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The structure of quantum geometry of spacetime at the Planck scale

  1. Sep 7, 2009 #1
    http://img233.imageshack.us/img233/5816/espaotempoquntico1.th.jpg [Broken]

    Two pictures of quantum spacetimes...

    The theory of quantum gravity (LQG) can perform calculations and predictions on the concrete structure of space and time at the Planck scale.

    Equations formulas that describe possible quantum state of geometry of space and time.

    The quantum states of the geometry of space and time are expressed in loop quantum gravity, in terms of loops. These states are the exact solution of the equations of quantum gravity, and where the cross leads to an infinite number of quantum states in loop: enabling an accurate description of the physics of the Planck scale at which space is constructed from bonds but there exist a set of discrete elementary objects, ie, inter-relationships defined space, which is expressed in a framework independent background. In the most important are the links and the topology of relationships, that is, how the loops interact, links to each other so as to give a description of the geometry of space and time at the Planck scale, so relational and dynamics. That is, a pure theory of loops that describe a real aspect of the universe in simple equations, which depends on the relations of the infinite ties together a theory of quantum gravity in loop (LQG).

    The smallest possible area of an intersection is about 10 ^ -66 centimeter square and the smallest possible volume is about 10 ^ -99 cubic of square.

    The nodes of spin networks has one meaning: they correspond to a quantized units of volume in which its structure is space.

    Events of space and time are the processes by which show how to change. The transition from quantum geometry of space, so a spin network can evolve over time, discrete and not continuous, ie, causal relationships between events, so you have light cones, but it have something that matches the moments time.

    So the image of space-time determined by loop quantum gravity agrees with the fundamental principle that there is not things but processes.

    Computer simulations of a universe with one space and one dimension of time - many are small universes, each edge corresponds to a Planck length, in which time to time the size of the universe suddenly jumps, ie, quantum fluctuations of geometry space -time. Each event in space-time is a simple change in geometry of space, that is, according to loop quantum gravity (LQG), or space-time considered the time scale of 10 ^ -43 of a second and a scale length of 10 ^ -33 of an inch: single elementary move.

    Is the theory of loop quantum gravity (LQG) right? We do not know ... will be decided through experiments designed to test the area and volume and other measures of space-time geometry, but their forecasts do not limit what else is there in the universe, how many extra dimensions there are and the fundamental symmetries (supersymmetry).

    Experience: way to test the Planck scale - the Method - using the entire universe as an instrument in order to find out experimentally the structure of 'atomic' space-time through the interference effect arising when waves associated with the photon that are spread by we discrete quantum geometry. Therefore, by detecting the photon, which to travel long distances accumulate effects (interference). And we can not detect the photon that has traveled large fraction of the observable universe? This requires carefully studying the images taken from X-ray and gamma-ray bursts in order to discover experimentally the discrete structure of space-time. That is, try to space-time has an atomic structure on the scale of Planck - will be a good discovery!

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  3. Sep 7, 2009 #2


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    Ola Uno!

    I'm looking at your posts for the first time. I see that you are asking questions related to quantum gravity and that you want discussion of LQG/Spinfoam (which is one of several interesting approaches being worked on.) I see that you have written 4 posts and that the other 3 seem to ask direct questions that we could answer. I think I will try to bring all the questions together here in this forum so we can look at them in one place. For me, the most immediately interesting question is the one you asked in the Computer forum about the possibility of calculating spinfoam amplitudes and doing computer simulations of a small LQG universe. There was a recent paper about this.

    So I will go and fetch the questions from the other 3 posts.
    (This asks about the possibility of doing a computer simulation---and Coin already gave you a good brief answer. They are working on it using a Beowulf cluster at University of Western Ontario, Dan Christensen on the faculty there is one of the co-authors of several papers. I will show you how to find his papers on line, in case you want to look at them. Also J Brunnemann and D Rideout have reported on some other computer effort somewhere else.)

    Here is the list of your posts, so we can see them all together:
    https://www.physicsforums.com/search.php?searchid=1739615 [Broken]
    Besides the question about computer simulation there is one about background---the background independence idea---and also one about quantum fluctuation of geometry.

    Here is your thread about quantum fluctuation (which is one very interesting idea about the origin of the big bang---a fluctuation in a previous spacetime region.)

    In case you happen to live in Brazil, a person on the web who knows about the QG research activity in Brazil is Christine Dantas. She posts here sometimes and also she has a blog. She can be contacted through her blog, which has the unusual name "Theorema Egregium". If she has time to talk with you, it could be helpful.

    Christine used to have a blog called "Background Independence". It was specifically about LQG and related approaches to QG. What happened was the arguments got too hot. There was too much controversy in the comments by other people. This took too much of her energy. (She works for the Brazil space agency and has not much free time.) So she closed that blog (I think it was around 2005) and after a while she opened the Theorema Egregium blog, which is more reserved and avoids controversy. However she herself knows a great deal.

    Anyway what you say about a quantum fluctuation is a possibility that a number of people have studied, but at the present time it is not the leading idea in LQC (Loop Quantum Cosmology). Here is a list of the most highly cited QC (quantum cosmology) papers since 2006.
    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    You will see that the top 50 papers are almost all Loop and almost all "bounce". Ten or twenty years ago quantum cosmology was dominated by other concepts and approaches (Hawking, Vilenkin, Veneziano, ...versions of the quantum fluctuation idea...ekpyrotic...brane-cosmology...bubble multiverse....etc.)

    So if you do the same keyword search for much earlier times, like the 1980s or 1990s you will find more string and other approaches etc etc. But there was a change in the main direction of QC research and Loop Cosmology became the main focus of work. The bounce idea became dominant even for people who were not specifically doing Loop. This doesn't mean it is correct! We don't know. But it currently interests the most people and they work on it. There has been a lot of computer modeling of how the big bang could have resulted from a collapse which bounced.

    If you look at that list of the top 50 papers, you will see that #4 is by Mario Novello
    It is called "Bouncing Cosmologies" and the preprint appeared in February 2008 and it has already been cited 38 times.
    The work was done in Rio at the CBPF (center brazil research physics) in the ICRA branch of that organization (institute cosmology relativity astrophysics)
    Here is the cosmology and gravitation group at the CBPF:
    You will see Mario Novello and you will see Nelson Pinto-Neto.
    I know of these people because of their work in cosmology. I don't know anything about their personalities or how approachable. Christine may know them.

    It is significant that out of 241 papers written after 2006, in the entire field of quantum cosmology, if you list them by citation count (how often other researchers have cited them as references---basically a measure of value and importance to the research community)---this paper by Novello comes up as #4. One of the most highly cited.

    It is not focused on Loop cosmology, which only comes in at pages 81-83. But he obviously knows something about Loop. It is one of the approaches that he reviews. If I lived in Rio I might go down to
    Rua Dr. Xavier Sigaud, 150 - Urca - Rio de Janeiro
    and visit Mario Novello in his office. Or maybe I would write him an email. You never know. Sometimes people like that answer emails. It's possible.
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  4. Sep 7, 2009 #3


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    You asked about computer simulation of quantum gravity and also computer modeling of quantum cosmology.

    I suggested you to look for papers by J Daniel Christensen.

    In case you want to do it, here is how to do the search. Go to arxiv.org
    and at the top there is a button called "search", which will get you this search form

    Type J D Christensen into the author box and press "do search". You will get:

    Of course this will be totally incomprehensible! Obviously. You are a beginner. But do not be frightened by this.
    Christensen has a supercomputer called a "Beowulf Cluster" at his university in Canada, and he likes to do Spinfoam calculations---that is calculations in LQG of the kind of things in your picture, that you copied from somewhere.
    A spinfoam shows how spin networks evolve in time. It is a 4D picture of how the quantum state of geometry evolves in time.
    Each "move" or change in the geometry is localized at some vertex in the spinfoam, and it has a complex number called an amplitude that can be calculated.
    These amplitudes are somewhat (but not exactly) like probability weights, and they can be used to make an average or a weighted sum, or to find the probability of a transition frome one geometry to another.

    But to calculate the amplitudes for the vertexes in a spinfoam is a big job. So Christensen and his co-workers use the computer at the UWO (university of western ontario) to do the calculation.

    And as a technical side remark, they are still using an out-of-date formula for the amplitude---the old Barrett-Crane formula! That must mean that the new formula (by Freidel Krasnov, or by Engle-Pereira-Rovelli-Livine) is hard to implement. They have not changed the computer program over to the new formula.

    When you see J Daniel Christensen's papers listed you will see that the most recent paper is:
    This will give an idea of the difficulty of calculating spinfoams.

    There is a much easier way to approach computer simulations of quantum geometry. Maybe ultimately it will not prove as good as LQG/Spinfoam. But it is simpler and progress has been much more rapid.

    This is the Triangulations approach of Renate Loll. There is even a popular article written about it---for wide audience---in the Scientific American.

    Try the link in my signature at the bottom of this post, where it says "Loll SciAm".
    This is the SciAm article by Renate Loll and her co-workers Jan Ambjorn and Jerzy Jurkiewicz. The full name of the approach is CDT (causal dynamical triangulations).
    The CDT approach is simple to explain, well suited for computer simulations, and Renate Loll writes very clearly. Her group in Utrecht, the Netherlands, runs QG simulations in the computer where little universes come into existence and evolve for a while and then go out of existence. They are able to "get inside" and explore these small primitive universes.
    They can make measurements of some basic quantities inside these random quantum universes. And calculate probabilities, correlations, averages. It is quite interesting.

    Of course people like Christine Dantas and Mario Novello would know about Loll's CDT work just as they would know about LQG.

    J Brunnemann has also done some computer work with LQG

    In particular there was this paper

    But I see now that this is probably not as interesting as the papers by Christensen.
    Last edited: Sep 7, 2009
  5. Sep 7, 2009 #4
    The 2 pictures and most of the opening statement have been taken verbatim from Lee Smolin's Three Roads to Quantum Gravity, Chapter 10, pp.107 to 145.
  6. Sep 7, 2009 #5


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    Is it directed at me? You know, this is where I live and CBPF, the name of the place in the address you mention, is not that far.
  7. Sep 7, 2009 #6


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    No!!!! MTd2, I had no idea you lived in Rio. That's great. Should I have known? Did you say this and for some reason I missed it, or forgot? I was thinking that Uno lives in Rio, maybe.

    Here is a photo and article about Novello, written in 2002.
    http://cienciahoje.uol.com.br/controlPanel/materia/view/1747 [Broken]

    BTW I know nothing about the biases of personalities of Novello and Pinto-Neto. Pinto-Neto seems to have a stringy background. But of course 6 years ago when they had Marcel Grossmann 10 in Rio everybody thought string was the Answer.

    In 2003 Pinto-Neto gave an invited plenary talk to Marcel Ten on the topic of bounce cosmologies.
    For whatever that is worth. I have no idea what he is thinking about now. Could be some idea of his own, off the beaten path.
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  8. Sep 7, 2009 #7


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    Grosquet, thanks for telling me. I wish Uno had put quotemarks "..." around parts that he quoted. I read Smolin's more recent book Trouble with Physics back in 2006 when it came out. But I did not read Three Roads.

    Quite a bit of Uno's post seems to be in his own words. Maybe the quoted sentences and/or phrases are interspersed.
    Last edited: Sep 7, 2009
  9. Sep 7, 2009 #8
    OK... exactly... Lee Smolin and his excellent book Three Roads to Quantum Gravity ... and I consider this information essential to a better understanding of our universe, as it raises the hypothesis of a geometry 'atomic' of spacetime at the Planck scale ... is essential.

    "What is space and time?"
  10. Sep 7, 2009 #9


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    Uno you have quoted from Smolin what is exactly the right question!
    I invite and urge you to read this of Loll:
    http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf [Broken]

    Can you get it on your computer.

    She says that geometry is not a flock of birds, but that it is LIKE a flock of birds, the changing shape in which they organize themselves during flight as each responds to his neighbors.

    She says that geometry is not a web of triangles but it can be APPROXIMATED by an active web of triangles.

    At Utrecht they make small universes come into and go out of existence, in the computer, using this type of approximation.

    Do you have a Portuguese translation of the SciAm? If you have July 2008, I would like a link to it because I have some Portuguese-speaking friends.
    EDIT: the correct Brazil issue is August 2008.
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  12. Sep 7, 2009 #11


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    That July issue is not the right one. The article by Loll came out in the AUGUST 2008 issue in Brazil. (It came out one month later in the Portuguese translation.)

    Universo Quântico Auto-organizado
    Por Jan Ambjørn, Jerzy Jurkiewicz e Renate Loll
    Uma nova abordagem da gravidade quântica, que perdura há décadas, retorna à física básica e mostra como estão dispostos os tijolos que constroem o espaço e o tempo.

    Instead, please try to read the English version. I already gave the link.
    Here is the link again.
    http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf [Broken]
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  13. Sep 7, 2009 #12
    It's a good article... but what led to the discovery of the structure 'atomic' of space and time: the significance of the temperature and entropy of a black hole.

    The conclusion about the quantum geometry of space-time and organization, not to determine the time between points in space-time - need to quantize - where the solution is to make something dynamic and relational, where space-time is seen as dynamic change, not as something fixed, as an evolving network of relationships defined by gravitational fields, where time is described in terms of change in a network of relationships that describes the space...
  14. Sep 8, 2009 #13


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    Yes and no!

    Compare to ordinary quantum mechanics: first Planck derived that radiation energy is quantized w/o knowing that it is quantized not only due to the geometry of the black body but also due to the quantization of energy levels within atoms - and do to the inherent quantization of energy. Planck thought that the quantization somehow comes from geometry and emission; but later it was discovered that quantization of energy applies to each photon w/o looking at its emission process.

    Thermodynamics of black wholes is a hint that the underlying structure should be quantized, but it was (e.g.) an approach like LQG which revealed how this spacetime quanta look like. Hawkings calculation is based on smooth GR, not QG!

    So it somehow compares to the development of ordinary QM.
  15. Sep 8, 2009 #14
    Hi Marcus,

    Although I was born, studied, and lived (until 1994) in Rio, and know the main (physics and astrophysics) centers there, I no longer live there, and also I am quite unconnected with the Brazilian physics and astrophysics community, for a few years now. So, although I have attended previous talks of some of the people that you mention, I do not know them personally at any extent, and vice-versa, so I am not a good source for that kind of information...

  16. Sep 8, 2009 #15


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    Sorry about my various misperceptions. From my perspective you are still the most knowledgeable person I know about QG and cosmology in Brazil (and possibly some other places because of web connections). So I still turn to you for information about the prospects of LQG in Brazil.

    You have sometimes told us about conferences in Brazil where, for example, Smolin has come to speak. I seem to remember that Bojowald (Loop Cosmology) may have given lectures in Brazil but my memory is hazy about this. Has there been any recent conference activity?

    Who in Brazil is doing quantum gravity or quantum cosmology of any sort (not necessarily Loop)? Do any names stand out? I would be interested in people doing string-inspired work as well, but don't want to put you to too much trouble.

    A couple of things impressed me. One was that Brazil hosted the Marcel Grossmann 10 in 2003. As you know MG11 was in Berlin and MG12 was in Paris. It's a big conference. Another thing was that Mario Novello's review paper on Bounce Cosmologies is #4 on the list of top-cited quantum cosmology papers appearing since 2006. For me this puts ICRA on the map. So I want some way to gauge the LQG "temperature".

    Also your Background Independence blog was the only LQG blog on the planet, for the years it existed, but alas it is no more :cry:
    Last edited: Sep 8, 2009
  17. Sep 8, 2009 #16


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    Marcus, why don't you open a blog for LQG?!? I know you love this place, but it is confusing when it comes to specific topics, and they just become buried with non-LQG stuff. BTW, you can open a thread about Lewandowski new paper.
  18. Sep 8, 2009 #17
    Well, I am very flattered by this, but also very disturbed at the same time.

    Yes, Smolin visited Brazil sometime back recently, but I only exchanged a brief email with him at that time; I was unable to meet him in person. Some other people working on LQG came here as well at other times. Whenever I eventually learn about this, I usually post here and/or at my blog.

    Concerning conferences in Brazil, yes, sometimes "big" conferences are placed here. I'm not sure what weights for those decisions: the attractive turistic cities, or some other reason more directly involved with relevance of the work (probably the former is not uncommon).

    See, e.g., the very impressive XXVII GENERAL ASSEMBLY of the IAU, recently in Rio:

    http://www.astronomy2009.com.br/ [Broken]

    There are certainly Brazilians working in string theory and string-inspired work in cosmology. Unfortunately, I do not know how many are abroad and how many work here and who they are exactly, apart from what I have already posted some time back in this forum. The same concerns LQG or other quantum gravity approaches. I'm not certain, but I would guess there are less people in those lines of research than in strings, but this is just a guess.

    Perhaps one that may have this information is Rivelles, who is organizing this

    NEW TRENDS IN QUANTUM GRAVITY II (begins just today).

    program: http://www.fma.if.usp.br/~rivelles/Rede_Tematicos/program.html

    Some Brazilian physics departments that you might want to check out:

    CBPF/Cosmology and gravitation:


    IFT/UNESP: http://www.ift.unesp.br/


    UNB:http://www.fis.unb.br/pesquisa.htm [Broken]


    Brazilian Physics Society: http://www.sbfisica.org.br/

    A recent statistics was raised about the Brazilian astronomy/astrophysics (not physics) community and the data tells that there were 219 published papers in refereed journals in 2008; 17.4% of them were on theoretical cosmology. I don't know how many of these were in quantum cosmology, I'd say the minority, if at all. I believe most of them concern dark energy, f(R) theories, Chaplygin gas models, CMBR, structure formation, inflation, topology... I do not know the data concerning the work on theoretical cosmology by the physics community (and quantum cosmology as a sub-field, no idea). I'd have to do much more research to learn about the Brazilian picture.

    There is an annual workshop that joins together the physics as astronomy/astrophysics communities, so you can have a snapshot on recent work (I'm not certain whether this is representative, but it's a nice conference that I was able to participate and give a talk a couple of times back):


    The Brazilian forum on cosmology is here (in Portuguese), in which eventually one might learn about upcoming conferences in Brazil:

    http://www.astro.iag.usp.br/~cosmobr/forum/index.php [Broken]

    "It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness", Charles Dickens, A Tale of Two Cities.

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  19. Sep 8, 2009 #18


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    This was an enormously informative post. Thanks!
    If the Dickens quote refers to your time running the Background Independence blog, I sympathize. I don't think I was one of those who argued (and made it hard for you) I remember mostly staying out of the fights. It is actually a beautiful quote from Dickens.

    I think you also have published some fiction writing, some novels, yourself.

    Well, it's great to hear of the Rivelles "New Trends" workshop that just started today! I will check it out.

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