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For many years, US physicists have been
experimenting with something which intrigues Liz and I. They have
been slowing down light, which we all have been taught travels at a
constant velocity. Initially, in the late 1990's a team at Harvard
University slowed it from its constant
velocity in vacuum, 299,792km/s (186,282mps) to a leisurely 61km/h
(38mph). Then,
in 2001 they stopped it altogether.
The
experiments involve shining light into a cloud of sodium atoms trapped
in a vacuum and cooled to just above absolute zero (-273 Celsius), the
theoretical state of zero heat. At
this temperature the atoms coalesce to form what is know as a
Bose-Einstein condensate, an exotic quantum entity first predicted by
Albert Einstein and created in the lab in 1995. A second laser
tuned the tiny atomic cloud to slow the pulse of light. In 2001,
working with a team from the
Harvard
Smithsonian
Center
for Astrophysics, the same group brought light to a halt, by slowly
turning off the second control laser, which has very exciting prospects
and also, we would suggest raises some interesting questions for
astronomy.
In
the latest experiments, instead of using just one cloud of sodium atoms,
the group used two, a fraction of a millimetre apart. Effectively
the two atom clouds were separated and had never seen each other
before. A pulse of light was shone on the first cloud, impressing
a "cast" of the pulse into a clump of spinning sodium atoms,
nudged in the direction of the second condensate. This
slowly-moving clump was composed entirely of sodium atoms, effectively
turning light into matter. Once the "messenger" group
had merged with the second cloud, a second laser was shone through the
condensate to revive the original pulse of light.
From
a standing start, the reconstructed beam sped back up to the normal
speed of light. Analysis showed that it possessed exactly the same shape
and wavelength as the original beam, although it was slightly
weaker.
There
are fascinating implications for computers, micro-control and optical
storage devices,
because instead of light shining through optical fibres into boxes full
of wires and semiconductor chips, intact data, messages, and images
could be read directly from the light.
Implications
for astronomy? Well, we assume the expansion of the Universe is
real, and that assumption is based predominantly on
red-shift,
the reddening of galactic light on its millions of years journey towards
us. What if something else happened to the light on way?
What if something else made the light look redder than we would
have expected? We have always assumed it couldn't, but we do know
that a large proportion of the mass of the Universe is made up of
something we can't even see - dark matter. So there's an awful lot
of "stuff" out there about which we know nothing. If a
Bose-Einstein condensate in a laboratory on the third rock from the sun
can stop light in its tracks, goodness only knows what dark matter might
be able to do. |