Quantum Tunnelling Through Energy Barriers

How can a particle reach the other side of a barrier it cannot classically climb over? A ball lacking enough energy to roll over a hill must turn back, yet quantum theory predicts a non-zero chance of finding a particle beyond a finite energy barrier. The central mystery is whether “crossing” means gaining extra energy or whether quantum physics describes the situation in a different way. The answer begins by separating an energy landscape from an ordinary solid wall.