Does LQG Do Nothing Cool Even If It Is Right?

[ohwilleke]

In "Loop quantum gravity and observations" by A. Barrau and J. Grain at http://arxiv.org/abs/1410.1714 the bottom line seems to be that the phenomenological implications of LQG relative to existing predictions from GR are so extremely subtle that they have virtually no practical effects that can be discerned without ultrahigh precision instruments. It makes the tweak to the orbit of Mars predicted by GR relative to Newtonian gravity look veritably spectacular and vivid by comparison.

Am I missing something, or is the correct conclusion that LQG, if proven right, is basically just wrapping up loose ends that would otherwise lack a mathematically rigorous treatment?

 

[Demystifier]

You are missing the fact that the same can also be said about even more revolutionary theories of the 20'th century, such as theory of relativity and quantum theory. Compared to the 19'th century theories, the 20'th century theories make little difference for phenomena seen under normal everyday-life conditions. And yet, they make a very big difference under certain very extreme conditions, such as velocities close to the velocity of light (special relativity), strong gravitational fields (general relativity), and small distances (quantum mechanics). Likewise, LQG makes a very big difference under extreme conditions of a sufficiently high energy concentrated within a sufficiently small volume (Planck scale).

 

[marcus]

...
Am I missing something, or is the correct conclusion that LQG, if proven right, is basically just wrapping up loose ends that would otherwise lack a mathematically rigorous treatment?

I agree with Demystifier about the importance of understanding phenomena that involve extremely high energy density.

Also I wouldn't knock "wrapping up loose ends" i.e. resolving paradoxes and inconsistencies between theories.

As a comment on your shorthand "LQG, if proven right, is basically..." I'd like to suggest Wieland's July 2014 paper and September ILQGS talk as exemplifying the current development of the theory and ask (if that extension of Spinfoam QG is "proven right" and gains acceptance, which is assuming a lot!) what do you think that would entail?

For one thing, unless I'm mistaken, since it joins Spinfoam QG with the Cortês-Smolin energetic causal set (ECS) picture, it addresses issues such as the passage of time, and the experience of the present moment---but does so in a polychronistic manner rather than the monochronistic one of conventional quantum mechanics and quantum field theory---suggesting a fundamental alternative to the block universe picture of the world.

You've probably already checked out the relevant papers by Wieland and by Cortês-Smolin. For convenience, I'll get links. They're on Part II of the third quarter MIP poll and in some other threads as well.
Both abstracts are here, scroll down, they are #9 and #10 on the list:
https://www.physicsforums.com/threa...portant-qg-paper-part-ii.773590/#post-4867362
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland

Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin

One of the cool things about Wieland's paper is that he extends LQG in such a way that it makes ECS able to connect with it.

 

[marcus]

Wieland's "New Action" paper does some other cool things as well. It resolves a conceptual tension around the Hamiltonian in Spinfoam QG and arrives at a new way of looking at the 4d simplicial complex. Here is a quote from page 1:

"...We will propose such a theory, and prove an intriguing correspondence: The entire simplicial complex represents a system of free particles propagating in a locally flat auxiliary spacetime, with every tetrahedron representing one of those particles, the volume-weighted time-normals representing the particles’ four-momenta, and the entire discretized action turning into an integral over the worldlines of those auxiliary particles..."

Just to keep it handy, here's the link to the July 2014 "New Action..." paper:
http://arxiv.org/abs/1407.0025
The number is 1407.0025 is fairly easy to remember in any case.

This might be the Loop theory paper currently best to focus on
(theory, that is, as contrasted with observations/phenomenology where a lot else is happening).

 

[julian]

"just wrapping up loose ends that would otherwise lack a mathematically rigorous treatment?"

According to the no hair theorem for black holes in classical GR the entropy of a black hole is zero and so seems to be in contradiction of physical arguments that it should have entropy. In LQG it is realized that the entropy of a black hole is accounted for by purely quantum mechanical degrees of freedom of the gravitational field.

 

[julian]

Being able to formulate mathematically rigorous QFTs shouldn't be sniffed at:

http://en.wikipedia.org/wiki/Clay_Mathematics_Institute#Millennium_Prize_Problems

You get $1,000,000 prize if you prove that Yang-Mills exists (plus it has a non-zero mass gap)!

 

[julian]

Lee Smolin:

… someone might earn a Clay prize by rigorously constructing quantum Yang-Mills within LQG. It will certainly not be me, but there are people working on exactly that program. The conjecture is that background independent QFTs are more likely to exist rigorously in 3+1 dimensions than Poincare invariant QFTs.

 

[marcus]

I think the relation of the LQC bounce to inflation is one of the remarkably cool things it does. And LQC has been derived from LQG so the cosmological tests serve to test the full LQG theory, not only the LQC application of it.
The Barrau Grain paper that Ohwilleke cited actually outlines this the predictions for cosmology and astrophysics observations (cool if you happen to be interested in cosmology and astrophysics, various unexplained signals in the gammaray and microwave and radio wavelength sky, understanding the start of expansion, the CMB etc). And the Barrau Grain paper has references if anyone wants to pursue such coolness further : ^)

But what currently intrigues me is this, particularly the new version of causal sets that results from Cortes-Smolin putting ECS together with Spinfoam QG. I think it can lead to an improved understanding of the passage of time and the experience of the present moment .
I'll try to explain that.

http://arxiv.org/abs/1407.0032
Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin
(Submitted on 30 Jun 2014)
Energetic causal sets are causal sets endowed by a flow of energy-momentum between causally related events. These incorporate a novel mechanism for the emergence of space-time from causal relations. Here we construct a spin foam model which is also an energetic causal set model. This model is closely related to the model introduced in parallel by Wolfgang Wieland in arXiv:1407.0025, and this construction makes use of results used there. What makes a spin foam model also an energetic causal set is Wieland's identification of new momenta, conserved at events (or four-simplices), whose norms are not mass, but the volume of tetrahedra. This realizes the torsion constraints, which are missing in previous spin foam models, and are needed to relate the connection dynamics to those of the metric, as in general relativity. This identification makes it possible to apply the new mechanism for the emergence of space-time to a spin foam model.
Comments:16 pages, 4 figures.

arXiv:1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
(Submitted on 30 Jun 2014)
We develop a proposal for a theory of simplicial gravity with spinors as the fundamental configuration variables. The underlying action describes a mechanical system with finitely many degrees of freedom, the system has a Hamiltonian and local gauge symmetries. We will close with some comments on the resulting quantum theory, and explain the relation to loop quantum gravity and twisted geometries. The paper appears in parallel with an article by Cortês and Smolin, who study the relevance of the model for energetic causal sets and various other approaches to quantum gravity.
26 pages, 2 figures

http://relativity.phys.lsu.edu/ilqgs/
Tuesday, September 16th
Wolfgang Wieland, PennState
Title: Covariant loop quantum gravity: Its classical action, phase space and gauge symmetries
PDF of the talk (1Mb)
Audio [.wav 36MB]

The 16 September seminar talk presents the results of http://arxiv.org/abs/1407.0025. Download the PDF first, before you start the audio. The talk is well-organized and the slides are rather complete so that one can understand a lot just by reading the slides PDF.

Both regular Causal Sets (e.g. as developed by Rafael Sorkin, David Rideout, Fay Dowker and others) and regular Spin Foam QG (covariant LQG) are active lines of research with important results. This makes it especially interesting to see how the two recent tweaked versions of each can be closely related.

 

[marcus]

The causal set idea of the passage of time is well described in this talk by Fay Dowker which summarizes work by Rafael Sorkin and collaborators:
google "dowker passage arxiv" to get the arxiv copy.
http://arxiv.org/abs/1405.3492
The birth of spacetime atoms as the passage of time
The idea is that there is no 3D instantaneous version of reality, what we experience is a confused disorderly process of causation at the frontier of the past where space-time cells are proliferating and the order in which they appear is pure gauge that is physically meaningless.

Any experience of reality I may have involves this current process of causation. Say I want to see if the wall is firm so I put the computer down and walk over and push on it. At every step this experience involves the birth of space-time atoms and the occurrence of events. Which go to build up the past, a kind of crystal of dead events which can no longer be influenced by our actions or experienced.
Best if you have a look at Dowker's paper, it is short and written for a wide audience, not just for physicists.

The relevance is that Cortes-Smolin have devised a version of causal sets in which the space-time cells or atoms are 4-simplices and they are undergoing Pachner moves . This means that the authors can ascribe dynamics to the growth of the past (and this disorderly proliferation we call the present) based on Wieland's action .

 

[marcus]

That May 2014 paper by Fay Dowker used brief dialog to point out the need for a change in our theory of space-time to include the experience of the present and the passage of time. She compares the present situation to an anomaly in Newtonian gravity. Newtonian gravity was at odds with experience in that it suggested that gravity was a force. But we do not feel the force of gravity the way we feel other forces. People put up with that curious anomaly for 300 years and then in 1915 it was discovered that gravity was not a force after all, it was the curvature of space-time geometry. By the principle of equivalence one can zero it out. It is not felt in free fall.
So one had a mismatch of theory with experience that eventually required a fundamental change in the theory. Here's the dialog:
==quote Dowker http://arxiv.org/pdf/1405.3492.pdf ==

... challenges the view that quantum gravity effects can only show themselves as phenomena in regimes far beyond our immediate reach. The idea is that we may have access, through our intimate experience, to a physical phenomenon that is not present in General Relativity but is part of a more complete theory of spacetime. Lest this seem far-fetched, let me introduce an example from history ...

Every day in the centuries between Newton’s discovery of the Law of Universal Gravitation and Einstein’s discovery of General Relativity, every human being was making an observation with no explanation within the Newtonian theory but which correlates perfectly with GR. This observational fact, was, throughout that time in history, “hidden in plain sight.” ...

17th Century Scientist: There is a physical force of weight on you. Look at all the data, celestial mechanics, etc. The Newtonian theory of gravitation accounts for all that data.

17th Century Sceptic: But I don’t experience this gravitational force of weight whereas I can feel mechanical forces of comparable magnitude. Why?

Scientist: The force of weight is physical. So your sense-experience of no force must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of lack of gravitational force of weight.

Sceptic: Maybe. But maybe this is telling us to look again at our theory, with the lack of gravitational force of weight as a heuristic.

The following is a parallel conversation that may, in the future, make similar sense:

21st Century Blockhead: There is no physical passage of time. Look at all the data, celestial mechanics, etc. The theory of General Relativity with spacetime as a Block perfectly accounts for all that data.

21st century Sceptic: But I don’t experience a Block. I experience a sequence of moments. Why?

Blockhead: The Block is physical, the passage of time is not physical. So your sense-experience of time passing must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of the passage of time.

Sceptic: Maybe. But maybe this is telling us to look again at our theory, with a physical passage of time as a heuristic.
==footnote==

5 If there is only a force upwards on you from your chair and no weight acting down on you then you must be accelerating upwards, away from the earth. And so must someone sitting on a chair on the other side of the planet. For everyone on the surface of the Earth to be stationary and yet also accelerating away from the centre of the Earth requires that the spacetime around the Earth be curved.
===endquote===

 

[marcus]

I think it's a safe conclusion that whatever physical world-theory arises from the current confusion will definitely have a place in it for the experience of the present and the passage of time.

==quote Dowker http://arxiv.org/pdf/1405.3492.pdf ==

Blockhead: The Block is physical, the passage of time is not physical. So your sense-experience of time passing must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of the passage of time.
Sceptic: Maybe. But maybe this is telling us to look again at our theory, with a physical passage of time as a heuristic.

===endquote===

So this is a big deal. Major change in store. Physics has so far been using a Block universe, or basing models on simple flat Lorentz spacetime--even on a naive global time variable.
This provides a way to respond to Ohwilleke's question that started the thread: What's the potential importance of Spinfoam QG, assuming it passes astrophysical tests described in Barrau Grain http://arxiv.org/abs/1410.1714?

What's the potential importance? It's different from the Block universe world trajectory--the past has a frontier where it is taking shape. A frontier which (if I understand Wieland's presentation) does not correspond to a reading on anyone's physical clock.

Because the ORDER one imagines new space-time simplices being added to the past is pure gauge.
There's a swarm of tetrahedra moving on time-oriented edges that form a branched manifold and the evolution parameter is not a physical time. Where paths come together, and where they branch out into several, are vertices corresponding to spacetime simplices. Cells from which the spacetime past is built, or by which it grows. there is no synchronicity about the weaving worldliness of this swarm of tets. Spacetime arises from their interactions (Pachner moves). But you can't specify a state at some particular instant.
This accumulation of interactions ( = pentachorons = 4-simplices) does not take place IN time because there is no time. One only gets proper time for the individual path of a particular tetrahedron after the fact.

Maybe I can make this clearer. I'll try to say it better. I'm trying to paraphrase from the two papers:
(google "cortes causal spin foam" and "wieland new action")

 

[ohwilleke]

You are missing the fact that the same can also be said about even more revolutionary theories of the 20'th century, such as theory of relativity and quantum theory. Compared to the 19'th century theories, the 20'th century theories make little difference for phenomena seen under normal everyday-life conditions. And yet, they make a very big difference under certain very extreme conditions, such as velocities close to the velocity of light (special relativity), strong gravitational fields (general relativity), and small distances (quantum mechanics). Likewise, LQG makes a very big difference under extreme conditions of a sufficiently high energy concentrated within a sufficiently small volume (Planck scale).

I don't agree that this is true for the 20th century modern physics discoveries.

SR & GR implied: (1) time can be slowed down by moving something very fast, (2) light is influenced by gravity, (3) time is influence by gravity, (4) E=mc^2 which is at the heart of nuclear weapons and nuclear energy, (5) black holes exist and indeed turn out to be central of the structure of galaxies, and (6) interstellar travel during human length lifetimes faces fundamental barriers different in kind from those involved in breaking the sound barrier (i.e. change in velocity takes more additional momentum at higher velocities and nature has a speed limit).

QM implied: (1) quantum tunnelling which makes transistors possible, (2) QM makes it possible to understand and predict superconductivity, (3) QM provides an easily accessible example of non-deterministic behavior in the physical world, (4) radioactive decay is essentially perfectly understood in principle and critical to nuclear fission power generation, (5) QM provides an easily demonstrable example of non-local phenomena (entanglement), (6) phenomena from heat to X-rays are fundamentally the same, (7) lead can be changed into gold (in principle), (8) the last "1% of phenomena" have an immensely elaborate explanation, (9) pure energy can give rise to matter and routinely does, (10) anti-matter exists, and (11) there is an arrow of time in the laws of physics.

 

[Demystifier]

Ohwilleke, excluding (11) (because it is very questionable that QM implies arrow of time), which of these phenomena can you see in normal everyday-life conditions?

 

[Paulibus]

Barrau and Grain wrote (on Page 4 of their paper, see Post # 1 )

"In bouncing cosmologies, either from the loop approach or any other, the question
of anisotropies is very important for a clear reason: the shear term varies as
a^(-6) where a is the "mean" scale factor of the Universe . When the
Universe is contracting, the shear term becomes more and more important and
eventually drives the dynamics.", so for them shear is an important factor. By way of emphasis, they add, on
Page 6:

"The phase and shear are the initial variables to set." but conclude onPage 7:

that "An important issue however remains: what would be a "natural" initial value for the shear? "

Shear is a spatially relative displacement gradient associated with spatial inhomogeneities like vortices in a finite liquid, or structural
defects in a finite continuum solid (see V. Volterra Ecol. Norm. Sup., 324 (1907), pp. 401––517) How could shear possibly arise on a non-localised bouncing cosmological scale, that includes by definition all that exists (whether the geometry be open or closed) and serve as an important issue in the present context?

 

[marcus]

In "Loop quantum gravity and observations" by A. Barrau and J. Grain at http://arxiv.org/abs/1410.1714 the bottom line seems to be that the phenomenological implications of LQG relative to existing predictions from GR are so extremely subtle that they have virtually no practical effects that can be discerned without ultrahigh precision instruments. It makes the tweak to the orbit of Mars predicted by GR relative to Newtonian gravity look veritably spectacular and vivid by comparison.

Am I missing something, or is the correct conclusion that LQG, if proven right, is basically just wrapping up loose ends that would otherwise lack a mathematically rigorous treatment?

This is an interesting topic in part because it points to the fact that our PERSPECTIVE on quantum gravity research may be changing. As interest in string theorizing declines, more people have been looking at the alternatives, including LQG-SF, but also Causal Sets, CDT, Shape Dynamics, AsymptoticSafetyQG...
and may have been revising their perspectives.
So what Ohwilleke is prompting is an examination of LQG, how has it been progressing? How has it been changing? What can we expect from it, if a version of LQG is tested observationally and checks out well enough to gain provisional acceptance?

There was just recently (26 October) a brief review of the situation consisting of a few comments by Lee Smolin made in Peter Woit's block. He gave a number of links to papers touching on these issues. It might be helpful here to quote.
Lee began the discussion by commenting on a recent narrowly-focused conference as follows:

Lee Smolin says:
October 24, 2014 at 2:53 pm
Dear Peter,

If the conferees want an answer to the question “Are there any plausible alternatives to string/M-theory as a fundamental theory of physics?” they ought to invite some of the people contributing to the development of such alternatives and have an honest examination of the question. They have plenty to choose from, they could start with the speakers at Loops 13, which 200 people attended: http://www.pirsa.org/C13029 [Broken].

Fifteen or twenty years ago it was possible to say that string theory was more promising than its alternatives, and indeed 15 years ago I switched my research from LQG to M theory. But I think that any objective evaluation of the evidence has to credit much more substantial progress has taken place since concerning the alternatives; especially spin foam models, but also other background independent approaches including shape dynamics, CDT, group field theory and tensor models, etc.

To support this I would point to the fact that much more is known from spin foam models about the challenges it faced 15 years ago, including substantial recent results on the emergence of GR in the semiclassical limit, the entropy and temperature of generic black holes, the elimination of cosmological and black hole singularities, finiteness, etc.

So, I would urge my string theory friends who think they know the answer to the question to have another look. There is a whole generation of brilliant young theorists working on alternatives to string theory you should meet!

Thanks,

Lee
=======================
The following series of comment then ensued:

• Jesper says:
  October 25, 2014 at 5:27 pm
  Dear Lee
  could you provide links/references to the recent results you mention on emergence of GR in a semi-classical limit?
  Thanks

• Lee Smolin says:
  October 25, 2014 at 6:36 pm
  Dear Jesper,
  There are many papers which discuss the emergence of GR from spin foam models; here is a small selection of them, in no particular order.
  
  http://arxiv.org/abs/1108.2258, http://arxiv.org/abs/1304.5626, http://arxiv.org/abs/1109.6538, http://arxiv.org/abs/1308.4063, http://arxiv.org/abs/0809.2280, http://arxiv.org/abs/0902.1170
  
  Thanks, Lee

• vmarko says:
  October 26, 2014 at 10:03 am
  Jesper, Lee,
  Just for completeness, there are also some criticisms regarding the way GR emerges from spin foam models (and ways to get around that). See for example
  arXiv:1307.5352, arXiv:1104.1384, arXiv:1111.2865, arXiv:1201.2187.

• Bob says:
  October 26, 2014 at 1:03 pm
  Dear Lee,
  Thanks for those paper references. They look interesting. I have a question for you. LQG, spin foams, etc., are characterized as a quantum theory of pure gravity, without matter and matter interactions. Can you discuss the ways in which Loop Quantum Gravity addresses the criticisms that quantum gravity needs to unify gravity with the other interactions because gravity cannot be decoupled at Planck scales from all other interactions (if there are any) at Standard Model, and higher, energy scales?
  Thanks very much!

• Lee Smolin says:
  October 26, 2014 at 2:16 pm
  Dear Bob,
  Actually there is a moderate sized literature on matter coupling and unification within LQG. To summarize, let me emphasize that there are several different questions.
  
  1) It is just not true that “LQG, spin foams, etc., are characterized as a quantum theory of pure gravity, without matter and matter interactions.” From very early in its development, it was worked out in detail how to include gauge fields, fermions and scalars in LQG. For example, see Thomas Thiemann’s book.
  
  2) Extension to supergravity is also straightforward, at least for N=1, including D=11. See papers by Yi Ling and myself as well as recent papers by Thiemann and collaborators,
  
  3) Hence, even if LQG doesn’t constrain the matter or gauge field content of what it is coupled to, LQG provides a framework in which matter gauge fields and gravity are all coupled together.
  
  4) There are issues that need to be addressed with fermion doubling which are the subject of work in progress with Jacob Barnett.
  
  5) You can ask in addition whether, “quantum gravity needs to unify gravity with the other interactions”. This from a LQG point of view is an open question.
  
  6) But you can ask if LQG gives a compelling framework for unifying the different interactions? Here the answer appears to be yes. The simplest possible extension of the theory, gotten by extending the gauge group of gravity, which is the Lorentz group, to a larger group, G, yields a natural extension of Einstein-Yang Mills. This comes about from a built in spontaneous breaking of the gauge group G to the product of the Lorentz group with a compact factor H, which becomes the Yang-Mills gauge group. See http://arxiv.org/abs/0712.0977, older papers by Peldan and later papers by Krasnov et al. See also http://arxiv.org/abs/1212.5246 which explores implications for a unification of the electroweak interactions with gravity.
  
  7) Going beyond all these solid results are indications that the chiral fermions of the standard model may emerge from topological excitations of spin network states, http://arxiv.org/abs/hep-th/0603022.
  
  Clearly there is much still to do in this direction, but these are sufficient to assure us that LQG provides a framework within which to describe matter and gauge fields coupled to quantum gravity which can suggest hypotheses as to further unification.
  
  Lee

 

[marcus]

...
SR & GR implied: (1) time can be slowed down by moving something very fast, (2) light is influenced by gravity, (3) time is influence by gravity, (4) E=mc^2 which is at the heart of nuclear weapons and nuclear energy, (5) black holes exist and indeed turn out to be central of the structure of galaxies, and (6) interstellar travel during human length lifetimes faces fundamental barriers different in kind from those involved in breaking the sound barrier (i.e. change in velocity takes more additional momentum at higher velocities and nature has a speed limit).

QM implied: (1) quantum tunnelling which makes transistors possible, (2) QM makes it possible to understand and predict superconductivity, (3) QM provides an easily accessible example of non-deterministic behavior in the physical world, (4) radioactive decay is essentially perfectly understood in principle and critical to nuclear fission power generation, ... (9) pure energy can give rise to matter and routinely does, (10) anti-matter exists, ...

The analogies with GR and QM may be more pronounced than you suggest. You mention a MISCELLANY of significant commercial applications (e.g. transistors) and implications (e.g. antimatter exists, speed limited by growing inertia) which impress astonish fascinate normally curious people.
Most of the things you mention are things we do not actually SEE in everyday life. We see transistors but we do not actually see quantum tunneling or antimatter or nuclear fission.

"(9) pure energy can give rise to matter and routinely does..." The reverse would be a possible application of QG. We suspect that very small black holes could convert ordinary matter (like sand or water) completely into pure energy. This is a QG process and small BH are QG systems par excellence. But we don't really understand BH do we? There are a lot of puzzles still. So if you want to imagine a commercial application.

Actually it's hard to foresee commercial applications for really new theoretical developments and it WOULD HAVE BEEN hard in the cases of 1915 GR and 1925 QM. The motivation of the original researchers is probably more likely to have been philosophical, that is "to tie up conceptual loose ends." To understand geometry better, or time, or inertia, or energy, or atoms, or the thermodynamics of light...

In "Loop quantum gravity and observations" by A. Barrau and J. Grain at http://arxiv.org/abs/1410.1714 the bottom line ...LQG, if proven right, is basically just wrapping up loose ends that would otherwise lack a mathematically rigorous treatment?

The comparison with the two major developments (GR and QM) of the 20th century is apt. As I suggested, the original motivation for them was, more likely than not, philosophical---to understand the concepts better, in effect "tie up loose ends".

So what about the problem of "now" and the passage of time? Is our experience of the specialness of the present not an effect of quantum space-time geometry? Classical theories only give us a Block Universe worldview in which time does not pass and there is no representation of the present moment. which we constantly experience.

For discussion's sake, I propose that spacetime is constantly being made from the interactions of Planck-scale quanta of geometry. And it does not exist for us because (since we cannot interact with it) we cannot experience it. It is the past--something we know about but do not experience. We do experience the present---the interactions of quanta of geometry and matter which are forming the past, as we speak : ^) this is a QG phenomenon.
The present moment is a QG phenomenon which we constantly experience and which to understand is one of the jobs of LQG and other theoretical attempts at QG.

Seriously Ohwilleke, you toss out a miscellany of commercial applications on one hand and impressive fascinating IMPLICATIONS on the other. I think it is not hard to imagine, in the case of QG (i.e. quantum space-time geometry and how it interacts with quantum matter) applications and implications that are just as fascinating and impressive as those of 1915 GR and 1925 QM.
: ^)

 

[marcus]

It occurs to me to suspect that when you said "Am I missing something, or...?" you were playing "duffle's advocate" in this thread,
and were hoping that your challenge would provoke just the sort of responses that Demystifier, others, and I have been offering. ; ^))

 

[Demystifier]

Marcus, what is a "duffle's advocate"? oo)

 

[marcus]

Marcus, what is a "duffle's advocate"? oo)

http://en.wikipedia.org/wiki/Devil's_advocate
misspelling

 

[MathematicalPhysicist]

Don't let the people that invest the money for this research know that there's nothing spectacular about it...

 

[marcus]

Marcus, what is a "duffle's advocate"? oo)

Duffle is the title of Chapter 89
of the classic novel Cat's Cradle by Kurt Vonnegut
http://books.google.com/books?id=w25sx0G6nRsC&pg=PA199&dq=89+Duffle#v=onepage&q=89 Duffle&f=false

The term is defined at the beginning of the chapter. You can get the chapter simply by googling "89 duffle"

Frank was giving a classic illustration of what Bokonon calls duffle. Duffle, in the Bokononist sense, is the destiny of thousands upon thousands of persons when placed in the hands of a stuppa. A stuppa is a fogbound child.​

Alternative link: http://catscradle.neocities.org/chapter-89.html

I guess the etymology is somewhere between the German word Teufel, and the dull carry-all shapelessness of a kind of canvas suitcase called a "duffle bag" : ^)

 

[marcus]

Part of answering the topic question involves what one thinks is "cool". the OP cited a paper by Aurelien Barrau et al. about opportunities for observational testing of LQG and the LQC application to cosmology. ("Loop quantum gravity and observations by A. Barrau and J. Grain at http://arxiv.org/abs/1410.1714 ")

Today another Aurelien Barrau paper came out which I think has some rather cool consequences IF it turns out that LQG gives a reasonably good picture of the start of expansion (the bounce) in the early universe. Look especially at FIGURE 5, on page 8, where you see light cones slimming down to straight lines as the density increases to critical. In effect space momentarily consists of isolated points right around the bounce, and space-time geometry becomes a congruence of lines ("asymptotic silence", effective speed of light --> 0.)

http://arxiv.org/pdf/1411.0272v1.pdf

See also page 12, one of the conclusions:
==quote==
The main results of the paper are the following:

• The state of silence provides a natural way to set initial conditions for the cosmological perturbations. This state is the beginning of the Lorentzian phase, and, in some sense, the beginning of time. This is qualitatively a new feature in cosmology.
...
...
===endquote===

It should say "This state is the resumption of the Lorentzian phase, and, in some sense, the resumption of time. This is qualitatively a new feature in cosmology."

Before the bounce time proceeds normally and there is a normal Lorentzian phase. The difference is the U is contracting rather than expanding. It's one of two ways the Friedman equation can go (H(t) can be negative as well as positive, the equation only governs the square of H(t))

So then towards the end of contraction there is a signature change. Density rises to critical level, in effect c --> 0 and the light cones slim down to lines. The Lorentz signature is Euclidianized---but only temporarily, there is a bounce, density declines, Euclidian changes back to Lorentz, the lines open out to cones again.

Here's the abstract:
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)
Recent calculations in loop quantum cosmology suggest that a transition from a Lorentzian to an Euclidean space-time might take place in the very early Universe. The transition point leads to a state of silence, characterized by a vanishing speed of light. This behavior can be interpreted as a decoupling of different space points, similar to the one characterizing the BKL phase.
In this study, we address the issue of imposing initial conditions for the cosmological perturbations at the transition point between the Lorentzian and Euclidean phases. Motivated by the decoupling of space points, initial conditions characterized by a lack of correlations are investigated. We show that the "white noise" initial conditions are supported by the analysis of the vacuum state in the Euclidean regime adjacent to the state of silence.
Furthermore, the possibility of imposing the silent initial conditions at the trans-Planckian surface, characterized by a vanishing speed for the propagation of modes with wavelengths of the order of the Planck length, is studied. Such initial conditions might result from a loop-deformations of the Poincaré algebra. The conversion of the silent initial power spectrum to a scale-invariant one is also examined.
12 pages, 8 figures

 

[inflector]

Very interesting new developments gentleman. I can't but wonder if this doesn't tie into Jim Gates's work given the interesting four-node connections in the network diagrams he pulls out of the equations of Supersymmetric String Theory, he calls them Adinkras, the Peacock in particular shows this and it makes one wonder if the tetrahedral substrate of CDT and the 4-simplices of the above mentioned theories are not dancing around the same underlying mechanisms.

 

[marcus]

Inflector, I cannot say about the relation to the "equations of Supersymmetric String Theory" that you mention, but whatever it might be related to I find the Mielczarek Linsefors Barrau paper fascinating! According to MLB during the Loop cosmology bounce the lightcones slim down to one dimensional "time lines" (spacetime becomes a "congruence of timelines" as they say, a bundle of parallel worldlines of isolated points out of contact with each other).

And then immediately after the bounce they open out into lightcones again. there is a brief interval when the highest density is reached where the the speed of light decreases to zero (visualized as the slimming of the lightcones which they depict in figure 5) so that even neighboring points are out of communication.
This has been written about, as I recall, in a dozen or so LQG papers under the heading "SIGNATURE CHANGE". One way to describe the speed of light briefly shrinking to zero is to say that there is a change from Minkowski (- +++) signature to Euclidean (++++), and then normal Minkowski signature is resumed.

I'm wondering how to relate this to another recent paper which describes something going on at the BLACK HOLE singularity. This is not a bounce, as in the cosmology case, but has the broad outlines of normal BH evaporation. However because of the underlying LQG degrees of freedom unitarity is preserved and there is no firewall. The paper is by Alejandro Perez.
http://arxiv.org/abs/1410.7062
No firewalls in quantum gravity: the role of discreteness of quantum geometry in resolving the information loss paradox
I think both the MLB paper and the Perez paper should be considered for the fourth quarter 2014 MIP poll.

 

[marcus]

I mentioned Alejandro Perez' paper that just came out, as a possible cool one. I should give the abstract so you can see what it is about. He seems to have a remarkably simple solution to the BH information loss problem which has plagued theorists for well over 30 years and has been the occasion of much drama. Can Perez' solution be right? It looks so simple.

http://arxiv.org/abs/1410.7062
No firewalls in quantum gravity: the role of discreteness of quantum geometry in resolving the information loss paradox
Alejandro Perez
(Submitted on 26 Oct 2014)
In an approach to quantum gravity where space-time arises from coarse graining of fundamentally discrete structures, black hole formation and subsequent evaporation can be described by a unitary evolution without the problems encountered by the standard remnant scenario or the schemes where information is assumed to come out with the radiation while evaporation (firewalls and complementarity). The final state is purified by correlations with the fundamental pre-geometric structures (in the sense of Wheeler) which are available in such approaches, and, like defects in the underlying space-time weave, can carry zero energy.
12 pages, 7 figures.

 

[julian]

Hi marcus. Have you looked at the paper "A loop quantum multiverse?" http://uk.arxiv.org/pdf/1212.5150 where Bojowald talks about the possibility of signature change and implication for the initial conditions and the entropy problem?

 

[marcus]

Hi marcus. Have you looked at the paper "A loop quantum multiverse?" http://uk.arxiv.org/pdf/1212.5150 where Bojowald talks about the possibility of signature change and implication for the initial conditions and the entropy problem?

Hi Julian,
I looked at that paper briefly in 2012 when it came out. It was not the first paper of Bojowald to talk about signature change, nor the last, so that aspect did not make it stand out for me. By "entropy problem" I think you mean the issue about why, from our perspective in the expanding phase, looking back, the entropy in early universe (as we measure it) is low. I should probably take another look at that paper and see what he says. Thanks for the reminder!

It could be that mentioning "multiverse" in this context is just an unnecessary and distracting complication. It could be fun to consider what the putative signature change might do to entropy or entropies (as defined by which observers?)

It seems that entropies are measured in a way that depends very much on the observer---how he coarse-grains, what variables he interacts with, what he is necessarily ignorant of, what outside his horizon and beyond his ken.

BTW Bojowald co-authored with Barrau et al very recently. Just this year I think. Signature change was discussed. I don't know if you looked at that paper. It might give an up to date indication of how Bojowald currently sees it. I did not get into that paper. Maybe I should, but right now I'm more excited by what Barrau is making of it, as in the MLB paper. I like the echo of the classical BKL conjecture, asymptotic silence.

A propos BKL and asymptotic silence, in case not familiar to some readers, here's a tidbit from Steve Carlip. Julian you may have other background material you want to quote that could shed light on the topic! Anyway Carlip connects it to the signs of dimensional reduction that have come up in QG:
http://arxiv.org/abs/1207.4503
Spontaneous Dimensional Reduction?
S. Carlip
(Submitted on 18 Jul 2012)
Over the past few years, evidence has begun to accumulate suggesting that spacetime may undergo a "spontaneous dimensional reduction" to two dimensions near the Planck scale. I review some of this evidence, and discuss the (still very speculative) proposal that the underlying mechanism may be related to short-distance focusing of light rays by quantum fluctuations.
9 pages. Proc. of the Sixth International School on Field Theory and Gravity, Petropolis, Brazil, 2012
==quote Carlip==
The BKL behavior of the preceding section has been carefully studied in a different context, that of classical cosmology near a generic spacelike singularity [37]. There, the essential feature is what is called “asymptotic silence.” Light cones along world lines orthogonal to the singularity collapse to lines as the singularity is approached, leading to causal disconnection of nearby points. This behavior can be described as an anti- Newtonian limit: rather than approaching infinity, as in the Newtonian limit, the effective speed of light shrinks to zero. If this causal disconnection occurs rapidly enough, the structure of the field equations leads almost automatically to BKL behavior.

The ultralocal nature of the short distance Wheeler-DeWitt equation provides one indication of asymptotic silence in quantum gravity. Let us search for others. Note that unlike the cosmological setting, in which the initial singularity defines a preferred set of world lines, we now need light cones to collapse along world lines orthogonal to an arbitrary initial surface...
==endquote==

 

[julian]

Hi Julian,
I looked at that paper briefly in 2012 when it came out. It was not the first paper of Bojowald to talk about signature change, nor the last, so that aspect did not make it stand out for me. By "entropy problem" I think you mean the issue about why, from our perspective in the expanding phase, looking back, the entropy in early universe (as we measure it) is low. I should probably take another look at that paper and see what he says. Thanks for the reminder!

It could be that mentioning "multiverse" in this context is just an unnecessary and distracting complication. It could be fun to consider what the putative signature change might do to entropy or entropies (as defined by which observers?)

It seems that entropies are measured in a way that depends very much on the observer---how he coarse-grains, what variables he interacts with, what he is necessarily ignorant of, what outside his horizon and beyond his ken.

BTW Bojowald co-authored with Barrau et al very recently. Just this year I think. Signature change was discussed. I don't know if you looked at that paper. It might give an up to date indication of how Bojowald currently sees it. I did not get into that paper. Maybe I should, but right now I'm more excited by what Barrau is making of it, as in the MLB paper. I like the echo of the classical BKL conjecture, asymptotic silence.

Hi Marcus

Was it it not the first? Hmmm.

In this context the "multiverse" may be an unnecessary and distracting complication. But the question of its physical prediction being valid and its implications is another issue!

How the observer course-grains and questions of entropy is an issue brought up by Rovelli I think? Is that right? And I wish to understand this better.

I have not yet looked at the paper co-authored with Barrau et al. This may be another idea Bojowald maybe pursuing, but I would be surprised if he doesn't have intention of following his "Multiverse" idea and its implications - even if this may a difficult programme to complete.

I will look at the new papers you have mentioned - very interesting.

p.s. I still mean to review a paper by Thiemann on the limitations of LQC - I've partly written it up.

 

[marcus]

Was it it not the first? Hmmm.
...

For example
http://arxiv.org/abs/1112.1899
"...the emergence of the universe from Euclidean space at high density. This new version of signature-change cosmology suggests a natural way of posing initial conditions, and a solution to the entropy problem..."

I'm not suggesting that what someone e.g. Martin Bojowald says in 2011 or 2012 should be taken as authoritative final word. But the signature change thing does go back a ways with him and I guess others, even Steven Hawking?
I haven't thought about this enough to be helpful but I'm glad you have raised these issues.
Bojo collaborated with Barrau and friends in an April 2014 paper.
I really like where Barrau et al are going with this! I can't say more (at present) that would make any sense : ^)

 

[julian]

The No-Boundary Proposal of Hartle and Hawking?

 

[marcus]

The No-Boundary Proposal of Hartle and Hawking?

That's what I was vaguely referring to : ^)
Don't want to overemphasize that though. You may understand it better. Have to admit I never paid much attention to that idea.
Bounce always seemed more straightforward to me, ockham-minimal, no special assumption magic.

It is just what happens when you extend the quantization of gravity "into the UV", that is, to extremely high energy density, and then trace the history of the universe back in time.

Instead of finding a singularity at the start of expansion you find a rebound from a contracting phase.

So you don't have to "make up" any new physics. You just do your best you to quantize geometry and the result tells you quantum geometry resists being pinned down and says there's a bounce.

 

[julian]

I agree.

 

[marcus]

Julian, I'm glad you agree! We have similar perspectives on the cosmological bounce, I think. Today I was exploring the idea that the bounce may solve the (early universe low) ENTROPY PROBLEM

It's the idea that for the gravitational field more and more structure means increasing entropy. More clumping more collapsing, more intricate varigated structure... But if you keep writing on a slate eventually you get a blank slate! There comes a time when every pixel is marked and you recover the "tabula rasa" of no structure at all: zero entropy.

An important part of this is the discreteness of geometric information---the LQG "area gap" and such---which is the root cause of the bounce as well.

I want to bring forward the abstract of the 2014 Mielczarek Linsefors Barrau paper from post #23. And also post the abstract of a 2012 Mielczarek paper that is related. That way we will have these things in front of us to look over. I think the condition of total causal disconnection where every point is causally isolated from all the others (and thus no memory storage is possible) constitutes zero entropy---from the standpoint of anyone point.
Entropy is observer dependent and the the isolated points are the only observers. For each point, it is all that exists and it knows all there is to know.

Part of answering the topic question involves what one thinks is "cool". the OP cited a paper by Aurelien Barrau et al. about opportunities for observational testing of LQG and the LQC application to cosmology. ("Loop quantum gravity and observations by A. Barrau and J. Grain at http://arxiv.org/abs/1410.1714 ")

Today another Aurelien Barrau paper came out which I think has some rather cool consequences IF it turns out that LQG gives a reasonably good picture of the start of expansion (the bounce) in the early universe. Look especially at FIGURE 5, on page 8, where you see light cones slimming down to straight lines as the density increases to critical. In effect space momentarily consists of isolated points right around the bounce, and space-time geometry becomes a congruence of lines ("asymptotic silence", effective speed of light --> 0.)

http://arxiv.org/pdf/1411.0272v1.pdf

See also page 12, one of the conclusions:
==quote==
The main results of the paper are the following:

• The state of silence provides a natural way to set initial conditions for the cosmological perturbations. This state is the beginning of the Lorentzian phase, and, in some sense, the beginning of time. This is qualitatively a new feature in cosmology.
...
...
===endquote===

It should say "This state is the resumption of the Lorentzian phase, and, in some sense, the resumption of time. This is qualitatively a new feature in cosmology."

Before the bounce time proceeds normally and there is a normal Lorentzian phase. The difference is the U is contracting rather than expanding. It's one of two ways the Friedman equation can go (H(t) can be negative as well as positive, the equation only governs the square of H(t))

So then towards the end of contraction there is a signature change. Density rises to critical level, in effect c --> 0 and the light cones slim down to lines. The Lorentz signature is Euclidianized---but only temporarily, there is a bounce, density declines, Euclidian changes back to Lorentz, the lines open out to cones again.

Here's the abstract:
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)
Recent calculations in loop quantum cosmology suggest that a transition from a Lorentzian to an Euclidean space-time might take place in the very early Universe. The transition point leads to a state of silence, characterized by a vanishing speed of light. This behavior can be interpreted as a decoupling of different space points, similar to the one characterizing the BKL phase.
In this study, we address the issue of imposing initial conditions for the cosmological perturbations at the transition point between the Lorentzian and Euclidean phases. Motivated by the decoupling of space points, initial conditions characterized by a lack of correlations are investigated. We show that the "white noise" initial conditions are supported by the analysis of the vacuum state in the Euclidean regime adjacent to the state of silence.
Furthermore, the possibility of imposing the silent initial conditions at the trans-Planckian surface, characterized by a vanishing speed for the propagation of modes with wavelengths of the order of the Planck length, is studied. Such initial conditions might result from a loop-deformations of the Poincaré algebra. The conversion of the silent initial power spectrum to a scale-invariant one is also examined.
12 pages, 8 figures

Here's the 2012 Mielczarek paper:
http://inspirehep.net/record/1207431?ln=en
http://arxiv.org/abs/arXiv:1212.3527
Asymptotic silence in loop quantum cosmology
Jakub Mielczarek
(Submitted on 14 Dec 2012)
The state of asymptotic silence, characterized by causal disconnection of the space points, emerges from various approaches aiming to describe gravitational phenomena in the limit of large curvatures. In particular, such behavior was anticipated by Belinsky, Khalatnikov and Lifshitz (BKL) in their famous conjecture put forward in the early seventies of the last century. While the BKL conjecture is based on purely classical considerations, one can expect that asymptotic silence should have its quantum counterpart at the level of a more fundamental theory of quantum gravity, which is the relevant description of gravitational phenomena in the limit of large energy densities. Here, we summarize some recent results which give support to such a possibility. More precisely, we discuss occurrence of the asymptotic silence due to polymerization of space at the Planck scale, in the framework of loop quantum cosmology. In the discussed model, the state of asymptotic silence is realized at the energy density ρ=ρ_c/2, where ρ_c is the maximal allowed energy density, being of the order of the Planck energy density. At energy densities ρ>ρ_c/2, the universe becomes 4D Euclidean space without causal structure. Therefore, the asymptotic silence appears to be an intermediate state of space between the Lorentzian and Euclidean phases.
4 pages, 3 figures, talk presented at the Multiverse and Fundamental Cosmology Conference, 10-14 September, 2012, Szczecin, Poland

BTW this paper gives some nice background on the asymptotic silence limit, which Levy-Leblond dubbed the "Carrollian limit" in honor of the the author of Through the Looking-glass.
http://arxiv.org/pdf/1402.0657v5.pdf
One of the authors is Gary Gibbons (DAMTP), others based in Marseille, Tours...

 

[marcus]

To recall and summarize some of the cool things LQG does which we have been focusing on here:
1. The Loop cosmology bounce makes adequate inflation (with modest assumptions) a near certainty without incurring the excessive persistent inflation that leads to a multiplicity of universes.
Google "linsefors duration inflation arxiv" or see http://arxiv.org/abs/1301.1264 and references therein.
2. Indications are that LQG is testable (see Ohwilleke's post #1 or google "barrau loop observations"). Loop cosmology has been derived from LQG.
3. A brief episode of signature change at the bounce may clarify the imposition of initial conditions and the low entropy at start of expansion.(Mielczarek et al 2014, see preceding post, #33)
4. Causal spin foam QG addresses the passage of time and growth of space-time.
To get the July 2014 papers on this, google "wieland new action arxiv" and "cortes causal spin foam".
To emphasize the need to address the passage of time, I'll quote from a new paper by Ruth E. Kastner and also bring forward a pointed dialog quoted earlier in this thread.

That May 2014 paper by Fay Dowker used brief dialog to point out the need for a change in our theory of space-time to include the experience of the present and the passage of time. She compares the present situation to an anomaly in Newtonian gravity. Newtonian gravity was at odds with experience in that it suggested that gravity was a force. But we do not feel the force of gravity the way we feel other forces. People put up with that curious anomaly for 300 years and then in 1915 it was discovered that gravity was not a force after all, it was the curvature of space-time geometry. By the principle of equivalence one can zero it out. It is not felt in free fall.
So one had a mismatch of theory with experience that eventually required a fundamental change in the theory. Here's the dialog:
==quote Dowker http://arxiv.org/pdf/1405.3492.pdf ==

... challenges the view that quantum gravity effects can only show themselves as phenomena in regimes far beyond our immediate reach. The idea is that we may have access, through our intimate experience, to a physical phenomenon that is not present in General Relativity but is part of a more complete theory of spacetime. Lest this seem far-fetched, let me introduce an example from history ...

Every day in the centuries between Newton’s discovery of the Law of Universal Gravitation and Einstein’s discovery of General Relativity, every human being was making an observation with no explanation within the Newtonian theory but which correlates perfectly with GR. This observational fact, was, throughout that time in history, “hidden in plain sight.” ...

17th Century Scientist: There is a physical force of weight on you. Look at all the data, celestial mechanics, etc. The Newtonian theory of gravitation accounts for all that data.

17th Century Sceptic: But I don’t experience this gravitational force of weight whereas I can feel mechanical forces of comparable magnitude. Why?

Scientist: The force of weight is physical. So your sense-experience of no force must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of lack of gravitational force of weight.

Sceptic: Maybe. But maybe this is telling us to look again at our theory, with the lack of gravitational force of weight as a heuristic.

The following is a parallel conversation that may, in the future, make similar sense:

21st Century Blockhead: There is no physical passage of time. Look at all the data, celestial mechanics, etc. The theory of General Relativity with spacetime as a Block perfectly accounts for all that data.

21st century Sceptic: But I don’t experience a Block. I experience a sequence of moments. Why?

Blockhead: The Block is physical, the passage of time is not physical. So your sense-experience of time passing must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of the passage of time.

Sceptic: Maybe. But maybe this is telling us to look again at our theory, with a physical passage of time as a heuristic.
==footnote==

5 If there is only a force upwards on you from your chair and no weight acting down on you then you must be accelerating upwards, away from the earth. And so must someone sitting on a chair on the other side of the planet. For everyone on the surface of the Earth to be stationary and yet also accelerating away from the centre of the Earth requires that the spacetime around the Earth be curved.
===endquote===

 

[marcus]

I mentioned a recent paper by Ruth E. Kastner and want to quote from it. It's part of a trend to retire the BLOCK spacetime in which time does not pass and replace it with a GROWING spacetime ontology in which the experience of time's passage and the present moment have a place.
http://arxiv.org/abs/1411.2072
The Emergence of Spacetime: Transactions and Causal Sets
Ruth E. Kastner
(Submitted on 8 Nov 2014)
This paper discusses how the transactional interpretation of quantum mechanics can provide for a natural account of the emergence of spacetime events from a quantum substratum. In this account, spacetime is not a substantive manifold that becomes occupied with events; rather, spacetime itself exists only in virtue of specific actualized events. This implies that spacetime is discrete rather than continuous, and that properties attributed to spacetime based on the notion of a continuum are idealizations that do not apply to the real physical world. It is further noted that the transactional picture of the emergence of spacetime can provide the quantum dynamics that underlie the causal set approach as proposed by Sorkin and others.
14 pages, 4 figures. Contribution to The Algebraic Way, edited by Ignazio Licata.

===quote Kastner conclusions section==
5. Conclusion

The possibilist transactional picture can be viewed as a physical basis for the emergence of the partially ordered set of events in the causal set formalism. This formalism is currently being explored as a means to constructing a satisfactory theory of quantum gravity, and it has much promise in that regard. However, even apart from general relativistic considerations, the formalism breaks new ground in showing that, contrary to a well-entrenched belief, a block world ontology is not required for consistency with relativity. The causal set structure is a ‘growing universe’ ontology which nevertheless preserves the relativistic prohibition on a preferred frame.
Likewise, the transactional ontology proposed here is a variation on the 'growing universe' picture. The account is consistent with relativity theory in that the set of events is amenable to a covariant description: no preferred frame is required. This is because the transactional process is inherently Poissonian, and therefore preserves the relativistic covariance of the causal set model.
==endquote==
Googling "kastner spacetime causal" will get this one, if the link is not readily at hand.

The relevance to Loop and Spinfoam QG is that, given Wieland's "new action" formulation of spin foam the July 2014 Cortes Smolin paper (google "cortes causal spin foam") shows how Wieland's model implements the passage of time and the "growing universe", as Kastner calls it.