Loop Quantum Gravity: Where is it Going?

[wolram]

Where is loop quantum gravity going? I am sure you few have left most in the
dark.

 

[wolram]

It seems to me that LQG has more flavours than a flea can taste in a life time,
Almost as spegetified as string theory.

 

[marcus]

It seems to me that LQG has more flavours than a flea can taste in a life time,
Almost as spegetified as string theory.

no it only looks that way

LQG is spaghettified, admittedly, but string is WAY more spaghettified


LQG has about 100 researchers and still fairly coherent,
they are working on similar sorts of things or a few varieties of things and
you can get them all together in the same place once a year and they can still understand each other pretty much

the actually mathematical theory or theories are SUPPOSED to change and adapt, as they explore new possibilities and improve. so "LQG" has no fixed definition except what LQG people are doing at the moment and call their field of research

 

[Curious6]

Also, apart from LQG, I wonder what will happen to string theory after the recent paper by Kachru which showed that the landscape has an infinite number of vacuums. Are there any other directions string theory can actually take, or is this just the signpost that it is really not valid anymore?

 

[wolram]

By Marcus.
LQG has about 100 researchers and still fairly coherent,
they are working on similar sorts of things or a few varieties of things and
you can get them all together in the same place once a year and they can still understand each other pretty much.
As you know i am skeptical ,but open to new ideas, but having read
as much as i can," understanding some", i am lost as to the direction
LQG is taking.

 

[wolram]

By the by i would love an audio

 

[marcus]

Where is loop quantum gravity going?...

it is always a mistake to try to predict research, but since you ask I will try

right now it is extremely urgent for LQG to link up with
Renate Loll CDT-----the "triangulations" approach to quantum gravity that uses assemblages of "simplex" building blocks to approximate spacetime

it possibly a year of crisis, and change, in LQG
it will be fascinating to see how things sort out at the October 2005 conference
there are even some fights brewing, or serious rivalries

it has come time now for the full LQG theory to be applied to cosmology.
the LQC of Martin Bojowald was a simplified version of the full theory (assuming the universe is uniform and looks roughly the same in all directions) and LQG got several dramatic results in the period 2001 to 2004.
now they have to drop the simplifying assumptions and apply the full apparatus to cosmology and see if they can duplicate or modify those results. there may be fights (involving Bojowald and Thiemann) about this. but I think that Bojowald is basically a gentle mild personality who will not want to be quarreling, so it may not come out in the open.

I think we can ignore the fracas over cosmology. it will follow well-established lines and come eventually to some satisfactory resolution with more in the way of testable predictions.

What is much more explosive and unpredictable is the collision or merger between LQG and Loll's "triangulations" approach. this is even slightly scary to me.

I think Loop people should make every possible effort to learn and assimilate CDT which I think has some new mathematics contained in it.

sometimes the new mathematics comes from the humble applied grass roots and not from the monumental mathematically topheavy abstract oak trees.
CDT is basically 3 people who found out how to run simulations of the universe.

Bianca Dittrich has been Thomas Thiemann's righthand assistant for 2 years or so and has rendered him very valuable assistance on his Core-LQG program to construct official LQG dynamics. she is very smart. it would be a great blow to Thiemann to lose Dittrich as his assistant. But there are not enough smart people to go around. Suppose Bianca were to go over to the "Triangulations" group at Utrecht? Then there would be 4 CDT people

Wolram, people go back and forth between core-LQG and CDT. The theories involve similar kinds of thinking (but some different symbols or formalism). It might not be too hard for them to assimilate each other. But I absolutely cannot see how this would happen, at the mathematical level. Only at the human level

 

[wolram]

What is much more explosive and unpredictable is the collision or merger between LQG and Loll's "triangulations" approach. this is even slightly scary to me.

i have tried to follow this, but having just grasped some inkling of what
spinfoams are, the math seems to take a quantum leap in some other
direction, and left me, and I am sure others gasping for breath.

 

[marcus]

What is much more explosive and unpredictable is the collision or merger between LQG and Loll's "triangulations" approach. this is even slightly scary to me.

i have tried to follow this, but having just grasped some inkling of what
spinfoams are, the math seems to take a quantum leap in some other
direction, and left me, and I am sure others gasping for breath.

all that means is you are trying to assimilate too much in a hurry.

you already have some grasp of ordinary LQG and (you say) spin foams.


"Triangulations" (Loll style) is a bit like spin foams but here is a difference.

spin foams are mapped or projected or imbedded into some surrounding 4D continuum (technically a differentiable manifold, damn George Riemann for making up such a clumsy name for it)

spinfoams are made of pieces (triangles and stuff) that "live" in some surrounding t,x,y,z space

In Loll-style, the blocks don't live in a surrounding manifold. They ARE it. You use a whole lot of identical building blocks (actually two kinds, slightly different, think of them as male and females, but otherwise identical) and the assemblage of all these block IS the spacetime.

technically there is a topological space R x S³ which the union of this assemblage of half a million blocks is supposed to equal, but the topological space has no differential structure, no calculus to it, just a formality.

morally and intuitively the assemblage of glued together blocks is the space itself.

and then two things happen
1. you consider all the other ways the blocks could be glued together and you get this fantastic blur, this swarm of possible geometries. (they invented a "shuffling" process in the computer that imitates this blur)

2. you imagine reducing the size of each block and increasing the number of blocks, and you make this quantum swarm of geometries, getting finer and finer, APPROXIMATE the real spacetime you want to know about.


that is it,


so the upshot is that WHEN IT COMES TIME TO CALCULATE you can accept a finite degree of approximation and use enough blocks of sufficiently small size and simply don't worry about going to the limit. You just calculate with some finite degree of precsion. And then, all you need to consider is this assemblage of blocks!
And it turns out that Loll and the others figured out how to calculate with that assemblage like a sona*****, they can calculate stuff to beat the Dutch, oh my mistake, they ARE the Dutch.

for this reason it is cannot be permitted for core-LQG not to make contact with this little project they have at Utrecht.

 

[marcus]

I have been gradually been working on understanding CDT since April 2004.
You will just give yourself fits if you try to understand it too much in a hurry.

Relax, it is fairly basic. It will soak in to the wolram brain, in time.

I remember April last year when "Emergence of a 4D world from Causal quantum gravity" appeared
http://arxiv.org/hep-th/0404156
that was my first exposure to it.
at that time John Baez had not even started calling attention to it (that was later, in May)

It has been a year and I am still soaking it in. Not good to be impatient.

 

[wolram]

technically there is a topological space R x S3 which the union of this assemblage of half a million blocks is supposed to equal, but the topological space has no differential structure, no calculus to it, just a formality.

A swarm going to infinity?

 

[marcus]

when mathematicians talk about the limit as something goes to infinity or goes to zero, what they PRACTICALLY mean is you take it large enough or small enough to get close to a realistic answer, and decide to be satisfied


dont worry about mathematical formalities

you take enough blocks, (right now they are up to a third of a million)
or. what for a given volume amounts to essentially the same thing, you take small enough blocks
and you decide to calculate with that and be satisfied

the business of imagining being able to put an infinite number of block in the computer is a formality or a fantasy.
you want to have ENOUGH so your results can be realistic.

right now Ambjorn and Loll are using a WORK STATION and one of their runs will often take several weeks.
It is infuriating that some of the major research establishments, with supercomputers, have not realized how interesting this work is and offered them time on a big fast parallel process computer.

this kind of work is MADE for a fast parallel process computer. like they simulate airplane wings and nuclear explosions etc.

it is crazy that the CDT authors have to do this work like on a desktop workstation.

i would like to see the results of modeling spacetime with a couple million blocks, not just 1/3 million. and then put matter into the blocks (which would make the computing heavier)

 

[marcus]

once you decide on how many blocks, then you load that many blocks into the computer and you make the "swarm" or "blur" of geometries by
SHUFFLING
you cause the blocks to put themselves together differently again and again so that the range of possible geometries is explored

 

[wolram]

A desk top ? kick the weather forecasters out and use theirs, I am sure we
would not mind not knowing if it is going to rain or shine for a week or two