Archive for the ‘Gravitational Waves’ Category

In the states

June 17, 2006

I’ve arrived to College Park (in the Washington DC area) today, after a rather trying trip from Paris to Dulles (eight hours flight) and from there to the nearby hotel (3 (sic) hours taxi!). I’m participating in the 6th International LISA Symposium, which starts next Monday. I’ll present a poster, initially devised as a talk, giving an overview of the software aspects of Lisa Pathfinder’s development, which is currently well underway. To judge from the number of presentations (50), posters (75) and participants (250) in the symposium, there is a quite active community pushing hard to make LISA a reality, and i expect to learn a lot about the project and its prospects during the week. So the following days will probably be quiet ones here at physics musings. But i’ll be back ;)

Update: Over at Not Even Wrong, Peter Woit is reporting about some news claiming that, somehow, LISA will test string theory. My impression from the field is that nobody in the symposium is seriously talking about that, with the exception of Odylio Aguiar’s talk (mentioned here), which i couldn’t attend.

Update: Finally, there was some talk about string theory in the symposium. Yesterday morning, Craig Hogan gave a talk entitled New physics with LISA where he discussed the potential detectability of a stochastic GW background which would be a relic from an inflationary period and gave some qualitative analysis of cosmic strings as generators of gravitational waves. To be honest, i understood nearly nothing, and was left with a feeling of handwavyness, if only because in the conclusions Craig jokingly admitted that, most probably, in the next LISA Symposium he will give a talk with the same title but totally different contents!


June 3, 2006

Reading the recent article Electrons Act Like Waves (from the Physical Review Focus series, a highly recommended, lay[wo]man-friendly feed for your newsreader), i've discovered one of those peculiar stories that make the history of physics even more enjoyable than it would be by its purely scientific side alone.

The article tells the story of Davisson and Germer's discovery of the so-called wave-like nature of electrons. As explained in every textbook, they set up an experiment consisting in scattering electrons through a (nickel) crystal, and observed the familiar fringes that one obtains when a wave crosses a gratting with alternating slots in a wall.

SlitIt was 1927, and i always pictured Davisson and Germer as intrepid experimenters boldly trying to confirm de Broglie's 1924 ideas about the wave nature of matter [1]. (Justly enough, an idealisation of this experiment has become the de facto standard presentation of the quantum mechanical world!.) The funny thing is that this romantic picture has nearly nothing to do with what really happened. As it comes, D&G were looking for evidence of the atomic structure of metals and knew nothing about de Broglie. After the experiment had been going on for a somewhat sterile period, one of their widgets broke and overheated the nickel plaits, which crystalised and made (when used again for scattering electrons) the interference patterns apparent. The experimenters were all but bewildered, and only after Davisson discussed his results with other colleagues during a holiday in England, did they realize the importance of their discovery.

That's serendipity at its best. And, of course, it was not the first nor the last time that serendipity gave physicists a helping hand. The Oxford Dictionary gives a precise definition of this beautiful word:

serendipity noun the occurrence and development of events by chance in a happy or beneficial way.

or, even better, this one from Julius H. Comroe (as quoted by Simon Singh)

Serendipity is looking for a needle in a haystack and finding the Farmer's Daughter.

and its all too apt etymology:

ORIGIN 1754: coined by Horace Walpole, suggested by The Three Princes of Serendip, the title of a fairy tale in which the heroes “were always making discoveries, by accidents and sagacity, of things they were not in quest of.”

which in my opinion captures extremely well the kind of discoveries we're discussing. They were by chance, that's true, but not just chance: one needs to be in the quest of something, to begin with.

Another famous (and probably better known) example of serendipity at work is Penzias and Wilson's discovery of the cosmic microwave background. As explained by Ivan Kaminow,

He [Ivan] joked that Penzias was an unusually lucky guy. "Arno Penzias and Bob Wilson were trying to find the source of excess noise in their antenna, where pigeons were roosting," he said. "They spent hours searching for and removing the pigeon dung. Still the noise remained, and was later identified with the Big Bang."He laughed, "Thus, they looked for dung but found gold, which is just opposite of the experience of most of us."

The experiment was being conducted at Bell's Labs and its aim was to tune an ultra-sensitive microwave receiving system to study radio emissions from the Milky Way. It was only after Penzias talked with Robert H. Dicke (see also this nice memorial (PDF) for more on Dicke) that the misterious radiation was recognized as the relics of the Big Bang hypothesized by George Gamow some time before. I read the whole story for the first time in Weinberg's marvelous book (required reading), and I've always found a bit unfair that the Nobel prize went only to Penzias and Wilson.

My third serendipitous example comes also from the skies. In summer of 1974, Russell Hulse was a 23-year-old graduate student compiling data from the Arecibo Observatory radio telescope in Puerto Rico. The job was a little bit tedious: he was trying to detect periodic radio sources that could be interpreted as a pulsar [2]. One of the pulsar's earmarks is its extraordinary regularity (a few nanoseconds deviation per year for a period of about a second). Around 100 pulsars were known back then, all with a stable period with a extremely slow tendency to increase. At the end of the day, the data obtained by the telescope was processed by a computer program written by Russell, which selected candidate signals based on the stability of their period. Those were correlated with later or former observations of the same sky zone, to rule out earth-based, spurious sources. One night, Russell boringly noticed a very weak candidate, so weak that, had it been a mere 4 percent fainter, it would have passed unnoticed. On top of that, its period was too short (about 0,06 seconds) and, even worst, it was variable. Russell was on the verge of discarding it more than once during the following weeks, but eventually he persevered and, helped by his supervisor, Joe Taylor (a.k.a. K1JT), correctly interpreted the observation as a binary pulsar. The rest is history, and a Nobel prize [3] one. Russell tells the amazing story in his delicious Nobel lecture (PDF), which starts with these telling words:

I would like to take you along on a scientific adventure, a story of intense preparation, long hours, serendipity, and a certain level of compulsive behavior that tries to make sense out of everything that one observes.

I specially like this instance of serendipity, for it shows that, many a time, lucky strikes befall on those who work hard enough to get hit.

Update: I've just found an excellent article by Alan Lightman, Wheels of Fortune, which gives some very nice examples of serendipitous discoveries, as well as a nice discussion. After reading Michael post on serendipity in HEP, i was wondering about non-experimental lucky strikes, and Lightman gives an excellent example: Steve Weinberg's electroweak theory:

Serendipitous discovery strikes not only in the photographic plates, test tubes, and petri dishes of the laboratory. It also can strike in the pencil-and-paper world of theoretical scientists. In the fall of 1967, theoretical physicist Steven Weinberg was working out a new theory of the so-called “weak force,” one of the four fundamental forces of nature, when he discovered, to his surprise, that his new theory was actually two theories in one. Weinberg was approaching the weak force with the seminal idea that pairs of particles it acted upon, electrons and neutrinos for example, might be identical as far as the force is concerned, just as yellow and white tennis balls are identical as far as the game of tennis goes. When he cast this idea into the mathematical language of quantum physics, Weinberg found that his theory necessarily included the electromagnetic force as well as the weak force. The mathematics required the union of the two forces. As he later remarked, “I found in doing this, although it had not been my idea at all to start with, that it turned out to be a theory not only of the weak forces, based on an analogy with electromagnetism; it turned out to be a unified theory of the weak and electromagnetic forces.” 

[1] As an aside, i find the constant chatter about matter being some sort of schizophrenic mix between particles and waves misleading, if not outright wrong. As stressed (to no avail, it seems) by Feynman (see and hear him on this and much more in his Vega Lectures, for instance), electrons (and photons, for that matter) are particles. You never detect half an electron, or a pi-fold-photon. There's always ticks in a detector (a photo-multiplier, a photographic plate, or trails in a Wilson chamber, for instance). The wave function is not real (neither in the physical nor in the mathematical sense of real), and it 'oscillates' in an imaginary space which is not even 3-dimensional when more than a particle is described. The interference patterns observed (which arise from the addition of complex amplitudes which are squared afterwards) are not associated with single electrons, the only thing wavelike (with a twist) about them being the statistics of their hits on the wall. Even if you believe in Bohm's pilot waves, the particles are still particles! Of course, there's ample room for analogy, but i still find the typical discussions misleading.

[2] The discovery of pulsars had also its share of serendipity. They were found, also unexpectedly, by Jocelyn Bell and Anthony Hewish while they were looking studying scintillating radio signals from compact sources. Jocelyn has written a lively report of their discovery, including the funny story of how they were on the verge on attributing the signals to extraterrestrials, and jokingly use monikers starting with the prefix LGM (for little green men) to name the misterious radio sources. There's also a good review of the tale over at the Hitchhiker's Guide to the Galaxy funny website.

[3] The pulsar discovery also won a Nobel in 1974. But, curiously enough, the undergraduate hero of the story (Jocelyn Bell) was not awarded this time. One wonders.

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The LISA Newsletter

May 14, 2006

The LISA International Science Community has just announced its LISC web portal. It will hopefully become the place for coordinating all LISA related resources, and, although it’s still much under construction, you’ll already find there the pretty neat first issue of the LISA Newsletter, which covers, in a very accessible way, a lot of interesting topics:

  • Introduction to Extreme-Mass-Ratio Inspirals. EMRIs (the capture of a stellar-mass body by a super-massive black hole) are one of the most important gravitational wave sources detectable in the frequency range (peaked at 3 mHz) covered by LISA. Here you’ll find a short and well-written introduction to their properties. If they capture your fancy, Eric Poisson‘s living review and this review article by Kostas Glampedakis are a good way of learning more. For non-experts, see also the less specialized presentations by Poisson.
  • Status Report on LISA Pathfinder. Pathfinder (the project i work on) was started in 2004 and aims at testing some of the technologies to be used in the real LISA experiment. The article is written by the mission’s principal investigator Stephano Vitale and (besides a good overview of our current efforts) contains nice pictures of some of the widgets that we plan to put up there by late 2009.
  • A Year of Breakthroughs in Numerical Relativity. This article gives an overview of the impressive progress last year in calculating waveforms of gravitational waves (see, e.g., Goddard’s black hole mergers). Knowing what kind of signals we are looking for will be extremely helpful in data-analysis activities once LISA is operational. Warm-up work in the latter area is also reviewed.

And there’s more! As i said, the newsletter is beautifully edited and aimed at non-specialists. Thus, it’s an excellent way of getting acquainted with the current status and prospects of our quest for these elusive waves.

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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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Gravity’s shadow

April 12, 2006

Ordering a book online is always an excellent excuse to order another one in the same pack, on the basis of amortizing the packing and sending expenses. Thus, Rovelli’s book came with good company: Harry Collins’ “Gravity’s Shadow : The Search for Gravitational Waves”, an awesome history of the search for gravitational waves from Weber to LIGO. Interestingly, it’s written by a sociologist, but a well-informed one: Collins has spent the last 30 years among physicists, so i expect he got the science right. Of course, by awesome, i mean that it looks awesome, since i haven’t read it yet (albeit it’s been inserted near the top of my growing reading queue). But i’m confident it will live up my expectations, specially after reading Lee Smolin’s opinion on it. One can also read some chapters available at the book’s site.
Weber and his detectorBy the way, you don’t need to be an expert to read this book, only have an interest in gravitational waves and/or the sociology of science serious enough to swallow around 900 pages. Unfortunately, i’m not aware of any other popularization on gravitational wave physics, but you can read this little introduction from ESA’s site (with links to the projects i work on, LISA and LISA Pathfinder) as a quick start, take a brief yet nice course at Caltech, read about them in the BBC, visit a museum, learn about the strong (albeit indirect) evidence on their behalf, bet on their discovery, or participate in it via Einstein@Home (the site also contains useful information about what GW are and how we try to detect them). You can also get a little more serious and go for this living review of GW detection or follow Kip Thorne’s course on Gravitational Waves (videos of the lectures included).

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