Wrestling the Demon: the Physics of Free Will

At the intersection of physics and philosophy, there’s a question that’s weighed on the minds of great thinkers for centuries: Is there truly such a thing as free will? When we make a choice, are we fundamentally any different than a calculator “choosing” which segments of its display to light up when the = button is pressed?

The question has its roots in the acceptance that humans are, for all our astounding complexity, purely physical systems. Once you drop the notion that we’ve got an intrinsic, metaphysical soul that sits behind the eyes and pulls the levers, the question of why we make the choices we make becomes urgent…if only philosophically.

Barring the existence of a “ghost in the machine”, the factors that comprise the self and influence our decision-making processes can largely be broken down into two categories: nature and nurture (or genetics and environment, if you prefer). The first one is something that we obviously have no control over—nobody chooses their parents—and while we manipulate our environment, we can only do so to the extent that we are already manipulated by it.

The simplified example I like to give involves a fetus in the womb, just reaching the point where its nerves and muscles are developed enough to give a kick. Here, it has a choice—kick, or don’t. Which one it chooses clearly depends on things entirely beyond its control—the genes that encode its calcium channels, the ion concentrations in its neurons. But it makes that first choice, and—let’s say it chooses to kick—the world responds. Its mother puts her hand on her belly, and perhaps the fetus feels the extra warmth, or hears her cry out in surprise. Now, the child has the beginnings of a personality—it has learned something about what happens when you kick, and its next choice will be determined not just by its genes, but also by what it knows. Here, at the very beginnings of its life, it’s easy to see that—even though the child is making choices, they still depend on factors that it had no control over.

From there, that first pre-conscious choice, through birth, into adulthood—that person’s life will become a never-ending dance of stimulus and response, input and output, growing ever more complex but never breaking the chain of cause and effect. A rebellious teenager, making choices in apparent defiance of everything they’ve been taught, is being guided by their peers instead; by hormones, and ideas of what adolescence should be, ideas that they’ve been exposed to through books, or television, or the internet. Ask someone to think of a random noun and, while the answer might not be something you’d have expected, there will always be a reason (more likely multiple reasons) why they chose it.

The premise of the determinist argument against free will is that, theoretically, if you ask why someone made a choice, and then ask why those factors influenced their decision, and then ask why those factors were the way they were, you can go on and on, back along the chain of causality until you eventually find that the answer comes down to the laws of physics and the shape of the universe at its very beginning. Of course, that’s theory. In reality, if you keep asking someone why like that, you’re more likely to find yourself missing a few teeth.

While you can argue against free will from a biological and psychological perspective, those are messy and really complicated—but the same argument can be made with physics. We’re all just collections of particles, and particles obey rules. Even if we don’t fully understand them, or even if we can’t fully understand them, the rules are there, and they don’t get broken. Even if the laws of physics change over time, they change in accordance with other, higher laws that are equally inviolable.

Imagine a box full of bouncy balls, shaken up and then frozen in time. If you know the position and the momentum of every bouncy ball in the box, and you know the laws of physics, you can calculate where the balls must have been one second ago, to lead to their current arrangement; there’s only one possible setup that could end up with them in their present positions. And just as easily, you can calculate where they’re going to be one second into the future, or two seconds…or a thousand.

The point of this thought experiment is to demonstrate that, if you know the position and momentum of every particle in a closed system at one point in time, you know it at every point in time: past, present, and future. And if the future can be predicted, then every decision that could be made inside that box has already, in a sense, been made. Philosophically, it works equally well no matter what you’ve got in your box: bouncy balls, air molecules, or an entire universe.

Applied at the ultimate scale, the thought experiment is called “Laplace’s Demon”, after the French mathematician who popularized the argument. The “demon” bit refers to the fact that, to know the position and momentum of every particle in the universe, you’d need a brain bigger than the universe itself, necessarily outside it. Being from the 1700s and lacking the concepts of computer simulations and larger universes, a religious analogue was a natural choice for Laplace.

The physics of it isn’t so simple, of course—keen readers might point out that you can’t know both the position and momentum of a subatomic particle, thanks to Heisenberg’s uncertainty principle. Quantum mechanics seems to have probabilities built in, which means randomness, as well—and true randomness is the antithesis of determinism and causality, to the point that people have gone to the trouble of inventing theories where whole extra timelines branch into existence, one for each possible outcome of a random process.

But this strikes me as silly. When we say “there is a 50% chance of rain here tomorrow”, does reality split in two at the stroke of midnight, one universe where it rains and one where it doesn’t? Of course not. It’s either going to rain or it isn’t, depending on atmospheric conditions, and that probability statement is only saying something about our knowledge of the system, not about the system itself. Just because we cannot know the position and momentum of a particle simultaneously does not mean it doesn’t have definite ones. It seems like a fine hair to split, but it’s the difference between the future being set in stone—albeit effectively unpredictable, thanks to chaos—or truly unhinged from the present. This is where, even today, the demon comes in handy; if something is definite, then the demon knows it, taking away the messiness of measurement problems and seeming randomness at the ultra-small scale. (It should be noted that, while true randomness—”God playing dice“—is a feature of the Copenhagen interpretation of quantum mechanics, there are equally valid alternative interpretations that maintain a deterministic universe.)

So to our demon, the universe is perfectly predictable, and—being parts of that universe—we are, as well. At first, maybe this thought is existentially horrifying; we’re all just machines, making the choices we’ve been programmed to make. The infinite branching possibilities of choice are illusory—the present has always been inevitable, and the future, while it still depends on the choices we make, is already written. This line of thinking is a threat to the feeling of control that’s central to most people’s notions of the self, and many reject it out-of-hand, because a lot of things can start to unravel once you accept it.

But if you can, it grants a kind of enlightenment, full of absolution, wonder, and humility. When we see someone make bad decisions, we have to realize that, given what they were in life, there’s no other choice they could have made. The phrase “If I were you…” stops making sense; if you were them, you’d have done exactly as they did. Rather than blaming, or judging a person for things that are ultimately beyond their control, we’re instead inclined to ask how we might influence them to make better decisions next time. That doesn’t necessarily mean giving someone a pass on bad behavior because they can’t help being like that—after all, it’s the consequences of our actions that “program” us in the first place. Instead, it means we have to make our own choices wisely, to be one of the factors beyond another’s control that leads them to make the right choice in the future.

This line of thinking goes both ways, too; when it stops being useful to blame someone for their failings, taking credit for accomplishments goes out the window as well. With one fell swoop, adopting a deterministic worldview frees us from both egotistical and judgmental thinking. Vanity is replaced with gratitude to our parents and role models, to the society that shaped us into who we are. Judgment and blame are traded for empathy and pragmatic action.

When we see ourselves and every thought we have—down to the ones these words are provoking in you—as the inevitable outcome of all of history, the “self” doesn’t have to disappear. It can expand, instead, to encompass the entire universe, and everyone on Earth. All of it conspired to create this, the only possible now, the world that we’re a part of, where you clicked this link and read these words.

Your destiny is an echo of the big bang. Your story has been written since the beginning of time—and possibly before, since the universe owes its shape to something. Where does the story go from here? The choice, paradoxical as it may be, is yours.


—Stephen Skolnick

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