Nature Precedings

Nature now has a service of preprints much like arXiv for biology, chemistry and earth sciences:

Nature Precedings is a place for researchers to share pre-publication research, unpublished manuscripts, presentations, posters, white papers, technical papers, supplementary findings, and other scientific documents. Submissions are screened by our professional curation team for relevance and quality, but are not subjected to peer review. We welcome high-quality contributions from biology, medicine (except clinical trials), chemistry and the earth sciences.

The address: http://precedings.nature.com/

Delta Functions

An interesting paper about history of physics:

• On the Dirac-Infeld-Plebanski delta function – O. Rosas-Ortiz

SOC and Gravity

Sometime ago, I found these interesting papers about self-organised criticality and quantum gravity:

• Cosmology as a problem in critical phenomena – L. Smolin
• Causal evolution of spin networks – F. Markopoulou, L. Smolin
• Propagating spin modes in canonical quantum gravity – R. Borissov, S. Gupta
• Self-organized criticality in quantum gravity – M.H. Ansari, L. Smolin

In the first two, self-organised criticality is introduced as an idea of how the physical constants could have their current values without any external fine tunning, once that the universe should be self-contained. Self-organised systems naturally evolve to a critical state where any perturbation sufficiently large to drive the system away from one of its metastable states is dissipated by avalanche phenomena ocurring without a typical scale . Then, the paper by Borissov and Gupta (which I will call BG in the following) introduces a very simple model of a trivalent spin network with a discrete dynamics inspired by the sandpile model of Bak, Tang and Wiesenfeld appearing in the first paper about SOC:

 

• Self-organized criticality: An explanation of the 1/f noise – P. Bak, C. Tang, K. Wisenfeld

The main feature of SOC is the scale invariance of the avalanche phenomena which is reflected in the power-law distribution of their sizes and durations. The paper by Ansari and Smolin (AS) continues to explore different dynamical rules in the trivalent spin network introduced in BG. Although BG doens’t verify the existence of SOC in the analysed rules, AS does, showing that this is a valid possibility. The only other paper I found about the subject was very too similar to AS

 

• Self-organized critical behavior: the evolution of frozen spin networks model in quantum gravity – J.-Z. Chan, J.Y.Zhen

As far as I know there are no further developments.

 

An interesting feature of SOC is that it accomodates very well the possibility of macroscopic non-locality in spin networks of the kind that appears here:

 

• Disordered locality in loop quantum gravity – F. Markopoulou, L. Smolin

SOC is very robust to this kind of non-locality which, in a square lattice for example, could be viewed as an interaction beyond first neighbours.

 

This is an interesting path to be followed as SOC seems to me a more reasonable mechanism than the anthropic principle and a simpler scenario than cosmological natural selection. It is however fair to say that SOC is not completely understood and not rigorously defined as well and, as usually happens, both fields could benefit from the interaction. I am trying to work a little on that, but with so many replica calculations to do in my present job, it is gonna take a little while.

 

For those interested, a basic introduction to SOC can be found in Bak’s book ‘How Nature Works‘.

Life, the Universe and Everything

Life changes. So do we.

 

Almost two years ago I started my second tentative to write a blog. My first tentative was a little miscalculated: I started three blogs with different themes. Needless to say, it took me very little time to realise that I would never be able to update regularly all three. But my initial idea, which was sharing with anyone who would care all interesting things which I would find, was kept deep inside my mind and didn’t leave me. After thinking about the matter a lot, I came out with Skepsisfera. I was at least a little more focused now. It was meant to be mainly a science blog. I could say that I succeeded. Skepsisfera, although not a ‘hit’, has today some flow of people from every part of the world. But my mind changed in these almost two years.

 

One of the best science blogs I know is John Baez’s This Week Findings. It is just a collection of html files, with almost no picture and no graphic design at all. However, it has a superb content. Also, during the time I was writing Skepsisfera, a number of science blogs started to flourish. Of course, those which caught my attention were about Physics and Mathematics. High quality sites appeared and, among them, high quality technical sites. I could cite, particularly, Christine Dantas’ Background Independence. Christine is a Brazilian physicist like me, but infinitely more inspired. She stopped writing it sometime ago, but I believe she couldn’t resist the blogging calling and came back with Theorema Egregium. Others also made a big impact on me: Cosmic Variance and Not Even Wrong are notable examples. You can find a lot of them in Mixed States, an honorable attempt to gather all this wonderful blogs of physics and math together. I read them and I was inspired by them. I decided I would like to have a blog like them. So, I tried to make Skepsisfera a little more technical, but that blog is not like that. It already had a personality of its own and I could not change it. Well, then I decided to change. Although I like Skepsisfera, I doubt I can update both blogs, but I will still leave it alive for a while. It will continue to be my sandbox.

 

Skepsisfera moto was “Learning is breaking a symmetry.” Never explained there, it was inspired by my Ph.D. thesis about Machine Learning. In there, my programs started to learn from a completely symmetric state and when that symmetry was broken, they generalised, which technically means, they learned. I learned with Skepsisfera. Symmetry breaking is a powerful concept in theoretical physics, and a beautiful one. It is a powerful tool of Nature. Without it we probably would not be here. There would be just some homogeneous stuff all around, whatever ‘all around’ would mean in that case. It is present everywhere: in the freezing of the lake, in the creation of the forces and in the decisions of the brain. Symmetry probably would not be so beautiful if it would never be broken.

 

When a symmetry is broken, it usually means the onset of some deep change in the system. It is the precursor of a new phase. Like the drops that break the homogeneity of the gas phase in its transition to the liquid or, more fashionably, when the ferromagnetic order gives way to a glassy phase.

 

This site is meant to be a collection of my thoughts about physics and math. It will be a technical blog most of the time, for I probably will not be able to be technical all the time. Although it is just about physics, physics is large enough to touch a little of everything, from questions about the complexity of life to the search for the origin of the universe. And so it goes.