FTL goes Coax

From: Spudboy100@aol.com
Date: Fri Jan 25 2002 - 21:47:57 MST

I have never been sure what such claims really mean, however...


Electrical pulses break light speed record
22 January 2002 Pulses that travel faster than light have been sent over a
significant distance for the first time. Alain Haché and Louis Poirier of the
University of Moncton in Canada transmitted the pulses through a 120-metre
cable made from a coaxial 'photonic crystal'. The achievement raises hopes
that data could travel through electronic communications systems at almost
the speed of light (A Haché and L Poirier 2002 Appl. Phys. Lett. 80 518).

When a pulse of radiation travels through a 'dispersive' medium, different
wavelengths in the pulse move at different speeds and the pulse becomes
distorted. Ordinary dispersion arises when the refractive index of a material
changes with increasing wavelength. This stretches out the pulse and reduces
the group velocity - the speed at which the peak of the pulse travels. But
'anomalous dispersion' can occur in materials that absorb radiation in a
certain range of wavelengths. The refractive index on either side of this
absorption band changes sharply with wavelength. In these regions, the
components of radiation at the tail of the pulse interfere destructively, and
the peak of the wave is effectively pushed forward. To create their cable,
the Canadian researchers joined together five-metre sections of coaxial cable
with alternating electrical impedences. Radiation in the frequency range 9 -
11 MHz is partially reflected at the boundaries of these segments, which
gives the cable its absorption band. Haché and Poirier sent electromagnetic
pulses with frequencies between 5 and 15 MHz through the cable, and found
that the group velocity reached three times the speed of light for
frequencies in the absorption band. Haché and Poirier emphasize that their
experiment does not break any laws of physics. Although the group velocity
exceeds the speed of light - an effect permitted by relativity - each
component of the pulse travels slower than light. It would be impossible to
transmit information faster than light because it would be encoded onto a
single frequency component. But as Haché explains, many existing information
systems are based on coaxial cables, and the current top speed for data is
just two-thirds the speed of light. If the impedance of such cables were
adapted, pulses sent at frequencies close to the absorption band could
transmit information at speeds approaching that of light. 'Oddly, the reason
no one has done this before is that we are using what most people are trying
to avoid - the back-reflection caused by impedance mismatch', Haché told
PhysicsWeb. 'But as far as superluminal propagation goes, this is the key'.
Katie Pennicott is Editor of PhysicsWeb


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