What is quantum tunelling?

Answer 1

Short Explanation:

Quantum tunneling is one of the traditional examples of something that is permitted by quantum physics and is completely forbidden by classical physics and does indeed happen as quantum theory predicts. It is manifested only for small light particles where classical physics breaks down.

When the motion of a particle is confined, usually by some potential energy barrier, it can not cross that barrier if it does not have a kinetic energy that is sufficient to exceed to potential energy requirements of the barrier. Quantum theory says that a quantum system prepared in one region that is separated from another by such a barrier can traverse the barrier even if it does not have sufficient kinetic energy. It does this by "quantum tunneling" and there is a finite probability that the particle can be detected in the region where the potential energy is actually greater than the kinetic energy.

Perhaps a longer example and explanation:

A "voltage" between two points represents the amount of energy per unit charge that is needed to move a charge particle between the two points. In other words, it takes twice as much energy to move a charged particle between two points of 10 volts than the same particle between 5 volts.

The energy unit "electron-volt" (eV) is the amount of energy that is required to move one electron between a potential difference of one volt. It's a pretty small amount of energy.

If there is a potential difference of 2 volts between two points, and an electron with kinetic energy of 3 eV reaches the first point, it has enough kinetic energy to get to the second point. However, if its kinetic energy is only 1 eV, then it does not have enought kinetic energy to do so. Certainly makes sense, right?

Quantum tunneling is an unusual fact seen in sub-atomic interactions. Although this is VASTLY over-simplified, it basically states that an electron with LESS kinetic energy than that needed to overcome a voltage barrier (say, one with 1.99 eV of energy reaching a 2.00 volt barrier) has a certain probability of overcoming the barrier. The probability can be calculated, but ONLY the probability. In other words, we can never know for certain if a SPECIFIC particle will (or will not) get through the barrier, we can only calculate the probability of it doing so.

This fact has been confirmed in experimental results, and agree completely in keeping with predictions. In classical mechanics, an electron either does or does not have enough energy to go through a barrier. In quantum mechanics, the electron has a certain probability of doing so.