Tunneling, also known as the tunnel effect, is a quantum mechanical phenomenon by which a tiny particle can penetrate a barrier that it could not, by any classical or obvious means, pass. Though seemingly miraculous, the effect does have some intuitive characteristics. For instance, thin barriers allow more particles to tunnel than do thick ones, and low barriers permit more tunneling than do high ones.
Tunneling does not generally show itself in the macroscopic world. It only starts to become a factor for microscopic items. Atoms can tunnel, as can electrons, but things such as tennis balls and grapes, easily seen with the naked eye, will not. For microscopic particles, the barrier heights are described in terms of energy instead of distance, but for conceptual purposes there is little difference.
It is important to note that the effect can only be understood with the aid of quantum mechanics. Classical mechanics, the system pioneered by Isaac Newton that stood unchallenged until the early twentieth century, has no way of explaining tunneling. In the Newtonian philosophy, all particles, even the tiniest of microscopic particles, can be located precisely. Any uncertainty is seen as the result of an imperfect measuring device or a sloppy scientist. In addition, each microscopic particle is considered to be like a tiny pebble and there can be nothing wave-like about it.
The quantum view of the universe is fundamentally different from the Newtonian view. Each particle is said to have both corpuscular (pebble-like) and wave-like properties. Furthermore, a quantum particle cannot, in general, be located precisely. It has a built-in uncertainty that cannot be taken away by the best of measuring instruments. For these reasons, a particle in quantum mechanics is often treated as a wave function. This is another way of saying that the particle is akin to a small bundle of waves.
Representing a quantum particle as such a bundle has two advantages. For one, it reveals that the particle is, in some sense, blurry and can never be exactly pinned down. It exists over a range of space, not a specific point. For another, the wave function format allows particles to exhibit wave-like properties (see interference and diffraction). Tunneling is a one of these wave-like properties. By the more general name of "barrier penetration," it is a well-established characteristic of waves. Light waves, for instance, have long been observed to overcome daunting optical barriers.
In a fundamental sense, the quantum mechanical explanation of tunneling can be illustrated by an analogy. If we roll a ball very slowly toward, say, a cement speed bump, we confidently say that it will not surpass the barrier and predict that it will roll back toward us. However, if a very modest ocean swell approaches an offshore sandbar, we are not as sure of the results and rightly so. The character of the wave, even if diminished in size, often makes its way past the sandbar. Similarly, treating a microscopic particle with the mathematical model of a ball clearly tells us tunneling is impossible, while using the mathematical model of a wave just as clearly states that particles will always have a chance to tunnel.