On The Verge of Controlling The Velocity of Light

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.

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