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

[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

 

[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.

 

[marcus]

...

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

...

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

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

but Bojowald is special for me because he is NOT visionary. I trust him not jump to conclusions. he still has not, about this thing.
what I like is that I can tell that from where he stands he can see it very clearly but he will not jump at it. his example steadies me.

 

[marcus]

Turns out that Loll and Westra and Masters student of Loll's named Stefan Zohren will be giving a paper 20 July at the big Paris Einstein conference

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

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

Some photos from the Utrecht Inst. of Theor. Physics

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

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

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

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

...

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

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

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


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

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

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

[no abstract available yet]

15h30 - 15h45: Coffee Break

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

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

...

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

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

...

Friday 22 July

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

[no abstract available yet]

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

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

Fay Dowker is one of the featured (Plenary Session) speakers not only at Einstein2005 this month in Paris but also at Loops05 this October in Potsdam.

 

[marcus]

...BTW here is a picture of Fay Dowker. ...http://www.stp.dias.ie/events/2004/causal_sets_photos/WorkshopOnCausalSets-FayDowker-1.jpg

Fay Dowker is one of the ...Plenary Session...speakers not only at Einstein2005 this month in Paris but also at Loops05 this October in Potsdam.

dammit nobody has remarked on how beautiful Fay Dowker is!

do I have to paste this in as an attachment?

 

[selfAdjoint]

dammit nobody has remarked on how beautiful Fay Dowker is!

do I have to paste this in as an attachment?

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

 

[marcus]

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

I agree that three generations would be highly commendable
but we have to wait and see because her kids are only 3 years old and 7 years old

 

[marcus]

this lady does not stop
http://arxiv.org/abs/hep-th/0507012

it's her fourth paper this year

 

[selfAdjoint]

From the abstract:

This shows that one can in principle make sense of a gravitational path integral which includes a sum over topologies, provided suitable causality restrictions are imposed on the path integral histories.

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

 

[marcus]

... The CDT papers proper avoid that criticism because those paths do not exist even in theory in their model, causality is prior to their whole scheme...

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

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

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

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

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

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

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

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

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

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

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

 

[marcus]

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

2003   3
2004   4
LTM    8

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.

 

[marcus]

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]).

 

[marcus]

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---

 

[selfAdjoint]

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).

 

[marcus]

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).

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.

 

[marcus]

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.

 

[marcus]

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.

...>>

 

[marcus]

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...

 

[marcus]

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.

 

[marcus]

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.

 

[marcus]

an effective introduction to Causal Sets is this 2004 set of lecture slides by Dowker

www.dpf2003.org/xx/qg/dowker.pdf[/URL]

the title of the talk is
"Causal Sets as the Deep Structure of Spacetime"

the slides are written-out enough to understand on their own, without the talk. there is an essential reference to a paper by Sorkin et al
Ahmed, Dodelson, Green, Sorkin
[url]http://www.arxiv.org/abs/astro-ph/0209274[/url]
and indirectly to a 1993 Sorkin talk published in 1997
[url]http://www.arxiv.org/abs/gr-qc/9706002[/url]

from Dowker slides one gets the impression that Causets is not a quantum theory yet, it has some probabilities you can calculate but no hilbertspace and no complex amplitudes. seems conceptually nice (if you like discrete finite sets and relations defined on them) but so far not so good for calculating or for imitating General Relativity spacetime dynamics.
however it has a strong appeal for philosophers

Dowker intimates that Sorkin got an amazingly close estimate of the size of the cosmological constant (which if true might be an accident or might mean something) back in 1992 or 1993 just using Causal Sets reasoning.
Causets is still somewhat nebulous as a theory IMO, so this thing about predicting the size of the cosmological constant, even if just an order of magnitude guess, is iffy. Also it is based on a strange picture where Lambda oscillates and we just happen to be in a era when it is E-120.

but even discounting the claim of predicting Lambda, causets is a lively contender for the Nonperturbative Limelight.

 

[marcus]

In case you are interested in that 1997 Sorkin paper mentioned in the previous post

http://www.arxiv.org/abs/gr-qc/9706002
Forks in the Road, on the Way to Quantum Gravity
Rafael D. Sorkin (ICN-UNAM and Syracuse University)
29 pages
Int.J.Theor.Phys. 36 (1997) 2759-2781

'In seeking to arrive at a theory of "quantum gravity'', one faces several choices among alternative approaches. I list some of these "forks in the road'' and offer reasons for taking one alternative over the other. In particular, I advocate the following: the sum-over-histories framework for quantum dynamics over the "observable and state-vector'' framework; relative probabilities over absolute ones; spacetime over space as the gravitational "substance'' (4 over 3+1); a Lorentzian metric over a Riemannian ("Euclidean'') one; a dynamical topology over an absolute one; degenerate metrics over closed timelike curves to mediate topology-change; "unimodular gravity'' over the unrestricted functional integral; and taking a discrete underlying structure (the causal set) rather than the differentiable manifold as the basis of the theory.

In connection with these choices, I also mention some results from unimodular quantum cosmology, sketch an account of the origin of black hole entropy, summarize an argument that the quantum mechanical measurement scheme breaks down for quantum field theory, and offer a reason why the cosmological constant of the present epoch might have a magnitude of around 10^{-120} in natural units.'

Sorkin is one of the invited plenary speakers at the Loop 05 conference at Potsdam in October

it could be an accident but 10^{-120} is about right.

 

[Spin_Network]

an effective introduction to Causal Sets is this 2004 set of lecture slides by Dowker

www.dpf2003.org/xx/qg/dowker.pdf[/URL]

the title of the talk is
"Causal Sets as the Deep Structure of Spacetime"

the slides are written-out enough to understand on their own, without the talk. there is an essential reference to a paper by Sorkin et al
Ahmed, Dodelson, Green, Sorkin
[url]http://www.arxiv.org/abs/astro-ph/0209274[/url]
and indirectly to a 1993 Sorkin talk published in 1997
[url]http://www.arxiv.org/abs/gr-qc/9706002[/url]

from Dowker slides one gets the impression that Causets is not a quantum theory yet, it has some probabilities you can calculate but no hilbertspace and no complex amplituded. seems conceptually nice (if you like discrete finite sets and relations defined on them) but so far not so good for calculating or for imitating General Relativity spacetime dynamics.
however it has a strong appeal for philosophers

Dowker intimates that Sorkin got an amazingly close estimate of the size of the cosmological constant (which if true might be an accident or might mean something) back in 1992 or 1993 just using Causal Sets reasoning.
But causets is not really a numerical theory yet, so this thing about predicting the size of the cosmological constant, even if just an order of magnitude guess, is iffy. Also it is based on a strange picture where Lambda oscillates and we just happen to be in a era when it is E-120.

but even discounting the claim of predicting Lambda, causets is a lively contender for the Nonperturbative Limelight.[/QUOTE]

What I like about the images, you can see Fay embedded into the background of the photographic paper, taking photo's! :!) that..is she taking photo's of slide's, or is she really taking photo's of herself? :rolleyes:

I think its a deliberate action!..a precise action..a double take!

 

[marcus]

Definitely deliberate. hip lady. your term "double take" is nice. I assume you saw that snapshot of Fay posing at the blackboard---had a link to it around post #54 back a ways in this thread.

https://www.physicsforums.com/showthread.php?p=665506#post665506

Let's take a closer look at the program at this month's Paris conference,
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 »
...
14h00 - 14h40: Brian Greene « The State of String Theory»
...
14h45 - 15h30: Alain Connes « Noncommutative geometry and physics»
...
...
15h45 - 16h25: Fay Dowker « Causal sets and discrete spacetime. »

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

Friday 22 July
...
15h45 - 16h45: Gerard t'Hooft « Conclusion Talk »
----------------------------------------------

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 ... speakers not only at Einstein2005 this month in Paris but also at Loops05 this October in Potsdam.

 

[marcus]

Last update was 20 july, time to update again

20 July Paris conference Zohren presented a Loll-Westra-Zohren paper about "sum over topologies"---including topology change in the nonP quantum gravity path integral

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/
possibly at this page
http://loops05.aei.mpg.de/index_files/Programme.html
there has already been some understandable delay.


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

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 mostly 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

http://www.phys.uu.nl/~helios/commissies.php#kas (Jaap)

Here is Loll homepage:
http://www.phys.uu.nl/~loll/Web/title/title.html

A clear brief non-technical description of CDT approach, written by Loll for general audience is here
http://www.phys.uu.nl/~loll/Web/research/research.html

CDT coverage in press here:
http://www.phys.uu.nl/~loll/Web/press/press.html

A commissioned general audience article on CDT, in English, is expected to appear in the forthcoming issue of Contemporary Physics
and should be listed here when it comes out
http://www.ingentaconnect.com/content/tandf/tcph
(unless the publisher is forgetting to update the website)

 

[marcus]

we still have a scarcity of popular English-language media coverage of triangulations gravity, compared with what's available in Dutch and German. So I translate general audience science journalism stuff sometimes.

Here is this popular Austrian magazine article from November 2004
http://science.orf.at/science/news/130119

As for the brow-level (hi-brow, middle, low-brow) get a load of this
http://orf.at/
it definitely looks Cross-section Austrian to me---sports, politics, celebrities, photomodels, online lottery, advertisments

And yet in their science section
http://science.orf.at/science/
which looks mass-audience enough
they had this thing about CDT last november.

what did Americans know about CDT last november?
Well maybe we will catch up. Something in newsweek? or the NYT? I didnt see it yet.

Anyway, I better translate at least a sample from this Austrian magazine article---anybody who wants to help, please do. Just post a chunk and I will merge it in with mine.

 

[marcus]

Here is a short exerpt from the article by Robert Czepel, 11 November 2004, for ORF.at "World of Science" section, as a sample of CDT coverage in Austrian popular press. I will just translate the beginning paragraphs:
http://science.orf.at/science/news/130119

HOW TO BUILD A FOUR-DIMENSIONAL UNIVERSE

With the help of so-called Quantum Gravity, physics researchers are trying to bring the large and small alike under one roof. ["under one hat", as the hat-loving Austrians like to say]. Until now this effort has been flawed, in that the theory could not explain why the universe's spacetime exhibits exactly four dimensions. Three physicists have now, with the help of Quantum Gravity, been able for the first time to establish why that is so.

A team led by Renate Loll at the University of Utrecht has succeeded in this by integrating a simple principle of cause and effect into the equations of the model. In the medium-scale world of everyday life, it is hardly a surprise that events have causes. But applied to the model of Quantum Gravity, this principle frankly works wonders.

CAUSALITY - A DISCOVERY?

Is causality an inherent property of our universe, or only an illusion which emerges from how we interpret the world?

The Scotish philospher David Hume proposed the latter view. In his opinion, we can never do more than merely observe that two events A and B regularly follow one another, and cannot say why the cause A leads to the result B.

According to Hume's sceptical interpretation the notion of causality is an invention of our minds, which has no counterpart expressed in the real world.

[Czepel has a link to an online encylopedia of philosophy here]

LARGE AND SMALL IN A SINGLE THEORY

A current study by the three physicists comes close to suggesting that Hume might just have been a little too skeptical. Renate Loll at the University of Utrecht, together with two collaborators, has designed a model which describes space and time as an assemblage of extremely small quantum particles.

[this is not quite true but Czepel is a journalist]

This is already of considerable interest because the principles of large scale physics (General Relativity Theory) on the one hand, and small scale physics (Quantum Mechanics) on the other, have until now scarcely been brought under one roof.

The two theories, although both well verified by experiment, do not immediately let themselves be "married"---that is, combined in a single coherent theoretical concept...

...
...
[there is lots more in case anyone else is curious, or wants to translate it for us.)

 

[marcus]

Last update was 8 august, time to update again

At the July Paris ("Einstein Century") conference Stefan Zohren presented a Loll-Westra-Zohren paper about "sum over topologies"---how to incorporate topology change into the nonperturbative quantum gravity path integral. Here's a snapshot of Stefan

[PLAIN]http://www1.phys.uu.nl/wwwitf/fotopagina's/Studenten/Stefan%20Zohren.htm[/URL] [/PLAIN]

there has been some delay in posting the schedule for October's Loops 05 conference. It should eventually appear at this site:
http://loops05.aei.mpg.de/
possibly at this page
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.

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 mostly 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

http://www.phys.uu.nl/~helios/commissies.php#kas (Jaap)

[PLAIN]http://www1.phys.uu.nl/wwwitf/fotopagina's/Studenten/Stefan%20Zohren.htm[/URL] [/PLAIN]

here is the whole staff of the Utrecht ITP (inst. theor. phys.) where Loll and co-workers and students are based.
http://www1.phys.uu.nl/wwwitf/Members/members.staff.htm

Here is Loll homepage:
http://www.phys.uu.nl/~loll/Web/title/title.html

A clear brief non-technical description of CDT approach, written by Loll for general audience is here
http://www.phys.uu.nl/~loll/Web/research/research.html

CDT coverage in press here:
http://www.phys.uu.nl/~loll/Web/press/press.html

A commissioned general audience article on CDT, in English, is expected to appear in a forthcoming issue of Contemporary Physics
and should eventually be listed here
http://www.ingentaconnect.com/content/tandf/tcph

We have a PF a thread polling people's predictions about whether the QG path integral including sum over topologies will prove extendable from the 2D case up to 3D and 4D. the thread is here:
https://www.physicsforums.com/showthread.php?t=81626

 

[marcus]

this thread is to keep track of the news/current events/essential links about Loll triangle gravity. (CDT path integral)
see preceding post for basix

something interesting I just became aware of:

people who have been watching CDT (since that April 2004 paper) know that one of the surprises is that microscopically the CDT spacetime continuum has this "fractal-like" highly non-classical stucture with dimensionality going down steadily from macro 4D down to 3.5 down to 2.9 etc etc down to (at very small scale) sort of 1.9 to 2.1----roughly 2D.

this is measured by creating a spacetime by Monty Sims in the computer and then RUNNING A RANDOM WALK DIFFUSION PROCESS in that spacetime
the dimension you measure that way is called the "spectral" dimension.
to distinguish it from the "hausdorff" dimension that you measure by comparing radius and volume.

this business is summarized in the paper I've had in my sig for much of this year:
http://arxiv.org/hep-th/0505154
and also in this other short paper http://arxiv.org/hep-th/0505113

So probably others, I for one at least, were thinking it was pretty weird that Loll gravity has a fractally wrinkly feathery kinky smallscale spacetime structure.

the thing about it is that with Loll gravity you DON'T PRESUPPOSE HOW SPACETIME IS GOING TO LOOK, you set up a dynamic action principle of how little spacetime chunks arrange themselves together and stick together and LET IT RUN and SEE what kind of continuum comes out, whatever comes out that's it, and if it doesn't look and act like our space macroscopically, then you did something wrong.

so you can't be sure you are going to get a nice smooth spacetime or even something that is four dimensional! for some 10 years they were trying to do this and the dimension kept coming out really wrong. But now by the grace of nature and the coolness of Renate Loll you do get something that is largescale 4D. however as we learned earlier this year, it is kinky at microscopic scale

OK NOW SOMEONE USING AN ENTIRELY DIFFERENT APPROACH at the University of Mainz, a guy named Martin Reuter HAS ALSO FOUND A KINKY FRACTALLY MICROSCOPIC STRUCTURE

he is coming from an entirely different direction and he says he has "MADE CONTACT" with Loll results.

so I am paying attention to this and emphasizing it. I think it could be significant.
Here is Martin Reuter paper that came out today. I already put it on the Rovelli thread

http://www.arxiv.org/abs/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity
O. Lauscher, M. Reuter
20 pages

"Four-dimensional Quantum Einstein Gravity (QEG) is likely to be an asymptotically safe theory which is applicable at arbitrarily small distance scales. On sub-Planckian distances it predicts that spacetime is a fractal with an effective dimensionality of 2. The original argument leading to this result was based upon the anomalous dimension of Newton's constant. In the present paper we demonstrate that also the spectral dimension equals 2 microscopically, while it is equal to 4 on macroscopic scales. This result is an exact consequence of asymptotic safety and does not rely on any truncation. Contact is made with recent Monte Carlo simulations."[/color]

Reuter approach is so different that the fact that the two approaches CONVERGE and actually AGREE on something makes me wonder if maybe in some sense REAL spacetime continuum could be micro-fractally, and only have the illusion at large scale, that we see, of having 4 nice dimension

in other words wondering if maybe it is not just the MODEL but the actual real continuum that is like this---but what that could mean is a relational thing. spacetime is the web of spatial relations and causalities among things and for it to be microfractally must have to do with how things can relate to each other at very close quarters---how they can be on top of each other and beside and between and inside and around, but at very small scale. these possibilities of relationship ARE space. and if Loll and Reuter picture is true (which would be pretty surprising because physics theories turn out so often to be false) then these possibilities of relation would have to be DIFFERENT at very small scale than over larger reaches.
well it is a lot of stuff to be puzzled with.

 

[arivero]

The original argument leading to this result was based upon the anomalous dimension of Newton's constant.

Yep, this makes such kind of results intuitive. G is an area, as Fermi constant. All the others are adimensional.

 

[Sam Owen]

What is meant by spacetime is a fractal, fractal like or has a kinky fractally structure ?

I know what a fractal is and what they look like, so is it an appearance of fracticality or actually fractal and does the reductablity of the pattern ever stop at a cut off volume ?

also how does one get half a spatial dimension or half a temporal dimension surely a half is still a whole when talking of dimensions ?

 

[marcus]

In her papers (like e.g. http://arxiv.org/hep-th/0505154 and the earlier http://arxiv.org/hep-th/0505113 ) Loll says "fractal-like". and "evidence of a fractal structure" --- she never says that spacetime actually turns out to BE a fractal. I think she is speaking more carefully than Reuter---who says his picture predicts that at small scales it IS a fractal. I think he is using word in a more general sense or just being careless. but I do not know reuter work at all well, so cannot be sure. Let us assume that he also means "fractal-like"

I am starting an auxilliary thread to talk about what fractal-like
versus just plain "fractal"
could mean. That way, discussing what is a fractal or what it means to say something is "like a fractal" will not gum up this CDT news-and-gossip thread.

Here is the auxilliary thread
https://www.physicsforums.com/showthread.php?t=86751

I hope it helps get some answers about what is means to not actually be a fractal but to be "like a fractal".

 

[marcus]

Hanno Sahlmann (the Sahlmann algebra is basic in LQG) is now a postdoc at Utrecht ITP
http://www1.phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Hanno.htm
(earlier he was at AEI-Potsdam and at Penn State, if I remember correctly.
Here is Hanno's Penn State webpage:
http://cgpg.gravity.psu.edu/people/igpg_hsahlmann.shtml )

Sergei Alexandrov is at Utrecht as well. He has done both string and LQG research. Some of his LQG research was with Etera Livine (now at Perimeter). Sergei Alexandrov used to be at Paris. after PhD he came to Utrecht.
http://www1.phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Serguei.htm

Utrecht has a capability in at least 3 kinds of quantum gravity research: string, LQG and CDT (Renate Loll's specialty). this is interesting because not very many places have this kind of diversity.


Stephon Alexander is now faculty in Abhay Ashtekar's department at Penn State (he was a postdoc at SLAC-Stanford)

 

[marcus]

because of Fall term changes, here's a CDT update:

schedule for October's Loops 05 conference should eventually appear here:
http://loops05.aei.mpg.de/

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

I believe there is a paper in the works by Loll with Dario Benedetti and Francesco Zamponi.

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

Here are some snapshots mostly 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

http://www.phys.uu.nl/~helios/commissies.php#kas (Jaap)

http://www.phys.uu.nl/~0404985/ (Stefan Zohren, alt. at Aachen)

here is the whole staff of the Utrecht ITP (inst. theor. phys.) where Loll and co-workers and many of her students are based.
http://www1.phys.uu.nl/wwwitf/Members/members.staff.htm

Here is Loll homepage:
http://www.phys.uu.nl/~loll/Web/title/title.html

A clear brief non-technical description of CDT approach, written by Loll for general audience is here
http://www.phys.uu.nl/~loll/Web/research/research.html

An excellent survey article for wide audience ("The Universe from Scratch") is here
http://arxiv.org/hep-th/0509010

CDT coverage in press here:
http://www.phys.uu.nl/~loll/Web/press/press.html

We have a PF a thread polling people's predictions about whether the QG path integral including sum over topologies will prove extendable from the 2D case up to 3D and 4D. the thread is here:
https://www.physicsforums.com/showthread.php?t=81626

It is interesting that Hanno Sahlmann is now listed as postdoc at Utrecht. Here is his earlier web page, describing his research interests, with sketches of ideas, from when he was at Penn State:
http://cgpg.gravity.psu.edu/people/igpg_hsahlmann.shtml .

 

[marcus]

...It is interesting that Hanno Sahlmann is now listed as postdoc at Utrecht. Here is his earlier web page, describing his research interests, with sketches of ideas, from when he was at Penn State:
http://cgpg.gravity.psu.edu/people/igpg_hsahlmann.shtml .

Hanno Sahlmann postdoc is working for Loll (!)

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

 

[marcus]

it looks like one of the best available LQG postdocs has just crossed the border over into CDT. I do not think this is bad luck for LQG, on the contrary it could be good luck.


hanno casts a wide net of ideas. the LQG uniqueness theorem associated with Lewandowski Okolow Sahlmann Thiemann actually started out with two solo papers by Hanno.

then it was picked up by Lewandowski Okolow (who called the algebra the "Sahlmann algebra") and carried further by them and by Sahlmann-Thiemann collaboration. So he is able to work at an efficient level of abstraction and connect the dots. It seems exciting to me that hanno should decide to work postdoc with Loll. things could come of it that one didn't expect. I don't want to say that he might see a connection between some variant of LQG-spinfoam and CDT because maybe that is the wrong idea to be thinking. but something valuable is likely to come from his crossing the border. I think it is a lucky move, him going to work with Loll!



OTHER NEWS BIANCA DITTRICH HAS JUST MOVED FROM POTSDAM TO PERIMETER

if she decides to do some CDT while she's there it will make a stronger CDT presence at Perimeter.

 

[marcus]

Frank Saueressig is also at Utrecht

an interesting group of people is assembled at Utrecht (at the ITP and the Spinoza Institute there)

Sahlmann has been giving seminars about LQG
and Saueressig about (Reuter asympt. safe) QEG

Saueressig was at Mainz working with Reuter in 2001, for instance
http://www.arxiv.org/abs/hep-th/0110054
Renormalization Group Flow of Quantum Gravity in the Einstein-Hilbert Truncation
M. Reuter, F. Saueressig
58 pages, 24 figures
Phys.Rev. D65 (2002)

mostly by cleverness, I suppose, the Utrecht people have assembled a Quantum Gravity group that is strong in LQG/CDT/QEG
It would be difficult to do it just by spending money.

In 2002 Saueressig went to Jena, in 2003 he was still collaborating with Reuter:
http://www.arxiv.org/abs/hep-th/0311056
Nonlocal Quantum Gravity and the Size of the Universe
M. Reuter, F. Saueressig

In spring 2004 he was still at Jena, and doing some string papers with Thomas Mohaupt, but in fall 2004 he was already at Utrecht.

some science historian could someday write how this Utrecht combination was assembled (if they get results) and by what chain of decisions.

Here are recent seminar talks there:
http://www1.phys.uu.nl/wwwitf/Seminars/seminars.html
http://www1.phys.uu.nl/spinoza/seminars.html
http://www1.phys.uu.nl/spinoza/seminars/Grafiti.html

especially in this last link notice 12 Sept Renormalization Group improved cosmologies: no strings attached by Frank Saueressig, and two seminars on Loop Quantum Gravity on 19 Sept and 26 Sept given by Hanno Sahlmann Loop Quantum Gravity, an (un-)conventional QFT

You see also that Bojowald spoke in March, and Kiril Krasnov in April, and Renate Loll in June (Kiril Krasnov, of Nottingham, does Laurent Freidel type stuff---more or less simultaneous with Freidel)

So if Loll wants to talk to someone about QEG and fractally smallscale structure of spacetime, she does not have to email Reuter at Mainz, she can just walk down the hall to Saueressig. If there is any new development in LQG, Sahlmann can tell her about it. very good place. I will get a picture of Frank Saueressig to help thicken the soup.

 

[marcus]

Here is Frank Saueressig 12 September talk abstract:

Renormalization Group improved cosmologies: no strings attached

"It is generally believed that in the very early universe, at times smaller than the Planck time, quantum gravity effects play a crucial role. Since the physics in the Planck era prepares the initial conditions for the subsequent classical evolution of the universe it is desirable to gain some understanding of the quantum gravitational processes which took place immediately after the Big Bang. Based on the Einstein-Hilbert approximation of asymptotically safe quantum gravity we discuss a consistent renormalization group based framework which allows for the inclusion of such quantum gravitational effects into the cosmological field equations and applies to all stages of the cosmological evolution. The very early universe is found to contain a period of "oscillatory inflation". Later on the cosmological evolution is classical and asymptotes to a de Sitter era at cosmologically late times."


Here is a snapshot:
http://www1.phys.uu.nl/spinoza/members/Frank.htm

 

[marcus]

More Utrecht gossip:

Abhay Ashtekar will be teaching a course there Spring 2006 term.
http://www1.phys.uu.nl/wwwitf/courses/main.htm

It is a regular course for credit, with exam. Now is fall semester which runs thru the first week of January. So he is teaching NEXT semester which goes
February--June 2005

The course is Black holes in fundamental physics
lectures are Wednesday 9-11, with a tutorial on Monday.
A course description is available at the above link.

what intelligence is getting all these people together? As I said earlier, I don't think you could do it with just money---no matter how well funded Harvard or Princeton are, I don't think they are currently in position to assemble such a group. Or Stanford or UCSB-Kavli. What good is cash if you hire the wrong people? It is good luck for the graduate students at Utrecht---they can see several paths to QG where progress is being made and there are suggestions of convergence.

 

[Timbuqtu]

It's good to hear such positive things about Utrecht. I'm one of the students who enrolled in the Master's programme in Theoretical Physics in Utrecht this year. And I really enjoy the courses taught at the moment (QFT, Statistical Field Theory, GR) and I'm looking forward to next semester's courses (Cosmology, String, Black holes, Standard model, etc.). Certainly I will attend Ashtekar's course next semester.

Keep up the good work, Marcus! Your announcements are really useful.

 

[marcus]

.. And I really enjoy the courses taught at the moment (QFT, Statistical Field Theory, GR) :

interesting, if you go to GR lecture at 9 Thursday morning then you may be hearing classical GR from someone who has an idea of what could replace it.

 

[Spin_Network]

I would give anything to get into the lecture by :prof. dr.ir. P. Grassberger,:NS-TP450M: Kramers course: Phase transitions and anomalous scaling in non-equilibrium systems

then:NS-TP453M: Soft condensed matter theory:lecturer : dr. R.H.H.G. van Roij,

Great place to be!

 

[marcus]

Timbuqtu, I have a suggestion for you
the October 10-14 conference at potsdam could be historically significant
it might IMHO be of interest to you later to have been there and seen it.
I suggest you consider doing this:
go to Loll, or Westra and say "can you get me in as an observer to this conference? I know the registration is past, but i think it might be
a memorable conference and I would like to have seen it. would that work?"
http://loops05.aei.mpg.de

 

[marcus]

Perimeter is inviting applications for postdoc positions for 2006
http://www.perimeterinstitute.ca/index.php?lang=en
application must be done online (they supply a link)
and the deadline---in November 2005---will be extended until
all positions are filled.

 

[Timbuqtu]

interesting, if you go to GR lecture at 9 Thursday morning then you may be hearing classical GR from someone who has an idea of what could replace it.

It is interesting . But let me ask you something else: can you imagine Gerard 't Hooft, being one of the big string opponents, teached String theory for couple of years (until 2003/2004) here?

Timbuqtu, I have a suggestion for you
the October 10-14 conference at potsdam could be historically significant
it might IMHO be of interest to you later to have been there and seen it.
I suggest you consider doing this:
go to Loll, or Westra and say "can you get me in as an observer to this conference? I know the registration is past, but i think it might be
a memorable conference and I would like to have seen it. would that work?"
http://loops05.aei.mpg.de

Probably it's too late now (Potsdam is not exactly around the corner). But there will be enough oppurtunaties to see (some of) these people around here I think. And I guess I wouldn't understand a lot of what they are saying. But it sure must be impressive to see all these guys together.

 

[marcus]

It is interesting . But let me ask you something else: can you imagine Gerard 't Hooft, being one of the big string opponents, teached String theory for couple of years (until 2003/2004) here?

Yes Timbuqtu!
I think Gerard 't Hooft should be the perfect person to teach a course about String ideas.

I think that to all approaches to bring quantum physics together with GR he would be encouraging them to do their best. but I think he is also realistic and I like the quote where he is explaining that String ideas are not yet a Theory----that there is so far no scientific theory called String Theory, that makes predictions by which it can be tested---and he says in his book In Search of the Ultimate Building Blocks:

Actually, I would not even be prepared to call string theory a “theory” rather a “model” or not even that: just a hunch. After all, a theory should come together with instructions on how to deal with it to identify the things one wishes to describe, in our case the elementary particles, and one should, at least in principle, be able to formulate the rules for calculating the properties of these particles, and how to make new predictions for them. Imagine that I give you a chair, while explaining that the legs are still missing, and that the seat, back and armrest will perhaps be delivered soon; whatever I did give you, can I still call it a chair?[/color]


https://www.physicsforums.com/showthread.php?p=755232#post755232

https://www.physicsforums.com/showthread.php?p=761411#post761411

My impression of 't Hooft (please tell me if I am way off the mark) is that he wants people to get a quantum theory of spacetime, AND HE DOES NOT CARE WHO, OR HOW, OR BY WHAT METHOD IT COMES. So he is not protecting the interest or prestige of this method or that method, or the authority of this person or that person. I like to imagine that 't Hooft says to all the methods "go for it! go get 'em". And if this is his attitude and also the attitude of De Wit, then those people stand a good chance.]

 

[marcus]

here is a general question to anybody

Renate Loll has a new snapshot of herself at her website

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

and she is holding a doll.

Identify this doll.

 

[Kalimaa23]

Isn't that the Einstein action figure they realized for the 2005 World Year of Physics?

 

[marcus]

Isn't that the Einstein action figure they realized for the 2005 World Year of Physics?

Thank you Dimitri! Stupid me, I could tell it was Einstein but I didn't know there was an Official Horrible Taste Kitsch doll created especially for the World Year of Physics. what will they think of next.

 

[marcus]

this photo troubles me

maybe it is a good photo. what do you think? anybody.

what I see in the person at that moment is a mixture of
dignity and impudence

and maybe the picture is iconic, or symbolic of a true relationship, that already at this moment we (in combination with her) hold Gen Rel in the palm of a new model.
and that Gen Rel is the artifact of the quantum theory we watch developing