Wave velocities and synchronized events
It has long been known theoretically that it is possible for the "
group velocity" of light to exceed
c.
[12] One recent experiment made the group velocity of
laser beams travel for extremely short distances through
caesium atoms at 300 times
c. In 2002, at the
Université de Moncton, physicist
Alain Haché made history by sending pulses at a group velocity of three times light speed over a long distance for the first time, transmitted through a 120-metre cable made from a coaxial
photonic crystal.
[13] However, it is not possible to use this technique to transfer
information faster than
c: the velocity of information transfer depends on the
front velocity (the speed at which the first rise of a pulse above zero moves forward) and the product of the group velocity and the front velocity is equal to the square of the normal speed of light in the material.
Exceeding the group velocity of light in this manner is comparable to exceeding the speed of sound by arranging people distantly spaced in a line, and asking them all to shout "I'm here!", one after another with short intervals, each one timing it by looking at their own wristwatch so they don't have to wait until they hear the previous person shouting. Another example can be seen when watching ocean waves washing up on shore. With a narrow enough angle between the wave and the shoreline, the breakers travel along the waves' length much faster than the waves' movement inland.
[edit] Light spots and shadows
If a laser is swept across a distant object, the spot of light can easily be made to move at a speed greater than
c.
[14] Similarly, a shadow projected onto a distant object can be made to move faster than
c. In neither case does any matter or information travel faster than light.