MadSci Network: Physics

Re: Slowing of speed of light?

Date: Thu Dec 6 15:20:01 2001
Posted By: Michael Wohlgenannt, Grad student, Department of Theoretical Physics , University of Munich, Germany
Area of science: Physics
ID: 1005783908.Ph

Hi Carl,
the experiments are referring to are indeed fascinating. For references, search, e.g., in google for "slow light". A lot of interesting documents can be found there.

What is actually slowed down is the group velocity, i.e., the velocity of propagation of a light pulse, a wave packet consisting of many different frequencies. Information is transmitted with this group velocity. In vaccuum the speed of light is approximately 300 000 km/s, that's the limiting speed. Nothing can go faster than that. However in a medium light travels at a slower speed, this is described by the refractive index. The refractive index depends on the frequency (colour) of the light wave. If you consider a light pulse consisting of waves with different frequencies, these waves will travel at different speeds in the medium and will be bent differently. The colours will be seperated. That's what we observe when we look at a rainbow or the light passing through a prisma.
The scientists at Rowland use a technique called "electromagnetically induced transparency", where they have coupled extremely cooled sodium atoms to an external laser field. The sodium atoms are at a temperature of about 4.10^-7 K (about -273 centigrade). These atoms are in a very peculiar state of matter, called Bose-Einstein condensate. Somehow all the atoms together form one collective quantum mechanical state. The laser pulse coupled to the condensate makes the otherwise opaque medium transparent for a narrow band of frequencies. The refractive index is rapidly varying with the frequency (of light) in this range. And this very rapid variation is the reason for the incredibly low speed of light propagation. The single frequencies do still move at a very high speed (phase velocity), but the group velocity which depends on the rate of variation of the refractive index is incredibly low, a few centimetres per second! This velocity depends on the temperature of the condensate. The lower the temperature the slower light propagation. However, light propagation cannot be halted, the speed of light in the medium cannot be reduced to 0, since the temperature of the condensate would have to reach the absolute zero point, which is not possible.

This is also related to the possibility of optical black holes. These are atoms contained in a liquid and swirling around very fas. They are able to snag photons. At least in this respect they would be similar to gravitational black holes. In order to do so, the atoms must circulate at a speed faster than the speed of light in the liquid. Slowing down the group velocity of a light pulse as discussed before, may render optical black holes possible.
I hope I could help you,

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