Quantum Tunnelling: Getting Through a Wall Without Going Over It
A ball rolled at a hill either makes it over or rolls back. A particle at an energy barrier has a third option, and it takes it often enough to matter: it can simply turn up on the other side. This sounds like the kind of claim that should be treated sceptically, and it would be - except that it is measured routinely, engineered into consumer electronics, and required for the Sun to produce any energy at all. It is also, for exactly those reasons, one of the most frequently misused ideas in claims about energy.
Why a Particle Is Not a Ball
In quantum mechanics a particle is described by a wave function, and the square of that function gives the probability of finding it in a given place. When the wave function meets a barrier it does not stop dead. It decays exponentially inside the barrier, and if the barrier is thin enough, a small but non-zero amplitude survives on the far side.
That surviving amplitude is a real probability of finding the particle there. Nothing about the process is gradual or partial: the particle is either found on one side or the other, and repeated identical experiments give different answers with a fixed ratio.
The exponential dependence is what keeps the effect confined to small scales. Doubling the barrier width does not halve the tunnelling probability; it squares an already small number. This is why tunnelling dominates at the scale of atoms and is utterly negligible for anything you can see.
It is worth being clear that the particle never has more energy than it started with while inside the barrier. The classical question - where did it get the energy to climb? - does not apply, because the particle does not climb.
Where It Is Indispensable
George Gamow applied it to alpha decay in 1928. An alpha particle inside a nucleus is trapped by a barrier it cannot classically escape, yet nuclei decay with well-defined half-lives. Tunnelling explains both the escape and the extraordinary range of half-lives - from microseconds to billions of years - because the exponential sensitivity turns small differences in barrier height into vast differences in rate. This is the foundation of how radioactive decay is understood.
The Sun is the more striking case. Fusing two protons requires overcoming their electrostatic repulsion, and at the temperature of the solar core the nuclei do not have nearly enough kinetic energy to do so. Classically the Sun would not burn. It burns because a small fraction of collisions tunnel through the barrier, and that small fraction is enough.
In technology, the scanning tunnelling microscope images individual atoms by measuring a tunnelling current across a gap of a few atomic diameters, exploiting the exponential distance dependence as an extraordinarily sensitive ruler. Its inventors received the 1986 Nobel Prize.
Flash memory writes and erases by tunnelling electrons through an insulating layer, which is why the technology exists at all and also why memory cells wear out - each tunnelling event damages the insulator slightly. Tunnel diodes and superconducting junctions are further examples.
What Tunnelling Does Not Do
This is the part worth stating carefully, because tunnelling is regularly invoked in support of claims it does not support.
Tunnelling does not create energy. A particle that tunnels through a barrier arrives with exactly the energy it had before, and the barrier is unchanged. There is no net gain to anyone, and the process conserves energy exactly rather than approximately.
It also does not allow energy to be extracted from a barrier or from the vacuum. The uncertainty principle is sometimes cited as permitting a temporary energy loan, which is a loose way of speaking about a mathematical relationship between measurement precisions. It is not a mechanism for withdrawing usable energy, and no experiment has ever produced net energy this way.
What tunnelling does is change reaction rates. It makes processes possible at temperatures and energies where classical physics forbids them, which is enormously important - the Sun, radioactivity, chemistry, semiconductors - but it always operates on energy that is already present. Our page on free energy sets out the general form of this distinction, and tunnelling is one of the specific cases where it gets blurred most often.
For neutrinovoltaic research the relevance is limited and specific: tunnelling is part of how charge moves across engineered material interfaces, as it is in any semiconductor device. It is not a source of energy in that context and should not be described as one.
Frequently asked questions
How can a particle pass through a barrier it cannot climb?
Its wave function decays exponentially inside the barrier rather than stopping, so a small amplitude survives on the far side. That amplitude is a real probability of finding the particle there. The particle never has more energy than it started with.
Why doesn't this happen to everyday objects?
Because the probability falls exponentially with barrier width and with mass. For anything larger than atomic scale the number becomes so small that it would not occur in the age of the universe.
What does the Sun have to do with it?
At the temperature of the solar core, protons do not have enough energy to overcome their mutual repulsion and fuse. A small fraction tunnel through the barrier instead, and that fraction is what makes the Sun shine.
Is tunnelling used in real technology?
Yes, routinely. Scanning tunnelling microscopes image individual atoms with it, flash memory writes and erases with it, and tunnel diodes and superconducting junctions depend on it.
Can tunnelling be used to extract energy?
No. A tunnelling particle arrives with exactly the energy it had before and the barrier is unchanged. Tunnelling alters reaction rates; it does not create energy or allow extraction from a barrier or the vacuum.
