Physicist Sean Carroll has picked up the gauntlet dropped by Massimo and me, and has a nice post up on Cosmic Variance about determinism and its connection with free will: “On determinism.” Sean, of course, knows a ton more about physics than either of us, and his take is well worth reading. Here are some of the points he makes:
- All of our actions must obey the laws of physics. All of us agree with that, of course, but he realizes that this raises a question, “What room, then, for free choice?”
- Some people assert that classical mechanics may not be deterministic, though Sean says, “I personally don’t find the examples that impressive.”
- Conversely, quantum mechanics (QM) may be deterministic. It’s not deterministic under the bizarre Copenhagen interpretation, but is deterministic under the even more bizarre many-worlds hypothesis, in which all possible outcomes of quantum “indeterminacy” are actually realized in different universes (so there may be an infinite number of universes!). I was surprised to learn that Carroll adheres to the many-worlds interpretation of QM, which is a form of physical determinism.
- Carroll isn’t clear about whether “the lack of determinism in QM [if it's indeed probabilistic] plays any role at all in our everyday lives.” I, too, was dubious about that, although I can see how, as some readers have suggested, indeterminacy could have real effects on us, for example in creating mutations that affect our heredity or, somatically, causing cancer.
- The idea of chaos is irrelevant to this discussion, because, as I’ve pointed out as well, chaos theory is not probabilistic but deterministic. It affects predictability, since we can’t know things to such a precise level, but still allows results to be completely predetermined by initial conditions.
Of course, whether the laws of physics are deterministic or probabilistic is, to me, irrelevant to whether there’s free will, which in my take means that we can override the laws of physics with some intangible “will” that allows us to make different decisions given identical configurations of the molecules of the universe. That kind of dualism is palpable nonsense, of course, which is why I think the commonsense notion of free will is wrong. As Carroll notes:
We can imagine four different possibilities: determinism + free will, indeterminism + free will, determinism + no free will, and indeterminism + no free will. All of these are logically possible, and in fact beliefs that some people actually hold! Bringing determinism into discussions of free will is a red herring . . .
A better question is, if we choose to think of human beings as collections of atoms and particles evolving according to the laws of physics, is such a description accurate and complete? Or is there something about human consciousness — some strong sense of “free will” — that allows us to deviate from the predictions that such a purely mechanistic model would make?
If that’s your definition of free will, then it doesn’t matter whether the laws of physics are deterministic or not — all that matters is that there are laws. If the atoms and particles that make up human beings obey those laws, there is no free will in this strong sense; if there is such a notion of free will, the laws are violated. In particular, if you want to use the lack of determinism in quantum mechanics to make room for supra-physical human volition (or, for that matter, occasional interventions by God in the course of biological evolution, as Francis Collins believes), then let’s be clear: you are not making use of the rules of quantum mechanics, you are simply violating them. Quantum mechanics doesn’t say “we don’t know what’s going to happen, but maybe our ineffable spirit energies are secretly making the choices”; it says “the probability of an outcome is the modulus squared of the quantum amplitude,” full stop. Just because there are probabilities doesn’t mean there is room for free will in that sense.
But Carroll himself doesn’t adhere to this form of incompatibilism. He opts instead to redefine free will so it’s compatible with the laws of physics:
On the other hand, if you use a weak sense of free will, along the lines of “a useful theory of macroscopic human behavior models people as rational agents capable of making choices,” then free will is completely compatible with the underlying laws of physics, whether they are deterministic or not. That is the (fairly standard) compatibilist position, as defended by me in Free Will is as Real as Baseball. I would argue that this is the most useful notion of free will, the one people have in mind as they contemplate whether to go right to law school or spend a year hiking through Europe. It is not so weak as to be tautological: we could imagine a universe in which there were simple robust future boundary conditions, such that a model of rational agents would not be sufficient to describe the world. E.g. a world in which there were accurate prophesies of the future: “You will grow up to marry a handsome prince.” (Like it or not.) For better or for worse, that’s not the world we live in. What happens to you in the future is a combination of choices you make and forces well beyond your control — make the best of it!
With all due respect to Sean, whom I like a lot, I think this is a bit of a cop-out. What he seems to mean here is that “we can act as if we and others have choices, though we really don’t, because what we ‘choose’ is determined not by our will but by the laws of physics.” Yes, that’s useful, I suppose, but I think he’s wrong in saying that “a model of rational agents” accurately describes our world. What does that mean? Do people always act rationally? That depends on your definition of “rational,” I think, and he doesn’t define it. If by “rational,” Sean means “according to the laws of physics,” then his conception does become tautological. But of course one can make useful models, as do economists, assuming that most people act rationally—given that you specify the meaning of “rational.”
Finally, it may be irrelevant whether or not determinism affects our conception of “free will,” for we—as did Carroll—can always define free will so that it’s independent of determinism. But the question of how deterministic our actions really are remains vitally important. It’s important in our conception of how we dispense justice and hold people responsible for their actions. It’s important for many religious people as well, for whom the absence of determinism is pivotal for issues about salvation. Maybe philosophers and scientists know that there’s no dualism, but it’s important for us to get that message out to the general public, if for no other reason than it dispels the idea that there are supernatural forces like ESP and “souls” that can affect our fate.
In the end, everything must obey the laws of physics, whether they be deterministic or probabilistic. All else is commentary.