- #36
selfAdjoint
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Thank you Marcus!
Quantum Graffiti: MEDIA COVERAGE, JOB OPENINGS & Gossip Around Loll at Utrecht
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In summary: I just wanted to ask: is it really that different in the Netherlands, compared to the United States, when it comes to the level of media coverage of theoretical physics?
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selfAdjoint said:
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
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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.
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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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/
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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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
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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marcus said:
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
ohwilleke
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marcus said:
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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ohwilleke said:
sounds like a reasonable guess
the main obstacle to actually doing co-authorship analysis is my own laziness
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
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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.
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.
Attachments
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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
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
Brian Bell
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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 -
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 -
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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
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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
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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the tentative programme schedule for July 11-16 "Strings 05" has been posted.
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/program.html
the titles of many of the talks are listed on this separate page:
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/speakers.html
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/program.html
the titles of many of the talks are listed on this separate page:
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/speakers.html
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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
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
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nightcleaner
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
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
nightcleaner
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.
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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nightcleaner said:
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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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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dammit nobody has remarked on how beautiful Fay Dowker is!
do I have to paste this in as an attachment?
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selfAdjoint
Staff Emeritus
Gold Member
Dearly Missed
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marcus 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?
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selfAdjoint said:
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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- #59
selfAdjoint
Staff Emeritus
Gold Member
Dearly Missed
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From the abstract:
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.
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:
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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I use these links to check activity in CDT and to see what new authors are getting into this line of research. There has been some growth in the number of papers written per year
Using this kind of keyword search, I try to edit out anything they bring up by mistake. Like Lee Smolin's 2003 survey mentions dynamical triangulations but is not really ABOUT that, so I don't count it.
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/2003/0/1
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/2004/0/1
Last 12 months:
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/past/0/1
here are the authors who have recent CDT articles
Jan Ambjorn
Mohammad Ansari
Bianca Dittrich
Jerzy Jurkiewicz
Tomasz Konopka
Renate Loll
Fotini Markopoulou
Johan Noldus (postdoc with Loll at Utrecht)
Lee Smolin
Willem Westra
Stefan Zohren
this is probably not a complete list. it does not include all of Loll's graduate students at Utrecht,
http://www.phys.uu.nl/~loll/Web/students/students.html
and people like Arundhati Dasgupta who have written CDT papers but they were before 2003. but this is some of the people.
Using this kind of keyword search, I try to edit out anything they bring up by mistake. Like Lee Smolin's 2003 survey mentions dynamical triangulations but is not really ABOUT that, so I don't count it.
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/2003/0/1
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/2004/0/1
Last 12 months:
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/past/0/1
Code:
2003 3
2004 4
LTM 8here are the authors who have recent CDT articles
Jan Ambjorn
Mohammad Ansari
Bianca Dittrich
Jerzy Jurkiewicz
Tomasz Konopka
Renate Loll
Fotini Markopoulou
Johan Noldus (postdoc with Loll at Utrecht)
Lee Smolin
Willem Westra
Stefan Zohren
this is probably not a complete list. it does not include all of Loll's graduate students at Utrecht,
http://www.phys.uu.nl/~loll/Web/students/students.html
and people like Arundhati Dasgupta who have written CDT papers but they were before 2003. but this is some of the people.
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there is a German physics newsmagazine called P.U.Z.
Physik Unserer Zeit------Physics of Our Time. I think it could be a German version of "Physics Today".
and there is this physicist Claus Kiefer at Uni Köln.
and in January 2005 there was a one-page general audience article by Kiefer about CDT, in particular about the paper "Emergence of a 4D World" by Ambjorn Jurkiewicz Loll that appeared in fall 2004 in Physical Review Letters.
I didnt yet find this online in English. Does anyone have a link?
there is a horrible scarcity of (semi)POPULAR writing about CDT. In English there is almost nothing written about CDT for general audience, at least not online.
so I am contemplating translating this page of semipopular German science writing.
first let's see if I can just paste in the German text
-----------quote Kiefer from january P.U.Z.-----
QUANTENGRAVITATION: Die vierdimensionale Welt
Die klassische Raumzeit besitzt vier makroskopische Dimensionen. In einer zukünftigen Theorie der Quantengravitation ist damit zu rechnen, dass auch die Dimension zu einer dynamischen Variable wird, für die nur ein Erwartungswert angegeben werden kann. Unabhängig davon muss sich aus Konsistenzgründen I am semiklassischen Limes immer die Zahl vier ergeben. Dass dem tatsächlich so zu sein scheint, konnten Jan Ambjørn (Kopenhagen), Jerzy Jurkiewicz (Krakau) und Renate Loll (Utrecht) kürzlich I am Rahmen des Pfadintegralzugangs zeigen [1].
Eines der grundlegendsten offenen Probleme der modernen Physik ist die konsistente Vereinigung von Quanten- und Gravitationstheorie. Die Hauptschwierigkeit besteht hierbei darin,dass die Allgemeine Relativitätstheorie keine fest vorgegebene Hintergrund-Raumzeit kennt,sondern eine dynamische Geometrie. Bei den anderen Wechselwirkungen,beispielsweise der Elektrodynamik,quantisiert man aufeiner Raumzeit,bei der Allgemeinen Relativitätstheorie quantisiert man die Raumzeit selbst.
Den ehrgeizigsten Versuch,zu einer Quantengravitation zu gelangen,bietet die Stringtheorie,die davon ausgeht,dass dieses Ziel nur I am Rahmen einer Vereinigung aller Wechselwirkungen zu erreichen ist. Andere Zugänge versuchen,Einsteins Theorie direkt zu quantisieren. Hierzu gehören Quantengeometrodynamik und Schleifendynamik. Ambjørn und Kollegen wählten einen Zugang über das Feynmansche Pfadintegral.
In der Quantenmechanik summiert man I am Pfadintegral über alle möglichen Pfade für ein Teilchen zwischen zwei Orten und Zeiten. Die meisten Pfade sind stetig,aber nirgends differenzierbar. Das Ergebnis ist eine Übergangsamplitude,welche die Schrödinger-Gleichung erfüllt. In der Gravitationstheorie ist hingegen über alle möglichen vierdimensionalen Geometrien („Raumzeiten“) zu summieren,die zwischen zwei dreidimensionale Geometrien („Räume“) passen.
Formal leicht möglich ist eine so genannte Sattelpunktsnäherung,bei der sich I am semiklassischen Grenzfall als dominierende Raumzeit eine solche ergibt,die den klassischen Einsteinschen Feldgleichungen genügt. Für eine saubere Berechnung jenseits dieser Näherung muss die Summe über alle Geometrien aber zunächst definiert werden. Diese Regularisierung geschieht durch Diskretisierung und anschließenden Kontinuumslimes.Als Vorbild dienen die Gittereichtheorien für starke und elektroschwache Wechselwirkung. Dort ist allerdings die Geometrie festgelegt,die bei der Gravitation dynamisch ist.
Bisher hatte man das Pfadintegral zumeist I am euklidischen Bereich betrachtet,wo nur über vierdimensionale Räume integriert wird und nicht über Raumzeiten. Dieser Zugang wurde vor allem durch Stephen Hawking populär.Allerdings ergeben sich dort Probleme unter anderem I am Zusammenhang mit der Dimension. Man betrachtet den Erwartungswert für die effektive HausdorffDimension H. Dieser wird durch die Beziehung V(r) ~ <r>H definiert, wobei V(r) das Volumen einer Kugel mit Radius
r darstellt. Für einen dreidimensionalen Raum sollte sich also gerade H= 3 ergeben.
Die Hausdorff-Dimension ist aus der Theorie der Fraktale bekannt, allerdings als klassische Größe und nicht als Erwartungswert. Da in der Quantengravitation kein Hintergrund existiert,ist H a priori ungleich der Dimension d der Bausteine, über die I am Pfadintegral summiert wird. Merkwürdigerweise ergibt sich für das euklidische Pfadintegral der Wert H= 2 für d> 2.
Wegen dieses und anderer Probleme schlagen die oben erwähnten Autoren den alternativen Weg der „Lorentzschen dynamischen Triangulationen“ ein. Hier summiert man tatsächlich über Raumzeiten statt Räumen,was physikalisch vernünftiger erscheint [2]. Die Diskretisierung erfolgt durch Wahl von Tetraedern zur festen (diskretisierten) Zeit,die mit dem nächsten sowie vorangehenden Zeitschritt durch vierdimensionale Simplizes verknüpft sind. Simplizes repräsentieren also die (diskretisierte) Raumzeit. Abbildung 1 zeigt eine typische Konfiguration,die in dem gezeigten Beispiel aus 91100 Simplizes besteht. Die Summe über alle Konfigurationen I am Pfadintegral erfolgt durch Monte-Carlo-Simulation.
Die Autoren betrachten den Mittelwert des räumlichen Abstandes zwischen zwei Punkten I am räumlichen Volumen und finden für die oben definierte Hausdorff-Dimension den Wert H= 3,10 ±0,15. Dies ist
eine gute Evidenz für die Dreidimensionalität des Raumes (und somit für die Vierdimensionalität der Raumzeit). Hieraus folgt freilich noch nicht, dass es auf kleinsten Skalen tatsächlich einen glatten dreidimensionalen Raum gibt. Doch immerhin liefert dieses Ergebnis einen Hinweis auf die Existenz einer Kontinuumstheorie. Interessant ist noch,dass diese Methode nur bei einer positiven Kosmologischen Konstante funktioniert -- in Einklang mit Beobachtungen. Der numerische Wert wird allerdings nicht festgelegt. Die sich so ergebende dynamisch erzeugte Quantengeometrie kann dann als Hintergrund für die Quantenfluktuationen anderer Freiheitsgrade angesetzt werden.
[1] J. Ambjørn, J. Jurkiewicz, R. Loll, Phys. Rev. Lett. 22000044, 93, 131301.
[2] R. Loll, in: Quantum Gravity (D. Giulini, C. Kiefer, C. Lämmerzahl Hrsg.), SpringerVerlag, Heidelberg 2003. Claus Kiefer, Köln
ABB. 1 SIMULATION: Typische Konfiguration („Raumzeit“), wie sie in einer MonteCarlo-Simulation erscheint. Nach oben sind die Zeitschritte (hier insgesamt 40) aufgetragen, die beiden anderen Achsen sind Raumdimensionen (aus [1]).
Physik Unserer Zeit------Physics of Our Time. I think it could be a German version of "Physics Today".
and there is this physicist Claus Kiefer at Uni Köln.
and in January 2005 there was a one-page general audience article by Kiefer about CDT, in particular about the paper "Emergence of a 4D World" by Ambjorn Jurkiewicz Loll that appeared in fall 2004 in Physical Review Letters.
I didnt yet find this online in English. Does anyone have a link?
there is a horrible scarcity of (semi)POPULAR writing about CDT. In English there is almost nothing written about CDT for general audience, at least not online.
so I am contemplating translating this page of semipopular German science writing.
first let's see if I can just paste in the German text
-----------quote Kiefer from january P.U.Z.-----
QUANTENGRAVITATION: Die vierdimensionale Welt
Die klassische Raumzeit besitzt vier makroskopische Dimensionen. In einer zukünftigen Theorie der Quantengravitation ist damit zu rechnen, dass auch die Dimension zu einer dynamischen Variable wird, für die nur ein Erwartungswert angegeben werden kann. Unabhängig davon muss sich aus Konsistenzgründen I am semiklassischen Limes immer die Zahl vier ergeben. Dass dem tatsächlich so zu sein scheint, konnten Jan Ambjørn (Kopenhagen), Jerzy Jurkiewicz (Krakau) und Renate Loll (Utrecht) kürzlich I am Rahmen des Pfadintegralzugangs zeigen [1].
Eines der grundlegendsten offenen Probleme der modernen Physik ist die konsistente Vereinigung von Quanten- und Gravitationstheorie. Die Hauptschwierigkeit besteht hierbei darin,dass die Allgemeine Relativitätstheorie keine fest vorgegebene Hintergrund-Raumzeit kennt,sondern eine dynamische Geometrie. Bei den anderen Wechselwirkungen,beispielsweise der Elektrodynamik,quantisiert man aufeiner Raumzeit,bei der Allgemeinen Relativitätstheorie quantisiert man die Raumzeit selbst.
Den ehrgeizigsten Versuch,zu einer Quantengravitation zu gelangen,bietet die Stringtheorie,die davon ausgeht,dass dieses Ziel nur I am Rahmen einer Vereinigung aller Wechselwirkungen zu erreichen ist. Andere Zugänge versuchen,Einsteins Theorie direkt zu quantisieren. Hierzu gehören Quantengeometrodynamik und Schleifendynamik. Ambjørn und Kollegen wählten einen Zugang über das Feynmansche Pfadintegral.
In der Quantenmechanik summiert man I am Pfadintegral über alle möglichen Pfade für ein Teilchen zwischen zwei Orten und Zeiten. Die meisten Pfade sind stetig,aber nirgends differenzierbar. Das Ergebnis ist eine Übergangsamplitude,welche die Schrödinger-Gleichung erfüllt. In der Gravitationstheorie ist hingegen über alle möglichen vierdimensionalen Geometrien („Raumzeiten“) zu summieren,die zwischen zwei dreidimensionale Geometrien („Räume“) passen.
Formal leicht möglich ist eine so genannte Sattelpunktsnäherung,bei der sich I am semiklassischen Grenzfall als dominierende Raumzeit eine solche ergibt,die den klassischen Einsteinschen Feldgleichungen genügt. Für eine saubere Berechnung jenseits dieser Näherung muss die Summe über alle Geometrien aber zunächst definiert werden. Diese Regularisierung geschieht durch Diskretisierung und anschließenden Kontinuumslimes.Als Vorbild dienen die Gittereichtheorien für starke und elektroschwache Wechselwirkung. Dort ist allerdings die Geometrie festgelegt,die bei der Gravitation dynamisch ist.
Bisher hatte man das Pfadintegral zumeist I am euklidischen Bereich betrachtet,wo nur über vierdimensionale Räume integriert wird und nicht über Raumzeiten. Dieser Zugang wurde vor allem durch Stephen Hawking populär.Allerdings ergeben sich dort Probleme unter anderem I am Zusammenhang mit der Dimension. Man betrachtet den Erwartungswert für die effektive HausdorffDimension H. Dieser wird durch die Beziehung V(r) ~ <r>H definiert, wobei V(r) das Volumen einer Kugel mit Radius
r darstellt. Für einen dreidimensionalen Raum sollte sich also gerade H= 3 ergeben.
Die Hausdorff-Dimension ist aus der Theorie der Fraktale bekannt, allerdings als klassische Größe und nicht als Erwartungswert. Da in der Quantengravitation kein Hintergrund existiert,ist H a priori ungleich der Dimension d der Bausteine, über die I am Pfadintegral summiert wird. Merkwürdigerweise ergibt sich für das euklidische Pfadintegral der Wert H= 2 für d> 2.
Wegen dieses und anderer Probleme schlagen die oben erwähnten Autoren den alternativen Weg der „Lorentzschen dynamischen Triangulationen“ ein. Hier summiert man tatsächlich über Raumzeiten statt Räumen,was physikalisch vernünftiger erscheint [2]. Die Diskretisierung erfolgt durch Wahl von Tetraedern zur festen (diskretisierten) Zeit,die mit dem nächsten sowie vorangehenden Zeitschritt durch vierdimensionale Simplizes verknüpft sind. Simplizes repräsentieren also die (diskretisierte) Raumzeit. Abbildung 1 zeigt eine typische Konfiguration,die in dem gezeigten Beispiel aus 91100 Simplizes besteht. Die Summe über alle Konfigurationen I am Pfadintegral erfolgt durch Monte-Carlo-Simulation.
Die Autoren betrachten den Mittelwert des räumlichen Abstandes zwischen zwei Punkten I am räumlichen Volumen und finden für die oben definierte Hausdorff-Dimension den Wert H= 3,10 ±0,15. Dies ist
eine gute Evidenz für die Dreidimensionalität des Raumes (und somit für die Vierdimensionalität der Raumzeit). Hieraus folgt freilich noch nicht, dass es auf kleinsten Skalen tatsächlich einen glatten dreidimensionalen Raum gibt. Doch immerhin liefert dieses Ergebnis einen Hinweis auf die Existenz einer Kontinuumstheorie. Interessant ist noch,dass diese Methode nur bei einer positiven Kosmologischen Konstante funktioniert -- in Einklang mit Beobachtungen. Der numerische Wert wird allerdings nicht festgelegt. Die sich so ergebende dynamisch erzeugte Quantengeometrie kann dann als Hintergrund für die Quantenfluktuationen anderer Freiheitsgrade angesetzt werden.
[1] J. Ambjørn, J. Jurkiewicz, R. Loll, Phys. Rev. Lett. 22000044, 93, 131301.
[2] R. Loll, in: Quantum Gravity (D. Giulini, C. Kiefer, C. Lämmerzahl Hrsg.), SpringerVerlag, Heidelberg 2003. Claus Kiefer, Köln
ABB. 1 SIMULATION: Typische Konfiguration („Raumzeit“), wie sie in einer MonteCarlo-Simulation erscheint. Nach oben sind die Zeitschritte (hier insgesamt 40) aufgetragen, die beiden anderen Achsen sind Raumdimensionen (aus [1]).
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the German text by Kiefer can be found here
http://www3.interscience.wiley.com/cgi-bin/abstract/109860245/ABSTRACT
this is the HTML abstract, which has a link to the PDF file.
the Kiefer article can also be found by scanning the table of contents of the January issue of Physik Unserer Zeit
http://www3.interscience.wiley.com/cgi-bin/jissue/109860236
and also it is at Loll website
http://www.phys.uu.nl/~loll/Web/press/press.html
Because we have a shortage of general audience description of CDT in English, I translated the short Kiefer article:
--------transl. from Claus Kiefer january P.U.Z.-----
QUANTUM GRAVITY: The four-dimensional world
Classical spacetime has four macroscopic dimension. In a future theory of quantum gravity one must take into account that even dimensionality will be a dynamical variable, for which only an expectation value can be provided. For reasons of consistency the number four must arise independently in the the semiclassical limit. Jan Ambjørn (Kopenhagen), Jerzy Jurkiewicz (Krakau) and Renate Loll (Utrecht) were able to show that in the path-integral framework it actually works out that way.[1]
--------------------------------
One of the most fundamental open problems in modern physics is the consistent unification of the quantum and gravitation theory. The chief difficulty consists in the fact that General Relativity knows no fixed prior-given background spacetime, but rather a dynamic geometry.
One quantizes other interactions, for example electrodynamics, ON a given spacetime, but with General Relativity one must quantize the very spacetime itself.
The most ambitious attempt to arrive at quantum gravity has been on the part of string theory, whose point of departure is the assumption that this goal can only be attained in the context of a unification of all the interactions.
Alternative approaches attempt to directly quantize Einstein's theory. Among these approaches are quantum geometrodynamics and loop dynamics (LQG). Ambjørn and colleagues chose an approach via the Feynmanian path integral
In quantum mechanics the path integral is summed over all possible paths a particle can take from one point to another. Most of the paths are continuous but nowhere differentiable. The result is a transition amplitude, which satisfies the Schrödinger-equation. In quantum gravity, the job is to sum over all possible four-dimensional geometries ("spacetimes") which fit between two three-dimensional geometries ("spaces")
Formally, it's easy to write down a so-called saddle-point approximation which in the semiclassical limiting case is dominated by a spacetime satisfying the Einstein field equations. But to go beyond this approximation to a clean calculation, one must define the sum over all geometries. This regularization occurs by discretizing and a subsequent continuum limit. The lattice theories of strong and electroweak interactions serve as models. There, however, the geometry is fixed ahead of time, while in the case of gravity it is dynamic.
Up till now the path integral has mostly been used in the Euclidean context, where one integrates only over fourdimensional space, rather than spacetime, geometries. This approach was made popular above all by Stephen Hawking. However problems arise there, among other things with the dimension.
Consider the expectation value for the effective Hausdorff dimension H. This is defined by the relation V(r) ~ <r>H where V(r) is the volume of a ball with radius r. For a three-dimensional space this should come right out H = 3.
The Hausdorff dimension is known from the theory of fractals, however as a classical quantity and not a quantum expectation value. Since in quantum gravity there is no background, H is a priori NOT equal to the dimension d of the building blocks used in the path integral summation. Notably, in the Euclidean path integral the value of H turns out to be 2, even for d >2.
Because of these and other problems, the above-mentioned authors have introduced the alternative way of "Lorentzian dynamical triangulations". Here one actually sums over spacetimes rather than spaces, which seems physically more reasonable [2]. The discretization is accomplished by choosing, at some fixed (discrete) time, spatial tetrahedra which are then joined by four dimensional simplices to like tetrahedra at the next timestep. Thus the simplices represent the (discretized) spacetime. Figure 1 shows a typical configuration which in the example shown here consists of 91,100 simplices. The sum over all configurations in the path integral is performed by Monte-Carlo simulation.
The authors consider the mean value of the spatial separation between two points in a spatial volume and find, for the Hausdorff dimension defined earlier, the value H = 3.10 ±0,15. this is good evidence of the three-dimensionality of space (and thus the four-dimensionality of spacetime). From this it certainly does not yet follow that at the smallest scale there is actually a smooth three-dimensional space.
But nevertheless this result offers a pointer towards the existence of a continuum theory. Moreover it is interesting that this method only works with a postive cosmological constant--in agreement with observations.
The numerical value has however not been determined. The resulting dynamically produced quantum geometry can then serve as a background for the quantum fluctuations of other degrees of freedom.
[1] J. Ambjørn, J. Jurkiewicz, R. Loll, Phys. Rev. Lett. 22000044, 93, 131301.
[2] R. Loll, in: Quantum Gravity (D. Giulini, C. Kiefer, C. Lämmerzahl Hrsg.), SpringerVerlag, Heidelberg 2003. Claus Kiefer, Köln
FIG. 1 SIMULATION: Typical configuration ("spacetime") as it appears in a Monte-Carlo simulation. The time steps (here 40 in all) are upwards, the other two axes are spatial dimensions.[1]
----end quote from the Claus Kiefer article---
http://www3.interscience.wiley.com/cgi-bin/abstract/109860245/ABSTRACT
this is the HTML abstract, which has a link to the PDF file.
the Kiefer article can also be found by scanning the table of contents of the January issue of Physik Unserer Zeit
http://www3.interscience.wiley.com/cgi-bin/jissue/109860236
and also it is at Loll website
http://www.phys.uu.nl/~loll/Web/press/press.html
Because we have a shortage of general audience description of CDT in English, I translated the short Kiefer article:
--------transl. from Claus Kiefer january P.U.Z.-----
QUANTUM GRAVITY: The four-dimensional world
Classical spacetime has four macroscopic dimension. In a future theory of quantum gravity one must take into account that even dimensionality will be a dynamical variable, for which only an expectation value can be provided. For reasons of consistency the number four must arise independently in the the semiclassical limit. Jan Ambjørn (Kopenhagen), Jerzy Jurkiewicz (Krakau) and Renate Loll (Utrecht) were able to show that in the path-integral framework it actually works out that way.[1]
--------------------------------
One of the most fundamental open problems in modern physics is the consistent unification of the quantum and gravitation theory. The chief difficulty consists in the fact that General Relativity knows no fixed prior-given background spacetime, but rather a dynamic geometry.
One quantizes other interactions, for example electrodynamics, ON a given spacetime, but with General Relativity one must quantize the very spacetime itself.
The most ambitious attempt to arrive at quantum gravity has been on the part of string theory, whose point of departure is the assumption that this goal can only be attained in the context of a unification of all the interactions.
Alternative approaches attempt to directly quantize Einstein's theory. Among these approaches are quantum geometrodynamics and loop dynamics (LQG). Ambjørn and colleagues chose an approach via the Feynmanian path integral
In quantum mechanics the path integral is summed over all possible paths a particle can take from one point to another. Most of the paths are continuous but nowhere differentiable. The result is a transition amplitude, which satisfies the Schrödinger-equation. In quantum gravity, the job is to sum over all possible four-dimensional geometries ("spacetimes") which fit between two three-dimensional geometries ("spaces")
Formally, it's easy to write down a so-called saddle-point approximation which in the semiclassical limiting case is dominated by a spacetime satisfying the Einstein field equations. But to go beyond this approximation to a clean calculation, one must define the sum over all geometries. This regularization occurs by discretizing and a subsequent continuum limit. The lattice theories of strong and electroweak interactions serve as models. There, however, the geometry is fixed ahead of time, while in the case of gravity it is dynamic.
Up till now the path integral has mostly been used in the Euclidean context, where one integrates only over fourdimensional space, rather than spacetime, geometries. This approach was made popular above all by Stephen Hawking. However problems arise there, among other things with the dimension.
Consider the expectation value for the effective Hausdorff dimension H. This is defined by the relation V(r) ~ <r>H where V(r) is the volume of a ball with radius r. For a three-dimensional space this should come right out H = 3.
The Hausdorff dimension is known from the theory of fractals, however as a classical quantity and not a quantum expectation value. Since in quantum gravity there is no background, H is a priori NOT equal to the dimension d of the building blocks used in the path integral summation. Notably, in the Euclidean path integral the value of H turns out to be 2, even for d >2.
Because of these and other problems, the above-mentioned authors have introduced the alternative way of "Lorentzian dynamical triangulations". Here one actually sums over spacetimes rather than spaces, which seems physically more reasonable [2]. The discretization is accomplished by choosing, at some fixed (discrete) time, spatial tetrahedra which are then joined by four dimensional simplices to like tetrahedra at the next timestep. Thus the simplices represent the (discretized) spacetime. Figure 1 shows a typical configuration which in the example shown here consists of 91,100 simplices. The sum over all configurations in the path integral is performed by Monte-Carlo simulation.
The authors consider the mean value of the spatial separation between two points in a spatial volume and find, for the Hausdorff dimension defined earlier, the value H = 3.10 ±0,15. this is good evidence of the three-dimensionality of space (and thus the four-dimensionality of spacetime). From this it certainly does not yet follow that at the smallest scale there is actually a smooth three-dimensional space.
But nevertheless this result offers a pointer towards the existence of a continuum theory. Moreover it is interesting that this method only works with a postive cosmological constant--in agreement with observations.
The numerical value has however not been determined. The resulting dynamically produced quantum geometry can then serve as a background for the quantum fluctuations of other degrees of freedom.
[1] J. Ambjørn, J. Jurkiewicz, R. Loll, Phys. Rev. Lett. 22000044, 93, 131301.
[2] R. Loll, in: Quantum Gravity (D. Giulini, C. Kiefer, C. Lämmerzahl Hrsg.), SpringerVerlag, Heidelberg 2003. Claus Kiefer, Köln
FIG. 1 SIMULATION: Typical configuration ("spacetime") as it appears in a Monte-Carlo simulation. The time steps (here 40 in all) are upwards, the other two axes are spatial dimensions.[1]
----end quote from the Claus Kiefer article---
- #64
selfAdjoint
Staff Emeritus
Gold Member
Dearly Missed
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Marcus you are to be congratulated and you deserve our thanks for translating these articles. If there are no copyright issues in the way it would be nice to collect them into a permanent website for people wishing to getr some non technical info on CDT. (I like it that this article defines the Hausdorf dimension. That's something that a slightly technical public might know and respond to, from all the fractal stuff around).
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selfAdjoint said:
constructive idea,
for starters I will check with Loll
her "press" page at her website is something of a central spot
for popular writing
she already has some translations (dutch to german, german to dutch...)
that she links to, as well as the orig.
if she thinks my translation is accurate and colloquial enough to link to, or wants to correct it, that's fine. if not then a better translation may show up.
personally I think my translation is fine, of course, but I am not a professional tech translator and it is conceivable that there's a better one out there.
I suspect copyright is not an issue because of educational "fair use".
you are right BTW that Claus Kiefer's is more "educational" than usual because he actually defines things he talks about instead of doing what journalists usually do which is not define but give metaphors and analogies, which often fry the readers brain but give everybody literary satisfactions.
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time to update, today (20 July in Paris) Loll's grad student Stefan Zohren is giving a paper at the Einstein 2005 conference.
here is the schedule of plenary sessions of the conference, to give an idea of the scope:
http://einstein2005.obspm.fr/programmer.php
Stefan is presenting a Loll-Westra-Zohren paper about "sum over topologies"---including topology change in the nonP quantum gravity path integral---in one of the conference's parallel sessions, titled "The Nature of Space-Time"
If things go as planned, the schedule for the Loops 05 conference should be posted within the next week or so, at this site:
http://loops05.aei.mpg.de/
believe it when i see it, since delays can happen in scheduling a large conference. the list of invited speakers is already posted here:
http://loops05.aei.mpg.de/index_files/Programme.html
These are CDT papers that have come out in the past 12 months.
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/past/0/1
There is a paper in the works by Loll with Dario Benedetti and Francesco Zamponi. I would like to see another from Loll/Dittrich on Black Holes but I don't know of one in the works, and it would be exciting to get a new paper from Loll/Westra about sum-over-topologies.
Here at PF we had a thread to post predictions about whether the QG path integral including sum over topologies would be extendable from the 2D case up to 3D and 4D. the thread is here:
https://www.physicsforums.com/showthread.php?t=81626
The thread shows who predicted what. Only 36 percent of us thought that they would be able to reproduce the result in full 4D spacetime---4 out of 11 respondents. (I was one of the optimists, still not entirely comfortable with that since can't imagine how the result might be extended from 2D to higher dimension.)
This thread, Quantum Grafitti, is mainly about the people doing Loll-type quantum gravity, with random news and gossip. To get an idea of the CDT (Causal Dynamical Triangulations) scene check out Loll's website at the Uni Utrecht in Holland. Here is a list of Loll graduate students/postdocs:
http://www.phys.uu.nl/~loll/Web/students/students.html
Here are some snapshots 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
Here is Loll homepage:
http://www.phys.uu.nl/~loll/Web/title/title.html
OK I guess that is the update for now.
here is the schedule of plenary sessions of the conference, to give an idea of the scope:
http://einstein2005.obspm.fr/programmer.php
Stefan is presenting a Loll-Westra-Zohren paper about "sum over topologies"---including topology change in the nonP quantum gravity path integral---in one of the conference's parallel sessions, titled "The Nature of Space-Time"
If things go as planned, the schedule for the Loops 05 conference should be posted within the next week or so, at this site:
http://loops05.aei.mpg.de/
believe it when i see it, since delays can happen in scheduling a large conference. the list of invited speakers is already posted here:
http://loops05.aei.mpg.de/index_files/Programme.html
These are CDT papers that have come out in the past 12 months.
http://arxiv.org/find/grp_physics/1...gravity+AND+Lorentzian+quantum/0/1/0/past/0/1
There is a paper in the works by Loll with Dario Benedetti and Francesco Zamponi. I would like to see another from Loll/Dittrich on Black Holes but I don't know of one in the works, and it would be exciting to get a new paper from Loll/Westra about sum-over-topologies.
Here at PF we had a thread to post predictions about whether the QG path integral including sum over topologies would be extendable from the 2D case up to 3D and 4D. the thread is here:
https://www.physicsforums.com/showthread.php?t=81626
The thread shows who predicted what. Only 36 percent of us thought that they would be able to reproduce the result in full 4D spacetime---4 out of 11 respondents. (I was one of the optimists, still not entirely comfortable with that since can't imagine how the result might be extended from 2D to higher dimension.)
This thread, Quantum Grafitti, is mainly about the people doing Loll-type quantum gravity, with random news and gossip. To get an idea of the CDT (Causal Dynamical Triangulations) scene check out Loll's website at the Uni Utrecht in Holland. Here is a list of Loll graduate students/postdocs:
http://www.phys.uu.nl/~loll/Web/students/students.html
Here are some snapshots 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
Here is Loll homepage:
http://www.phys.uu.nl/~loll/Web/title/title.html
OK I guess that is the update for now.
Last edited by a moderator:
- #67
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http://www.phys.uu.nl/~loll/Web/title/title.html
I didnt hear back from Loll about this translation of the Kiefer article, guess she is busy. thinking about other places to offer (rough nonprofessional) translations. Should post (as selfAdj suggests) because there seems to be more in the european media than there is in US. where balanced QG coverage is scarce.
Loll press page has this interview with Hermann Nicolai from Die Zeit, I guess that is like the New York Times. so this is to compare with the articles by Dennis Overbye. (which tend to be "good" by Lubos Motl standards, that is what i would call onesided)
http://zeus.zeit.de/text/2005/16/Nicolai-Interview
Nicolai is a director at Albert Einstein Institute and in this interview he is critical of the German funding agencies for not giving enough support, however it is the other things that caught my attention---a research director can be expected to put in a plug.
<<Alles nur Fassade
...
DIE ZEIT: Hundert Jahre nach der Entwicklung der Relativitätstheorie haben Sie Ihre Kollegen zu einer Bestandsaufnahme ans Albert-Einstein-Institut nach Golm gerufen. Wie sieht die Bilanz aus?
Hermann Nicolai: Wir leben in spannenden Zeiten! Um es mit einem Wort des Astrophysikers Michael Turner zu sagen: Wir wissen viel mehr, aber verstehen weniger. Dank neuer Teleskope und moderner Beobachtungsmethoden ist die Kosmologie – anders als zu Einsteins Zeiten – heute eine empirische Wissenschaft. Wir haben eine Fülle von Daten über die Beschaffenheit des Universums. Doch eine umfassende Theorie, die all diese Beobachtungen erklären könnte, ist derzeit nicht in Sicht.
ZEIT: Hat Einstein das nicht mit der Relativitätstheorie versucht?
Nicolai: Diese kann zwar alles erklären, was mit der Schwerkraft zusammenhängt – die Struktur des Raumes, die Bewegung von Galaxien, Schwarze Löcher… Aber ebenso wie in der Elementarteilchenphysik fehlt uns eine fundamentale Erklärung, die sagt, woher alles kommt und warum das so ist. Warum dehnt sich das Weltall genau auf diese Weise aus, die wir beobachten? Weshalb hat es gerade diese Massendichte, warum gibt es Galaxien? Letztlich wissen wir das nicht. Das liegt auch daran, dass die Theorie fürs große Ganze – Einsteins Relativitätstheorie – und fürs ganz Kleine – die Quantenmechanik – unverknüpft nebeneinander stehen. Eine gemeinsame »vereinheitlichte Theorie« zu finden bleibt die große Herausforderung der Physik für das 21. Jahrhundert.
ZEIT: Als ein Kandidat gilt ja die Stringtheorie. Ihr zufolge gibt es keine punktförmigen Teilchen mehr, sondern nur winzige schwingende »Saiten«. Aus deren Schwingungen sollen sich sämtliche Elementarteilchen und ihre Kräfte erklären lassen.
Nicolai: Als die Stringtheorie in den achtziger Jahren aufkam, brach eine Art kollektiver Begeisterung aus, und die Physiker dachten: In den nächsten vier Wochen haben wir alles erklärt. Mittlerweile ist eine gewaltige Ernüchterung eingetreten. Seither haben über tausend kluge Köpfe weltweit an der Stringtheorie gearbeitet. Es hat wohl noch nie in der theoretischen Physik eine solche kollektive Anstrengung gegeben – doch wie die Stringtheorie schlussendlich aussehen soll, wissen wir noch immer nicht.
ZEIT: Gibt es Alternativen?
Nicolai: Eine Besonderheit des Albert-Einstein-Instituts hier in Golm ist, dass wir zwei Ansätze verfolgen: zum einen die Stringtheorie, die Einsteins Relativitätstheorie radikal modifiziert. Zum anderen die Quantengravitation, die Einsteins Theorie, so, wie sie ist, neu formulieren will – unter Berücksichtigung der Tatsache, dass die Größen der Natur nicht kontinuierlich erscheinen, sondern sprunghaft oder »gequantelt«. Allerdings wirft das gewaltige mathematische Probleme auf, die noch nicht gelöst sind. Mir kommt es so vor, als ob bei allen derzeit verfolgten Theorien eine entscheidende Einsicht fehlt, die wie ein Schlüssel ins Schloss passt.
...>>
I didnt hear back from Loll about this translation of the Kiefer article, guess she is busy. thinking about other places to offer (rough nonprofessional) translations. Should post (as selfAdj suggests) because there seems to be more in the european media than there is in US. where balanced QG coverage is scarce.
Loll press page has this interview with Hermann Nicolai from Die Zeit, I guess that is like the New York Times. so this is to compare with the articles by Dennis Overbye. (which tend to be "good" by Lubos Motl standards, that is what i would call onesided)
http://zeus.zeit.de/text/2005/16/Nicolai-Interview
Nicolai is a director at Albert Einstein Institute and in this interview he is critical of the German funding agencies for not giving enough support, however it is the other things that caught my attention---a research director can be expected to put in a plug.
<<Alles nur Fassade
...
DIE ZEIT: Hundert Jahre nach der Entwicklung der Relativitätstheorie haben Sie Ihre Kollegen zu einer Bestandsaufnahme ans Albert-Einstein-Institut nach Golm gerufen. Wie sieht die Bilanz aus?
Hermann Nicolai: Wir leben in spannenden Zeiten! Um es mit einem Wort des Astrophysikers Michael Turner zu sagen: Wir wissen viel mehr, aber verstehen weniger. Dank neuer Teleskope und moderner Beobachtungsmethoden ist die Kosmologie – anders als zu Einsteins Zeiten – heute eine empirische Wissenschaft. Wir haben eine Fülle von Daten über die Beschaffenheit des Universums. Doch eine umfassende Theorie, die all diese Beobachtungen erklären könnte, ist derzeit nicht in Sicht.
ZEIT: Hat Einstein das nicht mit der Relativitätstheorie versucht?
Nicolai: Diese kann zwar alles erklären, was mit der Schwerkraft zusammenhängt – die Struktur des Raumes, die Bewegung von Galaxien, Schwarze Löcher… Aber ebenso wie in der Elementarteilchenphysik fehlt uns eine fundamentale Erklärung, die sagt, woher alles kommt und warum das so ist. Warum dehnt sich das Weltall genau auf diese Weise aus, die wir beobachten? Weshalb hat es gerade diese Massendichte, warum gibt es Galaxien? Letztlich wissen wir das nicht. Das liegt auch daran, dass die Theorie fürs große Ganze – Einsteins Relativitätstheorie – und fürs ganz Kleine – die Quantenmechanik – unverknüpft nebeneinander stehen. Eine gemeinsame »vereinheitlichte Theorie« zu finden bleibt die große Herausforderung der Physik für das 21. Jahrhundert.
ZEIT: Als ein Kandidat gilt ja die Stringtheorie. Ihr zufolge gibt es keine punktförmigen Teilchen mehr, sondern nur winzige schwingende »Saiten«. Aus deren Schwingungen sollen sich sämtliche Elementarteilchen und ihre Kräfte erklären lassen.
Nicolai: Als die Stringtheorie in den achtziger Jahren aufkam, brach eine Art kollektiver Begeisterung aus, und die Physiker dachten: In den nächsten vier Wochen haben wir alles erklärt. Mittlerweile ist eine gewaltige Ernüchterung eingetreten. Seither haben über tausend kluge Köpfe weltweit an der Stringtheorie gearbeitet. Es hat wohl noch nie in der theoretischen Physik eine solche kollektive Anstrengung gegeben – doch wie die Stringtheorie schlussendlich aussehen soll, wissen wir noch immer nicht.
ZEIT: Gibt es Alternativen?
Nicolai: Eine Besonderheit des Albert-Einstein-Instituts hier in Golm ist, dass wir zwei Ansätze verfolgen: zum einen die Stringtheorie, die Einsteins Relativitätstheorie radikal modifiziert. Zum anderen die Quantengravitation, die Einsteins Theorie, so, wie sie ist, neu formulieren will – unter Berücksichtigung der Tatsache, dass die Größen der Natur nicht kontinuierlich erscheinen, sondern sprunghaft oder »gequantelt«. Allerdings wirft das gewaltige mathematische Probleme auf, die noch nicht gelöst sind. Mir kommt es so vor, als ob bei allen derzeit verfolgten Theorien eine entscheidende Einsicht fehlt, die wie ein Schlüssel ins Schloss passt.
...>>
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- #68
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DIE ZEIT: A hundred years after the development of relativity theory, you have invited your colleagues to the Albert Einstein Institute at Golm for a status report. How does the balance sheet look?
NICOLAI: We live in exciting times! As the astrophysicist Michael Turner puts it: we know a lot more, but understand less. thanks to the new telescopes and modern observation methods, cosmology is---in contrast to Einstein's time---today an empirical science. We have an abundance of data about the origin of the universe. But a comprehensive theory, that could explain all these observations, is so far not in sight.
Zeit: Didn't Einstein investigate that with his Relativity Theory?
Nicolai: Indeed this can account for everything related to gravity---the structure of space, the motion of galaxies, black holes...But we are lacking, just as we are in elementary particle theory, a fundamental explanation that says where does it all come from and why this is so. Why is the World arranged exactly in the fashion which we observe it to be? Why does it have exactly this density? Why are there galaxies? In the end, we just don't know. That is because the theory of the big total (Einstein's relativity) and that of the totally little (quantum mechanics) are not connected.
To find a common "unified theory" remains the great challenge of physics for the 21st century.
Zeit: String theory certainly counts as one candidate. According to it, there are no point particles, but rather tiny vibrating strings. From their vibrations the elementary particles and their forces are supposed to be explainable.
Nicolai: When string theory arrived in the eighties, a kind of mass enthusiasm broke out, and physicists thought: "in the next four weeks we will have explained everthing!" Then came the shock of disillusionment. Since then over a thousand clever heads worldwide have worked on string theory. There was never before such a huge collective effort in theoretical physics---but we still don't know what string theory will finally look like.
Zeit: Are there alternatives?
Nicolai: One of the special things about the Albert Einstein Institute here in Golm is that we pursue two different approaches:
One is string theory, in which Einstein's relativity is radically modified.
The other is Quantum Gravity, which attempts to reformulate Einstein's relativity exactly as it is, but taking into account the fact that quantitites in nature appear in steps or "quantized", instead of being continuous.
Either way tough mathematical problems turn up, which are not yet solved. My impression is that in all the theories that have so-far been tried there is a decisive insight missing, a missing key that will fit the lock...
NICOLAI: We live in exciting times! As the astrophysicist Michael Turner puts it: we know a lot more, but understand less. thanks to the new telescopes and modern observation methods, cosmology is---in contrast to Einstein's time---today an empirical science. We have an abundance of data about the origin of the universe. But a comprehensive theory, that could explain all these observations, is so far not in sight.
Zeit: Didn't Einstein investigate that with his Relativity Theory?
Nicolai: Indeed this can account for everything related to gravity---the structure of space, the motion of galaxies, black holes...But we are lacking, just as we are in elementary particle theory, a fundamental explanation that says where does it all come from and why this is so. Why is the World arranged exactly in the fashion which we observe it to be? Why does it have exactly this density? Why are there galaxies? In the end, we just don't know. That is because the theory of the big total (Einstein's relativity) and that of the totally little (quantum mechanics) are not connected.
To find a common "unified theory" remains the great challenge of physics for the 21st century.
Zeit: String theory certainly counts as one candidate. According to it, there are no point particles, but rather tiny vibrating strings. From their vibrations the elementary particles and their forces are supposed to be explainable.
Nicolai: When string theory arrived in the eighties, a kind of mass enthusiasm broke out, and physicists thought: "in the next four weeks we will have explained everthing!" Then came the shock of disillusionment. Since then over a thousand clever heads worldwide have worked on string theory. There was never before such a huge collective effort in theoretical physics---but we still don't know what string theory will finally look like.
Zeit: Are there alternatives?
Nicolai: One of the special things about the Albert Einstein Institute here in Golm is that we pursue two different approaches:
One is string theory, in which Einstein's relativity is radically modified.
The other is Quantum Gravity, which attempts to reformulate Einstein's relativity exactly as it is, but taking into account the fact that quantitites in nature appear in steps or "quantized", instead of being continuous.
Either way tough mathematical problems turn up, which are not yet solved. My impression is that in all the theories that have so-far been tried there is a decisive insight missing, a missing key that will fit the lock...
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a wide-audience account of CDT written in English (finally) has been commissioned by the UK based magazine Contemporary Physics
and should be appearing shortly.
the TOC of Contemporary Physics is online, but I don't think the text is available---will take a trip to the library for one or more of us.
for what its worth, here is the link to the online TOC
http://www.ingentaconnect.com/content/tandf/tcph
They haven't posted the contents of the May-June 2005 issue (vol 46 no 3)
and that is the one I'm expecting to see this article in. So the link is to check periodically to see if it's out yet.
and should be appearing shortly.
the TOC of Contemporary Physics is online, but I don't think the text is available---will take a trip to the library for one or more of us.
for what its worth, here is the link to the online TOC
http://www.ingentaconnect.com/content/tandf/tcph
They haven't posted the contents of the May-June 2005 issue (vol 46 no 3)
and that is the one I'm expecting to see this article in. So the link is to check periodically to see if it's out yet.
- #70
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this is a pretty good short essay
giving intuitive explanation of CDT
and how it arose in the context of other approaches to QG
to circumvent difficulties the others encountered
http://www.phys.uu.nl/~loll/Web/research/research.html
it is in English and also the best introduction for nonspecialist audience
I didnt realize this earlier.
giving intuitive explanation of CDT
and how it arose in the context of other approaches to QG
to circumvent difficulties the others encountered
http://www.phys.uu.nl/~loll/Web/research/research.html
it is in English and also the best introduction for nonspecialist audience
I didnt realize this earlier.
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