What phenomenon allows particles to pass through a barrier they classically cannot overcome?

Quantum tunneling allows a particle to pass through a potential barrier that classical physics says it does not have enough energy to overcome. The effect arises because quantum particles are described by wavefunctions, which can extend into and beyond the barrier.

When an electron encounters a sufficiently narrow barrier, most of its wavefunction may be reflected, but a smaller portion can emerge on the other side. That transmitted portion represents a probability of finding the particle beyond the barrier. The probability falls rapidly as the barrier becomes higher, wider, or involves a more massive particle.

Tunneling is not a particle secretly borrowing energy to jump over a wall; it is a specifically quantum transmission effect. Friedrich Hund studied an early tunneling problem in 1927, shortly after Schrödinger published his wave equation. The phenomenon later became central to semiconductor tunnel diodes, Josephson junctions, and scanning tunneling microscopes.

It also helps explain nuclear fusion in stars: atomic nuclei can tunnel through the electrostatic repulsion that otherwise makes fusion at stellar-core temperatures unlikely.