Modern physics has turned the old clockwork picture of the universe into something far stranger, filled with quantum probabilities, emergent patterns and deep puzzles about what counts as a “cause.” That shift has reopened an ancient question in a new key: if the world is governed by physical law, is there any meaningful sense in which our choices are genuinely ours. I want to trace how today’s best physical theories sharpen that question, and where, if anywhere, they leave space for a robust idea of free will.
From clockwork determinism to quantum uncertainty
Classical mechanics encouraged a hard-edged determinism: given the positions and velocities of every particle, the future seemed fixed. That picture underpins Stephen Hawking’s famous worry that if behavior is fully dictated by physical law, then we are “no more than biological machines” and what we call free will might be “just an illusion,” a concern captured in his widely cited remark that “it is hard to imagine how free will can operate if our behavior is determined by physical law” where he adds that free will could be “just an illusion” of self-awareness, a line preserved in a popular Hawking quote. On that view, every decision is simply the unfolding of prior conditions, no more open to genuine alternatives than the orbit of a planet.
Quantum mechanics disrupted that certainty by building probabilities into the basic formalism. As one analysis of the “edge of randomness” in physics puts it, quantum mechanics is the one place in fundamental physics that seems to have randomness built in, and the same discussion suggests that without this element “we would have a lifeless deterministic universe.” That does not automatically rescue free will, but it does mean that the future is not simply a rigid extrapolation of the past, and any serious account of human agency now has to grapple with both lawlike structure and genuine unpredictability.
What physicists actually think about free will
From the outside, it is tempting to imagine a single “physics answer” to the free will question, but working scientists are more fragmented and often more pragmatic. In one widely shared discussion on r/AskPhysics, a commenter admits “I’ve never heard of compatilism until you and u/OrboOhChao have mentioned it here, so I look forward to learning more about it,” before another voice notes that “most physicists don’t worry” about free will because they are “busy doing science stuff,” a candid snapshot preserved in an AskPhysics thread. That exchange captures a quiet reality: there is no consensus, and for many researchers, the metaphysical stakes feel remote from day-to-day calculations.
When physicists do weigh in, they often import philosophical labels rather than derive new ones from equations. Some, like the cosmologist Brian Greene, lean toward a hard-line view that free will is incompatible with a universe governed by impersonal laws, a stance described in a profile that lists him as “another prominent free will-denier” and notes that science journalist Philip Ball has tried to bring that skepticism into dialogue with psychological accounts of agency in a piece on finding the freedom in free will. Others adopt some form of compatibilism, arguing that even if the microphysics is fixed or probabilistic, there is still room for meaningful choice at the level of persons, a position that shows up as much in online debates as in formal papers.
Quantum superposition, randomness and the limits of “could have done otherwise”
One of the most common lay arguments for free will invokes quantum superposition: if particles exist in multiple possible states until measured, perhaps our brains can “choose” among those possibilities. A detailed exchange on Philosophy Stack Exchange, which begins with “Mar 12, 2024 — * 9 Answers. Sorted by: 7,” pushes back on that hope by stressing that quantum probabilities are still governed by precise mathematical rules and that invoking them to explain choice risks smuggling in “some non-physical concept,” a concern laid out in a thread asking whether quantum superposition enables free will. On that reading, indeterminism at the particle level does not automatically translate into a coherent story about agents who could have done otherwise for reasons.
Even among enthusiasts of quantum indeterminacy, there is recognition that randomness alone is not enough. In a r/freewill discussion titled “Just got suggested this sub. Why is there any debate?,” a user named LordSaumya posts in Dec that if someone has “proved that quantum mechanics is indeterministic, then it is a simple matter” to push through to free will, only to meet resistance from others who argue that unpredictability is not the same as control, a clash preserved in the Dec thread. The tension is straightforward: if my neurons occasionally fire one way rather than another because of quantum noise, that may break strict determinism, but it does not obviously make me more responsible for the outcome than if they had followed a fixed trajectory.
Observers, measurements and the “Free Will Theorem”
Modern physics also complicates the picture of measurement and choice in more subtle ways. In quantum experiments, the decision about what to measure seems to affect which properties of a system become definite, which has led some to wonder whether the experimenter’s freedom is built into the theory. A classic example is the “free will theorem,” which states that, given certain axioms, “if the choice about what measurement to take is not a function of the information accessible to the experimenters, then the outcome of the measurement is also not a function of anything previous to the experiments,” a striking claim summarized in the Free Will Theorem entry. The theorem does not prove that humans have free will, but it does show that if we treat experimenters’ settings as genuinely free, then certain particles must behave in ways that are not fixed by prior information either.
Debates on specialist forums highlight how slippery this line of thought can be. In a Physics Forums thread titled “Modern Physics, The Observer, and free-will,” one contributor writes that “the freewill issue is about imagining ourselves going to either simple extreme,” and then contrasts “either being completely determined by s…” with the opposite fantasy of total randomness, a framing preserved in the Jun discussion. That comment captures a broader lesson from quantum foundations: the mathematics forces us to abandon some naive pictures of causation, but it does not straightforwardly tell us whether the choices of observers are metaphysically special or just another part of the physical story.
When mathematicians and popularizers claim to “prove” or “disprove” free will
Occasionally, technical results are promoted as decisive verdicts on human freedom. The “Free Will Theorem” itself is sometimes presented this way, and so are more speculative arguments that try to combine quantum mechanics with information theory. A skeptical commentary on one such claim, which reports on a mathematician who says he has proven free will, begins with the line “But what makes me think that I don’t understand Conway’s proof of free will comes from the way it’s characterized in, say, Wikipedia,” and then unpacks how the informal summaries overreach what the formal result actually shows, a cautionary note recorded in a critique of Conway and the way his theorem is described on Wikiped. The upshot is that even elegant theorems about measurement choices and correlations do not, by themselves, settle the messy psychological and ethical questions that surround free will.
On the other side, some science communicators lean into a strong denial of free will, treating it as a straightforward consequence of physics. The Psychology Today analysis that introduces Brian Greene as “another prominent free will-denier” notes that Philip Ball tried to explore what, if anything, modern physics has to say about “agent causation,” the idea that persons can initiate new causal chains rather than merely pass along prior influences, a theme developed in the same Psychology Today piece. Those arguments are powerful reminders that physics constrains what kinds of agency are possible, but they also risk flattening the layered structure of human decision making into a single, all-or-nothing verdict.
Emergence, brains and the case for compatibilist freedom
One way to reconcile physical law with meaningful choice is to treat free will as an emergent property of complex systems rather than a primitive ingredient of the universe. Work on neural networks and dynamical systems in cognitive science emphasizes how simple local rules can generate rich, flexible behavior at larger scales. A landmark paper on “Emergence of Structure in Cognition” invites readers to “consider the complex structure of an ant hill,” noting that it can have “tunnels that go underground to 25 feet into the sky,” even though no single ant has a blueprint for the whole, an image developed in a study of Emergence of Structure in Cognition Emergence. The same logic suggests that human decision making might be a higher-level pattern that is real and causally efficacious, even though it is fully implemented in neural hardware.
From this perspective, the key question is not whether we can step outside physics, but whether the structures described by psychology and neuroscience have the right kind of stability and responsiveness to count as “free.” Jim Al-Khalili, in a physicist’s reflection on choice, notes that “were it not therefore for the weird and probabilistic quantum rules according to which those atoms behave, and in the absence of a…” range of possible outcomes, we would face a single fixed future, a point he uses to argue that quantum indeterminacy at least opens the door to “different outcomes, and hence different futures,” a line of thought he develops in his physicist’s perspective. That does not prove that emergent agents are free in a morally robust sense, but it does show how probabilistic microphysics and structured macrobehavior can coexist in a way that leaves room for nuanced forms of responsibility.
Stripping away bad arguments and focusing on what matters
Online debates about free will often get bogged down in what even participants recognize as distractions. In a r/freewill post titled “Let’s at least strip away from the debate an irrelevant if not flawed …,” a commenter in the Comments Section named rickthiagomelo writes “I thought your point of view was brilliant. I totally agree and I believ…” before the thread turns to the idea that some popular arguments, such as appeals to vague “quantum magic,” should be set aside so that more serious positions can be evaluated, a sentiment captured in the Jun Comments Section. That instinct is healthy: if modern physics is going to inform our thinking about agency, it has to be through clear mechanisms and constraints, not hand-waving about spooky effects.
Specialist communities echo this call for discipline. The Physics Forums discussion on “Modern Physics, The Observer, and free-will” warns against imagining that either extreme, total determinism or pure randomness, can capture the lived texture of decision making, and urges readers to “look up for the answers, not down,” a phrase that suggests we should pay attention to higher-level organization rather than only to micro-level noise, as the Jun forum post puts it. I find that advice compelling: the most promising role for physics in the free will debate is not to deliver a yes-or-no verdict, but to clarify which pictures of the world are even coherent enough to host the kinds of agency we care about.
Where the live debate now sits
At this point, the most interesting arguments cluster around a few live options. Hard determinists, often inspired by classical mechanics and by voices like Stephen Hawking and Brian Greene, hold that once we accept a law-governed universe, the sense that we could have done otherwise is illusory, a stance that shows up in Hawking’s remark that free will “is just an illusion” and in Greene’s reputation as a “free will-denier,” both highlighted in the Hawking quote and the Psychology Today profile of Brian Greene and Philip Ball. Libertarians about free will, by contrast, look to quantum indeterminacy and results like the Free Will Theorem to argue that some choices are not fixed by prior physical states, though critics point out that this still leaves open the problem of how randomness becomes responsibility.
Compatibilists, finally, try to chart a middle path by redefining free will in terms of capacities that can exist in a deterministic or probabilistic universe: the ability to respond to reasons, to plan, to learn from feedback, and to integrate information over time. Tim Andersen, in a piece on “quantum mechanics and the return of free will,” suggests that quantum theory might underwrite a richer picture of agency without abandoning physical law, even as a commenter named Tomas Schutz writes on 1 August that the article “still doesn’t ad…” fully resolve the philosophical puzzles, a tension recorded in the Jul analysis where readers are invited to Sign in and where Tomas Schutz weighs in. I read that exchange as emblematic of the current moment: physics has cracked open new conceptual space, but the hardest work now lies in integrating those insights with what we know about brains, behavior and the social practices that make talk of choice matter in the first place.
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