Sunday, October 29, 2006

Diversity in Science

Last week I read Chanda's guest post at CV about diversity in science. I was about to write a comment, but it got way too long, so I decided to promote it to a post on its own.

Let me begin with a disclaimer:
All members of our society should have equal opportunities to become scientists, irrespective of their race, gender, religion, or affiliation to other minorities. If you really need a reason for this, read the human rights. Unfortunately, this is not yet the case, neither in the US, nor in Europe. The cause for this is partly that higher education is money-wise often not favourable1, and sadly in many cases there do exist correlations between belonging to a minority (e.g. immigrants) and being faced with a difficult social background. Other reasons are traditions (physics is a boy-thing), peer pressure (white and nerdy), conflicting ethical values (does the bible mention dinosaurs?), and of course prejudices against minorities (small town guys are stupid).

Since my weekend has only 48 hours, these are the points I don't want to discuss today. What irritates me about the discussion on diversity in science is the entanglement between the issues of minorities in our society, the social diversity in the community of scientists, and the diversity on the 'marketplace of ideas'. An entanglement that should be approached with caution.

Diverse Diversities

Chanda's writing reminded me of the time when I was a graduate student. For a year or so, I was lucky to be part of a more or less frequently meeting group in which we would just discuss physics-related stuff. I learned a lot in these meetings, which in my opinion make a very nice example for the merits of diversity.

There were five people who reliably participated2:

  • Dr. Who, with a PhD in astrophysics and a diploma in mechanical engineering, whose invaluable skill was making estimations with a minimum of input. Typical sentence: 'Obviously A goes with the 3rd power of B, then X is approximately Y - modulo factors of 2 Pi or so, don't trust me on the signs.'
  • Dr. 2B, about to finish his PhD on quantum field theory, who had a natural talent for teaching. He liked to pass on whatever he'd read in a didactic and understandable way. We'd let him talk whenever we were in need of some motivation, because his fascination for physics was truly contagious. Typical sentence: 'Last week I read this cool paper...'
  • Dr. No, a nuclear physicist who returned to Germany after some years in the land of plenty, only to find flaws in everything. Unfortunately, he was most often right. On the other hand, he was the kind of guy who had ~100 publications at the age of 30, and reportedly drafted, wrote and posted a research paper within 24 hours. Typical sentence: 'But then the universe wouldn't exist'.
  • Prof. Dr. Senior, with a permanent position and a little detached from the recent research. He efficiently provided us with missing links by always knowing who had worked on this or something like this, or giving references to original works on related context. He was also going on our nerves with political advises as to which topics were recommendable or not, and endless stories about the days back then. Typical sentence: 'This reminds me of the time when...'
  • Then there was me. My role was it to look confused, ask for details on the allegedly obvious statements, (which I usually ended up figuring out alone), or to blow completely unrelated sentences into thin air, which could make the whole discussion completely spin off into another direction. If I have a typical sentence, nobody told me, but it probably ends with a question mark.

Besides me, all members of this group where German speaking, Christian, heterosexual, white men without visible disabilities.

On the other hand, I count myself a member of various minority groups: I'm a vegetarian, have freckles on my nose, am proud to drive a car with manual gear, and my clothes size is seriously underrepresented in North American stores.

I think this example makes it very clear that one needs to distinguish between different types of diversity, that I will call 'job-related diversity', and 'demographic diversity', the latter of which can be divided in 'readily detectable' and 'less detectable'. The influence of diversity on the efficiency of work performance is subject to active scientific research since at least a decade or so, though mostly for management in companies.
Some references I can recommend are e.g.

Diversity in Science

Of course, these investigations do not all apply to the science community, but the categories of diversity are useful nevertheless. Let's give it a try:

  1. Demographic diversity

    • a) readily detectable:
      • gender
      • race
      • religion
      • citizenship
      • disabilities
      • social background
      • family status
      • ...

      b) less detectable:
      • life experiences
      • sexual preferences
      • moral and ethic values
      • social skills
      • political orientation
      • hobbies
      • ...

  2. Job-related diversity:
      • education (when/where/what)
      • job experience (how long, what)
      • rank (student/postdoc/tenured/permanent)
      • external goals ( publications/appreciation/income...)
      • internal goals: (wisdom/curiosity/contribution to wellbeing of society... )
      • tasks (teaching/researching/typesetting/numerics/laboratory work/public outreach...)
      • connections (integration in community, friends, coworkers...)
      • organization of work (always working on 5 topics at least/obsessed with zing-zong theory and nothing else3)
      • divergent/convergent thinking
      • teamwork (prefers single work or group work)
      • rigor (fast and furious sketch of ideas/slow and thorough through all the nasty details)
      • temper (secure player/high risk-taker)
      • ...

(These are some points that immediately came into my mind. Feel free to let me know if you think I missed something essential.)

There is a good reason why demographic diversity is desirable on a very general level: it makes for an open-minded community that embraces differences. I'd say demographic diversity reflects in the climate at the work place. I want my colleagues smiling and motivated, and not scared to go to work.

But what we also see from the above list are two things:

First, it is not at all clear why and how demographic diversity should relate to job-related diversity.

In some cases one might be able to establish correlations. E.g. I would expect that on the average people with children are less risk-taking, or that a difficult social background disfavours a high number of connections.

However, in general the relations will be weak and I don't think these are topics we want to consider when hiring people. I don't want to end up wondering if a preference for heavy metal music goes better with solid state physics, or whether classic music is a more appropriate choice for string theorists.

Second, it's not at all clear that diversity (of whatever type) is a guarantee to increase progress.

Indeed it isn't. Diversity is not a cure for every problem but it comes with difficulties of its own. To put it simple: scientific controversy is healthy, and constructive criticism is a challenge that prompts new ways of thinking -- which is good. But constant disagreement hinders progress and doesn't get us anywhere. If we want to use the merits of diversity we'll have to balance its use.

As an example, consider how job-related diversity has been promoted in inter-disciplinary categories, like theoretical biophysics. This is a great thing, and I find these seminars often very interesting and inspiring. I always appreciate to hear something new. But for my everyday work, I don't want to be tied to that biophysicist and his proteins.

What Can We do?

Job-related diversity:

The bottom-line of the above is that job-related diversity needs to be managed wisely. We want to use the whole variety of different approaches, but not wash out focused work on specific problems. This brings me back to what I've pointed out earlier (see Science and Democracy, Science and Democracy II): our community has grown very much, very fast, but lacks a proper administration and management. It's about time we investigate under which circumstances science works best.

Indeed, I do think that we have a lack of job-related diversity right now. You can improve the situation yourself. Try this:

  1. Go to your colleague three doors down, and have a discussion about his/her work4.
  2. Register to a conference that's not directly related to your work, even if that means, you won't get a talk. Drop the thought that this is a waste of time.
  3. If you're organizing a conference, invite researchers from related fields to give plenary talks.
  4. Read a paper by someone who you haven't met in person.
  5. Don't call someone a crackpot just because you don't like the subtitle of his book5.
  6. Do actually look up some of the references from 100 B.C. that your white-haired colleague mentioned.
  7. I await your suggestions...

Perimeter Institute is a prime example for an excellently balanced and managed use of job-related diversity.

Demographic diversity:

Chanda has given a very nice list of what you can do here. I'd like to add some points. The first is that a serious problem are financial barriers to higher education. If you live in a democracy, what you can do is register to vote, and vote the right person - or better, become politically active yourself.

However, the representation of demographic diversities in theoretical physics isn't a problem we can solve without taking into account sociological and traditional barriers that I mentioned in the beginning. As I've pointed out elsewhere, just demanding that demographic diversity should reflect in the scientific community without realizing that the society might not yet be ready for this, is pretty short sighted, and can actually be counterproductive.

But a considerable amount of these barriers is based on wrong or missing information about what it means to be a theoretical physicist. If I average over my experiences, many people seem to think a theoretical physicist is something between an astronaut and a daydreamer. After mentioning what I do for a living, I have actually been asked whether I could maybe repair the fridge, or explain the weather forecast. And then there are those who just conclude I am still a student, or something-like-that6.

What you can do is to spread the fascination of understanding the laws of nature, and examining the mysteries of our universe. And explain how that works in pratice.

It is in this regard that I appreciate every popular science book which succeeds in bringing our work into the attention of the wider public, and also books that feature theoretical physicists as main characters.

And finally I have made a nice circle to justify why I spend my time writing pieces for the blog.

Footnote 1: This was the reason why there were no tuition fees for German Universities for a long time. In principle a good idea, though with foreseeable backlashes. Lately it has been improved just to end up even worse.

Footnote 2: Names changed.

Footnote 3: Zeig mir den unteren Topf.

Footnote 4: Here, discussion doesn't mean you try to convince him of what you wrote in your last paper, but half of the time you should l-i-s-t-e-n. Even if he's a postdoc from Nowhereland.

Footnote 5: If you want to know what lets me doubt whether I want to stay a physicist, listen to this, e.g. at 22:10 min. How diverse were the opinions in this room that it seemed a good idea to laugh? How come that during the whole hour nobody spoke up and said, look, backstory or not, there's a considerable amount of truth in Swoit's and Wolin's books. See also Clifford's and Peter's post.

Footnote 6: Recently, I have been promoted to an expert on wireless networking.

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How to secure your WLAN

I live in an 8 floor building. My laptop detects 12-15 wireless networks when I am near the cable outlet. There is a network called 'Petes', another is called 'looproject', and then there is 'Pleasure Town' (no kidding). The other ones are fluctuating amounts of 'default', 'linksys' and 'NETGEAR'. At the bottom of the list there is also a 'GETNEAR'. So much about my neighborhood.

Until 2 weeks ago I was one of the linksys. Then my software began notifying me of IP-address conflicts with other computers in the network. So, whether by accident (which linksys is mine?) or on purpose, I wasn't surfing alone. Though I pretend that I'm a nice girl, I have a limit on data transfer, and I was repeatedly disconnected from my own wireless, which sucks. Therefore, I decided to secure my WLAN. For my friends and family that now makes me an expert on wireless networking, and I am kind of tired to repeat the same thing. So, here's once and for all, for mum and her colleagues, for Stefan's neighbors, and for everybody else:

How to secure your WLAN

  1. Go online.
  2. Remember your WLAN's administrator name and password. You were most likely prompted to enter it when you installed your software, the default being 'admin' or something like this. If you can't remember what you entered, well, call customer service or so.
  3. Click here. You should be prompted for said administrator name and password. If that doesn't work, try this, or this. If none of that works, well, call customer service or so.
  4. Enter administrator name and password, that should get you to the configuration page of your WLAN. If your password doesn't work, well, call customer service or so.
  5. Go to a section called Wireless and a subsection called Wireless Security.
  6. For the option Security mode chose WPA personal.
  7. For the option WPA Algorithm chose AES.
  8. In the field WPA Shared Key enter a password you can remember. I recommend you take the word 'password' so I can use your WLAN whenever I'm around.
  9. Click on a button saying Save Changes or likewise.
  10. In case you were online via your wireless, you should be disconnected now. Go to the 'available wireless networks' menu. Your network should now show up as secured network. Click on 'connect'. You should be prompted for the WPA key.
  11. Enter the password you set in step 8. If that works, that's it. If that doesn't work, congratulations! You have sucessfully secured your WLAN from yourself. Look for your ethernet cable and repeat steps 1,2,3,4 and 8.
  12. Call customer service and yell at someone just for fun.


Sunday, October 22, 2006

Dark Matter

Thanks to Clifford and Robert, I came to realize the awesome power of theoretical physics, and that a grocery store is an excellent place to look for it.

This of course justifies uninhibited shopping in every brain downtime. For instance yesterday, I heard a talk by Sean Carroll, after which I was so confused about the direction of time that I went to get some chocolate as antidote. And that's how I found a model of the universe. It came in a bar of aero chocolate, one of the numerous children from the Nestlé family. It basically consists of:

  1. An high amount of emptiness which puffs up the volume. As I learned from Wikipedia, exactly how this works is one of the best kept secrets on earth. ( "The exact procedure [...] is a closely guarded secret. A spokesperson for Nestlé provided some clues but there has been no definitive answer." )
  2. Since it's a Nestlé product, the actual solid part of the bar consists of an almost negligible amount of the real thing (that is in this case cacao), which leads us to the conclusion that...
  3. ... the rest is a pretty mysterious dark matter, which (ALLERGY ALERT) might contain traces of this or that.

And isn't that a nice way to model our universe, where the present day cosmological data lets us conclude that

  1. 73% of the universe's content is dark energy, responsible for the observed accelerated expansion, exactly how this works is one of the best kept secrets of the universe ("A spokesperson from Super Zing-Zong provided some clues...")
  2. Only 4% is baryonic matter, or the real thing that we are made of...
  3. ... and 23% is non-baryonic dark matter, a mysterious but apparently unavoidable ingredient which makes out a disturbingly high fraction of the matter content.

On Wednesday we had a very interesting colloquium by Stefan Hofmann about the small scale structure of dark matter. It was one of these talks that succeed to capture the fascination of understanding a part of how the universe works, a talk that reminded me why I studied physics - and convinced me that the end of physics is nowhere close by.

1. Evidence for Dark Matter

a) Rotation Curves

Stars and other objects that are bound to spiral galaxies rotate around a common center. The rotation velocity of the stars is such that the orbits are stable. The required velocity for this depends on the attractive force acting on the star, which results from the matter content in the galaxy. The larger the force, the higher the velocity has to be to allow for a stable orbit.

Measuring the velocities of stars as a function of their distance to the galaxy's center therefore allows to draw conclusions about the matter distribution inside the galaxy. As it turns out, the visible amount of matter is not remotely sufficient to explain the observations, which in the outer regions of galaxies show larger velocities than expected (its square remains constant instead of dropping inverse to the distance). The observations can be explained by assuming a significant amount of non-visible (dark) matter which is distributed in the galaxy.

The challenge here is that the ratio of dark matter to visible matter (the mass-to-light ratio, commonly denoted M/L) depends on the type of galaxy. E.g. globular clusters show little or no evidence for dark energy.

b) Virilization of Galaxy Clusters

In a similar way, clusters of galaxies have a dynamics that is related to the total mass of the cluster, a relation which can be estimated using the virial theorem. Again, one finds that the visible matter is not remotely sufficient to explain the observations. Measurements indicate a mass-to-light ratio of M/L~300. Though there is some uncertainty as to whether clusters of galaxies have had sufficient time to properly virialize their internal motion, this evidence is pretty strong.

c) Weak Gravitational Lensing

General Relativity predicts that travelling light is bent by mass distributions. If a large amount of mass lies between us and objects that we are looking at, the image of the object can be noticeably distorted. This is known as gravitational lensing.

If the light bending is strong, it can result in multiple images of the source (examples here), or change a point-like shape into arcs. In case the bending is not strong enough to actually produce multiple images, as is typically the case when the matter that causes it is not cleanly localized, one can still measure resulting fuzzy distortions of the background image. In this case, which is known as weak gravitational lensing, the significance for the observed effect has to be enhanced by accumulating sufficient statistics.

Dark matter, though not visible on its own, causes gravitational lensing as every other stuff. In this way, gravitational lensing provides evidence for Dark Matter, which has been reported e.g. by the Canada-France-Hawaii Telescope:

"Using a series of deep images obtained at the Canada-France-Hawaii Telescope over the past two years, the French team analyzed the shapes of some 200,000 faint galaxies spread over two square degrees of the sky (an area approximately 10 times greater than that of the full moon). They have determined that the galaxies appear to be elongated in a coherent manner over large regions of the sky. The measured effect is small, a percent or so deviation from a purely random distribution of shapes, but the accuracy of the results leaves no doubt that the signal is due to the gravitational lensing effect of the dark matter distribution. These results have been partially confirmed by subsequent reports from two teams, one English and the other American, who have studied different patches of the sky."

Recently, researchers have been able to use weak lensing data to produce a three-dimensional map of how dark matter is distributed.

d) Large Scale Structure

The stuff that builds up everything we sit on, live from, and can order at fails disastrously when it comes to explain what we see at the night sky. Ordinary matter, also called 'baryonic matter', just doesn't clump enough during the evolution of the universe. To be more precise, it does not clump on small enough scales. To explain the observed density contrast and fluctuations, it requires a non-baryonic type of matter (non-baryonic meaning it is not something from the standard model of particle physics, or a synonym for we-don't-know-what). Moreover, this type of matter has to be rather cold, otherwise its temperature also wouldn't allow sufficient clumping. So, what we really need is non-baryonic cold dark matter (CDM).

The best evidence for this comes from the WMAP data, where the third peak in the distribution of temperature fluctuations (Delta T, the vertical axis) over angular momenta (l, the horizontal axis) is the indicator for a large fraction of dark matter. This is very nicely to see in the animation below (borrowed from this website). It shows how the peaks in the WMAP data change with turning up and down the fraction of CDM, denoted with Omega_m, displayed in the pink bar on the right.

As one sees, the height of the third peak is the thing to look for. In the measured distribution, the third peak is almost as high as the second. For details, see astro-ph/0603451.

2. Candidates for Dark Matter

The model we use to describe the universe on large scales is classical General Relativity (GR), its ingredients being the background metric which describes space-time, and source terms from the matter that cause the background to be curved. There are then basically two ways to explain the above deviations from this model: either our understanding of GR or that of the sources is incomplete.

The first possibility sounds tempting at first but faces severe challenges. GR on distances of the solar system is extremely well confirmed, so any modification could only set in at larger distances. A modification becoming important at a fixed distance however could never explain the observed rotation velocities for spiral galaxies, whose constant asymptotic value depends on the luminosity of the galaxy, a relation which is known as the Tully-Fisher relation.

A way to escape this was provided by modified Newtonian dynamics (MOND), where the modification of GR sets in, not at a fixed distance, but at a certain acceleration. This worked very well for explaining the rotation velocities, and there have been someattempts to also approach structure formation. However, recent observations of the bullet cluster strongly disfavor MOND. Even if MOND is realized in nature, it doesn't seem to work without the additional assumption of dark matter particles. For more details on the bullet cluster observations, see Sean Carroll's excellent post Dark Matter exists on CV, or the discussion here.

This then leaves us with the second possibility of finding source terms with the right properties to explain the observations. Candidates that fulfill the requirements for CDM luckily appear more or less naturally in various extensions of the standard model. These candidates can be characterized as a) being weakly interacting (via gravity and weak interactions only) with each other as well as with baryonic matter and b) being more massive than the common particles of the standard model. Both points are necessary for a sufficient clumpiness as well as to explain why we haven't yet seen these particles.

The term used for these candidates is therefore WIMP - weakly interacting massive particles. The most common candidates are axions, heavy sterile neutrinos, and supersymmetric particles like the neutralino or the bino. (Side remark for the Germans around here.)

3. Experimental Detection

The problem with the above mentioned CDM candidates is that their exact nature doesn't play a role for the observed rotation curves or the large scale structure. Though these particle differ in their microscopic properties, these have no imprint on the above mentioned observations. The present concordance model of cosmology (Lambda-CDM) is basically a parametrization of our ignorance about the nature of the universe's ingredients.

There are ways to examine the nature of CDM directly. There is of course the possibility that the WIMPs will be found in high energy experiments on earth if the collision energy exceeds the necessary production energy. And even though the WIMPs are weakly interacting, it still happens every now and then that they do interact. Decay products of such reactions can in principle be detected. The neutralino for example, is its own anti-particle, and it can annihilate into photons. The probability for this to happen depends on the density of the CDM (the rate is proportional to the squared density) and is typically very low. This makes experiments a very challenging task: The expected flux of photons on earth is approximately the same 'as we would receive from a single candle placed on Pluto' (source: astro-ph/0501589).
For more details about experiments on direct detection, see e.g.

And then there are the imprints of the nature of CDM in the structure of our universe. Structure formation is a very involved topic, a big part of which is typically performed numerically, with just stunningly beautiful results.
If you have DivX, and a fast internet connection, look at these movies from the millenium simulation in low resolution (10.8 MB), med resolution(13.4 MB), high resolution (48.6 MB), -- more info on this website.

Wow, what a trip!

But here's the point: each point in this simulation corresponds to 106 solar masses. All smaller structures are not resolved. What Stefan Hofmann and collaborators showed in their work though was that CDM's smallest structures are 12 orders of magnitude smaller than that! This applies generally for those types of CDM that have been in thermal and chemical equilibrium with the radiation in the early universe. This is typically the case for neutralinos and binos, but not for axions, in which case the small scale structure would look differently (he says work is in progress).

Starting with a primordial initial power spectrum, they calculated the evolution of this spectrum, for the first time including collisional damping and free-streaming. As Stefan said in his talk, in principle there is a third contribution from heat conduction 'but heat is a pretty boring thing for cold stuff, so we drop this term'. In their work they showed that the spectrum has a sharp cut-off at about 10-6 solar masses, below which there are no smaller substructures. You find a very readable summary on the arxiv

    The first WIMPy halos
    Anne M. Green, Stefan Hofmann amd Dominik J. Schwarz

Cosmologists measure time in redshift, commonly denoted with z. We are today at z=0. The larger z, the further in the past an event was. Hofmann's analytical calculations hold down to z approximately 60, where the linear perturbation theory can no longer be applied because the density contrast has become too large. These analytical results however, can then be used as input for numerical calculations. This has been reported in a Nature article

where the numerical calculation goes down to approximately z=20. Results of this simulation are shown in the picture below, where the small structures are magnified

(If you have no access to Nature, the same article is also available at astro-ph/0501589)

These smallest CDM halos without further substructure are distributed over a size of roughly the solar system, which means they are extremely diluted. Their average velocity is approximately 1 meter per second. They are estimated to propagate through galaxies without being disrupted, which means that these CDM substructures could travel through our solar system and render the background we live in time-dependent!

To summarize: the microscopic nature of CDM has an imprint on the small scale structure of our universe. The examination of these small scale fluctuations therefore would allow us to distinguish between different candidates for CDM.

Update Nov. 14th: See also the PS on Dark Matter.

4. Further Reading

5. Epilogue

In the October Issue of Physics Today, Burton Richter - an experimentalist and Nobel laureate - commented on 'Theory in particle physics'. About cosmology he wrote that it is in

"[...] a kind of intermediate state in which all that is missing to make it practical knowledge is a mathematically sound microscopic realization."

Well, yes, that is ' all ' that is missing ;-)

But as a theoretical physicist in the 21st century, I have to give credits to the experimental achievements. We have plenty of evidence for physics beyond the standard model. Astrophysics and cosmology provide us with numerous puzzles to keep our days busy. In case someone got the impression, we theoretical physicists are not sitting around being depressed about the trouble with physics. We just don't have the time! The universe is waiting to be explored. And if you aren't yet convinced of the beauty of it all, go get some chocolate.

(Some comments on Richter's article, see here and here)

Updated on Jan. 14th 2007.

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Saturday, October 21, 2006


One side effect of frequent moving is that one ends up on numerous email lists with names like [Department], [All], [Postdocs], [LunchList]. Every second week or so, I receive an email telling me that the windows will be cleaned on Monday, or that some printer on the second floor is out of toner. That's good to know, but it would be even better if I roughly knew which building the printer is in, or which state to begin with.

So, I usually don't read all these emails. But yesterday I received an especially interesting one that made me laugh quite a lot. No matter what Institute I've ever been at, there's always a kitchen with a fridge you hesitate to open.

(The only exception to this rule is Perimeter, where the fridges are thoroughly cleaned every Wednesday by the staff.)

If you are reading this blog to figure out what the life of a theoretical physicist is like in reality, you'll find an essential part of it described in the email below ;-)

----- Original Message --------------------------------------------
From: "A***** N********" ***@***.uni-****.de
To: allusers@***.uni-***.de
Sent: Thursday, October 19, 2006 3:21 PM
Subject: [allusers] mould breeding competition

Dear Colleagues,

G***** and I have just spent an exciting half hour cleaning outshockingly live food from the refrigerator in kitchen 1.2 ?! Now everything's ready for a new round in the [Institute's] mould breeding competition: whoever breeds the finest fungus within the next four weeks will be declared Master Disgustoid at the Christmas party!

So, get out your half finished packs of cream cheese and liquified fruit of undefinable origin and start the fun! Best of luck to all participants,


PS: In case we destroyed a research project: better mark it next time.
PPS: People placing prerotten food in the refrigerator will be disqualified.
PPPS: The current champion is the owner of the "Kräuterphiladelphia" with singing ascomycota.

Thursday, October 19, 2006


My dear friends and fellow readers of this blog who you have patiently suffered with me through the weeks of waiting: on Sunday evening my phone rang to announce The End of Ouch.

I was told to appear for customs clearance on Monday morning 8 am, somewhere in the middle-of-nowhere Kitchener, Ontario. There I went, Monday morning at sunrise, to a dead end street in an industrial park, lost between an infinite amount of containers from all over the world. The only living soul I could detect was a women clinging to a coffee cup who didn't look like someone you would want to disturb before finishing this very cup.

So I wandered around between the containers until suddenly a man jumped out of a corner, looked at me and said back of the building, up the ramp, before he disappeared to where he came from, maybe my imagination. Nevertheless, that I did, and found an office the approximate size of an award winning pumpkin with equally appealing interior design. At third sight I noticed a to pumpkin color faded sign saying Canadian Customs.

When I entered, I saw someone sitting in there who looked confusingly familiar. I tried to place his face unsuccessfully until his BlackBerry began to beep, and I realized he's a new postdoc at PI. That's how large Waterloo is.

What unsurprisingly did not appear until 10 am was the moving van. Out of which stepped Viko-the-moving-guy just to point out that nobody was in that pumpkin office who knew anything about customs. A fact that had been hard to ignore the two hours we were staring either at each other or at the orange walls. When finally someone appeared who pretended to be someone that someone did so only to request $ 11.45, and send me to the airport. At the airport I was asked whether I import any firearms. I correctly answered with no, upon which the lady said 'Welcome to Canada', stamped and handed me a form. And that was customs.

The nasty surprise at my apartment though was that Viko-the-moving-guy did not only use an extremely disgusting aftershave, but simply refused to assemble my furniture, claiming it wasn't written in his contract. It didn't impress him zip that it was written in my contract. Pointing out that he was hired by a sub-company of a sub-company of a sub-company, he shrugged his shoulders and left me within a complete mess.

Consequently, I immediately went back to yelling at customer service representatives, and when I'm done with it, I'm afraid you'll have to endure a lengthy report with naming all the companies who are executors of Murphy's law on earth.

I managed to assemble desks and bookshelves. (Even though I found myself climbing into the dumpster in the darkness, because I had accidentally thrown away some screws. I startled some squirrels in there who had plenty of fun with the bubble wrap.)

Thanks to the help of our yesterday's colloquium speaker, now also the bed is usable again. As a pleasant side effect, I got from him all the explanations about dark matter I asked and didn't ask for, and I promise will write something about his very interesting talk... as soon as I am done with unpacking. All the essentials of civilization are back in my household: the can-opener! my towels! the hair dryer! the microwave! and my teddy-bear :-)

Sunday, October 15, 2006

The End of Physics?

The October issue of 'Discover' has an article titled 'The Final Frontier', by John Horgan. Horgan is author of the book 'The End of Science', which was published ten years ago. In the article he comments on raised arguments against his claim that we are nearing the end of Science.

I haven't read the book, so I can't say anything about it, but the article is interesting nevertheless. I don't think the end of science is anywhere near by, so I was already annoyed before I began to read. But after finishing, it turned out that Horgan's perspective is quite reasonable, or you could say scientific. In the last section he writes:

"I could simply be wrong - there, I've said it - that science will never again yield revelations as monumental as evolution or quantum mechanics. A team of neuroscientists may find an elegant solution to the neural code, or physicists may find a way to confirm the existence of extra dimensions."

I on the other hand have to admit that the question whether there are limits to our knowledge is a good one that I've asked myself. Repeatedly. Whenever I sit in a talk scratching my head.

Since there's been some fuss lately about the trouble with zing-zong theory, the fall of a science, and what goes next to the bottom of the ocean (glubglubglub), it is unsurprising that Horgan also comments on things that aren't even wrong:

"But some mysteries are probably unsolvable. The biggest mystery of all is the one cited by Stephen Hawking [...] 'Why is there something rather than nothing?' More specifically, what triggered the Big Bang, and why did the universe take this particular form rather than some other form that might not have allowed our existence?

Scientists' attempts to solve these mysteries often take the form of what I call ironic science - unconfirmable speculation more akin to philosophy or literature than genuine science [...] A prime example of this style of thinking is the anthropic principle, which holds that the universe must have the form we observe it because otherwise we would not be here to observe it. The anthropic principle, championed by leading physicists such as Leonard Susskind of Stanford University, is cosmology's version of creationalism.

Another example if ironic science is string theory, which for more than 20 years has been the leading contender for a 'theory of everything' that explains all of nature's forces. The theory's concepts and jargon have evolved over the past decade [...] but the theory comes in so many versions that it predicts virtually everything - and hence nothing at all [...]. This problem leads Columbia mathematician Peter Woit to call string theory 'not even wrong' in his influential blog of the same title [...]

Although Woit echoes the criticisms of string theory I made in The End of Science, he still hopes that new mathematical techniques may rejuvenate physics. I have my doubts."

(There is no mentioning of Peter Woit's and/or Lee Smolin's books in Horgan's article).

Well, I've extensively commented on the ironies of the anthropic principle elsewhere, so let me focus on the second point, that is the still to be found theory of everything. The fact that there hasn't been much progress in this regard during the last decades isn't necessary an indication for the end of science. To me it indicates instead that:

  • a) Science has left the regime in which we strive to explain effects directly accessible to our own, build in, senses. We need particle colliders and telescopes up in the earth's atmosphere to reach the frontiers of our knowledge. This makes research more complicated, takes more time, and slows progress down.

    There isn't much you can do about that, unless someone someone comes up with a pill that enables me to IR/UV vision or so. This limit however, indicates the inadequateness of our bodies to our mind's searches, and not a limit to our possible knowledge.

  • b) We just aren't doing our research very effectively. There's no doubt that science on the frontiers has become more and more complex, but we're not dealing with it the right way. There's got to be some rethinking about scientific education and selection of research projects. Early specialization into subfields works well when the direction is clear, but likely to result in many dead ends otherwise. Having an overview on present research topics on the other hand is an underestimated value. It is underestimated not only in the educational process, but more importantly in the selection of researchers which (with luck) are also those teaching the next generation.

    We should realize that the amount of stuff that can be pushed into the human brain in finite time is limited. It has to be choosen wisely. You might say it's a meta-problem that we have here: the neglect of scientific research on how to do scientific research. It goes hand in hand with the separation between science, sociology and philosophy that has taken place, the latter of which has historically always been close to the Whys and Hows of understanding nature.

  • c) Hogan writes: "Just over a century ago, the American historian Henry Adams observed that science accelerates through a positive feedback effect: Knowledge begets more knowledge. This acceleration principle has an intriguing corollary. If science has limits, then it might be moving at maximum speed just before it hits the wall."

    Well, all such unlimited growth scenarios only work under the assumption of infinite resources, which brings us back to a) the limits on our perception of nature and b) the finite number of researchers, and - sad but true - our own mortality. But besides this, such a statement is based on a dubious measure on the 'amount of knowledge'. I am actually not so much concerned with the 'amount' of it, but with the content of it.

Since it is in the nature of men to be curious, I don't see any end of science coming, not until we could literally explain everything that happens, will happen, and has happened - and bore ourselves to death. But I find it indeed possible that the human brain is just not capable to ever do this. It will most likely reach its limits before we get even close to the limits of knowledge that are invoven into the fundamental nature of the universe.

On the other hand, it would be sufficient if we were capable of understanding how to improve our own mind (biologically or technically), and thus kickstart evolution.

However, one way or the other, I don't think we have yet reached our limits, I don't even think we are remotely close by. But I think that we have put big obstacles in our way. If we don't succeed in removing them, then it doesn't matter if its a neurological, a sociological, or a financial end to science.

To summarize:

Proclaiming The End Of Science under the present circumstances for fundamental research is like proclaiming The End Of The Internet because you have seen a lot of dead links lately. It's a sign for inefficient organization, and missing maintenance, but there is much room for functionality to be optimized. So, don't sit around and read other people's blogs, but go look for the Theory of Everything, it might be just around the corner (but not on the internet, believe me).


Horgan ends his article with an suggestion that I want to echo here (though slighly off-topic), because it's a suggestion that I have made it myself repeatedly. He points out that "scientists might help find a solution to our most pressing problem, warfare. Many people today view warfare and militarism as inevitable outgrowths of human nature. My hope is that scientists will reject that fatalism and help us see warfare as a complex but solvable problem."

I share this hope. Even though it is in the human nature to defend one's own life and that of friends and family with violence when forced to, there is no way anybody can convince me this evolutionary imprint extends to modern warfare in which most of the battle is no longer face to face. Though war will probably always persist as the last and final option, there ought to be non-violent pre-stages to war that should be undergone prior to retreating to mass destruction.

War and terrorism happen if those who initiate it see no other way to pursue their goals, or defend their lifestyle. Terror is likely to come from countries who feel their interests aren't globally acknowledged, and are pushed into a corner where nobody listens to them. Follow that thought: What we need is a global government. Not some countries imposing their idea of civilization upon others.

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Saturday, October 14, 2006

Recent Arrival

The Perimeter Institute has a newsletter called 'Inside the Perimeter'. Its latest issue introduces me as a 'recent arrival'. Since I - and maybe also some of you - have wondered what I am doing here, here is what Angela Robinson wrote (including the photo, taken last week by my officemate Stefan) :

"After obtaining her PhD from Frankfurt in 2003. Sabine held postdoctoral positions at the University of Arizona and UCSB. At PI, she will be spending a lot of time on quantum gravity... as well as foundations... as well as keeping in touch with developments in string theory... as well as bothering everyone who knows something about cosmology.

In addition to her diverse interests in physics, Sabine is also an active blogger and painter, and likes spending time hiking.

As she settles into Canada, she is enjoying her new-found discovery of maple syrup, and eagerly awaiting the arrival of her furniture from California. If you know why her furniture seems to be circling the globe en route to Waterloo, you can find Sabine in Office abc, at ext. xyz to explain."

Indeed, maple syrup is a great stuff, totally addictive. I wonder how I could completely ignore its existence for the first 30 years of my life!

But what I miss far more than my furniture are my books, my files, CDs, and my clothes. I would really appreciate the opportunity to wear a different shirt next week. The bright side of this is (there is always a bright side!) that it makes a great conversation starter. When people meet me on the corridor, they keep asking for news from my furniture, instead of, say, to which precision the spin statistic of the top quark has been measured.

The newsletter is a very nicely designed more or less frequently appearing update on what is going on at the Institute, and related events at the Universities in Waterloo and Toronto. Besides the current visitors and the extensive scientific schedule, it lists all the other activities going on here, from movie nights over excursions to the public lectures.

The last page of the newsletter has a questionnaire. I don't know from whom it is, but we've been laughing a lot about #3 B, which tells us that

"3B: Super zing-zong theory is the only theory that successfully predicts the dimension of spacetime to within an order of magnitude."


Thursday, October 12, 2006

Oh Canada

A completely representative recent survey among my friends and relatives asked for their associations to Canada. The results in order of importance:

  1. Snow
  2. Maple syrup, maple leaves, maple trees, and other maples
  3. Forest
  4. You just moved there (well, I told you it was completely representative and double blind and all that)
  5. Snow

Further not top-ranking but interesting associations were: bears, nose-bleeding, Tim Horton's, palm trees (hahaha), ice wine and snow.

So, when I tell you I have had an overdose of Canada today, I don't mean I drank the bottle of maple syrup. No, today, we had a snowstorm of the kind where you can't see farther than 10 meters. I had to abuse a CD to scratch the ice of my windshield. Luckily, it's stopped snowing by now.

Here's the prove, photo taken 5 minutes ago in the back yard:

I am shocked, longing for a sunny beach in Santa Barbara, and hoping that my winter clothes will arrive in finite time. Another persistent inquiry today brought up the news that my furniture is still in Montreal. But today, I can nevertheless report progress: the truck company has figured out that the storage in Montreal begins to get expensive (STORAGE?! Yes, it turned out they were waiting for more stuff, so the truck would be fully loaded). Since capitalism demands it now, they consider actually bringing my stuff to Waterloo.

They don't work on weekends though.

Wednesday, October 11, 2006

Does String Theory explain Heavy Ion Physics?

Earlier this week, we could follow a quite heated debate about the applications of the AdS/CFT (Anti-deSitter/Conformal Field Theory) correspondence to strongly coupled QCD (Quantum Chromo-Dynamics, the theory of quarks and gluons) as observed in relativistic heavy ion collisions.

Or, in a more catchy phrase, whether or not "string theory explains RHIC physics". Or -- even more provoking -- as it was formulated in the recent Nature issue:

"When the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York, first produced a hot quark gas, it was string theory that correctly predicted, retrospectively, some of the gas's properties. "

Nature 443, 491(5 October 2006), Theorists snap over string pieces, by Geoff Brumfiel, see also here.

(Okay, I take the word 'explain' in the title instead of 'predict', but I want to bring that quotation with the alleged prediction somewhere. - If it's unavoidable. - It is.)

This is quite an experimental post written by both of us, trying to understand what is there about these claims. If you ever want to test how much your marriage can take, try to write a blog post together. When you see remarks in brackets, these were the issues we couldn't settle.

In an earlier post, Bee reported on a talk about the applications of AdS/CFT to heavy ion physics that she heard at the KITP. She was thrilled to see string theorists trying to get in touch with experiments! And isn't it ironic that after several decades string theory has come back to where it started from: explaining features of the strong interaction? That was, before it was promoted to be a promising and promising and promising approach to the theory of everything (TOE), which would become important at unobservable energies. (Isn't that too sloppy? - It is called cynicism.)

The use of the AdS/CFT correspondence for strongly coupled QCD is an extremely interesting and exciting project, and probably one of the hottest and densest topics that is currently out there. (haha - sorry, could not resist the temptation) It can provide us with a lot of important insights into QFT. But one should be realistic here:

From the side of the string theorist, realistic about what it can possibly tell us about string theory as a TOE, and what it can't. From the side of the nuclear physicist, what it can possibly tell us about heavy ion collisions. And what it can't.

So, this is an attempt to explain some of the physics involved, from the point of view of relativistic heavy ion physics - and since since Stefan has some background there, we figured we would make a good team, but he's definitely the one to ask what a horizontal flow is. (I should know that by now, but I keep forgetting it. - Does that mean I have to answer all the comments?!)

Here's the outline:

1. What is this all about?
2. Is it a Hype?
3. What does it mean?

1. What is this all about?

Heavy Ion Collisions have one big goal: To map out the phase diagram of nuclear matter. The question one would like to answer is: Under which conditions of temperature and density is nuclear matter made up of hadrons (of nucleons like neutrons and protons, of hyperons like Sigmas and Lambdas, and so on), and when and how will one find the constituents of hadrons, the quarks and gluons, as the relevant degrees of freedom? Where in this diagram is the phase boundary between the hadron gas and the quark-gluon plasma, as the state where confinement is lifted and quarks and gluons can move freely is called? And moreover, what are the properties (the equation of state, or transport properties such as viscosity) of the quark-gluon plasma?

(Phase diagram here? - Good idea. - Where's the figure from your talk? - Where's yours?)

On the experimental side, there is only one tool available: Accelerate nuclei of heavy atoms such as gold or lead, and let them collide. At the collision, the kinetic energy of the nuclei is dissipated, and goes into the compression and heating of the nucleons in the nuclei. If heating and compression are high enough, a quark gluon plasma can be formed.

On the side of theory, there is QCD which describes the interaction of quarks and gluons. There is only one big problem: QCD is a complicated theory, and its low energy limit, which contains the hadronic ground states, the protons and neutrons and so on, can not be handled analytically. The same is true for the deconfinement transition from the hadronic world to the quark-gluon plasma: There is no analytical method to describe deconfinement and hadronization in QCD. What one can do instead is to use lattice QCD, or apply approximation schemes that approach hadronization from high densities or high temperatures, where the theory is asymptotically free, and perturbative methods can be used. There are different techniques available to describe QCD at temperatures above deconfinement, with hard thermal loop re-summation as one example. This is a very active area of research in current nuclear theory. For the regime of heavy ion collisions, there still remains on problem: At temperatures above the deconfinement temperature Tc, say for T = 1 - 3 Tc, QCD is not yet completely free. Lattice calculations of energy density and pressure show a clear difference to the Stefan-Boltzmann limit, which corresponds to an ideal gas of quarks and gluons. So, this temperature regime is difficult to study with standard QCD techniques. Unfortunately, it is just this temperature regime that is reached in heavy ion collisions at RHIC, the relativistic heavy ion collider at Brookhaven.

(Do you have some fundamental problem with entering paragraphs? - But the context belongs together! - It looks completely unreadable. - Who reads that anyway? - I think I don't like your attitude.)

There was one big surprise in the experimental data from RHIC: it seems that the quark-gluon plasma created in the collisions has a very low viscosity, or is a most ideal liquid. At least, that is what can be concluded from the success of hydrodynamical simulations of RHIC collision simulating the quark-gluon plasma as an ideal liquid.

Here, one point is important to note: There is no way to measure the viscosity of the quark-gluon plasma directly. You have to infer it from the momentum distribution of final state hadrons, in this case, of the anisotropy of the momentum distribution of hadrons in the transverse plane for non-central collisions, which is called the elliptic flow.

(Insert explanation, link, graphics. - Good! Where is it? - I'm at home, can't access the journal. - Okay, lets do that tomorrow.)

Large values of elliptic flow are observed at RHIC, larger than what was expected from an extrapolation of the results from the CERN-SPS, where the collision energy is lower. As mentioned before, this RHIC elliptic flow can be reproduced using a hydrodynamical simulation of an ideal (zero viscosity) fluid for the deconfined phase. So, the conclusion is, the viscosity of the QGP is very low.

Here, there is one point to keep in mind: the actual viscosity is not known for sure, and model assumptions about the QGP go into it: Assumptions about the initial state used for the hydrodynamics simulation, for the equation of state and the properties of the hot and dense system, for hadronization, and for hadronic rescattering, i.e. the interactions of the hadrons in the still dense, but late phase of the collision. Moreover, the hydrodynamics code in use only now start to systematically investigate the effects of actual viscosity on the expansion dynamics.

The simulations using ideal hydrodynamics that are so successful in the reproduction of the elliptic flow use a so-called Glauber-dynamics initial state for the codes to run. But this initial condition is not the only game in town. For example, the so-called colour glass condensate (one model assumption for the high density, high-temperature initial state of the nuclear matter, where gluons are the main players) produces very high initial transverse momenta, which produce an elliptic flow consistent with data only if a viscosity is taken into account which is markedly higher than in the ideal fluid models used so far. So, a definite, uncontroversial answer about the the actual viscosity is still out. Obviously, lots of issues are not yet completely settled here.

When the first data on elliptic flow larger than expected before become known, Edward Shuryak pointed out that the very low viscosity which data seem to imply (but keep in mind that this fact as such is not completely waterproof yet) would be consistent with predictions of a very low viscosity of a supersymmetric Yang-Mills theory, and that this low viscosity corresponds to an absolute minimum of viscosity derived from the AdS/CFT duality and superstring theory. Hence, the term "most ideal liquid" was coined for the QGP created at RHIC, and it was argued that the strongly coupled QGP can be described using the analogy to the supersymmetric Yang-Mills theory.

Shuryak is a brilliant physicist, but it is also fair to say, we would say, that he is known in the community as someone who strongly promotes his ideas. And his ideas are often contested - as in this case, the idea of the "most ideal liquid" has been contested a lot, especially from the side of the promotors of the colour glass condensate. So, there is an ongoing debate in the community about these questions, the press releases about the ideal liquid notwithstanding. Anyway, this is our impression of how AdS/CFT entered the heavy ion community.

(It this the one who...? - Yes. - Do you really want to write that? I mean, I don't usally comment on people. - That is fair to say, believe me. And for the heavy ion people it's a compliment.)

Now, what does the AdS/CFT say, and where can it be applied? In brief - and corrections of experts on this are welcome - it helps to write down correlation functions in strongly coupled gauge theories from a duality to the dynamics of strings in an 10-dimensional AdS background with a boundary. Strings end on the boundary, which is Minkowski space, and end points of strings correspond to particles in the gauge theory. Problems of the mathematical exactness left beside, this is a unique and ingenious way to get information about correlation functions, which are very hard to obtain (or are not obtained yet) by lattice gauge theories or thermal field theory techniques.

Where has this duality been applied? The first case has been mentioned before: To calculate the viscosity of hot gauge theories, with the famous universal lower value of 1/4 pi. There are two more situations where it has been applied: To determine the screening of the interaction potential of a heavy quark-antiquark pair in a system moving in a background of hot gauge theory (An AdS/CFT Calculation of Screening in a Hot Wind by Hong Liu, Krishna Rajagopal, Urs Achim Wiedemann, hep-ph/0607062), and for jet quenching calculations, that is, to determine the energy loss of fast particles travelling through a hot medium. (Calculating the Jet Quenching Parameter from AdS/CFT, by the same authors: Hong Liu, Krishna Rajagopal, Urs Achim Wiedemann, hep-ph/0605178, now accepted as a PRL). As a sidenote, Wiedemann and Rajagopal are not string theorist, but have worked in heavy ion theory, QCD and nuclear theory. Hong Liu and Dam T. Son, one of the authors of the main viscosity reference, and also not a string theories by formation, will have plenary talks at Quark Matter, the main conference of RHIC physics.

Can these things be observed in heavy ion collisions? For the case of viscosity, we have discussed it before: there are some caveats, since viscosity can not be measured directly - you have to reproduce elliptic flow, and the inverse problem is not unique. The hot quark-gluon system may be a most ideal liquid, it may be something else, we do not know yet for sure. Screening of the potential is relevant for the so-called J/Psi suppression, but this is also something that has to be inferred backwards from the measured J/Psi yield, which is influenced by many other factors (the original idea iabout this is twenty years old now - however, there are still many open questions left).

At RHIC, there are chances from photons that may make these signals more waterproof than at CERN-SPS, but currently,. ambiguities remain. Jet quenching and energy loss is also a point where many calculations and models exist, but the inverse problem is very hard. So, we would say in all these three cases, you may have a very beautiful application of AdS/CFT to QCD at strong coupling, but the connection to experimental data is difficult and ambiguous.

You should not be disappointed: that is, unfortunately, very common in heavy ion physics. Take the original idea about J/Psi, or disordered chiral condensates, and many other examples: Signatures to check beautiful ideas are often washed out by lots of dirty QGP soup and hadron gas wind effects.

2.Is it a hype?

Does the Global Positioning System (GPS) work because of General Relativity? One often hears this statement in discussions of General Relativity, and it is meant, we guess, to create an awareness that this arcane theory is true, and moreover has applications to down-to-earth technologies which are in every-day use. And as a matter of fact, it is true: The systematic effects on atomic clocks in orbit when observed from points on the surface of the earth as predicted by GR are incorporated into the system, and it all fits perfectly well!

On the other hand, to say that the GPS work because of General Relativity is an oversimplification which neglects the actual intricate details of the system, and which are, from a practical point of view, equally important for the workings of the GPS. A look in a technical description of the GPS will discuss lots off effects of the ionosphere and the atmosphere on the propagation of the satellite signals that have to be taken into account and corrected for - relativity often is not even mentioned! So, clearly, to say that the GPS work because of General Relativity is not wrong, but it is not the whole story: It is a catch phrase to show that GR is not some abstract mathematics, but plays indeed a role in the real world.

There is also a kind of inverse problem in the GPS example: Could one reconstruct GR from the GPS system alone? Could clever physicists derive GR from the systematic deviations in GPS, if they would not have been taken into account from the beginning? Well, they would only be partially successful, since, in fact, only some form of the equivalence principle is tested with the GPS, and not the full Einstein equations. To establish GR, more observations, such as the perihelion precession, are necessary.

To us, this seems to be very analogous to the situation of AdS/CFT in RHIC physics (To us? It was your comparison. I really like it but I like to point out it was your creativity at work here!) : There are applications of this fundamental duality to the physics of hot and strongly coupled QCD, and they probably contribute to the outcome of experiments. But there are many more, mundane effects coming into play, which influence final state hadronic data, and which make it very difficult to solve the inverse problem - to unambigously conclude an initial state.

For sure, AdS/CFT does not explain all of RHIC physics, so far, it seems, in our understanding, applicable to the regime of strongly coupled QGP above the deconfinement transition. What does it say about hadronisation, for example? Can it say something about this? That would be extremely cool, but it seems that there is no solution yet. Moreover, there seem to be open quastions in how far results derived for the supersymmetric Yang-Mills theory can, indeed, be carried over to QCD, see for example hep-ph/0608062.

These limitations of the AdS/CFT approach should also be mentioned, in our opinion, if only to avoid the misleading impression of string theorists showing up on the scene like the FBI agents with suits and sunglasses, take over the case from the dumb local police, and solve immediately what the locals have been unsuccessfully investigating for years.

(Couldn't find a nice pic of FBI agents, but I wasted some time on that. -- This is great!)

Besides this one should keep in mind that the AdS/CFT correspondence is an outcome of years of research of string theory. But it is not equal to string theory. Even if the current results show the usefulness of this correspondence, and make use of many developed techniques, what could this possibly tell us about string theory as a TOE? And then, the spacetime used there isn't really one that we would be interested in as a description of the world we live in - we come back to this in the last point.

These are some words of caution, but as Clifford pointed out:

"[...] applying string theory ideas - the whole shebang of strings, branes, black holes, gravity, etc - to understanding the new forms of matter being discovered at Brookhaven. This may welll be a great way of testing the remarkably intricate structures that string theory puts together and give us lots of clues about how to develop the theory better."

(You sure that's a good idea? - I don't want that to come out wrong either, I really like his point of view, esp. regarding the teapots and so on. And the fig jam. But I think it's okay, I mean we've made quite clear we aren't anti-string in any regard. - You really sure? - I have the comments forwarded on my BB, you think I want the beeps to keep me up all night?)

And this is understandibly something to get really excited about! Nevertheless, we can't avoid having the impression that string theorists must be pretty desperate if they try to justify their work with the calculation of a viscosity using a conjectured (unproven) side effect of the theory they have been working on. I am not aware of any work on how it would be possible to learn something about string theory as the TOE from observables in heavy ion collisions.

(Isn't that a bit hard, desperate? - Yes. I am not a nice girl in case you haven't mentioned. I want them to get the message. There's no need to be desperate. Nobody wants to kill string theory. But they should stay realistic.)

3. What does it mean?

So, to us, it seems that AdS/CFT is a cool application and we would be happy to understand more about it. (? - !) On the other hand, there are caveats, and experimental verification of great ideas is difficult, as always, in heavy ion physics. But then, there is, we think, a more fundamental question about the ontological status of this duality: It is merely a computational tool, or should we really belief that the quarks and gluons we know and love are just the endpoints of strings in a 10-dimensional AdS×S5 space? (I am not really very much concerned with the ontology, honey. If it's the same, then it's the same, what's the point?)

This is probably a very general question, that can, and should, already be asked for the Ising model and the mother of all dualities, the Kramers-Wannier duality. In an experimental realisation of a two-dimensional Ising system, the elements of reality which are described by the Ising Hamiltonian are the magnetic moments of atoms. Or aren't they? Taking the duality serious, we could as well argue that no, not the magnetic moments are the real thing, but the dual plaquette variables. But does this make sense? Apparently not, especially since duality works for the Ising variables, the magnetic moments, but not for all other real things in the system, the atoms with all their electrons, and their nuclei.

Coming back to AdS/CFT, if it works for strongly coupled QCD, should we believe that the dual side, the strings, are real? Maybe, but then, the duality should work for all kinds of particles, not just strongly coupled quarks and gluons. Then, one could not discern any more between both sides of the duality mirror, and both sides could claim the same right to be the real thing. Or are we fundamentally wrong here? (Are we? What is reality anyhow? - I don't want to get into this right now. Can we just finish this &$%@ post?)

To summarize: Heavy Ion Physics does not equal strongly coupled QCD, and String Theory does not equal AdS/CFT. The calculations done using the AdS/CFT correspondence are wayleading and exciting. But the connections to string theory as a theory of everything, explaining quantum gravity, the parameters of the standard model, and more, are so far very weak and require more investigation.

Update: See also More on Ads/CFT and RHIC

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Monday, October 09, 2006

Happy Thanksgiving!

One of the more pleasant surprises of moving to another country is the unexpected occurrence of holidays. So I noticed yesterday evening that today, the second Monday in October, Canada celebrates Thanksgiving.

What is even less known is that Germany also has a Thanksgiving (Erntedankfest), which is not a national holiday, and I can't recall the appearance of lager amounts of turkeys. It's celebrated in some churches, and around this time of the year there are many markets where the new fruits, vegetables and wines are sold. It's a kind of gathering I like to call Fressfest, which -- I apologize -- is completely untranslatable. In Frankfurt, it will probably take place in the Fressgass.

Yesterday, it was an absolutely gorgeous day, and I took the photos below (click to enlarge). A happy Thanksgiving to all of you, no matter what part of the world you're at.

And just in case there are still people in the world who don't know this, here's

    How to Cook a Turkey


    Go buy a turkey. Take a drink of whiskey (or scotch). Put turkey in the oven. Take another 2 drinks of whiskey . Set the degree at 375 ovens.

    Take 3 more whiskeys of drink. Turn oven the on. Take 4 whisks of drinky. Turk the bastey. Whiskey another bottle of get.

    Stick a turkey in the thermometer. Glass yourself a pour of whiskey. Bake the whiskey for 4 hours.

    Take the oven out of the turkey. Take the oven out of the turkey. Floor the turkey up off of the pick. Turk the carvey. Get yourself another scottle of botch. Tet the sable and pour yourself a glass of turkey.

    Bless the saying, pass and eat out


Friday, October 06, 2006

On the couch

Since some weeks I live on the couch. Not only do I sleep on the couch, in the absence of any other furniture, I eat on the couch, I work on the couch, I do literally everything on the couch.

Last weekend, I went for some coffee with a friend, S. Unfortunately, the cafe was pretty crowded so we were offered the last free table in the corner they call 'the living room'. Where I ended up sitting, you get it, on the couch. The waitress was slow, but told us everything we ordered was 'Terrific!'. We spent the afternoon discussing this and that, and at some point the question was raised whether people are more nuts in Canada or California. To make a point for California I told S. a story from my last visit in LA:

I went for a walk in Venice Beach. Yes, it looks like in the movies with all the muscles and bikinis being shown off there, that's already nuts enough if you ask me. But what's much more interesting is the amount of weirdos you meet on the walkway. Like, people offering psychic healing within 2 minutes (guaranteed, only $20, special offer), play Beatles songs backwards (so they say, not that I could tell), or sell incredibly bad 'art' that allegedly their gifted dog painted.

So, I was walking there on the weirdway and came by this white haired guy, who looked like he had never heard of sun protection. He was wearing nothing but a pink mini skirt, standing on an upside-down turned skateboard with one missing wheel, beating with a spoon on the back of a bowl. He asked me how I am, and distractedly I said I'm fine, staring at the amount of white hair and beard falling over clearly visible rips. He asked for my name, and I choose to be Kate, not in the mood to explain the origin of my first name.

"I am Jesus.", he said, stopped beating his bowl to shake my hand, and grinned at me, displaying evidence for a missing dental plan. Okay, well, I mean I've learned that Jesus is quite a common name in Mexico, so I just nodded. Then he added "I died for your sins."

Being a polite person, I said "Thanks.", he definitely looked like recently crucified. I asked how his mother is doing. "Busy, busy" he said. Yeah, I guess, tough job being holy and all. Anyway. Unfortunately Jesus recognized my German accent and began asking me things about the pope. In case you wonder: no, I have never met the pope in person, despite growing up in Germany, but hey, there are roughly 80 Millions of us. And by the way, I am not catholic.

I was trying to get rid of Jesus, who began quoting things from the bible that I didn't understand for one reason or the other, when he suddenly leaned towards me scaringly close and asked:

"If God would answer one question for you, what would you ask?"

Hah! What a question!

Okay, now back on the couch in the cafe with my friend S.

Pretending to be intellectual, I should come up with a sensible answer to that question, shouldn't I? What about: How do we achieve world peace in 3 easy steps? But then, what would all the newspapers write about? I really don't want to be responsible for millions of unemployed journalists. But I thought about the question for quite some while. Eventually, I recalled that this is supposed to be a science blog, so maybe I should ask for the theory of everything or so.

But what if I'd just not understand the answer? What if nobody of us would? What if the human brain is just not capable of grasping the theory of everything, assumed there is one? If it's like your baby cries, and all you do is hand over the car keys. It doesn't help either if you add a map with a clearly visible red X on the closest mall, but car keys taste quite interesting, don't they?

So, what I would really like to know: If there's a theory of everything, are we able to understand it?

If we are, I am sure, sooner or later we will find it. I hope, rather sooner than later, but as always I am quite optimistic there. What really keeps me up at night is the question whether we would realize what we have found, should we stumble across it.

My friend S. began to look concerned as a result of my couch talk, so I felt like I had to explain myself. Yes, I do think that the capacity of the human brain at it's present state of evolution is most likely not able to finally explain everything there is in the universe. I mean, everyone who ever had to fill out a tax income return form, knows that there are things you just can't understand.

To prevent any misinterpretation at this point, I am not advocating intelligent design, instead I like to call it the principle of finite imagination. But like most scientists I know, I don't see any fundamental incompatibility between science and religion (though there is undoubtly some incompatibility between followers of each). I strongly believe that there is a theory of everything, but I also think nature is still way ahead of us. Let me put it this way:

There are more things between strings and loops than are dreamt of in your philosophy.

(Okay, that was the story about my intellectual answer to Jesus' question what I'd ask God. What I actually said was "I'd really like to know is why his son is wearing a pink skirt." )

What would you ask?

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