the Ambjorn Jurkiewicz Loll paper

[Martin]

this is a landmark paper that represents several firsts

 

essentially they programmed the emergence of the universe in a computer simulation

and ran it over and over again to get averages

 

the preprint was posted at arXiv in April (arXiv is where you read things first before they have been published in the journals)

here's the link:

 

http://arxiv.org/hep-th/0404156

 

-----

I guess there are several observations to be made about this.

Gravity is the geometry of spacetime so a theory of gravity should generate the geometry. You should not have to pick a fixed geometry ahead of time (as is done in string theory---it's the main shortcoming in fact)

instead, the geometry of the universe should emerge from the model.

 

so they set up a quantum version of the 1915 Einstein equation of Gen Rel,

and turned it into a computer model and ran it and for the first time they got an extended 4D world

 

now some things have to be checked, they have to make sure that their quantized model really does give standard 1915 Gen Rel in the largescale limit,

and they have to do much bigger runs

their biggest run so far had around a third of a million "pixels"

that is it used some 300,000 little 4D simplex blocks

 

BTW people have been trying to do this since the 1980s

just because you start with a bunch of 4D simplex blocks doesnt guarantee that the overall thing you get when they all stick together is macroscopically 4D-----fractal stuff happens and other bad things.

so like after 20 years Ambjorn Jurkiewicz Loll finally got it to work.

 

I am going to post links to photos of them.

[Martin]

some animations of some earlier computer simulations

 

http://www.nbi.dk/~ambjorn/lqg2/'>http://www.nbi.dk/~ambjorn/lqg2/

 

Ambjorn's homepage

 

http://www.nbi.dk/~ambjorn/

 

Matt Visser's commentary in the newsletter "Matters of Gravity"

 

http://www.phys.lsu.edu/mog/mog19/node12.html

 

A picture of Renate

 

http://www1.phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Renate.htm

 

http://phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Renate.htm

 

Ambjorn and Loll's work is very closely related to the "spin foam" approach to quantum gravity developed by John Baez and others.

Baez is also involved in this approach now. Here's a picture of him

 

http://www.edge.org/3rd_culture/bios/baez.html

 

that's enough pictures for now

[Martin]

Some quotes from the AJL paper

"Emergence of a 4D world from Causal Quantum Gravity"

http://arxiv.org/hep-th/0404156

 

 

---- page 2----

...This may be seen as a particular case of a more general truth, not always appreciated, that in any nonperturbative theory of quantum gravity

"dimension" will become a dynamical quantity, along with other aspects of geometry.

----end quote----

 

 

---- page 3----

 

In what follows we will report on the outcome of the first ever Monte Carlo simulations of four-dimensional causal dynamical triangulation...We will present strong evidence that the Lorentzian framework produces a quantum geometry which is both extended and effectively four-dimensional. This is to our knowledge the first example of a theory of quantum gravity that generates a quantum spacetime with such properties dynamically.

 

----end quote----

 

 

 

AJL use a simplicial quantum gravity model which they implement in a computer to generate universes with various spacetime geometries

and the universes have been tending to come out macroscopically appearing 4 dimensional as they report in their paper.

 

Because the effective dimension is "dynamical quantity" or outcome of the model means you don't pick what the apparent dimension of spacetime (observed at macroscopic scale) at the outset, instead the effective number of spacetime dimensions is one of those aspects of geometry that emerge by running the model, either by solving the equations explicitly (which has sofar not been possible) or running a computer simulation.

[Martin]

some comment on AJL paper from John Baez

----6 June post, sci.physics.research----

--------Baez post Sunday, quote----

 

In article <61773ed7.0405240822.1c7108de@posting.google.com>,

Charlie Stromeyer Jr. <cstromey@hotmail.com> wrote:

 

>Here are three other reasons to be skeptical of discretized approaches

>to gravity:

.......(part of Strohmeyer post omitted)....

Nobody has yet constructed a background-free quantum theory that has

general relativity as its limit at large distance scales. The Ambjorn-

Jurkiewicz-Loll model is the closest anyone has come. If they succeed,

this will be of interest regardless of whether their model displays

mathematical features that appear in certain other theories!

 

>2) How are such approaches to be made compatible with Bell-like

>correlations, non-locality and non-causality which are each present in

>the experiment described in this brief four page paper [2].

 

As a quantum theory, the Ambjorn-Jurkiewicz-Loll model automatically

has Bell-like "entanglement" and all that jazz.

 

>3) To paraphrase a sentence that Stephen Hawking once wrote, to not

>believe in the beauty and unity of the dualities of M-theory is like

>believing that evolution did not occur because instead God placed by

>hand all the fossils in the Earth just to play a joke on the

>paleontologists

 

We resort to theological arguments in physics only when better arguments

are lacking. If a scintilla of experimental evidence for M-theory is

ever found, people will instantly stop making arguments of the sort

you mention here.

 

Please understand what I'm saying:

 

I'm not saying that M-theory is "wrong" or that the Ambjorn-Jurkiewicz-Loll

model is "right". M-theory makes too few definite predictions to be wrong.

The AJL model does not include matter, so it cannot be right. But the

AJL model is *interesting*, because it represents the best attempt so far

to find a background-free quantum theory that reduces to general relativity

in the large-scale limit!

 

----------end quote, my emphasis----

[Martin]

some more comment on AJL paper, from a conversation

on sci.physics.research between John Baez and Thomas Larsson:

 

---------quote from Baez 6 June post, SPR----

 

In article <24a23f36.0405170344.69e74067@posting.google.com>,

Thomas Larsson <thomas_larsson_01@hotmail.com> wrote:

 

>1. Is the AJL model really quantum?

 

Yes! It has a Hilbert space of states, observables described

as noncommuting self-adjoint operators on this Hilbert space,

and discrete time evolution described by unitary operators on

this Hilbert space.

 

>Some time ago, Urs

>Schreiber argued that LQG, or at least the LQG string,

>fails to be a true quantum theory, and I tend to agree.

 

I disagree, but it's not really relevant here: we're not

talking about those other theories.

 

>However, the AJL model can be viewed as a statistical

>lattice model, and if such a model has a good continuum

>limit, it is AFAIK always described by some kind of QFT.

>What else could it be?

 

Right!

 

>2. Is the AJL model really gravity. The action is a rather

>straightforward discretization of the Einstein action with

>a cosmological term:

>

> sum over (d-2)-simplices

>

> det g = volume => sum over d-simplices.

>

>What is perhaps somewhat unusual is that all edges have

>the same length, which is different from Regge calculus.

>Nevertheless, I don't think that this really matters, but

>one could check if the results look different if you

>allow for variable edge lengths.

 

Right! But, the test of whether the model "is really

gravity" is to carefully examine its behavior in the limit

of large distance scales (i.e. lots of 4-simplices). One

can't easily guess this from looking at the action.

Nonperturbative effects are too important! So, in the

absence of good analytical techniques, one really needs

to run computer simulations - as AJL are doing.

 

>3. Is the measure right? Here is the place where AJL differ

>significantly from previous simulations. AFAIU, the crux is

>that AJL insist on a strict form of causality: they exclude

>spacetimes where the metric is singular, even at isolated

>points. This may seem like an innoscent restriction, but it

>rules out things like topology change and baby universes,

>which require that the metric be singular somewhere.

>

>It is not obvious to me whether one should insist on such a

>strong form of causality or not, but this assumption leads

>at least to better results, e.g. a reasonably smooth 4D

>spacetime. Thus, I believe that it is a fair chance that

>AJL have indeed succeeded in quantizing gravity.

 

The issue of the "right measure" is very tricky, so tricky

in fact that I again think the most efficient way to begin

tackling it is to run computer simulations and see if the

AJL model acts like general relativity at large length scales.

 

>They do so not by assuming a lot of experimentally unconfirmed

>new physics, but rather by strictly implementing the

>time-honored principles of old physics, especially

>causality. That is cool.

 

Yes! Very cool!

 

----end quote, I have emphasized parts with boldface----