Two World-theories (neither one especially stringy)

[marcus]

I was just now replying to a post of selfAdjoint in the "1-to-10" thread and the thought occurred to that a lot of people may not have realized that getting a new model of the continuum may turn out to be THE FASTEST WAY TO A TOE.

Causal Dynamical Triangulations has a limited goal of merely arriving at a quantum model of spacetime that reproduces Gen Rel at large scale.
(but is based on quantum spacetime dynamics at microscopic scale)

Once people have a new continuum, and start working on it, and building theories of matter on that instead of Minkowski space, then it can be argued that the new features of the continuum are likely to inspire and enable new matter physics.

 

[marcus]

The CDT quantum continuum is still very new and the preliminary results on it are still coming in, so it involves guesswork to look ahead.

Suppose people start to reconstruct the Standard Model on the CDT spacetime foundations instead of on static flat txyz Minkowski, or a smooth-metric manifold.

CDT continuum has extremely non-classical geometry at small scale, but classical geometry at large scale. Or so appears (so early it is hard to be sure)

what new ideas about matter are going to be inspired on the way to putting matter fields into the "triangulations" picture, and what new mathematics enabled?

we should keep clearly in mind that the CDT is not a LATTICE approach where you take some tame classical geometry (mostly flat) and cover it with a grid.
Triangulations has the blocks assembled everywhichway, in arrangements you could not embed into a conventional flat txyz space, and results in extremely wild non-classical geometries.

so what happens when people start painting matter into the triangulations?
I suspect it would be foolish to try to put preconceptions on the outcome---they would just blind us.

And BTW let's keep in mind the general concept "nonperturbative quantum gravity" theory for something that gives you a new quantum picture of spacetime with Gen Rel in the large scale.
CDT is one possible "nonperturbative quantum gravity".
It happens to be one where they have reached the point of computer
simulations of the new model spacetime, and where they are getting interesting results.

But if there was a broad effort focussed on getting this kind of thing there could be other "nonperturbative quantum gravity" approaches also running computer models of spacetime and getting interesting results about dimensionality and bigbang cosmology and soforth.

spinfoams and loopcosmology does some of that, though CDT seems to have moved ahead of the pack, at least for now.

it just happens that this thread topic is CDT (small and fast-developing compared with LQG)

 

[marcus]

Looks like in this thread I never gave the abstract for the new CDT paper
"Reconstructing the Universe". I is good to examine the abstract of a Loll paper because she carefully articulates what she is doing and it can convey some perspective

http://arxiv.org/abs/hep-th/0505154
Reconstructing the Universe
J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
52 pages, 20 postscript figures

"We provide detailed evidence for the claim that nonperturbative quantum gravity, defined through state sums of causal triangulated geometries, possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale, and at the same time provides a nontrivial consistency check of the method of causal dynamical triangulations..."

To me this suggests that NONPERTURBATIVE QUANTUM GRAVITY regardless whether you do it with triangles has these features.
There may be various ways to do this, and arrive at quantum models of the continuum. And they may CONVERGE on a picture with

1. 4D and Gen Rel in largescale limit

2. Semiclassical (HawkingHartle) wavefunction of the scalefactor or size of universe

3. some picture of quantum spacetime dynamics at very small scale, which cumulatively and collectively generates expected largescale behavior

The abstract does not start off by saying "CDT", it comes to that only later.
So it is putting forward a rather bold claim
It says the authors have evidence that IT DOESNT MATTER WHETHER YOU USE OUR EXACT METHOD OR NOT, we have found out something about spacetime
and if you do some OTHER method of nonpert. QG and get a quantum spacetime dynamics that reproduces Gen Rel at large scale then quite possibly you will get similar results because THAT IS HOW SPACETIME IS.

this is a bold claim and they don't present it as a certainty, but something that they offer "detailed evidence" for.

And in fact the paper in question is full of detailed evidence.

so it is not saying "our CDT is the unique only approach, you all have to change to our method", it is saying that however you do the approximation whether or not with triangles and pathintegral, or whatever, if you can open a quantum spacetime dynamics window on the small scale that reproduces Gen Rel spacetime at the large, then you will see similar things!

So please try a slew of other approaches! We will see you at the finish line.

It's confident, and seems at the same time to have a clear modest reasonableness.

Well, I started this CDT thread well before "Reconstructing" appeared, and it appeared right around post #67 of this thread. I quoted some from the first paragraph, in post #67. But I never quoted from the abstract yet in this thread, so it was high time to listen to it.

 

[marcus]

Let's remove references to any specfic method (in the above quote) and see what Loll's overall program might be:

http://arxiv.org/abs/hep-th/0505154
Reconstructing the Universe

"We provide detailed evidence for the claim that nonperturbative quantum gravity... possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale,..."

So the program can be called "quantum spacetime dynamics" in that you start with a dynamical principle at very small scale, and all of spacetime is supposed to GROW from the action of that principle at very small scale.

You don't make global assumptions----like it is a smooth manifold of some fixed chosen dimension with some number of coordinates-----you only specify how something works at the subsubatomic level. the whole shebang is supposed to HATCH from that seed.

and what hatches has to look and act right on the large scale

that is her program,

and IT SHOULD NOT DEPEND ON THE PARTICULAR METHOD
anything that deserves to be called "nonperturb. quantum gravity" should POSSESS A LARGESCALE LIMIT---should be able to put in place some microscopic dynamical principle and have a familiar 4D spacetime grow from it. It should be able to because they did this in one example of a "nonperturb. quantum gravity" and surely they do not have a patent on spacetime!

So now I think the bar has been raised. It should be possible to model the spacetime continuum with many different methods of nonpert. QG and get this kind of result. Because it is it---different ways of looking at it should converge.

This is perhaps rather radical to say and might be wrong, but it says the map of QG is now changed and the game is redefined with a new price of admission. The candidate methods can show who they are by reproducing 4D and the semiclassical cosmology result that was mentioned.
Or something like that. i am still not certain how exactly things have changed but i do believe we have a new game.

 

[marcus]

http://arxiv.org/abs/hep-th/0505154
Reconstructing the Universe

"We provide detailed evidence for the claim that nonperturbative quantum gravity... possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale,..."

So the program can be called "quantum spacetime dynamics" in that you start with a dynamical principle at very small scale, and all of spacetime is supposed to GROW from the action of that principle at very small scale.
...

just understanding the terms in which the major players see what they are doing can be a project in itself

what does the overall goal of nonperturbative quantum gravity mean to the people who are driving toward it along these various approaches like CDT?

Fotini Markopoulou gave a short definition in a recent paper she did with Mohammad Ansari

<<The failure of perturbative approaches to quantum gravity has motivated theorists to study non-perturbative quantization of gravity. These seek a consistent quantum dynamics on the set of all Lorentzian spacetime geometries. One such approach which has led to very interesting results is the causal dynamical triangulation (CDT) approach[1, 2]. In the interest of understanding why this approach leads to non-trivial results, in this paper we study...>>

this is from the introduction of
http://arxiv.org/hep-th/0505165
A statistical formalism of Causal Dynamical Triangulations
Mohammad H. Ansari, Fotini Markopoulou
20 pages, 19 pictures, 1 graph
Abstract:"We rewrite the 1+1 Causal Dynamical Triangulations model as a spin system and thus provide a new method of solution of the model."