Does physics say that free will doesn't exist?

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You are currently making a conscious, willful decision to read this article. But physics says every action has a cause. So did you really make this decision as freely as you thought?

One of the bedrock philosophical concepts under all of physics is something called causal determinism. It says that every effect has a cause, and that if you know the current state of a system, you can use the power of physics to predict how it behaves. If effects happened without causes, then there wouldn't be much need for physics. And if we couldn't predict how systems would behave, then we wouldn't be very good at our jobs.

With this philosophy, physics has made enormous progress in advancing our understanding of the universe, from subatomic quantum systems to the Big Bang. And a part of that universe contains these weird things called brains that have the curious property of consciousness and the ability to freely make decisions.

So, at first glance, it seems like our understanding of physics forbids free will. We don't really have a choice, because if we had perfect knowledge of all the molecules and electrical activity in our brains, then we must be able to determine our choices in advance.

But there are three aspects of physics that add some wrinkles to this line of thinking.

The first is chaos theory. Some systems are easy to predict. But others, like double pendulums and weather patterns, are much harder to tackle. In these special kinds of systems, even a minuscule amount of uncertainty in the measurement of the initial state of a system very quickly compounds into complete ignorance about its future behavior. Strangely, these systems are perfectly deterministic; causes always lead smoothly to effects, so there's no mystery there. But they are impossible to predict well into the future.

The second wrinkle comes from quantum mechanics, which tells us that it's impossible to predict the outcomes of many kinds of experiments involving subatomic particles. Probabilities rule the day there, and the best we can do is assign chances to certain outcomes. Quantum mechanics is still a deterministic theory of nature — but again, it places a layer of ignorance over our understanding. We can't say for sure where a particle will go or how it will behave; we can only say what might happen. But it's not clear if the probabilistic rules of quantum mechanics apply to things like neural connections in the brain and the rise of consciousness, which is an emergent phenomenon.

The last wrinkle is exactly that: emergence. Fundamental descriptions of nature do not automatically guarantee an understanding of more complex systems. For example, we have an incredibly sophisticated theory of particle physics, based on quantum field theory, but that sophisticated theory works only when describing quantum systems. We have no quantum field theory description of how a star forms, or why chocolate tastes so good. We have to adopt other laws and theories to describe the systems as a whole.

None of these wrinkles gives a clear-cut yes-or-no answer to the question of free will. But they do show that our understanding of physics is limited. Most philosophers believe in a class of ideas under the heading of "compatibilism," which says that free will and physics can live together in harmony. It might be that our understanding of nature is not yet sophisticated enough to explain how free will can work with causal determinism.

In other words, if we work hard enough, we might someday reach a level of understanding that preserves causal determinism and all the usual physics goodness while including things like free will in a framework that makes sense.

Either way, we have no choice but to keep asking.