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Thursday, 24 February, 2000, 16:11 GMT
Putting the brakes on light
A Bose-Einstein condensate: It slows down light
A Bose-Einstein condensate: It slows down light
By BBC News Online science editor Dr David Whitehouse

You think that light is fast? Well, think again. Sometimes it is slower than a crawl.

All schoolchildren know that light is the fastest thing there is. It zips along through empty space at 297,000 km per second (186,000 miles a second). Light from the Sun takes about eight minutes to reach us, from the Moon just over a second, and two million years from the nearest galaxy.

But now a Danish physicist and her team of collaborators have found a way to slow light down to less than 1.6 km per hour (one mile an hour) - slower than a slow walk.

The researchers, led by Dr Lene Hau of the Rowland Institute for Science, and Harvard University, both in the US, said last year that they had slowed light down to 60 km per hour (38 mph). Now, they have gone even further.

Addressing a conference in the US, Dr Hau said that you could almost send out a beam of light, go for a cup of coffee and return in time to see the light come out of the other side of her equipment. "You could almost touch it," she added.

Slow light

The way Dr Hau and her team have slowed down light by a factor of 600 million or so is to use a group of atoms called a Bose-Einstein condensate (BEC). These atoms are cooled to a temperature of only a few billionths of a degree above absolute zero, the coldest possible temperature, at which all motion stops.

In a Bose-Einstein condensate, atoms are hardly moving at all. This means that according to the uncertainty principle that rules atoms, they are spread out and overlap. This results in a group identity for the collection of supercold atoms.

And when light passes through such an environment, it will slow down.

Physicists have known for a long time that the speed of light is reduced when it travels though any transparent medium, such as water or glass. Lenses, for example, focus light by allowing it to pass through different thicknesses, thereby slowing it down by differing amounts.

By firing co-ordinated beams of laser light through the BEC, Hau and colleagues have slowed light down to a crawl. Inside the BEC, the so-called refractive index (which measures the slowing of light) becomes enormous: as high as 100 trillion times greater than that of glass.

Slowing down light may have many practical uses in communications, signal processing, television displays and night-vision devices.

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14 Jan 99 | Sci/Tech
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