tunneling

Tunneling is a quantum physics phenomenon where particles like electrons pass through barriers that would normally block them, due to their wave-like nature. Imagine a ball hitting a hill: classically, it would roll back unless it has enough energy. Quantum tunneling allows the particle to "appear" on the other side of the barrier without having enough energy to go over it, because there's a small chance of it "sneaking through" based on its wave function. This effect is fundamental in technologies like tunnel diodes, scanning tunneling microscopes, and nuclear fusion.