Cabi's Glasses

Sub-millimeter gravity

2005-10-13T23:55:00.000-04:00

Today's colloquium was given by Eric Adelberger from University of Washington on their experiments for small scale gravity. They are using an extremely sensitive torsional pendulum apparatus coupled gravitationally by two rotating discs. All of them have holes so that 1/r^2 effect of two discs are cancelled and there is only non standard effects left at the first order. There are many checks of course. For example they discovered micron level non-smoothness of the inner surfaces of the holes by gravitation before the microscope!!! I think this is enough reason to trust them. Their conclusion so far is that there is nothing new down to 0.1 mm. But things get interesting at that scale because that is the length scale corresponding to measured dark energy. And as rumored by Lubos some time ago there seems to be something interesting around there. Data they have so far is not clean enough but seems to eye that the observed force is a little bit less than the theory. Since it is at the limits of their instrument, you should not get excited right now. He thinks that they can have enough tests in a few months to figure out whether it is some systematics or real signal. It seems that they are well away from the theoretical limits of the instrument design and currently limited by how fine they can manufacture their parts. We can expect much more accuracy in the following few years. Fingers crossed and waiting.

National Geographic Wildcam

2005-10-13T23:21:00.000-04:00

There is a live webcam in National Geographic's website. You can watch a small pond in the middle of nowhere in Botswana, Africa. Many wild animals come there to drink water, wash themselves, have some rest or cool down. I sometimes leave it on my second monitor. It not only is a nonintrusive calming scene and an interesting watch but also reminds you how "unnatural" is our "modern" life. Everybody I showed it told me that being an animal there must be so peaceful. Well, at least when the lions around are full. Give it a try, it might be a better break than reading this blog.

OpenLaws

2005-10-03T20:40:00.000-04:00

Today I am starting a cyber-social experiment. In history, as the societies get bigger we had to move to representative democracy. Now with the modern ways of communication and tools of the cyber world we have new collaborative creation devices. I think the most unbelievable one is the wikis. I want to try if we can use to write laws for the society with a wiki open to everybody. I call it OpenLaws. Do I believe that it is going to work? Absolutely not! But I would say the same thing for Wikipedia. So let's try. Go ahead and write laws of your dream world. I won't say much, hope it will build itself beyond my imagination. Comments are welcome.

DEVONthink for Scientists: Part II

2005-09-30T23:04:00.000-04:00

In a recent post I gave an introduction to DT. Now let me give some examples of how to use it.

Here is one of the last things I did on my personal database. I put a new quotation from Sean Carroll. It was a small paragraph including the sentence "Things are not intrinsically interesting, they are found to be interesting by people". DT immediately suggested a small clipping from David Deutsch including the sentence "In the long run, the distinction between what is interesting and what is boring is not a matter of subjective taste but an objective fact". This is nothing but immensely thought provoking and I would never remember it. On the scientific side, it helps to locate standard references. For example, when I start writing something about renormalization, it reminds me to look at Peskin Chapter 12 which has a summary inside the database. I hand picked my notes and archive and organized them into folders very well. Hence it may not be very surprising.

Let's look at some bulk databases I built. Joanna Karczmarek of Rutgers kindly supplied me the abstracts of all the hep-th papers until July 2005, that makes 37586 items, 110,720 unique and 4,749,379 total words. This is not only a huge database but also very disorganized. Abstracts are collected into folders according to submission months. This is probably the best thing you can do to confuse an contextual search engine. But still DT did quite a nice job. Here is an example:

I chose the paper hep-th/9907001 of Parikh and Wilczek called "Hawking Radiation as Tunneling". First thing one can notice is that suggestions are dominated by the papers of the authors. This makes sense because authors' surnames are the most characteristic words for each document. But even this is not done blindly. DT offers bunch of Parikh papers, who wrote about the same subject extensively but no other Wilczek papers. Figuring out the authors' related other papers are not very interesting or useful, because I can do it myself. So here are the top five papers from other authors: hep-th/0505266 (Hawking Radiation as Tunneling through the Quantum Horizon), hep-th/0110289 (Radiation via Tunneling in the Charged BTZ Black Hole), hep-th/0504188 (Tunneling through the quantum horizon), hep-th/0503081 (Hawking Radiation as Tunneling for Extremal and Rotating Black Holes), hep-th/0207247 (Radiation via Tunneling from a de Sitter Cosmological Horizon). It is nice to see that some of the above papers are actually citing Parikh and Wilczek. Also the last one is interesting, in a sense that it is related but I wouldn't expect a computer to recognize that.

Another example is the database I built from a snapshot of the open mathematics encyclopedia PlanetMath with 4570 entries, 31,007 unique and 3,053,620 total words. This is a nice one because titles are organized into subject folders. When you look at entry for the Einstein field equations, here are the top five related entries: Ricci tensor, Einstein summation convention, Schrodinger's wave equation, Hartman-Grobman theorem, linear time invariant systems. Relation of the forth one is not apparent to me, may be this is something I should learn. Here is a different one: If I take Hamilton equations, it gives symplectic manifold, Hamiltonian vector field, cotangent bundle, Examples of symplectic manifolds, Poincare 1-form. (Yeah, I am too lazy to put a link for each word.) I leave it to you to decide whether these are useful enough. May be not, but the main purpose of the program is to help your memory, and it does this very well.

Some people argue that the best thing to do is to throw everything inside a single database. Besides the performance issues, if you put all of the above inside your personal database, they will naturally dominate the suggestion results. So my advice is to go for the Pro version and experiment with many databases till you are satisfied with the combinations.

Here is my bottom line; DEVONthink may not be perfect (when compared to a human who does not forget) but there is nothing better or even something to compare out there.

I know that there some services which try to do the same with the whole internet. I tried blinkx and it gives awful results, nothing useful and I haven't tried Watson yet, it is Windows-only. Although, I think this is a beautiful idea and we will use and enjoy such tools soon. That will be the next Google.

The Efficient Academic

2005-09-30T21:35:00.000-04:00

There is a new online discussion group for productivity in scholarly work. It is called The Efficient Academic. I don't know why, but my impression is that people in the academia is much more disorganized than the "business" people. We try to cover it as a source for creativity, but is it really a legitimate excuse? Is there anything we should learn from the business self-help books (like the excellent GTD)? It is nice to hear others' experiences, solutions and tools.

Twenty-five Questions for String Theorists

2005-09-25T00:59:00.000-04:00

Gordon Kane and his collaborators have just published a review paper on the phenomenological aspects of string theory: hep-th/0509157. If you are an outsider to string theory (like me), you may wonder what are these people are really working on. String theory is advertised as a very beautiful solution to the unification of physical laws (or the question to the answer 42). But if they established such a theory what are those thousands of strings theorists trying to figure out? One answer is to find the M-theory, the non-perturbative completion of the string theory. The other answer is to connect proposed Planck scale physics to our experimental capabilities. This new paper is a compilation of 25 open questions in this second category and also it is a good resource for outsiders to understand the cutting-edge issues and where the problems may lie. I just wonder what percent of string theory is along these lines.

Extracting numbers from AdS/QCD

2005-09-24T02:29:00.000-04:00

This Wednesday's seminar at CTP was given by Emanuel Katz of Boston University (see him here). He talked about his recent work on AdS/QCD correspondence, published in hep-ph/0501128. The main program here is to find a 5D string theory such that QCD is its induced theory on the 4D boundary of this bulk. So far nobody could came up with such a theory. QCD has a very rich structure with asymptotic freedom, chiral symmetry breaking and many particle fields. Unless you came up with the right matter content and exact SU(3) symmetry there is no hope that you can have reasonable calculations for low energy. (Low energy is interesting because high energy QCD is easy to due to asymptotic freedom.)

Katz and his collaborators took a simpler and different approach. Instead of finding a full fundamental theory, they tried to find a few relevant terms in the bulk, fix their coefficients by QCD data and compute other data and see if there is anything matches (which is not very likely a priori). They took an AdS space with a boundary where we live. Fifth dimension in the bulk, corresponds to the energy scale of the interactions on the boundary. As you move away from the boundary energy scale gets smaller (length scale gets larger). So if you want to study confinement you need to study the region near the boundary. In their model this is done by putting another brane parallel to the boundary as an infrared cut-off and study the region in between. Physics is imposed by the boundary conditions on the 'infrared brane'. There are four free parameters in the Lagrangian which are set by number of colors and three experimental values like the rho meson mass, the pion mass, and the pion decay constant. Surprising thing is after building this simple but very unrealistic model they can calculate many other experimental parameters very accurately (within %10 in most of the cases). He showed additional results with strange quark in the talk and even they are still very good (within %30). Actually this is better than the first versions of computationally intensive lattice-QCD.

I always thought that even if they find a dual theory for QCD, it would be much more complicated and will not be practical for anything beyond intellectual joy. But this work made me step back and take a second look. May be there is something here.

Even more important question is whether it has a fundamental meaning or not. Spring-mass system and an LC circuit have dual theories (actually same theories with different coordinate names). But it does not mean that one is made out of the other. Are we really living on a brane and is standard model just a projection of a (5+n)D theory? Probably I will never learn the answer.

Update: I just saw that Jacques Distler has a slightly more technical review of this paper.

Notes on making science

2005-09-23T18:32:00.000-04:00

Read this and replace every "art" with "science". It surprisingly makes sense.

Update: Another short piece I would recommend along these lines is some "Frank" advice on making right choices in life. Does anybody have a proof that the given algorithm is the best?

DEVONthink for Scientists: Part I

2005-09-20T03:34:00.000-04:00

Managing information is a very important part of a modern scientists life. Lab notebooks, ideas scribbled on papers, proceedings, papers to be written, papers written 10 years ago, papers to be read, papers to be refereed and of course physics blogs. Many people have many different ways to cope with it. Some are very tidy with classified hanging folders and highly maintained list of bibliographies and some are very messy. I personally hate bunch of papers flying around filling desk and shelf space. I can never find things there. What can I do, I was using Yahoo before reading my first paper! So for the last few years I was in a search for effective collecting techniques for information. My dear idea notebook served me quite a while but it was just better than nothing. I couldn't easily cross reference and it didn't integrate into paper folders. But finally I feel like the issue is settled for quite a long while. Today I will talk about my final decision: DEVONthink.

DT is a (Mac-only) database management software for every kind of electronic document. It lately became well-known (read fashionable) thanks to Steven Johnson's NYTimes article and following blogs (here and here). Probably journalists and writers are the primary market for this software but I believe it is an invaluable tool also for scientists. So I'll try to give review (or a praise) of it for you.

Structure of the database for your documents in DT, looks like the good old file structure of your hard drive with folders and subfolders. You put in any kind of document (like txt, rtf, pdf, html, tex as well as image and quicktime files and yes doc!) inside and classify in a dedicated place where you can search inside the documents, edit and give links (with wiki capabilities) to each other. Up to this point you have many alternatives that you can do the same. But DT shines now on with its unmatched, intelligent text indexing abilities.

DT looks (at least) two things in your database. Which words go with which words inside the documents you chose and which files go which files inside the folders you classified. Using these data it decides which are the words that is characteristic to a file or a folder. It uses this information for the two magic buttons. First one is "Classify". When you put a new document "Classify" suggests you where to put it in your database. I "usually" know where I want to put it but just for fun I always click and see with joy that my folder is among the top suggestions and it really helps when I am undecided. Second one is "See Also". This button is like your personal librarian. It suggests documents related to the current one. After a while it becomes a common thing that DT reminds you little notes written down from a book or an abstract from an ArXiv read months ago and already forgotten. I hadn't recognized how much I forget, until using this and I am 24.

Both functions start to give reasonable results after tens of documents placed in a few folders and get better and better as your structure get bigger and better organized into a few levels of subfolders. I am very satisfied with the speed. It works seamlessly on my 1.5GHz, 512MB Powerbook with my primary database which has about 1.5K documents with 75K unique and 1.7M total words. (There are so much unique words because I have Turkish documents as well in the database. I am planning to move them into a separate database later. And I should say that it works also perfect in Turkish. It (statistically significant) sometimes guesses the (folder) name of columnists from just the text!)

You can also ask for related documents for just the selected part of the text which is very useful. But unfortunately it can not do the reverse yet. I mean, it can not suggest a certain paragraph of a document. So putting into too long pieces (like books) probably won't be very useful (may be just for search). Steven johnson suggest that the ideal length is 50-500 words, but I think physics papers are OK in length. Getting a relevant paper is enough most of the time, you can figure out where to read (which is the conclusion).

What I put in? Notes for my ideas, notes from books and papers I read, daily musings, abstracts (sometimes whole papers), blogs, news, columns, chapter summaries for Sakurai and Peskin, This Week's Findings in Mathematical Physics archives, theorems from Wikipedia etc etc etc.

One more unique feature is the fuzzy search. When you turn it on it not only searches for the words you supplied but also "related" words and words that look like yours with some suffixes and misspellings.

What else I like about it?

* There is a full screen edit mode. It takes you away from all the distractions of internet. Just you and your thoughts. I write everything here now. Plain texts can be seen green on black background which is very easy on the eye.

*If you like adding equations (preferably LaTeX) to your notes (like any physicist). You can use universal RTF format and Equation Service to put inline LaTeX created mini PDF formulas. You can go back and forth LaTeX and PDF as many times as you like. Mac services menu makes this process just a single key combination. (I use cmd+.)

* Integrates perfectly with your Safari. Again thanks to services menu any selection goes to your database with single key combination (I use cmd+[ for txt and cmd+] for rtf). There is also a browser in DT good for html capture and site sucker for whole site downloads.

* You can read your RSS and Atom feeds and save items as separate files with a script. There is a great Apple script support if you know, if not there is big library of them.

* You can have replicates (synchronized mirrors) of files in different folders.

* Exporting files as you put them in. This is very important if you want to move somewhere else one day. For example you can't do it with OneNote.

* They have a fast tech-support and lively forums with lots of writers, journalists, students and academicians.

* And many other things I haven't yet used so far; like index, summarize, concordance, searchable file comments etc.

DT comes in two flavors Personal (single database, no site sucker, no RSS etc) for $40 and Pro (everything above) for $75. There is %15 discount for students (yes grads also). You can try it for 150 using hours for free.

I hope I have convinced that you have never seen a software like this before, highly recommended. I wished there were some competition.

In the second part of my review (praise) I will give additional examples for using it as a scientist. Please share your experience in comments if you are a DT user.

UPDATE: Here is the second part.

Three dimensions from strings, or may be seven...

2005-09-16T00:02:00.000-04:00

First we thought that strings live in 26 dimensions. Then it turned out that it is 10. Now string theory says that we should either be in a 3 or a 7 dimensional brane. That's what Karch and Randall's analysis claim in hep-th/0506053. You can also read the recent AIP news article here.

The argument is that if the universe (of course the 10D one) is initially filled with branes of all dimensions and sizes, eventually only 3 and 7 dimensional ones will survive and dominate the universe. So the dimension of our world is not chosen by landscape or some fundamental principle, but by the cosmological dynamics. I can't go into the details here, but it is interesting that branes with dimensions have different reasons to dilute away.

I am certainly not an expert, but the arguments here seems to be very basic. If they survive it might be a major milestone in string theory. All we need is an anthropic principle to choose 3D ones ;) Actually these things are already in the market. Structures will not be stable in universes with 7 space and 1 time dimensions. But Karch and Randall suggest that if the expansion of the universe is dominated by something other than branes, there is quite a good chance that 3D ones dominate 7D ones anyway.

It looks interesting, I'll keep an eye on these.

Glasses are back

2005-09-15T22:28:00.000-04:00

Nice to write you again! After months of silence Glasses are back. I see from site statistics that, there are people falling here from various links even these days. I hope I still have a few regular readers left.

There was really no reason not to write blogs. Actually there were lots of reasons to write. Anyway it just happened, a gigantic blogger's block! What happened in the last couple of months? Well, I had a wonderful vacation in Turkey, asked my girlfriend an important question, spend two weeks in IAS Princeton, read and thought some physics, switched from Windows to Mac (highly recommended) and started TA'ing undergrad relativity class here at MIT.

Many bloggable things came and went along the way, but let's look at the future. Let's see what the life brings on.

To make your reading a little worthwhile, I would like to point you here. Wolfram's cellphone tunes, uniquely composed for you by a cellular automata. Quite geek but not very appealing for me. My loyal readers would remember that I gave some hints about this service months ago.

SU(3)xSU(2)xU(1) is special!

2005-04-04T19:59:00.000-04:00

I was always puzzled by the fact that nature chooses this particular combination of symmetry groups. Today Victor Kac showed an incredible way of realizing this symmetry in his talk at CTP. Let me try to summarize my humble understanding.
He managed to extend Cartan's classification of simple Lie algebras to superalgebras some years ago. Here instead of 4 (parametrized) series of algebras one gets 10 series, plus only 5 exceptional ones. You can understand these exceptional ones in terms of their maximal subgroups. Two of them have SU(3)xSU(2)xU(1) and one has SU(5). And if you believe in SU(5) unification you are left with a unique choice since only one of the preceding ones can be embedded in the latter. We can name them now: E(3|6) and E(5|10).
He and Rudakov studied the representations of E(3|6) and the result resembles nature surprisingly much. There are three families of fermions, five families of quarks and the same gauge sectors but no superpartners. Though there are important problems like anomaly cancellation issues due to the mismatch of lepton and quark families, weird neutrino sector, charged gluons and so forth.
Despite its immature physics right now, I found it extremely compelling, because this is the first place standard model appears as a special structure in mathematics. Also there are naturally occurring families. Most important question now is how to write Lagrangians with these symmetries.
Results are new even for mathematics community and I don't think that they find their way into physics literature. Most accessible introductions by Kac are math.QA/9912235 and math-ph/0302016. Also there is a rather diverse discussion in sci.physics.research. And finally since it is my specialty, I can tell you a place where you can watch a very similar talk by Kac along with some lecture notes.
Does anybody know any reasonable introduction for physicists? Feel free to leave comments and references.

Is QFT vacuum real?

2005-04-04T19:00:00.000-04:00

A latest notable paper from CTP is from our director-on-leave, Bob Jaffe. You can read it at hep-th/0503158. Almost everybody cite the Casimir effect as the proof of reality of quantum ground state of standard model fields. Lately it has become a commonsense especially in the dark energy literature, as an aspect deepening the problem.

Jaffe shows in his paper that you can formulate the Casimir effect without giving any reference to the vacuum energy. Hence it gives no further insight than any other one-loop effect in QED like the Lamb shift.

It makes sense to me and I won't refer to Casimir effect in this context anymore. But as he says in the end, this does not help much, because symmetry breaking condensates, like Higgs, are still there.

My Nobel Nomination

2005-03-22T02:18:00.000-05:00

As Frank got his well deserved and long awaited Nobel this year I can announce my second group in my nomination list. Well, Nobel committee do not send me nomination forms but I can use the freedom of blogs. I would suggest; Edward Lorenz and Benoit Mandelbrot.

Their contribution to physics is well above the Nobel standards. They opened up a totally new field which evolved into one of the major branches in physics. With the insights of chaos and fractal geometry we understood a great deal in everyday physics and beyond. Since their work is incredibly original, firm in mathematical background, tested experimentally in thousands of intrinsically different situations and the influence on physics is immense; I can not understand why they haven't got it yet. As they are both over 80, I would suggest 2005 to be their year. I hope somebody reading this has a nomination form.

Please feel free to send your Nobel suggestions as comments.

Coleman Fest: Day 2

2005-03-20T19:45:00.000-05:00

Lubos, Peter and David very well summarized the second day so I don't need to spend your and my time. Instead I want to focus on another question, Gross briefly and Witten extensively talked about. Can we make spacetime emergent?

I think the question needs better definition. What they are giving as examples right now is indeed extremely interesting. Two theories living in two different spacetime dimensions can be "dual". This means that every calculation done in one way, can be done in the other and give the same result. I personally would not call these dual theories as emergent. Material continuum is an emergent realization of atoms. So if you want to make a theory with emergent spacetime I would think that dimensions are build from discrete geometries or some other totally different mathematical structure which effectively geometric at large "distances", whatever it means in this case. My humble intuition for the current situation is that it is a manifestation of the gauge redundancy in our theories. They can be reduced (somehow) not only by changing the field degrees of freedom but a combination of spacetime and gauge degrees, since the former is also dynamical in the gravitational theories. But you may need to introduce new redundancies in the second theory. Zee speculates in the last chapter of his marvelous book that our gauge language may not be the best one to represent nature since it includes unavoidable redundancies. Most probably dual theories will have the same or trivially related representations in the ultimate language when we find it.

I don't want to alert anybody out there unnecessarily. I should say that everything Gross and Witten said is accurate for our current understanding, but the word emergent might be confusing since it is used in many different contexts these days. And there is no sign of inaccuracy of the gauge language; it is just hard to play with for the human mind despite its all elegancy.

I also had an opportunity to eat the lunch with David Gross, Frank Wilczek, Stephen Wolfram (Mr. Mathematica), Leonard Parker (who wrote the apparently useful but ridiculously expensive Mathematica package MathTensor) and Jack Ng (who wrote interesting papers on unimodular gravity which I am currently interested in). Wolfram was trying to convince Gross and Wilczek for the enumerability of the physical laws. (Well, they were not amused.) He would like to give brute force search. I am quite skeptical that we can recognize even if we hit the right one. Wolfram is unquestionably a smart guy. I can't say that I buy his cellular automaton ideas but community might be too harsh on him. We always need someone outside the crowd. He also announced us the latest upcoming service from the Wolfram companies but probably I should not say it hear. One teaser: it is about cell phones!

Lively debate (clockwise): Ng, Parker, Gross, Wilczek, Wolfram, Cabi

It was sad to hear that Coleman does not feel well. Shame on me that I have not read his book yet. This week should be the time.

Coleman Fest: Day 1

2005-03-19T01:49:00.000-05:00

Today I've seen the Mount Olympus and came back to my humble room. Harvard organized a conference in honor of great mentor Sidney Coleman. I have never seen a greater concentration of physics talent. First day's speakers were David Gross, Frank Wilczek, Paul Steinhardt, Murray Gell-Mann and Sheldon Glashow!

Gross (who made me so flattered by calling me "grandson") gave the recent talk on "The Future of Physics". It is blogged elsewhere and you can watch it yourself here or here. Even though most approachable and promising question seems to be the dark energy, I am also deeply interested in the possibility of emergent spacetime. I am told by Gross that IIB matrix models in string theory gives four spacetime dimensions without any preliminary reference to space. Even if string theory will not survive eventually, these dual theories will certainly have great theoretical and philosophical implications in the future. There is also Wilczek's question of training artificial intelligence physicists, but this is whole another blog. (Related: Take a look at New Scientist's list of puzzling phenomena.)

Secondly Wilczek gave an illuminating talk on the past of QCD which was very similar to his Nobel lecture. Next, it was interesting to listen Steinhardt on cyclic universes. This was my first exposure to the idea beyond some press releases. This is much better grounded than I initially thought, but still confused why the asymptotic behavior of the scalar field potential is almost zero. This seems to me a crucial step to be explained.

Gell-Mann and Glashow gave entertaining anecdotes. Gell-Mann had a remarkable critic of the current institutes that they do not promote collaboration between post-docs and graduate students enough. Gell-Mann and Glashow had a wonderful partnership in this way. Glashow's story of a Russian meeting was very funny but unfortunately I can not reproduce it here.

Tomorrow's speakers will be Erick Weinberg, Steven Weinberg, Gerard 't Hooft and Edward Witten. This time I'll be in Mount Olympus with my camera.

A final comment: Can Harvard make videos of Sidney's QFT lectures available online? They are only in the Harvard library for overnight loan. Sharing is not a Harvardish policy but I am still hopeful from the Jefferson Lab gang ;)

Rapid Physics History Test

2005-03-17T21:35:00.000-05:00

As I was sitting on my desk, thinking on the near future of physics I found myself writing keywords for the decades future and past. Though it is incredibly incomplete and one (high energy) sided I put it here untouched for your thought provocation. I did not mean to give the discovery dates but the areas which were most lively and exciting those days. Take it as physicist psychology test where you should give the first thing you remember in physics for the decade. Feel free to attack and put your own versions. By the way, what is your guess for 2030's?

1890's Electron
1900's Special Relativity
1910's General Relativity
1920's Quantum Mechanics
1930's Relativistic Quantum Mechanics
1940's Nuclear Physics
1950's QED
1960's Electroweak Theory
1970's QCD
1980's Electroweak Experiments
1990's Neutrino Physics
2000's Cosmology
2010's LHC
2020's Gravitational Waves
2030's ???

I found a much more professional timeline here by APS.

Universe accelerated beyond the horizon

2005-03-16T18:39:00.000-05:00

As I was trying to keep myself busy with modifications of general relativity, I came across to the new paper of the famous cosmologist Kolb and colleagues. They claim that inflation today is created by the primordial fluctuations stretched beyond the horizon. Approach is extremely conservative and the final diagram is very impressive. My first intuition about the super-Hubble modes is that they are just flat ground states here inside the horizon, but this is not enough to drive acceleration it has to dilute much slower than matter. The statistical reason for that is not crystal clear to me, but may be I shouldn’t feel bad because it even puzzled Sean Carroll. He thinks that nothing beyond the horizon can influence us.

For the records, I should say that I did not like the proposed solution. We always thought that solution of the acceleration of the universe puzzle will also solve two other great mysteries. Namely; why we do not see a cosmological constant in the Einstein-Hilbert action although the known symmetry principles allow it, and why the QFT vacuum condensates (ground state) do not gravitate; or why they cancel out. If the acceleration is due to super-Hubble fluctuations, I think we will be in a worse trouble. At least they are offering observable differences from a pure cosmological constant and nature will say the last word.

Physics of computer games and PhysX

2005-03-10T01:12:00.000-05:00

I miss the good old days when I had time for playing computer games for long hours. I remember that time when I first saw a book on game physics. I had never thought of them as a physics problem. It seems that the market in this area have grown since then. Now I see that the problem can be very interesting. Find an algorithm for classical mechanics so that you can allow low precision in position and momentum but still the long term behavior 'looks' reasonable and the complexity is optimized. What is the best compression (or in physics language: coarse-graining) algorithm for (classical) physical reality? There might be something deep lying there, but I assume people are just using the enormous power of today's computers. Do you remember playing less than 1MB games for days on a 286? Programming must be much more fun and challenge at that era.

That was the software part. On the hardware side, specialized processors for celestial mechanics simulations are not new for physicists. Now there is a commercial physics processor available for game physics calculations: PhysX. Do you think that a physics processor will be a standard component of computers like math and graphics chips? I believe it is an essential component for computers that interact with its physical environment. Any way, this chip is a nice event for the year of physics.

Gravity from symmetry breaking

2005-03-08T18:42:00.000-05:00

Ingo Kirsch, a post-doc from Harvard, submitted an interesting paper on arXiv today. Goal is to get gravitational structure from almost nothing and symmetry breaking. He shows that a spacetime just made out of a manifold and a symmetry breaking Higgs field under certain potentials can give rise to a Riemann structure. In other words connection and the metric can be Goldstone modes of the theory. This work is a follow up and generalization of ghost condensation idea from the same (Arkani-Hamed) group.
I could not follow all the group theoretic ideas in the paper. I need more patience and to go back to the study desk for basics. However it reminds me the earlier work of Wilczek, where he gets the metric field out of symmetry breaking of a gauge structure and a preferred volume.

Something for everybody

2005-03-08T12:52:00.000-05:00

Yesterday Wilczek gave a wonderful public talk on the triumphs and prospects of our deepest understandings of nature. I am amazed that he can always maintain a level such that it is educative for the layman as well as motivating for the scientists. So probably it is a better idea to keep it short and watch the great mind.

Bosons from Fermions or Fermions from Bosons!?!

2005-03-07T04:07:00.000-05:00

I want to take your attention to a recent proceeding paper by the grandmaster Anthony Zee. I didn't have a chance for an in-depth look but it is certainly thought provoking at the first sight.

He is challenging the idea of thinking fermions as more fundamental than bosons since you can make bosons as bound states of fermions but not vice versa. He is reminding us a very old result that "eigenstates of a charged boson in the presence of a magnetic monopole have half-integral angular momentum". So why can't the fermions be bound states of monopoles and bosons? He wildly continues other speculations. Have fun!

Axiomatic

2005-03-07T03:56:00.000-05:00

I had a silent week. Besides working on my research and finally reading the last chapters of Zee's masterpiece, I read the sci-fi book Axiomatic by Greg Egan. He is a physicist, a gifted writer and dreamer. I first met him with Permutation City last year. Dust hypothesis proposed there is still a brain-twister for me. To make a long story short, the idea is; what if there is no connection between different spacetime points with different field configurations. They all exist separately and our universe is just one of its organizations. Like a big bunch of letters actually contain all the novels on earth simultaneously.

Axiomatic is a collection of short stories. Each one of them is a snapshot of quite an original universe. Make your brain a delightful favor and read this book. Mr. Egan is certainly making his way to the top of my list of greatest sci-fi writers of all times.

Have fun with physics on the web

2005-02-26T15:01:00.000-05:00

Recently I learned how tomography works. I was wondering it for a long time. How couldn't I figure it out? It is so easy when you see it. Thanks go to Physics 2000 website of University of Colorado. They have a nice collection of physics Java applets. Probably Java applets are the best tools for science education available now.

Next fun site I encountered is The Museum of Unworkable Devices. It is an extensive collection of perpetual motion devices. There are very challenging ones. Don't spoil the fun by looking at the solutions right away.

Both links are via Uncertain Principles. Thanks!

Mannheim's Conformal Gravity: Revisited

2005-02-24T15:41:00.000-05:00

After my recent post, yesterday we were lucky to have Mannheim here at MIT. He is currently collaborating with Alan Guth on brane gravity models. We went thorough my blog entry. So I should summarize it before forgetting.

First of all he is not working in the field for thirty year but about fifteen now. He did start to study conformal gravity just for its mathematical beauty but in the end it turned out to be a solution for dark energy and dark matter. (I wonder if there are solutions for dark galaxies.) He initially named it as Weyl gravity but changed the name quickly to prevent confusions with Weyl geometry.

Variation with respect to conformal scaling field does not make any sense because it is not a new degree of freedom. Actually degrees of freedom in the theory are not different than GR.

He does not accept the von Neumann criticism because galaxy curves fit with just two universal constants instead of different dark and baryonic matter ratios for different galaxies in the standard theory. Interesting thing is these parameters as well as G itself is effective in this theory. You have to match interior and exterior of the source to find the "constant". For example you need the details of the QCD energy momentum tensor distribution inside the proton. Here comes the basic motivation that struck him many years ago. If QFT teaches us that inertial mass is dynamical then gravitational mass must be dynamical. This theory satisfies him in this respect. But I need to think more. His mass generation mechanism is similar (or exactly the same?) to Higgs.

Regarding to cosmology he has a significant prediction. There should not be a deceleration period in the universe history. Universe was always accelerating! I reminded him the current claims about the evidence for deceleration. He said he is well aware of them and they are not precise enough yet. He claimed that the data fits both possibilities still in equal amount of certainty. Two-sigma contours cover his range also. He also told me that his cosmological equations are not fourth order contrary to my first understanding.

He did not get important criticism related to the fundamental principles after many talks across the country for many years. But he accepts that the theory is not completely studied needs further work. He is the only man in the field. He finds it amazing that a theory which is so beautiful can predict dark matter and dark energy observations even though it is not designed to be.

He believes that the community is not ready for a theory that strikes had on GR and might be right. Cosmology hopefully will give better tests soon.