Have fun with physics on the web

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

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.

Dark Galaxy

Let me take a break from my homework marathon. Physicsweb and PPARC announced that a candidate for "dark galaxy" is observed. Paper is already on ArXiv astro-ph/0502312. It was first recognized in 2000 in radio astronomy hydrogen gas survey. From the size and rotation velocity, its mass seems to be about 10^8 solar mass. Subsequent observations in other wavelengths showed no sign of stars inside or nearby galaxies which can give out debris. So the most plausible explanation is a gigantic cloud almost entirely made out of dark matter! If it is not hidden by a cold dense cloud we have a very interesting object.

I was skeptical at first sight about how to generate these objects. Can dark matter clump without sucking lots of matter inside? As I went through the paper, I saw references to simulations that predict these objects. Claim is there must be more dark halos than galaxies.

If we can find a family of these objects, it will be a great challenge for any alternative gravity theories against dark matter. I can't think of any way to have such gravitation with no dark matter and almost no baryonic mass. Wouldn't it be so exciting if it turns out to be a SUSY or axion galaxy! I love astronomy; it is always full of surprises.

What do Feynman diagrams mean?

Speaking of Freeman Dyson, reminded me an old debate between him and Feynman. What do the Feynman diagrams really mean?

Do Feynman diagrams in particle physics represent real processes happening there in superposition as Feynman claimed or are they merely a bookkeeping tool for the perturbation calculations? There are Feynman diagrams in condensed matter physics and even in fluid mechanics (called Wyld diagrams) which certainly does not reflect any superposition according to my understanding (at least not in quantum sense). Also loop diagrams always confused me. Can such processes be "real"? I don’t think that any collider experiment probed those virtual processes, only outgoing particles are observable.

So, what do you think? Are they real or just calculation tools? Does it matter? Is it possible to discriminate or should we “shut up and calculate”? Please leave your comments.

The Darwinian Interlude

I want to take your attention to great physicist Freeman Dyson's thought provoking recent article in MIT's innovation magazine Technology Review.

He is presenting ideas of biologist Carl Woese on evolution. He is conjecturing an era before the Darwinian evolution where the primordial life forms do not have species but all the organisms share the genes they have and evolve together. Once in a while, an organism cease to share become a species. Gene becomes selfish though it was not all the way back. Claim is this way of evolution is much efficient and be able to form complex cell mechanisms faster.

Dyson takes the idea from here and speculate that as the genetic engineering advances we will be able to move genes freely between species so the boundaries will get blurred. Evolution will not be based on competition for food and land, therefore the end of Darwinian evolution. It is just an interlude.

I think the idea of sharing instead of compete for faster evolution might have very interesting social analogs. What happens when every patent is freely shared? Does physics advance faster than technology? Is competition the best motivation? Can sharing be better not only in social but also in economical aspects? Is Creative Commons the future of intellectual property?

Coming back to evolution, I am attending a course on statistical physics in biology. I am learning a lot but still desperate about state of the art. I would love to calculate how much mean time we need for a complex organ like eye to evolve from the first multicellular organisms. But it seems this is almost impossible right now. I am certainly disappointed.

Conformal Alternatives to Higgs

Recently I came across with two independent but apparently similar researches. They are both alternatives to Higgs mechanism and use conformal invariance as the main argument.

First is from Hung Cheng of MIT mathematics department: math-ph/0407010. He is doing a Weyl theory which eats the extra degree of freedom in the Higgs mechanism, so nothing left for it. His Weyl field is not the electromagnetic vector potential and claimed as a dark matter candidate.

Second is from two Polish physicists Pawlowski and Raczka: hep-th/9407137, hep-ph/9610539, hep-th/9808021. They appear to send the missing degree of freedom to the gravitation sector. It is too late at night to check whether there is a way to connect those two theories. Can somebody do that? Or can you see any obvious flaws?

What makes me excited here is the possibility of missing Higgs because of the beautiful conformal symmetry. Although I find Higgs mechanism very elegant and am enthusiastic about all the new physics behind and details of the symmetry breaking; secretly I want it not to be there. Wouldn’t it be simply more fun?

Hawking Radiation and Useful Neutrinos

Today I’ve seen two interesting works:
First is from my advisor Wilczek and my friend Mr. Robinson. After Frank's long rush time for obvious reasons their paper is finally on ArXiv: gr-qc/0502074. They are considering an effective field theory around a black hole horizon. They show that to cancel the anomalies that break general covariance, black hole must sit in a thermal radiation bath exactly at the Hawking temperature. It is a brand new way of proving Hawking radiation with lots of physical insight. Well done, Sean! We will soon congratulate you for your graduation ;)

Finally somebody found a use for neutrinos! If you can make a vertical beam line, you can probe Earth's interior density distribution with incredible accuracy from neutrino oscillations. Idea is due to Walter Winter of Advanced Study. See hep-ph/0502097 and PhysicsWeb coverage.

Physics Seminar Videos

As you might already noticed I am collecting links for physics talks available on the net. I also put a permanent link to my sidebar. I will try to update as much as I can (last update is today), please take a look and send your contributions.
I know how hard to be in a university where there are barely two talks a month. (Don't take me wrong, I learned and enjoyed a lot in my university.) I think it is important to make the information available for everybody on Earth. I thank all the institution who made it possible.
Please distribute the link to your friends. I would even give permission to copy them all, but it is probably much more convenient to keep them in a single place and make it grow there.
Enjoy, learn and spread the word!

Mannheim's Conformal Gravity

These days I am reading about alternative gravity theories and try to classify with Tegmark. I'll try to write brief summaries of some interesting ones in the coming days. I will start with Philip Mannheim's conformal gravity. I call it “Mannheim's” not because I am sure that he is the first to write this Lagrangian ('cos I am not) but he seems the only one writing about it for thirty years and review papers do not give it enough attention.

Idea is to write a fully conformal but generally covariant theory without introducing any new structure (like a gauge field as Weyl did or torsion). It is not possible at the first order in R. But in the second order there is a beautiful Lagrangian: C^2. It is the Weyl tensor properly contracted.

You get the field equation by varying with respect to the metric as usual. (Is it also meaningful to do variation of the conformal scaling field?) Of course the field equation is a mess with fourth derivative of the metric (gravitational potential in the Newtonian limit). But Mannheim managed to find a Schwarzschild like solution. As you can imagine it has three independent parameters instead of one (mass) in Schwarzschild. First thing you can do with this solution is to fit your galaxy curves. They seem to fit very good if you accept those two new parameters as new universal constants. I can't help remembering von Neumann's wise words: "With four parameters I can fit an elephant and with five I can make him wiggle his trunk".

Mannheim also proposes some accelerating cosmological solutions, though it doesn't surprise me in the vast probabilities of a fourth order equation. Physicists like second order equations :)

Like Weyl, he also needs to face with fact that particles are massive, so world is not conformal. He offers a kind of Higgs mechanism to generate mass. Another interesting property is G appears not in the fundamental theory but as an effective coupling constant.

If you are interested, start reading with gr-qc/9306025 for his criticism of GR. In conclusion, I don't see it as the strongest candidate before seeing the uniqueness (Birkhoff's) theorem of solutions and some testable predictions (which may already be in the literature that I did not notice). However it certainly deserves more attention in review papers.

Comments are welcome.

IMPORTANT UPDATE:
See my recent post for Mannheim's response.

Can Cosmology Test String Theory?

Today, I listened a talk by Richard Easther of Yale with the above title. I think their approach is certainly worth a try. Rather than looking specific string-cosmology like brane world scenarios or stringy inflation, they pursued a search by an effective field theory approach. They took a reasonable initial condition anzats with a minimum length scale and evolved it outside to the horizon. When it comes back to the horizon, we observe it as the CMB fluctuations. Since in any inflation model today’s fluctuation length scales were beyond quantum gravity scales, it is plausible that we can see some effects, but are they big enough?

Their anzats, which seems reasonable in for string theory, modulates the power spectrum and for some values in the parameter space it is observable. To quote from their paper astro-ph/0412613: "For the specific case we consider, we conclude that if the tensor to scalar ratio, r~0.15, the ratio between the inflationary Hubble scale H, and the scale of new physics M has to be on the order of 0.004 if the modulation is detectable at the 2σ level. For a lower value of r, the bound on H/M becomes looser." This means if the universe is very very kind to us, we can see something even with the second year WMAP data. (By the way, does anybody know what's going on in WMAP? Third year data should be almost ready but they haven't announced the second year.)

I want to mention one point. I couldn't see any point in this approach very specific to string theory. Any theory with a minimum scale is equally fine. Even if we could see a significant signal, string theory still has to predict a specific scenario.

Today I also learned that fitting the cosmological parameters is not being done by brute fore n dimensional grid search anymore. There is a well working Bayesian algorithm called Monte Carlo Markov Chain, first proposed for CMB analysis in astro-ph/0006401.

I hope coming years will be full of surprises in cosmology.

TOMB: New Boston Attraction

I had a fun night so I should write it, in the spirit of this blog. My great (but overpriced) MIT dorm, took us to TOMB.

It is something like a mini theme park but much more interactive and I guess unique in the New England area. Quite unique thing among the theme parks is that the game is fully interactive. You can really search for secret passages, push stones, etc.

Plot? OK, I'll try to make it spoiler free. Tomb is game played as a group and guide who doesn't help much but make it more fun by creating the story for you. Goal is to reach the burial chamber of the pharaoh, but he doesn’t want you to go there. There are various traps and puzzles you have to face with. Before going, I thought that the puzzles will be extremely easily, since it is aimed at a general community. But they turned out to be just easy. At least I can say that 12 MIT students are too much for the game :) The best part is, it is not guaranteed to see the final. You can die at any time, although we are told that dieing was also fun (we didn't see it). It takes about 40 minutes to 1 hour to complete.

After the game we talked with the MIT alumnus founder. While I was asking things like “What happens when a group catches the front one? What is the mortality rate?” people were curious about the cost, portability, insurance costs and maintenance costs. I realized that I am not entrepreneur minded. What a surprise!

Plan for now is to create a new game and transport this one to new location and grow organically this way. I am already looking for the new one. It will most probably be either 20000 Leagues Under the Sea or James Bond.

Only negative thing I can say is size. Even though the puzzles take enough time, size of the game is so small that you are not really feeling like going anywhere. I think they could rent a whole basement. Also while waiting for the group in front, you hear lots of voices inside and it feels like spoiling the surprise.

In conclusion, it well worth the money and time at least for the first time. If you are around Boston give it a try.

Space Station Toilet

Have you ever seen how the toilets in space station or shuttle works? Yes, it sucks! Nasa Brain Bites has fun videos about life in the orbit. Non-scientist friendly :) I hope they will add more of them.

Global Mini Array

Stimulated by the below talk, I was thinking about astronomy. I have question for my more knowledgeable readers.

It is a fact that amateur astronomers are sometimes more lucky in finding faint and rare events like comets or asteroids. Simply because they a big gang of people. My question is; what would be the cost of all time all (night) sky monitoring, done by small telescopes with CCDs, distributed to the universities around the globe. What would be the minimum size of telescopes for enough power to catch events like faint solar system objects or very very early supernovas to warn big brothers? And how many of them would we need?

Does anybody have an estimate?

Hmm, it seems that there are some useful information here.

Update: OK! How about this estimate? 10000 telescopes each for 10000$ making totally 100 million dollars. 5000$ for a nice small (around 14 inch) telescope and 5000$ for the tracking system, CCD and the computer. Every university can buy its own equipment and take software and technical help from the center as well as access to the data. Doesn't it worth that?

Magnetars

I love to be physicist when exposed to brand new observations totally out of imagination. Today is one fo those days. I attended a colloquium by Victoria M. Kaspi on magnetars.

This is a fairly new idea for explaining two exotic astrophysical objects "Soft Gamma Ray Repeaters" and "Anomalous X-ray Pulsars" and may be more. The problem with these objects is basically energy released by their rotational deceleration is not enough to explain observed brightness and occasional bursts. You need another energy source.

Latest fashionable idea is the magnetars. Claim is these objects are isolated neutron stars with extremely high magnetic fields about 10^15 Gauss (about thousand times the usual pulsars and more than anything we know so far!). Observed radiation is due to instabilities in the crust and the release and reorganization of the magnetic flux. It is a gigantic magnetic-neutron-star-quake. Everything seems to fit with many new observations. The problem is how to create these extraordinary magnetic fields at the first place. There is a dynamo idea in the literature but not really well studied so far.

There are skeptics like distinguished Turkish neutron star expert Ali Alpar. They are argue that it is quite unlikely to have such magnetic fields. Competing idea is the accretion disk model. But Prof. Kaspi argued that the disk will not be stable in that environment and whole thing seems to emit light many orders of magnitude more radiation than Eddington limit, which gives the upper limit for accretion radiation.

By the way, I also learned that my high school physics olympiad knowledge about neutron star acceleration is out of date. I thought that sudden increase in velocity of pulsars can be explained by crust crack and the decrease in the moment of inertia. But I learned that it did not fit the data and abandoned many years ago. Ooops! New mechanism is a superfluid core covered by a thin solid crust. Fluid inside rotate faster and occasional interactions between the core and the surface increase the crust velocity. There are gaps in my understanding like why the magnetic field moves with the crust but I’ll take experts’ words for a while.

You can find more about magnetars in the article of Bob Duncan, one of the originators of the idea.

Buy Blue

There is useful information in Buy Blue for the readers who care about politics. You can learn which company supported which party in the last election. Well, it can be used for both sides but I always support transparency in these issues anyway. Good idea although the list should be improved. It seems that they will add information about issues like child labor, environmental concerns etc.

Eternal Dissatisfaction of the Obsessive Mind

Here at MIT I have many friends working in the lab. I see many researchers giving talks. I am sometimes quite jealous. I see people working in the areas where the tests of theories are possible to do in a reasonable amount of time and people. A good friend of mine in biological sciences said to me a few days ago: "I have great idea for a certain mechanism but experiments needed to verify are very demanding. I need to work for months in the lab and I already have many other projects to do." I was shocked: "What?!? You can do it yourself in a few months and do you think that it is considered as hard! You must be kidding."

Today both high energy physics and cosmology require experiments and observations that are far beyond the ability of not only one but hundred scientists. If you have a very bright idea and need certain verifications you would probably need to wait for your whole career even if the whole community agrees with you. Higgs is waiting for his boson, Wilczek is waiting for axions, Guth is waiting for new evidences for inflation and the list goes on.

So I am asking myself: "Why would I be stuck here? Why would I chase endless questions in this field? I also see many interesting questions out there waiting to be explored in a personal lab. Am I masochist or something?"

Well, I know the answer. Gravity is what I really wonder. Even though there are many ad hoc theories that fit current data for dark energy but can not be tested further, even though there are no complete theories for quantum gravity and there is no hope for direct (not semi-classical) probing in the foreseeable future, I will inevitably continue to meditate on these question. I am doomed to try to understand the picture on a huge jigsaw puzzle with only few pieces in hand. Worst part is: I like it!

Emergent Particles?

How do we know that electrons and photons are fundamental and there is nothing beyond? No experiment can make us sure; anything we observe can be emergent and any understanding can be effective. Although we can not prove the contrary, we can one day find out that current theories is just a low energy or big scale description of something more profound. I know that low energy and big scale are used interchangeably in high energy physics, but I want to make a distinction for making my point more clear.

Understanding particles as a low energy configuration of a single entity is the current program of string theory. Everything we see around are very cooled strings and properties (particle spectrum) are determined by the specific the compactification of the extra dimensions which is a valid description only in low energies. But still one electron is made out of one string.

Another way of realizing the world would be seeing particles as a collective behavior of many small agents. They are emergent concepts in the big scale, but their degrees of freedom do not have any meaning at scale of fundamental building blocks. I am very sympathetic to this point of view because there is a particle-like object (that propagates, interacts and stays intact) but is actually just a collective motion of something smaller. This object is the phonon.

I think in the high energy physics community emergent concepts are highly underrepresented, compared to string theory. But certainly there were many people who tried it and failed and trust decayed over time. Simplest (best known) way of carrying this approach is the solitons. Most famous soliton particle is probably the Skyrmion, but it wasn’t very successful so far.

Today I want to talk about the most promising collective model of particles, I have seen so far. Idea is due to Prof. Xiao-Gang Wen of MIT. He was last year’s MIT Theory Retreat speaker and my interest in the subject is increasing since then. He started with the above motivations about the phonon. But no vacuum state we know (no matter how abstract) has the right gapless excitations of gauge bosons and fermions until the work of Wen. He studied a very interesting condensed matter system: condensation of strings. His strings are not (at least not necessarily) related to the superstrings. They are just excited states lying on curve in a lattice of ground state bosons with a certain simple interaction. Surprisingly these objects can give rise to fermions, gauge bosons of U(1) and SU(3) and probably many other things as a collective excitation. Main problem here is the chiral description. Theory does not naturally give chiral fermions therefore SU(2) couplings are problematic. If theory gives them in some limit or at some critical state approximately then it will be a big triumph. How about gravity? Theory does not have graviton excitations but it seems that loop quantum gravity “spin-networks” can be written in terms of these “spin-nets” and vice versa. Wouldn’t it be fun if loop quantum gravity has the standard model particles as collective phenomena!

If you want to read more about it start from cond-mat/0407140, cond-mat/0406441 and references therein. His latest book treating these issues in great detail is also recently published. Comments are welcome.

Weirdest physics auction

Somebody on eBay was selling Wilczeks' used glasses which claimed to have their DNA! Auction is over and the winner is Mrs. Wilczek or somebody who uses her name as nickname. It was gone for $40 plus shipping.
Hmmmm, maybe I should gather some precious DNA from Frank's office ;)
By the way is there anybody out there who would give $40 for Cabi's Glasses?

Update: They were really bought by Mrs. Wilczek.

Two useful services

Today I came across with two useful websites on the Lifehacker.
First one is a file transfer site: YouSendIt. You can upload a file to the server and they send a secure entrance link to the given email address. File is stored on the server for a week and can be reached as many times as you like. So what is so good about it? You can upload a file up to 1GB! I think you know what you want to do with it ;)
Second one is a public account database: BugMeNot. Have you ever want to reach a free site without losing time with that "free accounts" or give your name and email? This is site holds a database for free accounts. Brilliant! If you can not find what you want, please create an account and submit it here to do others a favor.

Newton vs. Mach vs. Einstein

I was always confused with the "Machianity" of general relativity. After partially reading Brans and Dicke's famous paper on their alternative gravity theory, I settled on the following view:

*Newton claims that space-time exists and it is absolute. Matter moves with respect to that.

*Mach claims that space-time does not exist as a physical entity. Matter moves with respect to other matter. Space-time somehow is an emergent entity used to describe the perceived change. Though I don't think that Mach himself would propose it this strong, because emergent concepts were not in the mental language and braveness to deny the existence of space and time was not common among physicist. He was just operational and believed only in observable things. I wonder how he would react to gravitational waves, Aharonov-Bohm effect or modern gauge theories. I certainly believe that Hulse-Taylor pulsar would puzzle him.

*Einstein claims that space-time exist as a physical entity. But both matter and space-time move (or change) with respect to the others. So it is a mixture of Newtonian and Machian views. When I go back I see that this is the explanation of the Einstein theory as told in Rovelli's book. (See here for the free draft.) His book also gives different realizations of Mach's principle and stating if they are true in general relativity or not.

It may not be quite right to summarize these giants' ideas with today's words, but this is pretty much the picture that is referred as Newtonian, Machian and Einsteinian point of view. I still don't feel that Einstein gravity is the last word on gravity, even in the classical level. But total Machianity is certainly losing some points.

Tegmark says that I am finally finding peace with Einstein and not worry anymore that his theory is not totally Machian. I should say that Gödel universe which has nonvanishing angular momentum (with respect to what) still worries me along with some other issues but I will wait for further settlement of dust in my mind.

For a beautiful discussion on Mach see also Wilczek's article in Physics Today, April 2004.