I have a degree in physics. To get that degree, I had to jump through a lot of hoops, many of them made from dry-erase markers, exams, and the tears of overworked graduate students. In the process of learning how to jump through those hoops, several principles were beaten into me and my classmates.
Among these was the principle that everything that happens in the universe is the result of consistent, explainable rules that we like to call Laws. This is perhaps the core tenet of physics: the Laws are the same no matter where or when you go in the universe. We don’t know what all the Laws are yet, but in attempting to figure them out we proceed with the expectation that the complete set will have some character, some nature that’s consistent. If it turns out that the Laws change based on your position in spacetime, well, that would probably ruin some people’s whole day. Others would be ecstatic.
Anyway, the implication of this tenet is that nothing can exist outside the realm of the Laws. Everything, we assume, must obey them. Otherwise, either our Laws would be incomplete, or we would be forced to conclude that some things are extraphysical (read: magic). That’s just no fun, so we assume that it’s not true. So far, there hasn’t been any good evidence to disprove that assumption, so after a few centuries of building we’ve become pretty confident in it.
Many systems in the universe are complex. That is, they’re so large and have so many pieces that we—collectively or individually—can’t take them up in our minds very well. They don’t fit, because our minds are more or less the products of evolution, and our ancestors didn’t have a lot of quarks to interact with in the savanna (at least, as far as they knew). For this reason, we often need to abstract systems into smaller pieces so we can fit them into our thinking mouths and chew on them. The problem with these abstractions is that they necessarily discard some of the information in their creation, sort of like how you have to discard some wood shavings in order to whittle a duck, or whatever. Discarding information is critical to their function, after all, but it’s still an unfortunate consequence. If we keep doing this and abstract away too much, then what we’re left with is an over-simplified model of the real thing.
Abstractions make it easy to forget our fundamental assumption (at least, in physics) that the universe operates according to physical Laws. That’s important, because assumptions like that have critical implications for questions that seem like they couldn’t be further from physics, such as the question of free will.
Can you do what you want? You may think so, but you’re wrong. You—your body, anyway, which is what you are—exists in the universe. You’re made of matter, and as far as anybody can tell empirically, your body obeys the same physical Laws as anything else in the universe. Inside your body is a little chemical computer that does some fantastic work in real time to take inputs from the rest of your body and send outputs back to it. That’s how you’re reading these words, and it’s how I’m able to send them to you, more or less.
Suppose you turn off the device you’re reading this on, perhaps because you think I’m a pretentious prick for trying to leverage a moderate level of science education into a definitive answer to the free will question. Did you decide to do that? Well, let’s work backward. Lastly, your body moved in such a way that the device turned off. That required that some muscles move around, converting stored chemical energy into motion and some waste heat. Before that, your brain sent signals to your muscles to expend that energy precisely in that way. Before that, your brain took in inputs from your surroundings via various sensors. Before that, you were reading the last paragraph.
Nowhere in that process is anything like a decision or a thought. We have names for those things because it feels to us like they’re happening—like our minds are places we can go to—but the reality is quite different. Really, we are wonderful machines that operate according to Laws, no different than stars or starfish. We don’t have free will, because nothing does. We just do whatever the matter in our bodies does.
Consider this: almost nobody would argue that an atom can think. Cells don’t make the cut either, and neither do muscles, bones, or any other organs besides the brain. And yet, the brain is made up of the same matter as everything else. The emergent properties of the complex system that is the human brain aren’t unexplainable magic. They’re the impressive output of a machine whittled over eons to do what it does.
You may think this is a frightening conclusion. Perhaps you think that being predictable, even in principle, lessens your existence. I don’t think so. By fantastic chance, we all have the sensation of making decisions and thinking, even though we’re not. That’s a wonderful thing, and we should be happy to have received such a beautiful gift. Still, remember that we are physical beings, and unless we allow extraphysical magic, there can be nothing more to our existence than the natural course of the complex play that is the universe and its Laws.