Tuesday, 24 January 2006

MaxEnt 2006

Entropy Trails

This year MaxEnt 2006 will be in Paris from 8th to 13th, July. MaxEnt is an annual conference on Maximum Entropy methods and their applications in science and this year it will be the 26th time it happens. The conference-chair is Ali Mohammad-Djafari and it has some notorious people in the organization: G. L. Bretthorst, A. Caticha, C. Rodriquez and J. Skilling among others.

Maximum Entropy methods are extensively used not only in physics but also in statistics and engineering since the landmark paper of Shannon, named A Mathematical Theory of Communication, where he found that the same quantity called entropy in statistical physics could be used as a measure of information.

One of the most interesting applications of MaxEnt is related to Bayesian inference. It is often said that the Bayesian interpretation is a "subjective" interpretation of probability theory and a lot of criticism come from this, some even say that due to this Bayesian probabilities are not even scientific. I'm in the course of writing a post about Bayesian inference, but while I cannot finish it, let me say that MaxEnt can be used as a method to generate Bayesian prior distributions in a totally objective way. The idea, roughly, is that as entropy is a measure of "uncertainty" or "disinformation", the correct prior distribution is obtained by maximizing the entropy of the distribution subject to constraints given by the avaiable information. This means that when using this principle we are trying to use just the avaiable inforamtion and assuming nothing about everything else, i.e., we assume that our disinformation about the rest is maximal.

An interesting and simple example of this method is related to the well-known Gaussian distribution. It can be shown that the probability distribution obtained by MaxEnt when the only information we have is the mean and the covariance is given by a Gaussian, i.e., if the only thing you have is a mean and a covariance matrix, your best guess is a Gaussian one.

Picture: Entropy Trails, by Tetragrammaton Productions.

Thursday, 12 January 2006

Renewed Homepage

After some months, I finally finished the "restauration" of my homepage. It does not have too much information yet, mainly because it has been renegated to second place after I started this blog, but I have some ideas and the new format is simpler for me to modify and include new things.

Any suggestion or critique is wellcomed, and I would like very much to receive a lot of both.

On my desk:

  • Vacuum Fluctuations, Geometric Modular Action and Relativistic Quantum Information Theory, Verch (gr-qc/0512053)

  • The Meaning of the Interaction-Free Measurements, Vaidman (quant-ph/0103081)

  • Relative entropy in quantum information theory, Schumacher & Westmoreland (quant-ph/0004045)

  • Quantum interference and Bayes theorem, Neri (quant-ph/0601061)

Wednesday, 11 January 2006


NASA's Stardust spacecraft is comming back to Earth bringing an important load from space: dust from the Comet Wild 2, that is believed to contain information about the conditions of our Solar System in the time of Earth's formation, and interstelar dust collect during the trip.

To analyse the dust, it is important to pinpoint the tiny particles in the aerogel in which they were collected using microscopy photos. It is important to know exactly where the dust is before trying to get it so as not to disturbing unnecessarily the rest of the sample. The odd thing is that NASA will recruit people all over the planet to do that by internet. All you have to do is to subscribe in the site with the name of the project Stardust@Home and as soon as the spacecraft arrive in Earth they will begin to apply a test to evaluate if you have the required ability to help the program.

Those who really find dust particles in the photos will even gain the right to name the dust particle as a reward for the discovery. A good oportunity to become a scientist if even you never thought in being one!

Some news about the project:

Internet users will hunt for Stardust@home

Public to look for dust grains in Stardust detectors

Stardust@home Project Brings Cosmic Dust to Your Desktop

Monday, 9 January 2006

Net Articles

Continuing my efforts to organize my stuff before leaving Brazil, I was organizing my bookmarks and found these links to interesting online articles:

Life in Extreme Environments
Why smart people defend bad ideas
Oklo (A natural nuclear reactor.)
Twistors --- What Are They?
Brief History of Artificial Intelligence
An Introduction to Lagrange Multipliers
A compendium of NP optimization problems
Notes on Functionals
On the Origins of Twistor Theory by Roger Penrose.

As with the links in the last post, finding more articles to post here is just a matter of time as my cleaning proceeds...

On my desk

Picture taken from: perso.wanadoo.fr/overlord59/