The Huygens' Principle,named after Christiaan Huygens, is an interesting propertry of the solutions of the wave equation:
You define the influence area of a point x at time t in Rn as the set
I(x,t) = { y in Rn | u(y,t) is not constant if the inital conditions are canged at x }. That's the set of points which depend on x at time t. If you solve the wave equation you would notice that for n = 1 or n even I(x,t) is a closed ball. But for n > 1 and n odd, it's a sphere.
This fact makes communication in the real world much easier: The signals (light or sound, anything transmitted by waves in space) reaching you from a point x at time t are the only signals you receive from that point. That's because we live in a world with 3 space dimensions (odd, >1 !). With 4 space dimensions this would be problematic: all signals from the past would interfere with new signals and after some time it might be troublesome to spot the new information in this mess.