  Tunnelling processes are thought to proceed via virtual waves due to observed
superluminal (faster than light) signal speeds. Some assume such speeds must
violate causality. These assumptions contradict, for instance, superluminally
tunnelled music and optical tunnelling couplers applied in fiber communication.
Recently tunnelling barriers were conjectured to be cavities, wherein the
tunnelled output signal is not causally related with the input. The tests
described here resolve that tunnelling waves are virtual, propagations are
superluminal, and causality is preserved.
