Is the future predetermined? And if so, does this imply humans do not have free will?
Throughout history, many explanations have been offered. The ancient Greeks believed in Ananke, a primeval force of compulsion, and in the Fates, three goddesses who controlled every moment of life from birth to death. In Eastern philosophy, the laws of karma are rooted in the idea that actions have determined consequences. In Catholic theology, scholars struggled with whether an omniscient God knowingly creates individuals He knows will be destined for condemnation, or whether human free will renders His predictive capacity incomplete. Philosophers like Hume, Hobbes, and Nietzsche have written on the issue. Albert Einstein found the idea of a random universe without a definite outcome to be unsavory.
For most of human history, the discussion fell squarely within the jurisdiction of philosophers and theologians. Today science, and physics in particular, can speak more forcefully on the issue.
Start with classical Newtonian physics. Particles in the natural world possess characteristics such as mass, position, velocity, charge, etc., the full set of which we refer to as the “state” of the system. The laws of physics are literally mathematical functions. Input the state of a system, and the output tells you the future, like where a ball thrown off a cliff will land or how much light will bend as it passes through water.
Of course, gathering all the information needed to perfectly describe the state of a system may be impossible. So too may be the quest to learn every last one of the laws of physics. But philosophically, classical physicists operate under the presumption that a full set of these laws do exist and with complete omniscience would be sufficient to predict all future events.
If you accept this simple premise, then the laws of classical physics render the universe fully predictable and hence, fully predetermined, unless one of two things are true: A) there is no definitive “state” of a system or B) the laws of physics are non-deterministic, i.e. the same initial conditions will generate different final results.
To date, scientists have encountered no evidence to suggest that the laws of physics are non-deterministic. Identical experiments yield identical results regardless of when and where they are carried out. Scientific progress is defined, in a sense, as taking events that are seemingly random, and pulling back the veil to uncover there was in fact a deterministic explanation all along.
Option A turns out to be much more interesting, but one physicists couldn’t say much about it until the advent of quantum mechanics (QM) in the early 1900’s.
To put it frankly, QM as we understand it is weird and unintuitive. It states that particles at the quantum (e.g. atomic) level don’t possess definitive positions, momenta, energies, or the like. Instead, they have their own “wavefunctions.” Manipulated this way or that, the QM wavefunction can provide information about its system. For example, square the wave function and you obtain the probability that a particle will exist in a particular place at a particular time.
The word “probability” is key because it demarcates the classical and quantum mechanical views of the universe. The QM wavefunction is presumed to be a statistical object where each outcome (e.g. position, momentum) has a certain probability of occurring, but no outcome is for certain. This statistical take on QM is referred to as the Copenhagen interpretation.
To use an analogy, the difference between classical and quantum physics is like the difference between the values of dice which have been rolled then hidden and the inherent state of those same dice before they are rolled. The former has a definite but unknown state while the latter is purely random. For the unrolled dice, like quantum mechanics, everything is possibilities and nothing is for certain until we roll.
To better tease out the implications of the Copenhagen interpretation, allow me to introduce two Socratic adversaries: Raúl, who will defend the position that the universe is random and Dante, who will contend that it is deterministic. They begin by discussing an observation of an electron.
Raúl asks, “Can you tell me, Dante, what the position of the electron was the instant before we measured it?”
Dante replies, “Well that’s trivial. Once we measure, we know where the electron is. It must have been in almost exactly the same location the instant before.”
Raúl retorts, “That’s where you’re wrong. Our measurement, like a roll of the dice, caused the electron to be in a particular position. Before that measurement is was everywhere and nowhere.”
Physicists have observed the position of an electron multiple times in quick succession and found its position stays mostly the same. The standard explanation is that the measurement caused the wavefunction to collapse to a particular state. Whatever the position the electron collapsed to, it will remain there, at least for a little while.
Of course, there is no way of observing the electron immediately before observing it for the first time. It would seem that no experimental physicist will be able to resolve Raúl and Dante’s debate.
So instead we turn to three theoretical physicists – Albert Einstein, Boris Podolsky, and Nathan Rosen – who in 1935 published an article containing a simple thought experiment. Their idea later became known as the EPR paradox.
The paradox considers a subatomic particle known as a pion. Occasionally, a pion will decay into two particles – an electron and a positron – which subsequently fly off towards observers (let’s make them Raúl and Dante) at opposite ends of the Universe. Electrons and positrons each possess a property known as “spin”. There are only two spin-states allowed – spin-up or spin-down.
According to a law of physics known as “conservation of momentum” the electron and positron must possess opposite spins. But how does a particle come to possess its spin? Raúl would claim that this occurs at the time of measurement. “Only when we examine the electron does it finally assume a spin-state. Before that examination, it could have been either/or. There’s no way of knowing,” he explains.
“Oh look! Here’s comes the electron now!” Raúl grabs his detector to take the measurement. “Dante, I just observed the electron and it is spin-up. Fancy that!”
Dante would say, “Of course! The electron was always spin-up and the positron was always spin-down. When you measured we simply learned which was which.” He would then go on to criticize Raúl further by saying, “If you caused the electron to be spin-up when you measured it, then you equally caused the positron to be spin-down as a consequence. But for that to be true, the ‘message’ that the positron must now be spin-down must have travelled across the Universe instantaneously! As you’re well aware, nothing can travel faster than the speed of light, so your suggestion is clearly impossible.”
Dante has introduced the concept of “locality”. Two systems cannot affect each other until information has had enough time to travel between them. Because light speed is the fastest any information is known to travel, until light has sufficient time to get from one to the other, the two systems remain independent of one another, or “non-local.”
At this point EPR and Dante would all feel somewhat victorious over this random assault while humbly admitting that their version of quantum mechanics wasn’t perfect. If QM particles have definite, non-probabilistic states, then there must be some information that tells us what those states are. We just haven’t found it yet.
This idea led scientists to concoct various “hidden variable theories.” They proposed that the wavefunction as they understood it was incomplete. Perhaps, they argued, there is some “hidden variable” inside it which we have yet to uncover. Find the hidden information, and maybe it will tell us the particle’s true state and dispel this statistical view of quantum mechanics once and for all.
And people tried. Many hidden variable theories were proposed, but none worked. Then, in 1964, Raúl’s champion, J. S. Bell, proved that any local hidden variable theory is fundamentally incompatible with quantum mechanics. Einstein once famously quipped that “God doesn’t play dice,” a statement which served as a slap in the face to the Copenhagen interpretation. Had he still been alive at the time of its publication in 1964, Bell’s theorem would have caused him to spit his drink all over the Baccarat table. (And in case you’re wondering, there have been many tests of Bell’s Theorem. They have all confirmed its veracity.)
Raúl explains, “Look, Dante, J. S. Bell tells us everything we need to know. There is no possible way that any hidden information (as represented by an unknown variable) is inside the wavefunction. The wavefunction as we see it right now is all there is. There’s no man behind the curtain. There’s no hidden reality. It’s just random. The electron’s spin is not determined until the time of its measurement and this reality is somehow communicated to the positron so it can assume the opposite spin.”
Dante is compelled to reemphasize his point. “Raúl, even if I’m wrong, you still can’t explain how information about the electron’s spin can travel instantly across the Universe and affect the positron’s spin. That would violate causality! I may not conclusively win this point, but neither do you.”
“In fact,” he continues, “you haven’t even considered the implications of relativity.”
Dante is referring to Einstein’s theory of relativity which describes the relationship between space and time. One of relativity’s stranger results is that a single event can be viewed by two observers to have occurred at two different times. For example, if a spaceship races away from Earth, I would see the spaceship’s clock ticking more slowly than mine. I would, in effect, be looking into the spaceship’s past. My present would be its past. Stranger yet, reverse the motion and my present can actually be its future!
“Past, present, and future must all exist, Raúl, because my past and my future can be somebody else’s ‘right now’,” Dante says. “If a definite version of my future exists for someone else, how can the universe be anything other than deterministic?”
Raúl is silent, wondering if Dante is pulling some twisted logic on him.
Dante continues, “I can see you’re not convinced, so let’s return to that Bell’s Theorem you respect so highly. Bell’s Theorem says there can be no ‘hidden variable theory.’ Fine, but then one of two things must be true – either physics is non-local or the wavefunction is physically real. There’s no other way for the electron and positron to know about each other’s spin states.”
Fortunately for Dante, a recent paper (i.e. not even peer reviewed yet) out of University College London provides evidence to support the deterministic view. It proves (allegedly) that the quantum wavefunction is not statistical, but rather is a real, physical object. This means when the two aforementioned particles speed away from one another, one is actually spin-up and the other is actually spin-down. This preserves locality but also means that quantum mechanics, and thus the universe itself (barring some hidden physical reality of which physicists are unaware), is deterministic.
“What you are suggesting is heresy,” Raúl blurts out. “If the universe is deterministic, then humans don’t have free will. It’s all just stimuli and genetics, biochemistry and brain waves leading our bodies to act and think in ways that are outside our control.”
“Well, there is some evidence to back that up,” Dante says. “Psychological research has revealed that, at least in some instances, the human brain makes decisions before we are consciously aware of them. There’s also the case of Transient Global Amnesia, a medical condition where people lose almost all short term memory beyond a minute or two. During these intervals patients will engage in almost exactly the same conversation over and over and over again, like machines.”
So is Dante right? Is the universe neat, ordered, and deterministic? Perhaps, but it would be arrogant to assume this is the end of the story. University College London’s physical wavefunction theory may yet prove incorrect. At very least, much more work will need to be done before quantum mechanics experiences a paradigm shift away from the Copenhagen interpretation. And even if quantum mechanics is proven deterministic, maybe consciousness is the exception to the rule for reasons we cannot yet understand.
Then again, maybe it is something we are simply incapable of understanding. In the same way that the brain cannot visualize new colors or explain why there is a universe rather than nothing at all, it could be that the true nature of free will will always lie beyond our comprehension.