In the course of a long life, each of us will occasionally encounter a difficult decision we must make. Stay single or get married? Go for a run or have another doughnut? Go to grad school or enter the real world?
Wouldn’t it be nice to be able to choose both sides, rather than picking one? Quantum mechanics suggests a strategy: whenever you have a decision to make, you can do so by consulting a quantum random-number generator. Indeed, there is an app available for iPhones called Universe Splitter that can be used for this very purpose. (As Dave Barry says, I swear I am not making this up.)
Let’s say you have a choice to make: “Should I get pepperoni or sausage on my pizza?” (And let’s say you have too much restraint to give the obvious answer of asking for both on the same pizza.) You can fire up Universe Splitter, where you will see two text boxes, into which you can type “pepperoni” and “sausage.” Then hit the button, and your phone will send a signal through the internet to a laboratory in Switzerland, where a photon is sent toward a beam splitter (essentially a partially silvered mirror that reflects some photons and lets others through). According to the Schrödinger equation, the beam splitter turns the photon’s wave function into two components going left and right, each of which heads toward a different detector. When either detector notices a photon, it produces a readout that becomes entangled with the environment, quickly leading to decoherence and branching the wave function in two. The copy of you in the branch where the photon went left sees their phone flash with the message “pepperoni,” and in the one where it went right, they see “sausage.” If each one actually follows up with your plan to do what your phone advises, there will be one world in which a version of you orders pepperoni, and another in which a version of you orders sausage. Sadly, the two persons have no way of communicating with each other to share tasting notes afterward.
Even for the most battle-hardened quantum physicist, one must admit that this sounds ludicrous. But it’s the most straightforward reading of our best understanding of quantum mechanics.
The question naturally arises: What should we do about it? If the real world is truly this radically different from the world of our everyday experience, does this have any implications for how we live our lives?
Largely—no. To each individual on some branch of the wave function, life goes on just as if they lived in a single world with truly stochastic quantum events. But the issues are worth exploring.
You are welcome to offload your hard decisions to a quantum random-number generator, thereby ensuring that there is at least one branch of the wave function in which the best alternative was chosen. But let’s say we choose not to. Should the branching of our current selves into multiple future selves affect the choices we make? In the textbook view, there is a probability that one or another outcome happens when we observe a quantum system, while in Many-Worlds all outcomes happen, weighted by the amplitude squared of the wave function. Does the existence of all those extra worlds have implications for how we should act, personally or ethically?
It’s not hard to imagine that it might, but upon careful consideration it turns out to matter much less than you might guess. Consider the infamous quantum suicide experiment, or the related idea of quantum immortality. It’s an idea that has been considered ever since Many-Worlds came on the scene—reportedly Hugh Everett himself believed a version of quantum immortality—but has been popularized by physicist Max Tegmark.
Here’s the setup: we imagine a deadly device that is triggered by a quantum measurement, such as sending a query to the Universe Splitter app. Imagine that the quantum measurement has a 50 percent chance of triggering a gun that shoots a bullet into my head at close range, and a 50 percent chance of doing nothing. According to Many-Worlds, that implies the existence of two branches of the wave function, one of which contains a living version of me, the other of which contains a dead version.
Assume for purposes of the thought experiment we believe that life itself is a purely physical phenomenon, so we can set aside considerations of life after death. From my perspective, the branch on which the gun fired isn’t one that any version of me ever gets to experience—my descendant in that world is dead. But my descendant continues on, unharmed, on the branch where the gun didn’t fire. In some sense, then, “I” will live forever, even if I repeat this macabre procedure over and over again. One might go so far as to argue that I shouldn’t object to actually going through this experiment (putting aside the rest of the world’s feelings about me, I suppose)—in the branches where the gun fired “I” don’t really exist, while in the single branch where it failed to fire time after time I’m perfectly healthy. (Tegmark’s original point was less grandiose: he simply noted that an experimenter who survived a large number of trials would have good reason to accept the Everett picture.) This conclusion stands in stark contrast to a conventional stochastic formulation of quantum mechanics, where there is only one world, and I would have an increasingly tiny chance of being alive within it.
I do not recommend that you try such an experiment at home. In fact, the logic behind not caring about those branches in which you are killed is more than a little wonky.
Consider life in an old-fashioned, classical, single-universe picture. If you thought you lived in such a universe, would you mind if someone sneaked up behind you and shot you in the head so that you died instantly? (Again, setting aside the possibility that other people might be upset.) Most of us would not be in favor of that happening. But by the logic above, you really shouldn’t “mind”—after all, once you’re dead, there’s no “you” to be upset about what happened.
The point being missed by this analysis is that we are upset now—while we are still very much alive and feeling—by the prospect of being dead in the future, especially if that future comes sooner rather than later. And that’s a valid perspective; much of how we think about our current lives depends on a projection into the rest of our existence. Cutting that existence off is something we are perfectly allowed to object to, even if we won’t be around to be bothered by it once it happens. And given that, quantum suicide turns out to be just as bleak and unpalatable as our immediate intuition might suggest. It’s okay for me to yearn for a happy and long life for all the future versions of me that will end up in various branches of the wave function, as much as it would be valid for me to hope for a long life if I thought there was just a single world.
This goes back to something we discussed in Chapter Seven: the importance of treating individuals on different branches of the wave function as distinct persons, even if they descended from the same individual in the past. There is an important asymmetry between how we think about “our future” versus “our past” in Many-Worlds, which ultimately can be attributed to the low-entropy condition of our early universe. Any one individual can trace their lives backward in a unique person, but going forward in time we will branch into multiple people. There is not one future self that is picked out as “really you,” and it’s equally true that there is no one person constituted by all of those future individuals. They are separate, as much as identical twins are distinct people, despite descending from a single zygote.
We might care about what happens to the versions of ourselves who live on other branches, but it’s not sensible to think of them as “us.” Imagine that you’re just about to perform a vertical-spin measurement on an electron you have prepared in an equal superposition of spin-up and spin-down. A random philanthropist enters your lab and offers you the following bargain: if the spin is up, they will give you a million dollars; if the spin is down, you give them one dollar. You would be wise to take the deal; for all intents and purposes, it’s as if you are being offered a bet with equal chances of winning a million dollars or losing just one dollar, even if one of your future selves will certainly be out a dollar.
But now imagine that you were a little quicker in your experimental setup, and you observed a spin-down outcome just before the philanthropist busts in. It turns out that they are a pushy deal-maker, and they explain that the version of you on the other branch is being given a million dollars, but you now have to give them one dollar in this branch.
There’s no reason for you to be happy about this (or to give up the dollar), even though the version of you on the other branch might be happy about it. You are not them, and they are not part of you. Post-branching, you’re two different people. Neither your experiences nor your rewards should be thought of as being shared by various copies of you on different branches. Don’t play quantum Russian roulette, and don’t accept losing bargains from pushy philanthropists.
That may be a reasonable policy when it comes to your own well-being, but what about that of others? How does knowing about the existence of other worlds affect our notions of moral or ethical behavior?
The right way to think about morality is itself a controversial subject, even in single-world versions of reality, but it’s instructive to consider two broad categories of moral theory: deontology and consequentialism. Deontologists hold that moral behavior is a matter of obeying the right rules; actions are inherently right or wrong, whatever their consequences might turn out to be. Consequentialists, unsurprisingly, have the alternative view: we should work to maximize the beneficent consequences of our actions. Utilitarians, who advocate maximizing some measure of overall well-being, are paradigmatic consequentialists. There are other options, but these illustrate the basic point.
Deontology would seem to be unaffected by the possible presence of other worlds. If the whole point of your theory is that actions are intrinsically right or wrong, regardless of what outcomes they lead to, the existence of more worlds in which those outcomes can occur doesn’t really matter. A typical deontological rule is Kant’s categorical imperative: “Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.” It seems like it would be safe here to replace “a universal law” by “a law holding in all branches of the wave function,” without altering any substantive judgment about what kind of actions might qualify.
Consequentialism is another matter entirely. Imagine that you are a no-nonsense utilitarian, who believes there is a quantity called utility that measures the amount of well-being associated with conscious creatures, and that this quantity can be added among all creatures to obtain a total utility, and that the morally right course of action is the one that maximizes this total utility. Imagine further that you judge the total utility in the entire universe to be some positive number. (If you didn’t, you’d be in favor of trying somehow to destroy the universe, which makes for a good supervillain origin story but not for good neighbors.)
It would follow that, if the universe has positive utility and our goal is to maximize utility, creating a new copy of the whole universe would be one of the most morally valorous actions you could possibly take. The right thing to do would then be to branch the wave function of the universe as often as possible. We could imagine building a quantum utility maximizing device (QUMaD), perhaps an apparatus that continually bounces electrons through a device that measures first their vertical spin, then their horizontal spin. Every time an electron undergoes either measurement, the universe branches in two, doubling the total utility of all universes. Having built QUMaD and turned it on, you would be the most moral person ever to live!
Something about this smells fishy, however. Turning on QUMaD has no impact whatsoever on the lives of people in this universe or any other. They don’t even know the machine exists. Are we really sure it has such a morally praiseworthy effect?
Happily there are a couple of ways out of this puzzle. One is to deny the assumptions: maybe this kind of no-nonsense utilitarianism isn’t the best moral theory. There is a long and honorable tradition of people inventing things that would nominally increase the utility of the universe, but don’t resemble our moral intuitions whatsoever. (Robert Nozick imagined a “utility monster,” a hypothetical being that was so good at experiencing pleasure that the most moral thing anyone could do would be to keep the monster as happy as possible, no matter who else might suffer thereby.) QUMaD is just another example along these lines. The simple idea of adding up utilities among different people doesn’t always lead to the results we might initially have imagined.
But there’s another solution, one that comports more directly with the Many-Worlds philosophy. When we talked about deriving the Born rule, we discussed how to apportion credences in conditions of self-locating uncertainty: you know the wave function of the universe, but you don’t know which branch you are on. The answer was that your credences should be proportional to the weight of the branch—the corresponding amplitude, squared. This “weight” is a crucially important aspect of how we think about worlds in an Everettian picture. It’s not just probability that goes that way; conservation of energy also only works if we multiply the energy of each branch by its associated weight.
It makes sense, then, that we should do the same with utility. If we have a universe with some given total utility, and we measure a spin to branch it in two, the post-branching utility should be the sum of the weights of each branch times their utilities. Then, in the likely event that our spin measurement didn’t affect anyone’s utility in a substantial way, the total utility is completely unchanged by our measurement. That’s just what our intuition might expect. It’s also what we would directly conclude from the decision-theoretic approach to probability we mentioned in Chapter Six. From this perspective, Many-Worlds shouldn’t change our ideas about moral action in any noticeable way.
It’s nevertheless possible to cook up a system in which the difference between Many-Worlds and collapse theories really would be morally relevant. Imagine that some quantum experiment will lead to equally likely outcomes A or B, with A being extremely good and B being just a little bit good, and that these effects apply to everyone in the world with equal measure. In a single-world view, a utilitarian (or any commonsensical person, really) would be in favor of running the experiment, since either the vast good of A or the minor good of B would raise the net utility of the world. But imagine that your ethical code is entirely devoted to equality: you don’t care what happens, as long as it happens to everyone equally. On the collapse theory, you don’t know which outcome will happen, but either one maintains equality, so it’s still a good idea to run the experiment. But in Many-Worlds, people in one branch will experience A while those on the other branch will experience B. Even if the branches can’t communicate or otherwise interact, this could conceivably offend your moral sensibilities, so you’d be against doing the experiment at all. Personally I don’t think that inequality between people who literally live in different worlds should matter that much to us, but the logical possibility is there.
Excluding such artificial constructions, Many-Worlds doesn’t seem to have many moral implications. The picture of branching as “creating” an entirely new copy of the universe is a vivid one, but not quite right. It’s better to think of it as dividing the existing universe into almost-identical slices, each one of which has a smaller weight than the original. If we follow that picture carefully, we conclude that it’s correct to think about our future exactly as if we lived in a single stochastic universe that obeyed the Born rule. As counterintuitive as Many-Worlds might seem, at the end of the day it doesn’t really change how we should go through our lives.
So far we’ve treated branching of the wave function as something that happens independently of ourselves, so that we simply have to go along for the ride. It’s worth asking whether that’s the proper perspective. Whenever I make a decision, are different worlds created where I chose different things? Are there realities out there corresponding to every series of alternative choices I could have made, universes that actualize all the possibilities of my life?
The idea of “making a decision” isn’t something inscribed in the fundamental laws of physics. It’s one of those useful, approximate, emergent notions that we find convenient to invoke when describing human-scale phenomena. What you and I label “making a decision” is a set of neurochemical processes happening in our brain. It’s perfectly okay to talk about making decisions, but it’s not something over and above ordinary material stuff obeying the laws of physics.
So the question is, do the physical processes going on in your brain when you make a decision cause the wave function of the universe to branch, with different decisions being made in each branch? If I’m playing poker and lose all my chips after making an ill-timed bluff, can I take solace in the idea that there is another branch where I played more conservatively?
No, you do not cause the wave function to branch by making a decision. In large part that’s just due to what we mean (or ought to mean) by something “causing” something else. Branching is the result of a microscopic process amplified to macroscopic scales: a system in a quantum superposition becomes entangled with a larger system, which then becomes entangled with the environment, leading to decoherence. A decision, on the other hand, is a purely macroscopic phenomenon. There are no decisions being made by the electrons and atoms inside your brain; they’re just obeying the laws of physics.
Decisions and choices and their consequences are useful concepts when we are talking about things at the macroscopic, human-size level. It’s perfectly okay to think of choices as really existing and having influences, as long as we confine such talk to the regime in which they apply. We can choose, in other words, to talk about a person as a bunch of particles obeying Schrödinger’s equation, or we can equally well talk about them as an agent with volition who makes decisions that affect the world. But we can’t use both descriptions at once. Your decisions don’t cause the wave function to branch, because “the wave function branching” is a relevant concept at the level of fundamental physics, and “your decisions” is a relevant concept at the everyday macroscopic level of people.
So there is no sense in which your decisions cause branching. But we can still ask whether there are other branches where you made different decisions. And indeed there might be, but the right way to think about the causality is “some microscopic process happened that caused branching, and on different branches you ended up making different decisions,” rather than “you made a decision, which caused the wave function of the universe to branch.” For the most part, however, when you do make a decision—even one that seems like a close call at the time—almost all of the weight will be concentrated on a single branch, not spread equally over many alternatives.
The neurons in our brains are cells consisting of a central body and a number of appendages. Most of those appendages are dendrites, which take in signals from surrounding neurons, but one of them is the axon, a longer fiber down which outgoing signals are sent. Charged molecules (ions) build up in the neuron until they reach a point where an electrochemical pulse is triggered, traveling down the axon and across synapses to the dendrites of other neurons. Combine many such events, and we have the makings of a “thought.” (We’re glossing over some complications here; hopefully neuroscientists will forgive me.)
For the most part, these processes can be thought of as being purely classical, or at least deterministic. Quantum mechanics plays a role at some level in any chemical reaction, since it’s quantum mechanics that sets the rules for how electrons want to jump from one atom to another or bind two atoms together. But when you get enough atoms together in one place, their net behavior can be described without any reference to quantum concepts like entanglement or the Born rule—otherwise you wouldn’t have been able to take a chemistry class in high school without first learning the Schrödinger equation and worrying about the measurement problem.
So “decisions” are best thought of as classical events, not quantum ones. While you might be personally unsure what choice you will eventually make, the outcome is encoded in your brain. We’re not absolutely sure about the extent to which this is true, since there’s still a lot we don’t know about the physical processes behind thinking. It’s possible that the rates of neurologically important chemical reactions can vary slightly depending on the entanglement between the different atoms involved. If that turns out to be true, there would be a sense in which your brain is a quantum computer, albeit a limited one.
At the same time, an honest Everettian admits that there will always be branches of the wave function on which quantum systems appear to have done very unlikely things. As Alice mentioned in Chapter Eight, there will be branches where I run into a wall and happen to tunnel through it, rather than bouncing off. Likewise, even if the classical approximation to my brain implies that I’m going to bet all my chips at the poker table, there is some tiny amplitude for a bunch of neurons to do unlikely things and cause me to make a snug fold. But it’s not my decision that’s causing the branching; it’s the branching that I interpret as leading to my decision.
Under the most straightforward understanding of the chemistry going on in our brains, most of our thinking has nothing to do with entanglement and branching of the wave function. We shouldn’t imagine that making a difficult decision splits the world into multiple copies, each containing a version of you that chose differently. Unless, of course, you don’t want to take responsibility, and turn your decision-making over to a quantum random-number generator.
Similarly, quantum mechanics has nothing to do with the question of free will. It’s natural to think that it might, as free will is often contrasted with determinism, the idea that the future is completely determined by the present state of the universe. After all, if the future is determined, what room is there for me to make choices? In the textbook presentation of quantum mechanics, measurement outcomes are truly random, so physics is not deterministic. Maybe that opens the door a crack for free will to sneak back in, after it was banished by the Newtonian clockwork paradigm of classical mechanics?
There’s so much wrong with this that it’s hard to know where to start. First, “free will” versus “determinism” isn’t the right distinction to draw. Determinism should be opposed to “indeterminism,” and free will should be opposed to “no free will.” Determinism is straightforward to define: given the exact current state of the system, the laws of physics determine precisely the state at later times. Free will is trickier. One usually hears free will defined as something like “the ability to have chosen otherwise.” That means we’re comparing what really happened (we were in a situation, we made a decision, and we acted accordingly) to a different hypothetical scenario (we wind the clock backward to the original situation, and ask whether we “could have” decided differently). When playing this game, it’s crucial to specify exactly what is kept fixed between the real and hypothetical situations. Is it absolutely everything, down to the last microscopic detail? Or do we just imagine fixing our available macroscopic information, allowing for variation within invisible microscopic details?
Let’s say we’re hard-core about this question, and compare what actually happened to a hypothetical re-running of the universe starting from exactly the same initial condition, down to the precise state of every last elementary particle. In a classical deterministic universe the outcome would be precisely the same, so there’s no possibility you could have “made a different decision.” By contrast, according to textbook quantum mechanics, an element of randomness is introduced, so we can’t confidently predict exactly the same future outcome from the same initial conditions.
But that has nothing to do with free will. A different outcome doesn’t mean we manifested some kind of personal, supra-physical volitional influence over the laws of nature. It just means that some unpredictable quantum random numbers came up differently. What matters for the traditional “strong” notion of free will is not whether we are subject to deterministic laws of nature, but whether we are subject to impersonal laws of any sort. The fact that we can’t predict the future isn’t the same as the idea that we are free to bring it about. Even in textbook quantum mechanics, human beings are still collections of particles and fields obeying the laws of physics.
For that matter, quantum mechanics is not necessarily indeterministic. Many-Worlds is a counterexample. You evolve, perfectly deterministically, from a single person now into multiple persons at a future time. No choices come into the matter anywhere.
On the other hand, we can also contemplate a weaker notion of free will, one that refers to the macroscopically available knowledge we actually have about the world, rather than running thought experiments based on microscopically perfect knowledge. In that case, a different form of unpredictability arises. Given a person and what we (or they, or anyone) know about their current mental state, there will typically be many different specific arrangements of atoms and molecules in their bodies and brains that are compatible with that knowledge. Some of those arrangements may lead to sufficiently different neural processes that we would end up acting very differently, if those arrangements had been true. In that case, the best we can realistically do to describe the way human beings (or other conscious agents) act in the real world is to attribute volition to them—the ability to choose differently.
Attributing volition to people is what every one of us actually does as we go through life talking about ourselves and others. For practical purposes it doesn’t matter whether we could predict the future from perfect knowledge of the present, because we don’t have such knowledge, nor will we ever. This has led philosophers, going back as far as Thomas Hobbes, to propose compatibilism between underlying deterministic laws and the reality of human choice-making. Most modern philosophers are compatibilists about free will (which doesn’t mean it’s right, of course). Free will is real, just like tables and temperature and branches of the wave function.
As far as quantum mechanics is concerned, it doesn’t matter whether you are a compatibilist or an incompatibilist concerning free will. In neither case should quantum uncertainty affect your stance; even if you can’t predict the outcome of a quantum measurement, that outcome stems from the laws of physics, not any personal choices made by you. We don’t create the world by our actions, our actions are part of the world.
I would be remiss to talk about the human side of Many-Worlds without confronting the question of consciousness. There is a long history of claiming that human consciousness is necessary to understand quantum mechanics, or that quantum mechanics may be necessary to understand consciousness. Much of this can be attributed to the impression that quantum mechanics is mysterious, and consciousness is mysterious, so maybe they have something to do with each other.
That’s not wrong, as far as it goes. Maybe quantum mechanics and consciousness are somehow interconnected; it’s a hypothesis we’re welcome to contemplate. But according to everything we currently know, there is no good evidence this is actually the case.
Let’s first examine whether quantum mechanics might help us understand consciousness. It’s conceivable—though far from certain—that the rates of various neural processes in your brain depend on quantum entanglement in an interesting way, so that they cannot be understood by classical reasoning alone. But accounting for consciousness, as we traditionally think about it, isn’t a straightforward matter of the rates of neural processes. Philosophers distinguish between the “easy problem” of consciousness—figuring out how we sense things, react to them, think about them—and the “hard problem”—our subjective, first-person experience of the world; what it is like to be us, rather than someone else.
Quantum mechanics doesn’t seem to have anything to do with the hard problem. People have tried: Roger Penrose, for example, has teamed with anesthesiologist Stuart Hameroff to develop a theory in which objective collapse of the wave functions of microtubules in the brain helps explain why we experience consciousness. This proposal has not gained much acceptance in the neuroscience community. More important, it’s unclear why it should matter for consciousness at all. It’s perfectly conceivable that some subtle quantum processes in the brain, involving microtubules or something completely different, affect the rate at which our neurons fire. But this is of no help whatsoever in bridging the gap between “the firing of our neurons” and “our subjective, self-aware experience.” Many scientists and philosophers, myself included, have no trouble believing that this gap is very bridgeable. But a tiny change in the rate of this or that neurochemical process doesn’t seem to be relevant to understanding how. (And if it were, there’s no reason the effect couldn’t be repeated in nonhuman computers.)
Everettian quantum mechanics has nothing specific to say about the hard problem of consciousness that wouldn’t be shared by any other view in which the world is entirely physical. In such a view, the relevant facts about consciousness include these:
1. Consciousness arises from brains.
2. Brains are coherent physical systems.
That’s all. (“Coherent” here means “made of mutually interacting parts”; two collections of neurons on two non-interacting branches of the wave function are two distinct brains.) You can extend “brains” to “nervous systems” or “organisms” or “information-processing systems” if you like. The point is that we aren’t making extra assumptions about consciousness or personal identity in order to discuss Many-Worlds quantum mechanics; it is a quintessentially mechanistic theory, with no special role for observers or experiences. Conscious observers branch along with the rest of the wave function, of course, but so do rocks and rivers and clouds. The challenge of understanding consciousness is as difficult, no more and no less, in Many-Worlds as it would have been without quantum mechanics at all.
There are many important aspects of consciousness that scientists don’t currently understand. That is precisely what we should expect; the human mind generally, and consciousness in particular, are extremely complex phenomena. The fact that we don’t fully understand them shouldn’t tempt us into proposing entirely new laws of fundamental physics to help ourselves out. The laws of physics are enormously better understood, and that understanding has been much better verified by experiment, than the functioning of our brains and their relationship to our minds. We might someday have to contemplate modifying the laws of physics to successfully account for consciousness, but that should be a move of last resort.
We can also flip the question on its head: If quantum mechanics doesn’t help account for consciousness, is it nevertheless possible that consciousness plays a central role in accounting for quantum mechanics?
Many things are possible. But there’s a bit more to it than that. Given the prominence afforded to the act of measurement in the rules of standard textbook quantum theory, it’s natural to wonder whether there isn’t something special about the interaction between a conscious mind and a quantum system. Could the collapse of the wave function be caused by the conscious perception of certain aspects of physical objects?
According to the textbook view, wave functions collapse when they are measured, but what precisely constitutes “measurement” is left a little vague. The Copenhagen interpretation posits a distinction between quantum and classical realms, and treats measurement as an interaction between a classical observer and a quantum system. Where we should draw the line is hard to specify. If we have a Geiger counter observing emission from a radioactive source, for example, it would be natural to treat the counter as part of the classical world. But we don’t have to; even in Copenhagen, we could imagine treating Geiger counters as quantum systems that obey the Schrödinger equation. It’s only when the outcome of a measurement is perceived by a human being that (in this way of thinking) the wave function absolutely has to collapse, because no human being has ever reported being in a superposition of different measurement outcomes. So the last possible place we can draw the cut is between “observers who can testify as to whether they are in a superposition” and “everything else.” Since the perception of not being in a superposition is part of our consciousness, it’s not crazy to ask whether it’s actually consciousness that causes the collapse.
This idea was put forward as early as 1939, by Fritz London and Edmond Bauer, and later gained favor with Eugene Wigner, who won the Nobel Prize for his work on symmetries. In Wigner’s words:
All that quantum mechanics purports to provide are probability connections between subsequent impressions (also called “apperceptions”) of the consciousness, and even though the dividing line between the observer, whose consciousness is being affected, and the observed physical object can be shifted towards the one or the other to a considerable degree, it cannot be eliminated. It may be premature to believe that the present philosophy of quantum mechanics will remain a permanent feature of future physical theories; it will remain remarkable, in whatever way our future concepts may develop, that the very study of the external world led to the conclusion that the content of the consciousness is an ultimate reality.
Wigner himself later changed his mind about the role of consciousness in quantum theory, but others have taken up the torch. It’s not generally a view you will hear spoken of approvingly at physics conferences, but there are some scientists out there who continue to take it seriously.
If consciousness did play a role in the quantum measurement process, what exactly would that mean? The most straightforward approach would be to posit a dualist theory of consciousness, according to which “mind” and “matter” are two distinct, interacting categories. The general idea would be that our physical bodies are made of particles with a wave function that obeys the Schrödinger equation, but that consciousness resides in a separate immaterial mind, whose influence causes wave functions to collapse upon being perceived. Dualism has waned in popularity since its heyday in the time of René Descartes. The basic conundrum is the “interaction problem”: How do mind and matter interact with each other? In the present context, how is an immaterial mind, lacking extent in space and time, supposed to cause wave functions to collapse?
There is another strategy, however, that seems at once less clunky and considerably more dramatic. This is idealism, in the philosophical sense of the word. It doesn’t mean “pursuing lofty ideals,” but rather that the fundamental essence of reality is mental, rather than physical, in character. Idealism can be contrasted with physicalism or materialism, which suggest that reality is fundamentally made of physical stuff, and minds and consciousness arise out of that as collective phenomena. If physicalism claims that there is only the physical world, and dualism claims that there are both physical and mental realms, idealism claims that there is only the mental realm. (There is not a lot of support on the ground for the remaining logical possibility, that neither the physical nor the mental exists.)
For an idealist, mind comes first, and what we think of as “matter” is a reflection of our thoughts about the world. In some versions of the story, reality emerges from the collective effort of all the individual minds, whereas in others, a single concept of “the mental” underlies both individual minds and the reality they bring to be. Some of history’s greatest philosophical minds, including many in various Eastern traditions but also Westerners such as Immanuel Kant, have been sympathetic to some version of idealism.
It’s not hard to see how quantum mechanics and idealism might seem like a good fit. Idealism says that mind is the ultimate foundation of reality, and quantum mechanics (in its textbook formulation) says that properties like position and momentum don’t exist until they are observed, presumably by someone with a mind.
All varieties of idealism are challenged by the fact that, aside from the contentious exception of quantum measurement, the real world seems to move along quite well without any particular help from conscious minds. Our minds discover things about the world through the process of observation and experiment, and different minds end up discovering aspects of the world that always end up being wholly consistent with one another. We have assembled quite a detailed and successful account of the first few minutes of the history of the universe, a time when there were no known minds around to think about it. Meanwhile, progress in neuroscience has increasingly been able to identify particular thought processes with specific biochemical events taking place in the material that makes up our brains. If it weren’t for quantum mechanics and the measurement problem, all of our experience of reality would speak to the wisdom of putting matter first and mind emergent from it, rather than the other way around.
So, is the weirdness of the quantum measurement process sufficiently intractable that we should discard physicalism itself, in favor of an idealistic philosophy that takes mind as the primary ground of reality? Does quantum mechanics necessarily imply the centrality of the mental?
No. We don’t need to invoke any special role for consciousness in order to address the quantum measurement problem. We’ve seen several counterexamples. Many-Worlds is an explicit example, accounting for the apparent collapse of the wave function using the purely mechanistic process of decoherence and branching. We’re allowed to contemplate the possibility that consciousness is somehow involved, but it’s just as certainly not forced on us by anything we currently understand. Of course, we will often talk about conscious experiences in our attempts to map the quantum formalism onto the world as we see it, but only when the things we’re trying to explain are those experiences themselves. Otherwise, minds have nothing to do with it.
These are difficult, subtle issues, and this isn’t the place for a completely fair and comprehensive adjudication of the debate between idealism and physicalism. Idealism isn’t something that’s easy to disprove; if someone is convinced it’s right, it’s hard to point to anything that would obviously change their mind (or Mind). But what they can’t do is claim that quantum mechanics forces us into such a position. We have very straightforward and compelling models of the world in which reality exists independently of us; there’s no need to think we bring reality into existence by observing or thinking about it.