  Tunneling is an important physical process. The observation that particles
surmount a high mountain in spite of the fact that they don't have the
necessary energy cannot be explained by classical physics. However, this so
called tunneling became allowed by quantum mechanics. Experimental tunneling
studies with different photonic barriers from microwave frequencies up to
ultraviolet frequencies pointed towards a universal tunneling time
(Haibel,Esposito). Experiments and calculations have shown that the tunneling
time of opaque photonic barriers (optical mirrors, e.g.) equals approximately
the reciprocal frequency of the corresponding electromagnetic wave. The
tunneling process is described by virtual photons. Virtual particles like
photons or electrons are not observable. However, from the theoretical point of
view, they represent necessary intermediate states between observable real
states. In the case of tunneling there is a virtual particle between the
incident and the transmitted particle. Tunneling modes have a purely imaginary
wave number. They represent solutions of the Schroedinger equation and of the
classical Helmholtz equation. Recent experimental and theoretical data of
electron and sound tunneling confirmed the conjecture that the tunneling
process is characterized by a universal tunneling time independent of the kind
of field. Tunneling proceeds at a time of the order of the reciprocal frequency
of the wave.
