This is the article replied to by D. H. Mellor's 'Too Many Universes'. It is the revised text of Martin Rees's contribution to a symposium with Mellor on 'Our Universe - and Others?' held at Darwin College Cambridge on 26 May 2000. This revised text is now in print on pages 211-20 of God and Design: The Teleological Argument and Modern Science (London: Routledge, 2002), edited by Neil A. Manson, by whose permission, and that of Martin Rees, it is reproduced here.
We do not know whether there are other universes. Perhaps we never shall. But I want to argue that 'do other universes exist?' can be posed in a form that makes it a genuine scientific question. Moreover, I shall outline why it is an interesting question; and why, indeed, I already suspect that the answer may be 'yes'.
First, a pre-emptive and trivial comment: if you define the universe as 'everything there is', then by definition there cannot be others. I shall, however, follow the convention among physicists and astronomers, and define the 'universe' as the domain of space-time that encompasses everything that astronomers can observe. 'Other universes', if they existed, could differ from ours in size, content, dimensionality, or even in the physical laws governing them.
It would be neater, if other 'universes' existed, to redefine the whole enlarged ensemble as 'the universe', and then introduce some new term - for instance 'the metagalaxy' - for the domain that cosmologists and astronomers have access to. But so long as these concepts remain so conjectural, it is best to leave the term 'universe' undisturbed, with its traditional connotations, even though this then demands a new word, the 'multiverse', for a (still hypothetical) ensemble of 'universes'.Current theories - based on well defined (albeit often untested) assumptions - have expanded our conceptual horizons, bringing the 'multiverse' within the scope of cosmological discourse. Our entire universe, stretching 10 billion light years in all directions, could (according to one widely-studied model) have inflated from an infinitesimal speck; moreover, this 'inflationary' growth could have led to a universe so large that its extent requires a million-digit number to express it. But even this vast expanse may not be everything there is: patches where inflation doesn't end, may grow fast enough to provide the seeds for other big bangs. If so, our big bang wasn't the only one, but could be part of an eternally-reproducing cosmos.
Other lines of thought (distinct from the generic concept of inflation) also point towards a possible multiplicity of universes. Some theorists conjecture, for instance, that whenever a black hole forms, processes deep inside it trigger another universe that creates a 'new' space-time disjoint from our own. Alternatively, if there were extra spatial dimensions that weren't tightly rolled up, we may be living in one of many separate universes embedded in a higher dimensional space.
The entire history of our universe could be just an episode, one facet, of the infinite multiverse. I shall try to argue that this is a genuinely testable and scientific hypotheses. But before doing so, let me briefly sketch why the 'multiverse' concept seems attractive.
Obviously we can never fully delineate all the contingencies that led from a big bang to our own birth here 13 billion years later. But the outcome depended crucially on a recipe encoded in the big bang, and this recipe seems to have been rather special. A degree of fine tuning - in the expansion speed, the material content of the universe, and the strengths of the basic forces - seems to have been a prerequisite for the emergence of the hospitable cosmic habitat in which we live. Here are some prerequisites for a universe containing organic life of the kind we find on Earth.
First of all, it must be very large compared to individual particles, and very long-lived compared with basic atomic processes. Indeed this is surely a requirement for any hypothetical universe that a science fiction writer could plausibly find interesting. If atoms are the basic building blocks, then clearly nothing elaborate could be constructed unless there were huge numbers of them. Nothing much could happen in a universe that was was too short-lived: an expanse of time, as well as space, is needed for evolutionary processes. Even a universe as large and long-lived as ours, could be very boring: it could contain just black holes, or inert dark matter, and no atoms at all; it could even be completely uniform and featureless. Moreover, the laws must allow the variety of atoms required for complex chemistry.
If our existence depends on a seemingly special cosmic recipe, how should we react to the apparent fine tuning? There seem three lines to take: we can dismiss it as happenstance; we can acclaim it as the workings of providence; or (my preference) we can conjecture that our universe is a specially-favoured domain in a still vaster multiverse. Let's consider them in turn.
(a) Happenstance (or coincidence)
Maybe a fundamental set of equations, which some day will be written on T-shirts, fixes all key properties of our universe uniquely. It would then be an unassailable fact that these equations permitted the immensely complex evolution that led to our emergence.
But I think there would still be something to wonder about. It's not guaranteed that simple equations permit complex consequences To take an analogy from mathematics, consider the beautiful pattern known as the Mandelbrot set. This pattern is encoded by a short algorithm, but has infinitely deep structure: tiny parts of it reveal novel intricacies however much they are magnified. In contrast, you can readily write down other algorithms, superficially similar, that yield very dull patterns. Why should the fundamental equations encode something with such potential complexity, rather than the boring or sterile universe that many recipes would lead to?
One hard-headed response is that we couldn't exist if the laws had boring consequences. We manifestly are here, so there's nothing to be surprised about. I'm afraid this leaves me unsatisfied. I'm impressed by a well-known analogy given by the philosopher John Leslie. Suppose you are facing a firing squad. Fifty marksmen take aim, but they all miss. If they hadn't all missed, you wouldn't have survived to ponder the matter. But you wouldn't leave it at that: you'd still be baffled, and you'd seek some further reason for your luck. Likewise, I think we would need to know why the unique recipe for the physical world should permit consequences as interesting as those we see around us ( and which, as a byproduct, allowed us to exist) Some of course would invoke:
(b) Providence or design
Two centuries ago, William Paley introduced the famous metaphor of the watch and the watchmaker - adducing the eye, the opposable thumb and so on as evidence of a benign Creator. This line of thought fell from favour, even among most theologians, in post-Darwinian times. But the seemingly biophilic features of basic physics and chemistry can't be as readily dismissed as the old claims for design in living things: biological systems evolve in symbiosis with their environment, but the basic laws governing stars and atoms are given, and nothing biological can react back on them to modify them. A modern counterpart of Paley, John Polkinghorne, interprets our fine-tuned habitat as 'the creation of a Creator who wills that it should be so'.
(c) A special universe drawn from an ensemble, or multiverse
If one doesn't believe in providential design, but still thinks the fine-tuning needs some explanation, there is another perspective -- a highly speculative one, however. There may be many 'universes' of which ours is just one. In the others, some laws and physical constants would be different. But our universe wouldn't be just a random one. It would belong to the unusual subset that offered a habitat conducive to the emergence of complexity and consciousness. The analogy of the watchmaker could be off the mark. Instead, the cosmos maybe has something in common with an 'off the shelf' clothes shop: if the shop has a large stock, we're not surprised to find one suit that fits. Likewise, if our universe is selected from a multiverse, its seemingly designed or fine tuned features wouldn't be surprising.
Science is an experimental or observational enterprise, and it's natural to be troubled by assertions that invoke something inherently unobservable. Some might regard the other universes as being in the province of metaphysics rather than physics. But I think they already lie within the proper purview of science. It is not absurd or meaningless to ask 'Do unobservable universes exist?', even though no quick answer is likely to be forthcoming. The question plainly can't be settled by direct observation, but relevant evidence can be sought, which could lead to an answer.
There is actually a blurred transition between the readily observable and the absolutely unobservable, with a very broad grey area in between. To illustrate this, one can envisage a succession of horizons, each taking us further than the last from our direct experience:
(i) Limit of present-day telescopes
There is a limit to how far out into space our present-day instruments can probe. Obviously there is nothing fundamental about this limit: it is constrained by current technology. Many more galaxies will undoubtedly be revealed in the coming decades by bigger telescopes now being planned. We would obviously not demote such galaxies from the realm of proper scientific discourse simply because they haven't been seen yet. When ancient navigators speculated about what existed beyond the boundaries of the then-known world, or when we speculate now about what lies below the oceans of Jupiter's moons Europa and Ganymede, we are speculating about something 'real' - we are asking a scientific question. Likewise, conjectures about remote parts of our universe are genuinely scientific, even though we must await better instruments to check them.
(ii) Limit in principle at present era
Even if there were absolutely no technical limits to the power of telescopes, our observations are still bounded by a horizon, set by the distance that any signal, moving at the speed of light, could have travelled since the big bang. This horizon demarcates the spherical shell around us at which the redshift would be infinite. There is nothing special about the galaxies on this shell, any more than there is anything special about the circle that defines your horizon when you're in the middle of an ocean. On the ocean, you can see farther by climbing up your ship's mast. But our cosmic horizon can't be extended unless the universe changes, so as to allow light to reach us from galaxies that are now beyond it.
If our universe were decelerating, then the horizon of our remote descendants would encompass extra galaxies that are beyond our horizon today. It is, to be sure, a practical impediment if we have to await a cosmic change taking billions of years, rather than just a few decades (maybe) of technical advance, before a prediction about a particular distant galaxy can be put to the test. But does that introduce a difference of principle? Surely the longer waiting-time is a merely quantitative difference, not one that changes the epistemological status of these faraway galaxies?
(iii) Never-observable galaxies from 'our' Big Bang
But what about galaxies that we can never see, however long we wait? It's now believed that we inhabit an accelerating universe. As in a decelerating universe, there would be galaxies so far away that no signals from them have yet reached us; but if the cosmic expansion is accelerating, we are now receding from these remote galaxies at an ever-increasing rate, so if their light hasn't yet reached us, it never will. Such galaxies aren't merely unobservable in principle now -- they will be beyond our horizon forever. But if a galaxy is now unobservable, it hardly seems to matter whether it remains unobservable for ever, or whether it would come into view if we waited a trillion years. (And I have argued, under (ii) above, that the latter category should certainly count as 'real'.)
(iv) Galaxies in disjoint universes
The never-observable galaxies in (iii) would have emerged from the same Big Bang as we did. But suppose that, instead of causally-disjoint regions emerging from a single Big Bang (via an episode of inflation) we imagine separate Big Bangs. Are space-times completely disjoint from ours any less real than regions that never come within our horizon in what we'd traditionally call our own universe? Surely not - so these other universes too should count as real parts of our cosmos, too.
This step-by-step argument (those who don't like it might dub it a slippery slope argument!) suggests that whether other universes exist or not is a scientific question. So how might we answer it?
At first sight, nothing seems more conceptually extravagant -- more grossly in violation of Ockham's Razor - than invoking multiple universes. But this concept follows from several different theories ( albeit all speculative).
Linde, Vilenkin and others have performed computer simulations depicting an 'eternal' inflationary phase where many universes sprout from separate big bangs into disjoint regions of spacetimes. Guth and Smolin have, from different viewpoints, suggested that a new universe could sprout inside a black hole, expanding into a new domain of space and time inaccessible to us. And Randall and Sundrum suggest that other universes could exist, separated from us in an extra spatial dimension; these disjoint universes may interact gravitationally, or they may have no effect whatsoever on each other. In the hackneyed analogy where the surface of a balloon represents a two-dimensional universe embedded in our three-dimensional space, these other universes would be represented by the surfaces of other balloons: any bugs confined to one, and with no conception of a third dimension, would be unaware of their counterparts crawling around on another balloon. Other universes would be separate domains of space and time. We couldn't even meaningfully say whether they existed before, after or alongside our own, because such concepts make sense only insofar as we can impose a single measure of time, ticking away in all the universes.
Guth and Harrison have even conjectured that universes could be made in the laboratory, by imploding a lump of material to make a small black hole. Is our entire universe perhaps the outcome of some experiment in another universe? If so, the theological arguments from design could be resuscitated in a novel guise. Smolin speculates that the daughter universe may be governed by laws that bear the imprint of those prevailing in its parent universe. If that new universe were like ours, then stars, galaxies and black holes would form in it; those black holes would in turn spawn another generation of universes; and so on, perhaps ad infinitum.
Parallel universes are also invoked as a solution to some of the paradoxes of quantum mechanics, in the 'many worlds' theory, first advocated by Everett and Wheeler in the 1950s. This concept was prefigured by Stapledon, as one of the more sophisticated creations of his Star Maker: 'Whenever a creature was faced with several possible courses of action, it took them all, thereby creating many ... distinct histories of the cosmos. Since in every evolutionary sequence of this cosmos there were many creatures and each was constantly faced with many possible courses, and the combinations of all their courses were innumerable, an infinity of distinct universes exfoliated from every moment of every temporal sequence'.
None of these scenarios has been simply dreamed up out of the air: each has a serious, albeit speculative, theoretical motivation. However, one of them, at most, can be correct. Quite possibly none is: there are alternative theories that would lead just to one universe.
Firming up any of these ideas will require a theory that consistently describes the extreme physics of ultra-high densities, how structures on extra dimensions are configured, etc. But consistency is not enough: there must be grounds for confidence that such a theory isn't a mere mathematical construct, but applies to external reality. We would develop such confidence if the theory accounted for things we can observe that are otherwise unexplained.
As the moment, we have an excellent framework, called the standard model, that accounts for almost all subatomic phenomena that have been observed. But the formulae of the 'standard model' involve numbers which can't be derived from the theory but have to be inserted from experiment. Perhaps, in the 21st-century theory, physicists will develop a theory that yields insight into (for instance) why there are three kinds of neutrinos, and the nature of the nuclear and electric forces. Such a theory would thereby acquire credibility. If the same theory, applied to the very beginning of our universe, were to predict many big bangs, then we would have as much reason to believe in separate universes as we now have for believing inferences from particle physics about quarks inside atoms, or from relativity theory about the unobservable interior of black holes.
'Are the laws of physics unique?' is a less poetic version of Einstein's famous question 'Did God have any choice in the creation of the Universe?' The answer determines how much variety the other universes - if they exist - might display. If there were something uniquely self-consistent about the actual recipe for our universe, then the aftermath of any big bang would be a re-run of our own universe. But a far more interesting possibility (which is certainly tenable in our present state of ignorance of the underlying laws) is that the underlying laws governing the entire multiverse may allow variety among the universes. Some of what we call ' laws of nature' may in this grander perspective be local bylaws, consistent with some overarching theory governing the ensemble, but not uniquely fixed by that theory.
As an analogy, consider the form of snowflakes. Their ubiquitous six-fold symmetry is a direct consequence of the properties and shape of water molecules. But snowflakes display an immense variety of patterns because each is moulded by its micro-environments: how each flake grows is sensitive to the fortuitous temperature and humidity changes during its growth.
If physicists achieved a fundamental theory, it would tell us which aspects of nature were direct consequences of the bedrock theory (just as the symmetrical template of snowflakes is due to the basic structure of a water molecule) and which are (like the distinctive pattern of a particular snowflake) the outcome of accidents. The accidental features could be imprinted during the cooling that follows the big bang - rather as a piece of red-hot iron becomes magnetised when it cools down, but with an alignment that may depend on chance factors.
The cosmological numbers in our universe, and perhaps some of the so-called constants of laboratory physics as well, could be 'environmental accidents', rather than uniquely fixed throughout the multiverse by some final theory. Some seemingly 'fine tuned' features of our universe could then only be explained by 'anthropic' arguments, which are analogous to what any observer or experimenter does when they allow for selection effects in their measurements: if there are many universes, most of which are not habitable, we should not be surprised to find ourselves in one of the habitable ones!
We may one day have a convincing theory that tells us whether a multiverse exists, and whether some of the so called laws of nature are just parochial by-laws in our cosmic patch. But while we're waiting for that theory-- and it could be a long wait - the 'ready made clothes shop' analogy can already be checked. It could even be refuted: this would happen if our universe turned out to be even more specially tuned than our presence requires.
Let me give two quite separate examples of this style of reasoning.
(i) Boltzmann argued that our entire universe was an immensely rare 'fluctuation' within an infinite and eternal time-symmetric domain. There are now many arguments against this hypothesis, but even when it was proposed one could already have noted that fluctuations in large volumes are far more improbable than in smaller volumes. So, it would be overwhelmingly more likely that we would (if Boltzmann were right) we would be in the smallest fluctuation compatible with our existence. Whatever our initial assessment of Boltzmann's theory, its probability would plummet as we came to realise the extravagant scale of the cosmos.
(ii) Even if we knew nothing about how stars and planets formed, we would not be surprised to find that our Earth's orbit wasn't highly eccentric: if it had been, water would boil when the Earth was at perigee, and freeze at apogee - a harsh environment unconducive to our emergence. However, a modest orbital eccentricity is plainly not incompatible with life. If it had turned out that the earth moved in a near-perfect circle, then we could rule out a theory that postulated anthropic selection from orbits whose eccentricities had a 'Bayesian prior' that was uniform in the range 0-1.
We could apply this style of reasoning to the important numbers of physics (for instance, the cosmological constant lambda) to test whether our universe is typical of the subset that that could harbour complex life. The methodology requires us to decide what values are compatible with our emergence. It also requires a specific theory that gives the relative Bayesian priors for any particular value. For instance, in the case of lambda, are all values equally probable? Are low values favoured by the physics? Or are there a finite number of discrete possible values? With this information, one can then ask if our actual universe is 'typical' of the subset in which we could have emerged. If it is a grossly atypical member even of this subset (not merely of the entire multiverse) then we would need to abandon our hypothesis.
As another example of how 'multiverse' theories can be tested, consider Smolin's conjecture that new universes are spawned within black holes, and that the physical laws in the daughter universe retain a memory of the laws in the parent universe: in other words there is a kind of heredity. Smolin's concept is not yet bolstered by any detailed theory of how any physical information (or even an arrow of time) could be transmitted from one universe to another. It has, however, the virtue of making a prediction about our universe that can be checked.
If Smolin were right, universes that produce many black holes would have a reproductive advantage, which would be passed on to the next generation. Our universe, if an outcome of this process, should therefore be near-optimum in its propensity to make black holes, in the sense that any slight tweaking of the laws and constants would render black hole formation less likely. (I personally think Smolin's prediction is unlikely be borne out, but he deserves our thanks for presenting an example that illustrates how a multiverse theory can in principle be vulnerable to disproof.)These examples show that some claims about other universes may be refutable, as any good hypothesis in science should be. We cannot confidently assert that there were many big bangs - we just don't know enough about the ultra-early phases of our own universe. Nor do we know whether the underlying laws are 'permissive' : settling this issue is a challenge to 21st century physicists. But if they are, then so-called anthropic explanations would become legitimate - indeed they'd be the only type of explanation we'll ever have for some important features of our universe.
What we've traditionally called 'the universe' may be the outcome of one big bang among many, just as our Solar System is merely one of many planetary systems in the Galaxy. Just as the pattern of ice crystals on a freezing pond is an accident of history, rather than being a fundamental property of water, so some of the seeming constants of nature may be arbitrary details rather than being uniquely defined by the underlying theory. The quest for exact formulas for what we normally call the constants of nature may consequently be as vain and misguided as was Kepler's quest for the exact numerology of planetary orbits. And other universes will become part of scientific discourse, just as 'other worlds' have been for centuries. Nonetheless (and here I gladly concede to the philosophers), any understanding of why anything exists - why there is a universe (or multiverse) rather than nothing - remains in the realm of metaphysics.