Universe Today
This is Part 2 in a series on the physics of free will. Check out Part 1.
All of physics rests on causal determinism. It’s like…how we do physics. It IS physics. It’s how we make predictions for experiments and determine what the universe was like in its past. If we didn’t have causal determinism, we….well life would be a lot more difficult. Yes, Laplace’s demon doesn’t ACTUALLY exist, and no we don’t have ALL the laws of physics…yet. And even if we never did, even if we stopped all of physics research and said pack it in everybody, we’re giving up, what matter is that Laplace’s idea works in PRINCINPLE. As long as you can imagine such a super-intelligence, then causal determinism is true and free will doesn’t exist.
But wait, didn’t Laplace live over two hundred years ago? Hasn’t physics advanced since then? Why do we care what some long-dead French guy has to say about the subject?
Fair questions. Physics has advanced a lot. And there are three branches of physics that…well they don’t exactly restore free will but they certainly put some cracks in the whole causal determinism THING.
The first of those branches is chaos theory. If I give you a pendulum, then I can perfectly predict its motion for all of eternity. You tell me the starting angle, the length of the string, the strength of Earth’s gravity, and I’m done. I can write down a single equation that will you exactly where that pendulum will be at any point in the infinite future. Causal determinism for the win.
But what if I gave you a DOULBE pendulum? First off you might think that I’m not the most thoughtful gift-giver, and if I’m gifting double pendulums I should just give you a card stuffed full of cash, which is fair. But anyway, a double pendulum is one pendulum attached to the end of another pendulum.
Here’s the wild thing. Causal determinism? Check. All forces of nature and possible interactions? Known. Math? Simple and straightforward. Ability to predict the future? Absolutely gone.
A double pendulum is CHAOTIC. These are physical systems that just go on to random and unpredictable places, despite knowing the initial state as well as we can. Because if there’s any amount of uncertainty whatsoever, even MICROSCOPIC uncertainty, then my ability to predict the pendulum’s behavior far into the future totally falls apart.
To be able to predict the future I don’t just need the laws of physics, I need to know the initial state. I need to know EXACTLY at what angle the pendulum started at. With a regular pendulum, if I’m a little off in my measurement of its position, it’s no big deal. That uncertainty, that little bit of fuzziness, doesn’t go anywhere or do anything – it doesn’t affect my ability to make future predictions.
But a double pendulum? A chaotic system? If its ACTUAL starting position is different by even a SUPER TINY AMOUNT, like a femtometer, from what I THINK is its starting position, then chaos comes roaring in. The uncertainties in a chaotic system AMPLIFY on themselves as time goes on. And what I see is that at first the double pendulum is kinda sorta close to where I predict it should be, but before long it gets wildly out of step, and before I know it I have no idea of what’s going on or how to keep track of it.
Causal determinism is still here. There’s no magic of divine finger that’s reaching down from the heavens to nudge around the pendulum when I’m not looking. It’s all just Newton’s laws and gravity and F=ma and all the usual stuff. It’s causal, it’s deterministic…and unpredictable.
Chaotic systems are everywhere. It’s one of the reasons that weather is so hard to predict. It’s not just because it’s complex (with lots of forces and interactions and particles involved), it’s because its chaotic: even a tiny tiny tiny mis-judgment or error in measurement spirals out of control to the point that long-term prediction becomes impossible.
Now chaos theory doesn’t sway the free will argument one way or another, but it does color it. Chaos theory tells us that in certain cases we can never make fully accurate predictions. Even Laplace’s super-intelligence is going to fall short, because it’s impossible to know the exact precise starting state of every particle in the universe down to infinite precision.
So chaos theory doesn’t DESTROY determinism, it just makes it more fun. So it could be that free will still exists, or it could mean that free will doesn’t exist, but that we’ll still never be able to predict the actions that we’ll take.
And, listen, I know I’m tying the question of free will to the question of determinism, which isn’t necessarily the right thing to do. Because if you say hey determinism doesn’t exist, and there can be effects without causes, then you can also say that we’re not responsible for our actions because we have no will over them. Our choices and actions just…happen, with no agency input of our own. So in a way you might NEED determinism to make free will happen, because that’s the only way that we can be responsible for the choices we make.
But there are, of course, other philosophical arguments around that (they’ve had a lot of practice in the past few thousand years). And, of course, I’m not a philosopher. So I’m going to keep focusing on determinism because that’s where physics has the most to say.
And quantum mechanics has a LOT to say about determinism.