excellent. thank you q "The meaning of a negative group velocity is illustrated in figure 2. Within the cell, the peak of the pulse travels backwards relative to the direction it is moving in outside the cell. Long before the incident light pulse reaches the cell, two peaks appear at the far end: one travelling away from the cell at c, the other travelling back towards the entrance. This second pulse travels 300 times more slowly and is timed to meet up with the incident peak. The transmitted pulse travelling at c appears to leave the cell some 60 ns before the incident pulse arrives, enough time for it to travel an additional 20 metres. What is shocking is that such an effect has been observed for the first time without a great deal of attenuation, amplification or distortion of the pulse. It appears as though energy has, in fact, travelled faster than light." http://physicsweb.org/article/world/13/9/3 *the article then goes on to say how this did not really happen. (i obviously was not paying attention when i read it) Please Register or Log in to view the hidden image!