Tunneling -- from Eric Weisstein's World of Physics

This entry contributed by Leonardo Motta

Tunneling is the quantum mechanical process by which a particle can penetrate a classically forbidden region of space (for example, passing from two separate points A and B without passing through intermediate points). The phenomenon is so named because the particle, in traveling from A to B, creates a sort of "tunnel" for itself, bypassing the usual route.

In 1927, F. Hund was the first to notice the possibility of the phenomenon of tunneling, which he called "barrier penetration," in a calculation of the splitting of the ground state in a double-well potential. The phenomenon arises, for example, in the "inversion" transitions of the ammonia molecule, which is allowed in quantum mechanics although forbidden in classical physics. In the same year, L. Nordheim applied the Schrödinger equation to the calculation of the reflection coefficient of an electron from various kinds of interfaces and noted that an electron, whose energy was insufficient to go over the barrier classically, could still tunnel through the barrier for the case of a rectangular potential barrier. Thus, Nordheim extended the case of tunneling between bound states noticed by Hund to the case of tunneling between continuum states. Oppenheimer Eric Weisstein's World of Biography subsequently performed a correct calculation of the rate of ionization of hydrogen by an external field in 1928. Following this, George Gamow Eric Weisstein's World of Biography and, independently, R. W. Gurney and E. U. Condon applied the tunneling phenomenon to explain the range of alpha decay rates of radioactive nuclei.

Although tunneling may seem abstract and far removed from reality, it is a actually a basic and important processes of nature. It is vital, for example, in the very first step of the thermonuclear reaction which powers the Sun.

The inflationary theory of the universe Eric Weisstein's World of Astronomy even proposes that the universe Eric Weisstein's World of Astronomy began at a state of no geometry (i.e., a universe Eric Weisstein's World of Astronomy with nothing, not even time) and then a tunneling occurred, allowing the universe Eric Weisstein's World of Astronomy to pass from the state of "nothing" to "something" (the "false vacuum") by tunneling (Guth 1997).

Tunneling Time

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