Quantum Gravity

[Martin]

Maybe the shortest clearest reporting on CDT is by Adrian Cho, writing for the American Physical Society publication "Focus" last September

 

http://focus.aps.org/story/v14/st13

 

The American Physical Society has a series of peer-review journals Phys. Rev. and Physical Review Letters. They pick out articles for highlighting journalistically in the accompanying publication "Physical Review Focus".

Here is what Cho had to say about recent CDT developments

 

 

<<The researchers added up all the possible spacetimes to see if something like a large-scale four-dimensional spacetime would emerge from the sum. That was not guaranteed, even though the tiny bits of spacetime were four-dimensional. On larger scales the spacetime could curve in ways that would effectively change its dimension, just as a two-dimensional sheet of paper can be wadded into a three-dimensional ball or rolled into a nearly one-dimensional tube. This time the researchers found that they could achieve something that appeared to have one time dimension and three space dimensions--like the universe we know and love.

 

"It's exceedingly important" work, says Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada. "Now at least we know one way to do this." Des Johnston of Heriot-Watt University in Edinburgh, Scotland, agrees the work is "very exciting" and says it underlines the importance of causality. "The other neat thing about this work is that you're essentially reducing general relativity to a counting problem," Johnston says. "It's a very minimalist approach to looking at gravity.">>

[Martin]

This thread is about "QGATS" (quantum gravity alternatives to string) research and people.

 

QGATS is no longer primarily LQG, it can cause confusion when people equate QGATS with narrowly defined LQG. And it is getting more prominent.

A rough idea of what non-string QG covers can be seen by looking at the topics on the program for the "Loops 05" conference

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

and list of invited speakers

http://loops05.aei.mpg.de/index_files/Programme.html

 

A rough idea of the recent higher visibility of non-string QG can be seen by looking at the program for the big Paris Einstein centennial conference

http://einstein2005.obspm.fr/indexr.php

You click on "conference" and get lists of invited speakers, both in the plenary session and the parallel sessions.

 

from the "loops05" page you can see the topics go well beyond Loop QG and the CDT approach of Renate Loll and co-workers is one of the main features (as well as that of Laurent Freidel and co-workers). For definiteness I want to keep track of what these people look like, and also links to their recent research papers.

Here is a good snapshot of Freidel.

http://cosmos.nirvana.phys.psu.edu/online/Html/Seminars/Spring1999/Freidel/freidel.jpg

 

In line with the increased prominence of Loll's CDT approach, she is giving a paper (with two of her students) at the Paris Einstein 2005 conference in July. Look at the schedule for the parallel session on "The Nature of Spacetime", for Wednesday, 20 July 2005 14h20-14h40 :

Here are some photos of Loll and students from the Utrecht Inst. of Theor. Physics

 

http://www1.phys.uu.nl/wwwitf/fotopagina's/foto's/Renate.jpg

 

http://www1.phys.uu.nl/wwwitf/fotopagina's/foto's/Willem1.jpg

 

http://www1.phys.uu.nl/wwwitf/fotopagina's/foto's/Dario.jpg

 

http://www1.phys.uu.nl/wwwitf/fotopagina's/foto's/Johan.jpg

 

for a list of Loll students and postdocs that includes some whose photos are here

http://www.phys.uu.nl/~loll/Web/students/students.html

here is Loll's homepage

http://www.phys.uu.nl/~loll/Web/title/title.html

[Martin]

this thread is mostly about the quantumgravity alternatives to string, which is research guided by a slightly different philosophy-----instead of going directly at the goal of a unified theory you say "the classical GR theory of gravity has no fixed geometry, no background metric, and we dont even have a background-independent quantum version of GR yet, so let's quantize General Relativity first, and get a quantum spacetime and then we can BUILD a field theory on that spacetime as a foundation."----so the philosophy is to develop a quantum theory of spacetime first and then maybe that will give some ideas about the right way to implement matter on that new spacetime basis.

 

it looks like a more gradual and modest way of proceding, rather than trying to do it all at once.

 

but even though the topic is non-string QG approaches, I want to keep track of the general direction hep research is going----meaning not to discuss, just keep some links handy. Here are some:

 

http://www.slac.stanford.edu/library/topcites/2004/annual.shtml'>http://www.slac.stanford.edu/library/topcites/2004/annual.shtml

 

http://www.slac.stanford.edu/library/topcites/

 

http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/program.html

 

http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/speakers.html

 

http://arxiv.org/Stats/

[Martin]

there are 3 "poll" threads to keep track of and recall in 6 months (around the first of the year 2006)

 

I will put links to them here to make them easier to find when we want to check.

A.

http://scienceforums.net/forums/showthread.php?t=12690

this is "string poll revisited" and it has two sets of predictions

"How many recent string papers will get 125+ citations in 2005?"(see Jan 1, 2006)

 

OUR PREDICTIONS OF HOW MANY RECENT STRING PAPERS WILL BE CITED 125+ TIMES IN 2005

 

Guesser  # recent string papers to get 125 or more citations in 2005

CPL.Luke     1 or none
Dave         2-5
Ed84c        1 or none
MacSwell     2
me           5
Yourda       3

 

 

Remember in 1999 this citations number was 15. So it is [possible] that we are all under-estimating.

the thread has links to where you find the citations numbers, and details

 

A second set of guesses was to be checked much farther off---July 4, 2007.

 

"By the gauge described here, where do you think string research will be in two years?"

 

Guesser         M/B index for July 2007

Severian              620
dave                  550
DQW                   540
me                    500
J.C. MacSwell         400

 

The index we are guessing here is the keyword M-theory/brane papers published July 2006 through June 2007, a number which (if past years are any guide) will be easy to get from the Harvard data base

http://adsabs.harvard.edu/ads_abstracts.html

 

the thread also has details on that, and some history of that keyword-class of paper over past 12-month periods.

============================

 

B.

http://scienceforums.net/forums/showthread.php?t=12699

"Geometric Quantum Gravity poll" was about a fairly new approach to quantizing general relativity called CDT. This is to check any time after Jan 1, 2006

"What's your guess as to 2005 postings of CDT research?"

OUR PREDICTIONS FOR CDT RESEARCH POSTINGS THIS YEAR

 

Mister Nine        9
Yourdadonapogos    8
MacSwell          14
me                 8

 

Basically we are just forecasting how many new CDT research articles will be posted on arxiv.org. Arxiv is an online library of preprints, where researchers put their papers even before they get accepted for regular publication. In the past CDT postings were 3, in 2003, and 4 in 2004. the question is whether or not CDT research go into "exponential growth" mode now (which we seem all to be predicting) or just continues doodling along.

 

the thread has details, including link to arxiv search engine that will give the number we need in January. Actually for convenience I will stash that link here too

 

http://arxiv.org/find/grp_physics/1/OR+OR+abs:+AND+triangulations+AND+Lorentzian+dynamical+abs:+AND+triangulations+AND+causal+dynamical+ti:+AND+gravity+AND+Lorentzian+quantum/0/1/0/2005/0/1

 

 

=======================

 

C.

http://scienceforums.net/forums/showthread.php?t=12710

the third forecast thread was about Quantum Cosmology, specifically the Loop approach LQC. It is to check any time after Jan 1, 2006.

"What do you think the LQC postings on arxiv will be in 2005?"

 

Here is the link to arxiv search engine that we will need to check.

http://arxiv.org/find/grp_physics/1/abs:+AND+Cosmology+AND+Loop+Quantum/0/1/0/2005/0/1

 

Here are our predictions:

Guesser    Loop Quantum Cosmology papers on arxiv in 2005

luc           40-49
MacSwell      53
me            46
Yourda        40-49

 

I think it would be all to the good if luc and Yourda wanted to refine their predictions down from a range 40-49 to particular numbers. but we can cope either way. And there is certainly room for a few more forecasts here, if anyone else gets interested in Cosmology and how current reserarch is quantizing it. (they got rid of the big bang singularity and found a possible mechanism for causing inflation without having to postulate a special "inflaton" field to do it----a quantum effect of very high density just turned up in the model without having been put in on purpose). the LQC field has potential for surprises so it seems worth checking into.

[Martin]

Update. Too late to simply edit the preceding post.

 

OUR PREDICTIONS OF HOW MANY RECENT STRING PAPERS WILL BE CITED 125+ TIMES IN 2005

 

Guesser  # recent string papers to get 125 or more citations in 2005

Bettina      1 or none
CPL.Luke     1 or none
Dave         2-5
Ed84c        1 or none
Jakiri       2-5
K9-47G       2-5
MacSwell     2
me           5
Severian     4
Yourda       3

 

 

Remember in 1999 this citations number was 15. So it could be we are all under-estimating.

[Martin]

the three recent polls are closed now, so we just have to arrange to fetch them back at the beginning of next year.

hopefully these links won't go bad. they are to the final results of the three poll threads

 

"string poll revisited"

http://www.scienceforums.net/forums/showpost.php?p=185657&postcount=29

 

"geometric quantum gravity poll"

http://www.scienceforums.net/forums/showpost.php?p=185630&postcount=9

 

"quantum cosmology predictions"

http://www.scienceforums.net/forums/showpost.php?p=185624&postcount=12

[garyr10000]

Something that has bothered me for a long time is my ignorance concerning the relationship between theoritical "requirements" that gravity be propagated by a force carrying particle and the notion that gravity is experienced because matter causes curvature in space. I want to say "You can't have it both ways". My intuition is that gravity is fundamentally different than the other 3 forces. I conceptualize gravitational effects as space collapsing into matter (space is growing as a whole because of the outward momentum of matter that decreases the mass density of the universe). Objects "obeying" gravity could be seen as at rest relative to their local space-time reference.

GRR

[Martin]

Something that has bothered me for a long time is my ignorance concerning the relationship between theoritical "requirements" that gravity be propagated by a force carrying particle and the notion that gravity is experienced because matter causes curvature in space. I want to say "You can't have it both ways". My intuition is that gravity is fundamentally different than the other 3 forces. I conceptualize gravitational effects as space collapsing into matter (space is growing as a whole because of the outward momentum of matter that decreases the mass density of the universe). Objects "obeying" gravity could be seen as at rest relative to their local space-time reference.

GRR

 

I think you have it right. It's hard to see how it could be both ways and the General Relativity way of looking at gravity has so-far gotten accurate results. In that picture' date=' gravity is not due to a force-carrying particle traveling in a fixed rigid space. As you indicate, the picture is that gravity IS the geometry of spacetime

 

that means, for instance, that the gravitational field is the geometry on which other fields are defined. objects in free fall follow geodesics in that geometry

 

as it happens, I do not consider myself an authority. my knowledge of this and several other areas isnt complete and I'd like to know more. anyone is welcome to disagree, and people DO take different views of this. Some people do picture gravity as the result of force-carrying particles, gravitons, moving in a rigid space. Or they picture space having some prior chosen fixed geometry and gravitons being little ripples or [b']perturbations[/b] superimposed on that geometry.

 

I dont think that "perturbative" description is fundamental. It may perhaps be used to get approximations in limited situations---it might work out in situations where spacetime is not very curved or not expanding very much and the gravitational field is in effect not very important. But in other situations, like gravitational collapse, a near collision of black holes, big bang events, then I wouldnt expect a "perturbative" description with graviton ripples superimposed on a static geometry to work.

 

I would expect a "nonperturbative" approach to be needed, with fully dynamic spacetime geometry.

 

If you are interested in this kind of thing, keep your eye on this website

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

 

as it says it's about the upcoming conference (in October) that is

"the annual international meeting on non-perturbative/background independent quantum gravity"

 

over the next few weeks and months, more information should accumulate there---people will be bringing the best ideas they have to that conference.

 

people actually do not understand non-perturbative quantum gravity very well yet. it is something to watch over time, as understanding develops

(but not expect a complete answer right away)

[Martin]

every few months I check the research output in non-perturbative approaches to QG to see how things are going overall.

 

I only have a very rough indicator which uses the keyword search engine at arxiv.org. It gets some papers it shouldn't (that just happen to have the right keywords in the abstract) and it misses some. In other words it's very noisy, but it still gives a crude idea of trends.

 

 

Year 2001:

http://arXiv.org/find/grp_physics/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2001/0/1

Year 2002:

http://arXiv.org/find/grp_physics/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2002/0/1

Year 2003:

http://arXiv.org/find/grp_physics/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2003/0/1

Year 2004:

http://arXiv.org/find/grp_physics/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2004/0/1

 

Last twelve months:

http://arXiv.org/find/grp_physics/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/past/0/1

 

2001    98
2002   121
2003   140
2004   184

 

Year to date, 2005:

http://arXiv.org/find/grp_physics/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravity+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phenomenology+OR+canonical+nonperturbative+abs:+OR+OR+spinfoam+AND+spin+foam+AND+OR+triply+doubly+special/0/1/0/2005/0/1

[Martin]

Bascule mentioned reading about the evolutionary scenario for the universe in the book called The Ancestor's Tale. To be prepared for possible discussion, we should have some online sources on CNS ----cosmological natural selection:

 

http://arxiv.org/abs/gr-qc/0205119

Is there a Darwinian Evolution of the Cosmos? - Some Comments on Lee Smolin's Theory of the Origin of Universes by Means of Natural Selection

Rudy Vaas

20 pages; extended version of a contribution to the MicroCosmos - MacroCosmos conference in Aachen, Germany, September 2-5 1998; finished in late 1998 and published in the conference proceedings

 

"For Lee Smolin, our universe is only one in a much larger cosmos (the Multiverse) - a member of a growing community of universes, each one being born in a bounce following the formation of a black hole. In the course of this, the values of the free parameters of the physical laws are reprocessed and slightly changed. This leads to an evolutionary picture of the Multiverse, where universes with more black holes have more descendants. Smolin concludes, that due to this kind of Cosmological Natural Selection our own universe is the way it is. The hospitality for life of our universe is seen as an offshot of this self-organized process. - This paper outlines Smolin's hypothesis, its strength, weakness and limits, its relationship to the anthropic principle and evolutionary biology, and comments on the hypothesis from different points of view: physics, biology, philosophy of science, philosophy of nature, and metaphysics..."

 

Smolin's book on CNS:

http://www.amazon.com/gp/product/0195126645/104-4065623-8351914?v=glance&n=283155&v=glance

 

A recent Smolin paper partly devoted to CNS---critical of the Anthropic Principle---offers evolution as an ALTERNATIVE to the Anthropic Principle which can explain values of the fundamental contants in the context of a testable theory.

 

http://arxiv.org/abs/hep-th/0407213

Scientific alternatives to the anthropic principle

Lee Smolin

Contribution to "Universe or Multiverse", ed. by Bernard Carr et. al., to be published by Cambridge University Press.

 

"It is explained in detail why the Anthropic Principle (AP) cannot yield any falsifiable predictions, and therefore cannot be a part of science. Cases which have been claimed as successful predictions from the AP are shown to be not that....

We show however that it is still possible to make falsifiable predictions from theories of multiverses, if the ensemble predicted has certain properties specified here. An example of such a falsifiable multiverse theory is cosmological natural selection. It is reviewed here..."

 

http://arxiv.org/abs/gr-qc/9404011

The fate of black hole singularities and the parameters of the standard models of particle physics and cosmology

Lee Smolin

27 pages

 

"A cosmological scenario which explains the values of the parameters of the standard models of elementary particle physics and cosmology is discussed. In this scenario these parameters are set by a process analogous to natural selection which follows naturally from the assumption that the singularities in black holes are removed by quantum effects leading to the creation of new expanding regions of the universe. The suggestion of J. A. Wheeler that the parameters change randomly at such events leads naturally to the conjecture that the parameters have been selected for values that extremize the production of black holes. This leads directly to a prediction, which is that small changes in any of the parameters should lead to a decrease in the number of black holes produced by the universe. On plausible astrophysical assumptions it is found that changes in many of the parameters do lead to a decrease in the number of black holes produced by spiral galaxies. These include the masses of the proton,neutron, electron and neutrino and the weak, strong and electromagnetic coupling constants..."

 

A number of recent papers deal with the quantum regimes which REPLACE the classical big bang singularity and the classical black hole singularity. here is one:

 

http://arxiv.org/abs/gr-qc/0509075

Quantum geometry and the Schwarzschild singularity

Abhay Ashtekar, Martin Bojowald

31 pages, 1 figure

IGPG-05-09/01, AEI-2005-132

 

"In homogeneous cosmologies, quantum geometry effects lead to a resolution of the classical singularity without having to invoke special boundary conditions at the singularity or introduce ad-hoc elements such as unphysical matter. The same effects are shown to lead to a resolution of the Schwarzschild singularity..."

 

that is a start. I will collect some more links to online sources as time permits.

[Martin]

More links about the removal of singularities at black hole and big bang and their possible connection. I need to sort through these and tag them selectively. There are more to chose from but this is probably an adequate sample.

 

======Modesto=====

 

http://arxiv.org/find/gr-qc/1/au:+Modesto_L/0/1/0/all/0/1

 

http://arxiv.org/abs/gr-qc/0509078

 

http://arxiv.org/abs/gr-qc/0504043

 

http://arxiv.org/abs/gr-qc/0411032

 

http://arxiv.org/abs/gr-qc/0407097

 

=======Husain Winkler=======

 

http://arxiv.org/abs/gr-qc/0503031

 

http://arxiv.org/abs/gr-qc/0412039

 

http://arxiv.org/abs/gr-qc/0410125

 

http://arxiv.org/abs/gr-qc/0312094

======Date========

 

http://arxiv.org/abs/gr-qc/0505030

 

http://arxiv.org/abs/gr-qc/0505002

 

http://arxiv.org/abs/gr-qc/0407074

(Genericness of Big Bounce in isotropic loop quantum cosmology)

 

http://arxiv.org/abs/gr-qc/0407069

(Genericness of inflation in isotropic loop quantum cosmology)

 

==========Bojowald===========

http://arxiv.org/abs/gr-qc/0506128

 

http://arxiv.org/abs/gr-qc/0505057 (survey)

 

http://arxiv.org/abs/gr-qc/0503041

 

http://arxiv.org/abs/astro-ph/0309478

 

http://arxiv.org/abs/gr-qc/0304074

 

http://arxiv.org/abs/gr-qc/0206054 (inflation)

 

http://arxiv.org/abs/gr-qc/0105113 (semiclassical limit)

 

http://arxiv.org/abs/gr-qc/0102069 (absence of big bang singularity)

[Martin]

this is a great talk about using double pulsars to test theories of gravity

http://vmsstreamer1.fnal.gov/VMS_Site_03/Lectures/Colloquium/041215Kramer/index.htm

 

It is by Michael Kramer of Jodrell Bank, one of a team that found the first double pulsar in 2004.

 

the talk was given at Fermilab. I found it works to "pre-cache" the slides and then press start. you will get a split screen with slide show on right and video of speaker on left

 

=============================

 

this is a great talk about a new preon model that produces numbers from a very simple basis that imitate the standard model of particle physics.

The inventor of this model is a young Australian at Adelaide who spent November 2005 at Perimeter Institute working on ideas to merge Loop Quantum Gravity with sundance preon model. go here where the seminar talks are listed

http://streamer.perimeterinstitute.ca:81/mediasite/viewer/?cid=a9b1d20a-efa7-485f-8d5d-3b62fb7d3e4c

and scroll down to 16 November 2005

where you see Sundance Bilson-Thompson talk

Topological preon models: a braid new world

 

this might work as a direct link---skipping the menu and selection process:

http://streamer.perimeterinstitute.ca:81/Mediasite/Viewer/Viewer.aspx?layoutPrefix=LayoutTopLeft&layoutOffset=Skins/Clean&width=800&height=631&peid=2e5425f4-3f47-4e5a-a86a-804a6d499b17&pid=e949d11b-a1a5-4365-a152-7e2014cb3867&pvid=1&playerType=WM64Lite&mode=Default&shouldResize=true

 

the talk was at Perimeter. it is split screen with slides and video.

 

===============================

 

Here is Renate Loll giving a talk at Perimeter

on the Emergence of Spacetime in Quantum Gravity

split screen slides and video

http://streamer.perimeterinstitute.ca:81/Mediasite/Viewer/Viewer.aspx?layoutPrefix=LayoutTopLeft&layoutOffset=Skins/Clean&width=800&height=631&peid=ae9950c3-33b8-46f3-a248-52d17f97bb86&pid=bba3089f-6509-485b-9ffd-379e4b8ffdf0&pvid=1&playerType=WM64Lite&mode=Default&shouldResize=true

 

the talk was given 18 November 2005 as part of a PI workshop on the emergence of spacetime

[Martin]

the two places with the most going on in QG, AFAIK, are Perimeter Institute at U.Waterloo in Canada, and the physics department at Utrecht University in the Netherlands.

 

Marseille, Penn State, Potsdam, Lyon and several other places are important too.

 

And these places are part of an informal network , so the researchers in each place are always visiting back and forth----collaborating with each other and spending a semester or two at each others locale. If you graduate at one place then you have an inside track to go for graduate study at one of the others. And if you get a PhD in one of the network then you are apt to be considered for a postdoc in another.

 

So for example Renate Loll is a leading QG person at Utrecht and her current postdocs came from places like Penn State, U Waterloo, Perimeter Institute, Potsdam...

 

Someone was asking about this---or some related question. What the leading QG places are. So I'm going to do a little QG sociology and refresh some of the links from earlier that have gone dead---and update some information about one or two of the main places.

 

no time to finish this tonight, have to get back to it tomorrow

[Martin]

I keep meaning to do some maintenance on this thread. I want to restore some links that have gone dead and bring some of the information up to date.

 

there have been some important recent developments.

 

Part of this you could say was Quantum Gravity maturing as a field. Part is research advances.

 

Lee Smolin has some online video lectures INTRODUCTION TO QUANTUM GRAVITY

 

he does two lectures every Wednesday (this may change to thursdays) for a total of about 2 hours and 40 minutes.

the Perimeter Institute puts it online as streaming media. It is splitscreen format with a large still of the current blackboard and a small movie of him talking at the blackboard.

 

To find the online lectures go here

http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False

and look at the sidebar menu (on the left) and scroll down to where it says "Introduction to Quantum Gravity"

 

Smolin has done 8 lectures so far, as of 10 February 2006.

[Martin]

It would be a good idea if anyone besides me was watching these lectures and wants to discuss what he is talking about. what topics are covered in the different lectures? what is the main ideas of lecture #1, of lecture #2, and so on?

 

We should make a kind of TOC (table of contents) of the lectures-----a sort of course outline----so that we can come back to particular ones and find our way around.

 

It is a graduate level course: for grad students (and advanced undergraduates). I have spoken with a student from the University of Waterloo who is taking the course for credit.

 

It helps a field to develop when people start teaching these Introduction courses because they think of new ways to explain and organize the material. I think Smolin has invented a good way to deliver the ideas of quantum gravity in this course. It is a landmark or milestone of a kind.

 

=========================

 

another thing that has been happening in QG is the appearance of some QG BLOGS.

 

these are blogs by people with an interest in QG. these are good things for sharing gossip and advice about where to go if you want to study QG and what are the latest directions of research and things like that

 

these blogs are pretty new and just getting established, I hope that some of them catch on and thrive. or if these do not make it, then I hope there are others that start up. the QG blog business is just getting started.

 

One is a Brazilian blog of Christine Dantas, she writes onboard computer code for Brazil satellites and does QG in spare time. The blog is in English.

 

http://christinedantas.blogspot.com/

 

The blog is called "Background Independence" because that is something that distinguishes NON-STRING approaches---like Loop Gravity, and Spinfoams, and Causal Dynamical Triangulations---from older stringy thinking. The new approaches are explicitly and manifestly background independent in that they do not assume a fixed prior spacetime metric at any point. They are able to get started without setting up any spacetime geometry as a framework.

[Martin]

I am currently excited about a Handbook of Philosophy of Physics

that a major science publisher, Elsevier, is bringing out this year.

 

the editors are Butterfield and Earman and they have chosen the cosmologist George F. R. Ellis

to do the chapter on Philosophical Issues in Cosmology

 

Early in his career, George Ellis was a collaborator of Stephen Hawking. Together they wrote the classic 1973 book The Large Scale Structure of Space Time. this has been often republished and has been used as textbook in Gen Rel and Cosmology by many generations of students.

 

right now the hot issues in cosmology all relate one way or another to

QUANTUM GRAVITY, including the buzz about multiverses and about the fortuitous "fine tuning" of the basic parameters of the standard models---the fact that our universe has extremely nice numbers.

 

So I expect that the Butterfield Earman handbook will turn out to be important and that George Ellis essay will have a major impact on discussions involving quantum gravity.

 

The good thing is that as of yesterday YOU DON'T HAVE TO WAIT FOR THE BOOK TO BE PUBLISHED YOU CAN READ ELLIS ARTICLE ONLINE NOW.

 

http://arxiv.org/abs/astro-ph/0602280

Issues in the Philosophy of Cosmology

George F. R. Ellis

To appear in the Handbook in Philosophy of Physics, Ed J Butterfield and J Earman (Elsevier, 2006)

 

"After a survey of the present state of cosmological theory and observations, this article discusses a series of major themes underlying the relation of philosophy to cosmology. These are:

A: The uniqueness of the universe;

B: The large scale of the universe in space and time;

C: The unbound energies in the early universe;

D: Explaining the universe -- the question of origins;

E: The universe as the background for existence;

F: The explicit philosophical basis;

G: The Anthropic question: fine tuning for life;

H: The possible existence of multiverses;

I: The natures of existence.

 

Each of these themes is explored and related to a series of Theses that set out the major issues confronting cosmology in relation to philosophy."

 

here are some other miscellaneous Ellis links:

some biographical detail. Ellis was born around 1940.

http://www.ictp.trieste.it/~sci_info/News_from_ICTP/News_107/profile.html

 

a few online Ellis writings in physics and cosmology in the preprint arxiv.

http://arxiv.org/find/grp_physics/1/au:+Ellis_George/0/1/0/all/0/1

 

Ellis homepage with list of publications honors and appointments

http://www.mth.uct.ac.za/~ellis/

 

BTW Ellis also has religious views! He is a Quaker. See this video talk:

http://www.counterbalance.net/ssq2/ellis-body.html

[Martin]

Carlo Rovelli has a set of 59 slides for an overview talk he gave at Lyon

in January 2006

 

http://www.cpt.univ-mrs.fr/~rovelli/Lyon2006II.pdf

 

there is some other stuff at Rovelli homepage

http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html

including a link to a free download of a draft of his book "Quantum Gravity" published by Cambridge University Press in 2004.

 

the slides are the most concise authoritative survey of LQG that I know of and they are new this year

 

=====================

 

I mentioned this earlier: Lee Smolin has a series of lectures online at the Perimeter Institute streaming media center

http://streamer.perimeterinstitute.ca:81/mediasite/viewer/

In the sidebar menu, scroll down to Introduction to Quantum Gravity

which gets you a Table of Contents for the lectures.

Lectures 11 and 12 are on page two of the TOC

 

These are video recordings of a course he is currently giving. Normally he gives two lectures each Wednesday. there are 12 Smolin Lectures online so far---as of 4 March.

 

these go into much greater depth than Rovelli's overview lecture, the two complement each other

[Martin]

Update. Too late to simply edit the preceding post.

 

OUR PREDICTIONS OF HOW MANY RECENT STRING PAPERS WILL BE CITED 125+ TIMES IN 2005

 

Guesser  # recent string papers to get 125 or more citations in 2005

Bettina      1 or none
CPL.Luke     1 or none
Dave         2-5
Ed84c        1 or none
Jakiri       2-5
K9-47G       2-5
MacSwell     2
me           5
Severian     4
Yourda       3

 

 

Remember in 1999 this citations number was 15. So it could be we are all under-estimating.

 

this was the absolute last list of people's guesses before we closed the poll. It is post #105 on this thread.

 

I just checked the 2005 SLAC/Stanford topcites list

http://www.slac.stanford.edu/library/topcites/2005/annual.shtml

and it turned out that Yourda predicted right----it was 3----and several others got the right range.

 

Since in 1999 this number was FIFTEEN it seemed reasonable to me that we were all forecasting low and that even my guess was overly pess.

But actually the whole bunch of SFN people showed up pretty well. A lot of us were in the right ballpark. Nobody was way off, either.

 

Thanks to all who took part.

 

If anyone wants to comment on the poll or suggest some future things to forecast, here is a thread for discussion

http://www.scienceforums.net/forums/showthread.php?t=19743

[Martin]

I expect this to turn out to be an important QG paper-----among the year's top 5 or 10 most influential for future research. need some place to put the link

 

http://arxiv.org/abs/gr-qc/0607014

Particles as Wilson lines of gravitational field

L. Freidel, J. Kowalski--Glikman, A. Starodubtsev

19 pages

 

"Since the work of Mac-Dowell-Mansouri it is well known that gravity can be written as a gauge theory for the de Sitter group. In this paper we consider the coupling of this theory to the simplest gauge invariant observables that is, Wilson lines. The dynamics of these Wilson lines is shown to reproduce exactly the dynamics of relativistic particles coupled to gravity, the gauge charges carried by Wilson lines being the mass and spin of the particles. Insertion of Wilson lines breaks in a controlled manner the diffeomorphism symmetry of the theory and the gauge degree of freedom are transmuted to particles degree of freedom."

 

It is a way that matter can arise from the geometry of spacetime-----particles can arise as "defects" in the gravitational field.

There is now no "ONE RIGHT" way to unify space time and matter. Several ways are being investigated (besides the stringy approach). Freidel in particular is someone to watch.

 

I am told he has another paper with Artem Staro in the works, and also one with Aristide Baratin. Perhaps this will be an important year in Freidel's line of QG research.

[Locrian]

You are too kind to keep this thread updated. Thanks Martin!

[Martin]

my pleasure! I appreciate the feedback, Locrian. It's nice to know someone else is checking in.

I should probably talk some about this paper. First off, I guess that you or anyone else reading this thread knows how to use ARXIV.ORG if you have the author's name.

So I don't always need to give links to new papers.

 

If someone says there's a new paper by John Baez, you can just go

http://arxiv.org/find

and type Baez, in the author slot, and get it. All the new stuff is on line.

Usually last name suffices, or sometimes that plus first initial.

=============

 

The field of non-string QG entered a NEW PHASE over the past year where it used to be people investigating approaches to quantum theory of spacetime-----and now it is approaches to quantum theory of space time and MATTER.

 

this is shown by several things

 

1. Laurent Freidel's papers last year with Etera Livine. His papers this year with Aristide Baratin, Shahn Majid, Artem Starodubtsev, Jerzy Kowalski-Glikman

 

2. John Baez papers with Alissa Crans, Derek Wise, Alejandro Perez, Urs Schreiber---also unpublished work with Aaron Lauda

 

3. Lee Smolin papers with Fotini Markopoulou, Sundance Bilson-Thompson

 

4. Cambridge University Press publishing a book this year edited by Daniele Oriti to be titled "Approaches to Quantum Gravity: towards a new understanding of space time and matter"

Some 15 different QG people have contributed chapters to the book.

Last year the title was supposed to be "...new understanding of space and time". And this year they changed the title, provisionally.

 

5. Rovelli and collaborators doing gravitons and scattering amplitudes within a spinfoam (non-string QG) context.

 

I would say that the indications of this change are getting clearer. Maybe this new paper of Laurent/Jerzy/Artem is the first really clear indication.

 

Because for example last year Laurent Freidel was working mainly on the 3D SPACETIME case. He was getting matter and Feynman diagrams to come out of his model of gravity, but it was only a kind of WARMUP exercise, or toy model. this new paper is the first one where he gets a lot of the stuff to happen in 4D spacetime.

 

(there was sporadic earlier work in 4D of a tentative sort by Smolin, Starodubtsev, and Freidel but one wasnt sure it would go anywhere)

==============

 

so now, after some tentative and confused beginnings (I realize I am skipping some past history) the QG people are waking up to the fact that their models are not merely going to be quantum spacetime dynamics---they are not going to just be quantum models of the dynamical geometry of spacetime---they are going to TELL WHERE MATTER COMES FROM too.

 

the model is supposed to show how matter arises from the geometry (analogous to creases or glitches or defects or tangles in the otherwise smooth) and to show, since it is already in a sense a feature of the geometry, how it INTERACTS with the rest of geometry.

 

(geometry is another name for the gravitational field---so the model should give some understanding of how matter interacts with the rest of the gravitational field )

 

================

so, with this change, putting matter much more in the picture, non-string QG has gotten a lot more interesting

 

One way to follow it would just be to register each of Freidel's papers as they come out. John Baez says that Freidel has several more in the works (with Baratin or with Starodubtsev). It is not the whole field, but it is a representative part at least.

[GutZ]

Ok let me me see if I understand this some what. In a nutshell (a very basic one) quantum gravity theory is one that takes space and time (3 + 1 dimensions) and tries to model the geometry of them at the quantum level, or quantizes space and time at a quantum level to bridge GR to QM?

[Martin]

Ok let me me see if I understand this some what. In a nutshell (a very basic one) quantum gravity theory is one that takes space and time (3 + 1 dimensions) and tries to model the geometry of them at the quantum level,

 

Hi GutZ,

I like the way you express that. That is the goal of most or all of the non-string QG research I have seen.

 

there is ONE variant approach called "Causal Sets" which tries to look deeper than the geometry of space and time. It looks at the world as being made of a causal network of events. A point in spacetime does not exist unless something happens there and all points are interconnected or not depending on which could have caused or influenced which others. And from this causality web they BUILD a 4D geometry.

 

I personally do not like to think about Causal Sets approach and I seldom read their articles, but they had a bunch of papers come out this year and workshops etc. It is a small but active line of investigation and it COULD pan out.

 

the way I think, what you said is EXACTLY RIGHT and we can forget about Causal Sets, the exception to the rule.

 

what you say is what non-string QG mostly is. The idea is that GRAVITY IS GEOMETRY, and what is space besides geometry? Space is all the GEOMETRICAL RELATIONSHIPS. So if you can make a quantum dynamics of geometry, then you have both a quantum theory of gravity

and also a quantum theory of what space IS.

 

Time is a delicate issue, some non-string QG theories are quantum dynamics of 4D geometry (cover the whole history, like a path integral) and others have a quantum theory of 3D space that allows for dynamic change correlated to some quantum observable serving as a clock (which exists in the 3D space) and they may also have a 'fake' time parameter as well.

 

But mostly what I see as the typical non-string QG approach is very straightforward-----build a quantum dynamics of spacetime (4D) geometry

 

And what has started happening mostly last year and continues this year is that THEY ARE FINDING WAYS TO INCLUDE MATTER AS A FEATURE OF THE GEOMETRY or more specifically the topology, that they are making a quantum model of.

 

So the picture that is emerging from the work of, for example Laurent Freidel, is that EVERYTHING is geometry-------spacetime, gravity, matter fields, the interaction of matter and geometry, etc.

 

You can find Freidel's papers at arxiv.org

 

He uses a model called the SPIN FOAM, which is basically a path integral covering the fuzzy history of the geometry of the universe----or a particular segment of history of a particular region. It portrays how the geometry of whatever world you are looking at could have evolved, to get from an initial shape to a final shape----and it looks at ALL THE WAYS that could have happened. Like a feynman path integral that looks at all the ways a particle could have gone from point A to point B.

 

And it gives little complex amplitudes to each way the geometry could have evolved. And then makes a big average.

 

The most exciting thing about Freidel's recent work is that this year he and Baratin came out with a paper showing how FEYNMAN DIAGRAMS OF ORDINARY PARTICLE PHYSICS---THE USUAL FIELD THEORY---COULD BE REPRESENTED BY a special kind of gravityless flat SPIN FOAMS. He found a way to INCLUDE MATTER AS A NATURAL FEATURE OF SPINFOAMS and to include the observed interactions of matter.

 

this work is not yet complete. I am waiting for the appearance of another paper by Baratin and Freidel. they had one in April 2006 and now I want to see the next step.

 

 

 

or quantizes space and time at a quantum level to bridge GR to QM?

 

in the end this what you say is really just the same as what you already said. Consider that it is probably not important what mathematical objects one uses to construct a model of space and time. What matters is to successfully quantize their GEOMETRY. however you do it, with triangles, or lego-blocks, with rubber bands or balls and sticks:-) . What matters is by what dynamical rules does your model of spacetimematter geometry evolve. Does the quantum dynamics work to predict what we see at large scale?

 

Ultimately quantum gravity IMHO should explain particles or fields of matter as geometric feature (like hairline cracks, tangles, twists, knotholes, tunnels whatever) in the fabric of geometric relationships. and it should explain how the matter AFFECTS the gross curvature at large scale, which we experience as gravitation.

 

thanks for the comment/question. AFAICS it didnt need all this elaborate reply because you have it right---so I can simply say "yes" to both alternatives.

 

leb' wohl

[GutZ]

haha ok, I am slightly starting to understand it (basic)! I will keep looking for your updates and links, I am very interested in QG. I appreicate all the effort in posting all this information; not only here, but all over the physics forum and sub-forums.

 

I don't comment much but hopefully I will start understanding alot more in the many fields/sectors/groups of physics, and then I can, but I do read whatever I catch.

[Martin]

Ok let me me see if I understand this some what. In a nutshell (a very basic one) quantum gravity theory is one that takes space and time (3 + 1 dimensions) and tries to model the geometry of them at the quantum level...

 

AFAICS that is exactly right.

and right now there are two main QG stories I am following

 

1. TECHNICAL developments. mostly papers by Laurent Freidel with various co-authors (this year it has been Shahn Majid, Aristide Baratin, Jerzy Kowalski-Glikman, Artem Starodubtsev). You can find representative papers here if you do a little hunting:

http://arxiv.org/find/grp_physics/1/au:+Freidel_L/0/1/0/all/0/1

 

Freidel is making a bid to actually rigorously put quantum dynamical geometry together with usual quantum field theory----to merge quantum geometry and matter---and have a testable model.

 

2. increased VISIBILITY in the media.

 

currently QG is starved for funds and career opportunity, it is being advanced by postdocs apparently motivated by love of the subject or the excitement of the challenge-----at least in the US to get a HEP-theory job you should do string. there are essentially no postdoc or faculty openings for non-string QG in the US.

 

so the field is being strangled by the string monopoly on research positions---at least in the US---in high energy physics theory (the arxiv abbreviation is "hep-th":-) )

 

admittedly public media often have low-grade info and hype!

However the VISIBILITY is a sign that the research support picture may change.

 

So this is an important story and i try to follow it as well.

======================

 

Here is an example of the recent play in the popular media:

Something from NEW SCIENTIST (which I normally stay away from, but it is also part of the story)

===quote from New Scientist===

Editorial: Loop quantum gravity increases its pull

12 August 2006

Magazine issue 2564

String theory's main rival has earned the right to be taken seriously - it could be the most profound scientific generalisation of all time

THE accepted idea of matter is that it is made up of minuscule particles guided by quantum force fields. This is already far removed from the common-sense view that matter is, well, just chunks of stuff. If that seems hard enough to take, then brace yourself for another step away from common sense.

 

Theoretical physicists working in the rarefied field of loop quantum gravity have developed a way to describe elementary particles as merely tangles in space (see "Out of the void"). If they are right, it could be the most profound scientific generalisation of all time, in which everything in the universe emerges from a simple network of relationships, with no fundamental building blocks at all.

 

Up to now, loop quantum gravity has seemed like a poor relation of string theory, which for years has been the most popular route to a "theory of everything" in which all ...

===end quote===

http://www.newscientist.com/channel/opinion/mg19125642.900-editorial-loop-quantum-gravity-increases-its-pull.html;jsessionid=FJMOPBBFGOIA