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Quantum Graffiti

  1. Jun 1, 2005 #1

    marcus

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    the topic of this thread is MEDIA COVERAGE, JOB OPENINGS and other items of gossip around Loll at Utrecht and the new model spacetime continuum coming out of the Triangulations approach.

    I think what Loll and CDT co-workers are doing is currently the most significant development in theoretical physics. So I want to assemble news about it off the web.

    Like Loll is getting research grants to start a "Triangulations" GROUP at Utrecht and has posted job openings (for pre-doc and post-doc positions). How much, how many? What media coverage? Real life details.

    And what is this European-wide network for random geometry research called E.N.RA.GE? this thread is to help keep up to date on the CDT-style triangulations quantum gravity story.
     
    Last edited: Jun 1, 2005
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  3. Jun 1, 2005 #2

    marcus

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

    some exerpts:
    "Renate Loll is an Associate Professor at the Institute for Theoretical Physics of Utrecht University. She received her Ph.D. from Imperial College, London, in 1989. In 2001 she joined the permanent staff of the ITP, after spending several years at the Max-Planck Institute for Gravitational Physics in Golm, Germany, where she also held a Heisenberg Fellowship..."

    "...In December 2004, Loll won a five-year VICI award by the Netherlands Organisation for Scientific Research (NWO) to pursue her investigations into the quantum structure of space-time and strengthen her working group at the ITP. She is also in charge of the European Research and Training Network ENRAGE, which brings together 13 European research groups working on theories of random geometry, with applications ranging from..."

    this information is dated 2004
    here is a list of Loll's current and former students and post-docs
    http://www.phys.uu.nl/~loll/Web/students/students.html
    I hope by now the list has grown some!

    here is a list of job openings, from 2004, related to her research group
    http://www.phys.uu.nl/~loll/Web/jobs/jobs.html
    (the deadline for applying was January 2005, I hope there is a new list but I do not see one)

    Here is a feature about Loll in a Dutch daily called "NRC Handelsblad", the article is dated January 2005 and has some fresher news than the pages at her Uni Utrecht site.
    www.phys.uu.nl/~loll/Web/press/knutselen.pdf

    Here is the German translation by Karola Loll. (sounds like a daughter or other kin)
    http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm
     
    Last edited: Jun 1, 2005
  4. Jun 1, 2005 #3

    marcus

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    Great short article by Adrian Cho for Physical Review focus

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

    The American Physical Society has the major peer-review journals series Phys. Rev. and Physical Review Letters. And they pick out articles for highlighting journalistically in the accompanying publication Physical Review Focus.

    Adrian Cho did a good job with this focus piece on Loll-type triangulations QG.

    Like a good journalist he included some reaction-quotes from other scientists:

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

    "It's exceedingly important" work, says Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada. "Now at least we know one way to do this." Des Johnston of Heriot-Watt University in Edinburgh, Scotland, agrees the work is "very exciting" and says it underlines the importance of causality. "The other neat thing about this work is that you're essentially reducing general relativity to a counting problem," Johnston says. "It's a very minimalist approach to looking at gravity.">>
     
  5. Jun 1, 2005 #4

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    Last edited: Jun 1, 2005
  6. Jun 1, 2005 #5

    marcus

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    the title of the thread is supposed to be a pun (in case anybody didnt get it)
    with "gra-FEE-tee" pronounced "GRAF-fi-ti"

    here are links to some other "Triangulations" Quantum Gravity threads:

    https://www.physicsforums.com/showthread.php?t=76674

    https://www.physicsforums.com/showthread.php?t=57311

    https://www.physicsforums.com/showthread.php?t=75472

    What I keep putting off, because it looks like a lot of hard work, is making a translation into English of some exerpts from the article about Loll and CDT published in January 2005 by the Dutch daily "NRC-Handelsblad"

    But morally, out of simple fairness, it should be done. In the United States we have heard almost nothing except string theory. With some occasional mention of Loop gravity. media coverage inversely proportional to importance. And ZERO coverage of Loll-type triangulations Qvantoom Graffitee. You have to be able to read GERMAN OR DUTCH if you want to follow the most significant developments in theoretical physics. At least it would seem so, if your source is the popular media!

    I will try a bit of this Handelsblad article by Dirk van Delft.
    It may sound dumb (because of my translation and also because that's how the popular media can often sound) but let's try:
     
    Last edited: Jun 1, 2005
  7. Jun 1, 2005 #6

    marcus

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    exerpts from Dirk van Delft article in NRC Handelsblad 22 January, the Science and Education section

    <<Renate Loll investigates the microstructure of space and time

    Probing the Quantum Foam

    [On the smallest scale, spacetime is not smooth-flowing, but choppy. The Utrecht phsicist Renate Loll has for the first time glued together out of microbricks a world with the right number of dimensions---Dirk van Delft]

    For Renate Loll the year 2005 couldnt go neater. Before Christmas, the theoretical physicist (born in Aachen and fellow at the University of Utrecht's Spinoza Institute) received from the Netherlands Scientific Reseach Organization (NWO) 1.25 million Euro to build up, over the next 5 years, her own Group, which investigates the quantum structure of spacetime.

    Shortly before that she had already brought in 2.93 million, to build a network of 13 European institutes which will work together in the area of Random Geometries

    This involves a technique which can be used to explain spacetime at extremely small scale, but which, as a mathematical method, can also be applied elsewhere.

    "I enjoy thinking," says Renate Loll in her workroom in the Minnaert building, on the border of Utrecht Campus Uithof.

    "It keeps my wits in shape. With the structure of spacetime you really find yourself at the edge of the imaginable. there are still a lot of riddles to solve. And it is exciting to open up paths that no one has traveled yet."

    "In physics it's clear what direction one is working towards. I was also at the London School of Economics for a year. there it was all about human relations, which is frustrating and difficult. My partner is also a physicist, which I consider an advantage, since you have someone to talk about your work with. And after being a while in this field, competition plays less of a role. Also I like it that I never have to explain when I'm staring at a piece of paper for hours in the middle of the night.">>

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

    http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm

    here is the German text of the part I translated:

    <<Renate Loll untersucht die Mikrostruktur von Raum und Zeit

    Basteln mit Quantenschaum

    [Auf der allerkleinsten Skala ist die Raumzeit nicht fließend, sondern zerstückelt. Die Utrechter Physikerin Renate Loll klebte als erste aus Mikrobausteinen eine Welt mit der richtigen Anzahl Dimensionen zusammen. Dirk von Delft]

    Für Renate Loll kann im Jahr 2005 schon nichts mehr schief gehen. Die in Aachen geborene theoretische Physikerin, Mitarbeiterin des Spinoza-Instituts der Universität Utrecht, bekam vor Weihnachten von der Niederländischen Organisation für wissenschaftliche Forschung (NWO) 1,25 Millionen Euro, um innerhalb von 5 Jahren eine eigene Gruppe aufbauen, die die Quantenstruktur der Raumzeit untersucht. Kurz zuvor hatte sie schon von der EU 2,93 Millionen eingeheimst, um ein Netzwerk von 13 europäischen Instituten aufzubauen, die auf dem Gebiet der sogenannten Zufallsgeometrien zusammen arbeiten. Dabei geht es um eine Technik, die Raumzeit auf extrem kleiner Skala zu erfassen, die aber als mathematische Methode auch anderswo einsetzbar ist.
    „Ich denke gerne“, sagt Renate Loll in ihrem Arbeitszimmer im Minnaertbau, am Rand des Utrechter Campus’ Uithof. „Das hält mich geistig fit. Bei der Struktur von Raumzeit befindet man sich wirklich am Rand des Vorstellbaren. Es gibt noch viele Rätsel zu lösen, und es ist spannend Wege einzuschlagen, die noch niemand gegangen ist. In der Physik ist es klar, in welche Richtung man arbeitet. Ich war auch ein Jahr lang auf der London School of Economics. Da geht es um das Verhalten von Menschen, was frustrierend schwierig ist. Mein Partner ist auch Physiker, was ich als Vorteil ansehe, da man mit jemandem über seine Arbeit sprechen kann. Und sobald man länger in diesem Fach dabei ist, spielt das konkurrierende Element eine geringere Rolle. Auch finde ich es angenehm, dass ich nichts erklären muss, wenn ich mitten in der Nacht stundenlang auf ein Blatt Papier starre.“>>
     
    Last edited: Jun 2, 2005
  8. Jun 1, 2005 #7

    marcus

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    more exerpts from Dirk van Delft article in NRC Handelsblad 22 January, the Science and Education section
    http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm

    <<Loll's field is Quantum Gravity. Of which the fundamental problem is that Einstein's description of gravity (the 1915 General Theory of Relativity) is in conflict with the quantum theory developed in the 1920s by Heisenberg, Schroedinger, Dirac, and others.

    According to Einstein, four-dimensional spacetime (three for space and one for time) is curved by mass, or energy. As a consequence, a lightray passing the sun will be diverted. When, during a 1919 solar eclipse, Eddington observed this effect, Einstein became instantly world-famous.

    According to Einstein, spacetime is flexible (malleable, ductile), but also at the same time continuous and smooth-----for example it does not have breaks (fractures, gaps, rips....)

    Planck scale
    In quantum theory, which describes the microscopic structure of the world, this view can no longer be considered correct. Among the pillars of this theory are the famous Heisenberg Uncertainty Relations ("Un-sharp relations"):

    the smaller the length-scale considered, the greater must be the accompanying energy fluctuations

    these fluctuations must express themselves, according to Einstein, in a bending of spacetime

    Loll: "At the Planck scale---the smallest length unit which one can meaningfully talk about in quantum theory, where it has to do with numbers with 35 zeros after the decimal point---the energy fluctuations are extremely big. In my theory spacetime becomes infinitely bent by them, with infinitely many wrinkles.

    but there are physicists who work with rips and gaps. Instead of a continuum, the spacetime splits up into isolated fragments, and many speak of Quantum Foam.

    The job of quantum gravity is to describe this turbulent distortion of spacetime, and the mathematics of Einstein's continuous spacetime with wavelike disturbances is no longer adequate....>>

    =================================
    here is the German source for that section:

    <<Lolls Arbeitsgebiet ist die Quantengravitation. Deren grundlegendes Problem ist, dass Einsteins Beschreibung der Schwerkraft, die allgemeine Relativitätstheorie von 1915, mit der Quantentheorie auf Kriegsfuß steht, die in den zwanziger Jahren von Heisenberg, Schrödinger, Dirac und anderen entwickelt wurde. Bei Einstein wird die vierdimensionale Raumzeit (drei für den Raum, eine für die Zeit) unter dem Einfluss von Masse (oder Energie) gekrümmt. Die Folge ist, dass ein an der Sonne vorbei laufender Lichtstrahl abgelenkt wird. Als Eddington 1919 während einer Sonnenfinsternis diesen Effekt beobachtete, war Einstein mit einem Schlag weltberühmt. Bei Einstein ist die Raumzeit dehnbar, aber zugleich auch kontinuierlich und glatt, sie enthält beispielsweise keine Risse.

    Planckskala. In der Quantentheorie, die die mikroskopische Struktur der Welt beschreibt, lässt sich diese Sichtweise nicht mehr aufrecht erhalten. Zu den Säulen dieser Theorie gehören die berühmten Unschärferelationen von Heisenberg: je kleiner die betrachtete Längenskala, desto größer die damit einhergehenden Energiefluktuationen. Diese Fluktuationen äussern sich jedoch Einstein zufolge in einer Krümmung der Raumzeit. Loll: „Auf der Planckskala – der kleinsten Längeneinheit, über die man in der Quantentheorie noch sinnvoll sprechen kann und wo man es mit Zahlen mit 35 Nullen hinter dem Komma zu tun hat – sind die Energiefluktuationen extrem groß. In meiner Theorie wird dadurch die Raumzeit unendlich gekrümmt, mit unendlich vielen Falten. Aber es gibt auch Physiker, die mit Rissen oder Löchern arbeiten. Anstelle eines Kontinuums zerfällt die Raumzeit in einzelne Bröckchen, manche sprechen von Quantenschaum. Die Aufgabe der Quantengravitation ist die Beschreibung dieser turbulenten Verformung der Raumzeit, und dazu reicht die Mathematik von Einsteins kontinuierlicher Raumzeit plus wellenartigen Störungen nicht mehr aus.“>>
     
    Last edited: Jun 2, 2005
  9. Jun 2, 2005 #8

    marcus

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    more exerpts from Dirk van Delft article
    http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm

    <<How to reconstruct our known Universe, with its four-dimensional spacetime, from wildly fluctuating quantum foam? In the fall of 2004 Renate Loll scored a breakthrough regarding this long-outstanding open question. Her point of departure was not a new one: take microscopic pieces of flat spacetime and join them to each other in every conceivable way. By combining all possibilities in a sum---a standard proceedure in quantum theory---one could build a macroscopic Universe.

    Loll: "I was working not with two-dimensional, but instead four-dimensional, triangles. this kind of geometrical modeling appeals to me. The problem of the summation method, up till now, was always that the resulting universe had either two, or infinitely many, dimensions. Something in the approach was fundamentally wrong."

    Rolled up A structure made out of four-dimensional "triangles" does not itself have to be four-dimensional. In the case of flat two-dimensional triangles, the result can, for example, be a kind of "rolled up" structure just like a familiar water-pipe that looks one-dimensional from a distance.

    Furthermore, the choice of triangular building blocks is not essential: rectangular blocks would lead to the same macroscopic end result. the geometric peculiarities occur because of the enormous number of microscopic pieces. However using triangles is mathematically convenient.

    Loll's decisive idea was to place an additional causal requirement on the triangles: cause must preceed effect. "That's how it goes in research: a fantastic idea develops in your head, and you determine that---ahhhh!---that just might work! The years of struggling, that it took to make you receptive to the insight, no longer matter. So then you talk it over with colleagues with reliable expertise in triangulation methods, and they say: 'Forget it. No way will that work!'

    But in this business one shouldn't let oneself be discouraged easily! Next I pursued the question of whether the same idea would lead to something interesting in the simple case of two dimensions, which in fact it did.

    From that point on, with the help of computer models, I made the step to three and four dimensions. With the result that we actually obtained a four-dimensional universe." this work appeared on 24 September in the renowned Journal Physical Review Letters>>

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

    here is the German source for the above:

    <<Wie rekonstruiert man unser bekanntes Universum mit seiner vierdimensionalen Raumzeit aus wild fluktuierendem Quantenschaum? In dieser seit Jahrzehnten offenen Frage gelang Renate Loll im Herbst 2004 ein Durchbruch. Ihr Ausgangspunkt war dabei nicht neu: man nehme mikroskopische Stücke flacher Raumzeit und klebe sie auf jede nur erdenkliche Weise aneinander. Indem man alle diese Möglichkeiten überlagert – ein Standardansatz in der Quantentheorie – baut man sich ein makroskopisches Universum. Loll: „Ich arbeite nicht mit zwei-, sondern vierdimensionalen Dreiecken. Diese Art geometrischer Formulierung spricht mich an. Das Problem der überlagerungstechnik war bisher immer, dass das sich ergebende Universum entweder zwei oder unendlich viele Dimensionen besaß. Man hatte also irgendetwas grundlegend falsch gemacht.“

    Aufgerollt. Eine Struktur, die aus vierdimensionalen „Dreiecken“ aufgebaut ist, ist selbst nicht wieder unbedingt vierdimensional. Im Fall von flachen zweidimensionalen Dreiecken kann das Ergebnis zum Beispiel eine Art „aufgerollter“ Struktur sein, die, genau wie ein von weitem betrachtetes Regenabflussrohr, auf großen Skalen eindimensional erscheint. Im übrigen ist die Wahl dreieckiger Bausteine nicht wesentlich: viereckige Bausteine würden dasselbe makroskopische Endergebnis liefern. Bei der enormen Zahl mikroskopischer Bausteine fallen deren geometrische Einzelheiten im Endresultat heraus. Die Wahl von Dreiecken ist lediglich mathematisch bequem.
    Lolls entscheidende Idee war, an die Dreiecke zusätzliche kausale Anforderungen zu stellen: Ursachen müssen Folgen vorausgehen. „So geht das in der Forschung: in deinem Kopf entwickelt sich eine fantastische Idee und du stellst fest – ahhhh! – das geht ja! Die Jahre der Anstrengungen, die dich für diese fantastische Eingebung empfänglich gemacht haben, spielen auf einmal keine Rolle mehr. Dann besprichst du dich mit Kollegen, die mit der Dreieckstechnik vertraut sind, und die sagen: ,Vergiß es. Das klappt sowieso nicht.’ Aber in diesem Fach darf man sich nicht zu schnell entmutigen lassen! Ich bin dann zunächst der Frage nachgegangen, ob dieselbe Idee im einfachen Fall von zwei Dimensionen etwas Interessantes ergab, was sie auch tat. Daraufhin machte ich mit Hilfe von Computermodellen den Schritt zu drei und vier Dimensionen. Mit dem Ergebnis, dass wir tatsächlich ein vierdimensionales Universum erhielten.“ Diese Arbeit erschien am 24. September in der renommierten Fachzeitschrift Physical Review Letters.>>

    It seems to me that Dirk van Delft may have taken liberties with some pronouns and substituted the First Person Singular where Loll might have said, or meant to say, the Plural. Anything can happen in popular journalism for mass media, especially if it makes the story more gripping. In the scholarly writings that I've seen, in contrast to this journalistic account, Loll says "we" and includes her co-workers Ambjorn and Jurkiewicz without exception.
     
    Last edited: Jun 2, 2005
  10. Jun 2, 2005 #9

    marcus

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    Last edited: Jun 2, 2005
  11. Jun 3, 2005 #10

    marcus

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    another piece of the Dirk van Delft article about Renate Loll

    <<Die große Herausforderung, die noch vor Renate Loll und ihrer im Aufbau befindlichen Forschungsgruppe liegt, ist nachzuprüfen, ob die so erzeugte vierdimensionale Raumzeit makroskopisch wieder schöne glatte „Einsteinsche“ Eigenschaften annimmt. Loll: „Sobald dies bestätigt ist, können wir daran gehen, die Quantengravitation im Bereich zwischen Planck- und Makroskala zu untersuchen. Fragen gibt es da zur Genüge. Verursacht die zusätzliche Raumzeitstruktur einen kumulativen Effekt, der seine Spuren in einem Lichtstrahl hinterlässt, der uns aus der Tiefe des Weltalls erreicht? Ein anderer Knackpunkt: wie sieht es mit der Quantengravitation schwarzer Löcher aus, Objekten, die so kompakt sind, dass absolut nichts, auch kein Licht, aus ihnen herausdringen kann, die aber dennoch aufgrund der Heisenbergschen Unschärfe strahlen? Da gibt es noch viele ungelöste Rätsel.“>>

    <<The big challenge, that still faces Renate Loll and the research group she is building up, is to verify that the spacetime now shown to be four-dimensional does in fact aquire smooth Einsteinian properties at large scale.

    Loll: "As soon as that is confirmed, we will be able to move on to explore the region between Planck-scale and macro-scale. There are enough research problems to satisfy anybody here. Does the spacetime structure we are assuming have a cumulative effect, that would leave measureable traces in the light reaching us from the depths of the universe?

    Another critical point: how does quantum gravity describe black holes---objects so compact that no light can escape them, yet which nevertheless radiate by reason of the Heisenberg Uncertainty Relations. there are a whole bunch of unsolved riddles.">>

    http://www.phys.uu.nl/~loll/Web/press/NRCdeutsch.htm
     
    Last edited: Jun 3, 2005
  12. Jun 3, 2005 #11

    marcus

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    as part of keeping track of gossip and real-life details of causal triangulations gravity, we could keep track of who potentially are the researchers who might be bringing out papers, and what the rate of posting is.

    I will use these links, and try to edit out anything they bring up by mistake. Like Lee Smolin's 2003 survey mentions dynamical triagulations but is not really ABOUT that, so I dont 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 (Text):

    2003   3
    2004   4
    LTM    6
     
    here are the authors who have recent CDT articles, or those like Bianca Dittrich who have one in the works already being cited but not on arxiv yet:

    Jan Ambjorn
    Mohammad Ansari
    Bianca Dittrich
    Jerzy Jurkiewicz
    Tomasz Konopka
    Renate Loll
    Fotini Markopoulou
    Johan Noldus (postdoc with Loll at Utrecht)
    Lee Smolin

    this is probably not a complete list. it does not include 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.
     
    Last edited: Jun 3, 2005
  13. Jun 3, 2005 #12

    marcus

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    The Dynamical Triangulations Sex Manual

    The Dynamical Triangulations Sex Manual is mainly to explain the pleasureful maneuvers called

    1. the Einstein-Hilbert action
    and
    2. the Wick rotation



    (relevant passages of the manual will be covered in the thread here called
    Introduction to Loop[-and-allied] Quantum Gravity
    https://www.physicsforums.com/showthread.php?p=589732#post589732
    which is not only about core LQG but lately has also a Causal Triangulations section.)
     
    Last edited: Jun 3, 2005
  14. Jun 5, 2005 #13

    marcus

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    including topology change

    topology change is included in the state sum in these two papers
    by Loll and her graduate student Willem Westra

    http://arxiv.org/find/hep-th/1/au:+Westra_W/0/1/0/all/0/1

    a paper in preparation by Loll and Dittrich giving a CDT black hole model
    must I suppose also deal with topology change

    here is the citation for that paper given in "Reconstructing the Universe"

    [20] B. Dittrich and R. Loll: Counting a black hole in Lorentzian product triangulations, preprint Utrecht, to appear.
     
    Last edited: Jun 5, 2005
  15. Jun 6, 2005 #14

    marcus

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    how can LQG accommodate Loll-style "Triangulations"?

    the common turf is "non-perturbative quantum gravity"---what does that mean?

    the goal of n.-p. QG was recently described as "consistent quantum dynamics on the set of all Lorentzian spacetime geometries"

    As I read it, "Lorentzian" in this context means "causal" or layered.

    In the usual version LQG, the set of spatial geometries appears as a set of connections on a differentiable manifold. At least IMHO, this could be an obstacle. It looks like usual LQG is pre-committed to space being 3D at all scales. At this moment I cannot imagine how the standard-version LQG set of geometries could be generalized to include ones with dynamic, or varying, dimension.

    On the other hand Loop-and-allied approaches includes a bunch of different things and is not permanently anchored to one standard formalism---it tends to be pragmatically definable only by what Loop people do: by what the people who will show up at the Loops 05 conference are interested in and working on at the time.

    In the past twelvemonths we have seen some papers by Laurent Freidel and Etera Livine and others in which spinfoams are modified to be "Lorentzian". It would seem natural for people in the Causal Sets line of investigation to accomodate with causal triangulations (which may even have been inspired by causal sets, to what extent I can't be sure)

    BTW as of 6June recent annual CDT posting
    (see earlier post https://www.physicsforums.com/showpost.php?p=591375&postcount=11 ) has been

    Code (Text):

    2003   3
    2004   4
    LTM    7
     
     
    Last edited: Jun 6, 2005
  16. Jun 6, 2005 #15

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    Hermann Nicolai perspective

    Hermann Nicolai's own research field is string, but he also has a broad balanced vision of quantum gravity and the section of AEI Golm that he directs is one of the most effective QG research institutions in the world.
    Past/present AEI members that you could consider to be AEI "products" include Thomas Thiemann, Martin Bojowald,
    Hanno Sahlmann, Renate Loll, Bianca Dittrich (many other wellknown QG people have passed through there as well). AEI is one place they do string and LQG in a balanced way, along with solid work in numerical relativity etc.

    in QG if somebody is doing something interesting and new there is a substantial chance that if you look at their CV you will find that the person has spent a year or a few years in Nicolai's department of AEI

    in some sense it is Nicolai perspective behind the October Loops 05 conference because he is one of the local organizers but also the other local organizing committee members are his AEI people.

    So even though the guy is a mere string theorist, :wink: I want to pay careful attention to his perspective on QG. Well he just posted an article today on arxiv that gives some in the introduction. It is Nicolai's contribution to Abhay Ashtekar's book "A hundred years of relativity":

    ----quote from Nicolai http://arxiv.org/abs/gr-qc/0506031 -----

    1. Introduction

    As we look back 90 years to take stock of what has been achieved since Einstein explained gravity in terms of spacetime geometry and its curvature, the progress is impressive. Even more impressive is the wealth of structure contained in Einstein’s equations which has been revealed by these developments.

    Major progress has been made concerning

    •Exact solutions (Schwarzschild, Reissner-Nordstrom, Kerr, axisymmetric stationary solutions,...)

    •Cosmological applications (standard FRW model of cosmology, inflationary universe,...)

    •Mathematical developments (singularity theorems, black hole uniqueness theorems, studies of the initial value problem, global stability results,...)

    •Conceptual developments (global structure and properties of spacetimes, horizons, black hole entropy, quantum theory in the context of cosmology, holography,...)

    • Canonical formulations (Dirac’s theory of constrained systems, ADM formalism, Ashtekar’s variables,...)

    • Higher dimensions (Kaluza Klein theories, brane worlds,...)

    • Unified theories ‘beyond’ Einstein (supergravity, superstrings, supermembranes and M(atrix) theory,...)

    Quantizing gravity (perturbative and canonical quantization, path integral approaches, dynamical triangulations, spin networks and spin foams,...)

    All these subjects continue to flourish and are full of promise for further and exciting developments (hinted at by the dots in the above list)....

    ----end quote---

    From this one would say that Nicolai sees quantizing gravity as just one of the main research directions stemming from Gen Rel, and that when it comes to specific approaches to quantum gravity he mentions DT, the spin networks of LQG, and spin foams more or less on the same footing, and then says dotdotdot...

    Gerard 't Hooft is another person whose perspective on QG and Triangulations gravity in particular is worth noting. Coincidentally Renate Loll is at 't Hooft's institute: the ITP at Utrecht is home to both.
    If you have a fast connection and want a 't Hooft perspective here is
    a video (but it is 202 MB)
    http://pitp.physics.ubc.ca/archives/CWSS/showcase/panels.html
    click on the first panel discussion, or download the video directly:
    http://pitp.physics.ubc.ca/archives/CWSS/showcase/panel1.wmv
    Peter Woit reported on what 't Hooft said at this May 2005 conference here
    http://www.math.columbia.edu/~woit/blog/archives/000201.html
     
    Last edited: Jun 7, 2005
  17. Jun 7, 2005 #16

    marcus

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    To update the earlier list, here are authors who have recent CDT articles, or one in the works, like Dario Benedetti and Francesco Zamponi who have a paper with Loll that is being cited but not on arxiv yet:

    Jan Ambjorn
    Mohammad Ansari
    Dario Benedetti (grad student at Utrecht)
    Bianca Dittrich
    Jerzy Jurkiewicz
    Tomasz Konopka
    Renate Loll
    Fotini Markopoulou
    Johan Noldus (postdoc at Utrecht)
    Lee Smolin
    Willem Westra (grad student Utrecht)
    Francesco Zamponi

    some of the authors are on this list of u.U. students
    http://www.phys.uu.nl/~loll/Web/students/students.html
    only recent authors are listed so we miss those like Arundhati Dasgupta who have written CDT papers but before 2003.

    these arxiv links the relevant articles

    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

    I'm unable to identify the graduate student with Loll and Julian Barbour in this picture. Anybody know?

    http://perimeterinstitute.ca/images/marseille/marseille103.JPG
     
    Last edited: Jun 8, 2005
  18. Jun 8, 2005 #17

    marcus

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    That was post #13 of this thread, of 5June.

    By coincidence the day after i posted that, the paper of Dittrich Loll appeared and now I have been reading it and have some impressions. I hope maybe some other PF posters have too.

    http://arxiv.org/gr-qc/0506035

    I would emphasize the word PRODUCT triangulations. Loll and Dittrich are defining a simplicial manifolds concept analogous to FIBER which they spell "fibre". Also they use the terms base and "tower"
    in a product triangulation there is a base triangulations B and on that base there are tower triangluations, like a vertex tower, an edge tower etc.

    Look at pages 6 and 7. So after the definitions are taken care of they have a product triangulation B x F, where F consists of towers over simplexes in B.

    And it turns out that a CAUSAL or Lorentzian triangulations is simply one simple kind of PRODUCT. In other words with the basic multipurpose concept of product triangulation, once you have defined it then it is just a throwaway one-liner to say what causal spacetime triangulations are.

    so that is good, one expects that in good mathematics.

    and this concept of product triangulation is also helping Dittrich and Loll get a handle on black holes-----here the base space B is 2D, one for time and one for radial distance. so they can triangulate their black hole and have a convenient handle on distance from center
     
    Last edited: Jun 8, 2005
  19. Jun 8, 2005 #18

    marcus

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    https://www.physicsforums.com/showthread.php?t=46806

    this thread is about 't Hooft talk at the KITP 25th birthday conference. it has links to audio and slides, also some slides are transcribed to save the reader having to download graphics

    A couple of posts back there was a hint of a Hermann Nicolai perspective on CDT Triangulations approach to quantum gravity. I would guess if you are a grad student and want to do QG then if you go to Uni Potsdam you are very welcome to do that----you dont have to only do string theory or loop gravity. path integral/CDT was the first specific line of QG research that Nicolai listed.

    so we have a glimmer of where AEI-Potsdam is, but I have also been looking for some perspective on Loll's work by 't Hooft. and I have not found anything. Only that in some sense his institute at Utrecht has placed a "BET" on Triangulations by getting Loll there.

    I'll keep an eye out for 't Hooft views of the QG roadmap.
     
    Last edited: Jun 8, 2005
  20. Jun 9, 2005 #19

    marcus

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    this is from Smolin's opinion piece in June2005 "Physics Today". the piece is called "Why no 'new Einsteins'?".

    ----------quote from Smolin June 2005 article------

    ...Alternatives to strings

    More worrisome, young theorists who pursue alternatives to string theory have had great difficulty finding any academic positions in the US. This is true of those who pursue alternative programs in particle physics...and...of those who pursue alternative approaches to quantum gravity such as dynamical triangulations, causal sets, and loop quantum gravity. These subjects are all pursued much more vigorously outside the US, because leading researchers in these areas are drawn to leave US universities...

    ---end quote----

    what I want to call attention to is a sudden shift in priorities among the alternatives and the increased prominence of CDT "triangulations". Remember that in terms of numbers of researchers and annual postings on arxiv, CDT is a much smaller, newer, and until recently less prominent effort than LQG. for whatever reason, when Smolin lists a few QG alternatives he mentions triangulations FIRST.

    This is a small matter, but it reminded me of what I just heard in Hermann Nicolai's perspective on QG. From post #15 of this thread:

    Not only on the same footing (in spite of the fact that CDT is a newer and smaller effort) but he mentions the triangulations approach before the other two.

    BTW the whole Smolin article is well worth reading. He explores why Europe is currently doing better than US at promoting progress in theoretical physics. (overcommitment to string dead end has cost the US lead), and Smolin looks at some policy measures aimed not only at breaking the institutionalized string stranglehold but preventing any one speculative program from monopolizing theory funding and career opportunities in the future.

    Hopefully the Smolin article will be made generally available online.

    Woit's blog discusses the Smolin article "Why no 'new Einstein'?"
    and gives some more and lengthier quotes.
    http://www.math.columbia.edu/~woit/blog/archives/000204.html
     
    Last edited: Jun 9, 2005
  21. Jun 9, 2005 #20

    marcus

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    Smolin's June 2005 "Physics Today" article is worth quoting some. It is 4 pages and towards the end, on page 4, he makes some proposals---here's a sample:

    ------quote Smolin-----
    To prevent overinvestment in speculatative directions that may end up as dead ends, departments should ensure that different points of view about unsolved problems, and rival research programs, are represented on their faculties.


    Research groups should seek out people who pursue rival approaches, and include them as postdocs, students, and visitors. Conferences in one research program should be encouraged, by those funding them, to invite speakers from rival programs. Instructors should encourage students to learn about competing approaches to unsolved problems, so that the students are equipped to choose for themselves the most promising directions as their careers advance.


    Funding agencies and foundations should take steps to see that at every level scientists are encouraged to freely explore and develop all viable proposals to solve deep and difficult problems. Funding should go to individual scientists for individual thought and not to research programs. A research program should not be allowed to become institutionally dominant until supported by convincing scientific proof of the usual kind. Before such proof is demonstrated, alternative and rival approaches should receive encouragement to ensure that the progress of science is not stalled by overinvestment in a direction that turns out to be wrong.

    ----end quote---
     
    Last edited: Jun 9, 2005
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