The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family
Page 44
Probability derivations aside, why do we only experience one world?
Splitting tigers
Building on Saunders’ reliance on decoherence theory to explain the emergence of a classical reality, Wallace appealed to philosopher Daniel Dennett’s concept of real objects defined as persistent patterns.37 The insight goes something like this: Think of a tiger and the jungle and the branching universe in which it hunts. The tiger and its environment (including us, the observers) are composed of entangled microscopic systems constantly exchanging photons and electrons and waves of energy. The tiger, and everything it interacts and correlates with, constantly branches into copies that embark upon different histories. Quantum mechanically, the tiger is a smear of patterns shape-shifting through a quantum jungle spread over multiple worlds and times: but for macroscopic observers (and our copies), the tiger (and its copies) persists as a pattern, an approximation of a sharply defined object at an atomically coarsegrained level of nature, but an entity, nonetheless: a living, breathing regularity emerging from a “swirl of molecules.”38
Wallace elaborates:
It makes sense to consider a superposition of patterns, but it is just meaningless to speak of a given pattern as being in a superposition…. Patterns are not superposed, but duplicated by the measurement event…. It is worth remembering the crucial role that decoherence is playing in this account: without it, we would not have the sort of branching structure which allows the existence of non-interacting multiple near-copies of a given process. Multiplicity occurs at the microscopic level, thus macroscopic objects … are genuinely multiplied in number.39
Wallace speaks to the problem of understanding the definiteness of individual universes inside the giant superposition of the multiverse in which interference terms between branching universes are not wholly lost.
Are consistent histories, and worlds which persist over time, real? Yes, in the sense that rivers, or animals, or persisting objects, are real: like worlds or instants they are not directly present in the formalism, and unlike worlds or instants they are only approximately definable, but that is no reason why they should not be seen as legitimate entities or used in our explanations (any more than we should expect to be able to describe zoology in any useful or explanatory way using only the language of quantum field theory).40
Following Saunders, Wallace also links general relativity and Everett’s relative states:
We are undoubtedly more at home with Minkowski space-time than with the universal state. Partly this may be because we have worked with the [relativity] concept in physics for rather longer, but more importantly we have long been used to the idea that multiple times exist (in some sense)—the innovation in relativity theory is the unification of these instants into a whole, and the identification of the instants as secondary concepts. Everett asks us to take both steps at once: to accept that there exist many worlds, and then to fuse them together into a whole and accept that the worlds are only secondary. Clearly this is a significantly larger conceptual jump; still, if we are prepared to accept the existence of many worlds and if we are happy with the step from many times to space-time, there seems no reason to avoid a similar step in the case of quantum theory.41
It’s all about information flow, says Deutsch, who likens the structure of the multiverse—with its classically separated, but quantum mechanically linked realities—to a computation run by a computer straddling parallel worlds.42 Taking a hard line, he says that talking about many worlds as an interpretation of quantum mechanics “is like talking about dinosaurs as an ‘interpretation’ of fossil records.”43
The burning question is: are the Everett worlds real? For Deutsch, Saunders, Wallace and their supporters, yes, according to quantum mechanics. They have yet to convince the world, but Saunders makes the excellent point that nothing similar to their logical arguments has been made for sustaining a one-world realist interpretation of quantum mechanics—or any one-world realist theory of probability, for that matter. The idea that indeterminism rules the quantum world in one-world theories rests on familiarity and tradition, rather than an understanding of what physical chance really is, say the Oxford Everettians.44
Everett@Fifty
Even as the July 2007 issue of Nature featured Everett’s many worlds theory on its cover in celebration of its half century anniversary, the Oxford philosophers organized an international conference, Everett@Fifty. About 30 philosophers and physicists gathered to hotly debate the pros and cons of the many worlds theory at Oxford’s faculty of philosophy just off Logic Lane. In October, the debate continued at a conference at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
The ground rules for the Everett conferences mandated a realist, physicalist approach, no idealists need apply:
Ask not if quantum mechanics is true, ask rather what the theory implies. Specifically, ask what quantum-state realism implies, with the state as given by the Schrödinger equation. What follows, in particular, when quantum theory is applied without restriction to the whole universe?45
But within realism, there is plenty of room for disagreement. Especially when it comes to critiquing what amounts to a new theory of probability being worked out by the Oxford Everettians. For instance, although the Savage axiom-based approach to deriving probability does not do any better than von Neumann and Morgenstern did in explaining what probability is (or how utilities are set in the real world), it, arguably, surpasses military-style game theory as Everett practiced it by validating the use of an ethically neutral probability measure—the Born rule.46 It explains why it is not necessarily irrational (in theory) to use this measure when making self-interested decisions in a deterministic universe; (Everett, in his own way, was aiming at the same target: explaining why using the Born rule is rational).
At the conferences, critics of the decision theory approach questioned the criteria used to define utility and rationality in a deterministic multiverse. Huw Price, professor of philosophy at the University of Sydney, made a case that if a bottom line of maximizing self-interest is replaced by a form of “distributive justice” (which might weight the concerns of poor people greater than the rich, or the lame as greater than the fleet) then the appeal to decision theory might lose its rational connection to the Born rule. Price also insisted that metaphysical questions of personal identity be kept separate from decision theory since it is difficult to define “self-interest” for multiple “selves”.47
As usual in Everettian conversations about splitting observers, the question of personal identity came to the fore. At the center of several conference debates was the question of how and to what extent should a decision-maker in an Everettian multiverse care about the fate of her or his “successors.” What if she takes an action that will impoverish all of her successors save one billionaire– but that’s the one that she identifies with (she only cares about winners). Is that irrational? Or what if it is the other way, and whatever else happens, she is concerned to defend the worst off—to alleviate their sufferings—at some expense to the great majority? If these matters seem esoteric, think about the question of mutual assured destruction that Everett contemplated: What would it mean to launch (or not launch) a nuclear first strike in a branching universe in which all physically possible events occur with some frequency?
Bohmians and metaphysicians chimed in with a range of objections to the arguments of the Everettians. Leading the charge, David Albert worried that if the Everett interpretation is true, there would be no such thing as probability, and we would not be able to do statistical physics. He strongly objected to the Deutsch-Wallace-Greaves argument from decision theory:
The fission [observer splitting] hypothesis (since it is committed to the claim that all such experiments have all possible outcomes with all possible frequencies) is structurally incapable of explaining anything like that. The decision-theoretic program seems to act as if what primarily and in the first instance stands in the way of need of being explained
about the world is why we bet the way we do. But this is sheer madness! … a bait and switch … an argument to the effect that if we held an altogether different set of convictions about the world [i.e. believing the Everett interpretation] than the ones we actually hold, we would bet the same way as we actually do.48
Love it or hate it, the influence of many worlds grows with age as science and philosophy continue to contemplate the strangely beautiful idea proposed by Everett a half century ago.
Epilogue
Beyond Many Worlds
Now, at the beginning of the 21st century, faced with scientific claims like neo-Darwinism and the multiverse hypothesis in cosmology invented to avoid the overwhelming evidence for purpose and design found in modern science, the Catholic Church will again defend human nature by proclaiming that the immanent design evident in nature is real. Scientific theories that try to explain away the appearance of design as the result of ‘chance and necessity’ are not scientific at all, but, as John Paul put it, an abdication of human intelligence.
Christoph Schönborn, Cardinal Archbishop of Vienna, 2005.1
Vatican attacks on a scientific theory usually are a sign that it is intelligent and correct—just ask Copernicus, Galileo, Bruno, Newton, Darwin, and Einstein. Their revolutionary insights survived the scorn of theologians and provided foundations for brave new experimental discoveries and increasingly explanatory theories. Assuming that the concept of a multiverse is here to stay: In addition to Everett’s relative states formulation, how many kinds of multi-verses are there? Is any multiverse experimentally verifiable? Even in principle?2
This was the subject of my conversation with an Albanian-born cosmologist, Laura Mersini-Houghton, as we traipsed a path along the rocky shore of San Miguel, an island in the Azores. It was the summer of 2009. We were attending a conference on foundational questions in physics, and Everett’s theory was on many minds.3 Mersini-Houghton believes there is newly discovered cosmological evidence that points to the possible existence of an Everett type multiverse. Popular science magazines are lionizing her theory because if there is even a smidgeon of proof of a multiverse: that is news.4
The history of physics is littered with strange ideas—but the fact that Everett’s theory is a tool in the kit of contemporary physics underscores the observation by Wheeler with which this book began: “We can believe that we will first understand how simple the universe is when we recognize how strange it is.”5 Most new ideas are strange—at first.
The Landscape
As the 21st century dawned, the marriage of string theory and cosmology derived a “Landscape” of 10500 universes. That is a 10 followed by 500 zeros: a rather large number considering that there are only 1080 atoms in “our” universe. Each of the universes in the (abstract, unseen, unverified) Landscape—think of it as a multiverse—differs in the arrangement of its physical constants (vacuum energy, gravity, fine-structure, etc.). Most of these universes would be inhospitable to life as we know it, because the configuration of constants that permitted life to evolve from collections of hydrogen atoms needed to be so precisely calibrated that our presence seems altogether improbable. Until the advent of the Landscape, scientifically inclined theologians eagerly fastened upon this cosmological improbability. Wielding the anthropic principle that an explanation of the structure of the universe must be compatible with our presence as observers, they claimed that our finely tuned existence is so unlikely to have occurred that nature must have been designed by a super intelligent Being!
In defense of secular reason, physicist and author Leonard Susskind has explained that the Landscape sets the emergence of our seemingly improbable world inside such a vast distribution of possible worlds that our puny place in the scheme of all things seems unremarkable, inevitable, mundane.6 It is hubris to assert that the universe is anthrocentric, focused on humans. If the Landscape (or any type of multiverse) exists, we are probabilistic accidents, not the children of angels, nor the toys of a Grand Inquisitor.7
The Landscape is related to “eternal inflation,” the constant bubbling into being of new universes. As a metaphor for inflation, think of two pencil marks on a balloon expanding away from each other as the balloon is blown up. In a forever-expanding balloon, Big Bangs are the result of local braking actions. In a highly metaphorical sense, an area of this cosmically inflating balloon underwent a probabilistic shift in the value of its constants, “nucleating” a bubble, a balloon within the balloon, in a shower of quantum sparks followed by a powerful burst of temporary inflation that shaped our particular universe. The configuration of the cooling sparks (post-inflation) is preserved as Cosmic Microwave Background Radiation (CMB), an observable record of the early history of our bubble—one of many bubble universes in the Landscape of eternal inflation.
Susskind reflects,
The many-worlds of Everett seems, at first sight, to be quite a different conception than the eternally inflating megaverse. However, I think the two may really be the same thing…. There are branches (as well as real worlds) for every location on the Landscape.8
Physicist Juan Maldacena of the Institute for Advanced Study also draws a parallel between the Everett worlds and eternal inflation: “In principle they are different, however it is important in the notion of inflation that there are some quantum fluctuations that change the parameters along the spatial slice of this universe. So perhaps the many worlds of eternal inflation that occur at different points in space should be viewed as the many worlds of Everett where you think about these things but occurring at the same point in space.”9 In other words, nucleating bubbles and branching worlds may be—somewhere, somehow—linked by the rules of the quantum mechanical game.
Naturally, scientists need more than mathematical models to validate ideas; they need evidence that physical reality is congruent with ideas.
Matthew Kleban at New York University thinks about what might happen when two bubble universes collide. He says that a signature of a past collision could be recorded in the CMB. One sign of a collision would be the existence of a record that the otherwise uniform directional flow of our expanding universe has been marred by a contrary flow—what Kleban calls, tongue-in-cheek, an “axis of evil.” (“Evil” because a significant collision with a neighboring bubble would vaporize us.)10
Enter the Dark Flow, a directional anomaly in the CMB—8 billion light years distant—discovered in late 2008 by astronomers. The Dark Flow could signify a movement of galaxies flowing in a direction opposite to the rest of our universe. That possibility has some cosmologists in a tizzy—wondering if it is evidence of the existence of a space-time outside our space-time? Mersini-Houghton thinks it might be a gravitational pull on our universe by a neighboring universe (a “brane” in the parlance of string theory).11
It could also be a blip in the data.
Cold spot
In 2007, NASA astronomers announced that they had discovered a giant void in the CMB, nicknaming it the Cold Spot. Mersini-Houghton’s take on it was intriguing from the Everett point of view. She speculated that the Cold Spot recorded a moment during the Big Bang before inflation really took off, in which a quantum-gravitational superposition of possible universes existed in a very tiny patch. A moment later, an unfathomable number of universes—proto-bubbles in the Landscape—decohered from the patch, leaving behind the tiny, hot void, which gradually grew to a large, cold void as the CMB expanded. When writing about the emergence of this quantum multiverse, Mersini and her colleagues used Everett’s idea of a universal wave function:
By proposing to place the wave function of the universe on the landscape and use its many-worlds interpretation, the quantum mechanical universe was thus embedded onto the string landscape thereby making the Everett and the Landscape multiverse equivalent.12
More than 50 years after Everett aired his controversial idea, it remains alive and kicking and deeply disturbing. Maldacena remarks, “When I think about the Everett interpretation in everyday life, I do not
believe it. But when I think about it in quantum mechanics, it is the most reasonable thing to believe.”13
In 1730, Isaac Newton—who knew nothing of Planck’s constant or the Born rule or string theory Landscapes—wrote, in Opticks:
And since Space is divisible in infinitum, and Matter is not necessarily in all places, it may also be allow’d that God is able to create Particles of Matter of several sizes, and in several proportions to Space, and perhaps of different Densities and Forces, and thereby to vary the Laws of Nature, and make Worlds of several sorts in several parts of the Universe. A least, I see nothing of contradiction in this.14
And when all is said and done, why should there be but one universe? Is not the notion that there is only one world just as strange as that there might be many? And what if there are many worlds? What then?
Let sea-discoverers to new worlds have gone;
Let maps to other, worlds on worlds have shown;
Let us possess one world; each hath one, and is one.15
The End.