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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Monday, June 19, 2006

Searching CPT Violation

At a fundamental level the nature is ruled by a law of symmetry called the CPT invariance. C stands for charge, P for parity and T for time. So, if we start with a collection of interacting elementary particles and create a mirror-image version (Parity reversal), change the signs on all electrical charges (C) of those particles, and let time run backward, the interactions among the particles would still remain identical to what it was before this CPT transformation.

Since CPT involves space and time operations, any evidence of violation of CPT invariance would mean disruption of he spacetime symmetries embodied in relativity theory and would have far-fetching effect on basic Physics and cosmology. In effect, it would mean that space is not 'isotropic' or that not all directions in space are equivalent. There would be one special direction in space, and the propagation of electromagnetic radiation would be different in different directions depending on its orientation relative to this special direction.

Photo credit: NASA/WMAP Science Team.
WMAP has produced a detailed picture of the infant universe. Colors indicate "warmer" (red) and "cooler" (blue) spots. The white bars show the "polarization" direction of the oldest light.

To look for this effect, scientists at the National Astronomical Observatories in Beijing carefully studied recent measurements of the cosmic microwave background radiation, the remnant of the energy that originated from the big bang and now surrounds the space around us in all directions at approximately 2.9 degree Kelvin. They used data from the satellite-borne Wilkinson Microwave Anisotropy Probe (WMAP) and the BOOMERANG instrument, flown by balloon over Antarctica. These data sets include detailed measurements across the sky of the direction of the electric field, or polarization, of the Cosmic Microwave Background. If CPT symmetry is violated, then all polarizations will rotate with respect to the special space direction as the radiation travels through the universe. In that case the distribution of polarization of all spatial points would not be just random but there would be a specific type of correlation between two points influenced by the existence of such a special direction.

The Chinese team has found signs of this correlation in their analysis, but statistically speaking, it is not yet significant enough and could have arisen from random chance. More detailed investigation would be needed but it's certainly a good brave step in the direction of that more detailed exploration.

Here is the paper: "Searching for CPT Violation With Cosmic Microwave Background Data From WMAP and BOOMERANG", B. Feng, M. Li, J. Q. Xia, X. Chen, and X. Zhang, Phys. Rev. Lett. 96, 221302 (issue of 9 June 2006)

Further Study:
WMAP's introductory page on Cosmic Microwave background
Wikipedia page on Cosmic Microwave Background

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