Event Cloak

I just bumped into this funny stuff [http://physicsworld.com/cws/article/indepth/46376]:

But imagine if we could make a cloak that operates not only in space but in time as well. To understand how such a "space–time" cloak might work, consider a bank housing a money-filled safe. Initially, all incoming light continuously scatters off the safe and its surroundings, revealing the rather dull scene of an undisturbed safe visible to surveillance cameras. But imagine, near some specified time, splitting all the light approaching the safe into two parts: "before" and "after", with the "before" part sped up, and the "after" part slowed down. This would create a brief period of darkness in the stream of illuminating photons. If the photons were a stream of cars on a motorway, it is as if the leading cars were to speed up and those trailing behind were to decelerate, creating a gap in the traffic edged by bunches of cars (a dark period with bright edges – see t₃ in figure 1).

Now imagine that during the moment of darkness, a safe-cracker enters the scene and steals the money, being careful to close the safe door before he leaves. With the safe-cracker gone, the process of speeding up and slowing down the light is reversed, leading to an apparently untouched, uniform illumination being reconstituted. As far as the light reaching the surveillance cameras is concerned, everything looks the same as it did beforehand, with the safe door firmly shut. The dark interval when the safe was cracked has literally been edited out of visible history.

To complete our motorway analogy, it is as if the cars have acted to first open up and then close a gap in traffic, leaving no disturbance in the flow of vehicles. There is now no evidence of that temporary car-free interlude, during which the proverbial chicken may even have crossed the road without getting squashed. So by manipulating how light travels in time around a region of space, we can, at least in principle, make a space–time cloak that can conceal events – an "event cloak", if you will.

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)

No physical signal travels faster than c

Einstein's special relativity theory stipulates that no physical signal (i.e., anything that carries energy and obeys physical laws and is measurable) can not go faster than the vacuum light speed. There have been many 'dissidents' (mostly crackpots) don't like this and want to disprove this law, but all have been defied. Now an experiment that was recently done in HKUST demonstrated that, even a single photon cannot break it.

Einstein taught us that the speed of light was the traffic law of the universe—nothing could go faster. The development of media in which atomic gases can slow down or speed up the passage of light pulses initially caused a stir, at least until the difference between phase velocity and group velocity could be carefully explained. But what about the behavior of single photons, the fundamental quanta of light? Reporting in Physical Review Letters, Shanchao Zhang and colleagues at the Hong Kong University of Science and Technology have shown that photons obey the law too.

Zhang et al. study optical precursors, which are signals preceding the main wave packet in a light pulse with a sharply rising leading edge (as in a step function pulse). Past work has shown that even in “superluminal” media where the group velocity may be faster than light speed, the precursor is always in front of the pulse. The authors extend this work to the single-photon level with the help of cold atomic gases: a photon generated in one rubidium gas traverses a second collection of rubidium atoms. With careful use of electromagnetically induced transparency, the researchers can separate the precursor from the main pulse and confirm it travels at the speed of light. The results add to our understanding of how single-photon signals propagate but also confirm the upper bound on how fast information travels. – David Voss [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.243602]

The UNiverse seems less smooth than theory

If so, there will need some new understanding. Since Einstein's application of his grand theory, GRT, to comprehending the cosmos, a lot of observations have been achieved during the past years, especially about the cosmological structure on large scales. This allows one to make relatively accurate estimate about the mass and energy distribution, by calibration with GRT. Great fitting has been found if dark matter and dark energy are presumed up till now, when people find that, the universe is much more clumpier than expected on larger scale.

Thomas et al. use publicly-released catalogs from the Sloan Digital Sky Survey to select more than 700,000 galaxies whose observed colors indicate a significant redshift and are therefore presumed to be at large cosmological distances. They use the redshift of the galaxies, combined with their observed positions on the sky, to create a rough three-dimensional map of the galaxies in space and to assess the homogeneity on scales of a couple of billion light years. One complication is that Thomas et al. measure the density of galaxies, not the density of all matter, but we expect that fluctuations of these two densities about their means to be proportional; the constant of proportionality can be calibrated by observations on smaller scales. Indeed, on small scales the galaxy data are in good agreement with the standard model. On the largest scales, the fluctuations in galaxy density are expected to be of order a percent of the mean density, but Thomas et al. find fluctuations double this prediction. This result then suggests that the universe is less homogeneous than expected. [http://physics.aps.org/articles/v4/47]

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.

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]