Wednesday, May 31, 2006

Excuse me

The organizers of the conference here in Paris announced there would be no wireless on location. They gave in after three days...

Yesterday, Keith Dienes gave a very interesting talk about his work with Mike Lennek that I will try write more about soon. Also, March-Russell's talk about the 'friendly string landscapes' (astro-ph/0604254) was very, uhm, entertaining.

But since I am very braindead today, now to something completely different.

Here in Paris, the streets are crowded with people from different nations. The French actually seem to be the minority. But it's pretty easy to detect an USAmerican: if you come close to one, you will inevitably trigger an 'Excuse me'.

I wrote in a previous post how weird it is to come back to Germany, and to notice all the small differences to the USA. Amusingly, I just found that the Spiegel intends to write a survival guide for visitors of the soccer world cup in Germany:

Help us write the German Survival Bible

starting from the questions "Why are the shops closed on Sunday?" (Cause things don't change in Europe) to "Are Germans rude?".

This used to irritate me. I always found Germans to be overly polite and discrete. But moving to the US, I had to realize that it's actually true that Germans are considered to be rude, just because of this! I also had to realize that the cultural and sociological differences between Europe and the US are far larger than I thought.

From the article Rules of the Street:

Not that Germans are intrinsically rude. No, mostly they've just learned to come to terms with more day-to-day physical contact that many of us. Walking down the street can often feel like a rugby scrum. In a crowd, many Germans will plow grimly ahead like Arctic ice-breaking ships. Boarding a subway, some Germans like to pretend no one else is there. The guy who tromps on your foot will look surprised -- as if you should be somewhere else.

Even better: if you tromp on an American foot, the foot owner will excuse on a 99% confidence limit :-)

So, excuse me, but I also have to tell you something about the American rudeness, the constant urge to ask 'How are you?'. For most Germans this is considered a very personal question and surely nothing you want to be asked by some Jim or Marc you've never seen before, working at a 7/11, who's just supposed to sell you cigarettes.

To give you an applied example: two months ago I came back to Santa Barbara from a 10 hour trip, my stupid bag had stayed in Denver. Meanwhile they had changed Hammond's lyrics to 'It always rains in Southern California' and I found that the roof in my apartment was leaking. Worse, at 5 to midnight I had to run through the rain -- needless to say I had no umbrella and the car didn't start -- to get to the next groceries store. Standing at the register with painkillers, a pack of tampons, and a Hershey's bar, rain dripping from my hair, the cashier asked me how I am!

I seriously thought about hitting him with a better homes magazine, but that would have been rude. Instead I said: Terrific.

He was lucky not to ask whether I found 'everything alright?'.

Monday, May 29, 2006

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.

Monday, May 22, 2006


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

    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


Thursday, May 18, 2006

Monday, May 15, 2006

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?)

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,


Friday, May 12, 2006

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.

Wednesday, May 10, 2006

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:

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:

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

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

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

Monday, May 08, 2006

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

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?

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

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

Tuesday, May 02, 2006

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.