|Quantum particles interacting with each other.|
You see, before quantum theory, one of the prevailing ideas of classical physics is that if you know all of the circumstances in which an event takes place, you can predict with 100% certainty what will happen in the future in perpetuam. Meteorologists with enough information could predict whether or not it will rain on your birthday three years from now. Gambling wouldn't work, because given a careful analysis of the conditions in which a game was played or dice were thrown would, after some pretty heavy computation, be obvious. (And if you want to know more about that, I got it from this book.)
It doesn't take a great leap to realize that though our thoughts and actions can seem spontaneous, the mechanism that produces those thoughts and actions is made up of the same elements a star makes when it explodes (Did you know most of the stuff in the universe is created when stars go supernova?) and must also follow the laws of physics. Predicting, directing, and manipulating the way we think and act is child's play to most people--we do it unconsciously. Marketing and advertising professionals do it purposefully, sometimes to great effect. Teachers do it, parents do it, governments do it--it is not a secret that even our most private thoughts and impulses can be strongly influenced by outside control.
We know that certain chemicals in the brain react with our neurons to create these impulses and that we react to stimuli we receive through photons bouncing off objects and hitting our retina, molecules vibrating across distances and twiddling our eardrums, etc. Even our motivations can be traced back to physics: what is life but elements bound together in a specific way and charged by electricity?
So, if all existence, and all reality is simply dominos flicked over by whatever caused the Big Bang, where is our free will? If all can be predicted via logical cause-and-effect, what says that we have any control over what we do at all?
Here is where I am reassured by the quantum theory: uncertainty.
So, a slight departure to illustrate a point. If a tree falls in a forest and there is no one to hear it, does it make a noise? Intuition says "Of course, you can't stop a tree from making noise as it falls. You can't stop the vibrations as it falls through the air and the leaves and branches jostle each other." And that is most likely true. But if it is a quantum tree, the answer is no! Quantum particles do not have a position before their position is measured. They are only more probably in a certain location than others.
So you can't reliably predict when or where a quantum particle is. You can only find the probability that it is in a certain place.
So maybe that uncertainty doesn't trickle up and allow us free will. But it is enough to reassure me that not all of nature is predestined. I am not just a domino who thinks it is choosing to fall at a certain time and in a certain direction. There is true randomness in the universe. And that's all I need.