Quantum tunneling is the quantum mechanical phenomenon where a wave function can propagate through a potential barrier. If you toss a ball at a wall, the ball bounces back because it does not have sufficient energy to penetrate the wall. However, according to quantum mechanics, the ball is represented by a fuzzy probability wave function that spills through the wall. It actually has an infinitesimally small chance of tunneling through the wall. If the ball were to be thrown at the wall every second, you would have to wait longer then the age of the universe to have a good chance of it tunneling through.
Particles can tunnel through such barriers due to Heisenberg’s Uncertainty Principle applied to energies. According to the principle, it is not possible to say that a particle has precisely one amount of energy at precisely one instant in time. Rather, the energy of a particle can exhibit extreme fluctuations on short time scales and can have sufficient energy to traverse a barrier.
Some transistors use tunneling to move electrons from one part of the device to another. The decay of some nuclei via particle emission employs tunneling. Alpha particles (helium nuclei) eventually tunnel out of uranium nuclei. Tunneling is also important in sustaining fusion reactions in the Sun. Without tunneling, stars wouldn’t shine. Scanning tunneling microscopes employ tunneling phenomena to visualize microscopic surfaces, using a sharp microscope tip and a tunneling current between the tip and the specimen.
It is possible for spin-zero particles to travel faster than the speed of light when tunneling. This apparently violates the principle of causality, since a frame of reference then exists in which the particle arrives before it has left. An experiment done in 2020, overseen by Aephraim Steinberg, showed that particles were able to tunnel at speeds faster than light. Since then, the basic principles of tunneling were used in the construction of faster than light devices.