There exists a condition of
quantum entanglement. Two photons may be "
quantum entangled" if produced by the same
particle interaction and emerging in opposite directions. They stay
quantum entangled even when separated by vast distances (even
light-years apart). Two quantum-entangled
photons will make identical decisions when forced to choose among two equally probable pathways. They make the same decision at the same
instant. If there is some unknown
communication path between the two
photons, it has been observed to function at least SEVEN TIMES the
speed of light.
Another explanation, of course, is that the effect is completely instantaneous.