It's not the first time that physicists claim that the speed of light can be modified, and even exceed the theoretical limit called c without violating Einstein's laws of relativity (check for example this article from two years ago). Now, researchers from the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Lausanne, Switzerland, claim that light can travel faster than light!. They were able to control the speed of light in an off-the-shelf optical fiber. They said that they did "slow a light signal down by a factor of 3.6 (or about 71,000 km/s), creating a sort of temporary "optical memory." On the other hand, they also did create "extreme conditions in which the light signal travelled faster than 300 million meters a second." As they don't give any numbers for this upper limit, you have to trust them. Anyway, these results are important because they were achieved using off-the-shelf optical fibers, opening the way for future super fast all-optical routers. Update (August 22, 2005): Luc Thévenaz sent me insightful comments about this post. You'll find them at the end of this entry.
So what have done Luc Thévenaz and his fellow researchers in the EPFL's Nanophotonics and Metrology laboratory (page in French)?
The telecommunications industry transmits vast quantities of data via fiber optics. Light signals race down the information superhighway at about 186,000 miles per second. But information cannot be processed at this speed, because with current technology light signals cannot be stored, routed or processed without first being transformed into electrical signals, which work much more slowly. If the light signal could be controlled by light, it would be possible to route and process optical data without the costly electrical conversion, opening up the possibility of processing information at the speed of light.
This is exactly what the EPFL team has demonstrated. Using their Stimulated Brillouin Scattering (SBS) method, the group was able to slow a light signal down by a factor of 3.6, creating a sort of temporary"optical memory." They were also able to create extreme conditions in which the light signal travelled faster than 300 million meters a second. And even though this seems to violate all sorts of cherished physical assumptions, Einstein needn't move over – relativity isn't called into question, because only a portion of the signal is affected.
Anyway, the real value of this research doesn't come from light travelling faster than c, but from light travelling slower.
Slowing down light is considered to be a critical step in our ability to process information optically. The US Defense Advanced Research Projects Agency (DARPA) considers it so important that it has been funnelling millions of dollars into projects such as"Applications of Slow Light in Optical Fibers" and research on all-optical routers. To succeed commercially, a device that slows down light must be able to work across a range of wavelengths, be capable of working at high bit-rates and be reasonably compact and inexpensive.
The EPFL team has brought applications of slow light an important step closer to this reality. And Thévenaz points out that this technology could take us far beyond just improving on current telecom applications. He suggests that their method could be used to generate high-performance microwave signals that could be used in next-generation wireless communication networks, or used to improve transmissions between satellites.
The research work has been published by Applied Physics Letters in its August 22, 2005 issue under the name "Optically controlled slow and fast light in optical fibers using stimulated Brillouin scattering" (Volume 87, Issue 8, Article 081113). Here is a link to the abstract which is reproduced below for your convenience.
We demonstrate a method to achieve an extremely wide and flexible external control of the group velocity of signals as they propagate along an optical fiber. This control is achieved by means of the gain and loss mechanisms of stimulated Brillouin scattering in the fiber itself.
Our experiments show that group velocities below 71 000 km/s on one hand, well exceeding the speed of light in vacuum on the other hand and even negative group velocities can readily be obtained with a simple benchtop experimental setup. We believe that the fact that slow and fast light can be achieved in a standard single-mode fiber, in normal environmental conditions and using off-the-shelf instrumentation, is very promising for a future use in real applications.
In this abstract, as in the news release, the researchers give a number for "group velocities" slower than c, but not a single one for those faster than c. I wonder why...
Update (August 22, 2005): Here are Luc Thévenaz's comments in reaction to the above note, which he nicely allowed me to reproduce.
Most of your comments are right, just be aware that what is really important for applications is delaying and advancing a signal, not the real speed of propagation. This makes possible a synchronisation of optical signals, that was impossible to realize so far with a control by light.
You look very suspicious about our capability to propagate faster than the speed of light in vacuum and you wonder why we mentioned no figure about this. Hmmm, I think you were a bit lazy and you did not read entirely our APL article. The answer is in the 3 last paragraphs, read carefully. We state clearly that we could achieve an infinite and even negative group velocity! We even show a graph of our measurements. We also give explanations why this does not break the principles of relativity and causality in the next paragraph and information still propagates slowlier than the vacuum light velocity.
I just want to mention that what we have just reported experimentally was already predicted theoretically and fully explained during the 1910s by Leon Brillouin and Arnold Sommerfeld. Nothing new and no paradox, there is nothing magic behind and no theory needs to be revisited.
Finally, Luc sent me a copy of the full APL paper. Here is a link to this paper (PDF format, 3 pages, 75 KB).
Sources: EPFL news release, August 19, 2005; and various web sites
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6:57:18 PM
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