On the morning of June 28, 1914, a Bosnian Serb student named Gavrilo Princip stood outside Moritz Schiller’s delicatessen near the Latin Bridge in Sarajevo. Sometime after 10:45 A.M., a motorcade carrying archduke Franz Ferdinand, heir to the throne of Austria-Hungary, passed within meters of Princip, who drew his 0.38-caliber pistol and fired. One bullet struck the archduke in the neck. He was rushed to the military governor’s residence for medical treatment, but by 11:30 A.M. he was pronounced dead.
Thank you for reading this post, don't forget to subscribe!The assassination helped spark World War I. Historians view history as a series of interconnected but highly contingent events—built of myriad and mostly unseen chains of cause and effect. If Princip’s gun had jammed, the thinking goes, the archduke would have lived, and Europe’s subsequent history may well have been very different. Fiction writers have long been enthralled with these what-ifs (known to philosophers as “counterfactual histories”): What if Hitler hadn’t flunked out of art school? What if the Germans had developed the atomic bomb before the Americans? What if John Lennon had never met Paul McCartney? What if an asteroid hadn’t wiped out the dinosaurs some 65 million years ago and reptiles still ruled the Earth?
Such contingencies presume, of course, that things could have been different—either because a person exercising their free will could have chosen another course of action (Princip could have chosen not to pull the trigger) or because random events (such as the asteroid strike) could have unfolded differently. But is this attitude compatible with physics? Do the natural laws of the universe allow for free will?
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Scientists and philosophers have been arguing over the question for centuries and are often torn between two competing poles. Some think, Yes, you obviously have free will. (Aren’t you already four paragraphs into a story that you freely chose to read?) Others think, No, you can’t possibly have free will because the laws of physics say that whatever happens was determined by what happened immediately before—and the happenings within human minds are no exception. Recently a new argument for why quantum mechanics is even more deterministic than physicists might have thought has sparked the debate anew.
The notion that physics and free will might be incompatible goes back at least to the ancient Greeks, but it was expressed most forcefully by French scholar and polymath Pierre-Simon Laplace. Perhaps, wondered Laplace, everything that happens is strictly determined by what came before. His thought experiment involved an entity, now known as Laplace’s demon, that can discern the position and momentum of every particle in the universe. For such a demon, the future is fixed: there can be only one way for the universe to unfold. The cosmos would be deterministic, meaning that the future is uniquely determined by the present, which in turn was uniquely determined by the past. If Laplace was right, the notion of contingency—the idea that regardless of what’s happening at any moment in time, what happens next is “up in the air”—would seem to evaporate.
Then at the start of the 20th century came the twin upheavals of quantum mechanics and relativity. Quantum mechanics, in particular, seemed to have profound implications for free will and contingency. The theory sees nature as inherently fuzzy: quantities that were clearly defined in classical physics, such as position or momentum, are indeterminate in quantum mechanics—until they’re measured. Upon measuring a system (at least in the so-called Copenhagen interpretation of the theory), its wave function (a mathematical description of the system) is said to “collapse,” leaving one unique outcome, such as a specific observed position or momentum. The theory tells you only the probability of various outcomes of each observation but not which result you’ll actually see. At first glance, this haziness might seem to rescue physics from the clutches of determinism. On the other hand, it’s not clear how quantum indeterminacy would enable free will because we don’t usually think of our decision-making processes as random any more than we think of them as wholly preordained.
But there is another twist in this story—one that crops up when physicists attempt to apply quantum mechanics to the entire universe (a field known as quantum cosmology). Some quantum approaches to cosmology, such as the one envisioned by theoretical physicists Jim Hartle and Stephen Hawking (and described by Hawking in A Brief History of Time), appear to dictate not only the rules governing the evolution of the universe but also its initial state. In this way of seeing things—physicist Roger Penrose called it “strong determinism” in his book The Emperor’s New Mind—the universe can have precisely one history. Nothing could have been different from how it actually was and is. Everything from the trajectory of Princip’s bullet to the fact that you’re now reading this sentence was prescribed, so to speak, at the dawn of time.
That’s one way to interpret quantum mechanics—but not the only way. Another popular take is known as the “many worlds” view (or the Everettian view, after physicist Hugh Everett III, who first wrote about it in detail). In this view, everything that can happen does in fact happen—but in a different universe. So rather than saying that the universe has precisely one history, proponents of many worlds would say that the “multiverse” has just one history. Within this multiverse, there are branches, or universes, in which Princip pulled the trigger and also ones in which he didn’t. There are universes where Schrödinger’s famous cat is alive and universes where it’s dead. But the cosmos as a whole is fully determined.
Eddy Keming Chen, a philosopher of physics at the University of California, San Diego, believes we should take the idea of strong determinism—and its implications—seriously. If we embrace a theory like the one put forward by Hartle and Hawking, in which both the dynamics and the initial conditions of the universe (or multiverse) are specified, then only one unique history is possible. From this perspective, quantum mechanics is even more deterministic than its classical predecessor, Chen argued recently in Nature. (In a related preprint, Chen developed the idea further, describing what he calls the “Everettian Wentaculus,” which he wrote is “the first realistic and simple strongly deterministic theory of the quantum world.”)
But it’s tricky: even if we live in an Everettian multiverse, we only see one branch—our universe—and within that branch, we still tend to imagine that multiple outcomes are possible. In his preprint, Chen admits that “it is an open question how to think about freedom and agency in a multiverse context.” At the very least, though, the way we usually understand decisions, choices and contingency would need a rethink, Chen says. He believes that under strong determinism, it no longer makes sense to speak of counterfactuals. “You can understand the counterfactuals as referring to different physical possibilities compatible to the laws of physics,” Chen says. “But if I tell you there’s only one single possibility, then there are no counterfactuals. All counterfactuals become meaningless or trivial or vacuous.” And if there are no counterfactuals, he says, there’s no freedom. As he wrote in his Nature essay, strong determinism “makes it harder to appeal to quantum theory to defend free will.”
While physicists continue to debate the idea of strong determinism, Emily Adlam, a philosopher of physics at Chapman University, agrees with Chen that it appears to present more of a threat to free will than traditional determinism, particularly because of its ties to the Everettian multiverse. “In a standard deterministic picture, sure, everything that happens was determined from the past—but your mind was a key part of the causal process by which future events get realized,” Adlam says. “So in some meaningful sense, future events—even though they were predetermined—were mediated through processes that you identify with yourself.” But in the Everettian picture, she says, it’s harder to see where decision-making would fit in. “If you always make every possible decision, that does seem to severely undermine the sense in which you are exercising any meaningful kind of choice,” she says. “So in that sense, you do seem worse off than in the standard picture, where one outcome occurs and you play a role in bringing it about.”
As troubling as quantum mechanics (or at least certain versions of it) may be for the idea of free will, relativity—the other pillar of modern physics—isn’t off the hook. Many theorists think of relativity as describing a universe in which past, present and future are all equally real: a static cosmos that just sits there like a big block of spacetime (sometimes called the “block universe”). It’s not that time disappears in this picture—but it no longer “passes” or “flows.” (As Albert Einstein famously put it, the passage of time is a “stubbornly persistent illusion.”) Conceptually speaking, the strongly deterministic quantum universe and the block universe of relativity may not be so far apart. The quantum version can be thought of as “a kind of enriched block universe,” says Alastair Wilson, a philosopher of science at the University of Leeds in England. “Imagine taking a block universe and adding an extra dimension to it—the dimension of possibility.”
Still, theories about the fundamental nature of space and time need to be taken with a grain of salt. Physicists have a reasonably good grip on most of the universe’s 13.8-billion-year history. As we rewind the tape, though, we find that our understanding of space and time becomes more tenuous as we get closer to the big bang. In the universe’s first moments, neither relativity nor quantum mechanics on their own can offer an accurate description of what’s happening, and there’s no agreed-upon unified theory of quantum gravity to take their place. In this realm, “notions of space and time themselves start breaking down at the fundamental level in ways we don’t understand,” says David Wallace, a physicist and philosopher at the University of Pittsburgh. “If the notion of time breaks down, then the sharp distinction between laws, which say how things change over time, and initial conditions, which say how things are at the initial time, starts breaking down as well.” Despite how much interest the Hartle-Hawking proposal has garnered, Wallace cautions that it is still “speculative.” And although Wallace is an ardent Everettian (as are the well-known scientists David Deutsch, Max Tegmark and Sean Carroll), the Everettian multiverse remains controversial as well.
Skepticism about free will is hardly new. Long before quantum mechanics and relativity came along, people wondered what sort of freedom, if any, could be found in a universe in which matter merely moves about in response to forces like balls in a never-ending game of cosmic billiards. The latest in a long line of free will skeptics is biologist and neurologist Robert Sapolsky, whose most recent book is entitled Determined: A Science of Life without Free Will. You are who you are, Sapolsky argues, because of everything that came before, both in your own life’s history and long before you were born. After decades of trying to see what wiggle room science might leave for personal freedoms, he has concluded that “we have no free will at all,” he wrote in his book.
The preferred “solution” to reconciling possibly deterministic physics and seemingly free personal choices—the quotation marks are important because not everyone is onboard—is a position known as compatibilism. Whatever the fundamental particles and forces might be doing at the subatomic level, the compatibilist says, human freedom can still exist because we live our lives in the macroscopic world, where very different rules apply. Yes, we’re made of atoms (or fluctuating quantum fields, if you prefer), but it would be absurd to try to describe any feature of human behavior by analyzing our atoms (or our quantum fields). And although a slight majority of philosophers identify as compatibilists (polls put the figure at around 60 percent), others see it as a cop out. Immanuel Kant, for example, dismissed compatibilism as “wretched subterfuge.” More recently, neuroscientist Sam Harris wrote in his book Free Will that “from both a moral and a scientific perspective, [compatibilism] seems deliberately obtuse.”
For compatibilists, it comes down to a matter of perspective. Wilson gives the example of astronomer Arthur Eddington, who, writing a century ago, pointed out that a table loses its tablelike properties when examined at the microscopic level. “He discovered there’s a lot of empty space between the particles in the table,” Wilson says. “Does that mean it’s not solid? Or does that mean that solidity is not what we thought it was?” He suggests looking at the Everettian multiverse in the same light. From one perspective, we might say that the very notion of probability has vanished—or we could say “that there are probabilities—they’re just not what we thought they were.”
For committed compatibilists, the issue of free will doesn’t depend on what physics says about atoms, forces, quantum fields or anything else that applies at the microscopic level, and strong determinism is no more upsetting than regular determinism. As Adlam puts it: “On one level of description, people are the source of their decisions, and on a different, physical level of description, the distant past and the laws of physics are the source of their decisions. And I think if you keep those two levels of description separate, as you should, then you don’t really have a problem with free will.”
