The Sun only shines because particles cheat.
By classical physics, the Sun's core isn't hot enough for protons to fuse: they should repel each other before they touch. Quantum mechanics lets them 'tunnel' through that barrier anyway. Without this loophole, there would be no sunlight and no us.
The Sun is powered by nuclear fusion. In its core, hydrogen nuclei, which are single protons, combine step by step into helium and release energy. But protons carry positive charge, and like charges repel. To fuse, two protons have to get close enough for the strong nuclear force to take over, and the electrical repulsion in the way, known as the Coulomb barrier, is formidable.
The Sun's core is about 15 million °C. That sounds extreme, yet by the rules of classical physics it is far too cool: almost no protons move fast enough to climb over the barrier. On those rules alone, the Sun would barely shine.
Quantum mechanics changes the picture. A particle is not a tiny billiard ball with an exact position; it is described by a wave of probabilities. That wave does not stop dead at a barrier. It leaks into it and a little beyond, so there is a small chance the particle simply turns up on the other side. This is quantum tunnelling.
For any single pair of protons the odds are minuscule, and a typical proton in the core waits billions of years before it fuses. But the Sun contains an enormous number of protons, so tunnelling happens constantly, and slowly enough that our star burns steadily for about ten billion years rather than exploding. The same effect makes flash memory and scanning tunnelling microscopes work. The rules that seem to make fusion impossible are rescued by rules stranger still.
Source: Wikipedia — Quantum tunnelling · Text summarised from Wikipedia (CC BY-SA 4.0)
