Neutrino not that fast !!

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

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.

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]

Light almost stopped

This is not a new piece of work [1] to be spoken of here. It was released some a decade ago. The reason I recalled it here is because of one possible interesting demonstration with extremely slow light. Before discussing the idea, I'd like to quote the abstract of [1] below :

Techniques that use quantum interference effects are being
actively investigated to manipulate the optical properties of
quantum systems1. One such example is electromagnetically
induced transparency, a quantum effect that permits the propagation
of light pulses through an otherwise opaque medium2±5.Here
we report an experimental demonstration of electromagnetically
induced transparency in an ultracold gas of sodium atoms, in
which the optical pulses propagate at twenty million times slower
than the speed of light in a vacuum. The gas is cooled to
nanokelvin temperatures by laser and evaporative cooling6±10.
The quantum interference controlling the optical properties of
the medium is set up by a `coupling' laser beam propagating at a
right angle to the pulsed `probe' beam. At nanokelvin temperatures,
the variation of refractive index with probe frequency can
be made very steep. In conjunction with the high atomic density, this results in the exceptionally low light speeds observed. By
cooling the cloud below the transition temperature for Bose±
Einstein condensation11±13 (causing a macroscopic population of
alkali atoms in the quantum ground state of the con®ning
potential), we observe even lower pulse propagation velocities
(17ms-1) owing to the increased atom density. We report an
inferred nonlinear refractive index of 0.18 cm2W-1 and ®nd that
the system shows exceptionally large optical nonlinearities, which
are of potential fundamental and technological interest for quantum
optics.

Now let me talk about a possible use of the above work. As we know, there is interactions between photons, although such interactions are usually very weak. In vacuum, such interaction happens at about a frequency inversely proportional to the eighth power of bare (vacuum) light speed, c. The interactions can be understood in either quantum electrodynamics or classical ones (QED or CED). For simplicity, let's illustrate this with CED. According Maxwell equations, the EM fields are produced by sources, which are electrical charges and currents. These charges and currents come from of course matters (vacuum is also a kind of matter in the quantum sense, that vacuum can also interaction with other things). The whole system is thus a complex of both matter and EM fields. The laws governing the dynamics of this global system are expressed as two schools of equations: Maxwell ones and Newton's equations, the former telling EM fields how to behave while the latter dictating the matter how to behave. These two schools of equations are coupled: (1)Maxwell equations containing sources in terms of variables of matter; (2)Newton's equations containing forces coming from EM field strengths. Now if one express the matter variables in terms of EM fields through solving Newton's equations, one is able to obtain a highly nonlinear and anharmonic equations for EM fields by substituting the sources in terms of EM fields as obtained. The anharmonicity directly results in photon-photon interaction, which means, the superposition principle does not hold exactly true. Such anharmonicity is quite hard to capture in usual experiments, because it is of the order of 1/c^8 in the vacuum. On the other hand, if we can reduce light speed, such interaction shall increase. Now my idea is this: fire two light beams opposite to each other upon the sodium gas as described in [1], and these two beams shall linger in the gas due to slowing down and interact repeatedly, via generating electron-hole pairs, much the same way as in vacuum. The calculations can be easily done.

[1]Light speed reduction to 17 metres per second in an ultracold atomic gas
Lene Vestergaard Hau*², S. E. Harris³, Zachary Dutton*²
& Cyrus H. Behroozi*§
* Rowland Institute for Science, 100 Edwin H. Land Boulevard, Cambridge,
Massachusetts 02142, USA
² Department of Physics, § Division of Engineering and Applied Sciences,
Harvard University, Cambridge, Massachusetts 02138, USA
³ Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305,
USA

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.