Happy Birthday Lara and Gloria!

Today our two beautiful girls are two years old! We have two cakes with two candles each and the apartment is full of balloons, awaiting the grandparents for a visit.

During the last year, Lara and Gloria have learned to walk and to run and to jump, to dance and to climb. Since a few weeks, they can climb out of their cribs, so time has come to upgrade the beds. We're browsing the IKEA catalog as I type, so to speak.  In their explorations, they have also suffered the occasional bruise or scratch, but luckily no major injuries. We too have gotten our share of bruises and scratches, mostly due to being hit with toys in a moment of inattentiveness.

For us, this year has been much more work than the first, because for most of the time we couldn't leave the kids unattended for even a second; they would inevitably tear something down, break something, spill something or topple over with the chair. It has gotten better during the last months. They know now fairly well what they can do safely, and they are careful not to touch anything that might be glass. We can let them walk around in the apartment now, so long as we recall to lock away the detergents and knives.

The girls are slow with learning to talk, though our pediatrician says this isn't so uncommon with twins. Gloria refers to herself as "Goo-kie" and to Lara as "Gah-kie" for reasons we don't know. They can now both eat by themselves, though one better doesn't leave them alone with the spaghetti.

When I contemplate the human brain, I am always torn between frustration about its shortcomings and amazement about how well it works. Watching the kids, what astonishes me most is how quickly and flawlessly they learn to identify objects. If you look at some picture book, the drawings are not usually too precise. Yet the kids have no problem to identify items from the books with our household items. And many items, such as most animals or large vehicles, they have never seen in reality, yet if they glimpse as much as a part of a tiger in an illustration, they'll announce "tee-ga". They also find Stefan's and my photos in the tiniest thumbnail versions from among dozens, instantly.

A source of amusement for us is how they construct causal and temporal relations. If they want to watch the washing machine for example, they sometimes pile laundry in front of it, not necessarily laundry that actually needs washing. If I forget to close the blinds for their afternoon nap, they'll scream and point at the window. When I come back from my morning run and the kids are already up, Lara will come and say "Mama. Ouh." and point to the shower. If they want Stefan to read them a book, they'll put a pillow on the floor where he usually sits.

The next year will bring many changes for the kids and for us, not only because of the new beds but also because they will spend more time with other adults and other children. We haven't yet completely solved our daycare problem, but it looks like it will resolve soon. Now it's time to light the candles :o)

Book review: “Phantoms in the Brain” by Ramachandran and Blakeslee

Phantoms in the Brain: Probing the Mysteries of the Human Mind
By V. S. Ramachandran, S. Blakeslee
William Morrow Paperbacks (1999)

Yes, I’ve read another neuroscience book. This one has been recommended to me as one of the classics in the field, though a little dated by now. It didn’t disappoint.

Ramachandran takes the reader on an engaging tour through the functions of the brain by using case studies on patients with brain damage. At first I thought this would be a collection of heart-wrenching stories and bizarre peculiarities – nobody really doubts shit happens if an iron pole leaves a hole in your head. But I was surprised to learn how reproducible the effects of localized brain damage are, how strange, and how much can be learned from the unfortunate patients.

The book starts with phantom limbs and our body image in general, followed by sections on denial, memory, religious thought, emotions, laughter, delusions and hallucinations. The chapters usually start with a patient or several, and are followed by an explanation of what brain regions are involved and what they do, to the extent that one knows. Ramachandran then often discusses some experiments that he and his collaborators did to shed light on puzzles going along with the condition, sometimes leading to insights that could help the patient or at least provide a basis for the development of treatments. He also adds his own speculations and hunches, which I find very interesting. He is usually very clear in demarking where actual knowledge ends and his speculation starts.

I was very pleased by his sober discussion of “qualia,” and his careful treading on the question of religion and the mind-body interaction in general. His argumentation is overall very balanced; he comes across as an open-minded scientist who isn’t pushing any particular agenda, but is simply driven by curiosity. I didn’t find his elaborations on the nature of consciousness too enlightening, but I guess consciousness is to neuroscience what the cosmological constant is to physics: everybody’s got an opinion about it and nobody finds anybody else’s opinion convincing.

The book is well written and reads very smoothly. It is however in places somewhat repetitive in that some patients reappear and one has to read through a summary. Some readers might appreciate that, especially if they had put aside the book for a bit, but it switches my brain into jah-jah-you-already-told-me-that mode. (The brain region for this is between the yawn-campus and the facebook-lobe.) I also have to complain that Ramachandran is quite vague on explaining what research has been done in the field apart from his own studies, and is too focused on his own work. Since the book is now more than a decade old, maybe it just wasn’t all that much. Still, I would have hoped for a somewhat broader survey.

(The co-author Sandra Blakeslee is credited in the acknowledgements for “making the book accessible for a wider readership.” The book is written in the first person narrative.)

Altogether, I learned quite a lot from this book, and especially the section on denial has given me something to think about. I’d give this book four out of five stars.

This TED talk by Ramachandran will give you a good impression what the book is about (the part about synesthesia is not in the book):

Merry Christmas!

We wish all our readers happy holidays and a merry Christmas, and if you're not celebrating Christmas, we wish you a good time anyway.

The girls are now old enough to take note of what is going on, so this year I've been thinking about our Christmas traditions.

In Germany, Christmas is celebrated on the evening of December 24th, the "holy night", with presents being deposited below the tree and opened either before or after dinner. A very common dinner on Christmas here is goose with red cabbage and dumplings. The presents are attributed not to Santa Claus but to the "Christuskind" (Christ child), usually depicted as a little angel. (I recall being quite confused as to whether its a boy or a girl.) Saint Claus' (Nikolaus) day on the other hand is in Germany not Christmas but December 6th. He delivers his goodies into boots that you place in front of the door over night. However, Saint Nikolaus comes with a dark brother, Knecht Ruprecht, who will slap the kids if they haven't been nice.

So tell me something about your Christmas tradition and how you celebrate!

Large Extra Dimensions - Not Dead Yet

15 years ago large extra dimensions were on vogue.

The idea that our universe may have additional spatial dimensions so small we have not yet been able to observe them dates back more than a century. This idea received a tremendous boost by the realization that String Theory actually requires such additional dimensions for consistency, but they were normally assumed to be wrapped up to sizes about a Planck length, or 10^-35m. That’s so small you can forget about it. (Forgetting about them being the reason to wrap them up to begin with.)

Then in 1998/99 some smart physicists realized that if there are extra dimensions, they could be much larger than the Planck length, and we wouldn’t have noticed. Better still, if these dimensions have the right size this would explain why gravity is so much weaker than the other interactions in the standard model, a problem called the “hierarchy problem” that causes physicists sleepless nights.

In these scenarios with large extra dimensions, the stuff that we are made of (quarks, electrons and so on) sits on a slice with three spatial dimensions, which is called a “brane”. This matter does not normally notice the additional dimension, but gravity does. This has the result that gravity is weak on long distances, but becomes much stronger on short distances, leading to a “lowered Planck scale” and quantum gravitational effects that are much larger than naively expected. Thus the excitement. (There are different models with different realizations of this, but the details won’t concern us in the following. For details read this earlier post.).

If one buys into this, one however has a new problem: The question why the extra dimensions have exactly this size. But sometimes finding a new way to formulate an old question can be a big step forward, so this should not deter us from exploring the idea.

Models with large extra dimensions also made predictions for the LHC due to the lowered Planck scale, most strikingly graviton and black hole production. In 2012, now that the end of the world is near, we know that nothing like this has been seen.

As I explained in this earlier post, it is quite rare that experiment can falsify a model, even if you might have heard so. Normally a model has free parameters that should be determined by experiment, or, if nothing is found, be constrained by experiment. That the LHC has not found evidence for large extra dimensions doesn’t falsify the idea, but it certainly “implausifies” it by constraining the parameters into an uninteresting range. Which is another way of saying, move on, there’s nothing to see here.

So you might think large extra dimensions are dead. But Cliff Burgess begs to differ. In two recent arXiv papers, he and his collaborators have put forward an extra dimensional model that offers an intriguing new perspective:

Accidental SUSY: Enhanced Bulk Supersymmetry from Brane Back-reaction
C. P. Burgess, L. van Nierop, S. Parameswaran, A. Salvio, M. Williams
arXiv:1210.5405

Running with Rugby Balls: Bulk Renormalization of Codimension-2 Branes
M. Williams, C.P. Burgess, L. van Nierop, A. Salvio
arXiv:1210.3753

The key point is that they are trying in the first place not to solve the hierarchy problem, but the cosmological constant problem: Why is the cosmological constant that we observe small and nonzero? The cosmological constant term in general relativity describes the energy of the vacuum and this energy should receive contributions all the way up to the Planck energy. This would be a huge contribution, leading to a very strong curvature of our universe, incompatible with observation. Why aren’t these Planck energetic quantum contributions there?

Burgess and his collaborators argue that a plausible reason is that space-time has additional dimensions, and the full space-time is not Lorentz-invariant. In other words, it’s a scenario with branes in higher dimensions. In such a situation, the troublesome quantum contributions, which normally, due to Lorentz-invariance, take on the form of a cosmological constant term, might not make themselves noticeable on the brane, which is where we live.

The example that they give is that of a cosmic string. If one calculates the metric that the string induces, one finds that space is flat but has a defect angle that depends on the string tension. The string itself however is unaffected by what it does to the background. The scenario that Burgess et al construct is basically a higher-dimensional version of this, where our universe plays the role of the string and creates a defect, but no curvature is induced in our universe itself.

Concretely, they have two additional large extra dimensions. (There might be more than that, but if they are much smaller their presence does not matter for the argument.) These additional dimensions have the topology of a sphere. On the two poles of the sphere, there are a brane each, one of which you can interpret as our universe. Like in the case with the cosmic string, the matter density on the branes induces a defect angle for the sphere, creating a manifold which they call a “rugby ball”. The radius of the sphere is flux-stabilized, which leaves one free parameter (a combination of the radius and the dilaton field).

These extra dimensions induce a vacuum energy on the brane, which is essentially the Casimir energy of this compact space, and this energy depends on the radius of the sphere. To use this scenario to get the right value of the cosmological constant, the radius should be of the order of about 5 μm, which is somewhat below current measurement precision (45 μm), but not so far below.

But what about the troublesome quantum corrections?

Supersymmetry must be broken on the brane (because we don’t see it) but is intact away from it. Supersymmetry solves the cosmological constant problem in the sense that it brings all the troublesome contributions in the bulk to zero. What remains to be shown though is that the cosmological constant on the brane does not receive large correction terms, which depends on the way the branes are coupled.

Cliff and his collaborators have shown that, in the scenario they constructed, the cosmological constant on the brane (read “in our universe”) does receive correction terms from high energies, but due to the way the branes are coupled these corrections are highly suppressed and do not ruin the smallness of the effective cosmological constant; they do not induce a large curvature. Think of the example with the cosmic string that stands in for higher dimensional branes. The geometry on the string (or brane) is flat regardless of the value of the tension. The large quantum corrections are there, but they contribute to the tension rather than inducing a curvature.

Now once you have fixed the radius of the “rugby ball” so that the cosmological constant matches with observation, you can use this to calculate the value of the lowered Planck scale. It turns out to be at least 10 TeV, so we wouldn’t see gravitons or black holes at the LHC. (Keep in mind that the LHC collides protons, which are composite particles. The average energy per individual collision of quarks or gluons is in most cases far below the total energy in the proton collision which is usually quoted. That’s why everybody wants a lepton collider.) However, since the string scale is somewhat below the Planck scale, one would expect to see string excitations at the LHC, though still at fairly high energies; we wouldn’t have seen them yet.

So to sum up, what this model achieves is the following: 1) It provides a setting in which there is a small cosmological constant whose small value is not ruined by large quantum corrections. 2) It makes the prediction that we should see corrections to Newton’s law not too far beyond present measurement precision. 3) It gives a plausible reason why we haven’t seen evidence for extra dimensions at the LHC so far but 4) predicts that we should see some glimpses of it in form of string excitations within the next years.

This doesn’t convince me to start working on large extra dimensions again, but it does convince me that large extra dimensions aren’t dead yet.

The Usefulness of Useless Knowledge

Abraham Flexner was one of the founders of the Princeton Institute for Advanced Studies. The other day I came across a wonderful essay, titled “The Usefulness of Useless Knowledge” (PDF) that he wrote in 1939, on the relevance of curiosity-driven basic research:

“Much more am I pleading for the abolition of the word "use," and for the freeing of the human spirit. To be sure, we shall thus free some harmless cranks. To be sure, we shall thus waste some precious dollars. But what is infinitely more important is that we shall be striking the shackles off the human mind and setting it free...”

Flexner goes through historic examples in which progress came about by scientists not thinking about applications but driven to understand nature, which resulted in unforeseen breakthroughs that changed our lives. Needless to say, his examples (Maxwell's equations, Bose-Einstein Condensation, atom spectroscopy...) could not take into account the most stunning developments in the later part of the century, based on our increasingly better understanding of quantum mechanics that underlies pretty much all the little technological gadgets we put under the tree.

But despite his essay being 70 years old, the points are as timely today as they were then, and they have only grown more pressing: Without basic research, progress is not sustainable and applications will eventually run dry. Believing that applied research produces technological advances is like saying electricity comes from the holes in your outlet.

Flexner was also ahead of his time in clearly realizing that science is a community enterprise, driven by social dynamics and the interaction of experts, and not by single individuals working on their own:

“[O]ne must be wary in attributing scientific discovery wholly to anyone person. Almost every discovery has a long and precarious history. Someone finds a bit here, another a bit there. A third step succeeds later and thus onward till a genius pieces the bits together and makes the decisive contribution. Science, like the Mississippi, begins in a tiny rivulet in the distant forest. Gradually other streams swell its volume. And the roaring river that bursts the dikes is formed from countless sources.”

AdS/CFT predicts the quark gluon plasma is unstable

The gauge-gravity duality is a spin-off from string theory and has attracted considerable attention for its potential to describe the quark gluon plasma produced in heavy ion collisions. The last news we heard about this was that the AdS/CFT prediction for the energy loss of quarks or gluons passing through the plasma does not agree with the data. The AdS/CFT community has so far been disappointingly silent on this issue, which has now been known for more than a year.

Meanwhile however, there has been an interesting new development pointed out by Brett McInnes in his papers

Fragile Black Holes and an Angular Momentum Cutoff in Peripheral Heavy Ion Collisions
Brett McInnes
arXiv:1201.6443

Shearing Black Holes and Scans of the Quark Matter Phase Diagram
Brett McInnes
arXiv:1211.6835

The dual description of the quark gluon plasma is a black hole in AdS space. Since the plasma resides in a beam pipe in a background metric that is to excellent approximation flat, the black hole that has to be used to describe it is a planar black hole. If one would use a “normal” spherical black hole, then the background for the plasma would have a spherical symmetry too.

These planar black holes appear alien at first sight because they have an infinitely extended planar horizon and are nothing like the real black holes that we have for example in the center of our galaxy. The planar black holes cannot in fact exist in an asymptotically flat space; they need the asymptotic AdS-space. So they might be alien in the context of astrophysics, but they make a lot of sense as a dual description for the quark gluon plasma.

Brett now notes the following. The quark gluon plasma that is created in heavy ion collisions generically has an angular momentum when the nuclei do not collide centrally. In particular, this angular momentum comes in the form of a shear, that is a non-trivial velocity potential in the direction parallel to the beam axis. The reason is, essentially, that the colliding heavy ions are approximately spherical (in their rest frame) and the amount of constituent particles that takes part in the collision depends on the distance from the beam axis. Thus arises a velocity profile.

So the quark gluon plasma has a shear. But this shear then should also be present in the dual description, ie for the black hole. In his paper, Brett studies such a sheared black hole in the AdS space – and the interesting thing is that he finds it to be unstable. If one takes into account that pairs of branes can be produced in the AdS background, then one can see that in fact an infinite amount of brane pairs can be produced because the brane action is unbounded from below.

But what does this mean?

The description of the quark gluon plasma that the AdS/CFT duality offers does not take into account that the formation, and subsequent fragmentation into hadrons, is a dynamical, time-dependent process. Brett thus argues that in a realistic situation after formation of the plasma it takes some while until the system is affected by the instability. He estimates the time it takes for the instability to develop and finds that for currently existing experiments at RHIC and at the LHC the plasma is stable for a time longer than it exists in the collision zone anyway. So there is nothing to observe in these experiments.

The relevant quantity here is the chemical potential. At RHIC and LHC it is very small, essentially because the collision is so highly energetic that very many particle-antiparticle pairs are created. However, for some upcoming new experiments, such as the ones planned at FAIR, that operate at a comparably low collision energy, the instability might become observable for realistic values of the impact parameter!

Brett is however very careful to point out that while the theoretic argument for the instability is solid, one should not take too seriously the numbers one obtains from his estimate. Since a truly dynamic treatment of the system is presently not feasible, what he does is instead is to calculate the time it takes for signals of an impending instability to propagate in the AdS background. One should not expect the result to be very precise.

Be that as it may, this opens the exciting possibility that upcoming experiments might observe an effect that could only be anticipated by use of the AdS/CFT duality.

How liquid crystals handle conflicting boundary conditions

Two weeks ago, we discussed nematic films: thin layers of liquid crystals in solution, dropped on a substrate. These thin films are pretty to look at in polarized light, but they also teach us a lot about the behavior of the molecules in solution. Because the thin films can be easily manipulated with electric and magnetic fields, or changes in temperature and boundary conditions, they are excellent experimental territory.

This is interesting physics not only because we use liquid crystals and other types of soft matter in many every-day applications, but also because of their closeness to biological systems: Most of your body is molecules in solution, and most of your body’s processes depend on the organization and interaction of these molecules. Granted, most molecules in biological systems are larger and more complex than the molecules in these thin layers, but one has to start somewhere.

So let us look again at the image of the nematic film that we have seen earlier. Note the not quite regular stripes. Interesting.

Image source: arXiv:1010.0832 [cond-mat.soft]

This, it turns out, is not the only type of regular structure that one can find in nematic films if they are thin enough. Sometimes one also finds little squares.

Source. Image Credits: Oleg Lavrentovich

This behavior has puzzled physicists since it was first observed, almost 20 years ago. Especially it has not been understood theoretically at which thickness of the film such modulations start to appear and what determines their size. The width of the film is typically at least 20 times or so larger than the length of the molecules, so this cannot be the relevant scale.

This puzzle is what Oksana Manyuhina, now a postdoc at Nordita, and collaborators studied in their paper

Instability patterns in ultrathin nematic films: comparison between theory and experiment
O. V. Manyuhina, A.-M. Cazabat and M. Ben Amar
Eur. Phys. Lett. 92, 16005 (2010)
arXiv:1010.0832 [cond-mat.soft] 

The neat thing is how straight-forward their analysis is.

The nematic films are described by a vector field for the molecules orientation. The direction of the vector field does not matter, only its orientation. The system tries to minimize energy, which depends on the orientation of neighboring molecules relative to each other. Up to 2nd order in derivatives of the vector field there is a handfull of terms that can be written down with constants to parameterize their relative strength.

The relevant new ingredient to understand the structures in the thin films are boundary terms. The substrate below the film and the air above it have different chemical properties that lead to conflicting preferences for the molecules: At the liquid interface the molecules want to be parallel to the surface while at the air interface they want to be orthogonal to the surface.

Surface terms had been investigated before to account for the appearance of quasi-periodic structures, but without success. It was found instead that there should be structures at arbitrarily long wavelengths, in conflict with what the experiments show. In the above mentioned paper now a new term was added that introduces an energy penalty for relative angles between neighboring molecules at the plane of the interface. This has the effect that solutions for the vector field are no longer isotropic in the plane.

Once the expression for the energy is written down, one considers a perturbation of the system by rotating each molecule by a small angle, and does a linear stability analysis. This way one finds the energetically preferred configuration, at least as long as the linear approximation is good. And indeed, these configurations show a quasi-periodic behavior that sets in at some specific width of the film! Exactly when it sets in depends on the coupling constant in front of the new term, which can be nicely fitted with the data.

Below is a schematic image of how the molecules try to arrange themselves with the conflicting boundary conditions. You have to imagine the liquid substrate on bottom and air on top. The little rods represent the molecules of the liquid crystal. Note how, at the bottom, they are parallel to the surface while at the top they alternate between trying to remain parallel to the lower layers and trying to be orthogonal to the air interface – that is what causes the quasi-periodic structures.

Image credits: Oksana Manyuhina

This is such a nice example for how theoretical physics is supposed to work: An experimental result that can’t be explained. A mathematical model for the system, and an analysis that shows it can correctly describe the observations. We learned in this process about the relevance of boundary conditions, and that one should keep in mind configurations of a system need not respect the symmetries of the Hamiltonian (here: isometry in the plane parallel to the substrate).

I'm populär

The December issue of the Swedish magazine "Populär Astronomi" (Popular Astronomy) has a researcher profile about me. You can download a PDF here. For all I can tell, it's nicely written and accurate. If, in contrast to me, you actually speak Swedish, I would like to hear your opinion... I meet with the journalist, Anna Davour, during our cosmology program last month, after a glass of wine or two. It was a pleasure to talk to her and she did an excellent job.

The reason I'm grinning so stupidly in the photo is that we were looking for a whiteboard that would serve as background, and, when we found one, realized that none of us actually knew what the equations were about. Good thing one can't read them anyway. (We got as far as: It's a Hamiltonian. And something with spin couplings.)

And I have no clue what the header is supposed to mean.