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).

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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Tunneling may refer to:

• tunneling, on a violin or violia
• Digging of tunnels (the literal meaning)
• Mining, or tunneling during a siege
• Quantum tunnelling, a quantum-mechanical effect of transitioning through a classically-forbidden energy state
• Tunneling (fraud), a fraud committed by a company's own management or by major shareholders
• Tunneling protocol, transmitting one computer network protocol that is encapsulated inside another network protocol
• Urban exploration, the exploration of public utility tunnels by unauthorized people testing
• Australian Rules Football, a tactic used in the AFL, Used when a player is going for a mark and the opposition player elbows' the opposite players knee.