It follows from the special theory of relativity that mass and energy are different manifestations of the same thing. Now researchers have stopped light cold - and then brought it back to life - or light – in an entirely different place. The Harvard researchers sent a light pulse into a sodium cloud cooled to about 460 degrees below zero. That's the theoretical point where matter stops all movement. The light pulse was compressed by a factor of about 50 million at the extreme temperature, and it actually changed state - sort of like frozen water. But it also split into two forms of matter. One stayed frozen in the sodium, while the other crawled along at just 200 meters per hour. That second form - called a matter wave - contains the exact information of the original light wave. And when researchers sent it into a second sodium cloud a fraction of a milimeter away, the wave was converted back into light. And since fiber-optic communications use light, that means that someday we may be able to stop them, store them and turn them back on, just like a light switch.