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Eletric pulses faster than light
Ian Crawford, a British astronomer from the University
College London hypothesized in 1995 that ETs could be
playing hide-and-seek with humanity, becoming invisible
by travelling faster than light. They could use "worm
holes" as shortcuts, circumventing the light speed
limit by distorting the fabric of space-time itself, as
imagined by Skip Thorne and Michael Morris from Caltech,
they could use some kind of "warp" (a "warp
drive" is a moving segment of space-time), or they
could turn matter into tachyons, which would be "born"
already travelling beyond light speed, and then somehow
recover its original state. But let´s talk about
information.
| The issue of September
2000 of Physics in Action presented "the observation
of a light pulse leaving a gas-filled chamber before
it had even arrived", and stated that this sparked
a media frenzy. Yet the laws of physics have remained
intact. The journal said that although nothing can
travel faster than light, it was time to reexamine
what we mean by "nothing", and that the
peak of the pulse is simply not the kind of "thing"
to which Einstein´s famous law applies. |
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The explanation is that the overall velocity (or "group
velocity") of an optical pulse passing through a
medium is determined by the way the refractive index varies
for the different frequencies that make up the pulse.
The refractive index increases with frequency, and this
"normal" dispersion reduces the group velocity
below c, but the behavior of the light pulse is very different
closer to the absorption line, where the refractive index
decreases with increasing frequency. This behavior leads
to a so-called anomalous dispersion in which the sign
of the delay changes, which means that the group velocity
can exceed c.
Now Applied Physics Letter presents in its January 2002
issue the description of an experiment by Alain Haché
and Louis Poirier, from the University of Moncton in Canada,
in which pulses that travel faster than light have been
sent over a significant distance for the first time. They
transmitted the pulses through a 120-meter cable made
from a "photonic crystal". To create their cable
the Canadian researchers joined together five-meter sections
of coaxial cable with alternating electrical impedances.
Radiation in the frequency range 9-11 MH is partially
reflected at the boundaries of these segments, which gives
the cable its absorption band.
The explanation of the second paragraph applies, when
"anomalous dispersion" can occur in a certain
range of wavelenghts. The refractive index on either side
of this absorption band changes sharply with wavelenght,
and in these regions the components of radiation at the
tail of the pulse interefere destructively and the peak
of the wave is effectively pushed forward, as it is explained
in PhysicsWeb. Haché says that many existing information
systems are based on coaxial cables, but the current top
speed for data is just two-thirds the speed of light.
If the impedance of such cables were adapted, as they
did in their "photonic crystal" coaxial cable,
pulses sent at frequencies close to the absorption band
could transmit information at speeds approaching that
of light. 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. And they explain, it would
be impossible to transmit information faster than light
because it would be encoded onto a single frequency component,
or, as stated in another place, the relativistic notion
of simultaneity makes it clear that no information can
travel faster than light without throwing all our concepts
of cause and effect in disarray, since most physicists
still believe that cause needs to precede effect.
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