Paris, Planck 2006

This week, I am in Paris at the conference

Planck 2006: From the Electroweak to the Planck Scale

which is so far really nice. The company is pleasant, the food is excellent, the seats in the auditorium are extremely comfortable to sleep in, and - oh yes - the talks are okay so far.

I can't pass a single chocolaterie without picking some pralines. The coffee tastes like coffee, the cheese tastes like cheese, the traffic is still suicidal, Notre Dame is still here, and the French still smoke in public. That's the nice thing about Europe: it's so reliable.

Vive la France, B.

Nonlocality

I am relieved to see that Germany is still Germany. The weather is rainy, people drive like nuts, and everyone is unfriendly. Each time I come back for a visit I find it harder to localize my interactions. Crossing the street without being run over has become a challenge. The currency looks funny. Where do I get coffee to go, or aspirin on a Sunday afternoon? I go through the aisles in the grocery store, mumbling 'excuse me, thank you' and people stare at me with wonder. Not so much because I excuse, but because there is nothing to excuse for. And have avocados always been green?

Besides this, Germany is going nuts because of the soccer world cup. No street corner without advertisements, no magazine without reports, no good German without excitement. Glad to say, I leave before the fun starts.

I mentioned some time ago that I stumbled across this paper:

Relational EPR
Authors: Matteo Smerlak, Carlo Rovelli
quant-ph/0604064

We argue that EPR-type correlations do not entail any form of "non-locality", when viewed in the context of a relational interpretation of quantum mechanics. The abandonment of strict Einstein realism advocated by this interpretation permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality.

It took me some while to make up my mind, but here is a rough summary of my understanding and my opinion.

The so-called collapse of the wave-function, caused by the measurement process in quantum mechanics, is commonly considered to be a problem because it is non-local. Even though such collapse does not allow to actually transmit information between observers, it is unpretty.

Take a box with two particles, about which you know that the total spin is zero. Then divide your system into two particles, sending one to observer A, one to observer B. Quantum mechanics tells you the state is entangled before any further measurement is made, meaning you can't say what the spin of each particle is. When observer A has made his measurement, the outcome has to be either + or - 1. Let's say, it is +1. Since the total spin is zero, the other particle then has to go into state with spin -1. Instantaneously.

Now the claim of the relational interpretation is that in the above it was omitted to take into account that the measurement device, or the observer himself, also are subject to quantum mechanics. Therefore, strictly speaking, the outcome +1 of the measurement for A is not global. It is +1 as seen by observer A, it is a measurement relative to A.

The main statement is that there is no need for a non-local collapse of the wave-function if one takes this quantum mechanical relativeness of observers really serious. Drop the assumption of the non-locality, and assume instead that A measures something, and B measures something. As long as observer A and B don't compare their measurements, there is no problem. But if they do, their comparison is an interaction, subject to quantum mechanics, and has to be treated like such. As shown in the paper, if you describe the comparison as an quantum mechanical interaction, both observers will always agree on their description of the experiment. "It is clear that everybody sees the same elephant."

As I understand it, this means roughly the following. If A measures +1 and B -1, or vice versa, there is no problem. But if that always were the case then we were back to the collapse. Now the new thing is, let A measure +1, and B measure +1. That doesn't worry us as long as they don't compare their measurements. When they do (back in causal contact) B will find A's measurement in agreement with his, meaning 'A as seen by B' is -1, and also A will find B's measurement in agreement with his 'B as seen by A' is -1.

Should I badly have misinterpreted this, I would be very grateful if someone could enlighten me.
You might also want to look up Alejandro's blog, Christine's blog and the Physicsforums.

It seems to me that this might be possible to write down on a paper. But I can't make too much sense out of it. In particular, I have a problem with the above elephant, alias, the classical limit. As long as they are not in causal contact, observer A and B make up their own stories about what is happening. Yet, when they talk to each other, they both always agree. Not because their stories are identical, but because they interpret to other observer's story through some measurement as being identical to their own story.

But suppose A has a dog, and he agrees with B to kill it when he measures +1. A and B separate, are out of causal contact. Both measure +1. A kills the stupid dog.

Then he comes back into causal contact with B, and of course he takes the dog, which is nothing but a macroscopic result of a quantum measurement. But no matter what, B will always have to find that the dog is alive.

This reminds me of some conversations I have with my grandmother. Being almost deaf she just hears what agrees with her story of the universe. I goes somewhat like this

Me: "I hear his mom is really worried"

My granny: "It's so great he's getting married!"

Me: "No, I mean, they split up recently."

My granny: "Yes, that's how it's supposed to be."

Me: "But he left - HE HAS MOVED OUT."

My granny: "She has every reason to be proud."

I admit that I envy my granny for her ability to make up her own reality. The bottomline of the relational interpretation is essentially, that we all have our own reality, and no matter how hard we try to talk to each other, we will never agree. But we will never notice that we don't. On a philosophical level, I find that to depressing to like it. In addition, I feel specifically non-local today. Looking forward to your feedback, B.

This used to be my playground
This used to be my childhood dream
This used to be the place I ran to
Whenever I was in need
Of a friend
Why did it have to end
And why do they always say
Don't look back
Keep your head held high
Don't ask them why
Because life is short
And before you know
You're feeling old

~Madonna

Dreamer

"All that you see or seem
Is but a dream within a dream."

~Edgar Allan Poe

The Principle of Finite Imagination

Airports have a strange atmosphere. I frequently receive emails from friends who sit at airports, and contemplate their lives, go into philosophical meditations about the meaning of reality, or the mystery of our existence. Yesterday, I was stuck on such a meditation place, even worse, I was stuck there with nothing else to read but Susskind's book, "The Cosmic Landscape".

The last dinners I had in the company of physicists inevitably ended up with a discussion of the anthropic principle. At which point I very suddenly got very tired and left early. Being asked recently for my opinion on the matter I said I have none. As Stefan told me, that's not good, I am supposed to have an opinion about everything! No matter if it is a sensible one.

To get to my opinion or it's absence, let me introduce you to Oliver. I got to know Oliver as 'Olli_6703' in some online discussion around 2001. Since then he lost his job, his long-time girlfriend, an had an unfortunate accident that left him with a permanent limp. By now he is an alcoholic. I guess, drinking is his way to arrange himself with the insight that life sucks.

It's hard to say why he ended up like this. He's been very sportive, and suddenly being confined to slow-motion was really tough on him. It might also have mattered that his girlfriend did better in her job than he did, and she chose to live alone instead of with him - something he repeated endlessly and was unable to understand. Had she cheated on him and moved in with someone else it might have made more sense to Olli. But she stayed ALONE! Or then why did he have to loose his job in the first place, which started the whole series of unfortunate events? The company he worked for installed phone systems. They went broke cause the companies that were in need of these systems went broke. I am neither a therapist, a sociologist, nor an economist and don't know what the reason is. Maybe it's "just" the genes.

Sorry for the sad story.


Now imagine you are an atom in one of Olli's liver cells. You are a smart atom and have figured out the Standard Model. You had no need for some weird assumption like gravity, but you have found very elegant laws that describe the exchange of molecules in you cell. You even managed to measure the extension of the liver! To your very surprise you found that it has been growing recently. You and the other atoms are very puzzled by this, and you try to come up with a theory to explain it.



Maybe you introduce some a-essence that causes the liver to grow, but where does it come from and why has it only become important recently? Some of your colleagues have suggested that there are other meta-livers outside yours which obey different laws, but hey, you are a serious scientist, and that's just too weird. Besides this, it doesn't really explain anything. Some insist on a fourth force, based on some kind of a principle (you keep forgetting the name). But this force would only be important on completely unobservable distance scales. Though they claim it's important for a theory of everything you fail to see the point. Some even work on an extension of this theory that implies that the cosmos is nothing but a braid, but you can't really follow their arguments.

Then there are some others who found a beautiful string-like winding structure which they claim contains all the information necessary to explain the liver and probably more! But they are unable to predict anything from it. They keep repeating it's elegant, and keep making conjectures about things they don't even know what they are.

However, none of these incredible theories was able to help you understanding the strange place that you live in. Recently, it's been dubbed a crisis in liver-physics.

How smart has the atom to be to imagine the existence of a human being? To imagine several billions of them? Of the world they live in? With all the global and sociological problems? How smart has the atom to be to imagine the existence of the earth, the solar system, our galaxy, the universe, or even multiverses? The poor atom was just looking for a theory of everything, just some few equations that extend the Standard Model such that they explain the observed liver growth.

To come back to the anthropic principle: It is certainly right, if we weren't here then we wouldn't worry why we are here. But I am a physicist because I hope that I can understand at least part of the games that nature is playing on us. Retreating to the anthropic principle means to me to give up the believe that there is something to understand.

Maybe I am just just stubborn.

But I am surprised that just because we currently can not imagine a way out of the so-called 'crisis' in theoretical physics, so much effort goes into explaining why we can't explain what we want to explain. So much time goes into arguing why we can't argue. And smart physicists declare bugs to features, instead of looking for other ways to find insights.

Needless to say, I believe that there is a reason why the universe is the way it is. It might just be very hard to find. Try to imagine there is an universe in every gluon, and our universe is a gluon in an atom in some liver-cell of an alcoholic cosmic terrorist, who aligns his angular momenta on the axis of evil while his followers are forcing cosmological natural selection on innocent citizens.

I would rather come to the (admittedly depressing) conclusion that the human mind just might not be able to solve the problems we are currently facing, than being satisfied with the statement that there is nothing to explain. Going anthropic is not a solution to anything. If anything it's reason to quit physics.

Fortunately, it is in the nature of human beings to never be satisfied. Therefore, I have no doubt that this crisis is temporarily.

That's the reason why I don't spend time on thinking about the anthropic principle and the meaning of the string landscape, not even at airports. There are just more interesting topics.

(Like, where is the next Starbucks, and what do the Americans do with their milk foam without spoons?)

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See also Alejandro's recent post about the lamdscape.

Perimeter Institute

This is the first time I see Waterloo not covered by snow. When I was here in December last year, I took the wrong exit from the building and found what I thought was a large untouched field of snow. After too much brain activity during the day, I wasn't in the mood to figure out how the key-card works. I decided I could manage 3 inches white fluffy stuff on the pavement. After some steps however, I stumbled down an invisible edge, and stood more than knee-deep in snow. I was cursing whoever constructed this stupid yard.

However, today I solved the mystery of the annoying edge: it was the pool! (see photo below).

Greetings from Canada,

B.

Emmy Noether

I made my PhD at Frankfurt University in August 2003. From 2001 on Stefan, Marcus, and I lead a group we called LXD - Large eXtra Dimensions. Its official supervisor was Horst. From the very beginning on this group was quite successful and it seemed to be very attractive for young students. Soon we had several undergraduate and graduate members who were very interested in working with us. Temporarily, we even had encounters with mathematicians from the other side of the building in our group meetings!

During the first years, we managed to use money from other sources to support the group members, but soon it became clear that we would need an own grant to enable ongoing research. We applied for support at the German Research Foundation (DFG). The support was declined at the end of 2003. In January 2004, I moved to the US.

Here is a photo of (most of) the group (missing: Katja Poppenhaeger), taken around Christmas 2003. From left to right: Marcus Bleicher, Stefan Hofmann, Sabine Hossenfelder, Christoph Rahmede, Ulrich Harbach, Joerg Ruppert, Sascha Vogel, Horst Stoecker.

While I was sitting in Arizona and cursing the blisters from my Flip-flops, I had plenty of time to wonder about the complete absence of future my life suddenly suffered from. Then I found that the DFG has an excellent program, named the Emmy-Noether-program. Its purpose is:

To provide outstanding researchers with the opportunity to rapidly qualify for a leading position in science and research or for a university teaching career by leading an Independent Junior Research Group and assuming relevant teaching duties.

To recruit young, outstanding postdocs working abroad (back) to Germany.

In summer 2004 I applied for this program, as it was one of the little choices that seemed to make sense to me. Not only would it have meant a 5-6 year position, but I would have been able to hire collaborators, and to actually pursue my research interests! (As opposed to, say, cursing my blisters). This was such a great opportunity that it seemed worth the effort to write a proposal despite the small chances to finally get the grant (all of a sudden I heard from about thousands of people who did not get it).

Unfortunately, during the time my application was being processed, the guidelines for the program changed significantly. (The original version consisted of two separate phases). In February 2005, I received the decision from the DFG. They wrote they had to reject my application. I did not fulfill the requirements for the new guidelines. According to these, I needed at least two years of stay abroad, one of which was missing. Instead, the DFG was very generous and offered me to instead provide a scholarship for this missing year. After this year, I could apply again for the Emmy Noether program. Joerg and I found that very funny. Essentially they said: "You are great, please stay in the US."

I applied again for the program in summer 2005. The regulations had become significantly more difficult and bureaucratic. Among other things, the application had to be typeset in Arial, printed on A4 paper and two-hole-punched (you don't believe that? Read Section IV: Proposal Formats and Submission). Living in the US, the latter two requests I found almost impossible to fulfil. (Well, if you live in Germany, try to print 50 pages on letter format and get them 3-hole-punched).

Since I had no problem picturing me being unemployed and sleeping on the beach any time soon, I also applied for regular postdoc positions around November 2005. To make a long story short, I accepted the offer from the Perimeter Institute in January 2006. Then, in February, the DFG told me that I got the grant for the Emmy Noether. I asked them whether it was possible to postpone the starting date for one year. They said no. PI assured me every possible kind of support for a collaboration with Germany, so I offered the DFG that I would initiate and support the project from Canada (though I had to be talked into making this offer). Again, they said no, the grant is attached to my person, and no one else but me can make Phenomenological Quantum Gravity (such the title of the proposal) in Germany. They asked me to reconsider my decision.

Yesterday, I wrote them a lengthy explanation why I would stick to my decision, go to Canada, and decline the German offer.

This, I guess, is the end of the story I started in a previous post. I find it very sad that the possibility to start a group on Quantum Gravity in Germany fails due to a lack of flexibility in program guidelines.

Dear reader: should you qualify for the Emmy Noether Program and consider applying for a grant, please feel free to contact me. ESPECIALLY, if you are interested in Phenomenological Quantum Gravity, as it seems, the DFG is in principle willing to support such research.

---

I am traveling the next 3 weeks, so the frequency of my posts will crucially depend on the speed of my internet connection.

The Minimal Length Scale

Yesterday, I gave a seminar here at UCSB about my recent work, and so I will use the opportunity for some self-advertisement. You find the slides (PDF) online:

• The Minimal Length Scale
  Sabine Hossenfelder, UCSB

I talked about quantum field theories with a minimal length scale, their interpretation, application, and their relation to Deformed Special Relativity (DSR). The talk is mainly based on my recent paper:

• Interpretation of Quantum Field Theories with a Minimal Length Scale
  [hep-th/0603032]
  

And the basics of the model are from our '03 paper

• Signatures in the Planck Regime
  [hep-th/0305262]
  

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Here is a brief summary about the main statements. In 2003 my collaborators and I worked out a model that includes the notion of a fundamental minimal length into quantum mechanics and quantum field theories. It builds up on earlier works, most notably by Achim Kempf, but extends these approaches -- and is less mathematical but instead focused on applications. The model turned out to be useful to derive modifications to Feynman rules, and allowed us to compute cross-sections (at least at tree level).

Modifications of such a minimal length should become important at energies of about the inverse of that length. The Planck length is expected to play the role of such a minimal length. In case there exist large extra dimensions, the 'true' Planck length might be about 10^-4 fm and be testable at the LHC. Actually, what one would find in this case is that there is nothing more to find. Once one reaches the minimal length scale, it is not possible to achieve a higher resolution of structures, no matter what. (And thus there's no point in building a larger collider.)

The motivations for the existence of such a minimal length are manifold, you can e.g. look up my brief review

• The Minimal Length and Large Extra Dimensions
  [hep-ph/0410122]

Essentially the reason for the emergence of a fundamentally finite resolution is that at Planckian energies spacetime gets strongly distorted and it's not possible anymore to resolve finer structures. Also, such a minimal length acts as a regulator in the ultra-violet, which is a nice thing.

The basic idea of my model is to effectively describe such a finite resolution by assuming that no matter how high the energy of a particle gets, it's wavelength never becomes arbitrarily small. To do so, just drop the usual linear relation between wave-vector and momentum. With this, one can then quantize, 2nd quantize, etc. Note, that in my model, there is no upper bound on the energy. There is instead a lower bound on the wave-length. The energy and the momentum of the particles have the usual behavior and interpretation.

While writing the paper in 2003, I could not avoid noticing some kind of a problem with Lorentz-invariance. Apparently, a minimal length should not undergo a Lorentz-contraction and become smaller than minimal. However, it turned out that the quantities with the funny transformation behaviour never entered any observables. It was thus completely sufficient to assume that such a transformation exists, without knowing how it actually looked like.

It was only 2 years later that I realized the connection of this model to DSR which apparently has become enormously fashionable over the last years. (I just found that by now there is even a textbook on DSR available.) By now there are a vast number of papers on the subject. The one half are very phenomenological, the other half are very algebraical investigations. However, as far as I am aware, DSR-theories are still struggling to formulate a consistent quantum field theory. (There was a very notable recent attempt by Tomasz Konopka to set up a field theory with DSR, but imo the model has more than one flaw.)

Most importantly, DSR faces two problems: the one is the soccer-ball problem, the other one is the question of conserved quantities. To briefly address those:

1. In DSR there is an upper limit on the energy of the particle. Such a limit should not be present for macroscopic objects (e.g. a soccer-ball). The problem is how to get the proper multi-particle limit for which the deformed Lorentz-transformation behaviour should un-deform. (There has been recent progress towards this direction, see. e.g. Joao's paper, but I think the issue is far from being solved).
2. The second problem is that since in DSR the 'physical' momenta do not transform linearly, they do not add linearly, and thus one has to think about what quantities are conserved in interactions - or, how they are defined in the first place. To take a very simple example: usually, the square of the center of mass energy s for two particles with momenta p and q is s = (p+q)^2. Usually, s is Lorentz-invariant (its a scalar), and since the tranformation is linear on p and q, the result of boosting p+q is the same as boosting p, boosting q and then adding both. Not so in DSR. So, which quantity is the center of mass energy? Is it still a scalar? Is it conserved? The situation gets worse for more particles. (Again there has been progress, see e.g. the paper by Judes and Visser, but I think the issue is far from being solved.)

I should admit that I maybe just don't understand that. At least, it is confusing to me, and even after thinking about it several months, I still can't make sense out of it.

Therefore, lets get back to what I understand and that is how things work in my model. Free particles do not experience any quantum gravitational effects. They behave and propagate as usual. When they make an interaction with high center of mass energy and small impact parameter, quantum gravitational effects can strongly disturb the spacetime. An exchange particle in this region then experiences the effects of DSR. It's wavelength has a lower bound, and there is a limit to the resolution that can be reached in such an interaction. This is schematically shown in the figure below

The quantities that are conserved are as usual the asymptotic momenta of the particles. Composite objects do not experience any effects since they are not usually bound such that the gravitational interaction is very strong. Thus, both of the above mentioned problems of DSR are not present using this approach.

I should be so fair to point out that in my scenario there is no modification of the GZK-cutoff, which is the most popular prediction from DSR.

To briefly recall the problem: protons propagating through the universe can make cosmic rays when they hit the earth's atmosphere. The total energy of these events can be measured (at least a lower bound). A proton with very high energy however, should be able to make pions by interacting with photons of the CMB. If the energy of the proton is such high, it can not travel far enough, will not reach the earth, and can not produce a cosmic ray. This cutoff is expected to occur at a certain energy, the so-called GZK-cutoff. There are some events that seem to indicate that cosmic rays above this cutoff have been detected (data has to be confirmed). This has lead to a huge amount of speculation for the cause of the non-presence of the GZK-cutoff.

To come back to main subject, the reason why there is no modification of the GZK-cutoff in my model is very easy to see: the cutoff is a sudden increase in a cross-section as a function of the center of mass energy. Both, the center of mass energy and the cross-section are Lorentz-scalars. The cut-off has been measured on earth at a certain center of mass energy (about one GeV). Boosting it into the reference frame of the fast proton does not change the necessary center of mass energy.

Here is something that still puzzles me: DSR is argued to be observer independent. Now I wonder how can it be that in the one system the cut-off is at a different center of mass energy than in the other system. And if it was so, couldn't one then use exactly this to distinguish between observers?

Anyway, the bottomline is that my model is an alternative interpretation of DSR. It has less problems, but is also less spectacular. I am a particle physicist, and I would really like to see a self-consistent formulation of a QFT with the 'usual' DSR interpretation - maybe that would help me to understand it.

Update Dec. 9th 2006: see also Deformed Special Relativity

Why do we live in 3+1 dimensions?

People warned me that writing a blog would take a lot of time. Hopelessly naive as I am, I thought, well, I would just not post anything if I am too busy. It seems, I underestimated the persistent interest of my fellow readers.

So, I wrote last week that I volunteered to bring an Honest Question to our gravity lunch. This meeting takes place here at the Department pf Physics at UCSB every Friday at noon. Lately, the discussion has been mostly about black thingies. I tried to come up with a question that would roughly fit into the string dominated atmosphere and the time constraint, and eventually settled on "Why do we live in 3+1 dimensions?"

The last time I wrote this question down, someone was so nice to tell me that 3+1=4. Therefore, let me point out that the question actually consists of two parts: a) why 3 spacelike dimensions and b) why Lorentzian signature -- I will only discuss a) in the following.

But of course I had to make the question more complicated to be appropriate for the gravity lunch. To do so, I picked the paper

Relaxing to Three Dimensions
Authors: Andreas Karch, Lisa Randall
Phys.Rev.Lett. 95 (2005) 161601 (hep-th/0506053)

Last time I was at PI, I happened to hear a seminar by Lisa Randall about the paper, you can find it online at the streaming seminars (click on "seminar series", then "filter by presenter" Lisa Randall - comes under L not R, and hit "search" -- they promised they are working on an improvement...).

While writing this post, I also found an article about the paper from economist.com

A braney theory: An explanation for the anthropic principle comes a little closer

---

Here is a quick summary: The idea is to take a 9+1 dimensional non-compactified spacetime. Fill it with gases of d-branes, each with an energy density and pressure. And let it expand with a Friedmann-Robertson-Walker (FRW) ansatz, i.e. homogeneous and isotropic. The paper is quite impressing, as it only contains 4 equations, which are the FRW equations in higher dimensions.

Now the question is what happens to the gases of the d-Branes.

1. When the branes don't interact, the energy density will dilute slower the larger the dimension of the brane - because it can dilute only into the dimensions it does not occupy. In terms of the FRW scale parameter a, the density goes with a power -9+d.


2. But they can interact, i.e. branes can meet anti-branes and decay. This decay goes slower the larger the dimensionality of the brane - because there is less space to decay into. In terms of the time t, the density goes with -9+d.


3. Then the question remains whether d-branes do find each other to interact. It turns out from dimensional arguments that they will generically find each other and attempt to decay when 2*(d+1) is larger or equal 9.

From 3. it follows that 3-branes are those with the largest dimensionality that will not interact. From those that will not interact, they are also those whose energy density will dilute the least. For d larger than 3, the 9-branes do always overlap and therefore are gone. The 8-branes are apparently more complicated, but can be argued away. The only argument for the latter that I understood was that in some scenarios there just are no 8-branes. Let's assume that works.

Then, in terms of energy densities, 3-branes and 7-branes will dominate. Such the conclusion of the paper. I understand why one would like to have 3-branes. As to the 7-branes, the paper states

A configuration that is a natural candidate for four-dimensional gravity is the intersection of three 7-branes where the intersection has spacetime dimension four.

Among others, a question I raised on Friday was why this is natural. I learned that the physics on such an intersection allows chiral fermions. That explains why they write it is natural to live at this intersection. But not why it is natural.

More importantly, I fail to see why the densities of the gases are relevant for the question why we experience three dimensions. Even if the higher dimensional thingies decay, the lower dimensional ones are still around, no matter what their density is. Why is the energy density the selection criterion?

And another point that I still don't understand is how it is possible that the ongoing time-dependence in the bulk does not influence the physics (locally localized gravity) on the brane or brane/intersection. I mean, one has to make sure that things we call constant actually are constant (restrictions apply).

Bottom-line: I like the paper, I like the idea and the minimalistic setup. Unfortunately, it seems to me some of the arguments are more wishful thinking than strict conclusions.

I have certainly heard weirder things. I once sat through a seminar while the speaker explained that our universe has 10 dimensions because Pi^2 is approximately 10, and we live on a 3-dimensional submanifold because Pi is close by 3. No, I can't recall the name of the speaker, and I never heard of him again.

On the other hand, it is indeed puzzling that dimensional regularization works only in 4 dimensions, isn't it?

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For those of you who are interested, here are some further references

Why do we live in 3+1 dimensions?
Ruth Durrer, Martin Kunz, Mairi Sakellariadou

Brane Gases in the Early Universe
S. Alexander, R.Brandenberger, D.Easson

On the dimensionality of spacetime
Max Tegmark

(thanks to Garrett for the reference)

Why is Space Three Dimensional
Ingemar Bengtsson

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Oh, and due to a certain lack of volunteers, I agreed to bring another question for the next gravity lunch. Any suggestions?

Honest Questions

I volunteered!

I volunteered to bring a question to our gravity lunch next Friday. Now I am wondering what to ask the brainy guys. How about: why are there hundreds of postdocs discussing the existence, properties and numerical investigation of black holes, black branes, black strings, black rings, or short: black things? (Though I have been told the technical term is black THINGIES.) Black thingies may be boosted, rotating, magnetic, deformed, bumpy or bumpier, potentially unstable, but mostly extreme, and certainly in extra dimensions.

Other questions that currently puzzle me is what a particle is, or why women plug out their eyebrows and then paint them back on. But I figure none of these interesting questions is really appropriate.

Besides this I am preparing a seminar I have to give next week here at UCSB. Since the unwritten law is that questions I can answer won't be asked, I currently wonder what a quantum algebra is (don't ask) -- not that it has anything to do with the seminar.

However, thinking about questions, I wasted time classifying the type of questions I have encountered in multiple seminars and parallel sessions:

1. The commenting question

Starts most often with the words: "It is more a comment than a question..." and serves to prove that the person asking knows something about something, most likely his/her own work. This is the most comfortable question to encounter, and can actually be very interesting, even if it has nothing to do with your talk.

2. The completely irrelevant question

Is the most common question, and one that you might want to practice when you like to draw attention to yourself. To give an example, I have been asked once how many pions a black hole emits (couldn't care less). Here are some very universally applicable members of this family

• What about the Cosmological Constant?
  (Yeah, what about it?)
• Can you couple a massive scalar field to your model?
  (Sure, but why would I?)
• Does that work in an arbitrary number of extra dimensions?
  (Some particular number? 10 or 11 by any chance?)
• Can you say something about chiral symmetry?
  (Plenty, but that's got nothing to do with my talk)
• Is there any relation to the recent work by XYZ?
  (No idea, never heard of them.)

3. The never-ending question

Starts most often as an interruption -- say on slide 2 -- and has the answer: I will come to that later -- say on slide 34. Not satisfied with this answer, the person asking will insist on further and further details, thereby completely destroying the structure of your talk which you have been thinking about for the last weeks. If the chair person fails to pull the pump-gun, my advise is to simply ignore the questions.

Another kind of never-ending question is asked in the end of your talk, and requires you to remember every detail of every calculation, parameters of the numerical investigation, factors 4 Pi, references to people whose names you can't pronounce, starting dates of experiments you can't remember, rotation directions in complex planes, and will make you ask yourself why the fu*k you did this work at all. Maybe we can discuss that in the coffee break.


4. The stupid question

Requires very much politeness, since we don't want to embarrass anybody, do we? Stupid questions are frequently asked by those who missed the beginning of your talk. Like: what is d, when you have been talking about the number of extra dimensions for 1 hour.

5. The dangerous question

Is most often asked by senior scientists and frequently comes with an attempt to appear harmless, like "It is probably a stupid question, but..." or "I must have missed something, but..." In case you encounter such a question, consider faking a heart attack on stage.

6. The honest question

And then there are the occasional honest questions from people who genuinely try to understand your work. Honest questions have made me realize many times what direction to look for further insights, and have brought up viewpoints I might have completely missed on my own.

See also: my new poem Honest Questions.

Lisa's Warped Passages

Some of my friends in Germany frequently send me newspaper articles whenever the word physics appears in whatever context. Some months ago they sent me an article about Lisa Randall's new book which started by describing the "world's most cited theoretical physicist" as "blond, small, wiry and - female". Jörg commented about this very nicely "When Lisa Randall is small and wiry, then what are you - a dwarf?" Thanks Jörg ;-)

Anyway, I bought the book and read it, I actually read more of it than I thought I would. For the simple reason that she starts every chapter with some lines of lyrics! (Something I did in all my failed attempts to write a book -- as the Stefans might remember, who very bravely made their way through all of them.)

Though I initially did not like the book, I changed my mind in chapter 5. ANDI!! It's the Billy Bragg quotation:

"The laws of gravity are very, very strict
And you're just bending them for your own benefit."
~ Billy Bragg

---

Lisa's book is titled "Warped Passages" and explains about everything you need to know about high energy physics and physics beyond the standard model.

From the cover: "Lisa Randall demystifies the science and beguilingly unravels the mysteries of the myriad worlds that may exist just beyond the one we are only now beginning to know."


I don't see any point in giving you another detailed review on the book's content, you might want to check out those at amazon.com instead. Let me just say that the book provides you with a fairly self-consistent introduction into the idea of extra dimensions, covering general relativity, quantum mechanics (even quantum field theory) the standard model of particle physics, and string theory.

It successfully captures the excitement and the beauty, but it makes also clear what we know - and what we don't. It is well written, entertaining and very structured.

For a popular science book, it is very precise in the statements and it covers a lot of ground. If you are not familiar with the subject, you probably will have to think about it for a while. On the other hand, this makes the book also interesting for those with an education in physics.

Most subjects are explained with analogies - sometimes a confusing amount of them. E.g. the bulk is "a huge tennis stadium", perturbation theory fails for strong coupling because "it's like trying to reproduce a Jackson Pollock painting by randomly pouring paint", duality works like a five-course menu "pre-organized and prepared into salad, soup, appetizer, main course, and dessert", a small black hole evaporates more quickly than a big one "just as a small drop of coffee evaporates more quickly than a full cup" (something I still wonder about) etc.

For me, the analogies were the true challenge of the book. I admit that I find papers with equations more easy to understand. On the other hand, these analogies are the reason why I find Lisa's book enormously useful. It gives my the words to explain my work to other people.

I admit, I had been hoping for more gossip, and of course I was curios to know if it ever mattered to be not only "blond, small, and wiry" but also female. However, having thought about it, I think I wouldn't have made a point out of being a women in my first book either.
(Relax, I am not planning on writing one.)

I don't particularly like the stories with Alice and Ike as I generally don't like characters without character, but a huge plus are the short summaries of the most important points at the end of each chapter.

To summarize:

The book is really dangerous. If you are not already an high energy physicist, you probably want to become one after having read it. So, if you had in mind giving the book to your daughter as a gift, think twice.

Your citation-dwarf, Bee.

PS: And I learned the word jungle gym.

Why the Quark Gluon Plasma is a Banana

When I was in 5th grade, our biology teacher dropped out for a longer period. For whatever reasons, she was temporarily replaced by our maths teacher. He was about to be retired soon, and had not taught biology for more than 20 years. Now he saw himself faced with the task of giving "sexual education" to a bunch of 11 year olds.

After having extensively discussed various forms of STDs, we all were completely sure we would never ever have sex in our life. Our teacher then proceeded to safe sex, and eventually produced a banana to explain the proper use of a condom. He was clearly struggling with words he hardly ever used in his life, and the banana seemed to us a very theoretical example for a very involved practical situation.

I thought about this banana, when I was in very interesting seminar at the KITP last week by Pavel Kovtun, titled

How Can AdS/CFT Be Useful for Heavy Ion Physics?

There, in this seminar room, all the stringy guys were talking about jet suppression, shear viscosity and central collisions! It was equally weird as hearing your maths teacher talk about sex.

In the talk, it was explained how the AdS/CFT correspondence can be used to model certain properties of heavy ion collisions. I find this an interesting topic, with a clear connection to phenomenology, which I often miss in talks at the KITP.

It also reminded me of a colloquium by Joe Polchinski that I heard about 3 years ago at LBNL. The only thing I can recall was that he said something like the quark gluon plasma is a black hole. He most likely said more, but I was too nervous to listen, since own seminar (the first seminar I ever gave outside my home institution) was scheduled for the following lunch meeting. Consequently, the only thing that was in my head while I was giving my seminar (which btw had nothing to do neither with heavy ions nor with black holes or qcd) was what the fu*k a black hole has to do with the quark gluon plasma, and why I ever agreed to give this seminar.

Had someone told me at this point that I would end up at UCSB three years later, no way I would have believed it.

---

1. AdS/CFT

So, here are the rough basics that I hope are sufficient to get into the spirit. Strongly coupled QCD is messy soup, and calculations are nasty. It turns out however, that above the critical temperature, a conformal field theory is a pretty good description of the messy soup (see slide 8 of talk or picture below). Plotted is the energy density (epsilon), normalized to that of a Stefan-Bolzmann gas (subscript SB). The colored curves are lattice calculations, the black line is a conformal field theory - the maximally supersymmetric yang mills theory, which comes with two parameters: the coupling constant g, and the number N of the gauge group SU(N).

So it seems, instead of QCD at finite temperature, it's a reasonable try to model some properties of the mess with a conformal theory, like the mentioned super yang mills theory (SYM).

That by itself does not make things less messy, but now one uses the AdS/CFT correspondence! And here, the miracle happens: the 4-dimensional SYM gets translated into 5-dimensional gravity in some space called AdS₅ x S⁵ . Which is much easier to deal with.

One can then translate quantities from the 4-dimensional field theory into the 5-dimensional gravity in this specific space. E.g. the

The stress-energy tensor in SYM translates to a graviton on AdS₅ x S⁵

A state in the field theory translates to a state in the dual geometry

An operator in field theory translates to a field in the dual geometry

Etc. if you want to specify the type of particles you are looking at (fermions, gauge fields), things get more complicated, but are doable. Most importantly

The thermal equilibrium state in SYM translates to a black hole in AdS₅ x S⁵

And so, Marcus, I am very proud to say that at least my time at UCSB enabled me to answer the question what the QGP has to do with a black hole.

However, the general relativity one has now translated the field theory into, is classical only for large N. For small N, one would need to know quantum gravity - or string theory. It is usually assumed that N is 'large enough', which might or might not include N=3.

---

2. Applications

Using the above translation, one can now examine observables. A prominent example is e.g. the shear viscosity and it's lower bound. But also the photon and dilepton emission rate, or the energy loss of particles traveling through the messy soup can be investigated. Besides Pavel's talk, you might want to check some of his papers. It seems to me, he is working on an enormous amount of related stuff. Some more references about the topic

• Minkowski-space correlators in AdS/CFT correspondence: recipe and applications
  D.T.Son, A.O.Starinets


• The Hydrodynamics of M-Theory
  Christopher P. Herzog


• Viscosity in Strongly Interacting Quantum Field Theories from Black Hole Physics
  P.Kovtun, D.T.Son, A.O.Starinets

---

3. Problems

There are most likely a whole number of theoretical, analytical and numerical problems with the approach that I am not aware of, and therefore will not mention. Starting from the question whether N=3 is indeed a large number, or the assumption of thermal equilibrium, I have the vague impression there are more subtleties than I can (or want to) imagine.

Still, I think it's a reasonable try to use AdS/CFT for SYM to model properties of heavy ion collisions. Instead of, say, some hydrodynamical description, un-quantized numerical models, or cost- and time consuming lattice calculations.

The biggest problem I have with the scenario is the formation and time evolution of the fireball. Thermal equilibrium might correspond to a black hole in the dual theory, but what describes the collision, or the following hadronization?

---

The intention to use often very abstract mathematical theorems for direct applications is definitely something I welcome, esp. since the front research in theoretical physics is falling more and more apart into sub-fields.

It requires quite some courage to go to a heavy ion conference as a string-theorist, something Pavel did last month, which really surprised me. I am not sure though whether the average heavy ion physicist would already consider the approach to be more than a theoretical example for a practically very involved situation.

To come back to the banana issue: I am very relieved to tell you that I, and most of my classmates, eventually recovered from my maths teacher's lectures. We all figured out how to properly use a banana.

---

Acknowledgements: I would like to thank Pavel for his patience with my questions, and I apologize for the banana. I could not resist the temptation.

Anti-Gravitation

Today, I gave a telephone interview to a very nice young man for the print version of Welt der Wunder about black holes - apparently a topic of fatal attraction. To my amazement, it turned out, he was born the same day as I, in the city where my parents still live (and no, Lubos, I am not a schizophrenic hermaphrodite - at least not that I know).

Puzzled by the guy knowing my birthday, I googled my name - and I was surprised to find (besides my birthday in the google-cache) this article about my recent work on Anti-Gravitation

• Assumptions are repulsive, especially about gravity
  Tom Siegfried

which gives a very accurate brief summary of the idea:

"Electric charges can be positive or negative. Opposite electric charges attract each other; like charges repel. Gravity, on the other hand, seems to be always attractive -- all masses move toward each other. Apparently all matter has a positive gravitational charge, and like gravitational charges attract. If there was such a thing as negative mass, it would behave in the opposite way and push away all ordinary matter, a prospect that most physicists find repulsive -- except for Sabine Hossenfelder. "

So, for those of you who always want to know more, here is the story about anti-gravitation that is not in the paper:

Briefly after I finished high school and started studying at Frankfurt University, I moved to the Bockenheim, where the old campus was located. Unfortunately, I made a wrong move and ended up lying in bed the next two weeks. Whenever I turned my head, several nerves between my head and toes went up in sudden fireworks. Essentially, I stared at the ceiling for two weeks, wishing I had some anti-gravitational device to lift my furniture.

Besides shouting at my boyfriend and eating too much chocolate, I started wondering why there are no negative "charges" in general relativity. At this time, I was blissfully unaware of several reasons why this would not be possible, the most pressing one being the stability of the vacuum, others you find addressed in the Anti-Gravitation FAQs.

I am afraid, I annoyed my roommate with my anti-gravitation theories for quite a while. And I was stubbornly insisting I did not understand his concerns why this would not make sense.

(Anti-gravitation was not the only 'theory' I kept discussing with him. At some point we named them SH's Incredible Theories - SHIT. )

Then I got distracted from the anti-gravitation for the next years. Well - I had to write a diploma thesis, and make my PhD.

But the topic kept coming back whenever I had some free time. Just that the time was never enough to make it publishable. My last summer was very frustrating in many regards, I was absolutely certain my research would never go anywhere, and I would be unemployed by next year. While I was conference hopping in Europe, I wrote together what conclusions I had so far about the anti-gravitation. And I put it on the arxiv, praying it would be enough, hoping that someone might find it sufficiently interesting to finish what I wasn't able to finish.

The reactions on the paper were: none.

Though the first version of the paper was crowded with typos, it turned out to be a very useful basis to annoy other people by handing them a copy, and asking for objections. There were far less objections than I was prepared to discuss. And those that came were easily to answer. The most common reaction was: "uhm. interesting. uhm. why don't you talk to. uhm... [someone else]."

The most reasonable discussion I had for a long time was with one of my ex-boyfriends (credits go to Andi). Who is not even a physicist, but who understood far more of the idea, and the problems, than several over-educated brains before him.

Meanwhile, the guy who I shared the apartment with when I had my back problems, and who I had bugged several years with my anti-gravitational obsession, got a postdoc position at Perimeter Institute. During a phone conversation last October, anti-gravitation slipped in (again), and after listening very politely to my lengthy explanations, he said "uhm. interesting. uhm. why don't you talk to. uhm, Lee."

I thought, great idea, much better some guy at PI thinks I am nuts than the people I have to talk to every day. So I sent my paper to Lee. I am not sure whether he actually read it. But the conversations I had with him, and several others, since that time where encouraging enough for me to answer several depressing referee reports, and to send the paper to PLB, after it was rejected by PRD.

The revised version which was accepted for publication last month, is considerably improved, and I am enormously grateful for all the discussions I had with the two Stefans, Jim, Lee, Petr and (in the early days...) with Marcus. Unfortunately, the version in PLB is extremely shortened. I have been told repeatedly that the content is indeed much clearer now, after several revisions and shortenings, than in the first version. Though that might be true, imo the latest version suffers from a severe lack of applications. Needless to say, there is work to be done.

I am still not completely convinced whether there is some point I have been missing all the time, but I guess I will never find out unless I keep on discussing the idea. The ansatz has several problems that I know of -- probably I am the one most familiar with all of them -- and every second day I get stuck in a dead end. The other days though, everything makes perfect sense.

I am still waiting for objections.

---

Note added: see also the discussion in the PhysicsForums.

Complicated

You have a problem, I bet you do. Be grateful for it. Humans are funny, when they don't have problems, they make some.

On Friday, I declined the offer from the German Science Foundation for the Emmy-Noether-Fellowship. Something I have been delaying for some while. After all, didn't I want to go back ever since I came to the US? I got up in the middle of the night to make the phone call. Just to realize that it's a holiday in Germany. So, I sent them a lengthy email, which essentially tells them that I am sorry and they are stupid. Hope they get the message.

I miss my mum esp. on weekends, she always did my laundry. The apartment complex where I live has in toto 3 mashines for ~ 100 apartments (one of which currently is blue and green cause some kiddo apparently forget wax colors in a pocket). In addition, everybody does their laundry on Saturdays. So, I am out of sensible choices and down to the stuff I never wear. I ended up wearing all white, and since I was already at it, I finished the outfit with white shoes.

I made some attempts to go to the beach, but each time I left the apartment it started raining. Then I thought, I could as well go to work and get some stuff done. Apparently, I missed the exit on the highway and when the 'check engine' light turned on, I found myself halfways between San Francisco and Santa Barbara. I ended up sitting in a Cafe on the highway, vaguely pushing some details on the Higgs-mechanism around in my head. Took about 2 minutes until some old men sat at my table, stating

"You look like a deep thinker."

I tried unsuccessfully to stop frowning my forhead and said

"Maybe botox would help."

I only write this here, so the joke is not completely wasted. Without any further introduction, the old guy said his name was Pete, and then he told me almost every detail about his life I never wanted to hear. Joerg calls that agressive friendliness.

Turned out, Pete lived in New Orleans with his wife for 20 years. Then she got cancer. She always wanted to see San Francisco, but never had, so they moved to SF. She died 3 months later. He stayed. That was 30 years ago. I estimated him to be 80 or so.

Now he is about to move to LA because he has meet a women (very hairy eyebrow wiggeling). She is married, but apparently her husband is not doing too well. So, Pete is waiting for him to die any time soon. I wonder where 80 year old people meet. At the cemetary? No, they met in the bus, standing next to each other. "Standig is much better for the back than crouching - like you do!" and I got a lecture about sitting upright.

Then another old guy with a dog appeared. He was a friend of Pete and completely bald. The dog did not hesitate to immediately push his head between my legs and I spilled cold coffee on my white shirt. After five minutes or so, I yelled at the guys to stop excusing, and found it a good time to leave. The bald guy remarked "You are not a dog person". I said "Very observant" which he found very flattering and I got to see a lot of brilliant white fake teeth.

I mumbled some nice-to-meet-yous and Pete asked:

"Why are you wearing white?"

"Cause it's complicated.", I said.

Back on the highway, I remembered that the reason why I studied physics was that I did not understand it. I always thought it must be possible to make all that confusing stuff less complicated. I am still working on it.

Life's like this you
You fall and you crawl
And you break
And you take
What you get [...]

Why'd you have to go and make things so complicated?
~ AVRIL LAVIGNE

Wine and Cheese Physics

On the far end of my desk there is a pile of papers with a post-it saying: READ ME. Yesterday, it reached the critical height and dropped off the desk. So, I made a brave attempt to sort it out, the result of which are now 8 smaller piles of papers, mostly unread except for abstract and conclusions.

(There was a 9th pile with trash, which finally only consisted of one paper that I printed twice by accident.)

Surprising for myself, the biggest pile was made of papers circling around Bell's inequality, hidden variables, and the interpretation of quantum theory. I conclude that part of my mind is occupied with something very weird, not that I know exactly what.

I can hear Horst sighing to PLEASE stick to 'butter and bread physics'. I call myself very, very lucky to have had the opportunity instead to mostly do my 'cheese and wine physics'. Amazingly it seems that some of the cheese and wine has turned into butter and bread lately. Like the 'weird' black holes at LHC stuff, or the 'useless' minimal length model, etc.

(You don't yet believe in Anti-Gravitation, come back next year.)

Anyway, here are two recent papers that I read and that I found quite interesting

• Emergent Probabilities in Quantum Mechanics
  Olaf Dreyer
• Relational EPR
  Matteo Smerlak, Carlo Rovelli

I like the first one better, it adds some new point to the "how" of the measurement process, though it seems to me it has some missing links - the picture is kind of incomplete.

The latter is nicely summarized in Alejandro's blog, you can also find some discussion in Christine's blog and in the Physicsforum. I am not sure it really says something new, but it is a nice starting point if you want to know something about relational quantum mechanics. The main idea is that the question whether a wave-function is 'collapsed' or not depends on the observer. Everything is relative, even quantum mechanics. This avoids problems with the non-locality of the collapse which one has in the usual description: there is no paradox because both observers have to compare their measurement before they can talk about the result.

What I also found in the pile of papers:

• A recipe for Pina Colada.
• A letter of recommendation from someone I don't know about someone I don't know. But it's such a nice American-style letter that I will keep it. (Meaning: for a German it appears as if the writer is desperately trying to get rid of the guy).
• A note on the new CD from Die Sterne, that I wanted to order (but it's not available on amazon.com). Their last album 'Das Weltall ist zu weit' (the universe is too large) was pretty good (after the previous one was rather mediocre). I especially like the song "Wir sind wie Du", and I will close today with some of it's lyrics

Wir sind der Morgen
Wir sind das Erwachen
Wir sind die Möglichkeit,
die Welt zu erschaffen




Wir sind die Lebenden
Wir sind die Vielen
Wir haben nichts,
also nichts zu verlieren.




Wir sind die Quelle,
Der Anfang der Welle
Und hör zu:
"Wir sind wie Du!"

Phase transition or cross over?

So, since last Friday, finally, I am a happy, new "Dr. phil. nat.". Of course, I am not yet allowed to call myself officially like that - I have to wait until I have in my hands the "Zeugnis". This is only possible in two weeks, after the Promotionsbüro's Easter vacation is over. Everything has to be very orderly in Germany ;-)

In the meantime, I can take a breath, and try to realize what has happened. Getting a PhD is - that is my impression just now - not a sharp phase transition, but a very smooth cross over, starting with the handing in of the written thesis, and ending late after the defense is over.

I was quite a bit nervous before the defense, mainly because I had prepared the final version of my talk only the night before, and thus had not really much time to practice the text I wanted to say. The problem was, I thought that I have 15 to 20 minutes for my presentation, and I knew that it was to long for that period. But then, fortunately, the first thing that Carsten, the head of the examination committee, said was that the procedure begins with a talk of 25 to 30 minutes. I immediately got relaxed.

I think I got through my talk quite successfully in 25 minutes, interrupted by Horst only once or twice, and was ready to face the discussion. The start was a little bumpy: first, there was only colored chalk to write on the blackboard (and remember always to start with the most backward board), and than, I needed a little help to figure out the De Broglie wave length of the particles involved in my code: Roskos open the discussion with the question why the purely classical calculations I am using in my simulations can be justified. Of course, it is easy to write "lambda = hbar/p" on the blackboard, but then, it was not so easy for me any more, at that moment at the blackboard, to expand this fraction by c, use the famous "hbar · c = 197 MeV · fm", and conclude that a nucleus with an energy of 160 GeV (and a corresponding momentum of 160 GeV/c) has a wave length of roughly 1/1000 fm. But with a little help, I reached that conclusion.

It got a bit better when Roskos started asking about Debye screening, a concept he was fond of recognizing from his solid state physics, but then he asked whether I had studied plasma oscillations in my model, which I had to admit I had not, although, of course, I said, this would be an very interesting thing to do, one would expect plasmons to exist in the model, that the search is numerically involved, and that plasma oscillations and plasma instabilities are a hot topic to understand thermalization in the early phase of heavy ion collisions, blah blah. Fortunately, nobody asked about the typical frequencies of plasma oscillations and how to calculate them...

Then, Horst asked me to explain the differences between phase transitions and cross over, and how all this relates to the Kosterlitz-Thouless transition. That later transition was the topic of my diploma thesis, and I was surprised that he really asked this question, since the relation is quite far-fetched, to say the least. Anyway, I was quite successful in plotting the phase diagrams of QCD matter and simple water on the blackboard - although, if you plot the diagram of nuclear matter as a function of density in temperature, keep in mind that there are two lines, tracing the coexistence region of the first order transition, which should meet at the critical point. My first guess of that plot on the blackboard, unfortunately, was a little different.. However, I was quite convincing, I think, in explaining the critical point, critical opalescence, and the absence of sharp differences in density or whatever in a cross over beyond the critical point. I also guess that the audience may have been impressed by my explanations of vortices acting like Coulomb charges and undergoing an ionization transition in the Kosterlitz-Thouless transition, and the analogy to the instanton liquid that drives the chiral transition.

Horst went on asking about some new mechanism to create super-heavy elements, not using the famous cold valley, which I could not answer, since I had skipped the relevant talk of Zagrebaev at last weeks ISHIP conference. Instead, I could make him happy with a schematic plot of collisions of heavy ions at energies in the 100-MeV range and the time scales involved, and the relevance to the creation of conditions suitable for the diving into the lower continuum of the Dirac sea. I guess that everyone who has spent some time in the Frankfurt institute has heard about the funny things that happen when "Z·alpha > 1" and the vacuum spits out positrons ;-)...

The next round of questions went to Klaus Peters, of the IKF, in Frankfurt since last year. He asked me about the different quarks, and the experiments that show the existence of the different families. It is a little bit funny, but one of the relevant plots, the number of hadrons versus dimuons produced in electron-positron annihilation as a function of energy, I knew that from my job as editor of Greiner's textbooks at Harri Deutsch. So, this was really useful for something :-). To his next question, about the prediction of the charm quark, I could, unfortunately, only propose the buzzword GIM mechanism and Glashow, Iliopoulos, Maiani, and that it is related to the weak interactions, but in fact I spent one hour of the afternoon last Sunday to figure out that its about the "suppression of flavor changing neutral currents", which are not observed, but are a consequence of naive Cabbibo mixing - this can even be found in the Wikipedia! His questions about pentaquarks (the reason why I had asked him to take part in my examination committee), again, I could answer. The funny thing is, like in all examinations, that sometimes it is just important to guess what the examiner wants to hear - in this case, "higher Fock states" which are mixed to the valence quark wave function, which can mix with pentaquark states. It took a while until I had figured out that he wanted to hear just this notion.

Finally, in the last round of "official" questions, Carsten came back to the details of my model, asked about comparisons to his Friedberg-Lee model, and about elliptic flow. There is, of course, a big fuzz about elliptic flow at RHIC, which seems to exhibit beautiful quark number scaling for a large number of hadrons. I could tell him something about all this - it was a hot topic at Strange Quark Matter the weak before. I would be happy if I could study it with my model. Unfortunately, it is hard to get enough statistics, and the initial pressure is not high enough, probably because there is no hard core repulsion between quarks in my model. I tried to impress everyone by mentioning the Alder-Wainwright simulations of the early 1960's that showed that hard core repulsion is enough to obtain a reasonable simulation of fluids, and that's why, i tried to argue, my model is not a strongly coupled QGP.

At that point, Carsten decided that the questioning had been long enough. I could have go on for a while, and Walter Greiner gave me the occasion, since in the round "questions from the public", he asked if I had heard Kampert's ISHIP talk about cosmic rays. I had not, but I said it was probably about the first runs of the AUGER experiment in Argentina, and made a plot of the spectrum of cosmic rays with the knee and the ankle and the questionable events at highest energies, where photo-pion production with photons of the microwave background should decelerate protons and prevent them from reaching the earth, and that this may be relevant to possible modifications of special relativity blah blah...

But then, eventually, it was really over. Usually, at this point, the audience has to leave the seminar room, so that the committee can decide about the outcome of the examination. This time however, since the seminar room was completely full with people, the committee left, and found its decision somewhere else. When they came back some 10 minutes later to announce that the result was "sehr gut", and to congratulate me, this was, strictly speaking, the moment of the phase transition to the PhD.

For me, however, it is washed out to a smooth cross over - I have not yet completely reached the new state.

Are You Loosing Your Edge?

Two weeks late I finally wrote the referee report I mentioned previously. For a change, it was a manuscript written in understandable English and the report was a very positive (though the paper was extremely boring). While doing so, I found the letter I wrote to the TIME article "Are we loosing our edge?", which I mentioned in a previous post. In this article, the necessity of Bush's American Competitiveness Initiative was emphasized. Though I believe that there is lots of stuff going wrong in science (not only in the US), the whole article is pretty short sighted.

Anyway, here is the letter:

One of the most attractive factor for me to come to the US for research was that my work would be appreciated. People in the US have a genuine interest and pride in their national research programs. The flip side of this is a lacking ability for self-criticism which severely worsens when funding drops. Who points out own shortcomings when fearing to loose financial support? Instead, the own achievements are overemphasized, which then misleads future decisions on funding.

I had to experience that the tolerance and open-mindedness foreign scientists are welcomed with is contrasted by an sometimes unbelievable arrogance and ignorance for the achievements of other countries. It surprises me every day anew that most US citizens really believe their "standard of living is far higher than that of any other nation". What the US needs is certainly not more competitiveness! What it needs is to acknowledge that there is intelligent life outside the US. What it needs is worldwide cooperation, in a world where progress is made most efficiently by exchanging - and not buying - knowledge and people. You conveniently forget to mention that the www, as we use it today, was invented by Tim Berners-Lee, a British scientist, and 'went public' at CERN, an international research organization in high energy physics, located in Geneva, Switzerland.

For those who did not read the article, the latter remark referred to a listing of innovations by US scientists, among other the internet

Internet:

After the Soviet Union beat the U.S. into space with the launch of Sputnik I, the first satellite, in 1957, the Department of Defense created the Advanced Research Projects Agency to kick-start innovation. It named Joseph Licklider to find ways to protec the U.S. against a space-based nuclear attack, and he believed a communucations network was key to those efforts. The first Net went live in Oct. 1969 with the University of California, Los Angeles, talking to the Stanford Research Institute. In 1990 the National Science Foundation expanded the system connecting university networks. It reached the public in 1992.

Had that been the whole story, you would not be reading this blog right now.

Congratulations

HE DID IT!!! HE DID IT!!! Very very special congrats to

DR. STEFAN P. SCHERER

...and thanks to my mother for sending the photos...

from left to right: Stefan, Carsten, Adrian, Brachmann, Alex

and -- judging from the blackness of clothes -- I guess the ear on the far left is Andi's

Stefan and Horst (this is *not* the seminar room)

Young and Stupid

This afternoon I got stopped on the 101 by a police car some 30 miles south of Santa Barbara. Stefan is convinced that I can smell radar controls, so I was driving exactly at the limit, but the guys were suspicious about my car. Pulling to the right, I forgot that I own a CA drivers licence and my accent got significantly worse. I waved around with my pinkish German licence, claiming that I just moved here recently and had no idea that a front plate is required, and I am so sorry, officer.

It might have helped that I was wearing my favourite dress. Still, I had to open the trunk and let the guy poke around in my trash. Stefan left me with the bottle of wine he won for his poster at the SQM. He had neatly packed it into a plastic box with some tacos and even a bottle opener - no disasters with my thoughtful future husband. At least I had managed to put the wine in the trunk, where it still is. Anyway, the officer was very puzzled about the bottle opener and made some remark that sounded like 'always be prepared, huh'. I gave him evil look No. 17, which he chose to misinterpret as a nasty girls look and his colleague began to giggle. I promised to get a front plate (as I have promised several times before) and check my signal lights (seems some aren't working) and ignored the speed limit the next 30 miles.

Back at the department, I was lingering around in the kitchenette, waiting for the coffee machine. Someone-whose-name-I-probably-should-remember came by, and asked me whether I was a new grad. stud. Having explained that I am neither new nor a grad stud, but actually a postdoc here since last year, he apologized, but left me wondering how old I have to get, before I am mistaken as the secretary instead.


This was kind of a lengthy introduction why I started reading popular science books. Again - I did not touch any since 10th grade or so, being fed up with always the same incomplete and impossible to understand explanations. Last year however, I was at some airport being faced with the omnipresent Nora Roberts and Dean Koontz products. Besides this, there was the usual selection of improve-your-life books and travel guides.

Therefore, I settled on Brian Greene's Elegant Universe, something that I would not have bought in a sensible book store.


Reading the first chapters -- most of which I skipped, because they explain very basis stuff -- a women sat next to me and started some smalltalk. She had also read the book, found it incomprehensible and much too complicated. Since she saw me skipping the first chapters, she assumed I had the same problem. This made me realize that I actually must have learned SOMETHING during the last TWELFE years of education. I did not tell her I found the book kind of trivial.

I mean, usually I read papers on the arxiv and it happens only rarely that I really understand one. In some sense that is depressing and sometimes I have the impression that I have not learned anything since I left high school.

So, since that time, I have read several recent popular science books, about which I might post sometime. I read them not so much because of the scientist-for-non-scientist-part but to remind me how important it is to step back every now and then, and focus on the basis that we share.

In my opinion, it is a huge problem that the front of research has split so much, that only small groups of people are really able to follow each others work. Is it good that most of the new papers are so specialized, that their content is no longer extractable without digging into a whole collection of papers, circling around some sub-sub-hybrid model and it's possible relationship with the zeros of the zeta function or whatever? It is good that it takes several decades, if not a whole career, to get an overview about the status on any field of interest? And why do I often find that papers seem to be written on purpose such that they are as incomprehensible as possible?

I keep thinking that I am just young and stupid. But I can't avoid having the impression that in a time of such rapid expansion in scientific knowledge, it is more important than ever to keep things together and enable people to get an overview without drowning them in details. The living reviews in physics are an excellent step into this direction.

Have a nice weekend,

B.

Mini Black Holes

The last days I was at the conference Strangeness in Quark Matter, which was strange indeed. It was nice to meet all the people, and surprising how many I knew. Walter Greiner's 70iest birthday was an occasion to tell some funny stories. As so often, I got the impression that the Heavy Ion community is actually a large family, where people stick together even though they can not stand each other, and bad habits get passed on to the next generation.

Anyway, before I begin to wonder why I kicked myself out of the field, let me come to the reason of this post. My PhD adviser Horst gave a talk about Black Hole production at the collider. I had an earlier post 'Risky Black Holes' where I told you about the governmental guy who was concerned about the danger of Black Hole production at the LHC. He wrote back several times, asking for exact probabilities for every possible scenario, and circled around the question what probability to destroy the whole planet when LHC is switched on is acceptable? 0.001% ? 0.000000000000000000000000001 % ?

Also, last week I received an email from a guy writing for a German newspaper who apparently is about to plan an article about extra dimensions and black holes for the Sunday edition.

Some of you might know that the reasons I worked on black holes are quite obscure, and that I repeat at least twice a year that I don't want to work on it any more. What's so complicated about quitting is that the interest in the topic is so large! Black holes always make a good topic at BBQs, and everyone thinks he has something to say about extra dimensions. Even though I appreciate the discussion, I can't avoid having the impression that the scientific content is in most cases entertaining but doubtful. E.g. I learned that cosmic rays are black holes that are created on our brane, then go into the extra-dimensions, grow there, then come back to our brane and make an ultra high energetic cosmic ray. Sounds exciting, huh? But is that scientific?

So here is a quite general statements:

If the Hierarchy problem (the gap between the Elektroweak and the Planck scale) is only an apparent problem and the Planck scale is much lower, then black hole production is one of the most general predictions that we can make about what is going to happen. The assumptions are plain and simple, too much energy in too little space leads to a collapse.

However, though we think we know how the Hawking radiation looks like, we have no idea what the final decay looks like. Therefore, most details of the black hole's signature are strongly model dependent and should be treated carefully.

In addition to this, the required models with extra dimensions are not really understood, esp. regarding the stabilization, the evolution in the early universe (or time dependence in general), and at least I don't feel comfortable with the mechanisms to confine particles to the brane. Also, extra dimensions are surely not a theory of everything, so there should be more about it.

This is not to say that one should not investigate speculative ideas, just that one should try to stay as close as possible to reliable assumptions. I don't think there is any point in examining weird and even weirder scenarios on shaky ground. That's nice for a BBQ party but not really scientific.

Hi!

Hello to all of you and thanks for the invite!

To start with (and in lack of any other ideas what to post) I want to introduce you to

The Physicists' Bill of Rights

We hold these postulates to be intuitively obvious,
that all physicists are born equal, to a first approximation, and are endowed by their creator with certain discrete privileges, among them a mean rest life, n degrees of freedom, and the following rights which are invariant under all linear transformations:

1. To approximate all problems to ideal cases.
2. To use order of magnitude calculations whenever deemed necessary (i. e. whenever one can get away with it).
3. To use the rigorous method of "squinting" for solving problems more complex than the addition of positive real integers.
4. To dismiss all functions which diverge as "nasty" and "unphysical".
5. To invoke the uncertainty principle when confronted by confused mathematicians, chemists, engineers, psychologists, dramatists und anderen Schweinehunden.
6. When pressed by non-physicists for an explanation of (4) to mumble in a sneering tone of voice something about physically naive mathematicians.
7. To equate two sides of an equation which are dimensionally inconsistent, with a suitable comment to the effect of, "Well, we are interested in the order of magnitude anyway."
8. To the extensive use of "bastard notations" where conventional mathematics will not work.
9. To invent fictitious forces to delude the general public.
10. To justify shaky reasoning on the basis that it gives the right answer.
11. To cleverly choose convenient initial conditions, using the principle of general triviality.
12. To use plausible arguments in place of proofs, and thenceforth refer to these arguments as proofs.
13. To take on faith any principle which seems right but cannot be proved.

I wish you all a Happy Easter!