Quantum Tunneling

Suppose that there is a wall in front of you. Now you have thrown a ball to that wall. Suppose that the height of the wall is greater than the height the ball is travelling. What will happen? -The ball will bounce back if it doesn't have sufficient energy to penetrate the wall, or may be the ball will be absorbed by the wall. Alright, what if the height of the ball is greater than the height of the wall? The answer is pretty simple, the ball will go beyond the wall.

What if I tell you that when the height of the ball is less than the height of the wall, then there is a probability for the ball to go beyond the wall even if its energy is not sufficient to penetrate the wall, or, when the height of the ball is greater than the wall there is a probability for the ball to bounce back from the wall. It sounds, classically, absurd. But in the microscopic (Quantum Mechanical) world it is a true phenomena. When there is a potential barrier of V, a wavefunction can propagate through the barrier even if the energy of the wave function is less than V. Its called quantum tunneling. In the above analogy, the ball is treated as a wave function, the wall is potential barrier.

Quantum Tunneling. Image By Becarlson - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=67889226

Quantum tunneling was first noticed in 1927 by Friedrich Hund while he was calculating the ground state of the double-well potential. Leonid Mandelstan and Mikhail Leontovich discovered it independently in the same year. They were analyzing the implications of the then new Schrödinger wave equation. Information Source: Wikipedia

Tunneling cannot be directly perceived. Much of its understanding is shaped by the microscopic world, which classical mechanics cannot explain.