One issue is that the speed of light is 30 cm per nanosecond. This limits the size of processors that can run at, say, GHz speeds. It's also a limitation of wir

One issue is that the speed of light is 30 cm per nanosecond. This limits the size of processors that can run at, say, GHz speeds. It's also a limitation of wires, which is one reason why the original Cray supercomputer had such a funky geometry. When I was in grad school, the nuclear physics lab had racks of cables of different lengths that were conveniently labeled in nanoseconds.