COSMIC ACCELERATORS DISCOVERED IN OUR OWN GALAXY
Analyzing the data from the largest cosmic ray detector in history, scientists from UCLA and Japan have discovered evidence of natural nuclear accelerators at work in our own Milky Way galaxy.
Cosmic rays of the highest energies were believed to come from remote galaxies hosting gigantic black holes capable of consuming stars and accelerating protons to "macroscopic" energies, comparable to that of a bullet from a modern rifle. However, earlier this year, Pierre Auger Observatory has published a surprising discovery: many of the energetic cosmic rays are, in fact, nuclei, not protons. And the higher the energy, the more nuclei per proton the observers detect. This was totally unexpected because the nuclei, more fragile than protons, tend to disintegrate into protons on their long journey through space. Moreover, it is very unlikely that a cosmic accelerator of any kind would accelerate nuclei better than protons at these high energies.
The resolution of the paradox came from the analysis to be published in an upcoming issue of Physical Review Letters. Stellar explosions in our own Galaxy can accelerate both protons and nuclei, but, while the protons leave the Galaxy promptly, the heavier and less mobile nuclei get trapped in the turbulent magnetic field and linger longer than protons. As a result, the local density of nuclei is increased, and they bombard Earth in greater numbers, as seen by the Pierre Auger Observatory.
Stellar explosions capable of accelerating particles to ultra‑high energies have been seen in other galaxies, where they produce gamma‑ray bursts. The new analysis provides evidence that such powerful explosions took place in our Galaxy as well, at least a few times per million years.
The ultra‑high energy nuclei observed today have been trapped in the web of Galactic magnetic fields for millions of years, and their arrival directions have been completely randomized by the numerous twists and turns in the tangled field. However, the researchers predict, the protons escaping from other galaxies should still be seen at the highest energies and should point back to their sources, providing Pierre Auger Observatory with a valuable data for charged‑particle astronomy.
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LR12740
Ultimate Laser power is limited by physical laws
During the last decades, the world’s maximal available intensity
of laser radiation has been growing impressively up to 10^22W/cm^2.
Now days there are some projects under discussion and funding which
are capable of its further increase in the nearest future. But are
there any natural fundamental limits for intensity of optical lasers?
In our paper we are showing that such limitations not only exist, but
will be very probably faced with the next generation of high‑intensity
laser facilities. The origin of limitation we are discussing in the
paper is related to electron‑positron pair production. It was believed
for a long time that such process in vacuum may become important only
at critical intensity level of 10^29W/cm^2, which is beyond the scope
of either present or perspective facilities. However, it was shown
recently, that since the focal spot of optical laser systems exceeds
the length scale, characteristic for QED, by many orders of magnitude,
the threshold for pair production from vacuum is generally lower than
it was commonly accepted before. Under special set‑up, this effect can
become observable already at the intensity level of 10^25‑10^26W/cm^2.
As intensity is increasing further, the pair production yield is growing
very sharply. Moreover, according to our paper, a new mechanism of pair
production, development of electron‑positron avalanche, must simultaneously
come into play. The latter effect can be viewed qualitatively in analogy
with the known phenomenon of breakdown in dielectric slab. As a result,
the avalanche of electron‑positron pair production must blow up the
focused laser field at intensities 10^26‑10^28W/cm^2 (depending on the
experimental set‑up), so that critical intensity becomes completely
inaccessible with optical lasers. Our conclusion confirms the early
conjecture of N. Bohr that critical QED electric field can never be created.
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LT12953
TESTABLE PREDICTIONS OF STRING THEORY
String theory has yet to make readily testable predictions in high energy physics or cosmology, but in this paper we invoke the ''stringy black hole/qubit correspondence'' to derive new results in the field of quantum information theory, that can in principle be tested in the laboratory. Einstein called the entanglement between two quantum bits (qubits) ''spooky action at a distance''. Qubit entanglement finds application in quantum computing, teleportation, cryptography and communication. In the apparently separate world of quantum gravity, the Hawking effect of radiating black holes has also occupied center stage. Despite their apparent differences, recent work by the authors has established a correspondence between the two, in which the Bekenstein-Hawking black hole entropy is related to the amount of three-qubit entanglement. In this paper, we take things one step further and use the classification of black holes in string theory to solve an outstanding problem in quantum information theory, namely the classification of four-qubit entanglement, a subject of recent interest to experimentalists.
