The Case for the Multiverse
In the past several decades there has been an increasing amount of work done, both by philosophers and scientists, dealing with multiverse theories and their relevant hypotheses. From the philosophical side multiverse theories have been motivated by a variety of factors, have been argued for in various ways, and have come in a variety of structures. Be they independently motivated, put forth by theists as a response to the problem from evil from atheists, or put forth by atheists as an objection to the fine-tuning arguments of theism, and be they comprised an infinite number or interconnected universes, a finite number of spatio-temporally disconnected universes, or any other kind of composition, a great number and variety of multiverse theories have been discussed amongst thinkers. Because of the amount of multiverse accounts that have been discussed over time, there is an equally large amount of competing views regarding the nature of the possible universes that are supposed to populate this multiverse. Of course, when discussing something of this nature, just exactly what “possible” means is a crucial question that must be answered, and one that can spawn an entirely separate discussion altogether.
Just as the discussion of multiverses has gone on in the philosophical world, so too has the discussion in the scientific world. Scientists have carried out work in attempts to determine the plausibility of different structures and compositions of competing multiverse theories. Similar to the models and compositions discussed by philosophers that are alluded to above, scientists have set out to try to determine what could have been before the Big Bang, what will happen if and when our current universe ceases to exist, whether it is even really possible to conceptualize and construct a multiverse, what the existence of a multiverse would mean for us in this universe, what the possible compositions and physical constants of other universes may be, and the like.
As I have put it thus far I may have given the impression that there is a clear divide between the work of philosophers and the work of scientists when it comes to exploration of multiverse theories, but this is not the case. Philosophical work on the multiverse and scientific work on the multiverse are not mutually exclusive and the two can and often do influence one another, with each side borrowing and implementing information from the other blurring the lines between the two disciplines. While the two sides are looking at the same ideas it is the methodologies by which they go about exploring these ideas that differentiates the two. So while philosophy, at one extreme of the spectrum, may be limited to making theoretical cases for a multiverse via inductive or deductive reasoning using a variety of previous knowledge, assumptions and postulations, physicists, at the other end of the spectrum have the ability to engage in actual experimentation and field studies to ascertain answers to questions of the plausibility and nature of different multiverse theories. Of course, as this is on a continuum, in the middle we will find differing account that appeal to varying degrees of hard science and philosophical outlooks, blending the two together. The philosophy of science, and the philosophy of math, for example, both lie closer to the middle of the continuum and can contribute to the multiverse debate through the philosophical examination of different principles of science and math that are being used by the scientists in their approaches.
This particular chapter aims to do several things, all primarily in the way of setting the stage and providing a bit of context for what will come to be discussed in further chapters. In Section 2.1 I discuss the philosophical case being made for a multiverse. I mention several of the predominant multiverse views and their motivations, outlining the methodology and structure behind these multiverse theories. In Section 2.2, I do the same for scientific approaches to the multiverse. This subsection, again, discusses the methodologies and structures of various proposed multiverse models, but this time from the scientific community. These two subsections, however, do not provide an evaluation of any of the theories proposed, rather they will serve simply to bring some of the recent and predominant work on the multiverse to your attention. Of course, the degree of plausibility of each particular multiverse account varies slightly, or perhaps even greatly, but that is not of prime importance in this case. What is important is the plausibility and worthwhileness of the multiverse discussion as a whole. Finally, in Section 2.3 I discuss the multiverse pursuit as a whole, and its overall plausibility given both philosophical and scientific approaches. While my aim in this subsection is not to confirm or disconfirm the truth of the existence of a multiverse, it is to show that the exploration of the discussion as a whole is worth continuing and advancing regardless of whether or not you ascribe to a particular multiverse account.
2.1 The Philosophical Case for the Multiverse
The role of this section is to serve as an introduction to some of the prevailing views in the world of multiverse-thought, outlining several of the predominant and well-known philosophical arguments for the multiverse. This is not intended to serve as a full-fledged survey paper, as others other substantial survey papers and edited volumes on the multiverse have already been written by others. Furthermore, the limitation of my discussion of various philosophical views is due to the fact that any broader exploration of competing viewpoints is simply irrelevant to the overall aim of this section, which can be achieved through the discussion of just several accounts. The aim here is simply to provide a brief introduction to the variety of multiverse theories that exist within philosophy, and to demonstrate that there is no single model to which philosophy as a whole ascribes. The variety of multiverse accounts present within philosophical discourse leads me to two conclusions; (1) that there are a number of highly intelligent academics working on different aspects of these ideas so we ought to afford, at least the initial idea of a multiverse, a certain level of plausibility and worthwhileness, and (2) that the sheer number of different accounts means that there is more likely to be one that strikes the reader as more plausible than others, meaning that even in the case that the reader rejects the vast majority of philosophical multiverse accounts there still exists the possibility that he finds one as plausible and ascribes to it. Now, let us move on to the discussion of some of the philosophical views.
Motivated as a response to the argument from evil, Jason Megill puts forward his account for the plausibility of a multiverse (Megill, 2001). While he does not explicitly outline exactly just what his multiverse would look like (ie. How it would be formed, what it would consist of, etc) he provides reasons for believing that a multiverse may exist, regardless of the particular composition-brand one may ascribe to. That is to say, the reasons that he provides are consistent with a variety of different multiverse compositions. The sole detail regarding any attribute of the multiverse discussed is the possibility of a threshold of inclusion that suggests that only universes that contain more good than evil will be created by God and thus will be the only universes contained within the multiverse. While he does not argue explicitly that this is the required threshold, it remains his sole discussion of and suggestion for what may constitute the composition of the multiverse (Megill, 2011, p. 133). Megill goes on to allude to several different scientific views on the multiverse to both act as evidence for his claim that the multiverse is a plausible position to accept, as well as to demonstrate that, even within the scientific community, there is no single viewpoint on just what a multiverse may look like. Megill gives five reasons for the acceptance of the plausibility of the multiverse: (1) that his premise that a multiverse is possible is so weak that it is plausible, (2) it has not been conclusively shown that a multiverse does not exist, leaving it epistemically possible that there is one, (3) that it is entailed by several current theories in physics, (4) given that it is entailed by current theories in physics is cannot be denied by some of the most likely opponents, and (5) that if the multiverse theory is denied then it strengthens the theistic position by denying the fine-tuning argument, making such a denial imprudent for the atheist (Megill, 2011, p. 131).
While the structure and overall argument forwarded by Megill varies, the type of multiverse that he argues for as well as the motivation behind it seems to echo a paper by Donald Turner (Turner, 2004). Much like Megill, Turner argues for a multiverse that is composed only of universes with a “preponderance of good over evil” (Turner, 2004, p. 158). So, while Turner acknowledges the possibility of a multiverse containing every logically possible universe, he argues that, if his account is to stand up to the problem of evil then the threshold below which God chooses not to create any universes is the balance between good and evil. That is to say, on Turner’s account, no individual universe will contain more evil than good, and thus no universe in the multiverse will be on-balance bad.
A further alternative viewpoint on the multiverse is presented by Hud Hudson where he suggests, rather than the series of spatio-temporally isolated universes that are typically proposed in multiverse theories, what he calls a hyperspace (Hudson, 2005). Hudson’s hyperspace is essentially an additional dimension to the three dimensions of time-space that we are accustomed to. Hudson describes his hypothesis of a plenitudinous hyperspace by saying that “spacetime is a connected manifold with more than three spatial dimensions, yet the manifold can be partitioned into subregions which vary independently with respect to their cosmic conditions” (Hudson, 2005, p. 40). So while the language used to describe Hudson’s hyperspace varies from the language of other multiverse theories, the end product still seems to be quite similar: a series of universes, or regions, that exhaust a wide variety of cosmic conditions. In discussing his hyperspace model in relation to the fine-tuning argument, Hudson says the following:
…given the fine-tuning, the hypothesis of a plenitudinous hyperspace should of itself lead us to very high expectations that the cosmic conditions will be life-permitting. Why? Presumably because, on the assumption of a sufficiently rich plenitude, there will be enough distinct regions in which the physical parameters differ independently to render it unsurprising that at least some of them fall in the life-permitting range (Hudson, 2005, p. 41-42).
So while this is a specific discussion of hyperspace that is preferential to a three-dimensional conception of time-space with relation to the fine-tuning argument, it does a good job in illustrating the composition of Hudson’s account. The two prime differences that differentiate Hudson’s account from others seem to be that, first, Hudson’s account speaks of different regions being found in different dimensions of space-time rather than simply different universes being found in different spatial or temporal locations that other multiverse accounts appeal to. In having a series of different universes existing at different space or time locations there does not appear to be any need to invoke an extra dimension of space-time to accommodate for the existence of other universes, but this is not the case for the hyperspace account. And secondly, while many other multiverse accounts often maintain that different universes within the multiverse are spatio-temporally isolated from one another, it is not the case that the same isolation is necessary or present between regions in Hudson’s account, in that it seems to leave open the possibility for accessing one dimension from another or one region from another. Of course, the practical implications of crossing such dimensional or regional lines is a completely different discussion but, that aside, there does not seem to be anything inherent in the hyperspace account that precludes such a traversal.
Coming from different motivations, Klaas Kraay proposes a unique multiverse account that he sees as a response to the “best possible worlds” objection to theism (Kraay, 2010). Kraay argues that the best possible world need not be equated to a single universe, and that the best possible world can actually be a set (possibly an extremely large or infinite set) of universes. Given the nature of God, an unsurpassable being, He would create all of the universes that are worth creating (Kraay, 2010, p. 360). That is to say, given some vague threshold of goodness that determines whether a particular universe is worthy or unworthy of God’s creation and sustenance, for any universe above that threshold, God would create and sustain it. The total collection of all of these universes that are worthy of creation and sustenance would then collectively comprise our world. A world which contains a number of worthy, spatio-temporally disconnected universes. Kraay calls this the “Theistic Multiverse” (TM), and in TM, “God creates and sustains all and only those universes which are worth creating” (Kraay, 2010, p. 363). Because of the nature of God and the structure of this world, Kraay concludes that if one accepts various principles that he has put forth “are plausible, and if TM is logically possible, the theist must maintain that the actual world is TM, and that it is the unique best of all divinely actualisable worlds” (Kraay, 2010, p. 364-5). So on this account, Kraay argues not only that a multiverse is a logically possible world, but that the theist ought to accept that it is the actual world, and subsequently that it is the only possible world, simultaneously rendering it the best possible world.
The final account to be discussed in this section is one that I am currently developing. If it is plausible that free will is the greatest possible good, and God is all-loving and perfectly good, then it seems that God would be required to maximize the amount of free will in the universe. Since free will is not something that can be measured qualitatively, since my free will cannot be better than your free will, it must be measured quantitatively. That is to say, for God to maximize the amount of free will in the universe, He must quantitatively maximize the amount of instances that free agents exercise their free will. Imagine that each free choice made by each individual in our universe counts as one unit of good. So, me choosing to have granola for breakfast is one unit of good, me choosing to put strawberry yogurt on that granola is a unit of good, me choosing to eat my granola and yogurt from a particular bowl is a unit of good. Similarly, I could have chosen to have oatmeal for breakfast, I could have chosen to put brown sugar on my oatmeal, and I could have chosen to eat it straight from the pot in which I made it. Supposing that each of these two different breakfast scenarios are mutually exclusive and the only options available, I could have only actualized 3 of 6 decisions, and thus only garnered 3 units of good while leaving 3 units of good untouched. In order to create a maximally good universe, however, God must have it so that no units of good are left untouched. For this to happen, another universe in which I decide to eat brown sugar oatmeal out of a pot for breakfast must be actualized somewhere. For this particular example, the existence of these two universes would exhaust the amount of free choices available, thus maximizing the amount of units of good that are enjoyed between the two, which creates a maximally good multiverse.
Of course, the example just presented is extremely simplistic and limited in scope, but is simply done to illustrate how this multiverse model works at its most basic level. We could imagine a similar situation in which eating Froot Loops, eating a bagel, and not eating breakfast at all are all available free choices and that for each one of these options there is an actual universe where it is actualized. Similarly, this process would be carried for each free choice made throughout the lifetime of each individual, and so too would each possible collective conjunction of free choices between other free agents. This would result in an enormously large number of distinct universes that ultimately exhaust every possible free choice and combination of free choices, leading to a multiverse that has realized the greatest possible amount of units of good. A multiverse that realizes every potential unit of good is a maximally good universe, one that a perfectly-good and all-loving God would be compelled to create.
Some may argue that even if God is required to create a maximally good universe, and that free will is the greatest possible good that we need not appeal to a multiverse model to reconcile the two. We could simply imagine one universe in which there are enough free agents actualizing enough free choices in order to exhaust and gain all possible units of good. Surely this would create a maximally good universe without the use of a multiverse. In response to an objection such as this, while there is no doubt that a universe model such as this would yield a good universe, perhaps one that satisfies proponents of a “threshold” model of greatest possible worlds, I am after a maximally good ontological view, and a single universe simply cannot satisfy that.
Another reason that a single-universe model would not satisfy the kind of maximal good that I am after is that no matter how good a particular universe may be, enjoying an enormously large amount of agents who make a variety of free choices, such a universe could always be improved upon with the addition of one more freely acting agent. The addition of one more agent would result in, minimally, one more free choice being made by that agent, increasing the overall goodness of that universe. For any single-universe model, no matter how many agents there are, there could always be one more added to increase the goodness of that universe, and yet even with that addition of any arbitrary number of agents there would still be a multitude of unclaimed units of good because of the inability to actualize each possible free choice. The same cannot be said of the multiverse model outlined above since, by its very nature, exhausts all possibilities of free choices as well as all possible combinations of free choices. So, yes, for any individual universe within the multiverse one more agent could be added to create a universe with more good in it, but such a universe already exists elsewhere in the multiverse. Having the existence of such a universe (with that added agent in it) means that the goodness is merely realized in another universe, yet still within the same multiverse, which is the scope on which the maximal goodness of concern ought to be measured.
For those who find it difficult to maintain that free will is the greatest possible good there are still several possible alternatives that could yield the existence of a multiverse. In the first alternative it seems that we can substitute something such as happiness, joy, or pleasure for free will, and the multiverse account may still remain the same. Surely we can agree that there are different types of happiness, and failing that, there are at least different ways of attaining happiness. This being the case there would seem to be different kinds of happiness that are incompatible with each other, such as the happiness or satisfaction felt from eating the last piece of your favorite cake immediately, and the happiness or satisfaction felt from saving that same piece of your favorite cake for later and enjoying it after a period of anticipation. Assuming that the situations that we are talking about both involve the exact same person and the exact same piece of cake then it seems that enjoying both of these kinds of happiness is not possible since one cannot both be happy that he is currently enjoying the last piece of his favorite cake while simultaneously enjoying the anticipation of being able to enjoy his favorite piece of cake at a later time. Returning to the old terminology of “units of good”, with each kind of happiness equating to one unit of good, then in this case only one unit of good could be recognized while the other is left as potential. In order to recognize both units of good and create a maximally-good world (or multiverse), a second universe would be needed so that both kinds of happiness can be instantiated.
The same principle follows for a wide range of other goods such as, for example, justice, existence, life, and so on. For each different good it seems that within it there are multiple variations of that good that may not be compatible with each other, thus requiring a multiverse in order to realize all of them. While I will not go on into such detail as to provide examples, as I have done with free will and happiness, it stands that such an account is at least plausible, and that for any good that is argued to be the greatest possible good a multiverse will be entailed by it.
Secondly, for those who do not want to maintain that there is a single greatest possible good, rather that there are a variety of inherent goods that are on-par with one another or that there is a greater goodness held in a variety of goods rather than a flood of one greatest possible good, this too could entail a multiverse. In this case it is not altogether clear that there is a single instantiation of a variety of goods is any better than any other instantiation, and given the principle of plenitude, it still seems that God would create a multiverse in order to realize all possible combinations of goods. For example, it is not clear that a universe whose maximal goodness is composed of 50% happiness, 30% justice and 20% free will is any better than a universe whose maximal goodness is composed of 40% life, 25% pleasure and 35% honesty. Whether one universe is better than the other or not, it seems that God would be obliged to create both of these universes, and a whole host more, since they are all on-balance good, and more of a good thing is a better thing. The universes that God would create would be ones that exhaust every possible composition of every inherent good that yields an on-balance good universe, thus resulting in a maximally-good multiverse. Again, while the individual universes themselves will not yield maximal-goodness, they comprise a multiverse that is maximally-good.
2.2 The Scientific Case for the Multiverse
As with the case in philosophy, much work is going on in the sciences (primarily physics, for our purposes) regarding multiverse theories. Tim Wilkinson notes that “[a]t the coal force of science…it is usually extra-ordinarily difficult to find even one theory that fits the facts. In the current context, we do have a few competing theories, but all imply broadly the same thing: a multiverse” (2013, p. 94), meaning that while a survey of the literature seems to suggest that there is scientific consensus on the existence of a multiverse the debate arises in just what kind of multiverse we happen to be a part of. For the scientific community the question is not whether a multiverse exists, rather there are questions about the composition of the multiverse, the origins, the physical constants of the multiverse and the individual universes within them, and so on. Similar to Section 2.1 and the treatment of philosophical discussions of the multiverse the following is a short introduction to several ideas within physics regarding the multiverse. While I do not generally go into detail regarding the methodology and justification for the various multiverse accounts presented (since they are typically quite complex and laden with mathematical jargon) I present the concluding prediction of each theory. The reason for including a section on the scientific perspectives of the multiverse, in this overall philosophical discussion, is twofold. In the first case, scientific approaches to the multiverse provide us with just that, a different approach, bringing to light different conceptions of the composition, formation, and general structure and nature of the multiverse. The second reason is that, while philosophical approaches to the multiverse may be logically consistent and conceivable, scientific approaches seem to have that extra level of justification given that they not only have to be logically consistent but physically plausible as well. That is to say, I think that any philosophical view of the multiverse, no matter how strong it may be, can be strengthened and given more credibility if it is supported by a scientific account of the multiverse.
Max Tegmark composed a paper that serves as somewhat of a survey paper, not of particular scientific theories of the multiverse, but of the different and more general types of multiverse theories available within physics (Tegmark, 2007). Tegmark argues that most (if not all) current scientific multiverse theories taken seriously propose or predict multiverses that fit into one of four distinct categories, or levels, as he calls them. Each level, beginning with level 1 progressively varies more and more (with regards to the physical constants, laws of nature, etc.) from our universe. Similarly, as the levels progress up from one to four the multiverse theories that fall under them become increasingly debated and less-accepted. That is to say, level one is the multiverse model that contains individual universes that are most similar to ours, and is also the multiverse model that garners the least objections within the scientific community whereas the level 4 multiverse is the model that contains individual universes that vary greatly from ours and is also the multiverse model that is met with the highest levels of scepticism.
Tegmark’s Level 1 multiverse is a series of parallel universes that maintain the same (or very similar) physical constants as our universe but realize all initial conditions. That is to say, all possible worlds, given our physical constants, are actualized in some other parallel universe. A universe in a multiverse such as this would look very similar to the one that we are currently in, save for different initial starting conditions. So, for example, while another universe would enjoy extremely similar physical constants and laws of nature as ours, it would have a different initial starting point, making it, perhaps, at a point in development 2000 years behind our universe, or perhaps 2000 years ahead of our universe. The universes within this multiverse are inaccessible to other universes, and due to the rate of expansion and the distance between the universes, it would not be possible, says Tegmark, for one to ever travel between universes.
The level 2 multiverse is essentially composed of an “infinite set of distinct Level 1 Multiverses, each represented by a bubble…some perhaps with different dimensionality and different physical constants” (Tegmark, 2007, p. 105). Each bubble contains a distinct parallel universe that displays not only different initial conditions than the next bubble, but it also displays different physical constants and laws of nature (Tegmark, 2007, p. 107). So this second level is similar to the first in its makeup, the difference coming in the potential variations of physical constants and laws of nature. On this level it simply seems that there is the possibility of a greater number of universes than on level one, since there is a wider range in which the physical constants of each universe can fall, thus yielding more possibilities, and more universes.
Tegmark’s level 3 multiverse, I think, is better understood in relation to the first two levels rather than through pure explication of it. The level 3 multiverse, while it adds no new storylines beyond levels 1 or 2, varies in how these storylines come to be. Whereas in level 1 and 2 the universes are far apart, in level 3 they are all spatially very close, so to speak. This is because rather than being a series of disconnected independent universes in the levels previously discussed, on this level the universes are merely different branches of the same tree. Tegmark does not explicitly describe whether or not these different branches are able to causally interact with each other, and I would expect him to say that they do not, but that they are still interconnected in some sense. On levels 1 and 2, we can think of our counterpart selves doing other than we are doing here in some distant universe, whereas on level 3 our counterpart is simply on another “quantum branch in infinite-dimensional Hilbert space” unknown to us simply by our epistemic limitations (Tegmark, 2007, p. 112-3), and this seems to indicate that these different branches may be connected yet causally isolated from one another.
Finally, the level 4 multiverse “involves the idea of mathematical democracy, in which universes governed by other equations are equally real. This implies the notion that a mathematical structure and the physical world are in some sense identical. It also means that mathematical structures are ‘out there’, in the sense that mathematicians discover them rather than create them” (Tegmark, 2007, p. 116). This seems to entail a variety of universes that are widely divergent from the one that we are currently in, resulting in a seemingly infinite amount of universes that would be unrecognizable to us. The universes in the Level 4 multiverse contain “different fundamental physical laws” (Tegmark, 2007, p. 121), which means that the range of individual universes that we could see on this level far surpasses the range of universes that we could see in levels 1-3. Not only would we be able to have all possible storylines be realized, as is the case for levels 1-3, but we would be able to see all possible storylines that are compatible with each possible set of fundamental physical laws.
So, while Tegmark does not exactly propose any new scientific account of the multiverse, what he does do is set up a framework by which we can differentiate and classify existing or future scientific multiverse accounts. Tegmark’s levels of classification allow us to see just how far particular multiverse theories range from what we know about our own universe, as well as allow us to see how different some multiverse theories are from other ones in terms of what they call for with regards to physical constants of the universes that they contain.
The second scientific approach to the multiverse that will be discussed is one similar to Tegmark’s level 3 multiverse discussed above, and is proposed as an answer to the problem that “Schrodinger’s Cat” thought experiment posed for what we can claim about our knowledge of the physical world (Norris, 1999). This particular account was put forth by David Deutsch (1997), and subsequently discussed by Christopher Norris. Of Deutsch’s account, Norris says
On this account – in brief – every possible outcome of every wavepacket collapse is simultaneously and actually realized through constant branching of alternative quantum worlds that are all of them equally ‘real’ though only one of them is epistemically accessible to any individual observer at any particular time. For the observer must likewise be thought of as having previously split into a whole multitude of observers, each of them consciously inhabiting a ‘world’ whose history is itself just one among the manifold world-versions that have eventuated up to the point through the exfoliating series of wavepacket collapses. Thus he or she will have any number of counterpart ‘selves’ distributed across those worlds and each possessing a lifeline which, if traced back far enough, will rejoin his/her own at some crucial point just before their paths forked off into henceforth divergent and non-communicating series (Norris, 1999, p. 312-3).
Furthermore, Deutsch does not want to limit these other worlds to maintaining our physical constants or laws of nature. While it is not explicitly stated just how far the laws of nature and physical constants of these other worlds or universes may vary from those in ours, I think that it would be safe to say that Deutsch may have in mind the kinds of variations found in Tegmark’s level 3 and 4 multiverses. In short, Deutsch’s account says that we need to adopt “the multiverse theory and assuming that all possibilities have been realized in one or another of the multiple worlds that diverge at every point where some particular world-specific event…happens to occur” (Norris, 1999, p. 314). Of course, just what exactly constitutes a possibility for Deutsch depends highly on his range of allowable divergence between the physical constants of our universe and those of other universes, but the principle still remains regardless of the lack of an explicit explanation.
The final scientific account to be mentioned here is that of A.D. Linde, who argues for a kind of self-reproducing multiverse (Linde, 1987). This brand of multiverse account can be seen as one in which not all universes contained within the multiverse are actualized or exist simultaneously, rather they all come to exist (generally one-by-one) over a period of time. Linde’s account, in particular argues that “…the large-scale quantum fluctuations of the scalar field…generated in the chaotic inflation scenario lead to an infinite process of self-reproduction of inflationary mini-universes.’ He suggests a ‘model of an eternally existing chaotic inflationary universe…” Linde goes on to cite scientific and mathematical reasons for how this particular multiverse model is predicted. Of this prediction, Linde explains that
In our case the universe infinitely regenerates itself, and there is not global ‘end of time’. Moreover, it is not necessary to assume that the universe as a whole was created at some initial moment…The process of creation of each new mini-universe…occurs independently of the pre-history of the universe…Therefore the whole process can be considered as an infinite chain reaction of creation and self-reproduction which has no end and which may have no beginning (Linde, 1987, p. 172-3).
So in this case the universe goes on regenerating itself eternally with, in theory, every possible universe instantiation eventually being realized at one point or another within this string of eternally self-reproducing universes. While Linde does not specify the exact process through which these individual universes come to pass in and out of existence (their lifespan, so to speak) he does make several suggestions that allow him to show that his model is consistent with various competing scientific theories regarding exactly just how such a process may be feasible.
2.3 The Intersection Between Scientific and Philosophical Approaches to the Multiverse
While, as I have presented it, it may seem as if scientists and philosophers are working independently and in isolation of one another on matters of the multiverse, but this is not the case. Increasingly, philosophers are adopting and importing scientific work into their theories, to help both guide the arguments that they initially set out to make, as well as to serve as important tangible evidence for the cases that they go on to make. It is no longer the case that multiverse theories are relegated to pure speculation and loose inductive reasoning, rather now philosophers have hard evidence to support their views regarding the nature of our existence, something that was once a far loftier ambition. That is not to say, of course, that the scientific views being employed by philosophers are not without controversy themselves. Just as various philosophical views face rival theories and conceptions, as we have just seen, so too do the scientific theories. We have just noted several different kinds of scientific theories that conclude in widely divergent types of multiverses, assuredly with each one being argued for by many experts in the field while simultaneously being argued against by many experts in the field. Scientific approaches to the multiverse will also sometimes borrow ideas from philosophical explorations, using various philosophical models to craft their hypotheses in the initial stages of experimentation to explore the possibilities and entailments of a multiverse. It is not often the case that experiments of this sort will simply begin with no aim, rather there must be an idea present, something that is being searched for, an inquiry for which some answer is sought…these are the ideas that philosophy can and does supply for the scientific world. Those aforementioned lofty philosophical projects serve as initial starting points for mathematicians, physicists and other scientists to begin their work, to aim to confirm or disconfirm some idea or thought. This is the role that philosophy plays in the science of the multiverse.
One of the bigger questions or obstacles to be dealt with when dealing with navigating between scientific and philosophical discussions of the multiverse, or in any discussion of the multiverse for that matter, is the task of figuring out whether the various sides of the discussion are even talking about the same thing. There are two key distinctions between scientific accounts of the multiverse and philosophical accounts of the multiverse that will need to be clarified before any meaningful discussion can go on between the two. The first distinction to be made, one which actually contains a wide variety of further distinctions that must be made within it is that of semantics. The multiverse discussion, unlike some other philosophical or scientific discussions, does not yet seem to have within it a precise set of terms that carry with them precise definitions of how they ought to be used in various contexts. One of the most common cases of this is distinguishing between terms such as “universe” and “world,” with Tim Wilkinson noting that “[c]onfusingly, philosophers and physicists use ‘possible worlds’ and ‘possible universes’ interchangeably” (2013, p. 89). In some cases “universe” is taken to mean, and is used to refer to, the totality of all things in existence. That is to say, nothing can exist outside of the universe (except perhaps God, on some accounts, but this is not important for us yet). In some cases with this usage of “universe”, “worlds” are considered to be smaller self-contained, often disconnected, sections within the “universe”. Each “world”, in this case, would exemplify a different way that things could have been, and collectively all of the “worlds” would compose the larger “universe”. On other accounts, it is the exact opposite, with the “world” being taken as the term to connote the totality of existence, and a variety of “universes” that make up this world. With this is mind, it is not always altogether clear just how to compare and discuss various multiverse accounts that, while they may be using the same exact terms, will suffer from semantic incongruence. Similarly, various scientists and philosophers will coin their own terms to describe different levels of encompassment of the “worlds” or “universes” that they aim to discuss, so these too must also be navigated and, in a sense, translated in order to undertake an accurate comparison amongst other accounts. Moving forward, the way that I will employ these terms will be as follows. When I speak of a multiverse I am referring to a collection of universes (at least two) that, together, comprise the totality of all that is in existence. When I speak of a universe I am referring to spatio-temporally and causally disconnected units that comprise the multiverse. Universes are entirely self-contained and, for those existing within their respective universes, represent the confines in which all causal relations can take place. And finally, when I speak of a world, I am referring to the totality of all that is in existence. In some cases this will equate to a universe, and in some cases it will equate to a multiverse, and this will vary depending on the particular ontological view being discussed. For a single-universe model, the world will consist of that single universe, whereas on a multiverse model the world will consist of all of the universes that comprise that multiverse.
Somewhat tied in with issues of semantics, and a potential influence of the differences that we see within the terminology of the discussion, are the motivations from which philosophical accounts and scientific accounts of the multiverse come. On the one hand, scientific approaches to the multiverse are often concerned with justifying or delving into accounts of how the various universes that comprise the multiverse can come to pass in and out of existence. That is to say, scientific accounts are generally concerned with exploring the physical constants, laws of nature, mathematical structures, and the like, of these possible universes to determine whether or not their existence is even physically possible. Building on whether these individual universes are physically possible or not, the scientific approach will also go on to investigate by what process it is possible that these universes come into existence, what relation they may have with one another, whether it is possible that multiple universes exist at the same time, whether or not it is plausible for them to fit together into a multiverse, what the structure of this multiverse would be, and so forth. Scientists are concerned less with the contents of the universe than they are with the structure of the universe itself, tending to focus purely on the aspects concerning the physical plausibility of various multiverse models.
Philosophical accounts of the multiverse not only concern themselves with different aspects of the multiverse than do scientific accounts, but they also begin with and are often motivated by very different goals. Philosophical multiverse accounts tend often to be motivated be a desire to provide a response to various atheistic arguments. Many philosophers have appealed to multiverse accounts in attempts to block various arguments for atheism, generally being some version of the problem of evil or the problem of no-best-world. On the other hand, some philosophers have appealed to multiverse theories in order to undercut theistic arguments for God’s existence, namely, the fine-tuning argument. Given these motivations, the philosophical multiverse account is generally concerned not so much with determining how the universe came to be, as the scientific account is, rather it is more concerned with evaluating the overall value of various aspects of the multiverse. That is to say, evaluating the overall value of particular universes within the multiverse, or the multiverse as a whole, and in determining whether or not certain universes ought to be considered as worthy or possible of being part of the multiverse. It should be noted that the common employment of multiverse theories by the philosopher as a response to some objection can be seen as detrimental to itself, in some cases, with some philosophers seeing the multiverse theory as an ad hoc response to some particular argument. And that seems to be one of the bigger differences between scientific and philosophical multiverse accounts. While philosophical accounts are often mounted as a theistic response to account for some atheistic objection to the existence of God, scientific accounts generally appear to be more independently motivated.
In this chapter we have seen the differences between several philosophical multiverse accounts, as well as the differences between several different scientific multiverse accounts. Furthermore, we have seen that, while philosophical and scientific multiverse theories may come from different motivations and argue for different kinds of multiverses, the two disciplines do not operate independently of one another, with each side often taking and employing information and ideas from the other for their own theories. The result of this information and idea-sharing is a wide range of divergent multiverse theories, each one varying in terms of composition, value, make-up and origins, among other things.
Overall, the aim of this chapter was to present an overview of some of the prevailing multiverse views at work today, and to demonstrate not only that there is a broad range of ideas at play, but also that these views ought to be given serious consideration when considering the adoption of an ontological view.
 For more in-depth survey pieces or additional discussions and accounts of the philosophical multiverse see Kraay (2012), Kraay (2015), Stewart (1993), Draper (2004), Forrest (1996), O’Connor (2008), Parfit (1998) and McHarry (1978)
 I understand that this may appear to be an appeal to authority, but I make no claim regarding the acceptance of any claims regarding the multiverse. I simply want to point out that there is a substantial amount of serious work being done in the area and that, given that, we not completely dismiss the idea of a multiverse as entirely implausible and outlandish as some might be tempted to do.
 See Vaidman (2008), Section 1, Tegmark (2003), Jacobs (2009), Section 2a for more detailed discussion.
 Which he calls a cosmos.
 Which he calls simple universes.
 While his discussion is typically confined to discussion of a 4th dimension of time-space, it is compatible with any larger finite number of space-time dimensions.
 See Kraay (2010), Section 4.
 This is essentially taken as an extension of free will theodicy which argues that free will is a higher-level good, but I go the extra step in asserting that it could be the greatest possible good. See Plantinga (1965).
 It can generally be argued that the kinds of free choices that free will theodicy values are those that are morally significant, but I make or require no such distinction here.
 For the sake of simplicity, I will refer to each of these as simply being a different “kind” of happiness.
 For additional scientific perspectives on the multiverse see Smolin (1997), Steinhardt & Turok (2007), Wallace (2012), Linde (2000), Veneziano (2006), Ellis (2011) and Carr (2007).
 See Linde (1987), Abstract.
 This is also touched on by Kraay (2008, p. 359-60).
 This will be discussed in Chapter 7.
 Kraay, K. Personal correspondence. April 22, 2014.