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21st Century

- Feature Story -


Slow Light? Fast Light? E=MC²?

January 22, 2001

1) Scientists Bring Light to a Halt

Physicist Lee Hau, heading a research team from the Rowland Institute for Science, announced last year that his team reduced the speed of a beam of light down to one mile per hour using super-cooled sodium atoms.

Now, the same research team and another team from the Harvard-Smithsonian Center for Astrophysics have brought light to a halt.

The Rowland-Harvard team used a dispersion of chilled sodium gas to store a laser pulse within electron orbitals. The gas is excited by another laser to promote electromagnetically induced transparency. Once the test pulse of light is within the medium, the transparency-inducing laser is cut off. The pulse remains within the gas as excited electron orbital states. When the transparency-inducing laser is turned back on, the pulse is reformed by electrons dropping their excited orbitals. Restored light remains representative of the original pulse characteristics for about one second before stored orbitals begin to decay.

This technique may lead to future applications processing information for quantum computers. In itself, it's a curiosity, but if a modified process could be adapted to crystalline structure, the potential for data storage might be significant, particularly if the state of each crystal in a matrix could be electrically or optically set and read as desired.

2) Light Goes Faster Than C

Numerous successful experiments to exceed Einstein's constant C - the speed at which light traverses a vacuum - have been set up using "gain tubes" or superluminal propagation chambers.
superluminal tube layout

In the diagram, a pulse of light is fired at source A. At B, the pulse enters the superluminal propagation chamber. Light exits the tube at C.

For a given set of conditions, we assign a group velocity index of n(g). Our chamber's speed of light propagation is v. The speed of light constant is c. Time is T. Time @ location B is T1. Time at location C is T2. The length from B to C is L.

Then v(g)=c/n(g) within the superluminal tube.
Delta T=L/v(g)-L/c so Delta T=(n(g)-1)L/c
n(g) is less than 1 whenever v exceeds c [faster than Einstein's light speed in a vacuum], so (n(g)-1) is negative.
So our Delta T [time] from the entrance of light at B until the light exits at C is negative.

The light pulse exits at C before it enters the tube at B. Time flow is negative as the pulse propagates. This has been confirmed by laboratory measurements.

An interesting thought experiment would be to place a mirror with a pinhole at B, place a full mirror at C, and allow the light to bounce back and forth while traveling backwards in time. (Note: this is a standard gas laser layout except for the source.)

If the output pulse occurs BEFORE the input source is fired, can a sufficiently fast computer watch for an output pulse then decide NOT to fire the source lamp? What would happen?

Is this a sum of possibilities problem and how does a Copenhagen Interpretation, i.e. the simultaneous universes view, deal with such time flow problems?


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