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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, January 17, 2007

Set-back for Dark Energy

Observational evidences suggest that the rate of expansion of the universe is increasing with time. This goes in contradiction to the expectation of some physicists that the finite energy of expansion would be continuously depleted by the gravitational attraction that holds the universe together. Some cosmologists tried to explain this increasing expansion with “dark energy” which may counteract the force of gravity at relatively short length scales – about 85 micrometres.

In order to explain the observed rate of expansion, dark energy must account for about 70% of all energy in the universe. But physicists still need a direct confirmation of its existence.

(photo of Dan Kapner, lead author of the paper; courtsey: the Eöt-Wash group )

In a recent paper in Physical Review Letters, a team of physicists from the Eöt-Wash group at the Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle reported their measurement of the force of gravity down to 55 micrometres and their conclusion that the inverse-square law remained valid well below 85 micrometres with 95% confidence. In a laboratory set-up, the scientists made very precise measurement of the gravitational attraction between two plates placed upon a torsion pendulum. Although a few other groups in various countries are engaged in such measurements, according to the Eöt-Wash researchers, their experiment offers the highest sensitivity at the length-scale associated with dark energy because it employs more interacting mass at the required separations than other setups.

Those who are familiar with such type of precision measurement will know that this puts a limit on the length-scale of any new type of interaction that can be theoretically predicted to exist. The experiment still does not rule out the existence of dark energy. But the potential implication of this experiment is very significant -- it's indeed a set-back for the theory of dark energy that could explain the increasing expansion of the universe.

Reference:
"Tests of the Gravitational Inverse-Square Law below the Dark-Energy Length Scale"
D. J. Kapner, T. S. Cook, E. G. Adelberger, J. H. Gundlach, B. R. Heckel, C. D. Hoyle, and H. E. Swanson,

Phys. Rev. Lett. 98, 021101 (8th January issue, 2007) Link to Abstract

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