In quantum physics, what phenomenon lets a particle cross a classically forbidden energy barrier?

In quantum physics, quantum tunneling lets a particle cross a classically forbidden energy barrier. A particle's wave function can extend into and beyond a barrier, giving a nonzero probability of detection on the other side even when classical energy rules would forbid crossing.

Tunneling is especially important when barriers are narrow or particles are light. Its probability generally falls rapidly as barrier width and height increase. The effect helps explain alpha decay, in which an alpha particle escapes an atomic nucleus, and it enables scanning tunneling microscopes to image surfaces at atomic scales.

Tunneling also powers practical devices. Tunnel diodes, flash-memory cells, Josephson junctions, and parts of modern semiconductor technology rely on it. Tunneling is not the particle secretly gaining enough classical energy; it is a consequence of quantum wave mechanics and the probabilistic interpretation of the wave function.