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).

scanning tunneling microscope, device for studying and imaging individual atoms on the surfaces of materials. The instrument was invented in the early 1980s by Gerd Binnig and Heinrich Rohrer, who were awarded the 1986 Nobel prize in physics for their work. The underlying principle of the microscope is the tunneling of electrons between the sharp tip of a probe and the surface of the sample under study. The flow of electrons is extremely sensitive to the distance between the tip and the sample. As the tip is swept over the surface the height of the tip is continually adjusted so as to keep the flow of electrons constant. A map of the "bumps" on the surface is then obtained by accurately recording the height fluctuations of the tip.

or barrier penetration

In physics, the passage of a particle through a seemingly impassable energy barrier. Though a particle's energy may be too low to surmount a barrier in classical physics, the particle may still cross the barrier as a consequence of its quantum-mechanical wave properties. An important application of this phenomenon is in the operation of the scanning tunneling microscope.

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