we should expect the inflaton either to vanish into oblivion or to find a home in quantum gravity.

(edit: removed "pretty sure" to avoid future embarassment:)

 

[wabbit]

Wabbit, I have a vague memory of your mentioning something by Dowker, or by Sorkin. Did you by any chance happen to have a look at http://arxiv.org/abs/1405.3492 ?

Yes, that's the one, I think I picked it up from another thread of yours. Very nice paper.

 

[marcus]

But here, in respect to MTd2's question, I just want to suggest in very general terms that there ARE things we can hope for from QG besides the prediction of yet another type of "particle". ("Is the graviton the only thing?" Probably not )...

Indeed. And specifically about other particles, actually there is one - or rather, a field: I think we should expect the inflaton either to vanish into oblivion or to find a home in quantum gravity.

(edit: removed "pretty sure" to avoid future embarassment:)

That's a really interesting idea! Inflation field (obviate or find a home for) should certainly go on the wish list of things QG might offer. The cosmological curvature constant seems to be increasingly at home either as minimum measurable angle or as curvature inherent in the simplexes that serve as the theory's geometric building blocks. (recent papers by Rovelli Vidotto and by Haggard Han Kaminski Riello).

So if Lambda can find a home in QG, maybe the inflation field (if not somehow made unnecessary) might too. I think we already mentioned the "LambdaCDM bounce" paper by Cai and Wilson-Ewing, one of several recent ones that explore the possibility of doing away with inflation when the initial singularity is replaced by a bounce.

 

[marcus]

Will-he nill-he we seem somehow to be answering MTd2's original question

 

[wabbit]

Will-he nill-he we seem somehow to be answering MTd2's original question

Indeed : ) Regarding the inflaton I recall seeing (in an LQC exposition by Ashketar ? ) an inflation period arising at the bounce but it was on a timescale lower than needed, but that's a bit hazy.

As for Montevideo, would you want to do a leisurely step by step reading in discussion form ? (I say leisurely because I could't follow your otherwise hectic pace : )) I would certainly learn a lot from that, I mostly skipped all the equations so far ! Not necessarily right now of course as you have other things going on

 

[marcus]

There's a possible linguistic confusion. By definition "inflation" is a period of expansion with constant or slowly declining H. H(t) is a percentage growth rate of distance. So what we are talking about is exponential growth (or near) with constant or near constant percentage rate.

Inflation scenarios don't ordinarily have increasing H(t). But the LQC bounce necessarily involves a period of sharply increasing H(t). H(t) comes in extremely negative (collapse) passes thru zero at the moment of bounce, and goes extremely positive at the start of expansion.
As a fractional growth rate it is briefly on the order of Planck frequency or 1/planck time.
So what do you call exponential growth with an increasing fractional rate H(t)? You can't call it inflation because that is for steady or slowly declining rates of growth. In papers by the Ashtekar group it is sometimes called super-inflation.

But when they solve the LQC bounce equations, or run the numerical models, they get that this period of super inflation right after the bounce does not last long enough to get the e-folds we are used getting.

So one thing the researchers have done is add an inflaton field to the picture and argue that then there are fewer problems with the inflaton, higher probability of success, less fine-tuning needed, in the context of a bounce cosmology. Ashtekar, Wilson-Ewing, Agullo have I think written about this.
Also Aurelien Barrau---one or two interesting papers arguing that the probability of adequate e-folds is almost 1, in the bounce case.

But I'm not following that work with as much attention as I was earlier, now, because I have gotten intrigued by the Cai Wilson-Ewing paper where they dispense with inflation entirely---the "LambdaCDM bounce scenario" paper.

 

[wabbit]

Ah yes that's much clearer thanks. It wasnt really linguistic confusion on my part. Just plain confusion : )

 

[MTd2]

The idea: start by simple a rule, and get patterns out of this. Like cellular automata or Mandelbrot, you get nice stuff that you cannot see by just inspecting.
So, you get LQG as it is and somehow, you get all the other particle fields, "magically". You just have to seek the correct approximation to see and find that that. Until now, I only saw the search for the graviton, the classical GR, but why not the rest, all fields? Not talking about extra rules about how edges should connect, but just the very basic theory.

 

[wabbit]

From my limited laynan perspective I can only add that there seems to be quite a bit of research that links spacetime and other field in different ways (marcus' threads provide many pointers) though perhaps so far not in the way you'd want : ). Also I don't really see an obvious reason why spacetime should be the fundamental field from which all others emerge as you seem to expect, but the fact that I don't see it certainly doesn't mean it isn't there. I'll leave it to others to provide better grounded answers.

Edit : I say spacetime you say graviton - that's just different aspects of the same object, the gravitational field.
Edit2 : The pedestrian answer to your "why not" would be "because LQG is by design and purpose Quantum GR, so its expected to find GR and gravitons there, but not by necessity anything else"

 

[MTd2]

"because LQG is by design and purpose Quantum GR, so its expected to find GR and gravitons there, but not by necessity anything else"
That's actually the thing I asked. Though my rhetoric was lost.
I was kind of thinking in Torsten's work. What do you think? Marcus?

 

[sshai45]

I suspect that to conceive of the consequences of Loop and such QG one has to think more deeply than what I see in MTd2's opening post.
Just to give an example of what I mean, here is a conceptual exploration: suppose that geometry consists of events--geometric interactions occurring in a present. (see Gambini&Pullin http://arxiv.org/abs/1502.03410 http://arxiv.org/abs/1502.03831 )
Suppose, as Rovelli Vidotto http://arxiv.org/abs/1502.00278 suggested, that the presence of a smallest measurable angle, or equivalently, a cosmological curvature constant Λ,
causes spatial and temporal geometric interactions to be discrete occurring in small quantum hops shakes spasms or repercussions to speak figuratively.
This then affects how we think of spacetime--the habitat of other quantum fields---as a growing heap of events where only the surface is interactive and actual---the past events can no longer be interacted with and so no longer exist. Gambini and Pullin estimate the thickness of the live layer for us.
So then the consequence is simply that we have to rebuild the theory of quantum fields in this new spacetime geometry residence. In terms of interaction events that so to speak inhabit the geometric ones. In this world there are only events and fields describing which are possible. No little pebbles.

I'm curious about this: how does the existence of this "surface" layer jive with Relativity's idea that there is no absolute "present"? Wouldn't that mean different observers would disagree on how the "heap" has grown?

 

[wabbit]

I'm curious about this: how does the existence of this "surface" layer jive with Relativity's idea that there is no absolute "present"? Wouldn't that mean different observers would disagree on how the "heap" has grown?

In my view it certainly does. The "evolving surface layer" is a quantum version of a 3+1 foliation in GR, associated with a class of observers.

Also of course the view of "a present" or "the passage of time" is, here as in GR, a matter of perspective, one can equally well adopt the "unchanging spacetime" viewpoint.

 

[Talleyrand]

I enjoyed reading this thread.