FQXi announced the prizes for the Essay Contest on the Nature of Time. Results are here.
My essay was not awarded.
I have submitted an essay to the FQXi competition. If you are interested in reading it, click here.
Title: On the Nature of Time – Or Why Does Nature Abhor Deadlocks?
This essay aims at introducing a novel point of view on the nature of time, inspired by a synthesis of three seemingly unrelated concepts: Bergson’s notion of duration, Dijkstra’s notion of concurrency, and Mach’s notion of inertia.
Edit (June 9th 2009): Apparently, the essays on the nature of time are no longer available at the FQXi site. I have made a very few small corrections and modifications in my essay and a new version is available here (pdf file).
Back in 2005, when I started to read more and more papers on quantum gravity, I have found one paper that really interested (and still interests) me a lot, and was the subject of one of my first posts back at my older blog.
- The Continuum Limit of Discrete Geometries, by Manfred Requardt [http://arxiv.org/abs/math-ph/0507017]
It is a well written paper, with lots of interesting mathematics (Gromov’s geometric group theory, random graphs, etc). As far as I know, it has not been much discussed in other blogs.
Recently, Requardt posted another interesting one, which I am presently reading:
- About the Minimal Resolution of Space-Time Grains in Experimental Quantum Gravity, by Manfred Requardt [http://arxiv.org/abs/0807.3619]
We critically analyse and compare various recent thought experiments, performed by Amelino-Camelia, Ng et al., Baez et al., Adler et al., and ourselves, concerning the (thought)experimental accessibility of the Planck scale by space-time measurements. We show that a closer inspection of the working of the measuring devices, by taking their microscopic quantum many-body nature in due account, leads to deeper insights concerning the extreme limits of the precision of space-time measurements. Among other things, we show how certain constraints like e.g. the Schwarzschild constraint can be circumvented and that quantum fluctuations being present in the measuring devices can be reduced by designing more intelligent measuring instruments. Consequences for various phenomenological quantum gravity models are discussed.
I like his writting style and the various points that he covers in his papers with sensible criticism.
I have a new paper accepted for publication in Physical Review B in the area of micromagnetism. Interested readers can access the paper via the arxiv number [0807.1978], already available.
From: Christine Córdula Dantas
Date: Sat, 12 Jul 2008 13:55:38 GMT (1211kb)
Title: Micromagnetic simulations of small arrays of submicron ferromagnetic
Authors: Christine C. Dantas and Luiz A. de Andrade
Comments: 24 pages, 8 figures, accepted for publication in Physical Review B
We report the results of a set of simulations of small arrays of submicron
ferromagnetic particles. The actions of dipolar and exchange interactions were
qualitatively investigated by analysing the ferromagnetic resonance spectra at
9.37 GHz resulting from the magnetization response of con- nected and
unconnected particles in the array as a function of the applied dc magnetic
field. We find that the magnetization precession movement (at resonance)
observed in individual particles in the array presents a distinctive behaviour
(an amplitude mismatch) in comparison to isolated, one-particle reference
simulations, a result that we attribute to the action of interparticle dipolar
couplings. Exchange interactions appear to have an important role in
modifying the spectra of connected particles, even through a small contact
A result still coming out from my short time in software engineering… My contribution was mostly from the mathematical side, and the first steps for making a connection with the MSCs.
I’m writting this post to keep track of books on quantum gravity (and closely related/helpful books). I own only the first one, which I recommend if you are interested in the main conceptual problems of quantum gravity. Regarding the others, I am presently considering purchasing Thiemann and Henneaux & Teitelboim’s books.
Rovelli – Quantum Gravity (errata)
Thiemann – Modern Canonical Quantum General Relativity
Kiefer – Quantum Gravity
Amelino-Camelia – Planck Scale Effects in Astrophysics and Cosmology
Gomberoff – Lectures on Quantum Gravity
Ambjørn et al. – Quantum Geometry: A Statistical Field Theory Approach
Carlip – Quantum Gravity in 2+1 Dimensions
Gambini & Pullin – Loops, Knots, Gauge Theories and Quantum Gravity
Henneaux & Teitelboim – Quantization of Gauge Systems
Rickles et al. – The Structural Foundations of Quantum Gravity
Callender – Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity
There is also Daniele Oriti’s book, which is an edition of several contributions about various approaches to quantum gravity. As it appears, the book has not been released yet. You can learn more about it over at my old blog, Background Independence (scroll down after my two book reviews to find Oriti’s invited post). At that blog you can also find my “Basic Curriculum for Quantum Gravity” (scroll down a little more, after Oriti’s post, or use this direct link to a backup copy of that post, with comments). There, I link to several other helpful books and downloadable tutorials/papers. That list have not been updated. I believe these two papers by Ashtekar are useful recent reviews:
There is also Smolin’s 2006 Lectures on quantum gravity. See here for more details.
I should warn the reader that all that follows is a personal interpretation from a curious person that is attempting to learn new subjects. Your corrections are welcome.
As I have previously mentioned, Garrett Lisi’s work sounds extremely interesting, and any reasonable person expects that it finds its way to a serious critical analysis before far reaching conclusions and speculations take over. There are many issues that need further work. This is what Garrett explicitly have said several times.
Lisi’s paper got attention: a relatively heavy media hype (considering perhaps the title of the paper and the fact that Lisi is not in the academia, and enjoys a different lifestyle) and inumerous discussions all over the blogosphere. Now that things appear to have calmed down (although we can still hear some fresh echoes of “Lisi’s wave”), a new paper by Smolin, citing Lisi’s work, appeared in the scene.
From a first reading of Smolin’s paper, the most interesting aspect of it (as I see it) appears to be the introduction of an extension of the Plebanski action that accepts any large group structure (containing the local Lorentz group), showing that, with a symmetry breaking mechanism, the resulting dynamics is that of a unified Einstein-Yang-Mills, plus corrections. Smolin suggests his framework could be used for building up the dynamical sector of Lisi’s proposal (by applying the group E8 in the proposed extended action), a part of Lisi’s work that has not been worked out satisfactorily. According to Smolin, it would give “a fully gauge invariant action, which has solutions which spontaneously break the symmetry and give, when expanded around, the bosonic part of Lisi’s action, plus corrections”.
Smolin also suggests techniques from LQG, more specifically, the notion of “disordered locality”, as an alternative for the description of fermions and scalars to that used by Lisi (BRST extension of the connection). That part of the paper is not sufficiently clear to me, but while I try to figure it out, I would gladly accept a clarification.
Although Smolin recognises that there are open issues in Lisi’s work, specially whether the three generations of quarks and leptons are realized in the proposed structure of the E8 group, he boldly advances Lisi’s idea into aspects that as far as I was able to follow have not emerged from the whole body of discussions elsewhere. But I guess these ideas have been exchanged privately for some time (as inferred from the acknowledgement section of both Smolin and Lisi’s paper, as well as the mentioning by Lisi in his paper that the “quantum E8 theory follows the methods of quantum field theory and loop quantum gravity — though the details await future work”).
In any case, the main idea of Smolin’s paper seems to be independent of Lisi’s proposal. That is, suppose the E8 group ends up to be the wrong route. And instead one finds another group that realises the unification. Would not Smolin’s framework be consistent enough in order to allow the study of the resulting dynamics of the “correct” group, whatever it is?
So I think we have two separate issues here. One is the validity of Lisi’s proposal first of all. The other is how far Smolin’s work is dependent on it. As I mentioned, it does not appear to be completely dependent on it. He uses Lisi’s proposal as an example of application. But I may be wrong.
In any case, it is of fundamental importance to go through all the necessary details and assumptions systematically before reaching any conclusions.
On that regard, Jacques Distler has posted twice about Lisi’s work on his blog (here and here) where he argues the impossibility of getting all three fundamental particle generations through Lisi’s embedding into the E8 group. What calls the attention is Distler’s purely mathematical argumentation involving group theory representation only.
So it seems that we have reached a very first bottleneck on Lisi’s idea, in which there appears to be concrete technical (mathematical) issues that are absolutely necessary to be cleared up before continuing. Since it is a purely mathematical issue, it is indeed a point of tension that should be released before anything else. Otherwise the whole edifice of unification through E8 symmetry seems to face an early collapse.
This is unambiguous — as far as mathematics is: is Distler right or wrong?
Is Smolin’s results dependent on Lisi’s work?
If you would like to discuss specifics of the first question, please do it here. To the second question, you are invited to go here. These posts are for technical comments only. For general or more qualitative comments, you may use the present post.
As a final remark, I wonder whether those papers are being considered for refereed publication. I think that well moderated blogs can be a remarkable arena where scientific works can be widely discussed. But at the same time they do have a limited scope. Also, the peer review system has its problems; this is well known and acknowledged. It is not clear whether the authors will submit their papers to a journal. So what are we left with?
Not every member of the scientific community, whatever the field, is connected to the blogosphere. And even if they were connected, would it be acknowledged that blogs are the place where a consensus on the validity of a scientific work is to be met? Where should they be met anyway? We live the revolution of information and communication that the internet has brought, so there is a visible change in the scientific arena of discussions. Things are not as before.
In any case, Nature is the final judge. If you have a theory that can be cleary tested, then that is what is needed at the end of the day.
Except, of course, if you have got wrong mathematics to begin with.
Update: Garrett Lisi invites a technical discussion on his paper in this thread over at PF.
We consider pure three-dimensional quantum gravity with a negative cosmological constant. The sum of known contributions to the partition function from classical geometries can be computed exactly, including quantum corrections. However, the result is not physically sensible, and if the model does exist, there are some additional contributions. One possibility is that the theory may have long strings and a continuous spectrum. Another possibility is that complex geometries need to be included, possibly leading to a holomorphically factorized partition function. We analyze the subleading corrections to the Bekenstein-Hawking entropy and show that these can be correctly reproduced in such a holomorphically factorized theory. We also consider the Hawking-Page phase transition between a thermal gas and a black hole and show that it is a phase transition of Lee-Yang type, associated with a condensation of zeros in the complex temperature plane. Finally, we analyze pure three-dimensional supergravity, with similar results
Witten has written some months ago a paper in 3D quantum gravity and the monster group (see previous post here). Apparently, such ‘unphysical’ models may be (are supposed to be?) useful to learn about full quantum gravity.
We study a unification of gravity with Yang-Mills fields based on a simple extension of the Plebanski action to a Lie group G which contains the local lorentz group. The Coleman-Mandula theorem is avoided because the theory necessarily has a non-zero cosmological constant and the dynamics has no global spacetime symmetry. This may be applied to Lisi’s proposal of an E8 unified theory, giving a fully E8 invariant action. The extended form of the Plebanski action suggests a new class of spin foam models.
Notice the citation to Lisi’s recent paper.
I read Lisi’s paper and I’ll read this one. Sounds very interesting indeed, and a quite “fast link” to Lisi’s idea, that is, a connection of that idea with the LQG framework.
If you would like to discuss that paper in technical terms, I’ll be glad to host your comments here. Recall that this is a highly moderated, very high signal to noise ratio blog. Be educated.
Update: A personal view can be found here.
Tegmark’s new paper… I have no comments for the moment. I didn’t read it.
Update: Just finished reading it. Bad philosophy. Sorry.
From a discussion on plagiarism over at Woit’s blog, came out an interesting spin-off discussion. I will not make a guess on what led those people to plagiarism, nor offer suggestions on how to improve the system, but from the comments that I read, there are certain general issues that are difficult to deny:
1- Science is now a highly competitive field (no longer practised by some few enthusiasts)
2- Publish-or-perish system (the present solution to trim the excess from #1)
I believe such a state of affairs will/is make/ing an imprint to science which, although is not enough to hinder the scientific activity in certain areas and for a certain amount of time, is more than enough to obstruct the pathway for new ideas and originality and ponderation. These, along with hard work, are the genuine ingredients for the advancement of science when at the edge of knowledge.
Hence, #1 and #2 are very worrisome and perturbing items, at least in my view. I would very much like to learn argumentations against those very basic statements.
These two important conferences on high energy theoretical physics and quantum gravity are happening this week. The websites are:
Some bloggers/forums commenting on these conferences:
If you would like to comment specifically on the recent Witten’s paper on 3D Gravity (and his related talk at Strings 07), you are welcomed to do so, but only technical comments will be allowed. All comments are subject to moderation. Thanks!
Update: A fascinating discussion on foundational physics, theoretical versus mathematical physics, and the role of mathematics in physics, spinning off from a post about Witten’s paper over at n-Category Café, is worth reading.
Update: Sabine (Backreaction) has a nice (but non-technical) report from the Loops 07 Conference.
Update: Alejandro Satz over at Reality Conditions has two nice reports on Loops 07.
This is an intriguing paper by Padmanabhan:
I saw this paper back in 2004, when it was submitted, but failed to see reactions to it at that time. In the comments section of a post about string theory over at Cosmic Variance, the issue of gravitons came about and made me wonder again about that “semi-classical” work by Padmanabhan. I wonder whether the issues raised by Padmanabhan are indeed correct and if so, how would they be interpreted in string theory.