Tunneling

Tunneling represents the most fundamental process in physics. According to our present understanding tunneling started the universe about 13 billions years ago. Nowadays we know that tunneling is involved in radioactivity and in nuclear fusion — the latter effect is heating the sun. Tunneling is the process of molecular inversion motion in chemistry and is important in modern microelectronic devices. Physicists introduced the name tunneling for a classical forbidden process, which the theory of quantum mechanics explained around 1927: A ball, for instance, cannot overcome a hill if its kinetic energy is less than the hill's gravitational potential energy. In this case the ball rolls back. However, quantum mechanics explained that the ball has a tiny probability of getting to the other side of the hill. Similarly, an α-particle leaves the attractive nuclear potential well despite having a small energy, thereby producing radioactivity. In figures 1 and 2 an α-particle is illustrated as a wave packet embedded in a valley between two hills, which represent the attractive nuclear forces. The energy of the particle is assumed to be too small to overcome the tops of the hills. However, radioactivity, which was observed a 100 years ago, i.e., the decay of an atomic nucleus, is explained by quantum mechanics as a probability that a low energy particle is observed at the other side of the hill.

The explanation of alpha-decay as quantum mechanic tunneling followed around 1928 by George Gamow and simultaneously, but independently, by Edward U. Condon and Ronald W. Gurney. Incidentally, in 1927, Friedrich Hund was the first to notice the possibility of the phenomenon of tunneling, which he called barrier penetration, in a calculation of the ground state in a double-well potential. The phenomenon arises, for example, in the inversion transition of the ammonia molecule.