Physics · Explainer
Why the Sun shines even though fusion should be impossible
To fuse, two hydrogen nuclei must touch, but their positive charges repel and the Sun's core is not actually hot enough to force them together. They fuse anyway because of quantum tunneling: each nucleus is a wave that can leak through the barrier it cannot climb. Without tunneling, the Sun would not shine.
The barrier that should stop the Sun
The Sun runs on fusion: hydrogen nuclei merging into helium and releasing energy. But nuclei are all positively charged, and like charges repel. To fuse, two of them have to be pushed close enough to touch despite that repulsion, which is like rolling a ball up a steep hill.
Run the numbers and there is a problem. The Sun's core, at around 15 million degrees, gives its nuclei nowhere near enough energy to climb that hill. By classical physics, the ball rolls partway up and always rolls back. Fusion should almost never happen, and the Sun should be dark.
How tunneling gets through
Quantum mechanics changes the picture. A particle is not a tiny hard ball sitting at one point; it is a spread-out wave of probability. That wave does not stop dead at a barrier. It leaks a little way in, and if the barrier is thin enough, a sliver of the wave comes out the far side.
That leak means there is a small but real chance the particle is simply found on the other side of a wall it never had the energy to cross. This is quantum tunneling. In the Sun, nuclei that could never climb the repulsion hill occasionally tunnel straight through it and fuse.
Rare, but there are a lot of tries
Tunneling through the fusion barrier is wildly unlikely for any single pair of nuclei. What saves the Sun is sheer numbers: its core holds an almost unimaginable count of nuclei colliding constantly, so even a tiny per-collision chance adds up to steady fusion.
The result is a slow, stable burn that has lasted billions of years and will last billions more. The sunlight warming your face right now is the payoff of a quantum trick, particles passing through a wall they could not climb.
Sources & further reading
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