In quantum mechanics, the tunneling effect refers to various processes whereby particles appear to violate classical physics by instantaneously changing their location within the space-time continuum without passing observably through the intervening space. Theoretically this is achieved by passing through one or more of the non-observable dimensions predicted by the general theory of relativity. The exact process by which it occurs is not fully understood or reproducible, but is most readily observed in the quantum leap of an electron when it absorbs or emits a photon.

Radioactivity results when a particle or group of particles is able to tunnel out of the atomic nucleus.