Although the work of Dr Wang's team is remarkable, it is not the first time that this sort of "trick" has been performed - but it is certainly the most dramatic demonstration. Earlier this year, a team of physicists made a microwave beam travel 7% faster than light speed. Last year, they announced that they had even slowed light down to almost a crawl.
Anomalous refractive index
To achieve their peculiar effect, Dr Wang's group fired laser beams through a trap of caesium atoms. By adjusting the frequency of the laser beams to match those of the energy levels in the atoms, the researchers were able to achieve an effect called "anomalous refractive index." This boosts the pulses' so-called "group velocity" to a speed faster than what we understand to be the speed of light - just short of 300 million metres per second. The group velocity of a light pulse depends upon the mixture of frequencies within the pulse and the medium through which it travels. It need not be the speed of the pulse itself.
The important thing, however, is that whilst the group velocity can be manipulated to be faster than the speed of light, it is not possible to use this effect to send information faster than the speed of light. Because of the fast group velocity, the leading edge of the pulse appears to leave the caesium-filled chamber 62 billionths of a second before it arrives.
Causality principle
And according to Dr Wang, this strange result does not threaten Einstein's theories - in particular, the causality principle, which states that a cause must precede its effect. Or so almost all physicists think - for now. Privately, some admit that experiments such as Dr Wang's may force a reassessment of some cherished ideas. According to Dr Guenter Nimtz, of the University of Cologne, who has carried out similar experiments, the NEC work is very exciting. He told BBC News Online: "The effect cannot be used to go back in time, only to reduce the time between cause and effect a little bit. "The reason for this," he said, "is because the light pulse has a finite length of time, much longer than any gain obtained by a faster-than-light speed."
http://news.bbc.co.uk/2/hi/science/natur…