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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" )

Wednesday, May 17, 2006

Beyond Big Bang

Abhay Ashtekar (photo courtsey: Pennsylvania State Univ)

The universe started with a bang about 15 billion years back and went on expanding since then. Classical theories like Einstein's general theory of relativity can hope to explain the beginning upto a time very close to the actual 'bang' at which not only matter but space-time itself was born. The process of this understanding has been underway for many years now with the aid of careful observation of light and other kinds of electromagnetic radiation coming from deep sky and by devising clever theories to explain those observations.

But classical theories offer no clues about existence before that moment. Recently a research team at Penn State led by Prof. Abhay Ashtekar (Holder of the Eberly Family Chair in Physics and Director of the Institute for Gravitational Physics and Geometry at Penn State) has used quantum gravitational calculations to find threads that lead to an earlier time. The team showed that, prior to the Big Bang, there was a contracting universe with space-time geometry that otherwise is similar to that of our current expanding universe. Using quantum modifications of Einstein's cosmological equations, they have shown that as gravitational forces pulled this previous universe inward, it reached a point at which the quantum properties of space-time caused gravity to become repulsive, rather than attractive and instead of a sudden classical 'big bang', a 'quantum bounce' took place.

The idea of another universe existing prior to the Big Bang might have come naturally to many physicists or even non-physicists before this research, but this is the first systematic mathematical description that establishes its existence and also deduces properties of space-time geometry in that universe.

The research team used loop quantum gravity, a leading approach to the problem of the unification of general relativity with quantum physics -- an idea developed by Ashtekar in late 80s. The theory treats the space-time geometry itself as a discrete 'atomic' structure which in macroscopic scale looks like a continuum. This basic concept is not really unfamiliar to us -- We must remember that even our body and all solid things are actually made of 'void' at an atomic scale because most part of an atom is just empty space between orbiting electrons and a tiny nucleus (a nucleus is about 10000th times smaller than a typical atom in which it resides).

According to Ashtekar (but pardon our simplification), the fabric of space is literally woven by one-dimensional quantum threads. Near the Big-Bang, this fabric gets distorted by strong effects of gravitation and the quantum nature of geometry becomes important. It makes gravity strongly repulsive and gives rise to the Big 'Quantum' Bounce.

This research is reported in the current issue of Physical Review Letters. The paper is authored by Ashtekar and two of his post-doctoral researchers, Tomasz Pawlowski and Parmpreet Singh.



At 8:45 AM, Blogger oceanskies79 said...

This is a good read. Thanks.


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