I’ll try to make an analogy of quantum mechanics with mountain hiking, so bare with me for a second.
It takes a lot of effort (a lot of energy) to climb up a mountain. If your goal is to sit at the top and admire the view, or simply enjoy the hike itself then it’s worth it. But if your goal is to pass the mountain and meet your friend on the other side for lunch, then the mountain is simply a barrier along your way. Wouldn’t it be wonderful if you didn’t have to climb all the way up and then down again? If you could just dig a tunnel and travel through the mountain as if it wasn’t there? What if you hadn’t had breakfast that morning and you don’t really have enough energy to go over the mountain? Unless there is a tunnel, you are stuck, and your friend will be eating lunch alone.
You might think this is a silly scenario, but it’s the easiest way to explain how “quantum tunneling” works. The two friends who are trying to meet can be two atomic nuclei in the core of a star. And the mountain they have to overcome is a repulsive force because they both have positive electric charges.
In the classic picture, if the temperature of the star is not very high then our nuclei cannot overcome the ``mountain’’ and will never meet; they just get close, feel the repulsive force, and move the other way. If the temperature is high enough, then our nuclei can climb over the mountain and have the intimate encounter that we call nuclear fusion.
Let’s say that the nuclei we are talking about are the hydrogen nuclei in our Sun’s core. As the temperature increases, it would reach a point when every single hydrogen nucleus in the Sun’s core could fuse with another hydrogen. Simultaneously! That’s a lot of hydrogen, and a lot of energy released in an instant, and our Sun would simply go boom! Of course this would have happened a long time ago so we wouldn’t be here to worry about it.
This is where quantum mechanics came to save the day. In quantum mechanics everything works with probabilities. So, even if our two nuclei don’t have enough energy to overcome the barrier we talked about, there is still a small, but non-zero, probability that they will meet up. This is called ``quantum tunneling,’’ because it’s as if one of the nuclei will tunnel through the mountain to get to his friend, instead of having to climb over it.
The implications of quantum tunneling are enormous, especially for life on Earth. Because the probability for quantum tunneling is small, it means that a tiny fraction of the hydrogen in our Sun will fuse at lower temperatures, and produce energy at a slow pace. So, instead of becoming a hydrogen bomb, our Sun lives on happily, fusing its hydrogen bit by bit for billions of years, giving us a fair chance to… well... hike up another mountain and gaze at the world.