Quantum mechanics (QT) is the branch of physics that describes how nature behaves at its smallest scales. It may be a small world, but QT is the most accurate physical theory ever devised. At the same time, this is a theory that is notorious for its weirdness.
Take quantum tunnelling as one example. Imagine you’re an electron with legs. There’s a chance you might walk right through a wall. It’s hard to make sense of this in classical terms, but the quantum explanation is that electrons, like other subatomic particles, behave not as particles but probability waves. You heard right – probability waves. Those waves obey an equation formulated in 1925 by the Austrian physicist Erwin Schrodinger. The details need not concern us here. The important thing is that the solution to the Schrodinger equation shows that a small portion of the electron probability wave exists on the far side of an impenetrable barrier. In other words, there is some small but nonzero chance that you, the electron, will be detected on the far side of the barrier, as if you had tunnelled through the wall.
With some calculus, physicists can calculate the rate at which such tunnelling events occur and subject that prediction to rigorous tests. Those tests have confirmed the predictions of the theory. Tunnelling is real.
But then, so what? Does something as esoteric as quantum tunnelling have any thing to do with real life, on the large scale of things far removed from the level of subatomic? Turns out that tunnelling indeed has more than meets the eye in terms of applications. The first example that comes to mind is radioactivity. When alpha particles tunnel out of uranium nuclei at the predicted rate, they produce radioactivity, which is the basis of nuclear energy (by the process of nuclear fission) and nuclear medicine. Radioactive isotopes in particular have proven particularly effective as tracers in certain diagnostic procedures.
Tunnelling plays an important role in the nuclear-fusion processes that make the sun shine. Therefore, you could say that life on Earth depends partially on tunnelling. Finally, quantum tunnelling is opening the door to all sorts of technology applications, from scanning tunnelling microscopes that enable scientists to image and manipulate individual atoms to nanotechnology and all the wonders associated with it.