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Signals Exceed Light’s Speed

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Dr. Flavio Fontana

An established concept in physics -- the impossibility of transmitting a signal faster than the speed of light in a vacuum -- is again up for debate. Various teams have transmitted superluminal light pulses, but experiments at the Italian Council for Scientific Research have achieved such feats in a less complex system and over a greater distance.

Other researchers have reported superluminal experiments, but they were typically related to evanescent wave tunneling phenomena. A common objection to such experiments is that the apparent superluminal propagation is actually a reshaping of the pulse, caused by the different attenuation of the leading edge with respect to the peak. The latest experiment, which sends signals through free space rather than through highly absorbing materials, is immune to this criticism.

An experimental setup used Bessel beams to achieve superluminal propagation in free space.

Anedio Ranfagni and his team detailed their technique in the May 22 issue of Physical Review Letters. They have achieved a distance of 30 wavelengths. The work deals with an exotic type of electromagnetic field distribution known as a Bessel beam, which is known to propagate over very large distances without significant diffraction. Its group velocity can be higher than the speed of light, at least in a zone relatively near the source.

Though the experiments could exploit optical lasers, the Italian researchers used microwave signals -- with a wavelength of 3.48 cm -- to make the superluminal effect more evident. The scientists placed an annular slit in the focus of a large metallic mirror and illuminated it with a collimated beam of microwaves. They placed a microwave receiver downstream to measure -- at different distances from the mirror -- the group delay of a modulation superimposed on the microwave carrier.

The result is consistent with the theory of Bessel beams: The signal seems to propagate 5.3 percent faster than the light in the region where the correct field distribution typical of Bessel beams can be expected. Because of the limited region of space where the effect is evident, this line of research is of more interest to fundamental physics than to communications.

Photonics Spectra
Jul 2000
CommunicationsResearch & TechnologyTech Pulse

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