How Quantum Mechanics Fuels the Sun
a look into the macroscopic effects of quantum mechanics
Fusion is the process in which light elements fuse together to form heavier elements in the cores of stars. Hydrogens (the lightest element) smash together to form helium (the second lightest element), and the process continues all the way up until iron can be formed. This ends up releasing a lot of energy, which is how a star is fueled. But how does this process actually go down?
Atoms are made of a core called a nucleus with electrons in orbit. The nucleus is positive, and the electrons are negative. The nucleus is made of positive protons and neutral neutrons, which is why it takes on a positive charge. Here is a diagram of an atom.
The electromagnetic force results in positive charges repelling other positive charges, just as negative charges repel other negative charges. Like repels like. Opposites attract. So what’s the deal with the nucleus? There are no negative charges to attract the positive charges, there are only positive charges clustered together in a very close proximity. Shouldn’t the protons in the nucleus repel each other and destroy the atom from within?
It turns out that there’s something that overcomes the electromagnetic force: the strong nuclear force. The strong nuclear force is 100 times stronger than the electromagnetic force, except it acts on much shorter distance scales than the electromagnetic force. This ends up chaining the protons in place in the nucleus, since they are packed tightly together in a very close proximity.
So, in order to fuse, two nuclei need to be in close enough range for the strong nuclear force to kick in and lock them together. But it’s not that easy. Even at the core of the sun, where there’s an extremely high temperature and pressure, there isn’t enough energy to force the nuclei close enough together. They just keep repelling each other. So how does fusion occur?
At the ultramicroscopic realm of atoms, things don’t have a definite location. An atom cannot be precisely defined to be at a certain place at a certain time - there is always an uncertainty about where it’s at. We do, however, know the probabilities of where the atom could be, which is defined by something called the atom’s wave function. A wave function can look something like this.
This shows the distribution of probabilities of finding an atom in a certain location. There’s a greater chance of finding it in the middle of the graph, where the wave function peaks, than finding it where the wave function tapers off at the ends.
The wave function tells us where the atom could be. And it could be at any of these points.
Since it can be at any point at any time, it can, in essence, jump from one point to another. This is called quantum tunneling.
Imagine an atom moving towards a wall, except the atom is represented by its wave function.
It begins in front of the wall, as such.
Then it moves towards the wall, but hasn't bounced off of it yet.
In the first picture, the possible locations of the atom are all in front of the wall. As it approaches the wall, as shown in the second picture, there is a tiny probability that its actual location will be on the other side of the wall. This is no mistake - the atom can genuinely “tunnel” through the wall and appear on the other side. This is a real thing that happens, and is actually a problem for computer scientists when it comes to creating micro transistors - when they get too small, the electrons will escape through the wall of the transistor!
We can now understand how fusion can occur. As two nuclei get closer and closer, the more energy is needed to push them together. This is what it looks like when it’s graphed.
The vertical line signifies where the strong nuclear force kicks in - here, no more energy is needed to battle the repulsion between the positive nuclei.
This can be thought of as the “energy barrier” the atom must overcome. And what can atoms do with barriers? Tunnel through them! When they tunnel through the energy barrier, they are close enough to the nucleus for the strong force to kick in. Thus, fusion occurs.
Now, this process fuels the entire sun, without which life would never have been able to manifest. The result of such counterintuitive quantum mechanical behavior can have huge effects on the macroscopic world. Atoms tunneling through barriers can power a star.