The barrier is crossed by probability, not by shrinking
A particle does not squeeze itself down to fit through a gap. In quantum mechanics, its state is described by a wavefunction: a mathematical description that lets physicists calculate where it may be detected. Even when the particle lacks enough energy to pass over a barrier, that wavefunction can extend a short distance into the barrier and emerge on the far side. A measurement there can then find the particle beyond it. This is quantum tunneling.
The wave inside the barrier fades rapidly rather than stopping at its edge. If the barrier is thin enough, some of the wave remains on the other side, giving the particle a chance to appear there. The chance falls sharply as the barrier gets thicker or harder to penetrate. Nothing has to push the particle through, and it does not follow a hidden path like a tiny ball slipping through a crack.
Why thickness matters in real devices
That sensitivity makes tunneling useful—and sometimes a limit—in electronics. In a tunnel junction, two conducting materials are separated by an extremely thin insulating layer. Electrons can cross the insulator even though ordinary electrical conduction would treat it as a blocker. Make the layer thicker, and far fewer electrons get through; make it thinner, and the current can rise dramatically.
The same effect lets a scanning tunneling microscope map a surface. Its sharp tip sits very close to the material, with a tiny gap between them. Electrons tunnel across that gap, and the measured current changes strongly with the distance between tip and surface. By tracking those changes as the tip moves, the instrument can reveal surface details at the scale of individual atoms.
So the useful image is not a particle shrinking to dodge an obstacle. It is a wave-like quantum state extending into a barrier, with a small but measurable chance of continuing through. At atomic scales, a gap that blocks ordinary flow can still carry a current.
