5
THE CHARGED ATMOSPHERE in the theoretical physics community at the start of the new millennium eerily reminded me of the desperation and uncertainty that my family experienced when my dad was exiled for the second time. Back then, another aftershock from the cultural revolution that had been imported from China in the 1970s was shaking Albania. My dad was working as a scientist at the Albanian Academy of Science, and his research results on the optimization of forestry, hydropower, and other difficult problems based on statistical probability were sufficient to draw the jealousy of some powerful colleagues. Once again, my dad’s enemies could exact their revenge by targeting his weakest point: his “bad biography,” our family history.
This time, my father was sent to work on a cooperative in Dukat, a remote mountain village in the south of Albania. On that particular occasion, my dad’s concern for our safety led him to embrace a lie, even though it would cause him great personal hurt. Such was the Albanian reality then.
During my dad’s second exile, my mom’s file—every Albanian had a file cataloging his or her life and behavior—was taken from the League of Writers and Artists and sent to the isolated mountain village where my dad had been banished. For months, my mom had been pressured every day at her office in Tirana to pack up her belongings, take her family, and follow her husband to the remote countryside. After her file was transferred, the request for her to join her husband was elevated to an official order. Terrified at the idea that his wife and two children would also be sent into exile, my dad begged my mom to denounce and divorce him, for show. It was the only way for her to distance herself quickly from the “enemy of the party” (my dad) and keep my brother and me safely in Tirana. But Mom wouldn’t have it, so my dad sent my maternal grandmother to Tirana to talk sense into her daughter. Eventually, the two of them wore down her defenses, and she agreed.
On one of my father’s brief visits home, my parents, grandmother, and I went to the courthouse, having agreed beforehand that my mother would lie and claim that my father had beaten her. It was the fastest way to get a divorce. A female judge was assigned to the case. My mother entered her chambers and said, “My husband beats me. He is here; you can ask him yourself. I want a divorce.”
But what had seemed like such a simple solution broke down the moment Mom uttered those words. It turned out that the judge was a childhood friend of my father’s, and she adored him. She was furious. Staring down at my mother from the bench, she thundered that she knew my father’s character, and he would never hurt anyone. “What kind of a woman are you? That man wouldn’t even hurt a fly!” the judge yelled. “These are vicious lies. How dare you? Do you understand what kind of trouble you can get him into with your lies, as if he isn’t in deep enough trouble already? Get out of my sight!”
My mother, who had not wanted to agree to this plan in the first place, fled the court in tears, and we had to run to catch up with her. The divorce scheme failed; the truth of my parents’ own small universe won out. My dad was returned to Tirana a few months later, and our family stayed together.
Such memories crossed my mind years later as I started to suspect that a drastically different way of thinking was required to solve the question of how the universe had been created. What if the thing we saw as the problem—the universe’s improbable creation story—was not, in fact, the true problem? I suspected that part of the solution was hidden in the paradox that I had found in my many thought experiments: no matter how many different types of conceivable universes I could possibly imagine, the problem of an exquisitely special and extremely unlikely origin was the same for each of them. Did all of these universe models, no matter how different, share the same logical flaw?
Very soon, I had an idea. But to pursue it, I would have to break away from the scientific community’s orthodox picture of a single universe described by a unified theory. And in the world of twenty-first-century physics, that amounted to nothing short of heresy.
I started researching the origin of the universe full-time when I arrived at the University of North Carolina at Chapel Hill in 2004. I had no preconceived idea of what the ultimate theory of the universe should look like, which worked in my favor. I also had no preconceived idea of where my investigation of the origin of the universe would lead.
What I had realized by this time was that every model and every calculation, whether promising or a failure, started with the same fundamental assumption: that there is only one universe.
Perhaps, I thought, by refusing to allow for the existence of anything other than our one universe, physicists had already made it “special.” Perhaps, I thought, we had been asking the wrong question all along. After all, how could we ask ourselves why we started with this universe if all we allowed ourselves to consider was a single universe?
I gradually convinced myself that the question “Why did we start with this one?” logically made sense only if we had a range of possible beginnings to choose from—a range of different infant universes, any one of which might have turned into the universe that we find ourselves inhabiting today. Otherwise, the answer would be obvious: We got this universe because it’s the only one there is. End of story.
From the beginning, I expected that the idea of starting with multiple infant universes was going to be an unpopular, if not an outlandish, premise. Since antiquity, the philosophical concept of a single universe has dominated and shaped nearly all efforts to understand the universe. In modern times, this belief had crystallized into a global scientific effort to unify quantum theory (which governs the microscopic, unseen universe) and Einstein’s theory of curved space-time gravity (which governs the visible universe) to produce a theory of everything—a scientific model capable of explaining all the workings of our universe. Indeed, it is possible to draw a nearly uninterrupted line from Plato and Aristotle directly to Einstein and Stephen Hawking in support of a theory of everything for a single universe. To go against this vision was to go against the titans of philosophical and scientific thought for three millennia.
I wasn’t the first one to encounter this problem. The founding fathers of quantum mechanics got the same answer I did, a family of universes instead of a single one, when they solved the Schrödinger equation. Nevertheless, some tried their best to force their new quantum theory to produce the answer of a single, deterministic universe. And they did that in some very creative ways.
Recall, for example, that the solutions of Schrödinger’s equation showed that a quantum particle can follow multiple paths, and we won’t know beforehand which path the particle will choose; each has its own probability of happening. If the quantum particle happened to be an infant universe, then a family of solutions implied the possibility of a family of infant universes, each with its own probability of existing, because, on the basis of a wave-particle duality, each wave solution could be thought of as its own infant universe. Since each wave solution corresponds to an infant universe with its own probability, from now on in this book I will refer to them as wave-universe solutions. The collection of all these wave-universes derived by solving the Schrödinger-type equation for the universe is known as the wave function of the universe, and each individual wave-universe is a branch in the wave function of the universe.
For the founders of quantum theory, then, the question became this: Which one of these wave-universe solutions was the real one? Simply ignoring and discarding all the solutions save for the one that they liked and presenting that as the correct one seemed outrageously arbitrary. However, nobody knew what it meant to keep them all or how to identify which one of them would become the “real” universe because, as it soon became clear, Schrödinger’s equation and Heisenberg’s uncertainty principle didn’t offer a selection criterion that singled out one “valid” infant universe among of the multitude of their solutions.
Niels Bohr was adamant that somehow a single large universe had to be identified out of the family of quantum baby universes produced by the Schrödinger equation. If not, all the predictability of physics seemed poised to be lost. But how to identify the winner of this cosmic lottery? Bohr proposed bringing in a hypothetical independent judge, someone who could observe the wave function of the universe and rule in favor of only one probability wave out of the multitude. Bohr argued that the moment the judge observes what course a quantum particle has taken, then we can know with 100 percent certainty that this particular particle is the real one in the family of wave solutions because we have just observed that it exists. And we can then discard the rest of the possible answers.
Bohr’s solution became known as the collapse of the wave function because all but one branch from the family that makes up the wave function of the universe vanished. In this manner, the whole wave function was reduced; it “collapsed” from a multitude to a single choice, the surviving branch. To many, it sounded plausible. Indeed, Bohr’s collapse of the wave function dominated theoretical physics for decades. It still has a few fans left today in the physics community. I, however, am not one of them, for reasons that I will explain.
Heisenberg, for his part, was on board with most of Bohr’s proposal. Indeed, the creator of the uncertainty principle is credited with naming Bohr’s contribution to quantum physics; Heisenberg called it “the Copenhagen interpretation of quantum mechanics.” As Heisenberg said in a 1924 lecture in Chicago, “The probability wave meant a tendency for something. It was a quantitative version of the old concept of potentia in Aristotelian philosophy. It introduced something standing in the middle between the idea of an event and the actual event, a strange kind of physical reality, just in the middle, between possibility and reality.” So, for Heisenberg and Bohr, multiple possibilities became a single reality at the moment they were observed.
In our large, visible world, we are used to having only one correct answer to each question. That’s all Bohr, Planck, Schrödinger, and Einstein wished to achieve: to identify a single, classical deterministic universe from the multitude of quantum uncertainty—a single wave function. Bohr’s suggestion that none of the wave functions corresponded to real particles or universes except the one we found when we observed it in some ways managed to evoke a visible world where everything was determined with 100 percent certainty. But it failed in one important way—Bohr’s reality didn’t give Einstein and Schrödinger what mattered to them the most: an objective reality. Another Bohr observer could conceivably observe the same particle (or the same group of particles) and declare a different outcome, also with 100 percent certainty. Different observers could produce different findings for the same particle, and each observer would state with total certainty that his or her finding was the real one. And they would all be right. How is that for subjective reality?
Bohr’s collapse of the wave function also committed an additional sin: It created a double standard. It treated the observer (not necessarily a human) as a large, visible entity—in other words, part of the world of classical physics, where there is only one answer to every problem—rather than as another quantum object. In so doing, Bohr’s collapse of the wave function mixed determinate, classical physics into the highly indeterminate, quantum world.
To understand why this double standard was such a problem, imagine a situation where all the judges (the observers) ruling on human “quantum particle” disputes and offenses in the courtrooms on planet Earth were imported from an alien civilization on the theory that these alien judges would be independent observers. However, having aliens judge humans would force the humans to modify their laws and rulings to obey alien law and alien legal rulings. In the same manner, if tiny quantum particles were being judged by large, classical observers, they would have to become and behave like large, classical particles.
Despite the inconsistency in the Copenhagen interpretation of quantum mechanics, the role of the observers in cosmology remains to this day open to fierce debate. But what ultimately undermined Bohr’s efforts to create a world run by observers was not this classical/quantum inconsistency. Rather, it was a thought experiment by Erwin Schrödinger, the Austrian physicist whose namesake equation is one of the cornerstones of quantum mechanics.
Schrödinger wanted to extract a single, classical world from the workings of quantum theory as much as Einstein, Bohr, and Planck did. But he resented the godlike status that Bohr gave to his arbitrary observers by granting them the power to decide what was real in the world and what wasn’t. Schrödinger corresponded extensively with Einstein over these issues and their paradoxes, and in 1935, he came up with his own thought experiment: Schrödinger’s cat experiment. It was designed to highlight the flaws in Bohr’s collapse of the wave function, but it has become one of the most famous thought experiments in all of popular culture.
In his thought experiment, Schrödinger imagined a cat locked in a box that also contained a tiny amount of a radioactive substance that might or might not decay after one hour, a hammer, and a flask of poison. If the radioactive substance did decay, it would trigger the hammer to break the flask of poison, which would kill the cat. An hour and a speck later, an observer would open the box and find out if the cat was dead or alive. If the atom had decayed, the cat would be dead. If it had not, then the cat would still be alive. Dead cat and alive cat were both allowed possible states—or wave functions—to describe the cat; each had a 50 percent chance of being correct.
If we relied on the reasoning in Bohr’s collapse of the wave function, Schrödinger argued, we wouldn’t know before we opened the locked box in which of these two states we would find the cat, and therefore, the cat, before we observed it, had to be in a superposition of both states, meaning it could simultaneously be both dead and alive inside the box. Only the observer who opened the box would know for sure what state the cat was in. In Bohr’s terminology, the observer would collapse the superposed wave function by reducing it to only one choice: the cat was either alive, with 100 percent probability, or dead, also with 100 percent probability. So, once observed, the cat could no longer remain in a quantum state where it could be both dead and alive; instead, the cat would be abruptly pinned to one state of being.
Schrödinger was trying to demonstrate how ridiculous it sounded to have an observer pick one solution among the superposition of wave probabilities by offering a paradox: If the cat was found alive, then how could the cat have been either half dead and half alive or completely dead a second before it was observed? The Austrian’s thought experiment had the intended effect: Einstein liked Schrödinger’s paradox so much that, for a special effect, he suggested that instead of poison, Schrödinger could use gunpowder, which would blow the cat to bits. This would make the paradox even more dramatic: How could the cat inside the box exist in a combined state of being both alive and also blown to bits? At the moment when the cat was found to be alive, would those recently blown-up bits suddenly reassemble themselves into a live cat?
With the unfortunate cat as proof, Einstein and Schrödinger remained adamant in their conviction that there could be only one universe, ours, and that our universe is deterministic—it behaves according to a set of fixed rules (the laws of nature) that exist independently of an observer. If their convictions were correct, then it followed that they could reconstruct the universe’s past all the way to its moment of creation and that they could also predict its future. The only thing they had to do was figure out what those fixed rules were!
Einstein and Schrödinger devoted their lives to the search for this rule book, the fundamental theory of the universe: a unified description of all the forces of nature that reinforced order and restored certainty to the universe’s past and future, its origin and its destiny. A theory of everything for a single universe would replace the “dangerous” probabilities of the many indeterministic worlds of quantum theory. It was a pressing and enticing goal that seemed within their grasp. After all, in the previous century, Maxwell had achieved the elegant unification of electric and magnetic forces in his theory of electromagnetism. How hard could it be to extend that to the rest of the forces of nature?
As it turned out, finding a theory of everything was harder than anyone could have anticipated. So hard, in fact, that in their race to achieve this goal, Einstein and Schrödinger fell out with each other to the point where Einstein nearly sued Schrödinger for plagiarism—and might well have done so had it not been for Nobel Prize–winning quantum physicist Wolfgang Pauli, who convinced Einstein to drop the fight.
These pioneers of quantum theory failed to find the unified theory of nature or successfully collapse the wave function. But embedded in their failure was a success story: their work built the intellectual foundations on which the search for a testable theory of the multiverse could begin.
I could not have envisioned and developed my own theory without Einstein and Schrödinger’s struggles—or without the work of another young physicist who was active around the same time. Yet the story of this intrepid scientist underscored the costs of straying too far from the herd of mainstream theoretical physics.
From 1953 to 1956, Hugh Everett III was a graduate student at Einstein’s longtime home of Princeton University. There, the young scientist worked with John Wheeler, a renowned theoretical physicist who was a student, friend, and collaborator of Niels Bohr (not to mention a key figure in the Manhattan Project, which produced the first nuclear bomb near the end of World War II).* Everett found Schrödinger’s half-dead, half-alive cat irresistible. It got him thinking hard about the role of the observer and the collapse of the wave function, and this investigation became his PhD dissertation. Everett brilliantly identified the fundamental discrepancy between Bohr’s collapse of the wave function and the paradox of Schrödinger’s cat: Bohr’s hypothetical observer, Everett noted, lived by the rules of a classical world, whereas Schrödinger’s cat and the box that imprisoned it were subject to the rules of the quantum world. Everett offered a simple way out of the cat’s paradox: Let us agree that everything in the universe, including the universe itself, is governed by one and only one set of rules, those of quantum theory.
The implications of this insight were huge. The observer, like the cat, could now be in a superposition of two states, both dead and alive. Likewise, the cat was now “watching” the observer according to the same laws that governed the observer while he or she had been “watching” the cat. (For all we know, the observer could have been another cat!) Furthermore, the observer’s states needed to be combined with the dead-or-alive states of the cat to form a single wave function. This meant that there could be a universe in which both cat and observer were alive, another universe in which the cat was alive but the observer was dead (and vice versa), and another one in which both cat and observer were dead. Also, the observer and cat were interacting by the very act of watching each other. This interaction of the cat with the observer allowed them to “communicate” their findings instantly (recall how our tiny, quantum, inflating universe communicates during cosmic inflation) and make adjustments accordingly, thereby giving rise to yet more possible worlds. Mind-bogglingly, quantum mechanics allowed for all of these universes to exist simultaneously.
After Everett’s theory, the short version of which was published as his PhD dissertation in 1956, certainty about the world was gone. If Everett was right, then not only the cat but also the observer was a quantum object. It meant the observer had the same status as the cat and played by the same rules. Moving from the realm of exploding cats to the entire universe, Everett showed that a straightforward application of quantum mechanics to the universe predicted a complex and bizarre cosmos of multiple worlds, intricately entangled and wrapped—or superposed (recall the double-slit experiment and our concert hall and the superposition of waves)—with one another into a universal wave function. If, in its infancy, the universe was of subatomic size or nothing more than a tiny particle, then, Everett reasoned, the whole universe, regardless of size, must be subject to the rules of quantum mechanics. Like any quantum particle, it can be represented by a wave function or, more accurately, a bundle of waves packed together into a wave function of the universe.
Just as quantum particles had a chance to take any of a number of trajectories, not just one fixed, predetermined trajectory, likewise (according to Everett) the wave function of the universe has no predetermined course. It can split continuously into many possible trajectories or branches, each of which can produce a different universe. And just like that, the possibility of many worlds returned, over two thousand years after it was considered by the ancient Greeks as a philosophical thought experiment. But which one of those worlds is real?
The answer, according to Everett, is that all possible quantum universes that are superposed into a grand wave function of the universe—that is, all the branches of the wave function that produces universes—have equal chances to exist. They have that right of existence simply because, as Everett argued, no law of nature existed to tell us otherwise. Everett named his theory “the universal wave function of the universe.”
The collection of Everett’s universes became known in popular culture as “parallel universes.” Every time you had to meet a deadline and worked late, there was an identical copy of you at home in another universe reading a bedtime story to your child instead of working. Every time you tweeted something that you wished you hadn’t, there was an identical copy of you in a parallel universe that decided not to. Every time you hesitated and weighed your decisions, copies of you in parallel universes were experiencing all the possible decisions that you could have made but didn’t in your current universe. The universal wave function of the universe kept splitting and branching out in maddening ways into many possible worlds that accommodated all possible events that could happen to any particle in the universe. As had happened so often before in the history of quantum theory, what was intended as an investigative mission to demystify the weirdness of the theory actually made it stronger and weirder.
Alas, Everett’s career was eventually ruined because he went against mainstream thinking. What was Wheeler, Everett’s supervisor, to do with these ideas? Bohr was his friend and his hero. Everett was his student. Wheeler couldn’t abandon either. Despite Wheeler’s penchant for exotic solutions to hard problems, despite his uncanny intuition for recognizing great ideas such as the one advanced by his student, and despite the fact that Wheeler understood Everett was making an excellent point about the faults with Bohr’s collapse of the wave function of the universe, it was a very tough choice. Bohr was a force to be reckoned with. Wheeler tried to arrange a meeting between Bohr and Everett so the two could hash out their disagreements. Wheeler asked his star student Charles Misner (who, in addition to being brilliant, was known for a kind and calm disposition) to accompany Everett on their trip to Denmark. (Misner was later my general-relativity professor at Maryland.) However, nothing went according to Wheeler’s plan; Bohr and Everett could not be reconciled. Bohr hated Everett’s criticism of his treasured, still-dominant theory of the collapse of the wave function. His mind was closed to Everett.
In the end, caught between the outrage of Bohr and his Princeton colleague Einstein, Wheeler decided he couldn’t publicly support Everett’s many-worlds interpretation of quantum mechanics, so he cut most of the controversy out of Everett’s 1956 PhD dissertation by condensing its one-hundred-plus pages down to about thirty. Everett was squeezed out of an academic career and ended up working in the defense industry.
The world would not have known of Everett’s many-worlds interpretation had it not been for one man, my colleague in the theoretical physics group at UNC Chapel Hill, Bryce DeWitt. As a scientist, he was as conservative as they come, and as a person, he was every bit as fearful. (He built a bunker in the backyard of his house in Chapel Hill to protect his family from a possible nuclear attack during the Cold War.) DeWitt had spent his professional life working toward the same goal his scientific heroes had—mathematically trying to unify quantum mechanics with gravitational forces. Yet in 1973, as editor of the Reviews of Modern Physics, he recognized the importance of Everett’s work, took the initiative, and had the courage to publish Everett’s complete dissertation.
When Everett’s unbowdlerized work was finally published, DeWitt decided to give his “universal wave function of the universe” a better name. He repackaged it as “the many worlds interpretation of quantum mechanics,” a name that has stuck to this day. Not that Everett cared. It was too late for him to restart his academic career. He died less than a decade later, in 1982.*
Quantum theory had come full circle, from Planck’s quanta of energy to Everett’s many worlds, and this evolution had profound implications for cosmology. In quantum theory, the fundamental nature of the world became an unpredictable collection of many possible quantum universes, each of which had a chance to exist. Sometimes they might bundle together in a fuzzy superposition producing many possible combinations. Other times, they might branch off individually. The result: an infinite number of such worlds and an infinite number of possible behaviors. Did our universe, and many others, really result from a game of chance? Until relatively recently, very few scientists were willing to risk their careers on the many-worlds quantum theory. Everett’s fate was a cautionary tale for many of us. Including me.