Researchers there have managed to bring a laser beam to a complete standstill as it passes through a very cold gas cloud 1 . They anticipate that the technique could be used to store information in the computers of the future. But this applies only to light travelling in a vacuum. When it passes through other materials, light is often slowed down, which is why a light ray bends as it passes from air to water or glass. This slowing is generally small, however, and light still passes through most materials at phenomenal speeds.
Light travels more slowly through materials that are said to have a large 'refractive index'.
The higher the index, the more the material's atoms interact with the light and slow its progress. Imagine trying to wade through a lake of syrup. If the 'syrup' is thick enough, motion becomes extremely sluggish. The syrup used by Lene Hau and colleagues is a gas of several million sodium atoms, cooled to within a few millionths of a degree of absolute zero. These atoms are held in a cloud within a magnetic 'atom trap'.