Putting Ourselves Back in the Equation, page 15
Quantum physics takes a pickaxe to this bedrock of agreement. The disputes over clock time in relativity theory are piddling compared to what happens with Wigner’s friend, where one observer denies that the other saw something happen at all. “Quantum mechanics is the discovery that facts are contextual,” said Carlo Rovelli at France’s Aix-Marseille University, one of the physicists who has been developing a perspectival view of quantum theory. “They are relative to physical systems. Facts are relative in the same sense in which velocity is relative: velocity is a property of an object relative to another object.”
This doesn’t mean that anything goes, or that reality is all in our heads. It just means that some types of observations can’t be reconciled. “That things are relational or perspectival does not mean that they are not real,” said the philosopher Dennis Dieks at Utrecht University. Furthermore, it takes an unusual situation, in which one observer performs experiments on another, to expose a conflict. As serious a problem as these discrepancies are for understanding nature at its roots, they don’t mean there aren’t empirical facts about climate change and presidential election results.
EVERYTHING EXISTS AT ONCE
Hugh Everett did the groundbreaking analysis of perspectival quantum theory in his PhD thesis in 1956. He then ran into a problem far more intractable than quantum physics: human arrogance. Bohr by this point denied that there was any measurement problem to solve. Despite the best efforts of Everett’s graduate advisor, John Wheeler, Bohr and his disciples basically canceled Everett for his temerity in questioning their views. Everett left physics and went to work for the US military, tasked with developing nuclear war strategies.19 (He was the one who figured out that the only winning move is not to play.) An irony is that Everett saw his perspectivalist views not as a repudiation of Bohr’s principle of complementarity, but as a generalization of it.20
Using the same reasoning as Copernicus and Kant, Everett argued that physicists had committed a basic logical fallacy. The quantum wave equation predicts that objects enter a superposition of conflicting possibilities, but we never see them in one, so physicists figured they must collapse. They didn’t stop to ask whether we could see such a superposition. Unless we can, our nonobservation of these superpositions tells us nothing. Absence of evidence is not evidence of absence. In fact, Everett went on, we can’t see an object in two mutually exclusive states. The reason is that, according to the rules of quantum mechanics, superpositions infect us and we become part of the system we are trying to study, leaving us unable to see its overall state.
To explain how this happens, Everett imagined a bare-bones observer—not much to it, just an eye and a memory.21 The eye watches a measuring apparatus, and the memory records what it sees. The memory can be abstractly represented as a quantum bit, or qubit, with value 0 or 1. For instance, if a photon reflects off a mirror, it takes a certain path and hits a detector. The eye notes this and stores a 0 in memory, representing reflection. If the photon instead passes through a piece of glass, it takes a different path and hits a second detector. The eye duly notes this and stores a 1 in memory, representing transmission.
Now suppose the observer watches a photon strike a half-silvered mirror. The photon both reflects off and passes through, and the observer’s memory ends up in a superposition of 0 (representing reflection) and 1 (representing transmission), accurately reflecting the condition of the photon. In essence, the photon passes through and the observer registers that it passes through, and the photon reflects off and the observer registers that, too. There is no such thing as “the” path of the photon or “the” perception of the observer. The photon and observer are in limbo. But they are in it together—that’s the key. The whole purpose of a measuring apparatus is to establish a correlation between the exterior world and our perception. If the world is X, we perceive X. So if the world is in a superposition, our perception must be in a superposition, too. And because the two superpositions are linked, the observer has a well-defined state relative to the photon. That is, each option in the photon superposition (pass through or reflect off) is matched with a corresponding option in the observer superposition (see it pass or see it reflect).
Furthermore, if you ask the observer whether it saw a definite result, it will look into its memory, find a value stored there, and answer, “Yes.” The observer is never presented with a discrepancy—seeing the photon both transmitted and reflected—so it has no inkling of its own conflicted condition. This is perhaps not so unexpected. We humans always have difficulty getting perspective on ourselves—otherwise we wouldn’t need therapists. Everett was suggesting that this lack of self-awareness enters into our most basic observations and is an unavoidable part of being embedded in a quantum world.
This is a classic case of the inside/outside problem. From the outside, both particle and observer are in a superposition of multiple measurement outcomes. From the inside, all the observer perceives is a single outcome. In other words, collapse occurs only on the inside. The observer sees a particle change from a superposition to a single outcome not because the particle itself has changed, but because the observer has become entangled with the particle and lost an outsider’s perspective.
This is all we need to explain the various features of collapse, according to Everett’s analysis. For instance, according to standard quantum theory, collapse is irreversible; Everett said it was enough that collapse look irreversible. From the outside perspective, it could, in fact, be undone, although that would require acting on all the particles in the superposition, and for more than a few particles, that’s hard, bordering on impossible.22
In addition, thinking of collapse as the product of an insider viewpoint accounts for the random outcomes we observe. Everett showed this by analyzing a series of measurements on particles that were all prepared the same way.23 The observer measures the first particle and stores the result. The memory is now in a superposition of seeing the particle go through and seeing it reflect. Then the observer measures the second particle and stores that result, too. Now its memory is in a superposition of four permutations: having seen the first particle go through and the second go through, having seen the first go through and the second reflect, and so on. On the next measurement, it has eight permutations to track, then sixteen, and so on. The overall state is entirely predictable—a ginormous tree of permutations—but the observer can’t see that. All it sees is one series of outcomes, which, for a typical observer, will look like a toss-up. Much debate has ensued over what “typical” means,24 but the basic point is that the notorious randomness of quantum physics is entirely mental, occurring because observers are stuck inside superpositions. In Everett’s interpretation, randomness is a statement about us, not about particles.
Everett’s way of thinking makes short work of Wigner’s experiment.25 Suppose you observe the observer. To you, it is in a superposition. If you ask it whether it saw an outcome, it will query its memory, find a value there, and reply, “Of course, silly.” The two of you have divergent perspectives, and that’s fine, since the state of a system is relative to who is measuring it. But as soon as you ask the observer what the outcome was—in essence, you use that observer as your own measuring device—its superposition will infect you. Now you will find yourself in the same position as the observer. You will think you saw an outcome, so you will think the observer and photon have collapsed. You will have given up your outside view for the inside view.
This analysis has a powerful appeal for physicists and philosophers because it dispenses with the collapse rule of textbook quantum theory. Columbia University’s David Albert, who trained as a theoretical physicist and is now a leading philosopher of quantum theory, recalled encountering Everett’s interpretation in the 1980s. “I had the most powerful conviction: this is so beautiful, this must be true,” he told me. Yet he came to realize that Everett’s presentation can’t be the whole story. For one thing, it has a peculiar, self-negating quality to it: It says we don’t see mutually incompatible outcomes because we’re deceived about our own state of mind.26 We think we’ve seen a single definite outcome when we’re really in a superposition of having seen every possible outcome. For Albert and others, that sort of self-deception makes The Matrix or brain-in-a-vat scenarios seem tame by comparison. It’s one thing to consider that we might not be seeing the world as it really is, quite another to entertain the possibility that we aren’t thinking what we think we’re thinking. “I could be hallucinating that there’s a chair in the room, but I can’t be hallucinating that I think there’s a chair in the room,” Albert said. If we were so profoundly deceived, we wouldn’t be able to trust anything, including the observations and reasoning that led to quantum theory. So Everett’s interpretation pulls the rug out from under itself.
Traditionally we expect scientific theories to satisfy two criteria: they should hang together, and they should match reality. By formulating theories mathematically, we can confirm they are internally coherent, and we can extract numerical predictions to compare with data. But there is a lesser-known third criterion: theories must not deny the validity of observations. A theory can be scrupulously logical and predictive, but if it covers its own tracks, then it fails the standards of science. It is, as philosophers say, empirically incoherent.
To check whether a theory negates itself, we need to insert ourselves into it—we need to ask whether it lets us test it. Albert and others argued that Everett’s interpretation, in its original form, doesn’t. Superdeterminism—the claim that we are unable to conduct a randomized, controlled experiment in quantum physics—comes close to empirical incoherence, too, but at least there are some ways to test for it. The next time someone tells you about a conspiracy theory, check for empirical incoherence. Sure, maybe cannibalistic pedophiles have taken over the government, and if ever you try to look for them, they stop you (or worse). It’s logically possible; it would explain certain observations. But if you believe such a theory, why believe anything? Aren’t you worried the conspiracy is to make you believe there is a conspiracy?
MANY MANYS
Everett left these loose ends because, for him, explaining our observations was enough. He didn’t care much about questions concerning what was really going on, what the waves and superpositions in the equations correspond to in reality.27 But as his views spread from 1970 on—becoming arguably the leading interpretation of quantum mechanics today—physicists and philosophers came up with various ways to plug this interpretive gap.
The best-known is the many-worlds interpretation. (Sometimes this term is applied to Everett’s original work, but it really dates to 1973.)28 The idea is that the universe splits into parallel universes. If a photon is superposed between passing through and reflecting off a mirror, you can think of it as two photons playing out both possibilities. Those photons ripple outward into the universe, and before long there aren’t just two photons, but two of everything.
Essentially, this interpretation equates superposition with multiplicity.29 A superposition of two options means two things exist out there. An observer sees one of them, and his doppelgänger in a parallel universe sees the other. Now there is no self-deception—each observer sees his world as it really is—but the reality we perceive is still relative to us.
A lot gets swept under the rug when physicists talk about “worlds.” Quantum theory itself doesn’t provide any guidance for how we’re supposed to divvy up superpositions into “worlds.” This is a consequence of the menu problem that I mentioned in chapter 4. In the case of the half-silvered mirror, we routinely talk as if the photon chooses from a menu of two options—it either reflects off or passes through the glass—and therefore there must be two worlds. In fact, there’s a literal infinity of other, surreal menus in which the photon is reflected to varying degrees. The definition of a “world” is an add-on—some extra feature of physics above and beyond quantum theory itself. Physicists and philosophers have put considerable effort into articulating what that could be. Most now think it has to do with the process of decoherence. But this issue remains contentious. The idea of parallel universes, forming what is often called a multiverse, faces other challenges that I will explore in chapter 6.
For a while in quantum physics, interpretations that began with the word “many” proliferated almost as fast as the universes they claimed to describe: many threads, many spaces, many histories, many maps.30 In 1988 Albert and colleagues developed one, the many-minds interpretation, in which it’s not the whole world that splits, just your mind.31 If you see a photon pass through the mirror glass and you also see it reflect off, there are two yous occupying the same brain and body. Your brain enters a superposition that amounts to two independent streams of consciousness. Those two selves go on to have other distinct experiences and will almost certainly never reunite. Put simply, all of us have a kind of multiple-personality syndrome.
The many-minds interpretation didn’t go very far. Even its originators thought it was weird. “It didn’t seem to us even at the time that anyone should really be prepared to accept it,” Albert told me. One drawback was that it assumed that minds have continuity over time—that the mental state at one moment can be identified with a corresponding mental state at another.32 Physics itself does not establish this continuity; it must be separately postulated. “Mind really is being treated metaphysically, ontologically, as a distinct object from, say, the brain or from anything in the physical world,” Albert said of his interpretation. The continuity of identity is a major puzzle that I will return to in chapter 6.
Still, the many-minds interpretation is historically interesting because it brought neuroscience and philosophy of mind into the physics conversation. In this interpretation, the options we see in our experiments—such as a photon either reflecting off or passing through a mirror—have nothing to do with parallel worlds; rather, they originate in the structure of our minds. There’s something about human thought that carves the world into options such as “reflect” and “pass through,” as opposed to all the other possible dichotomies. To understand why that is, physicists need a theory of consciousness—speculating about consciousness is no longer just a fun diversion for them, but an essential part of understanding experimental results.
A WORLD OF RELATIONSHIPS
The core idea underpinning the many many interpretations is that we have access to only a very small fraction of reality. Somewhere beyond our view are worlds or minds by the billions. Only a god, standing outside the temporal realm, could view them all. Still, they exist independently of us or anyone else. Carlo Rovelli, for one, aims to dispense with this last vestige of absolute reality by taking Everett’s approach in an even more thoroughly perspectival direction.
I became engrossed by his take on quantum mechanics when I was writing my first book in the mid-2000s. We had a long email exchange, and I visited him in the French Mediterranean village of Cassis. We went hiking along the rocky shoreline, first chatting about the pre-Socratic Greek philosopher Anaximander (on whom he was writing a book at the time), about our experiences of living in different countries, and about the difficulty of coping with uncertainty in science and in life. Only then did we get to the questions that had brought me there. For Rovelli, science is never just transactional. It is the establishment of relationships, both between people and between us and nature.
Throughout his career Rovelli has argued that the physical world, too, is a web of relations. He takes the essential lesson of modern theories of gravity and other forces to be that things have no properties in isolation, but acquire them only at their point of contact with other things. He extended the principle to quantum mechanics in 1996.33 Measurements, he argued, are relations that we establish with something.
Although physicists frame quantum measurement in terms of observers, Rovelli doesn’t think sentient beings have a fundamental role in reality. Their minds definitely don’t cause wave functions to collapse. To him, they are just one type of physical system to which the quantities of physics can be related; a table lamp would do just as well. His view makes quantum theory completely egalitarian. If we measure a particle, we establish a relationship with it; if a lamp interacts with the particle, it establishes its own relationship with it. Thus the lamp is no less of an “observer” than we are, and its relationship is a “measurement” of sorts. “When I say that things are true relative to a system O, this has nothing to do with O having a mind,” Rovelli told me.
Relations are necessarily specific to the involved parties. If you have a relation with something, and I have a relation with that same thing, and our experiences differ, no problem. You have your reality, I have mine, and we will have to agree to disagree. “If quantum mechanics is correct, every time we give a full description of what we think is the ‘reality’ of a situation, we are in fact only giving a partial picture,” he said. “It is a ‘reality as far as we are concerned.’” This is how he makes sense of the diverging views in Wigner’s experiment.
Rovelli’s relational interpretation of quantum mechanics has a purity and evenhandedness that leads to strange and, to skeptics, implausible conclusions. When he says that reality is strictly relational, he means it: when a particle is just on its own, not interacting with a person or a table lamp, it has no properties, period. The quantum wave function does not describe the condition of a particle at these intermediate times, but only the correlations that will occur once it finally does interact with something else. Not only does a tree falling in the forest with no one to hear it make no sound, but it doesn’t even exist. Rovelli said: “In the relational perspective, you are not supposed to ask, ‘What is the real state of affairs?’ but only, ‘How will an object manifest itself next?’” In between these interactions, there’s a whole lot of nothing. Our observations are not snapshots of a world that existed before we came along and carries on existing afterward. They are all there is. That means reality has a staccato existence; it winks in and out. Dennis Dieks, though broadly sympathetic to Rovelli’s approach, finds his picture of an intermittently existing world “rather outlandish.” Rovelli doesn’t dispute that it’s strange. “The resulting world is still weird, very much so,” he told me.
