Archive for April, 2006

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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The sooner, the better

April 12, 2006

Rovelli’s “Quantum Gravity” arrived today (well, yesterday actually), sooner than i expected. And, albeit my plans were to finish Penrose (see reviews, specially this one) before starting this one, i’ve just read the first chapter and am hooked. This first chapter is an overview of Loop Quantum Gravity, a roadmap of what’s in store in the rest of the book. Basically, it’s an extended version of the essay i mentioned in my previous post, and is, i think, quite readable by non-specialists. The good news is that a draft version of the book is available online, so you don’t need to spend money to read it. I’ll try to keep (the two of) you posted on my readings.

Skimming the first chapter’s bibliography, i’ve remembered a couple of ‘popular-science’ books on QG i own: “Three Roads to Quantum Gravity” (Lee Smolin) and “Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity”. I read them 3 or 4 years ago, and they may be a good companion while commuting to work, and an alternative for those of you with a moderate interest in these matters.

If you have a bit of mathematical background, this introductory article is also a good way to get acquainted with the basics of LQG and one of its central results, the quantization of space. Have fun!

Rovelli on quantum gravity

April 11, 2006

Carlo Rovelli, one of the fathers of Loop Quantum Gravity, has just published a short, non-highly-technical overview of the conceptual issues to be faced by any theory merging Quantum Field Theory and General Relativity. I found specially enlightening his observations about time in General Relativity:

In general relativity, when we describe the dynamics of the gravitational field (not to be confused with the dynamics of matter in a given gravitational field), there is no external time variable that can play the role of observable independent evolution variable. The field equations are written in terms of an evolution parameter, which is the time coordinate x0, but this coordinate, does not correspond to anything directly observable. The proper time τ along spacetime trajectories cannot be used as an independent variable either, as τ is a complicated non-local function of the gravitational field itself. Therefore, properly speaking, GR does not admit a description as a system evolving in terms of an observable time variable. This does not mean that GR lacks predictivity. Simply put, what GR predicts are relations between (partial) observables, which in general cannot be represented as the evolution of dependent variables on a preferred independent time variable.

A deep point, easily overlooked even by experts in General Relativity.

Carlo’s article also contains an interesting bibliography for further study. I found there his A dialog on quantum gravity, a fun comparison of our two main contenders for a merger of quantum mechanics and GR. Although it is probably too jargon laden for casual readers (and, in my opinion, gives a totally biased view of the subject), the final remarks in this dialog prettily summarise a position which i tend to favour:

I think that string theory is a wonderful theory. I have a tremendous admiration for the people that have been able to build it. Still, a theory can be awesome, and physically wrong. The history of science is full of beautiful ideas that turned out to be wrong. The awe for the math should not blind us. In spite of the tremendous mental power of the people working in it, in spite of the string revolutions and the excitement and the hype, years go by and the theory isn’t delivering physics. All the key problems remain wide open. The connection with reality becomes more and more remote. All physical predictions derived from the theory have been contradicted by the experiments. I don’t think that the old claim that string theory is such a successful quantum theory of gravity holds anymore. Today, if too many theoreticians do strings, there is the very concrete risk that all this tremendous mental power, the intelligence of a generation, is wasted following a beautiful but empty fantasy. There are alternatives, and these must be taken seriously. Loop gravity is pursued by a far smaller crowd; has problems as well, as you pointed out, but is succeeding in places where strings couldn’t get, and is closer to reality. And if you think at the quantum excitations bulding up physical space, you truly see quantum mechanics and general relativity talking to one another. And is beautiful. I have an immense respect for string theorists, but I think it is time to explore something else. Don’t you think, to say the least, that both theories are worthwhile exploring?

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Two myths about special relativity

April 10, 2006

In a recent article, Two myths about special relativity[1], Ralph Baierlein provides a lucid discussion on two subtleties of special relativity. You may try to answer these questions by yourself and have a little fun:

  1. Does the Lorentz transformation reduce to the Galileo transformation for v/c << 1?
  2. What does really state Einstein’s second postulate on the velocity of light? That it is independent from the observer?

As it happens (and despite many authors answering both questions in the affirmative) the answer to both questions is ‘no’. In the first case, just take a look at Lorentz’s time transformation:

t1' - t0' = (1 - v^2/c^2)^{-1/2} ((t1 - t0) - (x1 - x0) v/c^2)

and note that, while you can safely approximate the square root as 1 when v/c is small, the term proportional to the spatial distance cannot be so mindlessly dropped: you can always find points for which it’s first order. Dropping it would be like saying that a rotation about the X axis reduces to a rotation about, say, the Z axis for small angles.

As for the second question, Einstein’s original postulates were: a) the principle of relativity, and b) that the velocity of a light ray respect of a given observer is independent of the emitter’s velocity. From these two principles (quite reasonable as they stand, specially if you think of light as some sort of wave), Einstein deduced (as you can do yourself with a little thought) that the velocity of light is the same for all observers (which is a less intuitive result): you don’t actually need to take that as a postulate.

Admittedly, nothing earth-shattering here, but it’s fine to review the basis (and clear up lurking misconceptions) every now and then.

[1] Unfortunately, you’ll need a subscription, a library with one, or a kind soul [2], to access the article.

[2] I wouldn’t say i have a mean soul.

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The Chymistry of Isaac Newton

April 9, 2006

When i was young(er), i had the immense fortune of having a mentor, Dr. Fidel Antonio Alsina, who guided me into discovering the wonderful world of physics. His lessons (by letter at first and later, when he came to Barcelona, in person) were mostly centered on relativity, but there were lots of opportunities to talk about physics and epistemology as well. He taught me to keep an open and critic mind, and not to accept arguments on the grounds of authority alone: he was fond of remembering that Newton believed sunspots were windows used by the inhabitants of the Sun. But, at the same time, he warned me against becoming a crackpot (“sages are silly, but not that silly”), and to study hard and respectfully the writings of great physicists, like Newton himself. I remember Fidel recommended me reading Newton’s Queries at the end of his Optiks to get a glimpse of how Newton’s mind worked, an advice that i can’t but pass on to all of you with a serious interest in science. As it comes, i can do even better, and point you directly to the Questiones quaedam Philosophiae, which are available online as part of The Newton Project, an ambitious initiative to put online all of Newton’s manuscripts. A related project is The Chymistry of Isaac Newton, which is publishing Newton’s alchemical works. I’m not specially interested in the later, except for their publication of Newton’s most complete laboratory notebook, which includes many of his optics experiments. A great way of seeing a great mind in action.

If you have not yet read any biography of Newton, of course the canonical reference is Westfall’s awesome Never at Rest : A Biography of Isaac Newton, but let me also recommend (for those of you with less time in your hands, and a library at hand, since it seems to be out of press) Gale E. Christianson’s In the presence of the Creator: Isaac Newton and his times, a book i enjoyed immensely.

Experimental gravity

April 8, 2006

The LATOR experimentAfter a few days in the abstract realms of Quantum Gravity, reading (and seeing) a bit about experimental physics today has been a refreshing experience. I picked up a couple of talks from the awesome streaming seminars collection at the Perimeter Institute for Theoretical Physics. If you search for Slava Turyshev, you’ll find an enticing talk about LATOR, and ongoing effort at JPL to test General Relativy in the PPN regime up to G^2. In the talk, Slava reviews current bounds (obtained by Solar System-based experiments) to Einstein’s theory agreement with experiment, which have improved from Edington’s one percent to 10^{-4} with the latest Doppler effect measurements from the Saturn Cassini mission. LATOR consists basically on putting in orbit two satellites (separated by 5 million kms) and observing them, using interferometers located at the International Space Station, when the Sun is passing in between (click figure on the left). The space-time curvature created by the Sun will alter the apparent position of the two space-crafts as seen from the ISS, which will be able to measure angular variations of the order of a pico-degree, thus testing GR’s predictions up to 10^{-9}. Pretty impressive. Before entering in the details of LATOR, Slava spends some time discussing the intriguing Pioneer anomaly, and i have bookmarked this article and this one to learn more about those puzzling results.

The second talk, of much lighter tone and aimed at non-specialists, was by Clifford Will and entitled Was Einstein Right? (look for it in the ‘EinsteinFest’ section of PI streaming page). He has been giving these talks about experimental checks of General Relativity for many, many years (besides studying them rigorously as part of his academic work, and publishing a couple of books), and it shows. Clifford is by now a real showman who knows how to make his talks interesting and fun for the general public. But you’ll enjoy the jokes even if you’re a specialist!

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Quantum Gravity

April 6, 2006

My research has always trodden the classical realms of General Relativity. As a result, my knowledge of advanced quantum field theories is shaky to say the least. Not that i wasn’t interested in foundational questions or in the quest for unification, or that i didn’t read something about those issues. Now i’d like to learn a bit more.

I’ve always found the fixed-background approach taken by QFT and (to my limited knowledge) String theory at odds with General Relativity’s spirit. In my opinion, gravitation is not just another force to be quantized, but rather a description of the background were other forces live in geometrical terms. If anything is to be quantized to obtain quantum gravity, it’s geometry. And since we seem to live in a world where every relevant quantity is discrete, the idea of a granular structure of space-time itself is, to me, all but natural. Although i read (without really understanding) that String theory predicts a quantized, relational space-time, the repeated affirmations of people like Witten about our not yet (?) understanding M-theory, together with the dismal multiverse speculations i’ve read recently (Lee Smolin explains exactly how i feel about it), have turned my attention, again, to Loop Quantum Gravity as an appealing subject of study. These days, i’ve been skimming several introductory and review articles, including the delicious Quantum Gravity Seminar by John Baez, and Smolin’s PI Course. The latter will be specially useful when my copy of Rovelli’s Quantum Gravity arrives in a couple of weeks, which will be about time to start studying LQG seriously, most probably guided by Christine Dantas’ excellent reading list on the subject.

Roger PenroseIn the meantime, i plan to end Roger Penrose’s The Road to Reality, which i’m sure i don’t need to review for you. In my case, this amazing book is being an excellent way of reviewing and relearning a lot of stuff i was supposed to know from the old times, and then discovering a lot of new things. I respect Penrose opinions immensely, and after seeing these recent lectures (half-way the page), i’m specially curious about his critiques of both String theory and Loop Quantum Gravity.

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Physics fiction

April 6, 2006

Just to start on the light side, i like to read novels and fiction featuring scientists, provided they present convincing, well-documented characters and situations. An excellent example of such a work i read recently (and wholeheartedly recommend) is Neal Stephenson‘s Quicksilver, where you’ll find Newton, Leibniz and the whole Royal Society playing an important part in the (otherwise fictional) history. Quicksilver’s sequels, which conform the so called Baroque Cycle, are awaiting in my shelves.

My other physics-fiction all time favourite is Gregory Benford‘s Timescape. Benford is a theoretical physicist, and it shows: this history about time travel is as sound as modern physics can make it.

I’m not much of a comic reader, but today this post captured my attention. Matt Fraction shares a sneak-preview of his forthcoming graphic novel, The Five Fists of Science, starring Mark Twain and Nikola Tesla in a race to save the world from Thomas Edison and J.P Morgan. The PDF preview looks pretty nice, with a charming nineteenth century atmosphere all around. Surely, not as good as Stephenson or Benford, but worth a look nonetheless.

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You’re never too old

April 5, 2006

Once upon a time, i completed a thesis on gravitational wave detectors, and got a Ph.D. that culminated more than twelve years of my life devoted to physics. Normally, that would have been just the beginning of a researcher’s career, but sometimes life intervenes and, as was my case, the doctorate marks instead what seems its end. I shifted gears and began a professional career as a programmer, not an unheard of story. During more than eight years, i’ve had lots of fun discovering a new world, full of interesting people, with its own myths, heroes and battles. I’ve tried hard to learn and even give something back, and, until a few months ago i was pretty happy with the idea of going on that way for the foreseeable future.

But then a little miracle happened. Despite not being a pro anymore, i had tried to read every now and then about physics (many books, and things like Scientific American or Physics Today, you know the drill) and to keep current (at a not too technical level) in latest advancements in the field. Thus, when last year somebody told me that he was going to a popular physics lecture (by some unknown guy) at my hometown’s science museum, i gladly joined the expedition. Two surprises were awaiting my arrival to the lecture room.

First, the lecturer was no other than Noble Laureate Frank Wilczek, who gave a nice talk about Dirac’s equation, including reminiscences about the man himself (since he was Wilczek professor during some time). Not bad for an unknown lecturer. The second surprise also involved an unexpected encounter, this time with my Ph.D. advisor. I hadn’t seen him for more than seven years and, naturally, there were lots of news and gossip to exchange after the lecture. To make a long history short, as a result of that talk i joined the Catalan Institute for Space Studies, where Alberto leads the Spanish contribution to ESA’s Lisa Pathfinder project, a forerunner of the planned LISA gravitational wave detector.

Until this fall, my work at IEEC is centered on software development. But once our application enters the testing and validation phases, i’ll have time to work on physics again. The Institute being a primarily a scientific institution, the plan is to get involved in LISA at a scientific level. Let me say that again: i’ll work on physics again. Who said dreams never come true? At first, a nagging doubt clouded my mind every now and then, a little devil whispering “you’re too old, you missed that train,” but i remembered a quote by one of my favourite authors:

It is never too late to be what you might have been.

So i will try again. I’ll work hard to relearn what i once knew, and then more. To trod the old paths, only a little bit wiser. In the old times, the web was in its infancy (i remember using Mosaic, and drooling over Netscape 1.0), but now i have at my disposal an amazing amount of on-line information, and a way to share my forthcoming toils and discoveries. I know i would have loved this when i started learning physics. So this blog’s intent is to share the joy of discovering physics, and physics, instead of boring autobiographical notes, will be the theme of the forthcoming entries in this space.

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