Saturday, September 28, 2013

Dalio: How the Economic Machine Works

Ray Dalio put together this very nice video to paint the workings of an economy with a broad brush. It presents a compelling lens through which to view the relentless onslaught of "economic" news. For the first time, I think I have a rough idea of what Ben Bernanke is trying to pull off:

Wednesday, September 25, 2013

Links: War Edition

1. OPEN Magazine's "Playing at War: How two madmen brought the world to the brink of a third great war" on the international machinations during the liberation of Bangladesh, should make us skeptical of official government lies lines. Very skeptical.

2. Of course, we should also be very skeptical of the ongoing war cries to strike against Syria. Here is Mish Shedlock's interesting take.

Friday, September 20, 2013

Pindyck, EconTalk, Climate Change, and Uncertainty

Russ Roberts recently interviewed Robert Pindyck on EconTalk. I found the interview very fascinating, because it avoids the two extremes that people instinctively gravitate towards: (a) impose a big carbon tax, or (b) do nothing.

He poses the question more narrowly and grapples with the question of "how big should the tax be given the 'real' uncertainties and timescales involved?" Blurb on EconTalk:
Pindyck argues that while there is little doubt about the existence of human-caused global warming via carbon emissions, there is a great deal of doubt about the size of the effects on temperature and the size of the economic impact of warmer climate. This leads to a dilemma for policy-makers over how to proceed. Pindyck suggests that a tax or some form of carbon emission reduction is a good idea as a precautionary measure, despite the uncertainty.
The interview uses his working paper, "Climate Change Policy: What do the Models Tell Us?" (pdf) whose abstract starts with two words that summarize the paper "Very Little."

A more accessible paper is published at the Cato Institute site (pdf), entitled "Pricing Carbon When We Don’t Know the Right Price". Given the political leanings of the Cato Institute the subtitle is even more provocative: "Despite the unknowns, we should begin to tax carbon."

Sunday, September 15, 2013


1. The classic Feynman Lectures in Physics now available online for free.
Caltech and The Feynman Lectures Website are pleased to present this online edition of The Feynman Lectures on Physics. Now, anyone with internet access and a web browser can enjoy reading a high-quality up-to-date copy of Feynman's legendary lectures. This edition has been designed for ease of reading on devices of any size or shape; text, figures and equations can all be zoomed without degradation.
2. Since we are talking physics, here is a placeholder link at Mathworks for teaching physics using Matlab.

Saturday, September 14, 2013

Molecular Weight Distribution Posts

I wanted to put together four posts I did on molecular weight distributions (MWD) just for future reference:
  1. The relationship between size-exclusion chromatography (SEC) and MWD: The first post explored the relationship between the typical SEC plot (w(log M) v/s log M.
  2. The second post looked at a particular example to "particularize" some of the equations above.
  3. I also looked at two common parametric forms for MWD, namely the log-normal distribution, and the generalized exponential or GEX functions.

Friday, September 13, 2013

Markov Chain Monte Carlo Seminar

This semester my colleague Peter Beerli is running a seminar in a journal club format. We are trying to read the most seminal papers in MCMC.

Of course, one has to start at the beginning and read "the" famous paper by Metropolis et al., "Equation of state calculations by fast computing machines" which has been cited more than 25,000 times. It had been a while since I had read the paper (more than a decade ago, I think), and I enjoyed re-reading it after the hiatus.

It was like re-reading one of those books you read as a teenager, and taking completely new things away from it. When I first read it, I was caught up with the mechanics, since they were still very new to me. This time around, I got to marvel at some of their astute observations (and lament their choice of a horrible random number generator).

I also spent some time reading the background of the paper (some links on Peter's webpage). In a relatively recent interview, one of the paper's authors Marshall Rosenbluth had this to say about some of the other co-authors:
Barth: Your collaborators for these papers were Edward Teller and Nick Metropolis and your former wife?
Rosenbluth: Yes. She actually did all the coding, which at that time was a new art for these new machines. You know, no compilers or anything like that.
Barth: And it’s also listing A.H. Teller.
Rosenbluth: That was Teller’s wife, who during the war had been one of these computer [women] - he wanted her to get back into the work, but she never showed up. So she was basically -
Barth: Put on the paper for it?
Rosenbluth: Yes. As was Metropolis, I should say. Metropolis was boss of the computer laboratory. We never had a single scientific discussion with him.
Some of this recollected in a historical paper (probably behind a paywall).

It is very interesting that none of the coauthors made any subsequent use of the algorithm.

The last bit from that paper:
Obviously, Marshall’s revelations prompted hallway and diner discussions about whether the Metropolis algorithm should be called the Rosenbluth or at least the Rosenbluth-Teller algorithm. One of the organizing committee, Rajan Gupta, whose curiosity got the best of him, privately asked Marshall his opinion about the name of the algorithm. Marshall replied: 
“… life has been good to me. I feel rewarded in knowing that this algorithm will allow scientists to solve problems ranging from fluid flow to social dynamics to elucidating the nature of elementary particles.” The original name will stick. 
Three times during the conference, Marshall walked up to me and said, “It hard to explain how exciting it was to be at Los Alamos during those times. To be able to interact with Teller and von Neumann was very important to me.” Even heroes have their heroes.

Wednesday, September 11, 2013

Robert Krulwich: Commencement Speech at CalTech

Robert Krulwich's (co-host of Radiolab, among other things) commencement speech at CalTech is a call for scientists to engage with the public using stories, metaphors, and narratives.


Friday, September 6, 2013


1. WonkBlog @ the Washington Post: A 10-part series on "Why the Tuition is Too Damn High?"

2. "The MOOC That Roared" Georgia Tech offers a masters in CS for $6,600.

As an academic and father of two kids who I still need to put through college, this is an important subject.

There are a lot of things that MOOCs still have to work out (how do you replace physical wet labs? How do you grade non-multiple choice questions? How do you ensure no cheating is taking place?), but it seems very possible that tuition won't increase at the rate that it has been or may even deflate.

Monday, September 2, 2013

RIPE: Polymers and Entanglements

Let's start with three everyday examples of entanglements.

(i) I am love my iPod and listen to it quite a bit. Usually, after I am done listening, I just stick it, along with headphones, into my pocket. The next time I pull it out, the headphones are all tangled up, and I often wonder what the heck happened to them in my pocket.

(ii) A similar thing happens every time I use my leaf-blower, and don’t stow away the cables carefully. It is a mess!

(iii) As a father of daughters, I am a big fan of short hair. Why? Long hair gets tangled up more easily, and needs more effort combing (de-entangling).

Headphones, cables, and hair – they all have a natural tendency to get all tangled up.

The “longer” these things get, the more trouble they are.

What has this got to do with polymers?

Polymers are long chain-like molecules formed by stringing together units called monomers. As the number of units increases, their “molecular weight” increases, as does their propensity to get tangled up.

Indeed, there is a threshold molecular weight – different for different polymers – called the entanglement molecular weight, beyond which their rheology is strongly influenced by these entanglements. (This opens up a fascinating cascade of research questions that I have spent countless hours pondering!)

Below the entanglement molecular weight, the viscosity (the resistance to flow) of polymer melts increases gently. Double the molecular weight, and you double the viscosity.

Beyond the entanglement molecular weight, this linear increase in viscosity is abruptly disturbed (see a picture here). In this highly entangled regime, a doubling of molecular weight results in nearly a 10-fold increase in viscosity.

Take that!

Footnote: Polyethylene in your milk jars, and polystyrene in Styrofoam cups have entanglement molecular weights of about 1000 and 15,000 daltons respectively.

Footnote2: RIPE = Research In Plain English