Saturday, January 30, 2010

A Book about Prices

Priceless: The Myth of Fair Value (and How to Take Advantage of It) by William Poundstone is an entertaining summary of the recent scientific literature about prices and uncertainty. Unfortunately it also reinforces one's dismay over the limitations of human reasoning ability in real-life practical situations.

Cooking as Courtship

Cooking as Courtship: a manual of sorts, exploring the ins and outs of love and friendship in the context of food - a charming book, also available in blog format.

Comparison of Human and Chimpanzee Y Chromosomes

Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content in Nature.
The human Y chromosome began to evolve from an autosome hundreds of millions of years ago, acquiring a sex-determining function and undergoing a series of inversions that suppressed crossing over with the X chromosome. Little is known about the recent evolution of the Y chromosome because only the human Y chromosome has been fully sequenced. Prevailing theories hold that Y chromosomes evolve by gene loss, the pace of which slows over time, eventually leading to a paucity of genes, and stasis. These theories have been buttressed by partial sequence data from newly emergent plant and animal Y chromosomes, but they have not been tested in older, highly evolved Y chromosomes such as that of humans. Here we finished sequencing of the male-specific region of the Y chromosome (MSY) in our closest living relative, the chimpanzee, achieving levels of accuracy and completion previously reached for the human MSY. By comparing the MSYs of the two species we show that they differ radically in sequence structure and gene content, indicating rapid evolution during the past 6 million years. The chimpanzee MSY contains twice as many massive palindromes as the human MSY, yet it has lost large fractions of the MSY protein-coding genes and gene families present in the last common ancestor. We suggest that the extraordinary divergence of the chimpanzee and human MSYs was driven by four synergistic factors: the prominent role of the MSY in sperm production, ‘genetic hitchhiking’ effects in the absence of meiotic crossing over, frequent ectopic recombination within the MSY, and species differences in mating behaviour. Although genetic decay may be the principal dynamic in the evolution of newly emergent Y chromosomes, wholesale renovation is the paramount theme in the continuing evolution of chimpanzee, human and perhaps other older MSYs.

Chromosome Segregation

Towards building a chromosome segregation machine in Nature.
All organisms, from bacteria to humans, face the daunting task of replicating, packaging and segregating up to two metres (about 6 × 10^9 base pairs) of DNA when each cell divides. This task is carried out up to a trillion times during the development of a human from a single fertilized cell. The strategy by which DNA is replicated is now well understood. But when it comes to packaging and segregating a genome, the mechanisms are only beginning to be understood and are often as variable as the organisms in which they are studied.

Quantum Computers

Physics: Quantum computing in Nature.
The race is on to build a computer that exploits quantum mechanics. Such a machine could solve problems in physics, mathematics and cryptography that were once thought intractable, revolutionizing information technology and illuminating the foundations of physics. But when?

Martian Methane

Planetary science: A whiff of mystery on Mars in Nature.
Methane has been discovered in the Martian atmosphere. Could that be a sign of life?

Friday, January 29, 2010

No Anomalies in the Cosmic Microwave Background

Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies?
A simple six-parameter LCDM model provides a successful fit to WMAP data, both when the data are analyzed alone and in combination with other cosmological data. Even so, it is appropriate to search for any hints of deviations from the now standard model of cosmology, which includes inflation, dark energy, dark matter, baryons, and neutrinos. The cosmological community has subjected the WMAP data to extensive and varied analyses. While there is widespread agreement as to the overall success of the six-parameter LCDM model, various "anomalies" have been reported relative to that model. In this paper we examine potential anomalies and present analyses and assessments of their significance. In most cases we find that claimed anomalies depend on posterior selection of some aspect or subset of the data. Compared with sky simulations based on the best fit model, one can select for low probability features of the WMAP data. Low probability features are expected, but it is not usually straightforward to determine whether any particular low probability feature is the result of the a posteriori selection or of non-standard cosmology. We examine in detail the properties of the power spectrum with respect to the LCDM model. We examine several potential or previously claimed anomalies in the sky maps and power spectra, including cold spots, low quadrupole power, quadropole-octupole alignment, hemispherical or dipole power asymmetry, and quadrupole power asymmetry. We conclude that there is no compelling evidence for deviations from the LCDM model, which is generally an acceptable statistical fit to WMAP and other cosmological data.

My previous blog posts on the Axis of Evil anomaly here and here.
A diagram of the WMAP spacecraft:

Wednesday, January 27, 2010

The Perry Bible Fellowship

The Perry Bible Fellowship is a webcomic which can be found here. It is irreverent, to put it mildly.

Monday, January 25, 2010

The Reason Why the Earth is at the Center of the Universe

I'm reading Galileo at Work: His Scientific Biography by Stillman Drake. Galileo had literary interests as well as scientific. When he was younger (before he had even adopted the Copernican system) he was invited to lecture on Dante's Inferno. "Galileo expounded on God's reasons for having placed [the earth] at the center of the universe --- namely, to have it as far as possible from the sight of the blessed residents of Heaven, least they be offended by its grossness."
'In the midway of this our mortal life, I found me in a gloomy wood, astray'

Alternatives to dark matter

Alternatives to dark matter: Modified gravity as an alternative to dark matter by Jacob D. Bekenstein.
The premier alternative to the dark matter paradigm is modified gravity. Following an introduction to the relevant phenomenology of galaxies, I review the MOND paradigm, an effective summary of the observations which any theory must reproduce. A simple nonlinear modified gravity theory does justice to MOND at the nonrelativistic level, but cannot be elevated to the relativistic level in a unique way. I go in detail into the covariant tensor-vector-theory (TeVeS) which not only recovers MOND but can also deal in detail with gravitational lensing and cosmology. Problems with MOND and TeVeS at the level of clusters of galaxies are given attention. I also summarize the status of TeVeS cosmology

The Crab Canon by J.S.Bach

There's a discusion of the video here: This Week's Finds in Mathematical Physics (Week 291).

Saturday, January 23, 2010

Group Extensions

Some notes on group extensions by Fields Medallist Terance Tao, contains a "high-concept" overview of methods for extending mathematical spaces, a typical and powerful technique.

Wednesday, January 20, 2010

Minature Heat Engine and Refrigerator

Piezoresistive heat engine and refrigerator
Heat engines provide most of our mechanical power and are essential for long-range transportation. However, whereas significant progress has been made in the miniaturization of motors driven by electrostatic forces, it has proven difficult to reduce the size of conventional liquid or gas driven heat engines below 10^7 um^3. Here we demonstrate an all-silicon reciprocating heat engine with a volume of less than 0.5 um^3. The device draws heat from a DC current using the piezoresistive effect and converts it into mechanical energy by expanding and contracting at different temperatures. It is shown that the engine can even increase the mechanical energy of a resonator when its motion is governed by random thermal fluctuations. When the thermodynamic cycle of the heat engine is reversed, it operates as a refrigerator or heat pump that can reduce motional noise in mechanical systems. In contrast to the Peltier effect, the direction of the thermal current does not depend on the direction of the electrical current.

Dense Stars

Dense matter in compact stars - A pedagogical introduction
Cold and dense nuclear and/or quark matter can be found in the interior of compact stars. It is very challenging to determine the ground state and properties of this matter because of the strong-coupling nature of QCD. I give a pedagogical introduction to microscopic calculations based on phenomenological models, effective theories, and perturbative QCD. I discuss how the results of these calculations can be related to astrophysical observations to potentially rule out or confirm candidate phases of dense matter.

Tuesday, January 19, 2010


Logicomix is a graphic novel which features a fictionalized life story of the Philosopher/Logician Bertrand Russell. I thought the story was quite charming and did manage to give some nice hints about the controversies in the foundations of mathematics, including Russells' Paradox, one of my favorites. I've never tried to read Principia Mathematica (Russell and Whitehead's magnum opus) myself, though I once had a meeting at a professor's office who showed me a copy - he was a big fan of the work - it looked pretty hairy!

Necker Cube Illusion

Necker Cube.

Nucleon Excitations

Nucleon Excitations
The mass pattern of The mass pattern of nucleon and Delta resonances is compared with predictions based on quark models, the Skyrme model, AdS/QCD, and the conjecture of chiral symmetry restoration. and $\Delta$ resonances is compared with predictions based on quark models, the Skyrme model, AdS/QCD, and the conjecture of chiral symmetry restoration.

Baryon Spectroscopy and the Origin of Mass
The proton mass arises from spontaneous breaking of chiral symmetry and the formation of constituent quarks. Their dynamics cannot be tested by proton tomography but only by studying excited baryons. However, the number of excited baryons is much smaller than expected within quark models; even worse, the existence of many known states has been challenged in a recent analysis which includes - compared to older analyses - high-precision data from meson factories. Hence $\pi N$ elastic scattering data do not provide a well-founded starting point of any phenomenological analysis of the baryon excitation spectrum. Photoproduction experiments now start to fill in this hole. Often, they confirm the old findings and even suggest a few new states. These results encourage attempts to compare the pattern of observed baryon resonances with predictions from quark models, from models generating baryons dynamically from meson-nucleon scattering amplitudes, from models based on gravitational theories, and with the conjecture that chiral symmetry may be restored at high excitation energies. Best agreement is found with a simple mass formula derived within AdS/QCD. Consequences for our understanding of QCD are discussed as well as experiments which may help to decide on the validity of models.

Monday, January 18, 2010

Zipf's Law

On the Universality of Zipf's Law
Zipf's law is the most common statistical distribution displaying scaling behavior. Cities, populations and firms are just a few of the examples of this seemingly universal law. Although many different models have been proposed, no general theoretical explanation has been shown to exist for its universality. Here we show that Zipf's law is, in fact, an inevitable outcome of a very general class of stochastic systems. Borrowing concepts from Algorithmic Information Theory, our derivation is based on the properties of the symbolic sequence obtained through successive observations over a system with an unbounded number of possible states. Specifically, we assume that the complexity of the description of the system provided by the sequence of observations is the one expected for a system evolving to a stable state between order and disorder. This result is obtained from a small set of mild, physically relevant assumptions. The general nature of our derivation and its model-free basis would explain the ubiquity of such a law in real systems.

Friday, January 15, 2010

Two Dimesional Time

Physics With Two Time Dimensions
We explore the properties of physical theories in space-times with two time dimensions. We show that the common arguments used to rule such theories out do not apply if the dynamics associated with the additional time dimension is thermal or chaotic and does not permit long-lived time-like excitations. We discuss several possible realizations of such theories, including holographic representations and the possibility that quantum dynamics emerges as a consequence of a second time dimension

Wednesday, January 13, 2010

Where does the element Beryllium come from?

Beryllium abundances and the formation of the halo and the thick disk
The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.

dark matter and gravitational lensing

The dark matter of gravitational lensing
We review progress in understanding dark matter by astrophysics, and particularly via the effect of gravitational lensing. Evidence from many different directions now implies that five sixths of the material content of the universe is in this mysterious form, separate from and beyond the ordinary "baryonic" particles in the standard model of particle physics. Dark matter appears not to interact via the electromagnetic force, and therefore neither emits nor reflects light. However, it definitely does interact via gravity, and has played the most important role in shaping the Universe on large scales. The most successful technique with which to investigate it has so far been gravitational lensing. The curvature of space-time near any gravitating mass (including dark matter) deflects passing rays of light - observably shifting, distorting and magnifying the images of background galaxies. Measurements of such effects currently provide constraints on the mean density of dark matter, and its density relative to baryonic matter; the size and mass of individual dark matter particles; and its cross section under various fundamental forces.

Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of dark matter and dark energy in the universe. In this brief review we outline how weak lensing helps determine the structure of dark matter halos, measure the expansion rate of the universe, and distinguish between modified gravity and dark energy explanations for the acceleration of the universe. We also discuss requirements on the control of systematic errors so that the systematics do not appreciably degrade the power of weak lensing as a cosmological probe.

Microlensing as a probe of the Galactic structure; 20 years of microlensing optical depth studies
Microlensing is now a very popular observational astronomical technique. The investigations accessible through this effect range from the dark matter problem to the search for extra-solar planets. In this review, the techniques to search for microlensing effects and to determine optical depths through the monitoring of large samples of stars will be described. The consequences of the published results on the knowledge of the Milky-Way structure and its dark matter component will be discussed. The difficulties and limitations of the ongoing programs and the perspectives of the microlensing optical depth technique as a probe of the Galaxy structure will also be detailed.

the Milky Way's dwarfs

Determining orbits for the Milky Way's dwarfs
We calculate orbits for the Milky Way dwarf galaxies with proper motions, and compare these to subhalo orbits in a high resolution cosmological simulation. We use the simulation data to assess how well orbits may be recovered in the face of measurement errors, a time varying triaxial gravitational potential, and satellite-satellite interactions. For present measurement uncertainties, we recover the apocentre r_a and pericentre r_p to ~40%. With improved data from the Gaia satellite we should be able to recover r_a and r_p to ~14%, respectively. However, recovering the 3D positions and orbital phase of satellites over several orbits is more challenging. This owes primarily to the non-sphericity of the potential and satellite interactions during group infall. Dynamical friction, satellite mass loss and the mass evolution of the main halo play a more minor role in the uncertainties.
We apply our technique to nine Milky Way dwarfs with observed proper motions. We show that their mean apocentre is lower than the mean of the most massive subhalos in our cosmological simulation, but consistent with the most massive subhalos that form before z=10. This lends further support to the idea that the Milky Way's dwarfs formed before reionisation.

the Dirac Belt Trick

Understanding Quaternions and the Dirac Belt Trick
The Dirac belt trick is often employed in physics classrooms to show that a $2\pi$ rotation is not topologically equivalent to the absence of rotation whereas a $4\pi$ rotation is, mirroring a key property of quaternions and their isomorphic cousins, spinors. The belt trick can leave the student wondering if a real understanding of quaternions and spinors has been achieved, or if the trick is just an amusing analogy. The goal of this paper is to demystify the belt trick and to show that it implies an underlying \emph{four-dimensional} parameter space for rotations that is simply connected. An investigation into the geometry of this four-dimensional space leads directly to the system of quaternions, and to an interpretation of three-dimensional vectors as the generators of rotations in this larger four-dimensional world. The paper also shows why quaternions are the natural extension of complex numbers to four dimensions.

Tuesday, January 12, 2010

The Kondo screening cloud

The Kondo screening cloud: what it is and how to observe it
The Kondo effect involves the formation of a spin singlet by a magnetic impurity and conduction electrons. It is characterized by a low temperature scale, the Kondo temperature, $T_K$, and an associated long length scale, $\xi_K = \hbar v_F/(k_BT_K)$ where $v_F$ is the Fermi velocity. This Kondo length is often estimated theoretically to be in the range of .1 to 1 microns but such a long characteristic length scale has never been observed experimentally. In this review, I will examine how $\xi_K$ appears as a crossover scale when one probes either the dependence of physical quantities an distance from the impurity or when the impurity is embedded in a finite size structure and discuss possible experiments that might finally observe this elusive length scale.

Ultra High Energy Cosmic Rays (UHECR)

UHECR Maps: mysteries and surprises
The rise of nucleon UHECR above GZK astronomy made by protons is puzzled by three main mysteries: an unexpected nearby Virgo UHECR suppression, a rich crowded clustering frozen vertically (north-south) along Cen A, a composition suggesting nuclei and not nucleons. The UHECR map, initially consistent with GZK volumes, to day seem to be not much correlated with expected Super Galactic Plane. Moreover slant depth data of UHECR from AUGER airshower shape do not favor the proton but points to a nuclei, while HIRES, on the contrary favors mostly nucleons. We tried to solve the contradictions assuming UHECR as light nuclei (mostly He) spread by planar galactic fields, randomly at vertical axis. The He fragility and its mass and charge explains the Virgo absence (due to opacity above few Mpc) and the Cen A spread clustering (a quarter of the whole sample). However more events and rare doublets and clustering elsewhere are waiting for an answer. Here we foresee hint of new UHECR component: galactic ones. Moreover a careful updated views of UHECR sky over different (Radio,IR,Optics, X,gamma, TeV) background are also favoring forgotten revolutionary Z-shower model. Both Z-Shower, proton GZK and Lightest nuclei UHECR models have dramatic influence on expected UHE neutrino Astronomy: to be soon revealed by UHE tau neutrino induced air-showers in different ways.

Observation of Ultra-high Energy Cosmic Rays
The measurement of ultra-high energy cosmic rays is an unique way to study article interactions at energies which are well above the capability of current accelerators. Significant progress in this field has occurred during last years, particularly due to the measurements made at the Pierre Auger Observatory. The important results which were achieved during last years are described here. Also future plans for the study of cosmic rays are presented.

High Energy Radiation from Black Holes: A Summary
Bright gamma-ray flares observed from sources far beyond our Galaxy are best explained if enormous amounts of energy are liberated by black holes. The highest-energy particles in nature--the ultra-high energy cosmic rays--cannot be confined by the Milky Way's magnetic field, and must originate from sources outside our Galaxy. Here we summarize the themes of our book, "High Energy Radiation from Black Holes: Gamma Rays, Cosmic Rays, and Neutrinos", just published by Princeton University Press. In this book, we develop a mathematical framework that can be used to help establish the nature of gamma-ray sources, to evaluate evidence for cosmic-ray acceleration in blazars, GRBs and microquasars, to decide whether black holes accelerate the ultra-high energy cosmic rays, and to determine whether the Blandford-Znajek mechanism for energy extraction from rotating black holes can explain the differences between gamma-ray blazars and radio-quiet AGNs.

Galaxy Structure

Nearby Galaxies and Problems of Structure Formation; a Review
The relativistic hot big bang cosmology predicts gravitational gathering of matter into concentrations that look much like galaxies, but there are problems reconciling the predictions of this cosmology with the properties of the galaxies at modest distances that can be observed in greatest detail. The least crowded place nearby, the Local Void, contains far fewer dwarf galaxies than expected, while there are too many large galaxies in the less crowded parts of our neighborhood. The structures of large galaxies show little relation to their environment, contrary to the standard picture of assembly of galaxies by the gathering of material from the surroundings, and the continued accretion of extragalactic debris has prevented establishment of an acceptable picture of formation of common galaxies with the properties of our Milky Way. There is the possibility that the indirect evidence astronomy affords us has been misinterpreted. But the variety of different challenges makes a strong case that we need a better theory, one that does not disturb the agreement with the network of cosmological tests applied on larger scales and fits what is observed on the scales of galaxies. A promising direction is more rapid structure formation, as happens in theoretical ideas under discussion.

Monday, January 11, 2010

Lords of Finance

I recently read "Lords of Finance: The Bankers Who Broke the World" by Liaquat Ahamed. It was about the central bankers who controlled international finance in the period leading up to the Great Depression. Well-written and (perhaps surprisingly) witty and entertaining, given that this might seem to be a rather dry subject.

Very High Energy Cosmic Rays

A Faraway Quasar in the Direction of the Highest Energy Auger Event
The highest energy cosmic ray event reported by the Auger Observatory has an energy of 148 EeV. It does not correlate with any nearby (z$<$0.024) object capable of originating such a high energy event. Intrigued by the fact that the highest energy event ever recorded (by the Fly's Eye collaboration) points to a faraway quasar with very high radio luminosity and large Faraday rotation measurement, we have searched for a similar source for the Auger event. We find that the Auger highest energy event points to a quasar with similar characteristics to the one correlated to the Fly's Eye event. We also find the same kind of correlation for one of the highest energy AGASA events. We conclude that so far these types of quasars are the best source candidates for both Auger and Fly's Eye highest energy events. We discuss a few exotic candidates that could reach us from gigaparsec distances.

Note: an EeV is 10^18 electronvolts or 0.1602 Joule!

Phase change memory

Phase change memory technology
We survey the current state of phase change memory (PCM), a non-volatile solid-state memory technology built around the large electrical contrast between the highly-resistive amorphous and highly-conductive crystalline states in so-called phase change materials. PCM technology has made rapid progress in a short time, having passed older technologies in terms of both sophisticated demonstrations of scaling to small device dimensions, as well as integrated large-array demonstrators with impressive retention, endurance, performance and yield characteristics.
We introduce the physics behind PCM technology, assess how its characteristics match up with various potential applications across the memory-storage hierarchy, and discuss its strengths including scalability and rapid switching speed. We then address challenges for the technology, including the design of PCM cells for low RESET current, the need to control device-to-device variability, and undesirable changes in the phase change material that can be induced by the fabrication procedure. We then turn to issues related to operation of PCM devices, including retention, device-to-device thermal crosstalk, endurance, and bias-polarity effects. Several factors that can be expected to enhance PCM in the future are addressed, including Multi-Level Cell technology for PCM (which offers higher density through the use of intermediate resistance states), the role of coding, and possible routes to an ultra-high density PCM technology.

Anomalous X-ray Pulsar

The spectacular X-ray echo of a magnetar burst
The Anomalous X-ray Pulsar (AXP) 1E 1547.0-5408 reactivated in 2009 January with the emission of dozens of short bursts. Follow-up observations with Swift/XRT and XMM-Newton showed the presence of multiple expanding rings around the position of the AXP. These rings are due to scattering, by different layers of interstellar dust, of a very high fluence burst emitted by 1E 1547.0-5408 on 2009 January 22. Thanks to the exceptional brightness of the X-ray rings, we could carry out a detailed study of their spatial and spectral time evolution until 2009 February 4. This analysis gives the possibility to estimate the distance of 1E 1547.0-5408. We also derived constraints on the properties of the dust and of the burst responsible for this rare phenomenon.

Neutron Stars

Neutron Stars, the Most Exotic Nuclear Lab in the Universe
In this lecture, we give a first introduction to neutron stars, based on fundamental physical principles. After outlining their amazing macroscopic properties, as obtained from observations, we infer the extreme conditions of matter in their interiors. We then describe two crucial physical phenomena which characterize compact stars, gravitational stability of strongly degenerate matter and neutronization of nuclear matter with increasing density, and explain how the formation and properties of neutron stars are a consequence of the extreme compression of matter under gravity. Finally, we describe how astronomical observations of various external macroscopic features can give invaluable information about the exotic microscopic scenario inside: neutrons stars represent a unique probe to study super-dense, isospin-asymmetric, superfluid, bulk hadronic matter.

Pair-Instability Supernovas

Supernova 2007bi as a pair-instability explosion
Stars with initial masses 10 M_{solar} < M_{initial} < 100 M_{solar} fuse progressively heavier elements in their centres, up to inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion -- an iron-core-collapse supernova (SN). In contrast, extremely massive stars (M_{initial} > 140 M_{solar}), if such exist, have oxygen cores which exceed M_{core} = 50 M_{solar}. There, high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron-positron pairs occurs prior to oxygen ignition, and leads to a violent contraction that triggers a catastrophic nuclear explosion. Tremendous energies (>~ 10^{52} erg) are released, completely unbinding the star in a pair-instability SN (PISN), with no compact remnant. Transitional objects with 100 M_{solar} < M_{initial} < 140 M_{solar}, which end up as iron-core-collapse supernovae following violent mass ejections, perhaps due to short instances of the pair instability, may have been identified. However, genuine PISNe, perhaps common in the early Universe, have not been observed to date. Here, we present our discovery of SN 2007bi, a luminous, slowly evolving supernova located within a dwarf galaxy (~1% the size of the Milky Way). We measure the exploding core mass to be likely ~100 M_{solar}, in which case theory unambiguously predicts a PISN outcome. We show that >3 M_{solar} of radioactive 56Ni were synthesized, and that our observations are well fit by PISN models. A PISN explosion in the local Universe indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic limit, perhaps resulting from star formation processes similar to those that created the first stars in the Universe.

Simulated view of a black hole

Cosmic Acceleration

Gif Lectures on Cosmic Acceleration
These lecture notes cover some of the theoretical topics associated with cosmic acceleration. Plausible explanations to cosmic acceleration include dark energy, modified gravity and a violation of the Copernican principle. Each of these possibilities are briefly described.

Coordinate Systems for Global Positioning

Relativistic versus Newtonian frames: emission coordinates
Only a causal class among the 199 Lorentzian ones, which do not exists in the Newtonian spacetime, is privileged to construct a generic, gravity free and immediate (non retarded) relativistic positioning system. This is the causal class of the null emission coordinates. Emission coordinates are defined and generated by four emitters broadcasting their proper times. The emission coordinates are covariant (frame independent) and hence valid for any user. Any observer can obtain the values of his(her) null emission coordinates from the emitters which provide him his(her) trajectory.

A null frame for spacetime positioning by means of pulsating sources
We introduce an operational approach to the use of pulsating sources, located at spatial infinity, for defining a relativistic positioning and navigation system, based on the use of four-dimensional bases of null four-vectors, in flat spacetime. As a prototypical case, we show how pulsars can be used to define such a positioning system. The reception of the pulses for a set of different sources whose positions in the sky and periods are assumed to be known allows the determination of the user's coordinates and spacetime trajectory, in the reference frame where the sources are at rest. In doing so, the phases of the received pulses play the role of coordinates in the null frame. We describe our approach in flat Minkowski spacetime, and discuss the valididty of this and other approximations considered

Introduction to Loop Quantum Gravity

Introduction to Loop Quantum Gravity
The questions I have been asked during the 5th International School on Field Theory and Gravitation, have compelled me to give an account of the premises that I consider important for a beginner's approach to Loop Quantum Gravity. After a description of some general arguments and an introduction to the canonical theory of gravity, I review the background independent approach to quantum gravity, giving only a brief survey of Loop Quantum Gravity.

The Planck scale

Six easy roads to the Planck scale
We give six arguments that the Planck scale should be viewed as a fundamental minimum or boundary for the classical concept of spacetime, beyond which quantum effects cannot be neglected and the basic nature of spacetime must be reconsidered. The arguments are elementary, heuristic, and plausible, and as much as possible rely on only general principles of quantum theory and gravity theory. The paper is primarily pedagogical, and its main goal is to give physics students, non-specialists, engineers etc. an awareness and appreciation of the Planck scale and the role it should play in present and future theories of quantum spacetime and quantum gravity

Saturday, January 09, 2010

Preventing the return of fear

Preventing the return of fear in humans using reconsolidation update mechanisms
Recent research on changing fears has examined targeting reconsolidation. During reconsolidation, stored information is rendered labile after being retrieved. Pharmacological manipulations at this stage result in an inability to retrieve the memories at later times, suggesting that they are erased or persistently inhibited. Unfortunately, the use of these pharmacological manipulations in humans can be problematic. Here we introduce a non-invasive technique to target the reconsolidation of fear memories in humans. We provide evidence that old fear memories can be updated with non-fearful information provided during the reconsolidation window. As a consequence, fear responses are no longer expressed, an effect that lasted at least a year and was selective only to reactivated memories without affecting others. These findings demonstrate the adaptive role of reconsolidation as a window of opportunity to rewrite emotional memories, and suggest a non-invasive technique that can be used safely in humans to prevent the return of fear.

Experimental Simulation of Zitterbewegung

Quantum physics: Trapped ion set to quiver
The peculiar ultra-fast trembling motion of a free electron — the Zitterbewegung predicted by Erwin Schrödinger in 1930 when he scrutinized the Dirac equation — has been simulated using a single trapped ion.

Microbe Immune Systems

CRISPR/Cas, the Immune System of Bacteria and Archaea
Microbes rely on diverse defense mechanisms that allow them to withstand viral predation and exposure to invading nucleic acid. In many Bacteria and most Archaea, clustered regularly interspaced short palindromic repeats (CRISPR) form peculiar genetic loci, which provide acquired immunity against viruses and plasmids by targeting nucleic acid in a sequence-specific manner. These hypervariable loci take up genetic material from invasive elements and build up inheritable DNA-encoded immunity over time. Conversely, viruses have devised mutational escape strategies that allow them to circumvent the CRISPR/Cas system, albeit at a cost. CRISPR features may be exploited for typing purposes, epidemiological studies, host-virus ecological surveys, building specific immunity against undesirable genetic elements, and enhancing viral resistance in domesticated microbes.

Friday, January 08, 2010

Quadrocopter Drone!

Check out the video in the link below!
Parrot AR.Drone hands-on: a quadrocopter for the rest of us

The chromosphere:

The chromosphere: gateway to the corona, or the purgatory of solar physics?<
I argue that one should attempt to understand the solar chromosphere not only for its own sake, but also if one is interested in the physics of: the corona; astrophysical dynamos; space weather; partially ionized plasmas; heliospheric UV radiation; the transition region. I outline curious observations which I personally find puzzling and deserving of attention.

Grigori Perelman Biography

I just read "Perfect Rigor: A Genius and the Mathematical Breakthrough of the Century" by Masha Gessen a biography of Grigori Perelman, the eccentric Russian mathematician. Interesting depictions of the former Soviet system for training mathematicians and the international mathematics community.

Thursday, January 07, 2010

Polarized Proton Collisions - a problem with QCD?

Hard Collisions of Spinning Protons - History and Future

There will be a review of the history of polarized proton beams, and a discussion of the unexpected and still unexplained large transverse spin effects found in several high energy proton-proton spin experiments at the ZGS, AGS, Fermilab and RHIC. Next there will be a discussion of possible future experiments on the violent collisions elastic collisions of polarized protons at the 70 GeV U-70 accelerator at IHEP-Protvino in Russia and the new high intensity 50 GeV J-PARC at Tokai in Japan.

"To summarize, for the past 30 years QCD-based calculations have continued to disagree
with the ZGS 2-spin and AGS 1-spin elastic data, and the ZGS, AGS, Fermilab and now
RHIC inclusive data."

Wednesday, January 06, 2010

An Intermediate-mass Black Hole Discovered

An Intermediate-mass Black Hole of Over 500 Solar Masses in the Galaxy ESO 243-49
Ultra-luminous X-ray sources are extragalactic objects located outside the nucleus of the host galaxy with bolometric luminosities >10^39 erg s^-1. These extreme luminosities - if the emission is isotropic and below the theoretical (i.e. Eddington) limit, where the radiation pressure is balanced by the gravitational pressure - imply the presence of an accreting black hole with a mass of ~10^2-10^5 times that of the Sun. The existence of such intermediate mass black holes is in dispute, and though many candidates have been proposed, none are widely accepted as definitive. Here we report the detection of a variable X-ray source with a maximum 0.2-10 keV luminosity of up to 1.2 x 10^42 erg s^-1 in the edge-on spiral galaxy ESO 243-49, with an implied conservative lower limit of the mass of the black hole of ~500 Msun. This finding presents the strongest observational evidence to date for the existence of intermediate mass black holes, providing the long sought after missing link between the stellar mass and super-massive black hole populations.

Tuesday, January 05, 2010

A Survey of the Observational Evidence for Dark Matter

Dark Matter: The evidence from astronomy, astrophysics and cosmology
Dark matter has been introduced to explain many independent gravitational effects at different astronomical scales, in galaxies, groups of galaxies, clusters, superclusters and even across the full horizon. This review describes the accumulated astronomical, astrophysical, and cosmological evidence for dark matter. It is written at a non-specialist level and intended for an audience with little or only partial knowledge of astrophysics or cosmology.

Ultra Dense Deuterium?

There is recent experiment evidence of ultra-dense deuterium from Sweden. "Ultra-dense deuterium would be by far the most dense material ever produced by man - one cubic centimetre would have a mass of 140 kilograms" - Wikipedia.
See also the preprint Ultradense Deuterium.
An attempt is made to explain the recently reported occurrence of ultradense deuterium as an isothermal transition of Rydberg matter into a high density phase by quantum mechanical exchange forces. It is conjectured that the transition is made possible by the formation of vortices in a Cooper pair electron fluid, separating the electrons from the deuterons, with the deuterons undergoing Bose-Einstein condensation in the core of the vortices. If such a state of deuterium should exist at the reported density of about 100,000 g/cm3, it would greatly facility the ignition of a thermonuclear detonation wave in pure deuterium, by placing the deuterium in a thin disc, to be ignited by a pulsed ultrafast laser or particle beam of modest energy.

When does a pair of Fermions act like a Boson?

Entanglement and Composite Bosons
Under what circumstances can a pair of fermions be treated as an elementary boson? Many authors have done detailed studies of this question, as it applies, for example, to atomic Bose-Einstein condensates, excitons, and Cooper pairs in superconductors. In a 2005 paper, C. K. Law presented evidence that the question can be answered in general in terms of entanglement: two fermions can be treated as an elementary boson if they are sufficiently entangled. Consider, for example, a single hydrogen atom in a harmonic trap. Within the atom, the proton and electron are strongly entangled with respect to their position variables; for example, wherever the proton might be found—it could be anywhere in the trap—the electron is sure to be nearby. Law suggests that this entanglement is the essential property underlying the (approximate) bosonic behavior of the composite particle, allowing, for example, a collection of many hydrogen atoms to form a Bose-Einstein condensate.

Friday, January 01, 2010



Snowball Dances to the Beat

That Bird Can Boogie
Snowball, the dancing sulphur-crested cockatoo, is a big hit on YouTube--and now he's also a scientific sensation. Researchers have shown that the bird, who bobs his head and lifts his legs to the Backstreet Boys' song Everybody, is in fact listening to and following the beat. The findings--detailed in a pair of articles--challenge the notion that only humans have the neural wiring for dancing in time to music.