Archive for April, 2006

Lost causes in physics

April 30, 2006

Before you start your brand new research programme, you may be interested in at least skim over R.F. Streater‘s twenty-six lost causes in physics, if only to have a laugh at some of them (like, e.g., number seventeen, converting R. Penrose to the Copenhagen view). But make no mistake: the list is not intended as a humorous compendium and many of its entries come with a quite detailed rationale and pointers to the literature, which make it an interesting, if highly opinionated, reading. On a more positive note, in Regained Causes in Theoretical Physics, Streater gives advice on research topics, according to him, worth pursuing.

In case you need a refresher, Streater, now seventy, besides having Erdös number 3, wrote his thesis under Abdus Salam‘s supervision, and is the coauthor (with A. S. Wightman) of the classic book “PCT, Spin and Statistics, and All That”. Most of his almost five decades long research career has been devoted to axiomatic quantum field theory. Not that bad for getting an advice, i think. He has also a bit to say about many, many physicists, present and past. Pretty interesting.

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A Quantum Game

April 27, 2006

More quantum non-local amusements, this time in the form of A Quantum Game based on Greenberger-Horne-Zeilinger three particle entanglements: try to find the (perhaps by now not so) surprising answers before following the links to them. Those links are, by the way, to a blog, koantum matters, written by Ulrich Mohrhoff, a physics professor with some definitely non-mainstream ideas about the interpretation of quantum mechanics. As a matter of fact, i was on the verge of classifying him as a crackpot, but after reading his review on the so-called Pondicherry interpretation of QM, i must say that he’s an honest physicist doing a remarkable and original job. Of course, it’s for you to decide whether his ideas hold any water.

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Fun problem

April 23, 2006

In a recent post on sci.physics.research, Igor Khavkine proposes a fun problem (requiring no more than high school classical mechanics) whose solution, he claims, is ‘a little surprising and intriguing’. Here is the problem:

BallConsider a point particle sliding on a flat table (ignore friction). The table has a cylindrical hole of finite depth (vertical walls, flat bottom). The particle can approach the hole with different velocities and with different impact parameters (the particle’s motion need not be directed toward the center of the hole). As the particle falls into the hole, it starts bouncing off the walls and the bottom (assume elastic collisions). Sometimes it gets stuck in the hole forever, sometimes it escapes (bounces out). Determine the relation between the depth of the hole, its radius, the particle’s initial velocity, and impact parameter necessary for the particle to escape after it falls in.

Some people have already chimed in with comments, but you may want to try your hand at it first!

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The Feynman Lectures on Physics Website

April 21, 2006

If i had to pick a text on physics to bring with me to the proverbial island, that would be, of course, Feynman‘s. If you’re thinking of (re)learning physics, those are the books to get. I’ve just discovered The Feynman Lectures on Physics Website, which contains stories, problems, errata, links (a pretty good collection) and even a forum devoted to these epoch making books. Enjoy!

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New physics, old computing

April 21, 2006

I was reading about the recent black hole mergers simulations performed by the people at Goddard, more thoroughly described here and in this forthcoming article. These are, undoubtedly, beautiful results, and a testament to the complexity of Einstein’s equations when it comes to obtain realistic results: according to the reports above, thousands of lines of code (plus an impressive array of supercomputers) were needed to obtain them. An impressive achievement, but still there’s something in it that makes me uneasy: if i’ve read it right, those thousands of lines of code are actually lines of Fortran (or, in the best case, C) code (more concretely, they’re using a library called Paramesh, written in Fortran 90). Now, if you ask anyone with a solid background in computer science, she will probably tell you that nobody (except physicists, that is) programs these days in Fortran. We know better languages, and have developed far better ways of writing computer programs in the 50 years since Fortran was invented. That is, we physicists are using obsolete technologies. Those newer languages (Scheme, Haskell, OCaml, and so on and so forth) are better in many ways, but specially in one that i am sure is close to any physicist’s heart: they provide far, far better means of abstraction. That is, you can write much shorter programs in a language that is conceptually closer to the problem at hand. And shorter may well mean something like a ten fold reduction in the number of lines of code; not to mention the benefits on clarity, maintanability and extensibility that greater abstraction entails. To use a metaphor, it’s like we were using Levi-Civita’s books and notation as our standard way of calculating in General Relativity, instead of modern differential geometry.

Of course, there’re perfectly understandable reasons for our using antics like Fortran, legacy code being probably the most prominent one; and physicists not having the needed expertise might well an important one too (but let me rush to say that efficiency of the code is not a good excuse these days). But i’m convinced that numerical physics would be vastly improved if we imported some expertise from the professional computing world. I’m told by friends in the field that some of the most ‘advanced’ guys are trying things like C++ and Java (instead of Fortran) these days: i’m sorry, but these languages were current some 20 years ago, and we’ve learnt since then how to avoid many of the pitfalls and unnecessary complexities they carry on. Much more interesting is to use interactive languages like Python (to be on the conservative side) or, if you ask me, functional languages like Scheme or Haskell. To give you a glimpse of what i’m talking about, here is how you’d write quicksort in Fortran 95; in Haskell, it’s a two-liner:

qsort []     = []
qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++ qsort (filter (>= x) xs)

Fortunately, not every one sticks to Fortran these days: Michele Vallisneri’s Synthetic LISA is a beautiful example of a step in the right direction, and i’m glad to see that numerical libraries like PETSC do in fact provide Python bindings. But, as i said, i think (after nine years or so of earning a living writing computer programs) that there are even better ways. As a matter of fact, i’m seriously considering the possibility of writing some LISA simulation code using Scheme. What deters me, besides lack of time, is the enormous weight of tradition: almost everybody out there in the physics community is using those C and Fortran libraries, and that means millions of lines of well-tested code and wondrous results like those black hole mergers. The easiest thing to do is to go with the flow, but still…

(By the way, these comments are by no means intended as a critique of Baker et al. work, which is impressive as it stands. Besides, for what i know, they may well be using far more sophisticated techniques than plain Fortran or C. My rants are more geared towards many other cases i’ve seen of physics programmers which were anything but sophisticated.)

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The Pleasure of Finding Things Out

April 18, 2006

Lispmeister is a blog mostly about, you guessed it, Lisp. But every now and then, one can find (pleasurable) surprises like today’s entry, The Pleasure of Finding Things Out, devoted to the great Richard Feynman, where he points to a video interview that you can see below (it’s a 40 min documentary featuring a grown up Dick):

The only other video footage i knew featuring Feynman are these lectures at the University of Auckland, where one can get a glimpse of how exiciting having someone like him as a teacher can be.

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Mr. Doyle and Dr. Bell

April 17, 2006

I’ve been spending the evening reading An Entangled Web of Crime: Bell’s Theorem as a Short Story, a mystery history in the vein of Sherlock Holmes‘ that explores, in a very entertaining way, the non-locality of quantum mechanics, as exposed by the thought experiments of N.D. Mermin (allegedly, the most simple formulation of the EPR paradox). The article is an excellent way of learning about EPR-like problems, specially if you work out the proposed exercises before reading the answers. For more on Mermin’s ideas on these issues, one can read how he stopped worrying and loved Bohr.

As an aside, David Mermin also has an interest in the pedagogy of science and relativity, as shown in his Seven Principles for Teaching Relativity to Nonscientists, his recently published book It’s about time (which i haven’t read), and his very interesting essays on writing and talking physics.

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The philosophy of space and time

April 16, 2006

Isaac NewtonWith this brief post, i just want to draw your attention to a site i stumbled upon while writing my previous entry: the Philosophy of Space and Time pages at Kyoto University’s Philosophy and History of Science homepage. Despite the occasional sections in Japanese, most of the texts there are in English, very well written and usually accompanied by beautiful diagrams: their author, Prof. Soshichi Uchii, has a soft spot for painting and notable talent, as shown for instance in his portraits gallery). But, as i said, it’s Uchii’s essays and studies what’s really interesting in that site. I already mentioned the series on the Genesis of General Relativity, but there is much more, too much to list in here. For instance, the latest addition is an amazing collection of commented excerpts from the Clarke-Leibniz correspondence, which somehow commenced the still on-going debate between the relational and substantivists views of space-time. Definitely recommended reading, accessible to both experts and laymen.

Also worth noting are Uchii’s PHS Newsletters, which, among other things, contain carefully written book reviews: for instance here is the one devoted to two books that you may find interesting: Brian Green’s The fabric of the Cosmos and Lee Smolin’s Three Roads to Quantum Gravity, which describe the two ‘mainstream’ competitors in the race for a quantum theory of gravity (and, sometimes, everything). Also interesting is his review of Julian Barbour’s The End of Time, a provocative, to say the least, new way of bringing Mach’s principle to the forefront of our physical theories. Happy reading!

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Conceptual relativity

April 15, 2006

I knew this was going to be fun, but Rovelli’s book is surpassing all my expectations. I’ve just finished Chapter 2 of his Quantum Gravity, which is devoted to (classical) General Relativity. It starts presenting the formalism, using Cartan/Ehresman connections over fiber bundles, so one needs to refresh some of the advanced topics covered in differential geometry, topology and group theory courses. At least, i had to. There are lots of books around, but if you have a good background, you may find Maximilian Kreuzer‘s lectures notes on Geometry, Topology and Physics (parts one and two) interesting (i definitely did). Rovelli presents the needed equations at a quick pace, mildly entertaining but hardly exciting… the real fun begins afterwards, in section 2.2 (aptly called The conceptual path to the theory) and up to the end of the chapter. Everyone studying General Relativity should read these sections. More than once (and, while you’re at it, take a look at the Kyoto University’s Genesis of General Relativity to keep on the fun). And if you think you already know a lot about GR, read them too, please. I have yet to digest them, but they are the best discussion on the conceptual underpinnings of Einstein’s theory i’ve read in many, many years. There you’ll find a discussion of the physical tenets of GR and their interpretation, including in what sense the theory makes (inertial) acceleration a relative phenomenon (revisiting Newton’s famous bucket experiment–see also here), and the meaning of general covariance (the paradigmatic gedanken here being Einstein’s hole argument) and the key distinction between active and passive transformations. The discussion is illuminating in many respects, and has given me food for thought for many weeks. And the best of all is that no sophisticated maths are needed to follow it; besides, there are excellent (and original) examples illustrating the arguments in the main text, based mostly in Newtonian physics. The chapter closes with notes on Mach’s principle, the philosophical interpretation of spacetime (relationalism vs. substantivism), Kretschmann’s objections against the physical content of general covariance (which i used to share, to some extent, in the past) and a detailed and delightful description of how GPS works.

A must read for anyone interested in General Relativity’s foundations.

I must say that it’s refreshing to find theoretical physicists interested in conceptual and philosophical issues in these calculate-and-shut-up times. I hope it’s a sign that they are a’changin’.

(I just noticed that the draft PDF of the book has disappeared from Rovelli’s webpage, but you can get it here.)

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Bell’s theorem

April 14, 2006

John BellI just found a beautiful, elementary proof of Bell‘s inequality at sci.physics.research. It’s the more transparent and intutive i’ve ever read, and, as an additional plus, no quantum mechanics is involved in the argument. In fact, i’ve liked it so much that i have reproduced it as the first entry in my new ‘Bits and bolts’ section.

An excellent introduction to the spooky non-locality of quantum mechanics, worth reading for those in the know too (who, incidentally, may be interested in this recent article on EPR by Rovelli and Smerlak, which indirectly led me to this discovery).

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