Quantum tunnelling
The appearance of a particle, with some probability, on the far side of a barrier it lacks the energy to climb over. It happens because particles behave as waves.
A ball without enough speed cannot get over a hill. In quantum mechanics, though, a particle's whereabouts is described by a wave, and the wave seeps a little way into the barrier. If the barrier is thin enough the wave reaches the other side, and the chance of finding the particle there is no longer zero. In 1928 George Gamow[1] and, independently, Ronald Gurney and Edward Condon[2] applied the idea to the atomic nucleus and solved the puzzle of alpha particles leaving the nucleus with less energy than the barrier's height.
The probability of getting through is extremely sensitive to the barrier's thickness and height. It is thanks to this effect that nuclei at the centre of the Sun can overcome their repulsion and fuse. In semiconductors, an insulating film that is too thin lets electrons tunnel through, and current leaks. The effect has also been put to use, in electronic components and in microscopes that can make out single atoms.
Related terms
- Alpha decayThe process in which a heavy atomic nucleus throws out one alpha particle, a helium nucleus, and becomes the nucleus of a different element.
- Schrödinger equationThe basic equation of quantum mechanics, which governs how the wave of an electron or other particle varies in space and time. Written down by Erwin Schrödinger in 1926.
- Half-lifeThe time it takes for the number of radioactive nuclei to fall by half through decay. It is fixed for each kind of nucleus and does not change with temperature or pressure.