The speed of light in a vacuum is a constant: 300,000km a second. However, light does not always travel through a vacuum. In water, for example, photons travel at around three-quarters that speed.
In nuclear reactors, some particles are forced up to very high speeds, often within a fraction of the speed of light. If they are passing through an insulating medium that slows light down, they can actually travel faster than the light around them. When this happens, they cause a blue glow, known as Cherenkov Radiation, which is comparable to a sonic boom but with light.
Incidentally, the slowest light has ever been recorded traveling was 17 meters per second â€“ about 38 miles an hour â€“ through rubidium cooled to almost absolute zero, when it forms a strange state of matter called a Bose-Einstein condensate.
Scientists at the University of Darmstadt in Germany have stopped light for one minute. For one whole minute, light, which is usually the fastest thing in the known universe and travels at 300 million meters per second, was stopped dead still inside a crystal. This effectively creates light memory, where the image being carried by the light is stored in crystals. Beyond being utterly cool, this breakthrough could lead to the creation of long-range quantum networks â€” and perhaps, tantalizingly, this research might also give us some clues on accelerating light beyond the universal speed limit.
To stop light, the German researchers use a technique called electromagnetically induced transparency (EIT). They start with a cryogenically cooled opaque crystal of yttrium silicate doped with praseodymium. A control laser is fired at the crystal, triggering a complex quantum-level reaction that turns it transparent. A second light source is then beamed into the now-transparent crystal. The control laser is then turned off, turning the crystal opaque. Not only does this leave the light trapped inside, but the opacity means that the light inside can no longer bounce around â€” the light, in a word, has been stopped.
With nowhere to go, the energy from the photons is picked up by atoms within the crystal, and the â€œdataâ€ carried by the photons is converted into atomic spin excitations. To get the light back out of the crystal, the control laser is turned back on, and the spin excitations are emitted at photons. These atomic spins can maintain coherence for around a minute, after which the light pulse/image fizzles. In essence, this entire setup allows the storage and retrieval of data from light memory.