Einstein's theory of relativity tells us that nothing can travel faster than the speed of light - 30,000,000 metres per second. Any particle travelling faster than this cosmic speed limit would violate causality, leading to effects happening before their causes, which creates paradoxes.
So it came as a surprise when it was discovered that in certain materials light travels faster than the limit of 30,000,000 metres per second. The finding, reported here in Nature, raised concerns about whether it meant that information could be transmitted could be transmitted at greater than light speed - which, if true, would lead to the disturbing conclusion that it could be possible to send messages into the past.
Fortunately, a group of researchers at Hong Kong University have carried out the experiment using single photons, and shown that even though the group velocity of the wave is greater than c, individual photons travel at the usual speed of light in vacuum. This means that no information is transmitted at faster than light speed and causality is preserved.