Undergraduate Notes

I have just posted some notes about probabilities in my website:

http://robertoalamino.uuuq.com/resources.html

SciVee

The aim of this site is to work as a mixture of YouTube and arXiv. You put a preprint of your paper together with a video explaining it:

 

http://www.scivee.tv/

 

Back from Finland

I was in the conference ‘Statistical Mechanics of Distributed Information Systems‘ in Finland last week, which (for my surprise) was sunny and warm, very different from Birmingham. Slides of the talks as well as references and papers can be found in the conference’s site.

A lot of famous people in the area were there. For instance, Marc Mezard gave a nice talk entirely on the blackboard about how to go from Belief Propagation to Survey Propagation. John Hertz was one of the organizers, but he didn’t give any talk. Again, the list of participants is in the webpage.

The given talks can be arranged in three big areas: computer science and biology with some small deviations (like economics and game theory, being the former basically analysed from the point of view of the later) and interconnections. W.r.t. biology, some of the talks where about algotrithms and others about models. It seems clear that Statistical Mechanics (SM) is gaining confidence with biologists and physicians finally. Among the topics were stochastic models for cell differentiation, the popular theme of information transfer and processing in the genetic code and philogenetic threes.

In the computer science area, the main topics were coding theory and satisfiability problems. Indeed, I saw the phase dagram of the satisfiability problem even in a talk about economics and it appeared so many times during the conference that I think I can reproduce it by heart now.

The concept of agents seems to be a pretty unifying concept in this area, although it is more or less the same as the concept of an interacting body, or entity. It is obly more fashionable. To be fair, I guess that the term ‘agent’ has the advantage of transmitting the idea of some entity which can change its response, which in this context can be viewed as more general than body. For instance, the talk about swarms of agents is an example of the term ‘body’ do not fit so well.

What is still lacking is fundamental physics applications of the methods or at least an interest of bringing the communities together. I personally do not know of any meeting where both topics are entangled. Anyway, it was a nice conference and if you browse the conference’s website you will have an idea about the main tendencies in the area.

Galaxy Zoo

Galaxy Zoo is an online cosmology/astrophysics project by a team of cosmologists and other scientists in whichanyone can participate to help classify galaxies and other objects. The article in New Scientist points to Kate Land (a former PhD student of well known Joao Magueijo of Imperial College) from Oxford as one of the main involved in the project.

The idea is to use our natural machine learning skills to identify different kinds of galaxies and objects in photographs. Well, once we had not dominated machine learning at the high level we would like (although we are marching very fast in the direction), why not train the best learning machines that already exist. And as people do that voluntarily and for pleasure, probably they will do it very well. I have tried and passed in the initial trial. Two big advantages: you will see a lot of nice pictures of far regions of the universe and learn a lot in the way.

Nice project, guys.

—–x—–

PS: there is a more detailed account of it in Cosmic Variance: Galaxy Zoo!

BH Entropy as Entanglement

While I prepare the post on dimers, here is a brief one directly from a thread in physics forums. It is a talk given in Loops’07:

 

• Entanglement Entropy in Loop Quantum Gravity – William Donnelly

The paper seems not to be in arXiv yet. The idea is that the Bekenstein-Hawking entropy is the entanglement von Neumann entropy between the black hole and the rest of the universe. I am not an expert in black hole entropy, so if anyone would like to contribute with comments or explanations it would be very nice. For example, I could not identify in the text the connection between the entanglement entropy calculated using spin networks and the usual entropy formula for the black hole.

Reading the paper, I discovered one more thing that I don’t understand. If the black hole and the rest of the universe are entangled, a measure of the state of the universe would give complete information about the state of the black hole. Isn’t that impossible for an external observer? Well, maybe I put this question in physics forum…

Anyway, if someone wants a flash and pedagogical definition of entanglement and von Neumann entropy, this paper is a very good one:

• The physics of forgetting: Landauer’s erasure principle and information theory – Plenio, M., Vitelli, V.

It is very basic, undergraduate level I would say, but has also good references for more advanced readers.

—–x—–

I’ve just noticed that BH in the title are the initials both for Black Hole or Bekenstein-Hawking, so read it the way you prefer.

Science Videos

ScienceHack is a science video search engine. It is claimed to be “peer reviewed” in some sense, in their own words

every science video on ScienceHack is screened by a scientist to verify its accuracy and quality

Although they say that the videos are judged by their scientists, I couldn’t found who they are… Anyway, it is a nice tool to find some good scientific stuff.

 

 

Dyson’s View on Biofuture

Dyson wrote this fascinating view on the future of biotechnology for the New Yourk Reviews of Books entitled “Our Biotech Future”. Among other things, he writes en passant about the understanding of physical world as a complex environment. This view is becoming clearer as our understanding of complex systems increases and the necessity of statistical physics methods to deal with them is growing. We can see it even in the quantum gravity literature but I will try to write on it later as it is too off topic here. Dyson’s text cites the paper of the influent biologist Carl Woeser, who was responsible for our modern view of the tree of life, together with Nigel Goldenfeld named Byology’s Next Revolution .

Jupiter’s Stripes

This is a nice picture of the recent changing in Jupiter’s stripe pattern by Hubble Space Telescope. This is the first time that the changes are observed with such detail. New Horizons and some Earth-based telescope are also collecting huge amount of data in order to help to understand the atmospheric dynamics of the planet. New Scientist story is here: Jupiter changes its stripes.

Statphys 23

From 9th to 13th July it is going to be another Statphys conference, the most important meeting in the Statistical Physics area. This year’s venue is Genova, Italy. The program can be downloaded from the conference site. Giovanni Gallavotti and Kurt Binder are receiving the Boltzmann Medal this year.

There are a lot of talks about applications of SP methods in interdisplinary areas as Economphysics, Information Theory and Biology. Interestingly, there are no talks about QFT or Quantum Gravity. There is a talk to be given by M. Caselle entitled “String theory description of the interface free energy”. It is not exactly about String Theory, but seems a very interesting application of its formalism to a Statistical Physics problem. The abstract is:

A powerful tool to describe the physics of interfaces is the well known “capillary wave” model. It was realized a few years ago that this model is nothing else that the long range limit of the bosonic Nambu-Goto string theory. An interesting open problem is to understand if real interfaces are described by the whole NG theory or only by its capillary wave limit. The two expressions only coincide at the first order in the 1/(\sigma)A expansion (\sigma being the interface tension and A the interface area). By using standard covariant quantization of the bosonic string, we were able to derive an exact expression for the Nambu-Goto expectation for the interface free energy as a function of the geometry of the interface. We then compared our predictions with a set of very accurate Monte Carlo data for the interface free energy in the 3d Ising model. The Nambu-Goto result turns out to describe the data much better than the simple capillary wave model in a large range of values of the interface area thus making interfaces one of the most effective realization of string inspired models in nature.

The related paper is in arXiv: The partition function of interfaces from the Nambu-Goto effective string theory. I haven’t read the paper yet, but I’ll try to do that and comment it when possible.

Unfortunately, I cannot go this year. Maybe next one…

 

More about the Pioneer Anomaly

It still remains elusive. This paper is going to be published in Physical Review D:

• Could the Pioneer anomaly have a gravitational origin? – K. Tangen

The abstract:

If the Pioneer anomaly has a gravitational origin, it would, according to the equivalence principle, distort the motions of the planets in the Solar System. Since no anomalous motion of the planets has been detected, it is generally believed that the Pioneer anomaly can not originate from a gravitational source in the Solar System. However, this conclusion becomes less obvious when considering models that either imply modifications to gravity at long range or gravitational sources localized to the outer Solar System, given the uncertainty in the orbital parameters of the outer planets. Following the general assumption that the Pioneer spacecraft move geodesically in a spherically symmetric spacetime metric, we derive the metric disturbance that is needed in order to account for the Pioneer anomaly. We then analyze the residual effects on the astronomical observables of the three outer planets that would arise from this metric disturbance, given an arbitrary metric theory of gravity. Providing a method for comparing the computed residuals with actual residuals, our results imply that the presence of a perturbation to the gravitational field necessary to induce the Pioneer anomaly is in conflict with available data for the planets Uranus and Pluto, but not for Neptune. We therefore conclude that the motion of the Pioneer spacecraft must be non-geodesic. Since our results are model independent within the class of metric theories of gravity, they can be applied to rule out any model of the Pioneer anomaly that implies that the Pioneer spacecraft move geodesically in a perturbed spacetime metric, regardless of the origin of this metric disturbance.