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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" )

Saturday, June 07, 2008

Black Holes at the End of the World

Ulf Leonhardt [photo credit: Maud Lang]

[This is an invited article based on an ongoing work led by the author. -- 2Physics.com]

Author: Ulf Leonhardt
Affiliation: School of Physics and Astronomy, University of St Andrews, Scotland

Black Holes are the remainders of supermassive stars that have collapsed under their own weight, but now scientists at the University of St Andrews are using lasers and fibre optics to simulate black holes in the laboratory. They want to test Professor Stephen Hawking’s prediction that black holes are not black after all but glow in the dark.

According to an ancient legend, the Scottish university and golfing town of St Andrews is “the end of the world”. In the 6th century, Saint Regulus, a Greek monk, saw a vision: a dream commanded him to bury the bones of Saint Andrew at the end of the world. So he sailed up the coast of Britain in search of the right place and finally found the perfect spot, St Andrews in fact. Now a small team at the University of St Andrews are using fibre optics and lasers to create artificial black holes at this end of the world. To be absolutely clear: the experiment is perfectly safe. No harm will happen, because these artificial black holes only exist as tiny flashes of light that race through a few inches of optical fibre and are gone when they leave the fibre. The team wants to fulfil a modern type of prophecy, a vision of theoretical physics.

In 1974 Professor Stephen Hawking at Cambridge University published a famous prediction about black holes and quantum physics. Astrophysical black holes are the remainders of collapsed stars. They swallow everything that comes in their way. Their gravity is so strong that not even light can escape. And yet, as Professor Hawking’s flash of insight showed, black holes are not perfectly black; they glow in the dark. However, this Hawking radiation of black holes is so faint that there is probably no chance of ever observing it in space.

Hawking’s theoretical vision has been the stuff of modern legends, because it shows a mysterious connection between various branches of physics, between the physics of the very large, astrophysics, and the physics of the very small, quantum mechanics. According to quantum mechanics, the world is teeming with virtual processes where Nature tries out many things, before some of them turn into reality. At the event horizon of a black hole, virtual light particles are turned into real ones, light is created from nothing, which then radiates into space as Hawking radiation.

The St Andrews team, led by Professor Ulf Leonhardt and Dr Friedrich König, is creating artificial black holes made of light. These creatures resemble real black holes, but they are much smaller (and a lot safer), they have no gravity, but they affect light like their big astrophysical cousins. Professor Leonhardt has been working for a decade on developing and testing ideas of how to engineer optical devices that make Hawking radiation observable. Now he believes he has found the perfect method. In performing and analysing this experiment the scientists hope to understand more about the way Nature creates light quanta at the horizon, something from nothing.

The figure illustrates the principal idea of the experiment. A light pulse in a fibre adds a small contribution to the refractive index, as if an additional piece of glass would be added. This fictitious piece of glass moves with the pulse; so it moves at the speed of light: pulses in fibres behave like materials moving at the speed of light. Imagine that a continuous wave of light follows the pulse, light with a different wavelength. Due to optical dispersion, the velocity of light depends on the wavelength. Suppose that the continuous probe wave is faster than the pulse, but is slowed down by it. The place where the speed of the probe equals the speed of the pulse is the horizon.

Professor Leonhardt put all his eggs in one basket and convinced others to contribute as well. The "start-up capital" for the experiment came from a private donation by Leonhardt Group AG, the corporation of Ulf Leonhardt's cousins Uwe and Helge. Both are businessmen from the Ore Mountains (Erzgebirge) in former East Germany, close to the Bohemian border (at another end of the world). In the less than 20 years after the fall of the Wall they have created quite something from nothing, a multinational corporation. The Leverhulme Trust financed the theory, a charity of Unilever that supports innovative research in the sciences and arts, and that also supported Leonhardt's work on invisibility devices. After the foundations had been laid, the Engineering and Physical Sciences Research Council UK took over. The first results of the team have recently appeared [1], but it will still take time, hard work and further financial support until a legend may become reality at the “end of the world”.

Further information: http://www.st-andrews.ac.uk/~ulf/fibre.html

[1] "Fiber-Optical Analog of the Event Horizon"
Thomas G. Philbin, Chris Kuklewicz, Scott Robertson, Stephen Hill, Friedrich König, Ulf Leonhardt,
Science 319, 1367 (2008). Abstract Link.

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