A
classical particle or system
could not penetrate regions in which its energy
would be negative, that is, barrier regions in which the potential energy is greater than the system
energy. In the real world, however, a wave function of significant amplitude may
extend into and beyond such a region. If the wave function extends into another region of
positive energy,
the barrier is crossed with some probability; this process is termed
tunneling (since the barrier is penetrated rather than climbed)
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