Archive for January, 2009

One of the most important skills a physicist can have is to estimate the size of various effects without doing a detailed calculation. This basic skill is one of the hardest ones to learn, mostly because it is very easy to disbelieve: after all, you didn’t do a real calculation. But at the same time, it is one of the most important skills one can have. These estimates will tell you if you are going to be wasting your time doing a calculation or not. On the other hand, they migt give you an enormous amount of information without having a fundamental system of equations to solve: let us say, you don’t know if there is a relation between some set of phenomena, but order of magnitude estimates migt be able to do that for you. I will now describe various instances of these order of magnitude estimates to show you how powerful they are.


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Dennis Overbye, in the New York Times, has some thoughts about science . You should also read the letter by David Gross and Eric Kandel on the Financial Times about why the stimulus package should also include science research.

You should also be aware of the findings of the National Academy of Science ( I linked to the executive summary page) regarding the future of Science in the US.


Here are some further random comments that I picked up (with the understanding that I don’t really know who the writers are).




Of course, there is Cosmic Variance as well (and I know who these guys are).


I have a plethora of opinions about this subject, not just because I am a scientist and fashion myself as an armchair economist (and who isn’t?). However,  I would need to spend quite a lot of time articulating them and I have been rather short of free time lately. In the meantime, I look forward to more debate on the issues and I pray that this part of the budget doesn’t get scrapped as part of the usual horse trading in DC.

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Scientific Blog Limericks

As I was walking near the sea shores

Thinking about all my chores

I met some girls who where pretty to see

And as I talked about the Dirac sea

It turned out they where whores.


Your homework for the week is to come up with an original Limerick using the title of your favorite science blog as a building block. The one above is an example.

For your information, it turns out that Isaac Asimov, apart from being one of the great writer of science fiction, was quite the writer of Limericks. Some of them were very, very naughty. I’m in the mood to celebrate the English language being mangled and taken to ridiculously new heights of fun by holding a limerick contest. The rules are as spelled above: you have to use a science blog name as a base (and you should probably tell us which one it is). I wrote mine above using the title of this blog as a base (that you are allowed to choose again if you want to). Mine is not that good, but it gets the creative juices flowing.

Rhyming is a good exercise to help you think quick on your feet. It can also be a lot of fun in good company. 

The winner (selected by me) will get at least a mention in the blog and if I find the Limerick irresistibly funny, the winner might even get some material prize to be determined later.


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Climbing down

Following David’s advice I’ll climb down from my theoretical ledge and rant a little bit, since I have no time to do anything more substantive. Every semester reaches some point where your time fractionalizes to smaller and smaller bits, where the number of small but urgent tasks exponentiates, and the ability to hold a single thought for more than 5 minutes (not to mention 5 seconds), becomes a sweet and fading memory.

In such times good energetic music is the way to go, I can use all the adrenaline I can get. Yes, this is turning out to be another one of those posts where I bore all the readers with some obscure classical music (“classical music” refers of course to the overwhelming majority of music ever made, a vast landscape of styles and personalities).


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This has not been my week….

In this blog we occasionally show our humanity and climb down from our theoretical speculations to talk about some aspects of our lives. Here is a sample.

Yesterday I was late for one appointment (I plain forgot) and arrived very late in the end after bumping into someone who reminded me “where I should have been over a half hour ago”. It was fixed, but I felt very embarrassed. In the mess I also misplaced my lunch box, and although it was eventually found, I ended up wasting a lot of time tracking it down. Because of it, I missed the lunch train.

Today, I left part of my lecture notes for my class behind at home, so I had to work a lot on making sure I had those details straight. I had also forgotten to reply to various e-mails that were somewhat urgent so I was feeling rather embarrassed again. I got caught in all this mess and it did not seem to get any better by the end of the day. I’m sure I forgot some other things that have not shown up on my radar yet, but they will be very important and I will feel rather foolish (yet again). I’m counting on statistics for this one as there is always one such item pending… As a matter of fact, I just figured out such an item. Ouch!

The usual pastime of blaming it all on somebody else hasn’t worked as all the arrows point directly toward me.

On the bright side of life, it finally rained here and my banana tree seems to have decided to start making bananas. Of course, neither of these two is any of my doing, but I have to be grateful for the few bones that the universe is throwing in my direction. Finally, the monarch butterflies are in town overwintering and when I saw them this past weekend they were spectacular.
If you are in town, I recommend that you go tae a look before they migrate north (sometime in late February, early March).

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You might think I have gone bonkers. When have you ever heard of the stress tensor being everyday physics? Don’t let nomenclature fool you. The stress tensor is as a matter of fact something that we have a lot of familiarity with, just not by that name. Consider for example a steel cable made of filaments as in the figure. What is the total tension on the cable? You can say that the tension is built filament by filament, so the total tension (the capacity to pull an object evaluated as a force) is proportional to the number of filaments if all the filaments have the same individual tension. 


Each filament contributes to the tension

Each filament contributes to the tension

Since the number of filaments is another way to count areas, we find that the total tension is proportional to the area. If we calculate the tension per unit area, we call that the stress of the material. Now it starts to sound familiar, doesn’t it?


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As you might have noticed, some objects float and some others don’t. Here below I have a rendition of a boat and a cube of ice floating.


Various floating objects

Various floating objects



Today, I will go on a bit about flotation. As a matter of fact, some of you might remember a puzzle with an egg I wrote down a while ago. Of course, most of you have probably heard of Archimedes Principle as describing flotation, so I will explain some aspects of how that principle comes about.


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Back in 2001, in a truly beautiful paper, Juan Maldacena formulated a version of Hawking’s information paradox, which has the added advantage that it could be discussed and analyzed in the context of a complete background independent theory of quantum gravity, namely that of the AdS/CFT correspondence.

This variant is similar to the original paradox, formulated for black holes surrounded by flat space, in that it displays a sharp conflict between properties of black holes in classical General Relativity, and basic postulates of quantum mechanics. Alas, it is also different in many crucial ways from the original paradox. Despite that, Juan’s proposed resolution to his paradox seems to have led to Hawking’s arguments, who managed to convince himself (though I think it is fair to say not too many others, unless they were already convinced) that information is not lost after all in the process of black hole formation and evaporation.


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Comments on student evaluations

Over at Uncertain Principles, Chad asks for suggestions for minor tweaks in the student evaluation system. I thought I’ll add my two cents here, and ask the readers for their opinions. Just to be clear, like many others I think the current system serves no purpose whatsoever, and is often counter-productive. In my mind, any minor tweaks will not change things much, but some re-thinking of the system can make it useful.

First, for those of you who are not currently at a university, let me remind you that every course at a north American university ends in the ancient ritual that is called student evaluations. In this, students are asked some vague questions, all of which come down to “on a scale from 1 to 5, is the instructor any good?”. We tabulate the results, and pretend that we now distinguished the good professors from the bad ones, and in the same process allowed the student-consumer have some feedback on the system. Everyone wins!

And here is the thing, I don’t think students are in any position to evaluate teaching. Any rational system of evaluating teaching a specific course will have to specify the goals of that course and measure to what degree those goals were achieved. There are many ways of doing that, but student evaluation, of any kind, is not going to cut it. Students taking the course are missing by and large the context of the course, and I think their answer to any evaluation question will be pretty much independent of the question. They will invariably answer the implicit question “did you enjoy the course?”, because that is the only data available to them.

By all means, enjoying the course should certainly be one of the goals, and student satisfaction could be measured and tabulated to decide to what degree that particular goal is achieved. I just think that student satisfaction is not the only goal of the course, and that is the only parameter we are attempting to measure at the moment. Since I believe we do need a system of evaluating teaching, pretending we already have one is counter-productive if we want to improve the quality of teaching.

All that should not be interpreted as the statement that student feedback is not useful. This could be an essential tool in improving the course and checking your teaching ideas against reality. For example, having the students answer a set of a course-specific questions, during or after the course, could very helpful for this purpose. Just forget about the 1 to 5 scales, those are just silly.

As always, comments are welcome!

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Fading into obsolescence

So, as a matter of fact, I occasionally buy some computer games. Also I also occasionally buy new computers. Eventually it becomes hard to use an old computer if the computer is not able to operate the newest software available. 

If I get bored, I might even try to install old games in a new computer. I tried this a couple of weeks ago when I was not feeling up to doing work and I was on Holiday.

As it naturally  happens, the old software does not run in the new computer. Operating systems change, and as they change, the graphic engines and old routines stop working properly: they become obsolete.



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