If everything that happens is governed by the laws of nature, then in some sense things have to occur just as they do. And since people are situated within nature, that applies as much to us as billiard balls and comets. So while it might seem like your actions are to some degree “free,” from a physicist’s point of view, there is no room in nature for free will. Every event is law-governed and determined to occur just as it does.

Here then is the worry about determinism. Is it true? The short answer is “probably not,” but the strongest case for determinism has historically come from physics. Let’s consider why.

Newton, His Laws, and Determinism

Although concerns about determinism were around long before the scientific revolution, Newtonian physics put it in a stark light. Consider a football that has just been kicked on the way through the uprights. Given the momentum imparted by the kicker, the force of gravity, and the wind conditions, the trajectory of the ball is fully determined by Newton’s laws of motion. Had the kicker placed his foot a centimeter lower,Hence, it seems that all events are at a deep level determined by Newton’s laws, even if it does not feel that way to us. the trajectory would be different. Nonetheless, given the initial conditions, the ball’s behavior is fixed and any physicist with enough information could predict its path. This is a deterministic system. Given a set of initial conditions and the relevant laws of nature, there is only one way the system can evolve.

Physicist Pierre-Simon Laplace extended this idea to the whole of nature. Laplace argued that if a super intelligence knew all the forces involved and the state of every particle at a given instant in time, it could in principle predict how the universe would evolve arbitrarily far into the future.Pierre Simon Laplace, A Philosophical Essay on Probabilities, Translated by F. W. Truscott and F. L. Emory, (New York, NY: Wiley, 1902). Originally published 1814. This would be equally true of the particles in your body as those floating in space. Hence, it seems that all events are at a deep level determined by Newton’s laws, even if it does not feel that way to us.

Does this show that freedom is impossible under Newtonian physics? Not quite. There were known exceptions to determinism, such as when three particles collide at the same time. The particles will rebound in some way or other, but the laws of mechanics do not say how. Some physicists, like James Clerk Maxwell (1879, 757), looked to these exceptions for clues about free will.James Clerk Maxwell, “Letter to Francis Galton, February 26, 1879,” in The Scientific Letters and Papers of James Clerk Maxwell, edited by P. M. Harman, 3:756–758, (Cambridge: Cambridge University Press, 1879). Others thought that the existence of minds/souls solved the problem. Determinism only applies to fully material entities, like atoms. But if the will is a capacity of the immaterial part of our being, then it is not subject to the laws of nature. Physical determinism is thus compatible with free will, if mind-body dualism is true.

Quantum Mechanics and Indeterminism

In any case, proponents of free will are generally relieved that Newtonian mechanics has been overthrown by quantum mechanics. This is because quantum systems appear to be indeterministic: the prior states of the system plus the laws of nature do not specify how the system will evolve. If rolling dice were truly indeterministic, then not even Laplace’s super intelligence with full information about the momentum of the dice, the friction of the table, etc., could predict how they would stop.

This claim about quantum indeterminism is misleading in two ways, however. First, most of what goes on at the quantum level is deterministic. Second, quantum mechanics is subject to a variety of interpretations only some of which allow for indeterministic events. Let’s consider these in more detail.

More surprisingly perhaps is that not all physicists and philosophers of physics believe that such events really are indeterministic, even if they might appear to be. The question depends on what interpretation of the theory one holds.

Quantum systems evolve according to Schrödinger’s equation, rather than Newton’s second law of motion. But the two have something in common: they are both fully deterministic. Most of elementary quantum physics is about the application of Schrödinger’s equation, and so indeterminism plays a minor role. Where does it appear? Radioactive decay for one. While there is a known probability that a given uranium atom will decay over some period of time, there is no hidden “fuse” within the atom that triggers that event. The atom might decay in the next hour or not for centuries, but precisely when it does so has no specific cause. In some sense, it “just happens,” and not because we are lacking any information about how radioactive decay works.

More surprisingly perhaps is that not all physicists and philosophers of physics believe that such events really are indeterministic, even if they might appear to be. The question depends on what interpretation of the theory one holds. The most common of these, what is (somewhat unfortunately) called the “Copenhagen interpretation,” includes indeterministic events, namely “measurements.”The scare quotes highlight that while measurement is an important idea in quantum mechanics, it has always been ill-defined. In the famous Schrödinger’s cat thought-experiment, there is a 50 percent chance that the cat emerges alive and 50 percent dead. The example is engineered so that the cat itself constitutes the measurement device. Under the Copenhagen interpretation, the measurement is indeterministic in that nothing in nature causes the outcome to be one way rather than the other.

A more popular view among philosophers of science is the GRW (Ghirardi–Rimini–Weber) interpretation.Peter J. Lewis, Quantum Ontology: A Guide to the Metaphysics of Quantum Mechanics (New York, NY: Oxford University Press, 2016). For this discussion, the only important difference is that indeterministic quantum events spontaneously occur on this view, instead of requiring any sort of measurement.

What About Free Will?

While these interpretations allow for indeterminism in physics, they are not themselves accounts of free will. An act of will is not a random event. Often it is, as Tom McCall pointed out in his introduction to this series, a rational act. People make choices based on their beliefs.People make choices based on their beliefs. Hence, most proponents of free will believe that quantum mechanics merely makes room for freedom in nature, but physics does not explain free will. Hence, most proponents of free will believe that quantum mechanics merely makes room for freedom in nature, but physics does not explain free will. Quantum indeterminism is at best a necessary condition. Note, however, that there are other interpretations of quantum mechanics. Let’s consider two more.

Another part of the story of orthodox quantum mechanics is that particles sometimes take on wave-like properties. Indeterminism is introduced in this transition between the wave-like and particle-like, which happens during measurements. Under the Bohmian interpretation of quantum mechanics, there is no such transition. Particles are always particles with well-defined positions in space. That doesn’t mean that we can predict how the particles will behave, but on Bohm’s view there is always a hidden truth about their position. To account for wave-like phenomena, Bohmian mechanics introduces a new, unobservable entity—the pilot wave—that is responsible for moving particles around. Without any wave-to-particle transitions or mysterious measurement events, there is no indeterminism. While quantum weirdness is not thereby eliminated, it is a small step back towards the more intuitive world of Newtonian physics. The next interpretation is decidedly not.

Consider Schrödinger’s cat again. In the standard example, there is a time when the cat is in some sense both alive and dead, a so-called superposition state. Looking in the box constitutes a measurement, destroying this superposition, and the cat instantaneously becomes either fully dead or fully alive. Things are different under the many-worlds interpretation. Instead of the system randomly collapsing into either a dead-cat or alive-cat state, both occur, but within their own universes. According to physicist Bryce DeWitt, whenever a superposition collapse would have occurred under the Copenhagen interpretation, separate universes come into existence.DeWitt was building on a proposal originally made by physicist Hugh Everett in his doctoral dissertation. For Schrödinger’s cat, there is one universe with a live cat and another identical world except that the cat is dead. Each possible outcome of a measurement happens in its own universe.

The key point here is that the creation of the different universes is a deterministic process. There are no measurement events with random outcomes in this interpretation. Instead, everything that could happen in quantum mechanical terms does happen somewhere among the many worlds. The upshot is that as more recent versions of this view continue to gain in popularity, there might come a day when physicists again believe that nature is fully deterministic.See David Wallace The Emergent Multiverse: Quantum Theory According to the Everett Interpretation (Oxford: Oxford University Press, 2012).

Would this spell the end for free will? Not in my view. Physical systems adapt to changes regardless of whether those systems are deterministic. The hands of a grandfather clock are mechanically determined by its internal mechanism. Nonetheless, if you were to reach in and move the hands, the system would deal with this change. Freedom at the level of persons is not dependent on indeterminism at the level of physics, although in fairness, many scientists and philosophers believe otherwise.See Jeffrey Koperski, Divine Action, Determinism, and the Laws of Nature (London: Routledge, 2020).