Science › Quantum world
Quantum tunneling
1928· Discovery / experiment
ProvenTested many times; part of working physics.
A particle can pass through a barrier it lacks the energy to climb, because its wave leaks through.
See it on the Science map →How it works
In classical physics, a ball without enough energy to roll over a hill stays put. Quantum particles are waves, and a wave does not stop dead at a barrier; it fades inside it. If the barrier is thin, part of the wave emerges on the far side, giving some chance the particle appears there. George Gamow used this in 1928 to explain radioactive alpha decay.
Why it matters
It lets the Sun shine: protons tunnel close enough to fuse. It runs flash memory and scanning tunneling microscopes, and limits how small transistors can shrink.
Think of it like
Like sound through a wall: a thick wall stops it, a thin one lets some through. The thinner the barrier, the more gets across.
How we know
Alpha-decay rates match Gamow's 1928 formula across enormous ranges. Leo Esaki's 1957 tunnel diode and the 1981 scanning tunneling microscope rely on it. The 2025 physics Nobel honored tunneling in electrical circuits.
Still a mystery
How long a particle spends inside the barrier is still debated, with experiments and theories giving conflicting answers.
Connections (3)
▸The wave function predicts Quantum tunnelingProven
The relationship
In the wave equation, a particle's wave does not stop abruptly at an energy barrier; it shrinks steadily inside it. If the barrier is thin enough, some wave remains on the far side. The amount remaining gives the probability the particle turns up beyond the barrier.
How the link was found
Friedrich Hund noticed it in molecules in 1927; George Gamow, and separately Ronald Gurney and Edward Condon, used it to explain alpha decay in 1928.
Why it's proven
Predicted alpha-decay half-lives, ranging from tiny fractions of a second to billions of years, match the formula. Tunnel diodes and scanning tunneling microscopes work as predicted.
Open questions and mysteries
How long a particle takes to cross the barrier is still debated, with experiments and theories giving conflicting answers.
▸Quantum tunneling lets the Sun fuse in Stars forge the heavier elementsProven
The relationship
Protons repel each other electrically. At the Sun's 15-million-degree core, classical physics says they almost never get close enough to fuse. Quantum mechanics lets a proton occasionally 'tunnel' through the repulsion barrier without the energy to climb over it. Those rare successes, across enormous numbers of protons, are enough to power the Sun.
How the link was found
George Gamow explained tunneling in radioactive decay in 1928. Robert Atkinson and Fritz Houtermans applied it to stellar fusion in 1929.
Why it's proven
Solar models built on tunneling predict the Sun's brightness and its neutrino output, both confirmed, including by the Sudbury Neutrino Observatory in 2001.
Open questions and mysteries
Tunneling in stars is settled. Making fusion practical on Earth without a star's gravity is the remaining challenge.
▸Quantum tunneling is borrowed for A universe tunneling from nothingSpeculative
The relationship
In alpha decay, a particle trapped behind an energy barrier sometimes appears on the other side. Vilenkin applied the same math to the size of the universe. A closed universe of near-zero size faces a barrier before vacuum energy can make it inflate; quantum mechanics gives it a small chance to tunnel through, starting from no size at all.
How the link was found
George Gamow's 1928 tunneling theory of alpha decay supplied the idea. Vilenkin applied it in 1982, then from 1984 recast it with the Wheeler–DeWitt equation, the quantum equation for a whole universe that Bryce DeWitt wrote in 1967.
Is it proven?
The link is mathematical analogy. Tunneling itself is thoroughly tested in labs and electronics; whether it applies to space and time, with no prior state, is untested.
Open questions and mysteries
Ordinary tunneling happens in time; here time begins with it. What does a 'chance' of tunneling mean when there is no before?