Quantum Graffiti: MEDIA COVERAGE, JOB OPENINGS & Gossip Around Loll at Utrecht

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  • #31
4-page Smolin reply on Woit's blog

http://www.math.columbia.edu/~woit/blog/archives/000204.html

there has been a 75+ post discussion here of Lee Smolin's essay Why no 'new Einstein'?

post #76 is a four page (printed out) comment by Smolin, about his essay and about what people had to say about it and other side points that came up.

I think it is real interesting, and well-written

Naturally CDT (Loll-type simplex gravity, computer simulations) came up, I guess because CDT is a fundamentally new development in quantum gravity and a hot issue now, so we have some additional comment or viewpoint from Smolin on that.

post #26 of this thread gave a link supplied by Motl to text of Smolin essay

... you can try
http://waltf007.mindsay.com/
this provided by Lubos Motl
Smolin's proposals are a bulleted list right at the end...)
 
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  • #32
Smolin's comments are a must-read for anyone interested in several things that have been posted here in the past. Such items as Thiemann's LQG quantization of the closed string and the assertion that LQG quantization must be wrong because it doesn't produce an anomaly as "it is known" to be required. Smolin tracks this down to the difference between quantizing with a fixed background and in a background free environment. And Smolin says, in response to a request for a blanket term to cover everything from Thiemann to Loll by way of Rovelli and Smolin himself with all their coworkers, that he uses background-free approach to quantum gravity. I propose that WE declare Background Independent Quantum Gravity (BIQG) to be the term we use here on PF and that you, Marcus get behind it as you have 8\pi G so we can really sell it.

Would it be possible to get a copy of Smolin's comment posted here?
 
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  • #33
selfAdjoint said:
... I propose that WE declare Background Independent Quantum Gravity (BIQG) to be the term we use here on PF and that you, Marcus get behind it as you have 8\pi G so we can really sell it.

Would it be possible to get a copy of Smolin's comment posted here?

Hi selfAdjoint, you are kind to think that I am persuasive enough to sell notation or improvements to widely current terminology!

I approve of your idea. But I personally did not even succeed in "selling" 8\pi G . Which was Baez suggestion or someone before him. I am not ashamed of having tried though. We had fun trying to make Baez 8\pi G version of the Planck units usable.

I think your idea of reforming common parlance to say BIQG is basically a good one. Let me go look at the Loops 05 website and see what they say at the top of their conference homepage. If it says BIQG, or words to that effect, that would be a good sign.

I am going to
http://loops05.aei.mpg.de/
to check out the wording

be right back

Yes, you are in luck. at the top of the page it says LOOPS '05

and then under that it says<<In the Einstein year 2005, the annual international meeting on non-perturbative/background independent quantum gravity takes place from 10-14 October 2005 at the Albert-Einstein-Institute in Potsdam, Germany. The aim of this conference is to summarise the status and open problems of the various approaches and to present new ideas and research directions.>>

that language would have passed Hermann Nicolai and it is what you are suggesting BIQG
except that it leaves open the possibility of also saying Non-perturbative QG. Strictly speaking the AEI website says "non-perturbative/background independent quantum gravity"

I think we can ignore that other option

I will think about this. As a general rule a journalist or librarian should not ever try to innovate. only use off-the-shelf language. if you innovate terms do it only for fun and then go back to already established language.

but if the professionals are going to change terminology (like Smolin and Nicolai) then we should be alert and ready.
 
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  • #34
Chiming in, I entirely agree with SA. Without background independence I see no hope for a workable quantum theory of gravity. Any first cause proposal must be background independent in my mind. I realize I'm borrowing heavily from information theory, but I believe that is the only approach that has a fighting chance of being correct. Treating the universe as a quantum computer has attractive features.
 
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  • #35
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  • #36
Thank you Marcus!
 
  • #37
selfAdjoint said:
Thank you Marcus!

You are most welcome, selfAdjoint!
I was just at Not-Even-Wrong and noticed that Peter has reposted Smolin's response with its own heading

http://www.math.columbia.edu/~woit/blog/archives/000206.html

so now it is easier to link to it directly than before when it was imbedded in the original thread
 
  • #38
I want to keep the link handy to the Loops 05 program
http://loops05.aei.mpg.de/index_files/Programme.html

elsewhere in this thread there is this main link
http://loops05.aei.mpg.de
but it does not reveal the file address for the programme.

Alejandro also provided the link for the Strings 05 conference which starts 11July in Toronto. I will fetch that---it will be interesting to see the titles of the talks whenever the schedule is filled in

the Loops 05 Program page so far just has the list of invited speakers and says that more will be posted in July, which would be nice (plenty of time for participants to do their homework before the October conference date)

yes, here is the link Alejandro found for Strings 05 (July 11-14)
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/program.html

selfAdjoint started me thinking how one can describe the Quantum Gravity programs that are the main rivals to string. One could say simply "non-string QG" or "QG alternatives to string". the map of this research territory is gets complex and interesting the more I look at it.
I do not think the main contenders are typified by a conventional narrowly defined LQG.

It occurred to me to try defining the non-string QG people by something like "Erdos numbers". Let's say the people who have low Freidel number or Loll number. this is not to presume to judge that those two researchers have special merit, but simply to use the fact that they are WELL-CONNECTED by co-authorship with the group of people I'm trying to characterize.

these are the people whose work currently rivals or challenges string in the sense that if support in US physics departments were diversified (a "mixed strategy" instead of all the eggs in one basket) then it would be people like this who would get more job-offers and postdoc fellowships.

It would not be some string theorist's stereotype of a stock-LQG researcher because there are not very many of those---I can't think of any young ones.

Anyway, who are Freidel and Loll? Freidel got his PhD in 1994 (quantum groups and theory of knots) advisor was Jean-Michel Maillet. They have a paper or papers on arXiv from around 1992-1994 which can give an idea.
http://www.ens-lyon.fr/PHYSIQUE/Theorie/rapport-activite-94-96/node14.html
Here are photos of theory people at the Ecole Normale, Lyon, including Freidel and Maillet
http://www.ens-lyon.fr/PHYSIQUE/Theorie/trombinoscope2.html

Loll got her PhD in 1989 at London Imperial College. Chris Isham is there. At the moment I don't know what her thesis was about or who her advisor was.

If you wanted to include LQC-cosmology then you could add Martin Bojowald and then almost anybody you can think of would have co-authored with one of those three.-----would have Freidel number or Loll number equal to one (or two at the most) or else Bojowald number equal to one.

Maybe some has a different idea? Or would use different nodes to define the network?

It was interesting to see that the topic of Freidel's thesis (1994) was "Integrable models, quantum groups, and knot theory"-----so it was in the general area of quantum algebra.
 
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  • #39
I think there is a generational shift in non-string Quantum Gravity. Only a few years ago (I think as recently as two years back) the symbolic figures were unquestionably the LQG founding fathers like Rovelli and Smolin and Ashtekar. At a representative conference one or more would head the line-up and speak for the field.

now it seems slightly different, I think there could be a conference in non-string Quantum Gravity approaches where the question i would be asking is "Is Laurent Freidel going to be speaking?" and is Renate Loll going to be there? and what are THEY going to talk about?

Not too long ago with in a similar situation i would be asking about the work of people who today are more like generals---not on the front lines. the field is moving right along and this is a normal and good progression, it does not diminish my respect for Sm. Rov. Ash. and thankfulness for what they have accomplished, but it does mean that I want to know a little bit more about the generation that is coming up, and central figures in it, like Loll.

Maybe you, with your different perspective, do not care. But I am interested in where she came from, training and early work. As one of her earliest, Renate published this paper in Physical Review D vol 41, in 1990, "Noncommutativity of constraining and quantizing: a U(1) gauge model"
and she got her PhD in 1989 from London Imperial College. I will try to pick up more details as we proceed.

Loll's Utrecht website has a list of her students and postdocs in her program. I gave a link earlier. but beyond that she is picking up new CDT people, which i want to keep aware of. She has a paper in the works with these two:
Francesco Zamponi
http://glass.phys.uniroma1.it/zamponi/pagine/fisica.html
Dario Benedetti (was at Rome, student of Amelino-Camelia, now at Utrecht)
http://www.phys.uu.nl/~benedett/home/
 
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  • #40
91 teraflop computer should help with simulating spacetime

since this thread is for news and gossip bearing on CDT (Loll-type triangulations approach to quantum spacetime dynamics), here a relevant research computing development:

http://biz.yahoo.com/fool/050617/111903931827.html?.v=2

In CDT some of the simulations take weeks on an older work station. The new IBM computer "Watson Bluejean", or whatever they call it, does 91 teraflops or 91 E12 floating point arithmetic operations per second. Should speed a gravity researcher's simulations up considerably.

<<... strengthen IBM's already strong position in the burgeoning field of nanotechnology. By reducing the time it takes to run simulations from months or weeks down to mere days or hours, Watson Blue Gene holds the potential to exponentially increase our understanding of the complex fields of materials sciences, quantum chemistry, and molecular and fluid dynamics.

...

IBM officials are confident that they'll be able to develop a successor capable of a 1 petaflop performance level by the end of the decade. That's 1 quadrillion calculations per second -- 10 times as powerful as Watson Blue Gene.>>

it strikes me there is an analogy between quantum gravity simulation, with all these microscopic geometric "moves" where you swap simplexes around, and PROTEIN FOLDING

<<..The first area where the new computer will prove useful is in modeling how proteins fold...>>

Hope Loll et al can get some time on a Blujean

In case it works better, here is another link to the same article
http://news.yahoo.com/s/fool/20050617/bs_fool_fool/111903931827;_ylt=AtgSlcORRLSVEnePChhelZojtBAF;_ylu=X3oDMTBiMW04NW9mBHNlYwMlJVRPUCUl
 
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  • #41
marcus said:
I want to keep the link handy to the Loops 05 program
http://loops05.aei.mpg.de/index_files/Programme.html
...
http://loops05.aei.mpg.de
...

Alejandro also provided the link for the Strings 05 conference...---it will be interesting to see the titles of the talks whenever the schedule is filled in.
[The conference is 3 weeks away.]

yes, here is the link Alejandro found for Strings 05 (July 11-14)
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/program.html
...

I just checked the Strings 05 schedule. It now lists three events of broad public appeal (but unfortunately so far none of the regular talks)

Public Talks: Saturday July 16, 2-5p.m.

Robbert Dijkgraaf U. Amsterdam
"Strings, Black Holes, and the End of Space and Time"

Leonard Susskind Stanford U.
"Cosmic Landscape: String Theory and the Illusion of Intelligent Design"

Panel discussion: Tuesday, July 12, 19:00-21:00
"The Next Superstring Revolution"
 
  • #42
marcus said:
It occurred to me to try defining the non-string QG people by something like "Erdos numbers". Let's say the people who have low Freidel number or Loll number. this is not to presume to judge that those two researchers have special merit, but simply to use the fact that they are WELL-CONNECTED by co-authorship with the group of people I'm trying to characterize.

[snip]

Maybe some has a different idea? Or would use different nodes to define the network?

My guess is that if you really looked at the numbers you'd find maybe a dozen co-authorship clusters. But, at the point, you'd really need to take a further step. If you wanted to be content neutral about it, the next step would be to which which co-authorship clusters frequently cite each other. This would likely collapse the number of QG programs to less than half a dozen.

For example, to take the example of the MOND literature (which is not really a QG program, but just used to show the method), with which I am quite familiar and which would normally be viewed as one research program, there are probably three or four major co-authorship clusters, but all would cite each other frequently, and then there would be maybe two co-authorship groups (Mannheim, for example) who have similar research programs, but are pursuing separate theories.

But, if you looked strictly at co-authorship, you'd miss the fact that there are entire co-authorship groups writing about the exact same thing who have never authored a paper with the core investigator (Milgrom) or anyone who has co-authored a paper with him.

Another way to approach it would be to use the less rigorous measure of people who speak at the same conferences, rather than people who co-author papers.
 
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  • #43
ohwilleke said:
My guess is that if you really looked at the numbers you'd find maybe a dozen co-authorship clusters...

sounds like a reasonable guess
the main obstacle to actually doing co-authorship analysis is my own laziness
:smile:

at the moment nearly everybody I can think of in non-string quantum gravity has co-authored with either Freidel, or Loll, or Bojowald
so if I just want a rough and ready ad hoc way to say who the people are that I think of as "LQG people" then I can just use "erdos numbers" for those three.

some other three would also work about as well, and my "seive" would have failures that i haven't thought of, and it is completely unscientific

but you are proposing something much more objective---that would include for example Martin Reuter. I would miss him. but he's on the Loops 05 invited speaker list. I would make a lot of mistakes like that, with my quick and dirty subjective test.

BTW ohwilleke, I just posted a new Moffat in astronomy forum
Galaxy Rotation Curves Without Non-Baryonic Dark Matter
J. R. Brownstein, J. W. Moffat
43 pages, 7 figures, 4 tables, 101 galaxies
Abstract: "We apply the modified acceleration law obtained from Einstein gravity ...The fits are compared to those obtained using Milgrom's phenomenological MOND model and to the predictions of the Newtonian-Kepler acceleration law."
interested to know if you've any reaction
 
  • #44
http://math.ucr.edu/home/baez/week69.html

this 1995 piece by Baez has some gossip about Loll.
She got her PhD at London Imperial (where Chris Isham is) in 1989
and Baez says she was a postdoc working with Isham at some point after that.
Baez says he met Isham, Ashtekar, and Loll in 1991 at a conference in Seattle where they all three were giving LQG papers.
I am going to try to put this jpg snapshot of Renate Loll in

there may have been an article about CDT in Nature, last October. haven't found it on line but did find this
http://www.nature.com/news/2004/041004/pf/041004-17_pf.html
dated 8 October. it doesn't have the photograph I expected and seems to have been abridged
this greek-language version has the photon and is more complete (still haven't found the english version online)
http://www.physics4u.gr/news/2004/scnews1631.html

the physics blogger Dave Bacon (the "Quantum Pontiff") had a short intuitive description of what happens in CDT
http://dabacon.org/pontiff/?p=706#comments
he didnt get anything much in the way of perceptive or pertinent comments unfortunately. CDT too new.
 

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  • #45
Baez column on CDT (from May 2004 Marseille conference)

http://math.ucr.edu/home/baez/week206.html

this entry from Baez "this week's finds" also has an intuitive description of how CDT works and puts it into larger quantum gravity context. It is Baez report after the Marseille Loop/Foam conference where Loll spoke.

just to keep things in perspective, here is a home movie that Peter Woit posted link to. it was taken during a 10 minute break at the 1927 solvay conference in Brussels.

http://www.maxborn.net/index.php?page=filmnews

the camera belonged to Irving Langmuir, the chemist, and the voice-over narrative is supplied by Nancy Greenspan (author of a book about Max Born)

I clicked on the "realplayer" version and it took a while to download but it was worth it. the home-moving runs around 2 and a half minutes.

http://216.120.242.82/~greensp/solvay.ram
 
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  • #46
limits

The greatest obstacle to discovery is not ignorance,
but the illusion of knowledge.

Thank God there is but one infinity! or
Vertigo makes the world go 'round -
 
  • #47

Thank God there is but one infinity! or
Vertigo makes the world go 'round - says Brian


Hi Brian, I am glad the quote caught your attention! the person credited with observing that about the biggest obstruction to discovery was Daniel J. Boorstin. he was an American, born 1914, and became the Librarian of Congress---sort of the country's chief librarian. this page has some 30 more quotes from Daniel Boorstin:

http://www.brainyquote.com/quotes/quotes/d/danieljbo175243.html

In response to your saying about thanking God and so forth, I think it is a mistake to say there is just one infinity. I believe there are a lot of different infinities. But it is nothing to get dizzy about---just take a deep breath and the initial moment of surprise will pass
:smile:
 
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  • #48
Turns out that Loll and Westra and Masters student of Loll's named Stefan Zohren will be giving a paper 20 July at the big Paris Einstein conference

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

Wednesday, 20 July 2005 14h20-14h40 :
R. Loll, W. Westra, Stefan Zohren
« Nonperturbative sum over topologies in 2D Lorentzian Quantum Gravity »

Some photos 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

If anyone is at the Paris conference 18-22 July, and is interested in quantum gravity, keep an eye open for them. The CDT paper that deals with topology change in the spacetime path integral context is in the session called "The Nature of Space-time"

some mathematicians who need to visualize several dimensions in geometry on their computer screens use funnylooking R/B 3D goggles. Also biochemists use those 3D goggles, in this picture Loll is wearing funny goggles
http://www.phys.uu.nl/~loll/Web/title/title.html

here is a list of Loll students and postdocs
http://www.phys.uu.nl/~loll/Web/students/students.html

This thread is mostly about CDT-people (causal dynamical triangulations approach to quantum gravity) and random information, gossip, conferences etc.
But also there are other important parts of "QGATS" (quantum gravity alternatives to string) and one part is what Laurent Freidel and co-workers do (like the recent Freidel Starodubtsev, Freidel Livine, Freidel Louapre papers). So I should keep track of this link of a good snapshot of Freidel.
http://cosmos.nirvana.phys.psu.edu/online/Html/Seminars/Spring1999/Freidel/freidel.jpg
 
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  • #50
yesterday a very cautious paper by Martin Bojowald came out
https://www.physicsforums.com/showthread.php?p=618695#post618695

https://www.physicsforums.com/showpost.php?p=618695&postcount=347

the second link is in case the first one doesn't work.

in a very tentative, "oblique", way he is venturing to suggest that black hole bottoms may connect to inflations of new branches of the universe.

Bojowald I respect as a careful scientist and he does not seem to be media-phile. he does not do the attentiongetting bold speculations routine.

so this paper is very underplayed, careful almost grudging.

BTW it is already quite risky to do Loop Quantum Cosmology, but what other quantum model cosmology is there where you can actually calculate numbers?
LQC is very close to standard cosmology Friedmann model that almost everybody uses, only quantized. And you can actually calculate stuff that people have hopes of being able to check for---Parampreet Singh is one of the experts on the observational side of LQG, look him up if interested.

anyway LQG is already risky, and LQC which involves simplifying down from the full LQG model is also risky, but at least you can calculate and run computer simulations showing how contraction leads to expansion and how inflation naturally arises.

and now Bojowald and others have begun studying the black hole contraction---but in order to calculate they have to make simplifying assumptions about symmetry. they assume what collapses is nice and round----not shaped like a potato.

who is going to dare to say that black hole looks like it might lead to big bang? I don't mean say that as a "visionary" but say it on the basis of the way the mathematical models fit together. who is going to venture to hint this not on the basis of some mental image but on the basis of how the numbers look.

i remember standing at cliff-edge at Big Sur looking at the Pacific ocean one afternoon in 1961.
one is conscious of the solid ground under one's feet
one does not make any abrupt moves
 
  • #51
A beautiful and disquieting image, Marcus. I am very glad you were careful of your feet. Do you know the bar/restaurant called Nepenthes located near Big Sur? I have a close friend who worked there in the late 60's, and he never seems to tire of recalling the times he spent meditating on the ocean from Nepenthes' deck, hundreds of feet above the surf. But it seems the place is in the hands of developers now, and styles itself a resort. I wonder if the Black Angel still hangs above the gift shop door?

Well, I have been guilty of wild speculations about big bangs beyond black holes before, and I am glad to hear that the polymaths are beginning, with caution, to say it may be possible. The notion seems to me to have a beautiful symmetry. Time and space once again seem to extend themselves beyond the horizons.

Now what about tidal forces and information? I will boldly speculate that tidal forces will not be a barrior to passage of information for the simple reason that the compression is of timespace itself, and objects as we know them are completely dependent on the timespace background. If the background compresses smoothly, so will the objects embedded in it. Therefore there should be some physical conditions which would allow passage of information through the event horizon and then through the "singularity" itself.

Essentially I am speculating that the "singularity" is not a singularity at all, but merely another infinite spacetime, infinitely removed from our point of observation. All lines seem to converge at infinity, but if you translate your point of view to the infinite location, Euclid's fifth postulate still holds. Infinity, like its inverse, the singularity, is permanently shrouded in an event horizon. We are never allowed to look on G-d's naked face. It is for our own protection. If we ever evolved enough to see G-d's face directly, we would no longer exist...not that we would be ripped apart by tidal forces, but that the perfect definition excludes our imperfect existence. G-d naked is solitary, and it seems G-d is not amused by that.

I think G-d is amused when we stand upon the cliff, and pleased when we choose to step back. It is not G-d's fault, when we look down at the surf, full of doubts and fears, but our own.

Be well Marcus, and all...

Richard
 
  • #52
Oh yeah, about that potato thing. I recall reading in Kip Thorne about some Russian theorists early in the Black Hole argument (would that have been in the 1960's?) who showed that irregularites ("a mountain") on the surface of a black hole will quickly be reduced to the sphere. Of course this is part of the argument that information passing into a black hole will be lost. In a sense, the irregularities actually are the information.

However, I wonder if this smoothness is just on the outside surface of the horizon. The inside surface of the BH could be all crinkley, and we would never know, would we? The information may be lost, to us on the outside, but that may not mean it is lost, looking back on it from the inside. What would the event horizon look like from the inside? Ahem. Cosmic Microwave Background Energy.

Richard.
 
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  • #53
nightcleaner said:
...

Well, I have been guilty of wild speculations about big bangs beyond black holes before, and I am glad to hear that the polymaths are beginning, with caution, to say it may be possible. The notion seems to me to have a beautiful symmetry. Time and space once again seem to extend themselves beyond the horizons.

...

I've had some nice times at nepenthe's but it is expensive now.

You and Smolin. he has had similar speculations about big bangs beyond black holes.

but Bojowald is special for me because he is NOT visionary. I trust him not jump to conclusions. he still has not, about this thing.
what I like is that I can tell that from where he stands he can see it very clearly but he will not jump at it. his example steadies me.
 
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  • #54
Turns out that Loll and Westra and Masters student of Loll's named Stefan Zohren will be giving a paper 20 July at the big Paris Einstein conference

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

Wednesday, 20 July 2005 14h20-14h40 :
R. Loll, W. Westra, Stefan Zohren
« Nonperturbative sum over topologies in 2D Lorentzian Quantum Gravity »

Some photos 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

...

Let's take a closer look at the program at this month's Paris conference, mentioned in the earlier post.
http://einstein2005.obspm.fr/indexr.php
http://einstein2005.obspm.fr/programmer.php
The abstracts of some Plenary Session talks are posted. Here is a sample:

Monday July 18
...
11h45 - 12h25: Abhay Ashtekar « Gravity, Geometry and the Quantum »

"General relativity is both sublimely beautiful and incredibly successful. But it is incomplete because it ignores quantum physics. Its satisfactory synthesis with quantum mechanics would constitute the next leap in fundamental physics. In the first part of the talk I will discuss the primary challenges we face and summarize the strategies that have been devised to overcome them. In the second part, I will focus on loop quantum gravity, a background independent approach in which the continuum picture of space-time breaks down. I will discuss a few ramifications of the quantum geometry that replaces it. The goal is to provide a few glimpses of the exciting world-view in which gravity, geometry and the quantum merge."


14h00 - 14h40: Brian Greene « The State of String Theory»

"I will briefly review the motivation for and essential ideas of string theory, and then assess the progress the theory has made in a variety of critical areas."

14h45 - 15h30: Alain Connes « Noncommutative geometry and physics»

[no abstract available yet]

15h30 - 15h45: Coffee Break

15h45 - 16h25: Fay Dowker « Causal sets and discrete spacetime. »

"In 1905 the basic question of whether matter was continuous or discrete was still controversial and it was only decisively settled by the work of J.-B. Perrin who verified the quantitative predictions about Brownian motion made by Einstein and by Smoluchowski, ending any remaining scepticism about the physical reality of atoms and molecules. In 2005 our best theory of spacetime itself is General Relativity, in which spacetime is a continuum. But there is growing circumstantial evidence that spacetime is discrete at the tiny scales at which quantum effects on spacetime can no longer be ignored. Taking that evidence seriously, one approach to "quantum gravity'' proposes a fundamentally discrete substructure for spacetime: a causal set. The only structure carried by a causal set is a microscopic notion of "before'' and "after''. A simple model of particles moving on a causal set background implies that they undergo a Brownian motion in momentum. I will speculate on whether this phenomenon may be able to provide a mechanism for the production of the high energy cosmic rays whose origin remains a mystery. If causal set phenomenology can indeed explain the origin of high energy cosmic rays, then this observational data may turn out to be the Brownian motion of our age, convincing us finally of the atomicity of spacetime itself."

...

Tuesday July 19
...
10h15 - 10h55: Carlo Rovelli «Loop Quantum Gravity »

" I review the main ideas and the main results at the basis of the loop approach to quantum gravity. This is an attempt to construct a fully background-independent quantum field theory, where space and time emerge as quantum excitation of the gravitational field. In other words, it is an attempt to fully merge quantum field theory with the lesson of Einstein's general relativity."

...

Friday 22 July

...
15h45 - 16h45: Gerard t'Hooft « Conclusion Talk »

[no abstract available yet]

----------------------------------------------

BTW here is a picture of Fay Dowker. Interesting that both her mother and father were physicists, born c. 1966 undergrad major math, married to physicist Jerome Gauntlett---physics seems to run in the family---has two children.
http://www.stp.dias.ie/events/2004/causal_sets_photos/WorkshopOnCausalSets-FayDowker-1.jpg

Fay Dowker is one of the featured (Plenary Session) speakers not only at Einstein2005 this month in Paris but also at Loops05 this October in Potsdam.
 
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marcus said:
dammit nobody has remarked on how beautiful Fay Dowker is!

do I have to paste this in as an attachment?

Yeah, she's beautiful! How old are her kids? Are they headed for physics careers too? Wouldn't three generations be a record unmatched since the Bernoullis?
 
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selfAdjoint said:
Yeah, she's beautiful! How old are her kids? Are they headed for physics careers too? Wouldn't three generations be a record unmatched since the Bernoullis?

I agree that three generations would be highly commendable
but we have to wait and see because her kids are only 3 years old and 7 years old
 
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From the abstract:
This shows that one can in principle make sense of a gravitational path integral which includes a sum over topologies, provided suitable causality restrictions are imposed on the path integral histories.

I hope this analytical work doesn't fall into the Motl trap of being accused of bad physics for ignoring acausal paths in the path integral. The CDT papers proper avoid that criticism because those paths do not exist even in theory in their model, causality is prior to their whole scheme. But it seems this new analytical work is back to ordinary spacetime.
 
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selfAdjoint said:
... The CDT papers proper avoid that criticism because those paths do not exist even in theory in their model, causality is prior to their whole scheme...

good point about how they avoid that trouble.
in this case they manage to retain a causally layered model (continuing to avoid that type of vulnerability) by severely restraining the jitter in the topology. In the Loll-Westra model the wormholes exist only for an "infinitesimal" period of time. They barely exist---unable to register at macroscopic scale---and yet they seem to change the effective cosmological constant.

this is what I find hard to understand. Loll-Westra hardly change the CDT model, if at all. I can hardly believe that these microscopic infinitesimally-brief topology changes are actually taking place. (they seem to 'undo' themselves before any clock has had a chance to tick).
and yet.
and yet.
even though I don't see them really existing they seem to affect the Hamiltonian! so that where there used to be a Lambda (cosm. const) term there is now a effective Lambda, somewhat smaller.

BTW in 2D the Newton G is dimensionless. and spacetime volume is an area. and "density of microscopic wormholes in spacetime" being a number per unit spacetime volume has the same dimension as curvature----namely reciprocal area.

so "density of wormholes" has the same dimension as the cosm. const. Lambda.

they find that as (the 2D version of) Newton G increases there get to be more wormholes, so that the "density of wormholes" is growing almost linear proportional to G!

and as G is increasing and "density of wormholes" is growing, the effective cosm. const. Lambda is tailing off----see their Fig. 4.

The Catalan numbers get into the analysis. and some Laguerre polynomials.

all in all a bit remarkable. this is how it was in 1998. In 1998 Loll and Ambjorn tested a 2D model, with a 'causal' assumption, and found it worked. But it took roughly 5 years to get it up from 2D to 3D to 4D.

now Loll and Westra have something remarkable working at 2D. but it is not obvious how to picture these infinitesimal very brief wormholes (compatible with the causal restriction of CDT) in the 3D case.

Well, it is Westra's thesis, so I hope it does not take 5 years!

http://arxiv.org/abs/hep-th/0507012
Taming the Cosmological Constant...
 
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