Worlds Without End: The Many Lives of the Multiverse

What a comedy of errors! When the debate between science and religion is staged, adjectives are almost exactly reversed: it is of science that one should say that it reaches the invisible world of beyond, that she is spiritual, miraculous, soul-fulfilling, uplifting. And it is religion that should be qualified as being local, objective, visible, mundane, unmiraculous, repetitive, obstinate, sturdy.

BRUNO LATOUR, “THOU SHALT NOT FREEZE-FRAME; OR, HOW NOT TO MISUNDERSTAND THE SCIENCE AND RELIGION DEBATE”

At first blush, it looks as though the boundary between “science” and “religion” is clear when it comes to the multiverse. Just as the Atomist philosophers did 2,500 years ago, modern multiverse theorists proclaim an infinite number of worlds in part to avoid the conclusion that this world was somehow “designed” for us. If an endless number of all sorts of universes actually exist, the scientists reason, then it does not matter how improbable or razor’s-edgy our fundamental parameters might be. The random generation of universes throughout infinite time and space ensures that even a cosmological constant as absurdly small as ours was bound to arise at some point. In short, the multiverse does away with the need for a creator-god—at least as an explanatory principle. In this light, some theologians have disparaged the multiverse as “the last resort of the desperate atheist.”¹

Among these theologians is Cardinal Christoph Schönborn, the current Roman Catholic archbishop of Vienna, who accuses “the multiverse hypothesis” and “[n]eo-Darwinism” of having been “invented to avoid the overwhelming evidence for purpose and design found in modern science.” In response to this invention, he resolves, “the Catholic Church will again defend human reason by proclaiming that the immanent design evident in nature is real.”2 At first glance, Schönborn’s use of the adverb again might call to mind a history of torrid conflicts between religion and natural science, placing the multiverse in an unsavory lineage that stretches from the slandered Epicureans through the executed Giordano Bruno and the prosecuted John Thomas Scopes. On the surface, then, the question of infinite worlds seems to stage a simple, familiar drama between the forces of dogma and innovation, sacred doctrine and secular reason, God and the multiverse.

And yet these lines become increasingly crossed, twisted, and even knotted the more closely we examine the controversy. It is important to note, for example, that Schönborn does not accuse the multiverse of violating doctrine, but of violating “modern science.” In response, he commits the church not to defending God against the multiverse, but to defending reason against it: “Throughout history,” he writes, “the church has defended the truths of faith given by Jesus Christ. But in the modern era, the Catholic Church is in the odd position of standing in firm defense of reason, as well.” Standing firm in this “odd position,” Schönborn marshals neither the Bible nor the fathers or doctors of the church to assert the singularity of the cosmos. He never calls the multiverse hypothesis heretical, incompatible with scripture, or an insult to divine infinity. Rather, he calls it an “abdication of human intelligence.” In particular, he charges, appealing to an infinite number of invisible universes amounts to “giving up the search for an explanation of the world as it appears to us.”³ It is a claim that we may remember from Walter Charleton, who argued against the neo-Atomists in the mid-seventeenth century that many-worlds cosmologies not only violate common sense but also, and more troublingly, prevent scientists from gaining objective knowledge about the “real” world (see chap. 4, sec. “From Infinity to Pluralism”). What are we doing dreaming about other universes when we have not managed to address global warming? or malaria? or the common cold?

Insofar as the multiverse hypothesis is said to be neither provable nor “falsifiable,” to use Karl Popper’s term,4 the archbishop and like-minded critics accuse multiverse cosmologies of falling outside the proper purview of science.⁵ Of course, one can always accuse these critics of hiding fundamentally theological motivations behind a scientific smoke screen—of saying their concern is for “human intelligence” when it is really for intelligent design. But whether this is true or not, such an argument is of limited value, for a number of reasons. First, many schools of Christian thought (including the Catholic, Orthodox, and Anglican traditions, not to mention their deist, process, and Platonic offshoots) have never held the sacred and the secular to be in conflict in the first place.⁶ According to these traditions, human reason participates in divine reason, and the mysterious workings of God are partially revealed in the order of the universe that we measure and observe. So it makes very little sense to split theological motivations from “rational” motivations; if reason is functioning properly within these traditions, there is no rift at all between science and religion.

Second, to reduce Schönborn’s and others’ critiques of the multiverse to “secret theology” is to overestimate the metaphysical power of the position they criticize. After all, while the principles of accident and infinity may do away with the need for a creator-god, they hardly disprove the existence of such a god, nor are they even incompatible with one. As Thomas Aquinas argued immediately after positing the “teleological argument” (which eventually produced the “intelligent design hypothesis”), it does not matter what sort of material process an atheist posits as the creative force of the universe; the theist can always retort, “God made that material process.”7 For this reason, there are plenty of self-professed Christian physicists and philosophers who affirm the existence of the multiverse with no theological difficulty at all.⁸ As astrophysicist and “progressive creationist” Jeffrey Zweernick has argued, multiverse theories still leave us marveling at the infinitely generative force of inflation, or the perfect unitarity of the wave function, or the well-oiled mechanisms that keep the ekpyrotic cycles cycling. So, he concludes, “as research into multiverse scenarios advances, it appears that they may simply move the design ‘up one level.’ In other words,” he explains, “instead of just one universe requiring fine-tuning to support life, it appears that any multiverse-generating mechanism also requires a high degree of fine-tuning to reproduce the observable universe in which we live.”⁹ The process of inflation, for example, must have been designed in such a way that quantum fluctuations can form pockets of true vacuum from time to time rather than just churning out an eternal sea of nothingness.¹⁰ Simply put, a Christian can easily affirm that God created the multiverse that created the universe.

That having been said, such an argument, however updated its evidence, would in principle be no more forceful than the traditional argument from design and would be subject to the same logical and ethical critiques that Philo levies against it in David Hume’s Dialogues.¹¹ Depending on the thinker, it might even be subject to a trenchant theological critique because it is not clear that the god of the design argument bears much resemblance to the God who breathes into the nostrils of an earth creature, or who delivers an oppressed people from slavery, or whose prophets implore the nation to “do justice, love kindness, and walk humbly with your God” (Micah 6:8 [NRSV]). In short, there is a chasm between what some theologians, following a fragment in Blaise Pascal’s Pensées, call “the God of the philosophers,” on the one hand, and “the God of Abraham, Isaac, and Jacob,” on the other.¹² The argument from design, along with the other traditional proofs of the existence of God, are concerned strictly with the former—with a philosophical “God” understood in a fairly bloodless fashion as a first mover, a self-caused cause, a lighter of the cosmic fuse.

One of the most powerful critics of this “God of the philosophers” was the German theologian Dietrich Bonhoeffer (1906–1945). Writing from his cell in a Gestapo prison (he had been part of a failed plot to assassinate Hitler and was executed just days before the end of the war), Bonhoeffer assailed the kind of thinking that makes God into a metaphysical principle at the edge of the world rather than an ethical force in the midst of it. “Religious people speak of God at a point where human knowledge is at an end,” he explained in a letter to Eberhardt Bethge; “actually, [this God is] a deus ex machina that they’re always bringing on the scene, either to appear to solve insoluble problems or to provide strength when human powers fail.”13 The problem with appealing to God at the limits of human knowledge is that as the scope of human knowledge expands, the space for God progressively shrinks, until God is edged out of the world completely. Now that humanity does not need God to explain the fall of rain or the rotation of the planets or even the creation and distribution of matter, God becomes confined, at best, to that one little ocular mechanism that biologists cannot explain, or to the bacterial flagellum, or to the 10^–34 seconds before the big bang hypothesis kicks in to explain creation. God becomes nothing more than a first nudge, a “stopgap” invoked to plug the remaining few holes in human understanding. “Inevitably that lasts only until human beings push the boundaries a bit further and God is no longer needed as a deus ex machina,” Bonhoeffer predicted.¹⁴

So the “God of the gaps” recedes with each passing age, disappearing from view like some far-off galaxy. And in the meantime, we have learned nothing about how we are to live in relation to this stopgap god: how to respond to genocide, for example, or what to do in the face of escalating global poverty, or whether the existence of an infinite number of worlds means we can let this one warm and melt with impunity. For Bonhoeffer, then, the whole endeavor to prove the existence of a God outside the world is flawed from the outset. “I’d like to speak of God not at the boundaries, but at the center,” he wrote, “not in weakness but in strength.”¹⁵ Far from existing at the edge of the universe (or of the multiverse), Bonhoeffer’s God “is the beyond in the midst of our lives,” an indwelling force that, precisely in refusing to swoop in and solve our unanswered problems, calls people to “responsible action” in the world—even to life in a Gestapo prison—so that “the coming generation” might “go on living.”¹⁶ Perhaps needless to say, the god of the design hypothesis does none of this in his sporadic plugging of epistemological holes. It might be, then, that the argument from design constitutes not just unsatisfying science, but unsatisfying theology.¹⁷

Finally, to dismiss those who criticize the multiverse as secret theologians would be to miss the extent to which multiverse theories themselves function theologically, colliding with invisible divinities and realms in the very gesture of trying to avoid them. We have seen this collision rather dramatically enacted in the manufactured and simulated multiverses, each of which updates the old god or gods into teams of brilliant physicists—just like us but smarter and more powerful and with faster supercomputers. But even those multiverse scenarios that stop short of such anthropomorphic designers still display a remarkable faith in what St. Paul would call “things hoped for” and “things not seen” (Hebrews 11:1).18

Taking this allegation up through Max Tegmark’s cosmic hierarchy, we might recall, for example, that the Level I, or quilted, multiverse—the endless set of overlapping observable universes centered on any given cosmic body—relies on two unprovable claims. The first is that the universe is infinite. Although satellite observations have confirmed that the universe is either flat or very close to flat in three dimensions, they have not confirmed that this flatness goes on forever. It might be that the universe is shaped like a gargantuan doughnut (torus), so that if you were to run to the “edge” and throw Lucretius’s spear, it would come back around the other side.¹⁹ If this were the case, then light would eventually “lap” the universe, so that we would see the same galaxies repeated at different stages of their lives. Teams of researchers are currently seeking such patterns in the sky, but so far they have “found no repeating images within one billion light-years of the earth.”²⁰ If such images were ever to be found, they would demonstrate (as Aristotle knew)²¹ the finitude of the cosmos. But not finding them would never amount to demonstrating the infinity of the cosmos—just its enormity. It will always be possible that an infinite-looking universe is really finite but so large that we will never live long enough to see light looping back on itself. So it is impossible not just in practice but in principle to prove the infinity of the cosmos. As John Barrow puts it, “[Y]ou can discover whether the Universe is infinite, but the learning will take an infinite time.”²²

In response to this charge, multiverse proponents might argue that insofar as cosmic infinity and homogeneity are predicted by the theory of inflation, inflation delivers these two principles from the realm of the speculative and places them on solid ground. In predicting these principles, however, inflation also generates an infinite number of other universes, the ensemble of which constitutes Tegmark’s Level II. The problem with this “inflationary multiverse” is not so much that inflation has not yet been proved or refuted,24 but that the scenario assumes that the laws we have derived from our observable universe—quantum field theory, for example—hold not only throughout our (purportedly) infinite universe, but also throughout wildly different and forever separated domains of a haphazardly generated multiverse.²⁵ To be sure, this notion gains support from the 10⁵⁰⁰⁺ vacua solutions of string theory, but as many critics are quick to point out, string theory itself remains what one might call “hoped for and not seen.” As beautiful and internally consistent as it may be, string theory has not been confirmed or even supported by any experiment or observation.²⁶ The reason it has seemed so promising for so long is that it integrates gravity with the electromagnetic and strong and weak nuclear forces. But in the very gesture of tying up this one universe with a “theory of everything,” the critique goes, string theory ends up becoming a “theory of anything,” positing a mind-bending number of alternative universes that we can neither measure nor see. And, according to Paul Davies, “appealing to everything in general to explain something in particular is really no explanation at all. To a scientists, it is just as unsatisfying as simply declaring, ‘God made it that way!’”²⁷

In a recent Harper’s article, Alan Lightman distills the theological costs of the inflationary-plus-string scenario thus: “[N]ot only must we accept that the basic properties of our universe are accidental and uncalculable. In addition, we must believe in the existence of many other universes. But we have no conceivable way of observing these universes and cannot prove their existence. Thus, to explain what we see in the world … we must believe in what we cannot prove.”²⁸ What is ironic is that inflationary cosmology initially seemed like the remedy for believing in what we cannot prove; after all, it gave us a mechanism for flattening and homogenizing the universe without the interference of a god. String theory similarly seemed like the ultimate answer to the problem of the fine-tuning of the universe, initially promising to find a single vacuum that explained why the parameters must be what they are. But then, as we have seen Paul Steinhardt demonstrate, both inflation and string theory have ended up postulating a host of invisible kosmoi in the very process of delivering the world in which we live from an invisible god.²⁹ The situation is almost like a cosmological version of the old lady who swallowed the fly: the moment physicists ask, “Why is the universe this way and not another?” which is arguably a metaphysical question to begin with, they can produce answers that only open more metaphysical problems. So we can swallow string theory to catch eternal inflation to catch the cosmological constant, but as we try to digest 10⁵⁰⁰⁺ invisible universes, we would do well to ask why we swallowed the fly in the first place.³⁰

Responding to the pseudo-theological postulates of inflationary and string cosmologies, Martin Gardner argues, “[S]urely the conjecture that there is just one universe and its Creator is infinitely simpler and easier to believe than that there are countless billions upon billions of worlds, constantly increasing in number and created by nobody.”31 A number of theologians have issued similar arguments,³² which Richard Dawkins rejected several years ago by insisting, “[Y]ou can’t get much more complex than an Almighty God!”³³ I confess to having no idea which hypothesis is “simpler” or “easier to believe” than the other—or even how one would go about measuring such values, assuming that they are values. But many physicists find the anthropic multiverse to be at least as speculative and outlandish as the god hypothesis that it tries to avoid. In response, Paul Steinhardt, Neil Turok, and their colleagues posit a new cyclical model that preserves the oneness of the universe. As we have seen, however, it does so, first, by relying (to varying degrees) on the still-speculative string hypothesis of braneworlds and, second, by fashioning dark energy into what is essentially a nonanthropomorphic, pantheist god. According to the ekpyrotic model, dark energy is the sole cosmic regulator—an indwelling force that creates, animates, destroys, and re-creates the “endless universe” it suffuses. So the scenario is not not theological; to the contrary, it is (fascinatingly) pantheological.³⁴

Lee Smolin, for his part, calls the anthropic principle “unscientific” and builds his alternative on the allegedly sturdier principle of natural selection.³⁵ But even his black-hole scenario relies on another perpetually unobservable assumption—that the big bang singularity is the same thing as a black-hole singularity, so we can assume that there are universes inside black holes, which, in the words of William Dembski, “of all the objects in space … divulge the least information about themselves.”³⁶ Moreover, this reproductive multiverse reinscribes the traditionally biblical claim that the ultimate purpose of the cosmos lies in procreation—specifically, in producing descendants that eventually become as “numerous as the stars.”

As for cosmologies stemming from quantum mechanics, they rely on a very different series of “things hoped for” and “things not seen” than the lower-level scenarios in Tegmark’s cosmic hierarchy. Unlike string theory, quantum mechanics has passed every experimental test to which it has been subjected, and this holds for the Many-Worlds Interpretation (MWI) as well as for the Copenhagen Interpretation. The MWI crosses into speculative metaphysical terrain only when it asserts that all its possible branches are actual—that is, that there are physical universes “out there” in which each quantum “decision” plays itself out. On the one hand, it is this conviction that allows theorists such as Sean Carroll and Colin Bruce to argue that the MWI is “simpler” and therefore more scientifically desirable than the Copenhagen Interpretation.37 For the many-worlders, if every outcome actually happens somewhere, then the wave function never “collapses” in relation to experimental peculiarities. Rather, “it evolves smoothly and deterministically over time without any kind of splitting or parallelism.”³⁸ What this means is that particles, far from existing in a superpositional haze until they are observed, do have determinate properties independently of the experimental apparatus that measures them; it is just that these properties take on different values in different worlds. As Brian Greene summarizes the matter, “[T]he mathematics of Many Worlds, unlike that of Copenhagen, is pure, simple and constant.”³⁹

On the other hand, this very assertion can be said to reveal what physicist Evelyn Fox Keller diagnoses as a longing for a nonquantum universe among adherents of the WMI. By insisting that every possible outcome is actual, the many-worlders display what Keller calls “an unwillingness to let go of the basic tenets of classical physics: the objectivity and knowability of nature.”⁴⁰ According to her, proponents of the MWI are engaged in an act of “cognitive repression,”⁴¹ specifically a repression of Copenhagen’s radical proposal that insofar as experimental phenomena are produced by the “intra-actions” between the observer, the observed, and the mechanism of observation, there is no such thing as an “individual,” self-constituted entity—whether it be a particle or a physicist. As Karen Barad has made clear, this does not mean that there is no such thing as “truth” or even “objectivity” along the Copenhagen Interpretation; to the contrary, this reading of quantum mechanics demonstrates that all that “is” is ontologically dependent on a series of interrelated events and components; as she puts it, “we are a part of that nature that we seek to understand.”⁴²

Along this view, the MWI can be seen as an effort to deny the quantum entanglement of knower and known—to replace the Copenhagen Interpretation’s relational and contingent universe with a nonrelational, deterministic one (well, many). And, indeed, Bruce affirms that “only in [the MWI’s] quantum world does it become possible to measure something without affecting it at all.”43 This, then, is the major article of faith subtending the MWI: that entities must be self-constituted; that is, they must possess determinate properties independently of one another or anything else. This micro-individualistic metaphysic would mean, in turn, that all decisions are determined ahead of time: that everything that can happen will happen in one world or another. More precisely, it would mean that decisions are not really decisions at all, because if all possibilities happen somewhere, then none of them is ever unpursued. Nothing, in other words, is ever lost in the MWI. Rather, all that might be is eternally held within the wave function itself: the timeless, unchanging governor of all things that actualizes all possibilities according to its sovereign will, unaffected by those innerworldly forces it controls and transcends.

A very different scenario emerges when one extends the Copenhagen Interpretation out to the level of cosmic formation. Neither Keller nor Barad mentions this scenario, and, indeed, it gets very little attention from anyone, remaining what even its proposer calls a “frail reed” and an “idea for an idea.”⁴⁴ The theorist behind the idea is the legendary John Wheeler (1911–2008), who imagined a revision to the double-slit experiment. For a century now, this experiment has shown that particles behave differently under different experimental conditions. Fire a beam of photons toward a screen with two slits cut out of it, and the beam will produce a wavelike interference pattern on the photographic film behind it. Fire photons one at a time through the two slits, and, astonishingly, each photon will appear to pass through both slits at once, interfering with itself before landing somewhere in the wave pattern. But—and this is where things get very strange—if you install a “which-path” detector above the slits so that you can see how on earth a single photon can pass through both slits at once, the photon will straighten up and behave like a particle (or a baseball), heading through just one of them and lining up neatly on the film behind it.

Wheeler’s revision to this setup is called the “delayed-choice experiment.” In this scenario, the observer is able to decide after the photon has passed through the slit(s) how she will configure the final measuring apparatus. If she configures it in a way that produces a wavelike pattern on the screen, then the photon will have passed through both slits. If she configures it to produce a particle-like pattern, then the photon will have passed through just one slit. Wheeler’s prediction—remarkably confirmed by a team at the University of Maryland in the mid-1980s—was that the experimenter’s decision would condition the photon’s decision even though the photon’s decision would have preceded the experimenter’s decision. In other words, the observer retroactively determines the photon’s behavior: her decision obliges it already to have acted like a particle or a wave. Wheeler dubbed this mind-bending phenomenon “backward causation.”⁴⁵

The punch line, then, is that the universe might well not “exist if we’re not looking at it”—that observers might co-create the very cosmic phenomena they observe. As Wheeler puts it, “[T]he observer is as essential to the creation of the universe as the universe is to the creation of the observer.”⁴⁷ In this case, the answer to the question of why the universe seems so finely tuned to our existence would be that our existence retroactively tunes the universe, creating the cosmic conditions that, in turn, create our existence.⁴⁸ Wheeler calls this cosmological model the “participatory universe.”

To be sure, this participatory scenario runs the risk of a rather flagrant anthropocentrism. Along Wheeler’s account, at least, the scenario seems to suggest that humans can somehow make the universe they want to have—in the manner, say, of American politicians who proclaim, “When we act, we create our own reality.”49 It is at this point that the model, which everyone admits needs a bit of work, would benefit considerably from Barad’s “posthumanist” critique of the Copenhagen Interpretation. Humans, she insists, are just as much the product of specific material–discursive configurations (including, but not limited to, experimental arrangements) as the photons or electrons (or in this case, universes) they purport to measure. In fact, Barad explains, “what we usually call a ‘measurement’ is a correlation or entanglement between the component parts of a phenomenon,” which include the measured object, the measuring device, and the measurers themselves. Far from simply determining the shape and nature of the world, she counters, “humans are themselves specific parts of the world’s ongoing reconfiguring.”⁵⁰

Unlike the MWI, then, the ontological assumption of Copenhagen cosmology is that particles (and all other entities co-constituted by and as the universe) do not possess properties independently of one another; rather, phenomena emerge only through the relations among cosmic components. Grafting Barad’s language onto the theory of the participatory universe, one might say that the observer, the observed, and the instrument of observing “intra-act” to produce the universe “itself” as a set of provisional and shifting intra-actions.⁵¹

Finally, we make our way to Tegmark’s Mathematical Universe Hypothesis, or Ultimate Multiverse, which operates on the absolutely undemonstrable assumption that all mathematical possibilities must exist physically. As Tegmark explains his scenario, this collection of these universes exists “outside space and time,” much like the Platonic Forms or, for that matter, the divine ideas.52 Again, it should be noted that although multiverse proponents and critics tend to express varying levels of tolerance for the unobservable, almost all of them stop short of Tegmark’s proposal, calling it “pure speculation”—an “ecstatic … mathematical fantasy land” that lies “beyond any scientific support.”⁵³ Even Greene, who confesses to having a “taste … for the expansive” when it comes to cosmology, says that he “draw[s] the line at … the full-blown version of the Ultimate Multiverse” because it, unlike all the other scenarios, does not emerge organically from any physical theory and therefore offers “no possibility of being confronted meaningfully by experiment or observation.”⁵⁴ Moreover, unlike all the other scenarios, the Ultimate Multiverse offers no common generating mechanism (such as, according to Greene, “a fluctuating inflaton field, collisions between braneworlds, quantum tunneling through the string theory landscape, a wave evolving via the Schrödinger equation”).⁵⁵ In the absence of such a cosmogonic hypothesis, there is no way even theoretically to prove that these worlds exist. There is, in principle, no way to gain any knowledge about the Ultimate Multiverse at all … unless, of course, it becomes possible to simulate universes. In this case, Greene imagines, “an army of future computer users … could spawn this [mathematical] multiverse through their insatiable fascination with running simulations based on ever-different equations.”⁵⁶ So if we ever do become (or have ever before become) “posthuman,” then all mathematically possible, which is to say all “computable,” universes could, in fact, exist. But then we would be right back where we started this ascending cosmic meditation: with the new anthropomorphic gods of the multiverse.

The point of this journey has simply been to show that every multiverse hypothesis opens in one way or another onto uncannily metaphysical—even theological—terrain. Each scenario requires us to assent to worlds, gods, or generative principles that remain, in the words of an old English hymn, “in light inaccessible hid from our eyes.”⁵⁷ This is not to say that the theories are somehow scientifically invalid; as Helge Kragh reminds us, “speculations have always been an integrated part of the physical sciences, sometimes hidden under the more palatable term ‘hypotheses.’ “⁵⁸ It is simply to say that the distinctions between the purportedly inimical terms of “science” and “religion” are highly unstable when it comes to the multiverse, which, to borrow Bruno Latour’s string of adjectives, is perhaps the most “spiritual, miraculous, soul-fulfilling, uplifting” hypothesis that modern science has ever produced.⁵⁹ What the debate comes down to, therefore, is not an argument between theism and atheism or between sacred and secular reason, but between vastly different understandings of what counts as “science.”

As we have begun to see, critics of the multiverse accuse these scenarios of venturing too far beyond the bounds of observation and experiment, of causing more problems than they solve, and of demanding just as much “faith” in the invisible as does the design hypothesis. In response, multiverse theorists tend to remind their detractors that modern science is teeming with elements—from black holes to dark matter to neutrinos to superpositions—that lie beyond our ability to access them directly, but explain phenomena that no other theory can explain. As Max Tegmark argues, “[F]or a theory to be falsifiable, we do not need to be able to observe and test all its predictions, merely at least one of them … consider Einstein’s theory of General Relativity [sic]. Because this has successfully predicted many things that we can observe, we can also take seriously its predictions for things we cannot observe.”60 If, then, the multiverse solves observational and experimental problems that nothing else solves (and its proponents argue that it does), then no matter how inaccessible the “multiverse itself” might be, it should be taken seriously as a scientific hypothesis. Andrei Linde is perhaps the most energetic of these defendants, saying in an interview that

we don’t have any other alternative explanation for the dark energy, we don’t have any alternative explanation for the smallness of the mass of the electron; we don’t have any alternative explanation for many properties of particles. What I am saying is to look at it with open eyes. These are experimental facts, and these facts fit one theory: the multiverse theory. They do not fit any other theory so far. I’m not saying these properties necessarily imply the multiverse theory is right. But you asked me if there is any experimental evidence, and the answer is yes. It was Conan Doyle who said, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.”⁶¹

From the perspective of the most fervent multiverse proponents, it is this improbability, rather than a coherent scientific commitment, that motivates multiverse critics. As Tegmark charges, “[T]he principle arguments against [multiverse cosmologies] are that they are wasteful and that they are weird.”62 And, indeed, some physicists and philosophers do display an almost visceral disdain for the multiverse. When, for example, Paul Davies calls it a “fantasy-verse” and an “infinitely complex charade,” or when Nobel laureate David Gross says, “I hate it,” or when Paul Steinhardt calls it “a dangerous idea that I am simply unwilling to contemplate,”⁶³ we might hear echoes of St. Augustine’s anxious “God forbid” or even of the character in Giordano Bruno’s On the Infinite Universe and Worlds who exclaims, “[E]ven if this can be true I do not wish to believe it, for this infinite can neither be understood by my head nor brooked by my stomach.”⁶⁴ As far as some of the modern heirs to Bruno are concerned, however, the multiverse is no “weirder” or less stomachable than Copernicanism was in Galileo’s day or than multiple galaxies were in Kant’s. This multiverse is just the next step in what Bernard Carr sees as a progressive expansion of our understanding of the universe. “Every time this expansion has occurred,” he reminds us, “the more conservative scientists have said, ‘This isn’t science.’ This is just the same process repeating itself.”⁶⁵

Other multiverse theorists disagree that “this is just the same process” as the convulsions over Copernicanism and galactic pluralism, but claim right alongside their detractors that “the very nature of the scientific enterprise is at stake in the multiverse debate.”⁶⁶ The seismic shift that these rivals identify concerns the effort to understand the nature of the universe “from fundamental principles”—to determine why our universe had to be the way it is. As Davies explains the problem, “[W]e should like to understand the bio-friendliness of this universe. To postulate that all possible universes exist does not advance our understanding at all. A good scientific theory is analogous to a well targeted bullet that selects and explains the object of interest. The multiverse is like a blunderbuss—hitting everything in sight.”⁶⁷ It is this “bullet” model of the scientific enterprise that multiverse critics ranging from Walter Charleton and Cardinal Christoph Schönborn to Steinhardt and Gross are seeking to defend. And it is the same model that Linde suggests we are going to have to give up so that we can learn to work with the blunderbuss. “For a long time,” Linde explains, “physicists have believed that there is only one world and that a successful description of this world should eventually predict all of its parameters, such as the coupling constants and the masses of elementary particles. The fundamental theory was supposed to be beautiful and natural. This was a noble, but perhaps excessively optimistic, hope. One could call this period ‘the age of innocence.’ I believe we are now entering ‘the age of anthropic reasoning.’”⁶⁸

Linde seems happily—even impishly—reconciled to this new age. Leonard Susskind is even more insistent a harbinger, saying that it would be “the height of stupidity to dismiss” the anthropic principle “just because it breaks some philosopher’s dictum about falsifiability.”69 But other theorists display far more ambivalence toward Linde’s “age of anthropic reasoning.” Both Alan Guth and Steven Weinberg regard the anthropic principle and the multiverse it now entails as a “last resort,” “plausible only when we cannot find any other explanation” for the apparent fine-tuning of the cosmos.⁷⁰ Both of them confess that they are not sure whether even the appalling smallness of the cosmological constant warrants our appealing to the anthropic principle and its infinity of other worlds.⁷¹ But both of them fear that it might, and they confess to feeling let down by the new anthropic age it has ushered in. “It would be a disappointment if [an ensemble of universes] were the solution of the cosmological constant problems,” Weinberg says, “because we would like to be able to calculate all the constants of nature from first principles, but it may be a disappointment that we will have to live with.”⁷² As for Guth, he assures his interlocutors (not least of all himself) that “there will still be a lot for us to understand, but we will miss out on the fun of figuring everything out from first principles.”⁷³ And although it might seem like an unprecedented shift in the scientific project, both Weinberg and Martin Rees say that this multiversal letdown will simply be a cosmic extension of the disappointment that Newton experienced when he realized that planetary orbits could not be deduced from first principles.⁷⁴ Rees explains the comparison thus:

People used to wonder: why is the earth in this rather special orbit around this rather special star, which allows water to exist or allows life to evolve? It looks somehow fine-tuned. We now perceive nothing remarkable in this, because we know that there are millions of stars with retinues of planets around them: among that huge number there are bound to be some that have the conditions right for life. We just happen to live on one of that small subset. So there’s no mystery about the fine-tuned nature of the earth’s orbit; it’s just that life evolved on one of millions of planets where things were right.⁷⁵

In effect, Rees is arguing, all the multiverse does is to move this arbitrary positioning up from the level of planets to the level of the universe itself. “There is no mystery” about the cosmological constant, the mass of the electron, or the strength of the nuclear forces; it is just that life has evolved in one of an infinite number of universes where things are right. Even a blunderbuss loaded with wood chips and rocks is bound to hit something at some point.

In one of his numerous critiques of multiverse cosmologies, George Ellis issues a word of warning to physicists. “There are many other theories waiting in the wings,” he cautions, “hoping for a weakening of what is meant by ‘science.’”76 What Ellis has in mind, Helge Kragh explains, is a host of “pseudosciences such as astrology, intelligent design, and crystal healing.” Is an infinite number of all possible worlds not at least as speculative as the twelve signs of the zodiac (not to mention one lonely creator)? “That is,” Kragh asks, “if multiverse cosmology is admitted as a science, how can scientists reject pseudosciences … on methodological grounds?”⁷⁷

In response to this line of questioning, multiverse centrist Brian Greene concedes that at the moment there is no (widely accepted) observational or experimental evidence for any multiverse scenario. He reaffirms the importance of withholding scientific assent to any model “not supported by hard data.” And yet he is fascinated by the idea that our universe might just be one of a host of others—not because of the metaphysical or narrative implications of such an idea (at least this is what he claims), but because although the multiverse has not yet been confirmed by any scientific discipline, it has been predicted by a number of them. From quantum mechanics to inflationary cosmology to string theory, “numerous developments in physics, if followed sufficiently far, bump into some variation on the parallel-universe theme.” For Greene, this “bump” is what distinguishes contemporary multiple-worlds cosmologies from astrology, intelligent design, and crystal healing. “It’s not that physicists are standing ready, multiverse nets in their hands, seeking to snare any passing theory that might be slotted … into a parallel-universe paradigm,” he maintains. “Rather, all of [these] parallel-universe proposals … emerge unbidden from the mathematics of theories developed to explain conventional data and observations.”⁷⁸ In other words, no one was looking for the multiverse, and yet it has sprung up nearly everywhere.

On the one hand, this account risks mystifying the multiverse even further—it is hard not to hear in Greene’s language an echo of Karl Jung’s “bidden or not bidden, God will be present.”⁷⁹ On the other hand, it also has the potential to render the multiverse debate a bit less dramatic than it often seems. If these scenarios do emerge from theories that emerge from observations, then they should ultimately be accountable to observations. In other words, it might be that multiverse cosmologies represent neither a total transformation nor an obliteration of the scientific project, but an organic development that just happens to open physics onto metaphysics at more or less every turn. But in that case, the multiverse is going to have to pass (or fail) a few tests.

It is in this spirit that Nobel Prize–winning particle physicist and multiverse skeptic Burton Richter issues a challenge to the colleagues he calls “landscape gardeners.” “Calculate the probabilities of alternative universes,” he suggests, knowing full well what a monstrous task this is, “and if ours does not come out with a large probability while all others with content far from ours come out with negligible probability, you have made no useful contribution to physics.”⁸² What Richter is looking for, in effect, is a justification for the metaphysical extravagance of the landscape. If, of all these possible universes, ours turns out to be a fairly common one—if it is highly likely that any given universe will have the kind of cosmological constant, electrons, nuclear forces, and so on that our universe does—then the gazillions of universes on the landscape are a justifiable expenditure; we genuinely need them in order to understand our universe as it is. At the moment, however, even the most fervent proponents of the landscape admit that life looks possible only in a small subset of these hypothetical vacua.⁸³ And if this remains the case, then as far as Richter is concerned, there is no reason to keep cultivating the landscape model. “It is not that the landscape model is necessarily wrong,” he explains, “but rather that if a huge number of universes with different properties are possible and equally probable, the landscape can make no real contribution other than a philosophic one.”⁸⁴ In other words, it might be fun to think about all the other ways the world might have been and the extreme improbability of the universe as it is, but such a scenario would leave us just as befuddled as ever by the miraculous fine-tuning of the universe we happen to be in.

That having been said, the physics community does not have to await the statistical “population” of the landscape for evidence of some sort of multiverse. To the contrary, as we began to see in chapters 5 and 6, observational astronomers and cosmologists are already claiming to have found such evidence on the CMB. To be sure, the patterns in question differ from team to team, as do the models they allegedly support, but the very existence of such work means that the multiverse does not lie totally beyond the bounds of observation, as its critics often allege. In 2010, for example, one group of researchers reported having found four “disc-shaped” temperature variations that they attribute to collisions between our universe and others during the inflationary period.⁸⁵ If such collisions do not destroy the universes in question, they would leave “bruises” in the form of “inhomogeneities in the inner-bubble cosmology, which could appear on the CMB.”⁸⁶ Other researchers, seeking to confirm Roger Penrose’s cyclical cosmology, claim to have found different sorts of inhomogeneities on the CMB—“concentric circles” that purportedly record the black-hole collisions of the previous aeon, which is to say the universe whose end produced our beginning.⁸⁷ And four of Laura Mersini-Houghton’s major predictions have been found, including one void and the set of inhomogeneities now called the “dark flow.”⁸⁸ According to Mersini-Houghton, both of these cosmic anomalies provide compelling evidence of “the birth of the universe from the multiverse”; they effectively are scars—not of the collisions between universes, but of their “entanglement,” of the primordial boundupedness of our universe “with all other patches … beyond the horizon,” which is to say with the multiversal “bath” itself.⁸⁹

In one of his numerous defenses of multiple-worlds cosmologies, Max Tegmark reminds his queasy critics that “the borderline between physics and metaphysics is defined by whether a theory is experimentally testable, not by whether it is weird or involves unobservable entities.”⁹⁰ And, indeed, if the examples given here count as such “tests,” then it does not matter how extravagant their underlying multiverse scenarios may be; they ought to be taken seriously as scientific hypotheses. But I would respectfully argue against Tegmark that this testability does not deliver such scenarios from the treacherous realms of philosophy or religion. To the contrary, the very observations and experiments that promise to establish the multiverse as “physics” also establish it as metaphysics.

Here I am using the term metaphysics both in the broad sense of “beyond the physical” and in the narrower sense that Martin Heidegger uses it to refer to the Western philosophical tradition founded on the question “What is?”91 To the extent that multiverse theories can be called physics, they can also be called metaphysics—first, because they posit realms that, however imprinted on or entangled with our own, remain inexorably beyond it and, second, because they operate by means of a specific conception of what is. Viewed in one light (there are others), the history of Western metaphysics can be seen as an ongoing effort to understand whether “what is” is fundamentally one or fundamentally many, identical or different. Philosophers often trace this debate to the distinction between Parmenides and Heraclitus or Plato and Aristotle or rationalism and empiricism. In the introduction, we saw William James refer to it as a conflict between monism and pluralism. And throughout the book, we have seen this question play itself out cosmologically: Is “all that is” part of the same reality or different realities? As Timaeus asks, “[A]^re we right … to speak of one universe, or would it be more correct to speak of a plurality?”⁹²

The only possible answer for Timaeus is “one,” and a great deal of his “likely story” about the origin of the world is dedicated to demonstrating such oneness. His motivation in this regard stems from his insistence that the universe must be eternal; if there were another world beyond, before, or after this one, then it would render our own world vulnerable to destruction—whether by collision or by periodic annihilation and rebirth. In order to assure his audience of the permanence of the universe, Timaeus therefore assures us of its singularity. As we have seen, however, our Platonic astronomer can assert the unity of the cosmos only by establishing it as a “mix”—even, as Michel Serres phrases it, a “mix of mixes”⁹³—of the divisible and the indivisible, the same and the different, the many and the one. A strikingly similar thing happens in the Metaphysics when in the very process of proving the oneness of the world (again, to establish its eternity), Aristotle suddenly leaves us with the conclusion that there are either forty-seven or fifty-five worlds, but that the math is too hard for him.⁹⁴ Then there are Thomas Aquinas, who hinges the oneness of the world on the oneness of a God who also happens to be three, and René Descartes, who insists that “there cannot be a plurality of worlds” even as he provides a cosmology of a plurality of worlds.⁹⁵

The reason, then, that the multiverse is only physics insofar as it is metaphysics is that the only “other worlds” that might compose a scientific hypothesis would be worlds that are in some way, however mediated and strung along, ontologically bound up with ours. After all, if they were wholly “other,” sharing neither space nor time nor any generating principle and bearing no trace of one another, then they would be wholly inaccessible to observation and experiment (this is the reason that most physicists draw the line at Tegmark’s Mathematical Multiverse Hypothesis, however “expansive” their sensibilities may be). At the same time, however, the “other worlds” of the multiverse remain to an extreme degree other: they are forever separated by an unrecoverable past, an unreachable future, an uncrossable distance, or an irremediable split. So it may be that, from a god’s-eye view, there is only one world. It may be that there are many. It may be that every mathematically possible universe does in fact “exist outside space and time,” whatever such existence might mean. But the only way to make multiple worlds an object for science would be to configure them as neither one nor many—neither undifferentiated from one another nor indifferent to one another. And so the metaphysical claim that the multiverse necessarily makes is that “all that is” exists in “some possible or mediate connection” with all that is, rendering being itself many-one, pluri-singular: multiple.⁹⁸ As such, neither our universe nor anything else is self-constituted, nor is it invulnerable to destruction or radical transformation. And although this cosmic interdetermination of physics and metaphysics might scandalize the modern secularist, it would come as no surprise to the thinker whose demon stole into our loneliest loneliness to proclaim that “all things are entangled, ensnared, enamored.”⁹⁹

In the third essay of On the Genealogy of Morals, Friedrich Nietzsche marvels at the persistence of what he calls “ascetic ideals.”¹⁰⁰ These ideals include humility, restraint, poverty, chastity, meekness—that “whole train” of what David Hume calls the “monkish virtues”¹⁰¹—and are enforced through various practices of self-denial, such as fasting, flagellation, and sleep deprivation. For Nietzsche, the ascetic sees the physical world as some kind of mistake. Setting herself against everything that is life giving, which is to say everything that is, she is a literal nihilist, reducing the whole world to nothing. As Nietzsche explains it, these ascetic ideals are Europe’s Christian inheritance. It was Christianity, he says, that globalized the allegedly “Jewish” values of “poverty,” “impotence,” and “wretchedness,” spreading these values through the parts of the world that it conquered. By now, what Nietzsche calls “slave morality” has become the foundation of Western political and cultural systems to such an extent that “we no longer see it because it—has been victorious” (1.7).¹⁰² (“The same evolutionary course,” he says in a long parenthetical paragraph, has been followed “in India,” where “five centuries before the beginning of the European calendar,” the Buddha taught the ascetic ideal that would spread itself throughout the “Eastern” world as effectively as the Christians spread their asceticism throughout the “West” [3.27].) The point is that wherever and whatever we might call ourselves, “we” are not finished with asceticism just because we claim to be free from religion.

Almost no one escapes Nietzsche’s charge of asceticism, but the third essay of the Genealogy focuses on three particularly guilty classes of people. The first class, unsurprisingly, comprises priests, who “heal” their flocks (or “herds”) by making them sick in the first place, prescribing them a set of self-destructive practices to keep them docile and numb (3.15–18).103 The next class of ascetics is a bit less obvious: philosophers, Nietzsche claims, are subject to the same ascetic ideal. Although he initially offers the less than compelling evidence that real philosophers never marry (Socrates, he says, is the only exception, and he “married ironically, just to demonstrate this proposition” [3.7, emphasis in original]), his lasting charge is that philosophers are ascetics because, like priests, they believe in a world beyond this one: an eternal realm of Forms or Ideas or things-in-themselves, of which this world is at best a pale reflection. Insofar as philosophers declare that “there is a realm of truth and being, but reason is excluded from it!” (3.12, emphasis in original), they are no different qualitatively from the priests who proclaim the eternal, mysterious providence of a God before whom we must abase ourselves. And then finally, Nietzsche intones, there are “scientists,” a broad category denoting scholars of what we might consider the human, social, and natural sciences—any discipline one might study at a secular university. Of course, Nietzsche says, “modern science” thinks itself “a genuine philosophy of reality,” having “up to now survived well enough without God, the beyond, and the virtues of the eternal” (3.23). In other words, modern science believes that it has liberated itself completely from religious delusions. And yet, he insists, “such noisy agitators’ chatter … does not impress me: these trumpeters of reality are bad musicians…. [S]cience today … is not the opposite of the ascetic ideal but rather the latest and noblest form of it. Does that sound strange to you?” (3.23, emphasis in original).

Assuming the answer is yes, Nietzsche goes on to explain that “science today” remains ascetic (and even excels at asceticism) for two interrelated reasons. First, scientists believe that there is a truth outside themselves, and, second, they devote themselves to the unconditional pursuit of that unconditioned truth. Here we might begin to think of all those hours at the computer, in the archives, in the lab; the ritualistic precision over methods, materials, data, results; the caffeine that stands in for both sleep and food: What is all this if not asceticism? And so, as Nietzsche inimitably explains the asceticism of science,

these hard, severe, abstinent, heroic spirits who constitute the honor of our age; all these pale atheists, anti-Christians, immoralists, nihilists; these skeptics, ephectics, hectics of the spirit (they are all hectics in some sense or other), they certainly believe they are as completely liberated from the ascetic ideal as possible, these “free, very free spirits”; and yet, to disclose to them what they themselves cannot see—for they are too close to themselves: this ideal is precisely their ideal, too … they themselves are its most spiritualized product, its most advanced front-line troops and scouts…. [I]f I have guessed any riddles, I wish that this proposition might show it!—They are far from being free spirits: for they still have faith in truth. (3.24, emphasis in original)

Part of what makes the field of cosmology such a fascinating case study for this Nietzschean hypothesis is that until very recently, cosmology was not considered a “real science.”¹⁰⁵ The reason was not only that cosmology has traditionally been the purview of philosophy and mythology (after all, every field has traditionally been the purview of philosophy and mythology), but also that cosmology, unlike the other disciplines that branched off in the early modern period, was not considered objective. The problem is that whereas every other secular discipline studies objects within the universe, cosmology studies the universe itself. This means that cosmology, unlike every other discipline, remains hopelessly internal to that which it studies. Unlike the biologist, the anthropologist, the economist, and even the astronomer, the cosmologist cannot even pretend to stand as a knowing subject over against her known object. Rather, she is inescapably caught within and irreducibly constituted by the very thing she is trying to measure and observe.

There is also the problem of repeatability. For a hypothesis to hold water, it has to be tested again and again on a vast number of specimens. But cosmology’s specimen is the uni-verse, which is to say that cosmology’s specimen is all there is. Where might cosmologists find another “all there is”—much less hundreds of them—to make sure they get the same results each time? Here we might recall Philo’s battery of questions to Cleanthes: “Have worlds ever been formed under your eye? And have you had leisure to observe the whole progress of the phenomenon, from the first appearance of order to its final consummation? If you have, then cite your appearance and deliver your theory.”¹⁰⁶ So from the seventeenth century onward, cosmology was accused of subjectivism, imprecision, speculation—in short, of sounding more like philosophy or religion than science.

But then, as the legend usually goes, this scrappy little discipline finally came into its own with the accidental discovery of the CMB in 1965. The story is remarkable. Two radio astronomers at Bell Labs in Holmdel, New Jersey, kept hearing interference hissing through their high-powered antennas. When they climbed up to the roof to see what the problem was, they found piles of pigeon droppings congealed around the equipment. So they power-washed the roof and climbed back down to the lab, only to keep getting the interference. Then, in consultation with some colleagues, they eventually realized that the hissing was not a result of pigeon droppings at all; it was the remnant of the big bang.107 The Cosmic Microwave Background, as it has come to be called, is a snapshot of the temperature and density variations of the universe when it was just a few hundred thousand years old. Subsequent developments in telescopic and satellite technology led to the release in early 2003 of the Wilkinson Microwave Anistropy Probe’s ovoid image of the CMB (see figure 5.1; and a brand-new image was released from the Planck satellite in March 2013),¹⁰⁸ which suddenly became the Object that cosmology had needed. It is, in fact, an object that cosmologists can measure and observe as often as they like. Granted, this object is rather grainily compiled by inexorably situated satellites; like everything else, the CMB is produced by means of specific and perspectival material configurations. Nevertheless, it seems to have done the work of disaggregating the subject of cosmology from something said subject can regard as an “object,” because the field has come into wide acceptance as a “proper, quantitative science.”¹⁰⁹

Staying tuned with our Nietzschean antennas, however, we might notice that the moment cosmology entered the domain of “objective science,” it also collided head on with Christian creation theology. After all, it was the CMB that confirmed that the universe had a beginning, that it began in a burst of light, and that it came out of something like “nothing”—all ideas that Jews, Christians, and Muslims had been teaching for centuries (see chap. 5, sec. “Let There Be Light”). Although this “big bang” hypothesis scandalized decades of physicists (Georges Lemaître says that upon hearing the idea, Einstein shot back, “No, not that, that sounds too much like creation”),¹¹⁰ the resemblance is not all that surprising if we, alongside Nietzsche, think of “science today” as the “latest and noblest form” of the ascetic ideal. In fact, it is a remarkable sign of the entanglement of Western science and religion that when science finally had a creation story to tell, it told such a familiar one.

Nietzsche concludes the Genealogy by expanding this vision, promising that “all great things bring about their own destruction through an act of self-overcoming” (3.27, emphasis added). This promise, then, has me wondering. If science can be regarded as the self-overcoming of a particular form of religion, might multiverse cosmologies be something like the self-overcoming of science? Might they mark the end of the fantasy that “science” has wrested itself free from “religion,” “objectivity” free from subjectivity, and matter free from meaning?111 After all, we have seen each of these multiverse cosmologies open onto metaphysics and mythology not in moments of lapse or weakness, but precisely where they are scientifically most compelling. I would like to be clear here: by pointing out this confluence, I do not intend to say that philosophers have already solved all these scientific riddles or that any particular theology can account ahead of time for the tiger-striped/zebra-streaked being of the multiverse. To the contrary, by revealing the persistent entanglement of all these disciplines, multiple-worlds cosmologies condition and even necessitate a renewed engagement among them.

“Okay …,” you might be wondering, “but does the multiverse exist? And if so, which model is the right one?”

In the face of such questions, I find myself wanting to hide behind someone like Johannes Climacus, a character that Søren Kierkegaard dreamed up to write something that he could not quite write. In Philosophical Fragments, or a Fragment of Philosophy, Climacus presents himself as a trifler, a “loafer out of indolence,” totally unqualified to contribute to serious philosophy. The tone of the preface gets increasingly cranky until finally Climacus responds to an imagined interlocutor, “But what is my opinion? Do not ask me about that. Next to the question of whether or not I have an opinion, nothing can be of less interest to someone else than what my opinion is. To have an opinion is both too much and too little; it presupposes a security and well-being in existence akin to having a wife and children.”¹¹²

To be sure, a hunch is hardly the basis on which serious scholarship ought to be conducted,¹¹³ and so I offer mine as an afterthought, a possibility opened by the foregoing analysis—not at all as its foundation. My hunch is that “everything” probably works the same way as anything does. Just as light will behave as a wave or a particle, depending on the question you ask it, and just as chemical and biological and psychological experiments help produce the phenomena they measure, so will the universe appear to be one or many, or linear or cyclical, or infinite or finite, depending on the theoretical and experimental configuration that examines it. In other words, the shape, number, and character of the cosmos might well depend on the question we ask it. Of course, this is not to say that every theory is right; some will be more internally coherent, mathematically reliable, and observationally demonstrable than others—and those coherent, reliable, and demonstrable models, I imagine, will be the ones that survive the decades ahead. But I doubt very much that we will or should emerge with only one of these theories. Would it even make sense to have a single account of cosmic multiplicity? To arrive at the one truth of the multiple ways worlds can be multiple?