My household is on the way to Sweden already and I'm sitting in an apartment empty besides the bed and the modem. Yesterday was my last day at PI. Laurent Freidel said the good-bye words at the weekly wine & cheese, and everybody politely clapped their hands, though most of the people present I didn't know. Some of them where students from the recently started PSI program. Laurent did quite a good job, though I always find it weird to hear my CV summarized in a few words. It keeps getting longer. My CV looks so neat, whenever I look at it I wonder where is all the sweat and pain. Can I put them in an additional section?
Since there is no direct flights from Toronto to Stockholm anyway, I will have a few days stopover in Germany which also allows me to pick up some stuff I deposited there earlier. Not to mention that I should say hello to my husband occasionally so he doesn't forget he's married. In Stockholm, am staying the first month in a guest apartment. Experience tells me the first weeks in a new country are very busy, filled with bureaucracy, disorganization and frustration. So expect blogging to be sparse.
Some of you might have followed it on my Twitter account, two weeks ago I had a water damage in my apartment. The water tank in the storage room which preheats the warm water leaked. This must have happened sometime in the evening after I went to bed. I got up the next morning early to make a phone call to Europe and noticed all the carpet was soaking wet. Since I didn't know how to turn off the water and it kept running I had to wake the building manager. He looked at the disaster, turned off the water and reappeared some hours later with a vacuum cleaner sucking out what must have been about 100 liters of water. Later that day a plumber exchanged the water tank. It took a full week for the carpet to dry. Luckily nothing was really damaged, just that I had my moving boxes on the floor at that time. They soaked up the water and basically dissolved. I had to replace several of them and repack which was very annoying.
Though it was clean water, it turned out to leave stains in the carpet. An insurance guy came to assess the damage, later a "carpet expert" came to figure out where the spots came from. He said that there was probably rust in the tank which would have accumulated under the carpet and they'd have to take out the carpet, clean the floor, shampoo the carpet, and dry it. I was hoping they could do that after I've left, but of course it's more important to be nice to the new tenant who'll be moving in Tuesday. So yesterday evening I came home to find an empty living room with a loose carpet and wind machine tucked under it, such that the carpet moves in constant waves. Quite a dynamical inner design. I think it has potential.
The rest of my story in Waterloo will consist of somebody picking up the bed, somebody else picking up the car, my landlord picking up the apartment and house keys, and somebody dropping me off at the airport. You'll hear from me once I'm back in Europe. Meanwhile, a nice weekend to all of you.
Saturday, August 29, 2009
Wednesday, August 26, 2009
Paper Zapping
A nice quote from Strategic Reading, Ontologies, and the Future of Scientific Publishing, by Allen H. Renear and Carole L. Palmer (Science, 14 August 2009, p.829), on how scientists make use of the literature:
Now, as scientists search and browse, they are making queries and selecting information in much tighter iterations and with many different kinds of objectives in mind, almost as if they were playing a fast-paced video game. […] In a compelling analogy, Nicholas et al. describe a "slightly irritated" father watching his young daughter flick from channel to channel while watching television:
[the] father asks … why she cannot make up her mind and she answers that she is not attempting to make up her mind but is watching all the channels … gathering information horizontally, not vertically.
And they conclude
Now we see what the migration from traditional to electronic sources has meant in information seeking terms. We are all bouncers and flickers, and the success of Google is a testament to that, with its marvelous ability to enhance and amplify this flicking and bouncing (like a really good remote) … […]
Just as the aim of channel surfing is not to find a program to watch, the goal of literature surfing, is not to find an article to read, but rather to find, assess, and exploit a range of information by scanning portions of many articles.
Monday, August 24, 2009
A little less conversation, a little more science please
Last time I visited my parents, I was browsing through their magazines and happened to read a particularly upsetting commentary in "Stern," one of the three large weekly German magazines. Since I was sitting in front of my laptop anyway, I wrote a letter to the editor which indeed got published.Since my letter picked up a theme we've been discussing on this blog many times, the status of modern democracies and the future of politics, I thought I'll post a translation. The article I was replying to is here, (English via Google translate). It is one of the weekly appearing commentaries titled "Zwischenruf," which means roughly an interjection by the audience, written by Hans-Ulrich Jörges. He was addressing the ailing status of our democracies and the increasing frustration of citizens. The cure, he wrote, would be more grassroots democracy.
If you have read some of my posts you know why I couldn't let this uncommented. I've said a great many times that grassroots democracy works well only for a very limited amount of problems. Those in which there are clear and simple questions of large interest for the electorate, and the average person is able to make a qualified decision. Few questions in politics are of that sort, and few people have the time and the interest to deal with the cumbersome details of day to day politics. There are good reasons why we have representative democracies, none of which seems to have occurred to the author of that piece. But what annoyed me much more was that he didn't bother to back up his opinion with any sort of argument or evidence. All together, it was a completely useless ramble that wouldn't even have made a good blog post. If that's the hight of intellectual commentaries German magazines can provide, then poor Germany.
In any case, here's the letter. For whatever reason they dropped the first sentence in the print version.
"Hans-Ulrich Jörges addresses an important issue but draws the wrong conclusions. The decay of democracy and the incapacitation of the people is not a typically German phenomenon, but can be observed in an increasing number of modern democracies. It is a consequence of the inappropriateness of our political systems for increasingly complex tasks. Calling for more grassroots democracy is simple, but not a solution. What we need instead is a scientific, non-ideological, debate about how we can update our political systems to the status of the 21st century. We need less interjected opinions and more scientific studies examining how our political systems can be made more efficient and less frustrating."
What we need, in short, is thus something like the Lightcone Institute :-)
Friday, August 21, 2009
Anonymity in Science
I recently read a very provocative essay in Inside Higher ED
To put his point of view into perspective, I think it is useful to know where the author comes from, so here is some info from his website
Needless to say, I read his essay with the eyes of a physicist and found plenty to disagree. But
To put his point of view into perspective, I think it is useful to know where the author comes from, so here is some info from his website
Jeffrey R. Di Leo is Dean of the School of Arts & Sciences, and Associate Professor of English and Philosophy at the University of Houston-Victoria. He is also president of the Southern Comparative Literature Association.
Professor Di Leo has a dual Ph.D. in Philosophy and Comparative Literature from Indiana University. His teaching and research interests include ethics, contemporary innovative literature, classical American philosophy, aesthetics, philosophy of language, and literary and cultural theory.
Needless to say, I read his essay with the eyes of a physicist and found plenty to disagree. But
First some words on anonymity in general.
In an earlier post, I commented on anonymity in the blogosphere. Here too, I want to distinguish between anonymity and pseudonymity. The former is when the source of information is completely unknown, the latter is when the information is assigned to a name - a pseudonym - that is not connected to an actual identity but serves to address the person and also makes it possible to recognize them later. Most commenters on this blog are psydonymous rather than anonymous in that they have chosen a nickname and return with that same name.
I strongly discourage posting as "Anonymous" because as soon as there's two anonymousses in a comment section it becomes impossible to lead a decent conversation. In the long run, a pseudonym also allows me to learn what your background is and thus what reply to your questions might be useful. This saves all of us a lot of time and reduces misunderstandings.
Now back to Di Leo's essay against anonymity
Di Leo has a problem with anonymity of the peer review process. His main point is that "Anonymity is Anti-Dialogue," "Anonymous propositions are fundamentally monological, not dialogical," and that critical dialogue is the essence of progress in science. While this is correct, peer review is not actually anonymous but pseudonymous since the author can typically correspond with "reviewer 1" and "reviewer 2," mediated by the editor. Leaving that aside, Di Leo dislikes anonymity in general, and the one-sided anonymity of peer review in particular, since usually the reviewers know the identity of the author, but not vice versa.
Di Leo regards double-blind reviews in which the reviewer doesn't know the author's identity (which he calls "totally anonymous") as "not as problematic" as the standard partially anonymous peer review. However, as I said several times before, double-blind reviews are practically unfeasible in the 21st century. Not only because in an increasing number of fields papers can be found on pre-print servers long before they got published, but also because in specialized disciplines it isn't hard to figure out who wrote a paper on what topic. People have a distinct writing style (or absence thereof), and it is often not much of a secret what they have been working on recently. Let's thus forget about that option.
Di Leo then goes on to explain that "anonymous assessment is antithetical to the very idea of the academy," that "academia has created a culture and ethics of uncritical consent and has hidden it behind the cloak of collegiality," and further bold assessments about the status of academia. He writes
So let me comment on this.
First, science is done by humans. We know that humans are not perfectly rational, and that the human perception and opinion making process is easily biased. Disregarding such weaknesses leads to disasters as you can see for example in the ailing status of our financial systems. You can talk about the "idea of the academy" all you like, we don't live in the world of ideas. Reality isn't ideal, and the reason why utopias of all kind fail is that they envision an unrealistic human behavior. The academic system shouldn't be build upon somebody's ideal for scientific dialogue but upon reality. If we have good reason to think that some reviewers might feel the need to pay attention to the author's status and their own relation to it if their identity was revealed, then we can bemoan they feel this need but denying its existence isn't helpful.
Second, I completely agree that critical dialogue is essential to science, that anonymity makes it impossible, and that pseudonymity hinders it. However, I don't think peer review is the place to lead that dialogue. Peer review is the place to decide what fulfills the requirements of being published in a certain journal as a certain type of article. More often than not, the dialogue is lead through these publications and not in the review process.
Third, nobody forbids the reviewer to contact the author of a paper and reveal their identity should they wish so; I actually know people who do that frequently. It is thus factually wrong to say the peer review process "prohibits or prevents dialogue" as Di Leo states. One main reason why many reviewers do not engage in more dialogue with the author is not that they are afraid of it, but simply that they don't have the time. There are weeks when I receive 3 or 4 requests for referee reports. I can't sensibly lead an extended conversation with all of these authors, otherwise I wouldn't be doing anything else. I read the paper, write my report, and that's it. Some of the authors I know anyway, and they know my opinion. If you manage to reduce the number of written papers by a factor 100 or so we can talk about extended dialogue.
Fourth, the above statement that reviewers would not speak the truth if their identity was revealed and thus something must be wrong with the community is only partly correct. I strongly doubt any serious scientist would deliberately omit pointing out factual errors in a paper, whether anonymous or not. But that is only part of the review process. There are many journals for which factual correctness is a necessary requirement but far from being sufficient for publication. What also matters is whether a paper is appropriate for a journal, and how interesting it is. And that is, like it or not, to a large extend a personal assessment. It might very well be you find a paper more interesting if it was written by a Nobel Prize winner and that isn't even irrational. It is this part of the assessment that might make the reviewer feel uneasy if it wasn't anonymous because it is socially and politically very involved. Sure, scientists shouldn't take it personally if some of their colleagues don't find their work tremendously exciting, but some do.
Di Leo also briefly comments on blogs and scientific debate
While he means to point out the difference between blogs and scientific debates, he actually points out what's different between science and belief. You are not obligated to respond to persons who disagree with your believes, because you can believe whatever you want, may that be gremlins or the flying spaghetti monster. But that's not science. Likewise it's not science to ignore facts relevant to your research that have been brought into your attention, but it shouldn't matter by whom or where.
Where and from whom you obtained information might however affect how much attention you'll pay to it to begin with, and that is another reason against unnecessary use of anonymity. We live in the information age. Today's problem isn't obtaining information but filtering it, and the source of information is a very widely used and very effective filter.
Bottomline
Anonymity has its place in science, but it should be used sparsely and only if absolutely necessary. Anonymity has the disadvantage of making dialogue more complicated, I agree with Di Leo on that. But it has the advantage of avoiding unnecessary social and political baggage. I thus think anonymous peer review will remain essential to scientific publishing.
In an earlier post, I commented on anonymity in the blogosphere. Here too, I want to distinguish between anonymity and pseudonymity. The former is when the source of information is completely unknown, the latter is when the information is assigned to a name - a pseudonym - that is not connected to an actual identity but serves to address the person and also makes it possible to recognize them later. Most commenters on this blog are psydonymous rather than anonymous in that they have chosen a nickname and return with that same name.
I strongly discourage posting as "Anonymous" because as soon as there's two anonymousses in a comment section it becomes impossible to lead a decent conversation. In the long run, a pseudonym also allows me to learn what your background is and thus what reply to your questions might be useful. This saves all of us a lot of time and reduces misunderstandings.
Now back to Di Leo's essay against anonymity
Di Leo has a problem with anonymity of the peer review process. His main point is that "Anonymity is Anti-Dialogue," "Anonymous propositions are fundamentally monological, not dialogical," and that critical dialogue is the essence of progress in science. While this is correct, peer review is not actually anonymous but pseudonymous since the author can typically correspond with "reviewer 1" and "reviewer 2," mediated by the editor. Leaving that aside, Di Leo dislikes anonymity in general, and the one-sided anonymity of peer review in particular, since usually the reviewers know the identity of the author, but not vice versa.
Di Leo regards double-blind reviews in which the reviewer doesn't know the author's identity (which he calls "totally anonymous") as "not as problematic" as the standard partially anonymous peer review. However, as I said several times before, double-blind reviews are practically unfeasible in the 21st century. Not only because in an increasing number of fields papers can be found on pre-print servers long before they got published, but also because in specialized disciplines it isn't hard to figure out who wrote a paper on what topic. People have a distinct writing style (or absence thereof), and it is often not much of a secret what they have been working on recently. Let's thus forget about that option.
Di Leo then goes on to explain that "anonymous assessment is antithetical to the very idea of the academy," that "academia has created a culture and ethics of uncritical consent and has hidden it behind the cloak of collegiality," and further bold assessments about the status of academia. He writes
"The common rationale for academic anonymity is quite clear: if one were required to accompany one's assessment with one's true identity, one would not speak the truth."
So let me comment on this.
First, science is done by humans. We know that humans are not perfectly rational, and that the human perception and opinion making process is easily biased. Disregarding such weaknesses leads to disasters as you can see for example in the ailing status of our financial systems. You can talk about the "idea of the academy" all you like, we don't live in the world of ideas. Reality isn't ideal, and the reason why utopias of all kind fail is that they envision an unrealistic human behavior. The academic system shouldn't be build upon somebody's ideal for scientific dialogue but upon reality. If we have good reason to think that some reviewers might feel the need to pay attention to the author's status and their own relation to it if their identity was revealed, then we can bemoan they feel this need but denying its existence isn't helpful.
Second, I completely agree that critical dialogue is essential to science, that anonymity makes it impossible, and that pseudonymity hinders it. However, I don't think peer review is the place to lead that dialogue. Peer review is the place to decide what fulfills the requirements of being published in a certain journal as a certain type of article. More often than not, the dialogue is lead through these publications and not in the review process.
Third, nobody forbids the reviewer to contact the author of a paper and reveal their identity should they wish so; I actually know people who do that frequently. It is thus factually wrong to say the peer review process "prohibits or prevents dialogue" as Di Leo states. One main reason why many reviewers do not engage in more dialogue with the author is not that they are afraid of it, but simply that they don't have the time. There are weeks when I receive 3 or 4 requests for referee reports. I can't sensibly lead an extended conversation with all of these authors, otherwise I wouldn't be doing anything else. I read the paper, write my report, and that's it. Some of the authors I know anyway, and they know my opinion. If you manage to reduce the number of written papers by a factor 100 or so we can talk about extended dialogue.
Fourth, the above statement that reviewers would not speak the truth if their identity was revealed and thus something must be wrong with the community is only partly correct. I strongly doubt any serious scientist would deliberately omit pointing out factual errors in a paper, whether anonymous or not. But that is only part of the review process. There are many journals for which factual correctness is a necessary requirement but far from being sufficient for publication. What also matters is whether a paper is appropriate for a journal, and how interesting it is. And that is, like it or not, to a large extend a personal assessment. It might very well be you find a paper more interesting if it was written by a Nobel Prize winner and that isn't even irrational. It is this part of the assessment that might make the reviewer feel uneasy if it wasn't anonymous because it is socially and politically very involved. Sure, scientists shouldn't take it personally if some of their colleagues don't find their work tremendously exciting, but some do.
Di Leo also briefly comments on blogs and scientific debate
"If one, for example, posts on his or her blog a statement concerning one’s belief in gremlins, one is not obligated to respond to persons who disagree with this statement. However, in the academy, students, faculty, and administration are expected to answer to questions about their opinions."
While he means to point out the difference between blogs and scientific debates, he actually points out what's different between science and belief. You are not obligated to respond to persons who disagree with your believes, because you can believe whatever you want, may that be gremlins or the flying spaghetti monster. But that's not science. Likewise it's not science to ignore facts relevant to your research that have been brought into your attention, but it shouldn't matter by whom or where.
Where and from whom you obtained information might however affect how much attention you'll pay to it to begin with, and that is another reason against unnecessary use of anonymity. We live in the information age. Today's problem isn't obtaining information but filtering it, and the source of information is a very widely used and very effective filter.
Bottomline
Anonymity has its place in science, but it should be used sparsely and only if absolutely necessary. Anonymity has the disadvantage of making dialogue more complicated, I agree with Di Leo on that. But it has the advantage of avoiding unnecessary social and political baggage. I thus think anonymous peer review will remain essential to scientific publishing.
I would welcome however if the communication in the peer review process wasn't so cumbersome. It is feasible for example that the author and the reviewer lead a discussion without each exchange being mediated by the editor, eg in some interface similar to a chat. After some weeks, the reviewer could summarize his impression from the exchange. This would allow it to easily clarify some general questions or possible misunderstandings.
On the general theme, the problems Di Leo addresses are at the core caused by the four pressures that I have discussed here. Financial pressure, time pressure, peer pressure, and public pressure all skew scientists' opinions and make it necessary for them to pay attention to things other than their true scientific assesment. The solution is thus to reduce these pressures as far as possible.
Monday, August 17, 2009
Office Stuff
The other day I cleaned out my desk at PI. It is remarkable how fast clutter accumulates in offices, especially in places with a large turnover rate.
The desk drawers typically feature a selection of paper clips in all colors, sizes and shapes together with various other supply like colorful textmarkers, tippex, post-its, and undefined sticky substances. The first desk I got in a department of physics was passed on to be my by the previous only-women-at-the-institute. She nicely left some tampons. When I arrived at PI my desk drawer unit was full with my new office mate's files. It took me several months to convince him I really do need some space myself. (Nice girls don't only not get the corner office, they don't even get a desk drawer.) Looking into the bookshelf next to my desk, it was filled with notes and papers and folders and business cards from pre- and pre-decessors.
I changed office during my second year at PI due to an incurable disagreement with my officemate about what constitutes a life-friendly room temperature. This degraded me down to the 2nd floor where the view isn't so pretty, but also upgraded me to a window desk. While there was still plenty of space for my few things in the shelf, that office too already featured some folders and files of unknown origin, some filled, some empty. I also found a copy of Weinberg's first QFT volume that I happily used. It doesn't have any library stickers on it, and I have no clue whose it is. I'll leave it for whoever gets the desk next.
It is also always interesting what's under the keyboard or to check if there are sticky notes in the drawers. They will typically feature phone numbers, email addresses, or cryptic combinations of numbers and digits that probably are some of the dreaded random-generated keywords the insufficient human memory is sometimes forced to deal with.
And PI isn't even yet 10 years old! In the advanced stages, it will look like the ITP in Frankfurt did. Upon being shown your new desk, you realized all bookshelves were full already with books, papers, files and notes that had accumulated over decades and that nobody ever dared to throw out. After all, they belong to somebody, right? Offprints and proceedings seem to be particularly vulnerable to abandonment. The walls will be covered with helpful notes, like "if the phone doesn't work push the plug," or "if AC runs havoc call XYZ & chatter with your teeth."
And let's not even talk about hygiene. Shortly after I changed offices I accidentally spilled coffee on the floor. A full year later, the spills where still there. Acting out on a severe case of PMS I asked what the cleaning staff is actually payed for. Next day, the coffee spills where gone, but the floor is still covered with dead insects and spider webs in the corners. The occasional pencil stroke on the desk is also preserved for future generations. A study by the University of Arizona found the typical desk has hundreds of times more bacteria per square inch than an office toilet seat.
It's also amazing what people store in their offices. One prof I knew used to dry his bathing trunks in his office. Many have various sports equipment and clothes, towels, and shoes. I've also come across shaving supply, suitcases, plumber's helpers, silver- and dishware, condomes, ropes, empty pizza-boxes and leftover food in advanced stages of decay, plush toys, action figures, pillows, pet food, jumper cables, and toys, tools and technical equipment of all sorts. Somebody should make a movie about physicists being trapped in their institute, needing to come up with some ingenious construction to save their lives from what's to be found in their offices only.
I'm looking forward to my next desk :-)
Sunday, August 16, 2009
That Photon from GRB090510
A brief comment on the recent paper
which reports the detection of an high energetic photon of 31 GeV within the first second of onset of the Gamma Ray Burst (GRB) 090510. As I explained in earlier posts (here, here and here), some scenarios with Lorentz Invariance Violation (LIV) motivated by quantum gravity do predict an energy dependence in the travel time of photons. Over such long distances as ours to the origin of the GRB, tiny delays add up and can upon arrival of the photon in our detectors result in an energy-dependent modification of the signal. The case that has been favored in the last years is that photons with high energies would arrive later than the lower energetic ones even when emitted simultaneously.
[Via Stefan via Lubos].
- Testing Einstein's special relativity with Fermi's short hard gamma-ray burst GRB090510
By Fermi GBM/LAT Collaborations
arXiv:0908.1832v1 [astro-ph.HE]
which reports the detection of an high energetic photon of 31 GeV within the first second of onset of the Gamma Ray Burst (GRB) 090510. As I explained in earlier posts (here, here and here), some scenarios with Lorentz Invariance Violation (LIV) motivated by quantum gravity do predict an energy dependence in the travel time of photons. Over such long distances as ours to the origin of the GRB, tiny delays add up and can upon arrival of the photon in our detectors result in an energy-dependent modification of the signal. The case that has been favored in the last years is that photons with high energies would arrive later than the lower energetic ones even when emitted simultaneously.
Previously reported cases indeed indicated high energetic photons arrive with a measurable delay. The status of the constraints on LIV derived from these events was nicely summarized in Lee and Giovanni's recent paper. One has to keep in mind though, without a precise knowledge of the emitting source it is hard to tell whether a measured effect happened during propagation. Besides this, the LIV modification in the propagation of photons is only measurable if it is a first order effect (of order Energy/Planck mass). If it is quadratic or higher, the effect would be too small to affect the spectrum of GRBs.
To let you know what my stakes are on the matter, I don't find it plausible quantum gravity would affect the propagation of photons from GRBs. I wrote a paper on that some while ago that explains very clearly why. Without going to much into the details, whether or not quantum gravitational effects are relevant depends on the energy density, not the energy. The energy density even of the GRB itself, and certainly that of the traveling photons, is many orders of magnitude too small to cause any measurable effect. It adds to this that models with violations of Lorentz invariance do either break Lorentz invariance, on which there are strong constraints from many experiments already, or they "deform" Lorentz invariance (known as "Deformed Special Relativity"), which causes conceptual difficulties that are so far unresolved. For these reasons, I am not very convinced such a quantum gravitationally induced energy-dependence of the speed of light would be observable in GRBs. It is however an interesting and testable scenario.
In any case, the measurements of GRB090510 reported in the above mentioned paper do clearly not support the energy-dependence of the speed of light. In the paper, they derive a bound on the Planck scale from their measurements that is some orders of magnitude higher than we expect the Planck scale to be. This means if there was an effect it is much smaller than it should have been. However, it gives me the creeps if people draw conclusions from single photons. As also Lee and Giovanni pointed out explicitly in their paper, the propagation could have a stochastic component since it's a modification caused by a quantum gravitational effects of the background. In that case, only more statistic could allow conclusions. It also remains to be explained what caused the delay in the other measured gamma ray bursts.
Thus, stay tuned...
[Via Stefan via Lubos].
Friday, August 14, 2009
News from Other Worlds
This week, I came across some quite amazing news about planets at other stars in our galaxy. But it's not just the stories of planetary collisions and retrograde orbits that have fascinated me: It's also how all this has been learned, by closely analyzing light curves and spectra. So, here are a the plots behind the news:
Direct light from an extrasolar planet
So far, extrasolar planets have mainly been detected indirectly - by dimming the light of the star during transit for example, or by a periodically changing Doppler shift. It's very hard to see the light reflected by the planet itself. This week, the Kepler team has reported clear evidence of the light form planet HAT-P-7b, in orbit around a star in the constellation Cygnus. Here is the proof:
Light curve of the parent star of planet HAT-P-7b, shown over a bit more than one period. From Kepler’s Optical Phase Curve of the Exoplanet HAT-P-7b by W. J. Borucki et al., Science 325 (7 August 2009) 709.
The figure shows the light curve of the parent star, its apparent luminosity over a time span of about three days. As can be seen in the upper figure, the luminosity drops very clearly by about 0.6 percent every two days or so - that's when the planet transits in front of the star, and darkens a bit its disk.
But wait - there is a second slight dip in the light curve right halfway in between two eclipses. Looking at the curve on another scale in the middle of the figure, we see that the luminosity gently increases and decreases by 0.01 percent over one period, with a marked drop back to "normal" in the middle.
This gentle variation of the light curve comes from the light reflected by the planet! The drop in the middle occurs when the planet is hidden behind the star on its orbit.
If only it was possible to detect such small changes in the spectrum of the light, it may tell interesting stuff about the planet.
Planetary collisions at HD 172555
Speaking of spectra... The Spitzer Infrared Space Telescope has observed a dust cloud around the star HD 172555 in the southern constellation of Pavo. The resulting spectrum of infrared light can best be understood by assuming that quite a dramatic planetary collision has happened at this star a few thousand years ago.
Infrared Spectrum of the dust disk around star HD172555. From Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ∼12 Myr HD172555 System by C.M. Lisse et al., Astrophys.J. 701 (2009) 2019-2032, arXiv:0906.2536v2
The spectrum - the black, noisy curve - shows a general blackbody shape, with additional features that are typical for silicate particles.
Silicates, the matter of sand and dust, contain silicon-oxygen bonds that produce infrared bands whose shape and location also depend of the specific crystal structure, and thus are very characteristic for the different types of silicates. A unique feature in the infrared spectrum of the dust cloud of HD 172555 is the sharp peak at a wavelength of about 9 micrometre. This peak can be understood as produced by a mixture of mainly two types of silicates, tektite and obsidian.
There is something special about these two silicates: They are glass-like, and they are produced by melting and rapid cooling of other silicates materials. Tektite is a telltale sign of the impact of large meteorites on the Earth's surface.
Now, for finding such large amounts of tektite and obsidian in the dust cloud of HD 172555, there is just one plausible explanation: There must have been a collision of Moon- to Mercury-sized planets orbiting the star, similar to the collision that happened to the early Earth which is supposed to be the origin of the Moon!
Retrograde planet WASP-17b
A collision might also have caused the retrograde motion of planet WASP-17b, which orbits star WASP-17 in the constellation of Scorpius.
One usually assumes that a star and its planets originate by collapsing from the same rotating cloud of dust. Hence, the rotation of the star and the revolution of its planets should be in the same sense - the spin of the star and the angular velocity of the planets should be parallel. This is the case, for example, for all the planets in the solar system. A planet revolving "in the wrong direction" is called retrograde.
Surprisingly, it is possible to check if the revolution of a planet is normal or retrograde if the planet transits the star. Due to the rotation of the star, one half of it has a component of motion towards our line of sight, while the other half is moving away from us. This implies a Doppler effect towards the blue and to the red for the different halves of the disk of the star, respectively. When averaging over the whole disk of the star, this results in a broadening of the spectral features. But when a planet transits the star and thus blocks parts of its light, a net Doppler effect can bee seen. This is the so-called Rossiter-McLaughlin effect.
Doppler shift of the light from star WASP-17 during the transit of its planet, WASP-17b. From WASP-17b: an ultra-low density planet in a probable retrograde orbit by D. R. Anderson et al, arXiv:0908.1553v1.
The curve shows the Doppler shift of the light from star WASP-17, measured with the CORALIE spectrograph shortly before, during and after the transit of its planet WASP-17b. If the planet revolves in the same direction as the star rotates, there should first be a net redshift: The planet appears in front of star on the side which is moving towards us, thus blocking the blueshifted light. For the analogous reason, there should be a net blueshift at the end of the transit. This "normal situation" corresponds to the grey, dotted spike-like curve in the figure.
But the data points evidently fit much better to the opposite case: First a blueshift, then a redshift. This is the sign of a retrograde orbit!
I do not know if there is more than mere speculation as to what has happened to planet WASP-17b in the past to make it revolve in "the wrong direction" - but maybe we can learn more from future spectra.
... and a meteorite on Mars
After all these abstract curves which have such vivid interpretations, here is very concrete photo (thanks to Andi for the link):
It is an iron meteorite, lying on the surface of mars, and stumbled upon by the Mars rover Opportunity.
Without any detailed spectroscopy, it also seems to tell us something: When the meteorite fell on Mars, the Martian atmosphere must have been much denser than it is today, because otherwise the impact on ground would have been strong enough to destroy the meteorite and create a big crater.
TAGS: astronomy, spectroscopy, extrasolar planet
Direct light from an extrasolar planet
So far, extrasolar planets have mainly been detected indirectly - by dimming the light of the star during transit for example, or by a periodically changing Doppler shift. It's very hard to see the light reflected by the planet itself. This week, the Kepler team has reported clear evidence of the light form planet HAT-P-7b, in orbit around a star in the constellation Cygnus. Here is the proof:
Light curve of the parent star of planet HAT-P-7b, shown over a bit more than one period. From Kepler’s Optical Phase Curve of the Exoplanet HAT-P-7b by W. J. Borucki et al., Science 325 (7 August 2009) 709.
The figure shows the light curve of the parent star, its apparent luminosity over a time span of about three days. As can be seen in the upper figure, the luminosity drops very clearly by about 0.6 percent every two days or so - that's when the planet transits in front of the star, and darkens a bit its disk.
But wait - there is a second slight dip in the light curve right halfway in between two eclipses. Looking at the curve on another scale in the middle of the figure, we see that the luminosity gently increases and decreases by 0.01 percent over one period, with a marked drop back to "normal" in the middle.
This gentle variation of the light curve comes from the light reflected by the planet! The drop in the middle occurs when the planet is hidden behind the star on its orbit.
If only it was possible to detect such small changes in the spectrum of the light, it may tell interesting stuff about the planet.
Planetary collisions at HD 172555
Speaking of spectra... The Spitzer Infrared Space Telescope has observed a dust cloud around the star HD 172555 in the southern constellation of Pavo. The resulting spectrum of infrared light can best be understood by assuming that quite a dramatic planetary collision has happened at this star a few thousand years ago.
Infrared Spectrum of the dust disk around star HD172555. From Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ∼12 Myr HD172555 System by C.M. Lisse et al., Astrophys.J. 701 (2009) 2019-2032, arXiv:0906.2536v2
The spectrum - the black, noisy curve - shows a general blackbody shape, with additional features that are typical for silicate particles.
Silicates, the matter of sand and dust, contain silicon-oxygen bonds that produce infrared bands whose shape and location also depend of the specific crystal structure, and thus are very characteristic for the different types of silicates. A unique feature in the infrared spectrum of the dust cloud of HD 172555 is the sharp peak at a wavelength of about 9 micrometre. This peak can be understood as produced by a mixture of mainly two types of silicates, tektite and obsidian.
There is something special about these two silicates: They are glass-like, and they are produced by melting and rapid cooling of other silicates materials. Tektite is a telltale sign of the impact of large meteorites on the Earth's surface.
Now, for finding such large amounts of tektite and obsidian in the dust cloud of HD 172555, there is just one plausible explanation: There must have been a collision of Moon- to Mercury-sized planets orbiting the star, similar to the collision that happened to the early Earth which is supposed to be the origin of the Moon!
Retrograde planet WASP-17b
A collision might also have caused the retrograde motion of planet WASP-17b, which orbits star WASP-17 in the constellation of Scorpius.
One usually assumes that a star and its planets originate by collapsing from the same rotating cloud of dust. Hence, the rotation of the star and the revolution of its planets should be in the same sense - the spin of the star and the angular velocity of the planets should be parallel. This is the case, for example, for all the planets in the solar system. A planet revolving "in the wrong direction" is called retrograde.
Surprisingly, it is possible to check if the revolution of a planet is normal or retrograde if the planet transits the star. Due to the rotation of the star, one half of it has a component of motion towards our line of sight, while the other half is moving away from us. This implies a Doppler effect towards the blue and to the red for the different halves of the disk of the star, respectively. When averaging over the whole disk of the star, this results in a broadening of the spectral features. But when a planet transits the star and thus blocks parts of its light, a net Doppler effect can bee seen. This is the so-called Rossiter-McLaughlin effect.
Doppler shift of the light from star WASP-17 during the transit of its planet, WASP-17b. From WASP-17b: an ultra-low density planet in a probable retrograde orbit by D. R. Anderson et al, arXiv:0908.1553v1.
The curve shows the Doppler shift of the light from star WASP-17, measured with the CORALIE spectrograph shortly before, during and after the transit of its planet WASP-17b. If the planet revolves in the same direction as the star rotates, there should first be a net redshift: The planet appears in front of star on the side which is moving towards us, thus blocking the blueshifted light. For the analogous reason, there should be a net blueshift at the end of the transit. This "normal situation" corresponds to the grey, dotted spike-like curve in the figure.
But the data points evidently fit much better to the opposite case: First a blueshift, then a redshift. This is the sign of a retrograde orbit!
I do not know if there is more than mere speculation as to what has happened to planet WASP-17b in the past to make it revolve in "the wrong direction" - but maybe we can learn more from future spectra.
... and a meteorite on Mars
After all these abstract curves which have such vivid interpretations, here is very concrete photo (thanks to Andi for the link):
It is an iron meteorite, lying on the surface of mars, and stumbled upon by the Mars rover Opportunity.
Without any detailed spectroscopy, it also seems to tell us something: When the meteorite fell on Mars, the Martian atmosphere must have been much denser than it is today, because otherwise the impact on ground would have been strong enough to destroy the meteorite and create a big crater.
TAGS: astronomy, spectroscopy, extrasolar planet
Tuesday, August 11, 2009
Four Stages of Science
In the comments to our recent post And how open would you want your science? Arun pointed out an interesting paper
In this paper, Shneider suggests to distinguish four different stages of a scientific discipline, that I will briefly summarize below. It is somewhat ironic the author writes he believes "this analysis could be instrumental for individual researchers in their career planning," but then publishes his paper in a subscription journal that wants to charge you US $ 31.50 for a 6 page article.
The identification of different stages through which a research program goes is an approach that resonates with me. I have been previously referring to this vaguely as "the stage of creative process," and have pointed out many times that there can be no overall prescription for what amount of "transformative" and "conservative" research a discipline needs that does not take into account different fields are in different creative phases. Thus any call for more support of one or the other research style is an oversimplified panacea that might or might not work in one or the other case.
However, though Shneider's paper is interesting, I don't find it very well thought through. In particular, the author tends to speak of the characteristics of a "science" and of "scientists," though I doubt these ever occur in a pure form. It would be more useful to characterize a specific research project, and then identify the stage of the discipline by what sort of projects are mainly pursued, or similarly characterize a scientist by what sort of projects he mainly works on. You also wouldn't call a restaurant or its cook "spicy," you'd call a dish spicy and then say the restaurant offers many spicy dishes, and the cook is known for them.
In any case, here the characteristics of the four stages:
Stage 1:
Stage 2:
Stage 3:
Stage 4:
The paper also has some remarks on how these four stages relate to Kuhn's theory of scientific revolution. The author points out that Kuhn was aiming at characterizing paradigm shifts, not the life-cycle of scientific disciplines.
I think one should consider that a discipline might run into a case of arrested development in any stage, in which case too much effort goes into the wrong research direction. Unfortunately, such cases might become self-supporting due to the present organization of the academic system in which people go where money goes, and money goes where people go. This leads to the formation of the scientific analog to economic bubbles. As a result, the amount of people working in a field does not accurately reflect its actual promise.
Shneider provides many examples, but these are dominantly from past centuries and from biology and chemistry. If you have a current example, leave it in the comments.
- Four stages of a scientific discipline; four types of scientist
By Alexander M. Shneider
Trends in Biochemical Sciences
Volume 34, Issue 5, May 2009, Pages 217-223
In this paper, Shneider suggests to distinguish four different stages of a scientific discipline, that I will briefly summarize below. It is somewhat ironic the author writes he believes "this analysis could be instrumental for individual researchers in their career planning," but then publishes his paper in a subscription journal that wants to charge you US $ 31.50 for a 6 page article.
The identification of different stages through which a research program goes is an approach that resonates with me. I have been previously referring to this vaguely as "the stage of creative process," and have pointed out many times that there can be no overall prescription for what amount of "transformative" and "conservative" research a discipline needs that does not take into account different fields are in different creative phases. Thus any call for more support of one or the other research style is an oversimplified panacea that might or might not work in one or the other case.
However, though Shneider's paper is interesting, I don't find it very well thought through. In particular, the author tends to speak of the characteristics of a "science" and of "scientists," though I doubt these ever occur in a pure form. It would be more useful to characterize a specific research project, and then identify the stage of the discipline by what sort of projects are mainly pursued, or similarly characterize a scientist by what sort of projects he mainly works on. You also wouldn't call a restaurant or its cook "spicy," you'd call a dish spicy and then say the restaurant offers many spicy dishes, and the cook is known for them.
In any case, here the characteristics of the four stages:
Stage 1:
- Introduction of new subject matter
- New scientific language
- Often based on new observations and/or experimental results
- First stage scientists not necessarily the ones who discover new facts
- First stage scientists often need to be somewhat imprecise or inaccurate because not all necessary facts are known or properly comprehended
- Theory often contains uncertainty
- First stage scientists do not always possess exquisite technical skills.
- Philosophical, aesthetic and cultural views, analogies and literature are instrumental to the first stage scientists' mode of thinking
Stage 2:
- Development of major techniques
- Often re-applications of methods previously developed in another discipline (plus rethinking and adjustments to new task)
- Main characteristic of second stage scientists are ingenuity and inventiveness, an ability to implement ideas and a high risk-tolerance
Stage 3:
- Most of the actual data and useful knowledge is generated
- Re-description of subject matter, creation of new insights and questions
- Difficulties and unexplained phenomena often give birth to new first stage
- Most useful personal qualities of third stage scientists are detail oriented, neat, hard working
- Extensive knowledge of philosophy or art is not instrumental
Stage 4:
- Communication and carrying on of knowledge
- Reviews, organization of knowledge
- Without the fourth stage scientists, the explosion of new data generated at the third stage would be chaotic
- Development of applications
- Re-evaluation of the role of the discipline in a possibly changing social and cultural context
- Forth stage scientists use a broad spectrum of cultural and philosophical views
- Forth stage work serves to inspire new generations of scientists
The paper also has some remarks on how these four stages relate to Kuhn's theory of scientific revolution. The author points out that Kuhn was aiming at characterizing paradigm shifts, not the life-cycle of scientific disciplines.
I think one should consider that a discipline might run into a case of arrested development in any stage, in which case too much effort goes into the wrong research direction. Unfortunately, such cases might become self-supporting due to the present organization of the academic system in which people go where money goes, and money goes where people go. This leads to the formation of the scientific analog to economic bubbles. As a result, the amount of people working in a field does not accurately reflect its actual promise.
Shneider provides many examples, but these are dominantly from past centuries and from biology and chemistry. If you have a current example, leave it in the comments.
Saturday, August 08, 2009
Interna
My move is slowly making progress. Meanwhile I'm getting increasingly edgy because I haven't had time to do as much as read a single paper for almost a week. I think I have withdrawal symptoms.
I managed to sell almost all of my furniture, except for a lonely armchair that hasn't found a new friend yet. The woman who bought the bed luckily doesn't need it before next month. She agreed to pick it up the day I'm leaving so I won't have to sleep on the floor. I washed and vacuum cleaned the car and put "For Sale" signs in the windows. Today somebody took a testdrive and said he's very interested and will get back to me.
The moving company is scheduled to pick up my stuff in 10 days. From here it goes to the East Coast, on a ship and over to Europe. I'm told the port of first entry will be in the UK. Luckily, unlike in Canada, I don't have to appear in person at customs. (Though customs Canada was more or less a joke. After my household items arrived, I had to appear at customs at the closest airport. A women asked if I had imported any weapons. When I correctly replied "No," she stamped my documents, said "Welcome to Canada," and that was that.)
In the UK the stuff will go back on the road and then I don't know. Moving companies, I had to learn, work in mysterious ways. In particular, they like to collect sufficient stuff to get a truck full. My things don't fill an 18 wheeler, thus they are likely to sit around in various European cities for an undetermined amount of time.
Sorting through my stuff, I found a lot of notes for topics I meant to blog about but then didn't. I also found a napkin ring that I absendmindely played with and then took with me first time we went to dinner at Hannah's. Hannah's used to be the best restaurant around, but it closed recently.
If you have ever been in Waterloo, you will have seen its landmark, a pyramid of barrels in front of the old Seagrams Distillery (which now hosts the Center for International Governance Innovation). It's just opposite Perimeter Institute. The pyramid has been removed in the last days to make place for something called the "Balsillie Centre of Excellence," about to be build there. Residents could come and pick up a barrell, I hear the barrels were gone pretty fast; more than 500 were distributed to members of the community.
Perimeter Institute itself is also in the middle of changes. The constructions for the building extension have begun. Most of the parking lot is closed, fenced and filled with fancy working equipment. It is also the time of the year when new people come in and the expired folks, like me, go out.
I'm sitting in my apartment, surrounded by moving boxes, trash, and piles of unsorted papers. I've cancelled my electricity, water, and internet services. I've handed in my notice to move out. I dealt with the insurances, the automobile club. It feels like I just yesterday opened the bank account, now I'm trying to find a smart way to close it. Inbetween, I'm trying to learn a few words of Swedish. En, tva, tre, fyra, fem, sex, sju, atta, nio, tio. Jag talar bara lite svenska. I haven't figured out yet how to get the little circles over the vowels.
Things are changing. It's kind of comforting this blog stays the same.
I managed to sell almost all of my furniture, except for a lonely armchair that hasn't found a new friend yet. The woman who bought the bed luckily doesn't need it before next month. She agreed to pick it up the day I'm leaving so I won't have to sleep on the floor. I washed and vacuum cleaned the car and put "For Sale" signs in the windows. Today somebody took a testdrive and said he's very interested and will get back to me.
The moving company is scheduled to pick up my stuff in 10 days. From here it goes to the East Coast, on a ship and over to Europe. I'm told the port of first entry will be in the UK. Luckily, unlike in Canada, I don't have to appear in person at customs. (Though customs Canada was more or less a joke. After my household items arrived, I had to appear at customs at the closest airport. A women asked if I had imported any weapons. When I correctly replied "No," she stamped my documents, said "Welcome to Canada," and that was that.)
In the UK the stuff will go back on the road and then I don't know. Moving companies, I had to learn, work in mysterious ways. In particular, they like to collect sufficient stuff to get a truck full. My things don't fill an 18 wheeler, thus they are likely to sit around in various European cities for an undetermined amount of time.
Sorting through my stuff, I found a lot of notes for topics I meant to blog about but then didn't. I also found a napkin ring that I absendmindely played with and then took with me first time we went to dinner at Hannah's. Hannah's used to be the best restaurant around, but it closed recently.
If you have ever been in Waterloo, you will have seen its landmark, a pyramid of barrels in front of the old Seagrams Distillery (which now hosts the Center for International Governance Innovation). It's just opposite Perimeter Institute. The pyramid has been removed in the last days to make place for something called the "Balsillie Centre of Excellence," about to be build there. Residents could come and pick up a barrell, I hear the barrels were gone pretty fast; more than 500 were distributed to members of the community.
Perimeter Institute itself is also in the middle of changes. The constructions for the building extension have begun. Most of the parking lot is closed, fenced and filled with fancy working equipment. It is also the time of the year when new people come in and the expired folks, like me, go out.
I'm sitting in my apartment, surrounded by moving boxes, trash, and piles of unsorted papers. I've cancelled my electricity, water, and internet services. I've handed in my notice to move out. I dealt with the insurances, the automobile club. It feels like I just yesterday opened the bank account, now I'm trying to find a smart way to close it. Inbetween, I'm trying to learn a few words of Swedish. En, tva, tre, fyra, fem, sex, sju, atta, nio, tio. Jag talar bara lite svenska. I haven't figured out yet how to get the little circles over the vowels.
Things are changing. It's kind of comforting this blog stays the same.
Wednesday, August 05, 2009
This and That
- Clifford Johnson now makes into movies.
- For those of you who speak German: the German Science Foundation (DFG) now features video clips of scientists reporting on their research. As amusing as informative. (Thanks to Andi!)
- The Quark Matter, the largest regular nuclear physics conference, awarded this year a "Young Scientist Award," which went to Sarah LaPointe for the best poster presentation and to Mateusz Ploskon for the best oral presentation. I think such an award is a lovely idea. Makes me a little nostalgic, since I won the poster award at the Quark Matter 2004.
- The previously mentioned Quantum to Cosmos Festival in honor of PI's 10th birthday has a blog.
Monday, August 03, 2009
Röser's equation, again
Over the weekend, I thought again about Röser's equation relating the transition temperature of high-temperature superconductors to a characteristic length scale called "doping distance". Having reread the Proceedings paper where it is described, I wanted to add a few comments to our last week's post, only to realize that blogger has a restriction to the length of a comment. So, here is a second post.
Let me just repeat the main point of the "Röser formula":
It claims a simple linear relation between the inverse of the superconducting transition temperature of a wide range of superconducting materials, and a so-called "doping distance". This relation is shown in the following plot:
(from: A Correlation Between Tc of Fe-based HT Superconductors and the Crystal Super Lattice Constants of the Doping Element Positions by Felix Huber, Hans Peter Roeser, Maria von Schoenermark, Proc. Int. Symp. Fe-Pnictide Superconductors, J. Phys. Soc. Jpn. 77 (2008) Suppl. C, pp. 142-144)
Here are a few more thoughts of mine about this relation.
Thermal de Broglie wavelength
Using the standard definition of the thermal de Broglie wavelength for a Cooper pair of two electrons with free masses, the Röser equation
4 π k me(2 x)2 n−2/3 = h2/ Tc
actually boils down to
(2 x) × n−1/3 = λc
where λc is the thermal de Broglie wavelength of the Cooper pair at the critical temperature Tc, where superconductivity breaks down.
In the proceedings paper, n = 1, so the factor n−1/3 can be dropped. Moreover, it is said to take values of n = 2 or 3 for other superconductors with a layer structure, depending on the number of layers in the unit cell. It's not completely clear to me how it is motivated.
Substances Covered
In the proceedings paper, the authors discuss doped iron arsenides LO(1−Δ)F(Δ)FeAs, where L is a rare earth, L = La (Lanthanum), Gd (Gadolinium), Ce (Cerium), Pr (Praseodymium), Nd (Neodymium) or Sm (Samarium) - see table 1 of the paper. These materials are labeled as "LOFFA", "GOFFA", "COFFA", "POFFA", "NOFFA", and "SOFFA" in the plot.
The other data points in the plot refer to cuprate superconductors - the substances labeled ...CO, or Bi-2212, where the numbers denote the composition. These cuprates are discussed in several Acta Astronautica papers, for example Acta Astronautica 65 (2009) 489, which unfortunately I do not have access to.
Doping
In the examples of the rare earth iron pnictides, superconductivity can be reached by replacing, in an ideal LOFeAs lattice, some of the oxygen atoms by fluorine atoms. Depending of the amount of fluorine, the critical temperature for superconductivity can take different values. The Physics article on High-temperature superconductivity in the iron pnictides shows phase diagrams for "LOFFA" and "COFFA".
In "COFFA", CeO(1−x)F(x)FeAs, for example, superconductivity sets in only when at least 6 percent of the oxygen atoms are replaced by fluorine, and the transition temperature is highest for a replacement of about 16 percent of the oxygen atoms by fluorine:
(from: Structural and magnetic phase diagram of CeFeAsO(1-x)F(x) and its relationship to high-temperature superconductivity, by Jun Zhao et al., arXiv:0806.2528v1, and Nature Materials 7 (2008) 953-959.)
The percentage of replaced atoms is called the "doping", and it is usually denoted with x, which should not be confused with the "doping distance" in the Röser paper. In the formula above for the substances, I have denoted doping with Δ, as in the Röser paper.
The "doping distance" x
The Röser formula compares the thermal de Broglie wavelength of a Cooper pair at the superconducting transition temperature with a "doping distance" x and states that they are equal, up to a geometry factor. The crucial point, then, is how to arrive at the "doping distance".
In the proceedings paper about iron arsenides, it is argued that every two of the "doping" fluorine atoms group around one iron atom, and that these "decorated" iron atoms form, again, a regular lattice. Such a lattice is usually called a superstructure.
In the example of "COFFA" at the "optimal" doping of Δ = 0.16, this reasoning implies that 8 percent of the iron atoms are neighbored by two fluorine atoms each, and the superstructure of the decorated iron atoms has to comprise 1/0.08 = 12.5 standard unit cells. They argue that this means that 5 unit cells are put on top of each other, yielding a doping distance of 5 times the height c of the unit cell, or x = 5 c.
For the other doped lanthanide iron arsenides, a similar reasoning is used. The constructions of the superstructure and the resulting doping distances are documented in table 1 of the proceedings paper.
Open questions
To me, construction of the "doping distance" is not really comprehensible, and it seems that there are a few points where the paper just glosses over:
I am not sure whether (1) the dopant fluorine atoms group pairwise at iron atoms and (2) if the decorated iron atoms indeed form a superlattice. However, this may be checked experimentally. But then, (3), if there is a superlattice, it is not clear to me why it should be ordered in the way claimed in the paper, with a pattern stacking five unit cells on top of each other for "COFFA", for example. There can be many ways to arrange unit cells to form a superlattice with the right supercell volume. This is more obvious in the case of "LOFFA", where the supercell comprises 18 unit cells, and there seems to be no a priori reason to select a superstructure which has 6 unit cells stacked on top of each other.
In other words, the determination of the "doping distance" involves an arbitrariness which may be used to select x in a way to fit the formula, and there is no real discussion of the selection rules in the paper. This arbitrariness, however, could be resolved by experiments which actually measure the superstructure.
There is another point which puzzles me about the paper, which is the application of the formula to the cases of "optimal doping" only, i.e. those values of doping where the transition temperature for the substance at hand is maximal.
I don't see a good reason why the formula should not apply at any value of doping, if there is a relation between the transition temperature and the "doping distance". Then, however, there is the problem that around the maximum, different values of doping give the same transition temperature, hence should have the same doping distance. This can be seen very nicely in the phase diagram of "COFFA" above. Maybe this point can be arranged for somehow, and maybe it is discussed in the Acta Astronautica papers.
Conclusion
Thinking about it, while the Röser relation is indeed amazing, it's unclear to me how much of an "a posteriori" selection bias concerning the "doping distance" goes into it.
However, as the relation makes strong claims about the arrangement of the dopant atoms, this probably can be checked experimentally.
If it comes out that the relation indeed holds, it will be exciting to understand what it means for the physics of superconductivity.
Let me just repeat the main point of the "Röser formula":
It claims a simple linear relation between the inverse of the superconducting transition temperature of a wide range of superconducting materials, and a so-called "doping distance". This relation is shown in the following plot:
(from: A Correlation Between Tc of Fe-based HT Superconductors and the Crystal Super Lattice Constants of the Doping Element Positions by Felix Huber, Hans Peter Roeser, Maria von Schoenermark, Proc. Int. Symp. Fe-Pnictide Superconductors, J. Phys. Soc. Jpn. 77 (2008) Suppl. C, pp. 142-144)
Here are a few more thoughts of mine about this relation.
Thermal de Broglie wavelength
Using the standard definition of the thermal de Broglie wavelength for a Cooper pair of two electrons with free masses, the Röser equation
4 π k me(2 x)2 n−2/3 = h2/ Tc
actually boils down to
(2 x) × n−1/3 = λc
where λc is the thermal de Broglie wavelength of the Cooper pair at the critical temperature Tc, where superconductivity breaks down.
In the proceedings paper, n = 1, so the factor n−1/3 can be dropped. Moreover, it is said to take values of n = 2 or 3 for other superconductors with a layer structure, depending on the number of layers in the unit cell. It's not completely clear to me how it is motivated.
Substances Covered
In the proceedings paper, the authors discuss doped iron arsenides LO(1−Δ)F(Δ)FeAs, where L is a rare earth, L = La (Lanthanum), Gd (Gadolinium), Ce (Cerium), Pr (Praseodymium), Nd (Neodymium) or Sm (Samarium) - see table 1 of the paper. These materials are labeled as "LOFFA", "GOFFA", "COFFA", "POFFA", "NOFFA", and "SOFFA" in the plot.
The other data points in the plot refer to cuprate superconductors - the substances labeled ...CO, or Bi-2212, where the numbers denote the composition. These cuprates are discussed in several Acta Astronautica papers, for example Acta Astronautica 65 (2009) 489, which unfortunately I do not have access to.
Doping
In the examples of the rare earth iron pnictides, superconductivity can be reached by replacing, in an ideal LOFeAs lattice, some of the oxygen atoms by fluorine atoms. Depending of the amount of fluorine, the critical temperature for superconductivity can take different values. The Physics article on High-temperature superconductivity in the iron pnictides shows phase diagrams for "LOFFA" and "COFFA".
In "COFFA", CeO(1−x)F(x)FeAs, for example, superconductivity sets in only when at least 6 percent of the oxygen atoms are replaced by fluorine, and the transition temperature is highest for a replacement of about 16 percent of the oxygen atoms by fluorine:
(from: Structural and magnetic phase diagram of CeFeAsO(1-x)F(x) and its relationship to high-temperature superconductivity, by Jun Zhao et al., arXiv:0806.2528v1, and Nature Materials 7 (2008) 953-959.)
The percentage of replaced atoms is called the "doping", and it is usually denoted with x, which should not be confused with the "doping distance" in the Röser paper. In the formula above for the substances, I have denoted doping with Δ, as in the Röser paper.
The "doping distance" x
The Röser formula compares the thermal de Broglie wavelength of a Cooper pair at the superconducting transition temperature with a "doping distance" x and states that they are equal, up to a geometry factor. The crucial point, then, is how to arrive at the "doping distance".
In the proceedings paper about iron arsenides, it is argued that every two of the "doping" fluorine atoms group around one iron atom, and that these "decorated" iron atoms form, again, a regular lattice. Such a lattice is usually called a superstructure.
In the example of "COFFA" at the "optimal" doping of Δ = 0.16, this reasoning implies that 8 percent of the iron atoms are neighbored by two fluorine atoms each, and the superstructure of the decorated iron atoms has to comprise 1/0.08 = 12.5 standard unit cells. They argue that this means that 5 unit cells are put on top of each other, yielding a doping distance of 5 times the height c of the unit cell, or x = 5 c.
For the other doped lanthanide iron arsenides, a similar reasoning is used. The constructions of the superstructure and the resulting doping distances are documented in table 1 of the proceedings paper.
Open questions
To me, construction of the "doping distance" is not really comprehensible, and it seems that there are a few points where the paper just glosses over:
I am not sure whether (1) the dopant fluorine atoms group pairwise at iron atoms and (2) if the decorated iron atoms indeed form a superlattice. However, this may be checked experimentally. But then, (3), if there is a superlattice, it is not clear to me why it should be ordered in the way claimed in the paper, with a pattern stacking five unit cells on top of each other for "COFFA", for example. There can be many ways to arrange unit cells to form a superlattice with the right supercell volume. This is more obvious in the case of "LOFFA", where the supercell comprises 18 unit cells, and there seems to be no a priori reason to select a superstructure which has 6 unit cells stacked on top of each other.
In other words, the determination of the "doping distance" involves an arbitrariness which may be used to select x in a way to fit the formula, and there is no real discussion of the selection rules in the paper. This arbitrariness, however, could be resolved by experiments which actually measure the superstructure.
There is another point which puzzles me about the paper, which is the application of the formula to the cases of "optimal doping" only, i.e. those values of doping where the transition temperature for the substance at hand is maximal.
I don't see a good reason why the formula should not apply at any value of doping, if there is a relation between the transition temperature and the "doping distance". Then, however, there is the problem that around the maximum, different values of doping give the same transition temperature, hence should have the same doping distance. This can be seen very nicely in the phase diagram of "COFFA" above. Maybe this point can be arranged for somehow, and maybe it is discussed in the Acta Astronautica papers.
Conclusion
Thinking about it, while the Röser relation is indeed amazing, it's unclear to me how much of an "a posteriori" selection bias concerning the "doping distance" goes into it.
However, as the relation makes strong claims about the arrangement of the dopant atoms, this probably can be checked experimentally.
If it comes out that the relation indeed holds, it will be exciting to understand what it means for the physics of superconductivity.
- The August 2009 issue of the Scientific American gives a nice overview of superconductivity in iron pnictides in the article An Iron Key to High-Temperature Superconductivity? by Graham P. Collins (albeit without illustrations, it seems...)
- Browsing the APS' Physics Archive for the tag superconductivity is a good way to keep up-to-date with the developments in the iron pnictides.