God is not a Good Theory (Sean Carroll)

very good to be here as you can see from my title I'm going to be approaching the question of the role that the idea of God has to play in explaining the universe we see from the point of view of a scientist a physicist or cosmologists not the point of view of a professional theologian or philosopher I do think that the point of view of a physicist or a scientist is relevant to this question it's not the only possible point of view one can have there are aspects of the idea of God that do not reduce to the roles played by a simple scientific theory but there are also aspects of the idea of God that do play the roles of a scientific theory and those aspects can be judged by the same criteria that we use to judge theories in the original schedule if you have seen that it removed the word good from my title so it just said God is not a theory but that is the opposite of what I'm trying to say here so just so you know my point of view I'm saying that God is a theory the idea of God to the extent that almost all believers use the idea of God does have aspects of it that can be judged in precisely analogous ways to the way that we judge an ordinary scientific theory now I'm very well aware there is a thread of thought that says the opposite of that that says that the idea of God is something separates different kind of idea than the ideas that we use when we have physics theories so you know God is not reproducible or something like that so I just want to get around that particular definitional problem by putting forward the idea of theory is something very very broad I don't want to pick some very very specific philosophy of science definition of a theory to me a theory is just an idea about the universe that may be true or false and that we should try to figure out how to judge whether or not this particular idea is true or false so the one of the problems with God as a theory is that it is not a very precisely specified theory that is going to be one of my points is that even though it we sometimes try to have God play the role of a physical theory making predictions and explaining things preserving some explanatory role I'm gonna argue that it doesn't do a very good job now again I will even though I'm not a trained theologian I certainly know that we have different conceptions of God in fact this is what I would argue is one of the check marks against God as a very good theory is that if you try to say well here's why I don't believe in God someone else will say oh you just don't understand what God is God is completely different than that and I recognize that I will try to at least explain which versions of God I am talking about I can't guarantee they're your favorite versions of God so that's we can pick out three that's not an exclusive list passive active and emergent conceptions of God and what I mean by this under passive is the kind of roles that are given to God going back to some of the ancient Greeks going back at least to Aristotle a sort of philosophical role for God as something that helps explain helps sustain the universe and unmoved mover a first cause a necessary being various philosophical notions that are not necessarily empirical in the same way that scientific theories are this kind of God is one that just is something that we need in this conception to make sense of the universe not something that necessarily is going around poking around in the universe and intervening it and changing the physical laws the active conception of God is a more personal idea often people will define God as a kind of person just as often they will say they God is not a person but there's this conception of God as someone who cares about life here on earth cares about human beings cares both in the sense that you know he wants us to succeed but also sometimes he judges us and sometimes a little harshly this it depends on how far you want to go but God will pass down moral guidance will set up an afterlife for you that you'll be sorted once you die and so forth there's various uses that this sort of more active God has to perform and finally there's there's what I like to call an emergent idea of God which is to say that God not as a fundamental category but God as a way of talking about aspects of the universe that you could also choose to talk about in different ways God is the laws of nature God is the orderliness of the cosmos God is the universe itself where God is love where God is our feeling of all in transcendence as we approach the universe and these different conceptions of God have different justifications for them the passive notion of God just in the background can be attempted to be justified in purely logical grounds just by thinking about how the universe might be the more active conception of God is going to be essentially an empirical argument that the best way to explain the universe we see is to imagine that God is doing these things in an active sense whereas the emergent idea of God is a more rhetorical move and I don't mean that in any disparaging way it's a way of talking about the universe that we see so I'm actually not going to be talking about this third category my personal belief for those of you who are fascinated by it is that I don't see need to talk about the universe in that vocabulary but you know if you want to I won't stop you what I'm more interested in for the purposes of this talk are the the the more the conceptions of God that introduced God is a separate metaphysical category that we have something to do with and I'll start with you know these more philosophical metaphysical arguments in favor of God I think that as a scientist the second category is more relevant but I do have to get this one out of the way before we can move on to the second one so what does it mean and what is what is the role played by God as an unmoved mover as an Aristotelian first cause God is a necessary being very often if you asks theologians you know how would the universe be different if God did not exist they will say I cannot imagine a universe in which God does not exist therefore it cannot answer that question to me as a scientist there's a huge problem right from the start with this kind of reasoning which is that it is that the whole strategy is one based on some a priori metaphysics that is to say this is you know very much armchair philosophizing in you know in the best sense of the word sitting down thinking about all the possible ways the world can be and concluding that those ways must somehow involve the idea of God you don't ever there's no step in that process in which you actually go out and look at how the universe actually is you don't need to in this way of thinking about it because you can just argue logically that God must be part of the universe it is my firm belief this kind of reasoning has never taught us anything true and interesting about the actual world that is not to say that armchair thinking without going out and looking at the world is not useful in any way it's extremely useful mathematics logic other branches of philosophy and formal inquiry are not empirical in nature they don't involve going out and looking at the actual world they reason an a priori sense but they also don't reveal interesting truths about the actual world mathematics reveals consequences of axioms you say I have a certain axiomatic structure and I derived theorems on the basis of that it doesn't tell you which axioms are possibly true if you want to actually figure out our universe does it involve some notion of God that is an actual fact about the specific universe in which we live and I personally don't think of this rere kind of reasoning is ever going to get us there but let me take it seriously for just a little bit and sort of give you my feelings as a scientist about this how would we refute the claim that God is a necessary being for example and I should also admit that I'm happily conflating all sorts of ultimately different kinds of a priori metaphysical arguments first cause prime mover necessary being these are all subtly different in ways that I'm going to completely ignore so sorry about that there they have a similar flavor and I'm going to treat them similarly so if you did believe that God was a necessary being that you literally could not in imagine a universe in which God did not play an important role in order logically for me to refute that belief all I need to do is to invent a universe in which God does not exist you know if you say that all swans are white as a logical fact all I need to do is show you one Black Swan to show that your logical argument can't be right so all we need to do is invent a universe that God does not play a role in and that would improve your imagination enough so that you could imagine universes where God was not important I claim that it is not hard to invent universes possible universes hypothetical universe is not necessarily our own but universes that are self-contained consistent coherent and in which God plays no special role no role at all so here's the universe for apologies for the math this universe is three dimensional space evolving in time and in that space there is one particle that is the whole universe this particle moves according to Newton's laws of motion in the under the influence of some potential energy so here's space there's time there's our particle moving it has an equation of motion and this goes on forever into the infant far past or the infinite far future I claim this is a logically conceivable universe but you can't stop me from imagining that this is the universe it's clearly not our universe I'm not trying to claim that there's some complicated transformation that makes our real world look like this but this could be a universe and there is no God in this universe there's just a particle moving if you want a slightly more realistic example here is a universal I think might actually be right I replace instead of three-dimensional space I have a space of states which is some Hilbert space that is to say some quantum mechanical space of possible states the space of wave functions if you like there's also time evolution so the state is an element of that Hilbert space and it evolves with time if you want to know how it evolves while it solves this equation Schrodinger's equation where this capital H is an operator called the Hamiltonian which basically tells you how the state of Bowls with time so this universe is one that is mathematically isomorphic to a trajectory in a hilbert space according to this Schrodinger's equation it goes forever from the infinite far pass to the infinite far future it is self-contained there's no something outside this universe that is holding it up sustaining it keeping it going causing it to exist or allowing it to persist it is simply that mathematical structure full-stop I actually like I said think that this is a plausible mathematical structure for the real world in which we live clearly there are some details to be filled in I need to know what the actual Hamiltonian of the world is which I don't claim to know but this is a plausible framework for reality and just to give you a final idea of what kinds of universes I'm thinking about here is a world a universe it's one point this is actually bigger than a point because it's hard to show you a point so this is a circle but imagine a mathematical structure that is isomorphic to a single point I claim that is a possible universe once again it is not our universe in any interesting way but it is a conceivable universe and once again god plays no role so what am I trying to get at here what I'm trying to get at is that the if you sat down again a priori and tried to think about all the possible ways that existence could manifest itself we know that it to at least some extent there are mathematical ways of talking about existence talking about the real world I can easily imagine self-contained and consistent mathematical structures in which the notion of God or necessary beings or first cause first causes play no role whatsoever now I'm you know this is not some brilliant new insight the people who do believe in necessary beings and so forth have a response to this they will say sure you can imagine that but they're not really legitimate possibilities for what the world can be liked and that's because there's no explanation fundamentally there's no reason why the world is like that that the role of God is to ground why this world rather than some other or indeed why this world at all why a world at all so the response to my examples would be sure you can mathematically write those down but you haven't explained why that would be the universe and my response to those arguments is so what who is to say that I'm supposed to explain why there is a universe you might prefer that there be an explanation as to why this universe rather than that universe but you can't prove logically that there must be some explanation the universe could just be that is the fundamental disagreement between people who believe in a metaphysical notion of God and the more empirically minded scientists so let me flesh that out just a little bit just because we have a universe is it necessarily true that there is some explanation or reason or cause I think that the reason why you be tempted to think that is because in our actual universe you associate causes with events things happen to go I can explain to you why that happened here is the reason why now there's two things to notice about that one is that there are very often multiple different sounding reasons that you could give for a single event why did this football team lose their match well because the goalie let in that goal or well because they were ill-prepared or well because space-time is four-dimensional I mean there's many possible reasons you could give what counts as a reason depends on the framework in which you are speaking reasons are not existing once and for all uniquely all by themselves they appear within contexts and the context in which the whole universe appears is different whatever it might be then all the contexts in which things inside the universe appear even if it were true when I don't think that it is true but even if it were true that everything inside the universe can be associated with a reason or a cause that doesn't mean that the same thing applies to the universe itself it may or may not and so the second point is that you might want to event a principle you might want to invent a way of thinking the principle of sufficient reason or something like that that it's simply demand that things have explanations that nothing that occurs does so without a reason why but my response to that would be why why is it necessary that everything have a reason why and you could give an argument you say well when I look at the universe everything that happens has a reason why but the point is that you're now taking your metaphysical argument and turning it into an empirical one why is it necessary for things to have reasons or explanations well I see that things have reasons or explanations but that doesn't mean that you can simply draw the conclusion that everything that happens must happen for a reason the example that I like to use is conservation of energy because conservation of energy is something while you look around you see a lot of different things happening a lot of different processes dynamical happenings in the world you go look every single time I can associate a number the energy which is constant over time it doesn't change in the process of this dynamical event so I invent a law of nature conservation of energy but the point is the logic is not I see that energy is conserved therefore energy is conserved the logic is I see that energy is conserved therefore I make a hypothesis I imagine that energy is actually conserved and then I go out and test that hypothesis it's an empirical one I look for a logical foundation in the basis of some wider theory and I actually do tests I actually there are experimental tests of energy conservation it might be wrong likewise if you see that everything in the universe happens for a reason which I don't think is actually true but even if you saw that you would at best be able to say therefore I hypothesize that perhaps everything happens for a reason perhaps therefore the universe has a cause or a reason but that's not simply a metaphysical demand that is an idea that you should go out and test so it goes back to being an empirical claim and should be evaluated accordingly that's just one more example there's something called the Kalam cosmological argument that begins by saying that everything that has a beginning has a cause and therefore it says the universe has a beginning and then says the universe has a beginning therefore the universe must be caused but the response to this is maybe not I don't even need to say no it's not true that everything that has beginning has a cause all I need to be able to say is maybe there are things that happen without causes it then becomes an empirical claim that I need to go out and judge just like I judge every other scientific theory just like conservation of energy so the conclusion of this kind of reasoning is that we should think about God as an empirically testable hypothesis not as something that is completely opry re even the purported metaphysical arguments that God is necessary ultimately come down to contingent empirical claims so what we should actually be asking is does including God in our ontology does having God play a role in our conception of how the world works give us the best possible theory on conventional scientific grounds now admittedly we don't understand what those conventional scientific grounds are with perfect clarity the idea of what is the best possible explanation for something even within science is a bone of contention but I'm not going to get into that in any detail I think we all sort of know it when we see it when it comes to what makes a good scientific theory we're looking for something that is a simple coherent theory that explains the largest amount of data and when it comes to that it's not hard to imagine aspects of the observed world but you might say the best possible explanation for this observed phenomenon is the existence of God historically that has certainly been true so motion is one of them remember one of Aristotle's conceptions of God is an unmoved mover and this is the one when I was an undergraduate at my Catholic University taking all those religion courses made the least sense to me and it took me years later till I finally figured out why it made no sense to me why you needed to have an explanation for why things are moving and the answer was that in an Aristotelian metaphysics you're basing what happens on Aristotelian physics in which motion is something that needs to be explained right in Aristotelian physics in order to get something moving you have to push on it and keep pushing on it and we see things moving all over the place so there must be something doing some ultimate pushing in some sense but then along comes what we now know as conservation of momentum of the Sena and Newton and Galileo point out that left to their own devices things do not need to be kept to be pushed constantly to keep moving things will just keep moving so the idea that things are moving is not something that needs to be explained it is just something that is a law of physics it is an aspect of the world I think that you know the shift in theological thinking that sort of implicitly followed the invention of Newtonian mechanics is an interesting and underdeveloped thing to think about there is of course life and I think that for many people now and in antiquity the reason why we are tempted by supernatural explanations is because living beings look very different than non living beings living beings are alive and conscious and they respond in interesting ways it seems like the simplest explanation that you know when something dies when it goes from being a living object a living organism to being a corpse it seems like something changed something left it something is now absent that was there some sort of spiritual energy some lifeforce and that I think is one very obvious motivation in a primitive society for thinking that there's something beyond the merely physical but nowadays we think that there's nothing about life or death that cannot be in principle explained just by chemistry and just by biology there is of course design not just the existence of living creatures but the wonderful diversity of all the different species we have of creatures here on earth there is a British person called Charles Darwin who explained that in terms of natural selection and nowadays we're still left with some unanswered questions there's the question of consciousness what makes a person have a feeling of what it is to be themselves to reflect to think about things I'm hopeful that neuroscience is addressing and will continue to get better at understanding what consciousness really is I'm not a professional neuroscientist I'm not going to talk about that but given the empirical success of science and giving naturalistic explanations for features of the universe I see no obstacle to that happening in the case of consciousness there's also finally the origin of the universe itself and here I actually do have some expertise what does naturalistic scientific modes of explanation have to say about the origin of the universe it is a traditional role for God to play if you pick up the Bible it's right there in the beginning God is creating the whole universe so here's what we know from the modern perspective about the origin of the universe 13.7 billion years ago the universe was in a hot dense rapidly expanding state which we call the Big Bang a lot of the credit for this idea goes to a bay George lameta a Catholic priest after la Mettrie helped invent the idea of the Big Bang the Pope tried to use it and to try to to match on the scientific theory of the Big Bang to a theological view of God creating the universe out of nothing and lumetri very wisely advised him against doing that but nevertheless the Big Bang has left us with questions about the origin of the universe the Big Bang is not necessarily the beginning of the universe we often talk as if it is but what we're really doing when we when we talk about the Big Bang as being the beginning is we're using a theory to draw a conclusion when we have no right to believe what that theory is telling us that theory of course is classical general relativity Einstein's theory of space time and gravity if you extrapolate in classical general relativity back to the Big Bang and keep going backwards you hit a singularity a moment when the curvature of space-time the expansion rate the energy density are all infinitely big so in classical general relativity you would say that is a boundary to space-time that is a point past which you can no longer go however we have absolutely no reason to think the classical general relativity is an appropriate and correct description of what is happening near the Big Bang we should not think of Djenne of the Big Bang as the beginning of the universe we should think it as the end of our understanding of what is happening we need to do a better job at the very least a theory of quantum gravity before we can actually draw conclusions about what happened at that moment right now it's better to simply say we don't know we can of course say can we possibly know is it possible to imagine theories of the universe in which the beginning of the universe is explained or do we need to hold that idea separate giving it a opportunity for God to sneak in there and play a role so I don't know what the right theory of the origin of the universe is but I have no shortage of plausible theories so let me just give you some of them one is that the Big Bang really is the beginning that the universe was in some sense created from nothing Stephen Hawking obviously very famously has promoted this idea for a long time but it's not the only one on the market there is the cyclic universe the universe which the Big Bang is just a phase that the universe goes through that it expands and collapses again and bounces an infinite number of times there is eternal inflation we are Joe already talked about inflationary cosmology the idea that from a very tiny patch of space a negative pressure fluid can force the universe to accelerate a huge amount and grow greatly in volume well it turns out that that can happen forever and you can go from one little universe to many many little sub universes the neck and again happen an infinite number of times there is also my favorite idea which is that this kind of thing happens but it happens in both directions of time so there is some point in the universe which is sort of a middle and to the right and to the left of that the universe grows and gets bigger and creates many little offspring universes any one of these models is like I said self-contained it tells you the whole story of the universe there is no external influence that is bringing the universe into existence I'm not advocating any one of these models as correct I'm merely pointing out that there's absolutely no reason to fall on our swords and say we need some help to explain the origin of the universe outside conventional conventional naturalistic explanations we don't need God to explain the origin of the universe however it's still possible that God is a better explanation than any of the purely physical naturalistic ones that I was just talking about so we should be judging God by the same standards that we judge all the other theories in my personal opinion the best argument on the basis of empirics in favor of the existence of God comes from the fine-tuning of physical parameters in our universe it is a it seems to be true that if you look around in the universe we characterize what's going on by various numbers the energy density on average the expansion rate physical parameters like the mass of the electron and so forth it seems to be the case that if you changed many of these numbers by an appreciable amount things would be very very different I said that weakly enough that it's almost certainly true if you change the mass to give the electron by a substantial amount things would be very very different it is plausibly true that for many of these numbers if you change them by an appreciable amount life would not possibly exist that's one fact it seems likely or at least possible that the values that the physical and cosmological parameters we observe around us take on allow for the existence of life and not many would furthermore it seems as if some of these physical parameters are not natural that is to say if you just picked them from a bag mass of the electron energy density of empty space expansion rate of the universe you would get very very different numbers than what we actually observe so you see where the argument is going you need certain very specific the argument says parameters in the universe for life to exist these parameters don't seem to be generic they do not seem to be what you would easily get just by randomly choosing numbers for the universe and yet they are right there where you need them in order for life to exist why were we so fortunate to find ourselves in a universe that seems unnatural but in a way that is allowing the universe to be hospitable for the existence of life is a very natural sensible place to wonder whether or not there was some designing going on whether or not God helped the universe along a little bit in picking the appropriate parameters to allow for the existence of life the best example in my mind is the cosmological constant the enter density of empty space the vacuum energy this is something that we think is not zero for you know when I was your age we thought this was zero when I was in graduate school but in 1998 we found that the universe is accelerating in a way that is exactly compatible with a small but nonzero energy density in empty space now the reason why that's interesting is because you can sit down and ask yourself if I didn't look at the universe if I just estimated on the basis of what I know about particle physics and cosmology what would I guess the value of the vacuum energy should be my guess is larger than the observed value by a factor of 10 to the 120 that is to say if it is true that we have a nonzero but small vacuum energy we did not pick it randomly out of a hat we think we know what the natural value would be if we did pick it randomly out of a hat it is very very very different than the value we actually observe so either our notion of picking randomly out of out of a hat is very very wrong or there is some dynamical explanation for why the value for the vacuum energy is so small so what kind of possibilities might help to explain this apparent fine-tuning of the parameters of nature well one is that we just got lucky it is not ever completely clear to me that asking this question is a sensible thing to do aren't we fortunate that the laws of nature allow us to be here asking how fortunate we are I'm not sure that that's a sensible question to ask because if the laws of nature didn't allow us to be here we would not be asking the question everyone who asks the question is lucky enough to live in a universe that allows for them to exist nevertheless I don't find it quite plausible or at least you know as a working scientist it sounds like a cop-out to me to say well we just got lucky the laws of physics allow for us us to be here isn't that nice let's move on I suspect that this apparent fine-tuning of the vacuum energy and other constants of nature or Clues they are telling us something there's something we don't understand about the universe we should try to do better so I'm going put aside this possibility another possibility is that the parameters are not nearly as finely tuned as we think when it comes to explaining the existence of life maybe for very different values of the parameters there will be other kinds of life it is extremely presumptuous I think to think that we understand perfectly when life could exist maybe life is quite generic and then the the last two more substantive possibilities are maybe what we're seeing as a selection effect maybe the parameters our nature are very different from place to place maybe most places they are utterly inhospitable to the existence of life but of course there's an environmental selection we are only going to find ourselves in those regions of the universe that are hospitable to us existing therefore we observe these finely tuned parameters or someone made it that way God intervened made the parameters because God wanted to make a universe where we could exist what would be the point of the universe without us after all so let me just get a little bit of lip service to this second possibility the idea that life is a lot more generic than we think it is that the parameters don't need to be that finely tuned in order to get something like life if we were really serious about making the claim that the parameters of the universe are clearly finely tuned in such a way as to allow for the existence of life this is the kind of argument we would have to make we would have to say consider all possible physical theories consider all possible cosmological manifestations of those laws of physics consider all possible ways that life and/or consciousness or something like that could exist within those universes and then calculate the fraction of those universes compared to the total number of possible universes now nobody does this religious people who would argue that fine-tuning is I've been for God don't do this physicists and cosmologists who want to argue that the multiverse is necessary to explain things don't do this nobody does this because it's wildly impractical our knowledge of all possible laws of physics isn't anywhere close to allowing us to make this argument and the scientists are just as guilty of this as any of the theologians who want to use this as evidence for the distance of God there are some hand-wavy arguments that say you know if the mass of the neutron were smaller than the mass of the proton life could not exist I personally have almost no faith that we understand what the universe would be like if the parameters of nature were very very different so I'm open-minded about this but it's clearly a very hard problem I just want to admit that it's just as much a problem for scientists as for religious believers okay so let's go on to the multiverse the multiverse versus God let's see if we can actually adduce evidence for or against the existence of God from the fact the parameters are nominally finely tuned and here just to make things look science II we use bayes's theorem basis theorem is the way that we take our expectations for what should be true and update them on the basis of evidence that we obtain that we attain about the universe so what we mean is that we have some observations some data D and we have a set of possible theories that might be true T sub I and then we define different ways of attaching probabilities to these theories being true so P of X given Y is the probability that X is true once we have found out that Y is true and then very crucially we have the prior probability for a given theory we need to say that not all theories are created equal we give a larger prior probability to the theories that seem more powerful more simple more elegant theories that require a lot of special pleading and finally fine-tuning are given smaller prior probabilities that's just the way that science works and then bayes's theorem tells us that once we observe our data the probability we assigned to our theory being the right explanation for that data comes from taking the prior probability that theory was true before we got the data and then multiplying it by the probability that the data would be true in that theory so if the theory is true what's the chance that the data is going to happen that is the probability the theory is true given that we observe that data and this is just a normalization factor on the bottom so basically all this is telling you the thing you need to keep in mind is that when we're judging our theories we need to calculate two numbers need to calculate the the likelihood that theory was right even before we look at the universe and we have to update that likelihood by saying now that we've looked at the universe how does that probability change so let's apply this notion to both God and the multiverse starting with the multiverse the idea that in many different regions of space laws of physics are very different we find ourselves only in the hospitable regions of space so most people agree that the probability that life could exist if the multiverse is true is substantial it is close to 1 if there really is a multiverse that is very very big that has all sorts of things going on then somewhere there life is going to be able to exist and and any people that are talking about this universe are gonna exist in those little sub places where the life exists that's more or less uncontroversial among people who are both Pro and anti multiverse the controversy is on what prior probability you should give to this theory this is our judgment on how good a theory the multiverse is in the first place should we penalize the idea of the multiverse because it is not simple it is ontological extravagant you're imagining all of this stuff that you will never observe even in principle just to explain a few parameters in our little patch of observable universe so this was put forcefully by Richard Swinburne a theologian who says to postulate a trillion trillion other universes a trillion trillion is amusingly tiny compared to the actual number of universes that we generally do postulate but postulated trillion trillion other universes rather than one God in order to explain the orderliness of our universe seems the height of irrationality so height if your rationality is a fanciful way of saying that the prior probability should be very very small Swinburne is saying look this you're just postulating a many many many unseen entities to explain one or two features of yours or observable world that's not good science is what he is saying of course the the the problem with this is that one does not postulate trillions of trillions of universe that is not the way it works the multiverse this is the crucial point the multiverse is not a theory the multiverse is not something that people sat around and said hey you know what would be neat is they were like a billion universes or a trillion or ten to the five hundred different universes wouldn't that be interesting that is not how the idea came about rather the multiverse is a consequence of other theories so it's not that you postulate a bazillion different universes is that you postulate a small set of possible laws of physics and these laws of physics predict the existence of many many different universes in the early 1980s Vilenkin and Lynda and others invented what we call eternal inflation that if you believe inflation is possible inflation is a very well-grounded physical scenario we don't know whether it's true or not but a very plausible scenario for explaining the observed homogeneity and isotropy of the universe what valentyna Linda showed is that if you believe that then in a wide swath of possible inflationary models inflation never ends it ends in some region of the universe but elsewhere it keeps on going and it keeps creating new regions of universe it's not something that you need to put in it very generically comes out of your attempt to explain the observed modernity of our universe then in the late 1990s string theorists realized that as you go from 10 or 11 dimensions of space space-time to string theory posits down to the 4 that we know and love you can do that but if you can do it in one way you can do it in 10 to the 500 different ways and all of these different ways to compactify the hidden dimensions of space give us different low-energy laws of physics this is called the string theory landscape all sorts of different possible parameters that you can imagine getting a huge huge number of them so string theory says that the laws of physics could be very different from place to place inflation says it's easy to physically create regions which actually have those laws now both of these schemes are optional we don't know that they are absolutely right they're not as established but the point is that the number of things that you are postulating is not trillions of trillions it's more like two or three you don't you don't argue against statistical mechanics of the atmosphere in this room on the basis of gee you need a lot of molecules in this room to explain this you don't count theories you don't count against theories because of a number of elements in a certain set that the theory invokes you don't say that the integers are ontologically extravagant because there's an infinite number of them it's a simple pattern that generates all of the integers there's only a small number of kinds of molecules of air in this room there's only a small number of physical principles that underlie the multiverse the way to judge the likelihood of the multiverse scenario is to judge the plausibility of inflation and string theory or some different ways of getting the same kind of thing but once you have these dynamical laws you get the many many many different regions of the universe you don't separately postulate them they don't count as a strike against the theory so so I would conclude so I don't know what the answer is here that what we're trying to get here is what is P of multiverse what is the AA priori probability you should give on the possibility of a multiverse existing I wouldn't say it's close to one necessarily but I wouldn't say it's 1 over 10 to the 500 either you don't decrease the probability every time you add a new universe you decrease the probability the conceptual complication of your theory every time you add a new idea and by that measure the multiverse might not be of order almost almost probability 1 but it's not vanishingly unlikely either it's something is very sensible to consider so now let's turn to God what is the prior probability for God and the probability that God is true given the data so I'm going to talk about the prior probability later let's talk about the probability that God is the best explanation of the data that we see given that God exists do we expect to see this kind of universe so I have no trouble believing that if God exists then life should exist I am willing to grant the god theorists that the kinds of gods that we're talking about here are the kinds that would make it possible for us to exist I don't think that is a difficulty it's the other data the data other than that the fact that life exists that gets God into trouble as a hypothesis and this is a very very difficult thing to do right what we're trying to do here is ask ourselves the question if we didn't know anything about the actual universe but we had a theory that says that there's this thing called God that created the universe that cares about us human beings that has a large or infinite amount of power and so forth what do we expect the universe to be like it's very hard to do this because God is little big frankly God does not make unambiguous predictions and we know what the universe looks like so we tend to say well God would do it just like that just like we see the universe looks looking but that is not at all fair we need to answer as honestly as we possibly can what would we expect the universe to look like if god were responsible for it so my favorite example of this is the low entropy of the early universe this was the subject of the last Oxford mini course on philosophy and cosmology the arrow of time in the modern conception of the arrow of time we understand that the microscopic laws of physics are perfectly reversible the same running forwards and backwards in time but the reason why macroscopic physics has such a pronounced directionality is that there's a boundary condition in the early universe it has a much much lower entropy than it would if you again pick the universe randomly out of a hat a lower entropy by a factor of about 10 to the minus 10 to the 120 so this is not a typo so the cosmological constant is fine-tuned one part in ten to the hundred and twenty this is one part in 10 to the 10 to the hundred and twenty so this is enormous Lemoore fine-tuning involved to explain the low entropy of the early universe and crucially the cosmos will come from the vacuum energy you can make the argument that if it were larger by a couple of orders of magnitude that life would not exist I'm not sure I bought by that argument but you could make that argument for the entropy however that argument utterly fails there is no sense whatsoever in which the entropy of the early universe needed to be that low just for life to exist it could be enormous ly higher it could be almost of order one compared to this and we would still be fine so if you really were trying to be consistent I would argue that the probability that the entropy of the early universe could be that low given that God created the universe in order for us to be here is of order 10 to the minus 10 to the 120 so that is big evidence against God if we didn't know what the universe were really like and we just said God created the universe in order for us to be here we would predict an early universe with enormously higher entropy another way of saying this is here is the Hubble ultra-deep field this is what you get when you point your camera at the sky and just take a picture if your camera is attached to the Hubble Space Telescope you see all of these little blobs each one of which is a galaxy comparable in size to our Milky Way galaxy so every one of these galaxies has billions of stars of order a hundred billion stars there's a hundred billion such galaxies in the universe none of them are necessary for us to exist here on earth if you are just gonna make the universe and you were gonna solve fine-tuning problems enough for life to exist there's no reason to make any of these galaxies all the matter all the degrees of freedom in those galaxies should be in thermal equilibrium if you're going to claim that your explanation for fine-tuning is that God made it that way in order for life to exist you strongly predict that this should be blank or at least just a chaotic mess it should not have all these other galaxies in there and I know that you know many religious believers would look at a picture like this and go you know oh my goodness the glory and stupendous news of God's creation I look at a picture like this and say how in the world can you think that the reason for this is to let us be here we are very tiny compared to a very big universe so there's a course responses to this kind of argument people have thought of it they say well maybe you can't make us maybe you can't form our solar system without forming all those other galaxies and so forth this is just false everything we know about physics tells us that none of those other galaxies is necessary to explain what we have in our neighborhood here so you say well maybe the reason why you get all those other galaxies of this really lower entropy than you ever needed is because God is sort of procedurally thrifty God does not just make I mean so remember it's God we're talking about God would have no trouble making just the Milky Way if that's what God chose to do but you might say well God wants to play by his own rules so God plays by the laws of physics so God makes use of some physical mechanism for universe creation which as a byproduct of making conditions that allow for us to exist also make a hundred billion other galaxies so that is plausible but you see what's happening you see that by trying to explain this feature that we would not have predicted we are pushed to create a essentially physical naturalistic scientific explanation we are removing all the usefulness from God we're saying that the reason why the universe looks like that is because there's some physical mechanism that makes it look like that and that's what God used but that's a story I can tell without invoking God at all there's a physical mechanism that made the universe look like this and then I can just stop so the advantage that you might have had in invoking God in the first place has gone away this kind of argument that a priori you would not have expected the entropy of the early universe to be so low if God were the explanation it's the same kind of argument that has been used to argue against God for millennia now their probability that if God really exists the world wouldn't look like that is what we're talking about this is the problem of evil if God existed we wouldn't expect there to be evil this is the problem of random suffering even if you argue that evil is necessary for free will and human choice and so forth there's no reason to you know have natural disasters to strike down innocent children or anything like that God could prevent that from happening it's not what you would expect our priority my favorite is the problem of instructions if I didn't know what the universe were like but I thought that the universe were designed and created by a carrying on impotent God who wanted us to do well the first prediction I would make would be that God would explain himself to us very clearly as a textbook author who has read his amazon.com reviews I would expect God's text book to be perfectly clear it might not be easy to follow the instructions but I would know what the instructions were it would have been very easy for God in some Holy Scripture to give us some clues to say you know matter is made of atoms the universe is billions of years old people of different races and sexes and genders and sexual orientations should be treated equally governments should drive their power from the consent of the government a whole bunch of things that God could have told us and chose not to that's a prediction that I would have made otherwise finally I like to say this is one way I like to say it imagine your theologian in a world where there wasn't evil or random suffering and where God had given perfectly clear instructions there were holy scriptures that said you know be nice to each other blah blah blah blah if you were that theologian would you take the absence of injustice in the world as evidence against the existence of God I think you would not I think you'd go well this is what I would expect God to do he made a just society for us if he wouldn't count the absence of injustice as evidence against the existence of God then you should count the presence of injustice as evidence against the existence of God there's a couple of ways to salvage the idea that God would have made the data like this one is just erase God's fingerprints everywhere to say you know what God really likes to do act like God doesn't exist God likes to obey the laws of physics again that's possible but then you're removing the usefulness of God and the other strategy is vagueness to deny that we have any reason to expect anything of God to deny that we can make predictions for what the universe could be like so I have a quote here from Terry Eagleton he's first trying to say that God is the condition of possibility the answer to why there's something rather than nothing very abstract ethereal sounding concept but then he likes to say well God can have regrets God is an artist who does things out of love and therefore you can't predict what God is going to do the problem is you can't have it both ways you can't both get credit for explaining the finely-tuned value of let's say the cosmological constant and yet say that God makes no predictions for other things that we observe about the universe the multiverse theorists at least try here is a plot from a paper where people who think about the multiverse are predicting the relic density of dark matter accion's in the universe on the basis of the multiverse hypothesis I don't especially believe this is a reliable prediction in any sense but at least they are trying I've never seen a picture like this in a theology journal no one ever tries to use the god hypothesis they can try to claim credit for the low vacuum energy that allows us to exist no one ever tries to calculate the predicted density of accion's under the God theory so that is my conclusion which is that the as ontological extravagant as the multiverse seems to be God is much more problematic it's a whole separate category it's not a kind of physics a kind of natural process it's a whole other kind of thing it is ill-defined unnecessary as far as I can tell and seemingly you need to sort of take or leave different predictions that you might make depending on how they fit it's on the table as a logical possibility if I think as good scientists we've learned over the last few hundred years that we can do better in explaining the universe thank you you

Channel: PhilosophyCosmology

Views: 804,594

Rating: 4.5780983 out of 5

Keywords: Philosophy of Cosmology, Sean Carroll

Id: ew_cNONhhKI

Length: 53min 16sec (3196 seconds)

Published: Wed Jun 05 2013

Reddit Comments

[u/Strilanc]

A random quote I liked at 24:10:

We should not think of the big bang as the beginning of the universe. We should think of it as the end of our [current] understanding of what has happened.

[u/bigwhale]

I also really liked his AHA presentation. Purpose and the Universe