Thursday, April 30, 2009


I've just done a couple of dives at cenotes (sinkholes) in the Yucatan peninsula of Mexico. My favorite so far is Dos Ojos - extremely clear water, beautiful light, nice cave formations.

Friday, April 24, 2009

Carbon Tubes and Ribbons

Carbon nanoelectronics: unzipping tubes into graphene ribbons
We report on the transport properties of novel carbon nanostructures made of partially unzipped carbon nanotubes, which can be regarded as a seamless junction of a tube and a nanoribbon. We find that graphene nanoribbons act at certain energy ranges as a perfect valley filters for carbon nanotubes, with the maximum possible conductance. Our results show that a partially unzipped carbon nanotube is a magnetoresistive device, with a very large value of the magnetoresistance. We explore the properties of several structures combining nanotubes and graphene nanoribbons, demonstrating that they behave as optimal contacts for each other, and opening a new route for the design of mixed graphene/nanotube devices.

an extraordinary flare in the M87 jet

Hubble Space Telescope observations of an extraordinary flare in the M87 jet
HST-1, a knot along the M87 jet located 0.85 arcsec from the nucleus of the galaxy has experienced dramatic and unexpected flaring activity since early 2000. We present analysis of Hubble Space Telescope Near-Ultraviolet (NUV) imaging of the M87 jet from 1999 May to 2006 December that reveals that the NUV intensity of HST-1 has increased 90 times over its quiescent level and outshines the core of the galaxy. The NUV light curve that we derive is synchronous with the light curves derived in other wavebands. The correlation of X-ray and NUV light curves during the HST-1 flare confirms the synchrotron origin of the X-ray emission in the M87 jet. The outburst observed in HST-1 is at odds with the common definition of AGN variability usually linked to blazars and originating in close proximity of the central black hole. In fact, the M87 jet is not aligned with our line of sight and HST-1 is located at one million Schwarzchild radii from the super-massive black hole in the core of the galaxy.

Manta Rays

I've just returned from a wonderful experience with the Manta rays of San Benedicto Island, in the Pacific off the coast of Mexico. Thanks to Gregory Colbert, his film crew and friends, and the crew of the Nautilus Explorer for an amazing trip.

Saturday, April 11, 2009

Thermodynanics from Quantum Mechanics

Origin of the Canonical Ensemble: Thermalization with Decoherence
It is shown that a system embedded in a closed quantum mechanical system relaxes to its canonical ensemble equilibrium state if the environment drives this system to a fully decoherent state and the excess energy can release from the system to the environment or backward. Our findings show that the canonical ensemble is a special state that may result from pure quantum dynamics, suggesting that quantum mechanics may be regarded as the foundation of quantum statistical mechanics

Statistical mechanics is one of cornerstones of modern physics but its foundations and basic postulates are still under debate [1, 2, 3, 4]. There is a common believe that a generic “system” that interacts with a generic environment evolves into a state described by the canonical ensemble. Experience shows that this is true but a detailed understanding of this process, which is crucial for a rigorous justification of statistical physics and thermodynamics, is still lacking.

Classical to quantum transition

Classical to quantum transition of a driven nonlinear nanomechanical resonator
Much experimental effort is invested these days in fabricating nanoelectromechanical systems (NEMS) that are sufficiently small, cold, and clean, so as to approach quantum mechanical behavior as their typical quantum energy scale $\hbar\Omega$ becomes comparable to that of the ambient thermal energy $k_{B}T$. Such systems will hopefully enable one to observe the quantum behavior of human-made objects, and test some of the basic principles of quantum mechanics. Here we expand and elaborate on our recent suggestion [PRL 99 (2007) 040404] to exploit the nonlinear nature of a nanoresonator in order to observe its transition into the quantum regime. We study this transition for an isolated resonator, as well as one that is coupled to a heat bath at either zero or finite temperature. We argue that by exploiting nonlinearities, quantum dynamics can be probed using technology that is almost within reach. Numerical solutions of the equations of motion display the first quantum corrections to classical dynamics that appear as the classical-to-quantum transition occurs. This provides practical signatures to look for in future experiments with NEMS resonators.

An empirical Gravity law up to galactic scales

An empirical Gravity law up to galactic scales
Modelling on the MOND proposal, we introduce an empirically motivated gravity law for galactic and sub-galactic scales. This, circumvents the need for the cumbersome and ill-defined transition regime which exists in MOND, when passing from the standard gravitational regime at high accelerations to the MOND regime at low accelerations. We show that the proposed force law does not violate dynamical constraints at sub-galactic and solar system scales, does not degrade the good fit of the MOND proposal at large galactic scales, and in fact, slightly improves the accordance with observations at dSph scales. The proposed gravity law hence yields a good description of gravitational phenomena from terrestrial to large galactic scales, within a unique framework, without the need to invoke the presence of the still undetected and hypothetically dominant dark matter. Isothermal equilibrium density profiles then yield projected surface density profiles for the local dSph galaxies in very good agreement with observational determinations, for values of the relevant parameters as inferred from recent observations of these Galactic satellites. The observed scaling relations for these systems are also naturally accounted for within the proposed scheme.

The Origin of Labor Division

In Bert Hölldobler, E.O. Wilson: "The Superorganism: The Beauty, Elegance, and Strangeness of Insect Societies", p39.
... solitary bees behave like semisocial species when forced forced together experimentally. ... the coerced partners proceed variously to divide labor in foraging, tunneling and guarding. ... The division of labor appears to be the result of a preexisting behavioral groundplan in which solitary individuals tend to move from one job to another after the first is completed. In eusocial special, the algorithm is transferred to the avoidance of a job already being filled by another nestmate. It is evident that progressively providing bees and wasps are already "spring loaded" for a rapid shift to eusociality once ecological factors favor the change.

Friday, April 10, 2009

Flying Micro Robot

UW researchers develop world's first flying microrobot for microscale applications it flys using magnetic levitation.
Credit: University of Waterloo

Nothing to Be Frightened Of

The first chapter of Nothing to Be Frightened Of by Julian Barnes.
The exchange below is between Barnes and his brother, they are discussing their neice C. and her boyfriend R.
Instead, I asked, "But you approve of R.?"

"It's irrelevant," my brother replied, "whether or not I approve of R."

"No, it's not. C. might want you to approve of him."

"On the contrary, she might want me not to approve of him."

"But either way, it's not irrelevant to her whether or not you approve or disapprove."

He thought this over for a moment. "You're right," he said.

You can perhaps tell from these exchanges that he is the elder brother.

Wednesday, April 08, 2009

Mapa de Cuauhtinchan No. 2

A Lost World on the Map a review of
Cave, City, and Eagle's Nest: An Interpretive Journey Through the Mapa de Cuauhtinchan No. 2
edited by Davíd Carrasco and Scott Sessions
University of New Mexico Press, 479 pp., $65.00
The indigenous map is from the sixteenth century Valley of Puebla in Mexico.

Measuring measurement

Measuring measurement
Measurement connects the world of quantum phenomena to the world of classical events. It plays both a passive role, observing quantum systems, and an active one, preparing quantum states and controlling them. Surprisingly - in the light of the central status of measurement in quantum mechanics - there is no general recipe for designing a detector that measures a given observable. Compounding this, the characterization of existing detectors is typically based on partial calibrations or elaborate models. Thus, experimental specification (i.e. tomography) of a detector is of fundamental and practical importance. Here, we present the realization of quantum detector tomography: we identify the optimal positive-operator-valued measure describing the detector, with no ancillary assumptions. This result completes the triad, state, process, and detector tomography, required to fully specify an experiment. We characterize an avalanche photodiode and a photon number resolving detector capable of detecting up to eight photons. This creates a new set of tools for accurately detecting and preparing non-classical light.

Monday, April 06, 2009

The Strong CP Problem

CP Violation occurs in weak but not strong nuclear reactions. Why? In theory, there could be a nonzero CP violating angle theta, but experimentally it is zero. One way that shows up is the neutron electron dipole moment, which is also zero experimentally. One possible explanation is a new particle, the axion.

Sunday, April 05, 2009

Coronal Heating Problem

The temperature at surface of the sun is about 5800K, but the temperature of solar corona is millions of degrees kelvin. Why is the corona so hot? We don't yet know.
a solar coronal loop
The corona is a source for solar storms, in particular coronal mass ejections. The most powerful such storm on record was the solar storm of 1859.
Currently the sun is unusually quiet: Deep Solar Minimum.

Garbage Land

Garbage Land by Elizabeth Royte recounts the author's attempt to track the destiny of her household garbage and sewage.

The Bear Stearns Meltdown

I'm also reading House of Cards: A Tale of Hubris and Wretched Excess on Wall Street, about the Bear Stearns meltdown.

"when involved in a trade, look around the room and determine who the chump is and if the chump is not clear to you, assume it is YOU."

"So as an example, if a salesman and trader are talking about how they did a trade with a customer and they think there's a significant business opportunity that came of that trade, at Bear Stearns they might say, 'I just ripped that fucker's head off. I'm going to make a lot of money on this trade. That's fucking crazy.' But at Goldman Sachs a salesman and a trader would talk about, 'That's a great opportunity. That was a very attractive and commercial price you purchased those securities at and I think we'll have a very interesting economic opportunity in the near future.'

Whales Battle on the Silver Bank

There was a spectactular battle between two humpback whales on the Silver Bank this year (2009) here are the photographs.

The Baseline Scenario

The Baseline Scenario is a blog about the current global financial crisis, written by Simon Johnson and James Kwak. Here a couple of their other recent articles.
The Radicalization of Ben Bernanke in the Washington Post.
He is throwing trillions of dollars at the financial crisis. What happens if his gambles don't pay off?
The Quiet Coup in the Atlantic.
The crash has laid bare many unpleasant truths about the United States. One of the most alarming, says a former chief economist of the International Monetary Fund, is that the finance industry has effectively captured our government—a state of affairs that more typically describes emerging markets, and is at the center of many emerging-market crises. If the IMF’s staff could speak freely about the U.S., it would tell us what it tells all countries in this situation: recovery will fail unless we break the financial oligarchy that is blocking essential reform. And if we are to prevent a true depression, we’re running out of time.

Friday, April 03, 2009

The Global Oceanic Conveyer Belt

There's a nice simplifed explanation of global thermohaline ocean circulation at Oceanic Conveyer Belt. The rest of the site nicely explains other aspects of "ocean motion".

Social Motives for Syntax

Social Motives for Syntax in Science.
Book review of Origins of Human Communication by Michael Tomasello,
MIT Press, Cambridge, MA, 2008. 407 pp. $36, £23.95. ISBN 9780262201773.
Summary: Tomasello argues for the importance of social interactions in an evolutionary transformation from pointing and pantomiming to human vocal communication.

On the Origin of Flowering Plants

On the Origin of Flowering Plants
Summary: Which plant was the mother of all angiosperms? In the fourth essay in Science's series in honor of the Year of Darwin, Elizabeth Pennisi discusses efforts to answer Darwin's question about how flowering plants diversified and spread so rapidly across the globe.

Sleeping to Reset Overstimulated Synapses

Sleeping to Reset Overstimulated Synapses in Science.
Summary: In this week's issue of Science (pp. 105 and 109), two studies with fruit flies provide what some researchers say is the most compelling evidence to date for the provocative hypothesis that sleep dials down synapses that have been cranked up by a day's worth of neural activity, helping to conserve both energy and precious real estate in the brain.

The North Atlantic Oscillation and Medieval Climate

Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly in Science.
The Medieval Climate Anomaly (MCA) was the most recent pre-industrial era warm interval of European climate, yet its driving mechanisms remain uncertain. We present here a 947-year-long multidecadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. Supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker NAO conditions into the Little Ice Age (LIA). Globally distributed proxy data suggest that this NAO shift is one aspect of a global MCA-LIA climate transition that probably was coupled to prevailing La Niña–like conditions amplified by an intensified Atlantic meridional overturning circulation during the MCA.

Exploring the Quantum Vacuum with Lasers

Quantum Vacuum Experiments Using High Intensity Lasers
The quantum vacuum constitutes a fascinating medium of study, in particular since near-future laser facilities will be able to probe the nonlinear nature of this vacuum. There has been a large number of proposed tests of the low-energy, high intensity regime of quantum electrodynamics (QED) where the nonlinear aspects of the electromagnetic vacuum comes into play, and we will here give a short description of some of these. Such studies can shed light, not only on the validity of QED, but also on certain aspects of nonperturbative effects, and thus also give insights for quantum field theories in general.

A Solar Twin?

High-Dispersion Spectroscopic Study of Solar Twins: HIP 56948, HIP 79672, and HIP 100963
An intensive spectroscopic study was performed for three representative solar twins (HIP 56948, HIP 79672, and HIP 100963) as well as for the Sun (Moon; reference standard), with an intention of (1) quantitatively discussing the relative-to-Sun similarities based on the precisely established differential parameters and (2) investigating the reason causing the Li abundance differences despite their similarities. It was concluded that HIP 56948 most resembles the Sun in every respect including the Li abundance (though not perfectly similar) among the three and deserves the name of "closest-ever solar twin", while HIP 79672 and HIP 100963 have somewhat higher effective temperature and appreciably higher surface Li composition. While there is an indication of Li being rotation-dependent because the projected rotation in HIP 56948 (and the Sun) is slightly lower than the other two, the rotational difference alone does not seem to be so large as to efficiently produce the marked change in Li. Rather, this may be more likely to be attributed (at least partly) to the slight difference in T_eff via some T_eff-sensitive Li-controlling mechanism. Since the abundance of beryllium was found to be essentially solar for all stars irrespective of Li, any physical process causing the Li diversity should work only on Li without affecting Be.

See also Age and mass of solar twins constrained by lithium abundance

Neutron Stars

The Behavior of Matter under Extreme Conditions
The cores of neutron stars harbor the highest matter densities known to occur in nature, up to several times the densities in atomic nuclei. Similarly, magnetic field strengths can exceed the strongest fields generated in terrestrial laboratories by ten orders of magnitude. Hyperon-dominated matter, deconfined quark matter, superfluidity, even superconductivity are predicted in neutron stars. Similarly, quantum electrodynamics predicts that in strong magnetic fields the vacuum becomes birefringent. The properties of matter under such conditions is governed by Quantum Chromodynamics (QCD) and Quantum Electrodynamics (QED), and the close study of the properties of neutron stars offers the unique opportunity to test and explore the richness of QCD and QED in a regime that is utterly beyond the reach of terrestrial experiments. Experimentally, this is almost virgin territory.

Thursday, April 02, 2009

Local Void vs Dark Energy

Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae
It is now a known fact that if we happen to be living in the middle of a large underdense region, then we will observe an "apparent acceleration", even when any form of dark energy is absent. In this paper, we present a "Minimal Void" scenario, i.e. a "void" with minimal underdensity contrast (of about -0.4) and radius (~ 200-250 Mpc/h) that can, not only explain the supernovae data, but also be consistent with the 3-yr WMAP data. We also discuss consistency of our model with various other measurements such as Big Bang Nucleosynthesis, Baryon Acoustic Oscillations and local measurements of the Hubble parameter, and also point out possible observable signatures.

Antimatter Annihilation Anomalies

PAMELA and other experiments are searching for antimatter annihilation, as possible evidence for Dark Matter and other astrophysical phenomenon. These experiments have observed excess positron annihilation, but not antiprotons, which is inconsistent with many popular Dark Matter models. See also Dark Matter Annihilation and the PAMELA and ATIC Anomaly and Discriminate different scenarios to account for the PAMELA and ATIC data by synchrotron and IC radiation and Is the PAMELA anomaly caused by the supernova explosions near the Earth?
See also A hint of dark matter? in Nature.
Cosmic ray positrons are known to be produced in interactions in the interstellar medium. As well as originating from this 'secondary source', positrons might also be generated in primary sources such as pulsars and microquasars — or by dark matter annihilation. A new measurement of the positron fraction in the cosmic radiation for the energy range 1.5–100 GeV has been made using data from the PAMELA satellite experiment. Previous measurements, made predominantly by balloon-borne instruments, yield a positron fraction compatible with 'secondary source' production from interactions between cosmic ray nuclei and interstellar matter. Above 10 GeV the new measurements deviate significantly from this expectation, pointing to the presence of a primary source, either a nearby astrophysical object or dark matter particle annihilations.

The cosmic ray lepton puzzle in the light of cosmological N-body simulations
The PAMELA and ATIC collaborations have recently reported an excess in the cosmic ray positron and electron fluxes. These lepton anomalies might be related to cold dark matter (CDM) particles annihilating within a nearby dark matter clump. We outline regions of the parameter space for both the dark matter subhalo and particle model, where data from the different experiments are reproduced. We then confront this interpretation of the data with the results of the cosmological N-body simulation Via Lactea II. Having a sizable clump (Vmax = 9km/s) at a distance of only 1.2 kpc could explain the PAMELA excess, but such a configuration has a probability of only 0.37 percent. Reproducing also the ATIC bump would require a very large, nearby subhalo, which is extremely unlikely (p~3.10^-5). In either case, we predict Fermi will detect the gamma-ray emission from the subhalo. We conclude that under canonical assumptions, the cosmic ray lepton anomalies are unlikely to originate from a nearby CDM subhalo.

Radio Emissions from the Center of the Milky Way

Modeling Emission from the Supermassive Black Hole in the Galactic Center with GRMHD Simulations
Sagittarius A* is a compact radio source at the Galactic center, powered by accretion of fully ionized plasmas into a supermassive black hole. However, the radio emission cannot be produced through the thermal synchrotron process by a gravitationally bounded flow. General relativistic magneto-hydrodynamical(GRMHD) simulations of black hole accretion show that there are strong unbounded outflows along the accretion. With the flow structure around the black hole given by GRMHD simulations, we investigate whether thermal synchrotron emission from these outflows may account for the observed radio emission. We find that simulations producing relatively high values of plasma beta cannot produce the radio flux level without exceeding the X-ray upper limit set by Chandra observations through the bremsstrahlung process. The predicted radio spectrum is also harder than the observed spectrum both for the one temperature thermal model and a simple nonthermal model with a single power-law electron distribution. The electron temperature needs to be lower than the gas temperature near the black hole to reproduce the observed radio spectrum. A more complete modeling of the radiation processes, including the general relativistic effects and transfer of polarized radiation, will give more quantitative constraints on physical processes in Sgr A* with the current multi-wavelength, multi-epoch, and polarimetric observations of this source.

The Origin of Galaxies

Early assembly of the most massive galaxies
The current consensus is that galaxies begin as small density fluctuations in the early Universe and grow by in situ star formation and hierarchical merging. Stars begin to form relatively quickly in sub-galactic sized building blocks called haloes which are subsequently assembled into galaxies. However, exactly when this assembly takes place is a matter of some debate. Here we report that the stellar masses of brightest cluster galaxies, which are the most luminous objects emitting stellar light, some 9 billion years ago are not significantly different from their stellar masses today. Brightest cluster galaxies are almost fully assembled 4-5 Gyrs after the Big Bang, having grown to more than 90% of their final stellar mass by this time. Our data conflict with the most recent galaxy formation models based on the largest simulations of dark matter halo development. These models predict protracted formation of brightest cluster galaxies over a Hubble time, with only 22% of the stellar mass assembled at the epoch probed by our sample. Our findings suggest a new picture in which brightest cluster galaxies experience an early period of rapid growth rather than prolonged hierarchical assembly.

Wednesday, April 01, 2009

An Unusual Galaxy Cluster

Galaxy Zoo: an unusual new class of galaxy cluster
Thanks to the visual inspection of SDSS images afforded by the Galaxy Zoo project, we have identified a new class of galaxy clusters which possess number of unusual properties. These clusters are unusually elongated, possess young and highly dynamic galaxy populations, and most unexpectedly, present neatly typeset, leftjustified, messages written in the English language. One interpretation for the existence of these galaxy clusters is as conclusive evidence for intelligent life elsewhere in the universe. Conversely, however, they could indicate that many phenomena usually attributed to intelligent life on Earth actually occur spontaneously, without any thought necessarily being involved at all.