According to classical physics, the Sun shouldn't shine. The sunlight hitting your skin is the result of a massive, ongoing quantum miracle 93 million miles away.
The Sun generates energy by fusing hydrogen protons into helium. For fusion to happen, protons must smash together with enough force to overcome their natural electrostatic repulsion, known as the Coulomb barrier. Because protons are positively charged, they repel each other, much like two North poles of a magnet.
To force them together requires immense heat and pressure. The core of the Sun reaches about 15 million degrees Celsius. But from a purely classical standpoint, 15 million degrees is nowhere near hot enough to overcome the Coulomb barrier. In classical physics, protons in the Sun’s core bounce off each other without ever fusing. If the universe ran strictly on classical mechanics, the Sun would just be a dark, warm ball of compressed gas.
This is where quantum mechanics steps in.
At the quantum level, protons do not behave like solid billiard balls. Due to wave-particle duality, a proton's location is not a single fixed point. Instead, it is a smeared-out wave of probabilities. While a proton is highly likely to be found in one spot, its probability wave extends infinitely in all directions.
This means that as two protons fly toward each other, their probability waves overlap. There is an infinitesimally small chance that a proton will simply "find itself" on the other side of the repulsive barrier without having enough energy to climb over it. This phenomenon is called quantum tunneling.
The odds of any two colliding protons tunneling into each other are vanishingly small—roughly one in ten octillion. But the Sun contains an enormous number of protons. Because the individual probability is so low, the fusion process is heavily bottlenecked. This bottleneck is exactly what allows the Sun to burn its fuel slowly and steadily over billions of years, rather than detonating all at once or failing to ignite entirely.
