tunneling, quantum-mechanical effect by which a particle can penetrate a barrier into a region of space that would be forbidden by ordinary classical mechanics. Tunneling is a direct result of the wavelike properties of particles; the wave associated with a particle "decays" through a barrier—the amplitude decreases—but the amplitude of the wave on the other side of the barrier is large enough that there is a finite probability of finding the particle there. The theory of tunneling has been successfully applied to understand alpha decay, in which a heavy nucleus decays into a lighter nucleus and emits an alpha particle (helium nucleus).
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