Why CDT changes the map of quantum gravity

[marcus]

http://citebase.eprints.org/cgi-bin/fulltext?format=application/pdf&identifier=oai%3AarXiv.org%3Agr-qc%2F0210061

a 2002 paper by Fay Dowker etc.

http://www.imperial.ac.uk/research/theory/research/quantum.htm

A link that may be of interest.

From where i am, I cannot get the first link to work, but I guess that it is to this paper
http://arxiv.org/abs/gr-qc/0210061
which I can get.

thanks for both links.

I poked around a little at the Imperial College theoretical physics site and noticed that not only is Fay Dowker there but also
Chris Isham.

Renate Loll's degree is from Imperial College. She got her doctorate there in 1989. Could be that her strong interest in quantum gravity was helped by the influence of people like Dowker and Isham.

 

[wolram]

From where i am, I cannot get the first link to work, but I guess that it is to this paper
http://arxiv.org/abs/gr-qc/0210061
which I can get.

Thats the one Marcus.

 

[marcus]

It is an odd coincidence that several of the papers I am currently checking out are by women. I swear it is not me. I am not a philanderer skirt-chaser type at all!

Just got a notion that I'd like to see this. But not only is it in German, so not easy to read, but it is not even posted on arxiv. Will see if there is a Uni Potsdam link.

B. Dittrich: Dynamische Triangulierung von Schwarzloch-Geometrien (in German), Diploma Thesis, Univ. Potsdam (2001).

No luck. Dittrich thesis is not in digital form online anywhere, it seems. Looked several places.

the dittrich-loll paper they just posted, about dynamical triangulation of black holes, says that part of what is in that paper comes from dittrich's thesis.

http://arxiv.org/gr-qc/0506035

the nagging question for me is what happens AFTER the details of this new model of spacetime are worked out. that will take a while. they have substantially done the empty "ground state", and they have made a start with
1. adding some token matter (so far posted work does this only in lower dimension cases)
2. allowing for topology change, like with formation of a macroscopic black hole (so far posted work only in 1+1D)
3. modeling black holes (preliminary)

more is in progress about these than has been posted yet, judging by the mentions in the papers we do have, and citations to unpublished work

But assuming all that gets done. And suppose it continues to check out OK and be at least consistent with what has already been observed. that still leaves the real hard testing with make-or-break predictions about future experiments, but suppose it continues surviving.

then (and that is a lot to assume) we still only have a model of spacetime!

it is a different enough continuum that everything (quantum field theories) built on it will have to be radically different. It is not the rigid framework that field theories are used to. It is not a smooth manifold with differentiable coordinate maps. It is very rough and jagged at small scale, somewhat fractal looking, although conventional looking at large scale.

What will the standard particle model look like when rebuilt on CDT's new spacetime continuum?

AFAIK this continuum only exists as a limit of triangulations, as a limit of a sequence of finer and finer simplicial, jaggy approximations to it. or an idefinite quantum cloud thereof.
(reminiscent of how sqrt2 only exists as limit of closer and closer approximations by a series of wholenumber fractions i.e. rationals) what will field theory look like built on that kind of thing?

I suppose one would have to build the matter field up in stages too, define it on the jaggy triangly approximations and then pass to the limit as the simplexes shrink down finer and finer.

 

[selfAdjoint]

But assuming all that gets done. And suppose it continues to check out OK and be at least consistent with what has already been observed. that still leaves the real hard testing with make-or-break predictions about future experiments, but suppose it continues surviving.

then (and that is a lot to assume) we still only have a model of spacetime!

it is a different enough continuum that everything (quantum field theories) built on it will have to be radically different. It is not the rigid framework that field theories are used to. It is not a smooth manifold with differentiable coordinate maps. It is very rough and jagged at small scale, somewhat fractal looking, although conventional looking at large scale.

What will the standard particle model look like when rebuilt on CDT's new spacetime continuum?

AFAIK this continuum only exists as a limit of triangulations, as a limit of a sequence of finer and finer simplicial, jaggy approximations to it. or an idefinite quantum cloud thereof.
(reminiscent of how sqrt2 only exists as limit of closer and closer approximations by a series of wholenumber fractions i.e. rationals) what will field theory look like built on that kind of thing?

I suppose one would have to build the matter field up in stages too, define it on the jaggy triangly approximations and then pass to the limit as the simplexes shrink down finer and finer.

These are very pertinent concerns, that would apply with only slight rewording to LQG too. The original superstring program was more direct, to uniquely postdict GR-like gravitation AND the standard model at low energies out of the same underlying theory. But that enterprise seems currently not to be working (I don't think it's appropriate to junk string theory yet, though!) Even if Thiemann's Phoenix Program comes through with flying colors, it will still not uniquely determine particle physics, at least I don't see any signs in his papers that it will.

Currently I am looking for a corresponding all-in-one development from some combination of non-commutative geometry, Kea's informational categories, Kneemo's Jordan algebras, and whatever else. It is more and more borne in on me that spacetime is no more to be taken as like what we intuit about it than matter has turned out to be.

 

[marcus]

It is more and more borne in on me that spacetime is no more to be taken as like what we intuit about it than matter has turned out to be.

If you insist that your spacetime model start off by reproducing the Standard (Matter) Model then I wish you luck selfAdjoint

I picture progress as more apt to occur in stages----first getting a quantum dynamics of spacetime, then constructing particle physics on that basis.

It would be a happy surprise if some smart person were to skip the first stage and get an All-In-One, but i don't expect this to happen.

I hope that a good quantum spacetime dynamics (still with only token generic matter, not the full gamut of particles) will inspire changes in how quantum field theory is done and revolutionize the Standard Model by requiring it to be built on the basis of a new continuum.

And so I see the emergence of a new spacetime in CDT as promising, in fact as an important development.

By contrast, you seem to be watching a mixed assortment of bids for an All-In-One, assuming I understand you correctly.

It's certainly wise to diversify one's bets. since you gave a list of your favorite hopefuls, let me fetch a link to a different list----which I would guess include several Hermann Nicolai picks: the topics to be covered in the Loops 05 conference

 

[marcus]

...By contrast, you seem to be watching a mixed assortment of bids for an All-In-One, assuming I understand you correctly.

It's certainly wise to diversify one's bets. since you gave a list of your favorite hopefuls, let me fetch a link to a different list----which I would guess include several Hermann Nicolai picks: the topics to be covered in the Loops 05 conference

responding to selfAdjoint list of prospects, I will list somebody else's list of picks. Keeping open to diverse avenues of progress is essential. The Loops 05 conference

http://loops05.aei.mpg.de/

has these topics:

1. Background Independent Algebraic QFT
2. Causal Sets
3. Dynamical Triangulations
4. Loop Quantum Gravity
5. Non-perturbative Path Integrals
6. String Theory

Thomas Thiemann's Master Constraint Program ("Phoenix") which selfAdjoint referred to is not specifically mentioned and Thiemann is not on the list of invited speakers. (He will surely be speaking and could still be added to the list of plenary invited talks, but as of now has not been.)

I see that the Loops 05 has been added to quite recently! In the past couple of days even. It now has a list of participants who have registered so far---this includes others besides the invited speakers.

What to make of the differences in perspective? selfAdjoint has one list and Hermann Nicolai (director in charge of QG and Unified theories research at AEI) has a different list. Not too much can be made! It is just people's different perspectives.

 

[selfAdjoint]

My point is that a program which merely adds matter to some gravitational theory, like one that merely tacks gravity onto a preexisting particle theory, smacks of epicycles. Surely this is not what Einstein meant by the secrets of the Old One? Sure it's a harder row to hoe, but how can we really be statisfied with less in the end?

 

[marcus]

My point is that a program which merely adds matter to some gravitational theory, .. smacks of epicycles.

and that is a good point! I heartily share your distaste for epicycles.
what I imagine is that QFT and Std Mddle will be fundamentally transformed by the shock of encountering a new kind of spacetime continuum.

(with a completely new structure and even different dimensionality at short range)

these venerable antiques (QFT Std Mddle) were built on Minkowski space. Now if they are to be reconstructed on a foundation that is not even a differentiable manifold it is likely to change them in ways we cannot begin to anticipate.

the image of "tacking on" as one would stick on an epicycle, hardly seems to fit what is under discussion, or?

 

[Kea]

It is more and more borne in on me that spacetime is no more to be taken as like what we intuit about it than matter has turned out to be.

Beautifully put, selfAdjoint!

At present I am working hard, knowing that the Streetfest is only a few weeks away. After that there is a short Categories workshop and then the NCG school is after that. Can't wait!

 

[Chronos]

I realize this is a simplistic view, but I perceive a hierarchical emergence of the macroscopic universe. Spacetime emerged before matter, so it seems logical to try approaching the problem using spacetime as the canvas and matter as the paint.

 

[marcus]

...hierarchical emergence of the macroscopic universe. Spacetime emerged before matter, so it seems logical to try approaching the problem using spacetime as the canvas and matter as the paint.

sure makes sense to me
by analogy in first year calculus you meet functions y = f(x) defined on the x-axis or some other set

the first thing you need to understand about such a thing is its domain of definition. If it is defined on the x-axis or on the real line, then what is that? What are the real numbers and the axis.

Not to get philosophical, you have to know is this function defined on the line or on the plane or a region of xyz space, or what?

After that you can talk about the specific properties of the function---is it positive or negative increasing decreasing, continuous or not, does it have a derivative, does it have some formula or solve some equation.

A MATTER FIELD IS LIKE A FUNCTION y = f(x) defined on some domain of definition which is spacetime.

THE FIRST THING YOU HAVE TO KNOW is about that domain of definition. What is that spacetime continuum that the fields are defined on?

Till now a lot of us thought it was a differentiable manifold with some fixed whole-number dimension. All the matter fields were defined on that sort of thing. Now it looks like it might NOT be.

this is bound to have a deep effect on how matter fields are eventually defined, namely what they are defined on. seems simple enough.

canvas and paint puts it well!

 

[Terry Giblin]

How does one get an invite to an Physics Conf.

Can I start by thanking Marcus, for introducing me to CDT.

Last year, if I had known I would have tried to discuss this topic it when I last visited Utrecht.

It appears that CDT and CFD both use 3 as their primary parameter and produces a shape similar to a diamond shock wave pattern.

So how does one apply to a Physics Conference, the replies I have received todate, is by inivation only, which is fine, but how to I get invited?

Regards

Terry Giblin

 

[marcus]

...
So how does one apply to a Physics Conference, the replies I have received todate, is by inivation only, which is fine, but how to I get invited?
...

Dear Terry, After giving it some seriouss consideration, I would have misgivings about encouraging anyone to attend Loops 05 who hasnt been following the quantum gravity scene for some time as it is likely to be largely technical and scattered around among a number of diverse topics. You are the only one who can judge if it is for you.

Only some (like any talks by Renate Loll) will be sure to be about CDT.
Have you tried reading her papers? I must assume that you have.

In any case if you have decided to try to go then I am puzzled that you could not just register. Did you go to loops05.aei.mpg.de/ and try to register using the registration form provided? I assume that you did, from what you said.

Unfortunately I do not know of anything to do besides the obvious one of filling out and submitting the form.

 

[marcus]

How CDT changes the map of quantum gravity research

... The Loops 05 conference site

http://loops05.aei.mpg.de/

lists these topics:

1. Background Independent Algebraic QFT
2. Causal Sets
3. Dynamical Triangulations
4. Loop Quantum Gravity
5. Non-perturbative Path Integrals
6. String Theory

as a working assumption the Loops 05 conference IS the map of quantum gravity, and these topics suggest how that map looks

and what it looks like to me right now is a trapeze act where Laurent Freidel does an aerial somersault and is caught by Renate Loll who is swinging by her knees upside down.

the topic of this thread is how CDT CHANGES THE MAP OF QG and we should focus back on that and take a fresh look.

it changes it radically and fast. Please have a look at the recent Laurent Freidel paper.

http://arxiv.org/abs/gr-qc/0506067

He basically does spin-foams-with-matter and relates it to other mathematical techniques. Some of the language in this recent paper reminded me of CDT. It was like Freidel was extra aware of how his 3D spin foams work might connect with 3D Loll-type simplex gravity. In both Freidel and Loll's case it has been very important to work things out in 1+1D and 1+2D as a guide for what to do and expect in 1+3.

Freidel co-authors have included people at Perimeter such as Artem Starodubtsev, David Louapre, Etera Livine, Lee Smolin, and others such as Kirill Krasnov, Kowalski-Glikman, Carlo Rovelli (his PhD thesis advisor if I remember right), and now in this latest paper two Cambridge people Daniele Oriti and James Ryan

It will be interesting to see if Loll actually catches Freidel or whether they miss and he falls into the net.