Neutrino not that fast !!

I like this blog entry by Zz. He highlighted a recent article that did not see data indicating superfast!

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

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]

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)].

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.

Relativity starts your automobile

I have already mentioned this interesting work. But it is so funny as to deserve a further click from Nature [469:269]:

Cars are started using lead–acid batteries, which generate energy using electrochemical reactions between lead compounds and sulphuric acid. Rajeev Ahuja of Uppsala University in Sweden and his colleagues modelled the reactions and found that as electrons move at high speed around a lead nucleus, their energy levels change owing to relativity. The authors conclude that the change accounts for 1.7–1.8 volts of a standard 2.13-volt lead–acid cell.

Relativity and the Lead-Acid Battery

This relation may seem at first glance quite unusual. For most, relativity needs be concerned only when the considered speed becomes comparable to that of light. The Lead -acid battery seems having no attachment to this speed. However, if you look into the electrons that are busy inside the substance, you may change your mind. Lead is a heavy element, and relativity effects, as you can derive from Dirac's equation, scale as the quartic power of the atomic numbers. That is how relativity plays a role, which proves crucial for your cars to start, in Lead-Acid Battery [http://prl.aps.org/pdf/PRL/v106/i1/e018301].

The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at nonrelativistic, scalar-relativistic, and fully relativistic levels, and using several exchange correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7–1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

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.

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/

Daily relativistic effects

Albert Einstein came up his special and general relativity theory in 1905 and 1915, respectively. They have withstood various experimental tests, most amongst which are performed at high energy or on huge scale. This is because the SRT has significance usually at high speed while the GRT for massive objects. But, this is not the case any more. Actually, modern technologies have counted SRT for much long time in GPS. While for GRT, the impacts seem much far from daily life. Nevertheless, this recent work [Science 329 (5999), 1676-b.] made a breakthrough. It made use of atomic clocks whose accuracy can be achieved of 10^(-17) second.

Due to the lack of an accessible optical transition in ²⁷Al⁺ for efficient laser cooling and state detection, precision spectroscopy of these ions uses techniques developed in quantum information science. Here, an Al⁺ ion is sympathetically cooled through its Coulomb interaction with an auxiliary "logic" ion that is simultaneously held in the same linear radio-frequency (RF) Paul trap (14). The logic ion also helps prepare and detect the internal state of the Al⁺ ion via quantum logic protocols. In this work, the two Al⁺ clocks used a beryllium (⁹Be⁺) ion (14) and a magnesium (²⁵Mg⁺) ion (11), respectively, as the logic ion. The Al^{+ 1}S₀³P₀ clock transition with frequency f₀ near 1.121 PHz has a narrow (f = 8 mHz) natural linewidth and a corresponding intrinsic quality (Q) factor of f₀/f = 1.4 x 10¹⁷ that permits high sensitivity for detecting small frequency-shifting effects. However, the observed linewidth for the clock transition is limited by the linewidth of the probe laser. We probed the clock transition with a subhertz linewidth laser referenced to a high-finesse optical cavity (15). In the Al-Mg clock, with 300 ms probe duration we obtained a narrow, Fourier transform–limited linewidth, realizing a Q factor of 4.2 x 10¹⁴ with nearly 80% contrast (Fig. 1). This high-Q line provides the basis for high-stability clock operation and sensitivity to small frequency shifts.

Special relativity comes to the help

In an ideal non-relativistic fluid without viscosity no circulation can be generated from none. This fact makes a challenge in cosmology: the genesis of the magnetic field ubiquitous to galaxies. In this Letter [PRL 105, 095005 (2010)], however, the authors were able to show how this theorem can break because of special relativity. They show that, even for an ideal dynamic fluid, circulation can arise naturally without the help of ad hoc commotions.

We demonstrate that a purely ideal mechanism, originating in the space-time distortion caused by the demands of special relativity, can break the topological constraint (leading to helicity conservation) that would forbid the emergence of a magnetic field (a generalized vorticity) in an ideal nonrelativistic dynamics. The new mechanism, arising from the interaction between the inhomogeneous flow fields and
inhomogeneous entropy, is universal and can provide a finite seed even for mildly relativistic flows.

Visualizing relativity theory

(1) For anyone interested in illustrating relativity effects graphically, this is a good website:

http://www.spacetimetravel.org/

(2) Here is a news article illuminating the discovery of dark energy:

http://physicsworld.com/cws/article/indepth/31908

Doppler shifts tells how a star moves

Doppler effects used in astronomy.

Pulsar bursts move 'faster than light'

Don't be confused by this claim. It means nothing in violation of causality or relativity, according to which none physical signal travels faster than vacuum light speed. A physical signal is a physical object in a particular state. This object can be detected physically and carries energy that can be exchanged with matter. Many unphysical things (such as a shadow) can be faster, but none physical signals in this definition.

Every physicist is taught that information cannot be transmitted faster than the speed of light. Yet laboratory experiments done over the last 30 years clearly show that some things appear to break this speed limit without upturning Einstein's special theory of relativity. Now, astrophysicists in the US have seen such superluminal speeds in space – which could help us to gain a better understanding of the composition of the regions between stars.

Superluminal speeds are associated with a phenomenon known as anomalous dispersion, whereby the refractive index of a medium (such as an atomic gas) increases with the wavelength of transmitted light. When a light pulse – which is comprised of a group of light waves at a number of different wavelengths – passes through such a medium, its group velocity can be boosted to beyond the velocity of its constituent waves. However, the energy of the pulse still travels at the speed of light, which means that information is transferred in agreement with Einstein's theory.

Now, astrophysicists claim to have witnessed this phenomenon in radio pulses that have travelled from a distant pulsar.

Modified pulses

The discovery has been made at the University of Texas at Brownsville, where Frederick Jenet and colleagues have been monitoring a pulsar – a rapidly spinning neutron star – more than 10,000 light years away. As pulsars spin, they emit a rotating beam of radiation that flashes past distant observers at regular intervals like a lighthouse. Because the pulses are modified as they travel through the interstellar medium, astrophysicists can use them to probe the nature of the cosmos.

Several factors are known to affect the pulses. Neutral hydrogen can absorb them, free electrons can scatter them and an additional magnetic field can rotate their polarization. Plasma in the interstellar medium also causes dispersion, which means pulses with longer wavelengths are affected by a smaller refractive index.

Timing is off

Jenet's group thinks that anomalous dispersion should be added to this list. Using the Arecibo Observatory in Puerto Rico, they took radio data of the pulsar PSR B1937+21 at 1420.4 MHz with a 1.5 MHz bandwidth for three days. Oddly, those pulses close to the centre value arrived earlier than would be expected given the pulsar's normal timing, and therefore appeared to have travelled faster than the speed of light.

The cause of the anomalous dispersion for these pulses, according to the Brownsville astrophysicists, is the resonance of neutral hydrogen, which lies at 1420.4 MHz. But like anomalous dispersion seen in the lab, the pulsar's superluminal pulses do not violate causality or relativity because, technically, no information is carried in the pulse. Still, Jenet and colleagues believe that the phenomenon could be used to pick out the properties of clouds of neutral hydrogen in our galaxy.

'Solid result'

"It seems to be very interesting indeed...a solid and rather nice result," says Michael Kramer, an astrophysicist at the University of Manchester who was not involved with the study.

Andrew Lyne, a pulsar astrophysicist who is also based at Manchester, thinks it is an "interesting, if not unexpected result". However, he has doubts that it could help in the understanding of neutral-hydrogen clouds because there are often several clouds in the same line of sight. "It is not clear from the paper quite what extra information will be obtained," he adds.

The research will be published in the Astrophysical Journal. A preprint is available at arXiv:0909.2445v2.