Quantum Gravity

[Martin]

the aim of this thread is to gather essential information from all quantum gravity-related threads into one, to get some focus

Links to other SFN threads will be provided, and links to outside sources

 

Smolin: How far are we from the quantum theory of gravity?

http://arxiv.org/abs/hep-th/0303185

 

Rovelli: Quantum Gravity Cambridge Press 2004

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

 

Smolin: Invitation to Loop Quantum Gravity

http://arxiv.org/hep-th/0408048

 

Ashtekar Lewandowski: Background Independent Quantum Gravity; a Status Report

http://arxiv.org/abs/gr-qc/0404018

 

Smolin: Scientific Alternatives to the Anthropic Principle

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

 

Rovelli:Strings, loops and others: a critical survey of the present approaches to quantum gravity

http://arxiv.org/gr-qc/9803024

(Plenary lecture on quantum gravity at the GR15 conference)

 

Vaas:The Duel: Loops versus Strings

http://arxiv.org/abs/physics/0403112

 

to be continued

[Martin]

Quantum Gravity Phenomenology conference Feb 4-14, 2004

http://ws2004.ift.uni.wroc.pl/html.html

(some talks are downloadable in PDF at this site, others are posted

at arxiv.org)

 

Vaas: Beyond Space And Time

http://arxiv.org/physics/0401128

 

Vaas: The Inverted Big-Bang

http://arxiv.org/physics/0407071

 

Smolin: Atoms of Space and Time

Scientific American, January 2004

 

NIST fundamental constants website:

http://physics.nist.gov/cuu/Constants/

 

Click "Universal" and you will get

http://physics.nist.gov/cgi-bin/cuu/Category?view=html&Universal.x=74&Universal.y=9

 

which has values for planck units, among other things

Planck Length

Planck Mass

Planck Temperature

Planck Time

 

We discussed the fine structure constant (one of those

which Smolin has suggested may have evolved by

Cosmological Natural Selection or CNS) in this SFN thread:

http://www.scienceforums.net/forums/showthread.php?t=5289&page=1&pp=20

 

CNS is strengthened by Quantum Gravity's elimination of the black hole

singularity so that inflation of a new universe can begin at the bottom

of any black hole

 

Modesto: Disappearance of the Black Hole Singularity in Quantum Gravity

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

[Martin]

The planck scale is so basic to LQG that the flat limit in that approach to quantizing gravity will be apt to be a modified version of Special Rel in which more than just the speed of light appears the same to all observers.

 

this will involve a modification of the energy-momentum relation in SR.

 

predictions based on this are due to be tested by GLAST starting around 2007 and are discussed in several papers such as Smolin's "Invitation" and "How far are we..." links given earlier.

 

some papers about modified Special Rel and related matters:

 

Girelli Livine Oriti:Deformed Special Relativity as an effective flat limit of quantum gravity

http://arxiv.org/gr-qc/0406100

 

Smolin,Jerzy K-G: Triply special relativity

http://arxiv.org/hep-th/0406276

 

Moffat: Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter

http://arxiv.org/astro-ph/0403266

 

Notale: The Pioneer anomalous acceleration: a measurement of the cosmological constant at the scale of the solar system

http://arxiv.org/gr-qc/0307042

 

a seemingly obscure point: doubly and triply special rel are mathematically possible---one can have not just one scale (speed of light) invariant but one can have 2 and even 3 invariant scales (a special length a special energy...) but one cannot have 4 or more. triply special, or 3-scale relativity is the max. could that be what we have?

Chryssomalakos and Okon

Linear Form of 3-scale Relativity Algebra and the Relevance of Stability

http://arxiv.org/hep-th/0407080

[Martin]

Usenet Physics FAQ

http://math.ucr.edu/home/baez/physics/

 

some John Baez links:

 

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

 

How many fundamental constants are there?

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

 

The meaning of Einstein's equation.

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

 

General Relativity tutorial:

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

(includes "Short Course Outline")

 

From Baez Quantum Gravity writings,

an article on Planck scale physics:

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

and a continuation of that "Quantum Quandaries"

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

 

Baez Quantum Gravity seminar notes:

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

[Thales]

Wow, now thats alot of information to trudge through. Good work Martin. i'll get started on it after I finish the latest paper I am working on. Still a while off yet, trying to gain a qualative understanding of the universe is almost as difficult as a quantative one.

[Martin]

Ambjorn Jurkiewicz Loll: Emergence of a 4D World from Causal Quantum Gravity

http://www.arXiv.org/abs/hep-th/0404156

 

 

Gambini Porto Pullin: Realistic clocks, universal decoherence and the black hole information paradox

http://arxiv.org/abs/hep-th/0406260

 

Gambini Porto Pullin: No black hole information puzzle in a relational universe

http://arxiv.org/hep-th/0405183.

 

Fleischhack: Representations of the Weyl Algebra in Quantum Geometry

http://arxiv.org/abs/math-ph/0407006

(goes beyond earlier results of Sahlmann, Thiemann, Lewandowski, Okolow.)

 

three recent ones of Date Hossain:

Genericity of Big Bounce in isotropic loop quantum cosmology

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

 

Effective Hamiltonian for Isotropic Loop Quantum Cosmology

http://arxiv.org/abs/gr-qc/0407073

 

Genericity of inflation in isotropic loop quantum cosmology

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

 

an odd one by some people i havent heard of before:

Corrections to the Planck's radiation law from loop quantum gravity

http://arxiv.org/abs/hep-th/0407072

[Martin]

Wow, now thats alot of information to trudge through. Good work Martin. i'll get started on it after I finish the latest paper I am working on. Still a while off yet, trying to gain a qualative understanding of the universe is almost as difficult as a quantative one.

 

be selective Thales, no need to trudge thru it all.

I gathered the links so they wouldnt get mislaid and I could hopefully find something about future topics as they come up. Sorry if it gave the impression that one person should wade thru everything!

 

while I'm here I will stash some links to Stacy McGaugh's MOND site

(modified newtonian dynamics---to explain galaxy rotation curves w/o dark matter)

http://www.astro.umd.edu/~ssm/mond/'>http://www.astro.umd.edu/~ssm/mond/mondvsDM.html'>http://www.astro.umd.edu/~ssm/mond/'>http://www.astro.umd.edu/~ssm/mond/mondvsDM.html

http://www.astro.umd.edu/~ssm/mond/'>http://www.astro.umd.edu/~ssm/mond/litsub.html'>http://www.astro.umd.edu/~ssm/mond/'>http://www.astro.umd.edu/~ssm/mond/litsub.html

http://www.astro.umd.edu/~ssm/mond/'>http://www.astro.umd.edu/~ssm/mond/

http://www.astro.umd.edu/~ssm/

 

and here is an collection of QG links assembled by a string theory enthusiast who has acquired a taste for LQG as well. the Egyptian artwork is a bonus:

http://wc0.worldcrossing.com/WebX?14@2.gNrQc8lGC5Y.43@.1de0f3cf

I havent examined these links in detail so can't vouch for their overall quality or usefulness but I can see there are some goodies in the batch.

[Martin]

Thales, actually I am wondering now what paper(s) you are reading and what particular qualitative questions about cosmos you have in mind now.

did you start a thread about it? delighted if you'd start a thread about your own focus of interest

 

whoa! I answered my own question, sort of, by checking out the threads you'v been posting----double slit, entanglement, lightspeed, basic concepts.

So I have a rough idea.

 

if you ever have time and want discussion about quantum gravity basics, or an introduction to it, let me know.

[Martin]

in LQG the operators of measuring area and volume are quantized and have discrete spectra. measuring such geometrical quantities can only give as a result certain multiples of the planck area and the planck unit of volume.

the planck units play an essential and pervasive role in LQG, to an extent that the flat limit of the theory may have to be a modified Minkowski space in which one or more other quantities besides the speed of light appear the same to all observers. familiarity with planck units is helpful in understanding Quantum Gravity.

 

NIST fundamental constants website:

http://physics.nist.gov/cuu/Constants/

 

Click "Universal" and you will get

http://physics.nist.gov/cgi-bin/cuu/Category?view=html&Universal.x=74&Universal.y=9

 

which has values for planck units, among other things

Planck Length

Planck Mass

Planck Temperature

Planck Time

 

in a certain sense the Planck units are all one unit. the unit energy is that of a photon with unit wavelength and unit frequency, the unit temperature is the one associated by E=kT to unit energy, the unit mass is the one corresponding by E=mc² to unit energy, the unit of speed (one unit of distance per unit of time) is the speed of light.

 

there is a thematic quote from Italo Calvino from the first paragraph of chapter 3 of "Six Memos" where he recalls a lecture on the Egyptian worldview given by the philosopher George Santayana:

 

"For the ancient Egyptians, exactitude was symbolized by a feather that served as a weight on scales used for the weighing of souls. this light feather was called Maat, goddess of the scales. The hieroglyph for Maat also stood for the unit of length---the 33 centimeters of the standard brick---and for the fundamental note of the flute."

[5614]

now theres a lot and a lot of links, does anyone read ALL of them? if i saw one, two maybe three links, i'd read it, but that would takes hours to go through, which are the best?

[Martin]

now theres a lot and a lot of links, does anyone read ALL of them? if i saw one, two maybe three links, i'd read it, but that would takes hours to go through, which are the best?

 

the short list consists of just one paper

 

Smolin: An Invitation to Loop Quantum Gravity

http://arxiv.org/hep-th/0408048

[Severian]

I personally prefer String Theory to LQG. I am not terribly sure why to be honest, perhaps it is just more of a gut feeling (no pun intended).

 

Perhaps the biggest attraction of string theory is supersymmetry. Supersymmetry has lots of nice features and motivations for both low and high energy. For me, only one of these is sufficient: it is the only (non-trivial) way to extend the Poincare algebra. So far every particle physics theory which has been successfully confirmed by experiment is based on symmetry groups. The question automatically arises 'why these symmetry groups?'. I feel that this will probably be answered by finding that the symmetry of the universe is the largest symmetry we can have without being inconsistent, and the low energy symmetry groups we see now are the remaining fragments left over from some symmetry breaking mechanism. If that is true, then shouldn't space-time be maximally symmetric too, which in turn would naturally lead to supersymmetry.

 

Now while I admit that LQG can be consistent with supersymmetry, it is rather auxilliary. By contrast in string theory, gravity is local supersymmetry - supersymmetry is completely fundamantal.

[Martin]

I personally prefer String Theory to LQG. I am not terribly sure why to be honest' date=' perhaps it is just more of a gut feeling (no pun intended).

 

Perhaps the biggest attraction of string theory is supersymmetry...[/quote']

 

hi Severian, nice to hear from you! As you correctly point out, there does seem to be a debate around the String vs. Loop issue, with some partisanship.

(I consider string and loop to be inevitable collaborators in the search for a quantum theory of gravity---the rivalry is secondary and something of a distraction)

 

We should gather some links about the debate between rival viewpoints.

Here is the Susskind-Smolin debate at Edge:

http://www.edge.org/documents/archive/edge145.html

 

Here is a discussion involving Lee Smolin, Urs Schreiber, Thomas Larsson, and others at the Not Even Wrong site:

http://www.math.columbia.edu/~woit/blog/archives/000072.html

 

Here is a recent perspective on string theory from Warren Siegel, author of a textbook on Quantum Field Theory, and a substantial corpus of string theory research:

http://insti.physics.sunysb.edu/~siegel/research.shtml

 

Yesterday a continuation of the Not Even Wrong discussion:

http://www.math.columbia.edu/~woit/blog/archives/000076.html

---sample---

Serenus Zeitblom: "I suspect that this blog has a large audience of believers in string theory [like me]"

 

Thomas Larsson: "Serenus, while I don't doubt your sincerity, I seriously don't understand how you (and others) can believe in string theory.

 

We all know that string theory makes no hard predictions (hence the name of this forum), but it makes quite a few soft predictions, e.g. supersymmetry, extra-dimensions, new gauge bosons, a negative cosmological constant, new long-range forces (massless scalar particles associated with moduli), etc. The most striking thing with these soft-predictions is that none of them has been confirmed by observation, and some (the negative cosmological constant and perhaps also supersymmetry) may even be in direct conflict with experiments..."

 

Posted by Thomas Larsson at September 2, 2004 06:55 AM

---end quote---

[Severian]

Well, we only have to wait until 2008 to see if supersymmetry is true. If it is, the next big question will be 'How is it broken?'

[Martin]

Well, we only have to wait until 2008 to see if supersymmetry is true. If it is, the next big question will be 'How is it broken?'

 

Meanwhile, let's build up some online LQG sources as a reference or bibliography. (If you want to start a String or a Supersymmetry bibliography in another thread that would be great!)

 

Thales gave me this: Seth Major's page of Quantum Gravity links.

http://academics.hamilton.edu/physics/smajor/resources.html

 

this is the intense green page which has a lot of the links we have already plus others we dont.

[Martin]

Severian reminded us of the Particle Data Group annual

(the most cited pages in all high energy physics)

http://pdg.lbl.gov/

it has tons of data on particles, as the name suggests

here's a more detailed table of contents

http://pdg.lbl.gov/2004/reviews/contents_sports.html

 

earlier I put a link to the NIST fundamental constants site

which gives just one (recommended) value, with error bounds, for

each physical constant

 

the PDG report is far more inclusive and

lists all the measurements made of something

and says who measured it. it is a monument of information

but you may have to download many PDF pages

whereas with NIST it is all immediately available in HTML

and somebody has boiled it down some.

[Dave]

Like to say that this thread has some pretty much invaluable information in it, so I'd like to thank Martin for it. I'm also making the thread sticky.

[Thales]

I'll second that one Dave. Thanks Martin!

[Martin]

thanks also to Dave!

I'm very glad the thread met with his approval.

We can keep it a bibliography (online reference shelf) thread

in line with being a sticky, and start separate threads for

discussion, questions about QG.

[Martin]

Olaf Dreyer

Background Independent Quantum Field Theory and the Cosmological Constant Problem

http://arxiv.org/hep-th/0409048

 

this is a 4-page paper addressing the cosmological constant problem in a new way.

 

here is the author's summary:

"We introduce the notion of background independent quantum field theory. The distinguishing feature of this theory is that the dynamics can be formulated without recourse to a background metric structure. We show in a simple model how the metric properties of spacetime can be recovered from the dynamics. Background independence is not only conceptually desirable but allows for the resolution of a problem haunting ordinary quantum field theory: the cosmological constant problem."

[Martin]

Carlo Rovelli, Daniele Colosi

Global Particles, Local Particles

http://arxiv.org/gr-qc/0409054

 

---from the abstract---

The notion of particle plays an essential role in quantum field theory (QFT). Some recent theoretical developments, however, have questioned this notion; for instance, QFT on curved spacetimes suggests that preferred particle states might not exist in general....

---end quote---

 

 

---from the conclusions section at the end---

 

...the distinction between global and local states can therefore be safely neglected in concrete utilizations of QFT. However, the distinction is conceptually important because it bears on three related issues: (i) whether particles are local or global objects in conventional QFT; (ii) the extent to which the quantum field theoretical notion of particle can be extended to general contexts where gravity cannot be neglected; and furthermore, more in general, (iii) whether particles can be viewed as the fundamental reality (the “ontology”) described by QFT. Let us discuss these three issues separately. ...

 

...Can we base the ontology of QFT on local particles? Yes, but local particle states are very different from global particle states. Global particle states such as the Fock particle states are defined once and for all in the theory, while each finite size detector defines its own bunch of local particle states. Since in general the energy operators of different detectors do not commute ([hr, HR'] nonzero) there is no unique “local particle basis” in the state space of the theory, as there is a unique Fock basis. Therefore, we cannot interpret QFT by giving a single list of objects represented by a unique list of states. In other words, we are in a genuine quantum mechanical situation in which distinct particle numbers are complementary observables. Different bases that diagonalize different HR operators have equal footing. Whether a particle exists or not depends on what I decide to measure. In such a context, there is no reason to select an observable as “more real” than the others.

 

The world is far more subtle than a bunch of particles that interact.

---end quote---

[Martin]

Alejandro Perez posted this, just today:

Introduction to Loop Quantum Gravity and Spin Foams

http://arxiv.org/abs/gr-qc/0409061

 

it is intended to be an entry-level introduction to LQG

in about 40 pages (plus 20-some more for spin foams and bibliography)

he is a young researcher who has postdocked at Penn State with

Abhay Ashtekar and also been at Marseille with Rovelli, so he

has to know his stuff and he tries, I think, to make it as easy

as he can in a few pages

 

here's a snapshot of Alejandro Perez

http://perimeterinstitute.ca/images/marseille/marseille017.JPG

He's the one smiling and looking at the camera. The other person appears to be Martin Bojowald.

 

 

there is no real LQG textbook as yet

 

Smolin's An Invitation to LQG is for physicists already

working in some other field who want to move into LQG or

connect with it in their research---or for grad students in search

of research topics for thesis

 

same with the longer book by Rovelli: Quantum Gravity

except it also has a lot of interesting history, intuitive description,

concrete visual examples, and philosophy of science foundations.

 

but the actual technical meat in Rovelli's book is still for the

grad student or professional theoretical physicist.

 

so you can find "textbook-LIKE" material in rovelli and elsewhere

there still is no real textook, say, for undergraduates.

 

this of Alejandro Perez, that came out today, may help

[Martin]

at the May 2004 Marseille conference

 

 

Carlo Rovelli

http://perimeterinstitute.ca/images/marseille/marseille008.JPG

Rodolfo Gambini

http://perimeterinstitute.ca/images/marseille/marseille010.JPG

Renate Loll

http://perimeterinstitute.ca/images/marseille/marseille011.JPG

Etera Livine

http://perimeterinstitute.ca/images/marseille/marseille015.JPG

Martin Bojowald and Alejandro Perez

http://perimeterinstitute.ca/images/marseille/marseille017.JPG

Thomas Thiemann talking with Renate Loll

http://perimeterinstitute.ca/images/marseille/marseille028.JPG

Phillipe Roche

http://perimeterinstitute.ca/images/marseille/marseille031.JPG

Kristina Giesel

http://perimeterinstitute.ca/images/marseille/marseille038.JPG

Ted Jacobson and John Barrett

http://perimeterinstitute.ca/images/marseille/marseille041.JPG

Kirill Krasnov

http://perimeterinstitute.ca/images/marseille/marseille044.JPG

Daniel Oriti and Alejandro Perez

http://perimeterinstitute.ca/images/marseille/marseille046.JPG

Jerzy Kowalski-Glikman and Fotini Markopoulou at the seacoast near Marseille

http://perimeterinstitute.ca/images/marseille/marseille059.JPG

Jerzy Lewandowski

http://perimeterinstitute.ca/images/marseille/marseille075.JPG

Julian Barbour, Renate Loll, and Don Marolf

http://perimeterinstitute.ca/images/marseille/marseille103.JPG

Dan Christensen

http://perimeterinstitute.ca/images/marseille/marseille117.JPG

Abhay Ashtekar

http://perimeterinstitute.ca/images/marseille/marseille121.JPG

John Baez

http://perimeterinstitute.ca/images/marseille/marseille123.JPG

Jerzy Kowalski-Glikman

http://perimeterinstitute.ca/images/marseille/marseille130.JPG

Viqar Husain

http://perimeterinstitute.ca/images/marseille/marseille146.JPG

 

 

 

here are the thumbnails, showing everybody at the conference

http://perimeterinstitute.ca/activities/scientific/cws/marseille.cfm

 

 

+++++++++++++++++

Giovanni Amelino-Camelia

http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1999/Amelino-Camelia/

Don Marolf

http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Marolf/

John Barrett

http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Barrett/

Sergei Alexandrov

http://schwinger.harvard.edu/~motl/PeoplePixWeb/SergeiAlexandrov1.JPG

[Martin]

The best collection of Quantum Gravity links that I know of on the web

 

http://jdc.math.uwo.ca/spin-foams/

 

it was assembled by Dan Christensen, physics, math, computer science at

University of Western Ontario. Here's his homepage

 

http://www.math.uwo.ca/~jdc/cv/index.html

 

his QG links page has links to all you need to know to get started in

various lines of QG research

including a lot of good explanatory writing by John Baez

(from his "this week's finds" column)

and stuff on massive parallel computation and

computer models of spacetime.

 

Dan Christensen is obviously really smart in both physics and efficient algorithms for computation so he is on faculty in both physics and computer science departments at UWO.

He has co-authored some papers with John Baez.

 

UWO could be the place you want to be if you want to study Quantum Gravy.

[Martin]

someone at oxford has just posted a paper about using cosmic neutrinos to test QG

 

Joy Christian

Testing Quantum Gravity via Cosmogenic Neutrino Oscillations

6 pages

http://arxiv.org/gr-qc/0409077

 

"Implications of some proposed theories of quantum gravity for neutrino flavour oscillations are explored within the context of modified dispersion relations of special relativity. In particular, approximate expressions for Planck-scale-induced deviations from the standard oscillation length are obtained as functions of neutrino mass, energy, and propagation distance. Grounding on these expressions, it is pointed out that, in general, even those deviations that are suppressed by the second power of the Planck energy may be observable for ultra-high-energy neutrinos, provided they originate at cosmological distances. In fact, for neutrinos in the highest energy range of EeV to ZeV, deviations that are suppressed by as much as the seventh power of the Planck energy may become observable. Accordingly, realistic possibilities of experimentally verifying these deviations by means of the next generation neutrino detectors--such as IceCube and ANITA--are investigated."