Cool Water Fountain

Here is a show of a very cute water fountain in Japan.

Think about the physics in it !

Space is much smooth on Planck scale

This finding might be the most stunning and most fundamentally interesting and important in the past tens of years. Many garish, dazzling yet gaudy, speculations can hardly withstand this finding.
The space is just so smooth ! [http://www.physorg.com/news/2011-06-physics-einstein.html]

Einstein’s General Theory of Relativity describes the properties of gravity and assumes that space is a smooth, continuous fabric. Yet quantum theory suggests that space should be grainy at the smallest scales, like sand on a beach.

One of the great concerns of modern physics is to marry these two concepts into a single theory of quantum gravity.

Now, Integral has placed stringent new limits on the size of these quantum ‘grains’ in space, showing them to be much smaller than some quantum gravity ideas would suggest.

According to calculations, the tiny grains would affect the way that gamma rays travel through space. The grains should ‘twist’ the light rays, changing the direction in which they oscillate, a property called polarisation.

High-energy gamma rays should be twisted more than the lower energy ones, and the difference in the polarisation can be used to estimate the size of the grains.

Philippe Laurent of CEA Saclay and his collaborators used data from Integral’s IBIS instrument to search for the difference in polarisation between high- and low-energy gamma rays emitted during one of the most powerful gamma-ray bursts (GRBs) ever seen.

GRBs come from some of the most energetic explosions known in the Universe. Most are thought to occur when very massive stars collapse into neutron stars or black holes during a supernova, leading to a huge pulse of gamma rays lasting just seconds or minutes, but briefly outshining entire galaxies.

GRB 041219A took place on 19 December 2004 and was immediately recognised as being in the top 1% of GRBs for brightness. It was so bright that Integral was able to measure the polarisation of its gamma rays accurately.

Dr Laurent and colleagues searched for differences in the polarisation at different energies, but found none to the accuracy limits of the data.

Some theories suggest that the quantum nature of space should manifest itself at the ‘Planck scale’: the minuscule 10-35 of a metre, where a millimetre is 10^-3 m.

However, Integral’s observations are about 10 000 times more accurate than any previous and show that any quantum graininess must be at a level of 10^-48 m or smaller.

“This is a very important result in fundamental physics and will rule out some string theories and quantum loop gravity theories,” says Dr Laurent.

Integral made a similar observation in 2006, when it detected polarised emission from the Crab Nebula, the remnant of a supernova explosion just 6500 light years from Earth in our own galaxy.

This new observation is much more stringent, however, because GRB 041219A was at a distance estimated to be at least 300 million light years.

In principle, the tiny twisting effect due to the quantum grains should have accumulated over the very large distance into a detectable signal. Because nothing was seen, the grains must be even smaller than previously suspected.

“Fundamental physics is a less obvious application for the gamma-ray observatory, Integral,” notes Christoph Winkler, ESA’s Integral Project Scientist. “Nevertheless, it has allowed us to take a big step forward in investigating the nature of space itself.”

Now it’s over to the theoreticians, who must re-examine their theories in the light of this new result.

Provided by European Space Agency (news : web)

Einstein's Theory STANDS UP

Usually quoted as the most refined brain child ever seen in human history, Einstein's general relativity theory has been frequently under test and it passed. The latest test was initiated some half a century ago and eventually came up with a result: it passed again ! [http://www.physicstoday.org/daily_edition/physics_update/gravity_probe_b_concludes_its_50-year_quest]

Results of an experiment conceived around 1960 to test general relativity and launched in 2004 were announced at a NASA press conference earlier this month: Albert Einstein’s theory passed. The experiment featured four (for redundancy) gyroscopes—spinning, niobium-covered spheres—orbiting 642 km above Earth (see the figure). The goal was to measure the precession induced in the gyroscopes by two general relativistic effects. The easier-to-measure geodetic effect influences any spinning object orbiting a mass. The second effect, frame dragging, arises when the spacetime-distorting mass, here Earth, is itself spinning. Gravity Probe B was not the first to measure the two effects, but it was designed to measure them independent of each other and to extraordinary precision. The gyroscopes are the most perfectly spherical objects ever fabricated. They needed to be, lest the general relativistic precessions be swamped by those arising from Newtonian torques. To measure the spin of those featureless spheres, the experimenters cooled them below niobium’s superconducting transition; the superconducting metal then produces a magnetic field, parallel to its spin axis, that can be measured with a superconducting quantum interference device. In the end, the experiment was a qualified success. It measured the geodetic effect to 0.3% precision, but stray charges on the gyroscopes and their housings limited the precision of the frame-dragging measurement to 20%. In both cases other efforts have achieved comparable results. (C. W. F. Everitt et al., Phys. Rev. Lett., in press.)—Steven K. Blau

Inaugural Article By C M Will

I really like the nice article by physicist Will. It is titled "On the unreasonable effectiveness of the post-newtonian approximation in gravitational physics". I think, it is accessible to anyone with a fundamental knowledge of general relativity (GR). Personally, I suddenly feel the study of GR so handy. Before I read this article, GR to me is quite like an abstract. This article shows me the experimental aspects in light of GR equations. OK. read it for yourself. [PNAS, 108:5938(2011)].

A TED talk by Janna Levin

I would like to embed the link to her talk. In this film, she talks of black holes , dynamics of space-time in the form of gravitational waves and the big bang. Especially, she discusses how such things can be heard. She presented to audience some astounding examples. I'm really impressed in the wonders of the pictures she brought there. They were actually made by numerical simulations.
That is marvelous !

The orbit of photons around black holes

Black hole distorts the space-time on its periphery drastically. This distortion is manifest in everything moving nearby, including photons. A photon is a spin-1 boson, and its orbit can be computed using geodesic equation. Due to the distortion, a photon shall gain excess angular momentum in the course of orbiting. And the trajectory can be very spiral, as this numerical study exposes [http://www.nature.com/nphys/journal/v7/n3/full/nphys1938.html?WT.ec_id=NPHYS-201103].

A photon emitted near a rotating black hole feels the ground beneath it swirl around. Try to run over a rotating surface, such as the platform of a merry-go-round, and you will not only find yourself fighting the Coriolis force; your body follows the rotation and you stagger and stumble. A photon does not stumble, but rotating spacetime can impart to it an intrinsic form of orbital angular momentum (OAM) distinct from its spin. Like other forms of orbital angular momentum, the photon's OAM is quantized by integer multiples of ħ, not just ±ħ. One can visualize OAM by the wavefronts of this twisted light⁷, which are not planar but rather resemble a cylindrical spiral staircase, centred around the light beam (Fig. 1). The intensity pattern of twisted light transverse to the beam shows a dark spot in the middle — where no one would walk on the staircase — surrounded by concentric circles. The twisting of a pure OAM mode can be seen in interference patterns, which show a fork-like structure of partially broken mirror symmetry.

How does black holes sette in galaxies

Every galaxy is believed to harbor a compact massive body-black hole- at its center. How it gets in? A natural way is through galaxy evolution based on the Big Bang model. However, there is the other way around, which is said being enigmatic [Nature,469:305].

Bulges and their black holes seem to be a natural consequence of structure formation in the hot Big Bang theory of the expanding Universe. According to this theory, galaxies grew by gravitational assembly of matter into clumps that gathered into larger clumps, and so on to galaxies. In galaxies with bulges, including ellipticals, which have bulges and no disks, the mass of the central black hole correlates not only with the mass of the bulge, but also, as Kormendy, Bender and Cornell¹ note ( page 374), with the average spread of velocities of the bulge stars (see Fig. 2a on page 375). The plausible explanation is that part of the gas out of which bulge stars formed settled instead near to the black hole, in part increasing its mass and in part fuelling explosions that blew the gas away and suppressed bulge-star formation. That is, the growth of bulge and black hole may have controlled each other. The timing looks right. Bulge stars are old: they formed when the expanding Universe was roughly a third of its present size (redshift about 2). This is when the rate of star formation per unit of matter was near its maximum (more than 10 times the present rate³). It is also when quasars — explosions powered by the central black holes — were most abundant (100 times more common than now⁴), probably an explosive result of overfeeding of the black holes as the early generations of stars were forming.

........

In theory, galaxies both with and without bulges were growing by the gravitational collection of clumps of matter when the star-formation rate was near its peak. That would suggest that the clumps contained stars; a recent discussion puts roughly comparable masses in stars and gas⁶. So where are these early generations of stars? Not in disks, because there is nothing that would slow the motion of a star to allow it to settle onto a disk. Bulges contain old stars, and it has been suggested that this is where the early stars ended up. But we now see that this is not plausible: why would these old stars have avoided our bulgeless Galaxy and settled instead in the bulge of our neighbour M31? Maybe the old stars are in diffuse stellar haloes. If so, it seems curious that the stellar halo of our Galaxy is much less prominent than that of M31. But more studies of other nearby galaxies will be required to check for inventories of stars that are old enough and abundant enough to account for stars that formed before disks.

How charge is renormalized by gravity

Electrical charge (the bare one), g, measures the coupling strength between electrons and photons. In QED, g is a constant. However, if interactions of QED fields with other fields (particles) are taken into account, the g shall be renormalized in the sense of renormalization group theory. In this article [doi:10.1038/nature09506], the author looks at how gravitational field renormalizes the g. In his treatment, there assumes a cutoff, below which the Einstein's theory is a reasonable starting point for quantization. Going through the usual RG procedures, he arrives at the statement that, gravity results in QED asymptotic freedom at high energy scales: g tends to zero at very large energy.

The first term on the right hand side of equation (12) is that present in the absence of gravity (found by letting kR0) and results in the electric charge increasing with energy. The second termis the correction due to quantum gravity. For pure gravity with L50, or for a small value of L as suggested by present observational evidence40, the quantum gravity contribution to the renormalization group b-function is negative and therefore tends to result in asymptotic freedom, in agreement with the
original calculation13.

Still Quiet is Dark Matter

Cosmological observations suggest the existence of dark matter, which has not shown any traces of interacting with known baryonic matter. Yet, dark matter comprises over 80% of the total matter needed to explain the space-time structure. Scientists have not a clue regarding the nature of these matter. One proposal says they may be made of sort of particles, the so-called WIMPs (weakly interacting massive particles). Various experiments have been devised to detect them. NO positive results exist up to now. A latest effort came in PRL, still no activities of these particles detected. They are really quiet, should they be there. [Phys. Rev. Lett. 105, 131302 (2010)]

The XENON100 experiment, in operation at the Laboratori Nazionali del Gran Sasso in Italy, is designed to search for dark matter weakly interacting massive particles (WIMPs) scattering off 62 kg of liquid xenon in an ultralow background dual-phase time projection chamber. In this Letter, we present first dark matter results from the analysis of 11.17 live days of nonblind data, acquired in October and
November 2009. In the selected fiducial target of 40 kg, and within the predefined signal region, we observe no events and hence exclude spin-independent WIMP-nucleon elastic scattering cross sections above 3:4 10 44 cm2 for 55 GeV=c2 WIMPs at 90% confidence level. Below 20 GeV=c2, this result
constrains the interpretation of the CoGeNT and DAMA signals as being due to spin-independent, elastic, light mass WIMP interactions.

The Compositions of Neutron stars

Does a neutron star comprise primarily of neutrons and protons or there are some other particles ? Both options have been used to construct models to describe the behaviors of neutron stars. A great difference between these two options is that, they yield different maximum star masses. For a star of largely protons and neutrons, the mass can be larger, because including other matter will soften the star in response to gravitational field. Recently, a group studied a pulsar, which is a neutron star and has a companion [doi:10.1038/4671057a]. This group measured the so-called Shapiro delay and has determined with high precision the masses of both the pulsar and its companion. The as-measured mass is 1.97+/-0.04 times the solar mass. Such a massive star can hardly be harbored by models containing matter other than protons and neutrons [Lattimer, J. M. & Prakash, M. Nucl. Phys. A 777, 479–496 (2006). ].

The Shapiro delay is caused by the gravitation of the companion: the spinning pulsar emits pulses regularly and this pulse passes by the companion on the journey to the earth, and the companion distorts the space-time nearby and makes a time delay. This delay is expected periodic, since the pulsar is moving around the companion. This enables the determination of the masses.

Quantum grativity in its present status

This is a nice article that was originally used as lecture notes in the Stockholm workshop. It gives a concise account of the present understanding of quantum gravity and and the arguments go at the heart. It also maps the planing for experimental search. In addition, rich references can be found.
[http://arxiv.org/PS_cache/arxiv/pdf/1010/1010.3420v1.pdf]

Curved space generating mass ?

Since Newtonian times, physicists have to talk of mass, a quantity having its origin a deep myth. In Newtonian mechanics, mass is impedes the change of velocity. In relativity, mass (in the conventional sense) is no more than the static energy. In non-relativistic quantum mechanics, mass plays to make the entity more of a particle. In relativistic quantum mechanics, mass is the energy required to generate a pair of electron and positron. In condensed matter physics, mass is the minimum energy to excite a system. Besides, mass also measures some correlation length.

Although we know many things about mass, we don't have a clear clue where mass comes from. In the standard model, all masses are produced by Higss mechanism: every mass-less particle moves in some kind of ether that is the Higgs clouds and acquires mass. Another idea is, mass can be generated by curved space, or more accurately, compactified dimension. Compactifying a dimension yields finite motion, one that is confined. According to quantum mechanics, finite motion implies discrete levels and finite gaps, so comes the mass. Yet, a clear regime is missing.

Graphene provides a playground for studying this regime. These authors roll the graphene and obtain a massive 1D system from a 2D massless Dirac system [http://arxiv.org/ftp/arxiv/papers/1010/1010.3437.pdf]. This is no surprising and actually was known before. But this is an example showing how mass might be generated this way. However, back to elementary particle physics, where is the hidden dimension in addition to the 4D space-time we are all used to ? Another question is, how the as-obtained masses interact in a gravitational way ? Anyway, mass should be gravitationally active !!!!

A source for learning GRT

Einstein's theory has amazed many people including the inventor himself. As he said, anyone who comprehends it could not help being charmed by its beauty. Unfortunately, this excitement is still not conveyed in a lot of college courses. Here is a link by which one may learn this theory himself.
http://math.ucr.edu/home/baez/gr/

Cosmic censorship violation in 5D

Singularities null the predictability of a physical theory. This issue may be avoided by positing the cosmic censorship, which states that, all singularities are hidden behind the horizons and can not be seen from the rest of space-time. However, this although seems comfortable, but no more than a proposal. Counter examples have already existed [http://en.wikipedia.org/wiki/Cosmic_censorship]. Here is another counter example found in 5D space-time [Phys. Rev. Lett. 105, 101102 (2010)].

NO dark matter detected, yet

Without a wisp of exaggeration, the greatest myth in present physics might be about the so-called dark matter and dark energy. Physicists, fairly speaking, for the moment have not even the slightest definite clue about them. They were motivated for two observations: (1) the rotation velocity of a typical galaxy does not follow the pattern based on Newton's theory; (2) the universe is expanding faster and faster. Fact (1) leads to proposal of dark matter while (2) to that of dark energy. Interestingly, the dark energy term was first hypothesized by Einstein, who later on dismissed it for Hubble's discovery, to find a static and stable universe. This energy never dilute in the course of expansion. It permeates everywhere. People don't know where it comes from, although some suggested it might be vacuum energy (calculations rejected this idea). As regards the dark matter, it is usually hypothesized as some undetected particles other than baryons. They interact extremely weakly with visible matter. Some suggest these might be the so-called Weakly Interacting Massive Particles that are predicted by supersymmetric theory. Detectors have been mounted to settle this issue. A latest survey reports a failure [Phys. Rev. Lett. 105, 131302 (2010) ].

Although the above dark matter idea is popular, it is quite dubious to some physicists, who don't like extra assumptions. In 2004, a German group did a study which reveals running gravitational constant that goes bigger at astronomical scales [Physical Review D 70: 124028 (2004)]. This study might null the necessity of dark matter.

Electrons on the surface of topological insulators

Topological insulators are featured with massless surface states that are protected from impurity scattering. Electrons in such states move on the surface, which are usually curved. How would this curvature affect the motions ? This article [Phys. Rev. B 82, 085312 (Published August 12, 2010)] shows that, the electrons shall feel gravity-like force.

Electron scattering in solids is normally associated with impurities, defects, lattice vibrations, and electron-electron Coulomb scattering. Now, in an article published in Physical Review B, Jan Dahlhaus and collaborators from the Instituut-Lorentz at the University of Leiden in the Netherlands show that for surface electrons on a topological insulator, electron scattering can be dominated by a completely different mechanism: geodesic scattering. Geodesics are the generalization of straight lines in curved space. In general relativity, gravitational fields curve four-dimensional spacetime, and particle motion follows geodesic lines shaped by gravity. Strong enough fields cause the phenomenon known as gravitational lensing, an observable deflection of massless particles such as photons.

The surface electrons of a topological insulator behave as massless particles and are constrained to move in a two-dimensional curved space. The curvature is caused by random surface deformations that appear naturally during the growth of the material. Such a bump on the surface acts like a gravitational lens for surface electrons, resulting in trajectories that are analogous to geodesic motion. Considering that due to the special nature of topological insulators these surface electrons are protected from the ubiquitous impurity backscattering, this article likely reveals a previously unsuspected and important contribution to the resistivity on the surface of these materials. – Athanasios Chantis

2D on the planck scale ?

Carlip, a cosmologist at University of California, in his just submitted preprint argued that, the universe at the imaginably smallest scale is perhaps 2D, one being space while the other being time. He called this dimensional reduction. In his scenario, this reduction is random and non-systematic, and thus indicating Lorentz invariance violation: going in different direction encounters distinct physics. He called for further investigations. Here I give a roster of some key concepts:

(1)Definition of dimension by random walk: "In particular, the return probability K(x, x, s) is
K(x, x; s) ∼ (4πs)−dS/2." Here ds is just the dimension, and K(x,x,s) gives the return probability of a random walker in space s.

(2)Some claimed evidences:

• Causal Dynamical Triangulations;
• Renormalization Group Analysis;
• Loop quantum gravity;
• High temperature strings;
• Anisotropic scaling models

(3)Wheeler-DeWitt equation, which is the Schodinger equation for metric tensor.
(4)Strong coupling limit.

At much smaller scales, on the other hand, the proper description is far less obvious.
While clever experimentalists have managed to probe some features down to distances close to the Planck scale [2], for the most part we have neither direct observations nor a generally accepted theoretical framework for describing the very small-scale structure of spacetime. Indeed, it is not completely clear that “space” and “time” are even the appropriate categories for such a description. But while a complete quantum theory of gravity remains elusive, we do have fragments:
approximations, simple models, and pieces of what may eventually prove to be the correct theory. None of these fragments is reliable by itself, but when they agree with each other about some fundamental property of spacetime, we should consider the possibility that they are showing us something real. The thermodynamic properties of black holes, for example, appear so consistently that it is reasonable to suppose that they reflect an underlying statistical mechanics of quantum states. Over the past several years, evidence for another basic feature of small-scale spacetime has been accumulating: it is becoming increasingly plausible that spacetime near the Planck
scale is effectively two-dimensional. No single piece of evidence for this behavior is in itself very convincing, and most of the results are fairly new and tentative. But we now have hints from a number of independent calculations, based on different approaches to quantum gravity, that all point in the same direction. Here, I will summarize these clues, provide a further piece of evidence in the form of a strong-coupling approximation to the Wheeler- DeWitt equation, and discuss some possible implications.

Black hole theory relevant to non-fermi liquid problem ?

Although I have not fully read it, this interesting report has captured my curiosity. It is said that an idea emerging in gravitational theory, in the strong limit and the weak limit, provides insights in the strange metal behaviors of cuprate superconductors.

Fermi liquid theory explains the thermodynamic and transport properties of most metals. The so-called non-Fermi liquids deviate from these expectations and include exotic systems such as the strange metal phase of cuprate superconductors and heavy fermion materials near a quantum phase transition. We used the anti–de-Sitter/conformal field theory correspondence to identify a class of non-Fermi liquids; their low-energy behavior is found to be governed by a nontrivial infrared fixed point, which exhibits nonanalytic scaling behavior only in the time direction.
For some representatives of this class, the resistivity has a linear temperature dependence, as is the case for strange metals.

SCIENCE VOL 329:1043 (2010)

Resource Letter PTG-1: Precision Tests of Gravity

A package of current research topics on experimental gravity.

This resource letter provides an introduction to some of the main current topics in experimental tests of general relativity as well as to some of the historical literature. It is intended to serve as a guide to the field for upper-division undergraduate and graduate students, both theoretical andexperimental, and for workers in other fields of physics who wish learn about experimental gravity. The topics covered include alternative theories of gravity, tests of the principle of equivalence, solar-system and binary-pulsar tests, searches for new physics in gravitational arenas, and tests of gravity in new regimes, involving astrophysics and gravitational radiation.

Simulating metric signature effects with metamaterials

A funny work here.

We demonstrate that the extraordinary waves in indefinite metamaterials experience an effective metric signature. During a metric signature change transition in such a metamaterial, a Minkowski space-time is created together with a large number of particles populating the space-time. Such metamaterial models provide a tabletop realization of metric signature change events suggested to occur in Bose-Einstein condensates and quantum gravity theories.