What Exists？ A panel discussion with Don Page, Raphael Bousso, Laura Mersini-Houghton, Carlo Rovelli

The topic is what exists, and we're going to figure this out in, we have one hour. I want to say just one thing for introducing the discussion, which is something obvious, but I think it's good to start from there. To exist means all sorts of different things, right? So we can use exist, does a prince of Denmark who was very hesitant and killed the king to avenge his father exist? Of course it exists, right? It's Hamlet, right? Of course it doesn't exist, it's just a character in a Shakespeare play. So it does exist, it doesn't exist, both are true in a different sense in which things exist. So there are all possible different senses in which we can say things exist, and I want to keep this present in the discussion. I don't want people to fight about this existence that doesn't exist because they're just using exist in two different meanings, right? So last comment, there are two strong, I would say, extreme positions about what exists. One is Quine, right? Quine has a, who has sort of reopened metaphysics perhaps to some extent in modern philosophy. What exists? Quine's answer is very simple, everything exists, that's Quine's answer. Everything you can, sorry Jeremy for making it so low level, everything you're talking about exists, you're just talking about it, right? So for Quine, flying horses exist and so on and so forth. This is a decision of using exist in a particular sense, and there's another opinion which is also very well, in fact more widespread, which is Buddha. What exists? Nothing exists, right? Again, it's using exist, everything is illusion. So I would start by, the three of you have already said a lot what you think exists. Raphael thinks only things exist in his own past licon, sorry. I know you didn't say that. And we better never leave it or we cease to exist. Right, so please stay with us, otherwise we are fucked. No, you didn't say that. So the three of you have said already quite a lot about what you think exists, so I will start from the four of you, and I guess I will give you four, maximum five minutes each, the way you want. If you talk less, you will have more time later. And I would like you to say two things. One, give a definition of existence that you want to use, or possibly two definitions of existence that you want to use. So if you give me two, it's much better. I will mark them down. And then under this definition, or these two definitions, what exists and what does not exist. Which way do you, do you want to go first? I'm happy to, but I'm going to set a bad precedent because I have other things I want to say, and you'll see why that matters at the end of my four minutes. I want to start actually by introducing you to someone who was very influential to me in my, especially in my formative years, and that's myself 40 years ago in college. And he, like probably a lot of us did in college, we'd have these late night discussions with who knows what mind altering substances is going on. And you'd wind up with conversations like, does this pen really exist? And I felt really proud. I could say, come on guys, I'm a physics major. And all I need to know about this pen is what I do with it, and what happens when I touch it, when I drop it, and all of that. And that felt really good. And then here I am 40 years later, and I, to begin with, I wonder whether the Everett worlds exist. I wonder about pocket universes with different laws of physics. I, if some of you know my work on the clock ambiguity, I wonder about all kinds of different laws of physics coexisting within the same theory. And then even though I'm very uncomfortable with Max's, I call it maxing out, Max's level four multiverse where all mathematics exists, I realize that I'm coming very close to that in my own thinking. So what's, how exactly do I draw the line? I don't know. And so, and finally here I am sitting on a panel talking about what exists. So it all looks pretty bad from the point of view of that 40 year ago guy, who was me, except I'm actually thinking some more about it. I realize I actually don't think it helps to talk about what exists. And I'm going to go, I mean, with the Everett worlds, I can tell you exactly what I do to compute things. I'm a theoretical physicist. I can tell you how I use quantum mechanics. Pocket universe is the same thing. I can tell you how it works. And I think that's all I need to do. I'm a physicist. I try to connect theory with data. I can tell you how I do it. When I think about the clock ambiguity, it's something I'm confused about how theoretical physics works. And that's my job as a theorist is to figure out how it should work. So I'm working on it. I don't have to talk about what exists. And so now I'm feeling a little bit better from the point of view of this 40 year ago guy. I can sit on this panel and say, I don't think it matters to talk about what exists. And I'll say that I really enjoyed each of the three talks this morning. I think they would have been, each of them would have been even better if you'd just taken out the what exists part. They were small parts of each of the talks. And I think they would have been even better without it. So remember the reign of terror. So that's my two cents. Thank you. Before you go ahead, can I ask you a question? You think that data exists? I don't need to talk about what exists. I talk about what I do. I calculate my theory and I compare it to data. I know exactly how to do it. Thank you. It's like the pen existing. Thank you. Bianca. Yeah. So I probably agree. So I have a notion of existence, which I will apply here, which will also agree with Steve's and Rafael's talk to some extent. So this is, you know, whatever is a useful structure in a theory of quantum gravity, which is consistent and maybe at some point experimentally verified, ignoring the fact that the theory might not exist. To this extent, it's just, you know, structures which help us to formulate this theory usefully. In this context, so that's my definition. And in this context, to give you examples, I believe that something like quantum spacetime exists, the notion of quantum spacetime exists. And it's maybe not emergent as used in some discussions. But the important point, I mean, that points do not exist, but rather processes exist, in the sense that we can compute probabilities for processes. And importantly, I wish that chunks of spacetime exist. And that relates basically, that relates to Steve's talk, maybe. So it's really the question, do we always have to speak about the full universe, you know, the wave function of the full universe? Or can we kind of at least in some relational sense locate our computations and our answers and our questions? And I believe that that might require maybe, you know, an extension of existence frameworks beyond the usual global Hilbert space and these things. So that's kind of the question. Can we make sense out of chunks of spacetimes and in a certain sense, speak about subsystems without speaking about everything at once? Chunks of spacetime. I'm going to ask whether later, whether you think these things that Bianca say exist, do exist. But Laura. Yes. So to me, the answer, the question what exists is unanswerable. I'm completely happy with the Wikipedia definition of existence, an objective reality of being. And it's scary that we talk of ontology, being physicists, and we do that in front of philosophers. So let me switch. An objective reality? Of being. Yeah. So let me switch to familiar grounds and talk about the question what exists in the framework of physics. And of course, one associated aspect of what exists is can things unexist? And that brings us into a deeper question about nature, which is, is nature deterministic or indeterministic? If existence is eternal, then we are talking about nature being deterministic at its fundamental level. But if it's not eternal, then we are talking about indeterministic nature. And so in that framework of physics questions within the what exists aspect, I'm going to write a list of what bothers me as a physicist, what I wish I knew the answer to. So I don't have an answer. And just as an example to Raphael, mathematical theorems. You're posing questions? Yes. No, no, no, I'm posing questions. Okay. I don't know the answer. Okay. I'm a physicist. Vacuum or nothingness. Thoughts or consciousness. That's enough to get started. Wonderful. Can you anticipate the tentative answer to this or not? I can give you... Maybe yes, I think, or perhaps no. I can give you my favorite answer for singularities. I would vote for no. For time, I would vote for yes. Mathematical theorems, yes. Vacuum, yes. But that again brings us to the discussion, is nature deterministic or indeterministic? Thought. Can something unexist? I'm a quantum theorist, so yes, I believe in an indeterministic nature. And the last question I didn't write there... Yes. And the last question I didn't write there, but which is related to the theme, is do we need an observer to bring something into existence? Multi-universe, you didn't say yes or no. Definitely yes. Oh, boy. That was an easy one. Wonderful. Don, you have your five-plus minutes. Okay. So I tend to go along with Weinberg that we say what exists is what we give a certain respect to. So rather than try to define it more precisely, let me just give some examples of what I believe as fundamental existence. I'll leave out things that I think can be derived from these, but the fundamental existence also leave out truisms like mathematical theorems. So I'll talk about a contingent world, the part that could logically not exist. And for me, that includes, well, first, I guess as a Christian, I believe in God, the Father, the Son, and the Holy Spirit, and with properties such as ethical requiredness for actualizing the best possible world, love of simple, elegant lawfulness, and of sentient beings, and creation of the universe and other entities from nothing. And then for our universe, what I think is fundamental is observations as conscious perceptions or sentient experiences, the first-person subjective sentient experiences. And then I believe that for each of these possible sentient experiences, there's a positive quantum operator, and then there's a quantum state for the universe or multiverse that gives measures for observations as the expectation values for the observations. And then going beyond, I believe God could have, well, I believe he's created new heavens and new earth and will resurrect humans in this new creation and other possible universes that I don't know, I think, whether they exist or not, but that's quite possible. But so as a physicist, I focus more on this, this physics part. I think you can circumvent the middleman of histories and decoding histories. You could just go from the quantum state to having operators, or sorry, having measures of conscious perceptions as being expectation values of positive operators. And these, there's no correlations between them, so in that sense, it's a bit like Raphael Bousso's idea. So in some sense, I'm taking the basic elements to be even much smaller than Raphael, not a whole causal patch, but just a causal, just a causal, sorry, a conscious perception. But also, of course, because this just depends on the expectation value of operators in the quantum state, it's not tied to their existing space-time or being able to decompose the Hilbert space into subsystems, the worry that Steve raised about. So I think it's a framework, I think, that can transcend a lot of these things and doesn't have the problems of the limitations of quantum gravity dividing up into subsystems. Of course, the big hitch is it's a framework, it's not a theory because I do not know what these operators are. So it's very much a framework for constructing a theory, but you maybe could have toy models and maybe you wouldn't even abstract away from consciousness and say you could just have the archive, what's the measure for the archive? If you put the information in the archive into a single string of digits, in other words, convert it into a binary number, what's the measure for all the numbers of the content of the archive? Okay, maybe, is that my four minutes? Yeah, that's wonderful. So I got your definition of what exists, it's what we give a certain respect to. It's a very good one, I think. It's Weinberg, yeah. It's Weinberg. But I like it. Don Weinberg. You mentioned a number of things that you think do exist. One is God, one is observables, one is the quantum state, one is heaven. And can atoms exist? Well, a certain, I mean, yeah. I guess I'm agnostic about whether there are other preferred operators besides these ones, these so-called awareness operators whose expectation values give the measures for consciousness. That's an interesting question. Are there other fundamental operators, such as quantum field theory operators, or something such that out of them maybe we build these awareness operators? Or are they fundamental and it's just that we abstract, that we can have simple descriptions in terms of effective other operators? But yeah, I'm agnostic as to whether atoms, chairs, well, even persons. I don't, in some sense, don't even really believe, I'm a really far-out Christian of not believing in the existence of persons other than God. I think we're images of a personal God, but we're not really persons. What we are is a sequence of, in some sense, a branching sequence of perceptions. The perceptions are fundamental, but not the persons. But there are sequences going forward and back in the ever-multiverse. There are a lot of approximate sequences that go from mine to yours if we go forward and backward enough. So I think we're all connected. And in some sense, I think if I could really realize in a psychologically deep way, it would help overcome the sinful selfishness I tend to feel intending to act to value my own conscious perceptions and their future value, whereas if I really do intellectually think that all are equally important, and so I should act to try to increase the value of everybody's conscious perceptions and not just my own future ones. Thank you. Good. Great. So I go to the right-hand side of the aisle. Again, please, one or two, or even more, if you have useful definitions of existence that we can make some use of, and possibly a list of things that you think exist and don't exist. Steve. Well, defining the existence is a tricky one, I think has been illustrated. At the basic level, something that exists is, well, arises if you have some kind of construct from which other things can be extracted. And so I gave sort of my quick answer regarding what I was suggesting I think exists to be the wave function or the space of states in quantum mechanics. And that is a construct we could think of as a fundamental construct from which it looks like we can get pretty much everything else in principle. So that would be my starting point. Wonderful. Janan. So I think I give Quine's answer to what exists, everything, but with a bit more elaboration. I think what it is for various things to exist depends on what kind of thing they are. So Hamlet exists, but what it is for Hamlet to exist is a little bit different from what it is for other worlds to exist or for electrons to exist. And I think I agree with much of the spirit of what Andreas and Bianca were saying, except I would say what our theories do is they tell us what it is for these various kinds of things to exist. They tell us what it is for the multiverse to exist in terms of the ways in which we can interact with it or can't and so on. So the list of things that exist in this sense is very large for you. You would basically say yes to almost everything I listed down. Everything. Everything. Wonderful. All right. So I mainly want to agree with Andy, who has liberated me from tyranny. I obviously try to be a good sport by using the word what exists, but I have ultimately a very impoverished view of the world, I suppose. To me, what matters is to have a successful and economical description of reality. And by reality, I mean experiments we can do. So a good example would be the ether. People thought it existed. It was there. It was part of the mathematical description. And it didn't do so well. And the description got more and more complicated because we insisted on keeping this thing around. And then when you throw it out, all of a sudden, not only does your description simplify, but you've liberated yourself to recognize a completely different space-time structure actually describes nature. And so in that sense, I think it's good to have an impoverished view. It's good to try to be minimalist and just care about what you need to have in your theory to make successful predictions. It sounds really dry, but that's the sort of thing that gave us special and general relativity, that gave us quantum mechanics, that gave us these great revolutions which completely changed the way that we thought about our world. So one thing that I love about physics is this interplay between a very dry and sort of, well, selfish attitude about how to think about the world. You just throw everything out you don't need and the sort of miracle of what you get back. And so I think that it's actually good not to worry too much about what exists, but just what is the most economic set of structure that I need to describe the world. Wonderful. So I sense a big agreement, a strong agreement in something and a strong disagreement on something else. So let me try to say this. About the how to define existence or what is the useful definition of existence, I think there is a remarkable agreement because everybody agrees that we can use existence in different ways. There is a strong deflationary, strongly deflationary attitude among all of you. The question, the strong question, what exists and what does not exist is not a question that interests us. Probably doesn't even make sense. We can just resolve it by saying, well, everything exists, but what is interesting in which sense different things exist. And then all of you suggest, okay, there is one way of thinking about existence which we might be interested in, which is you formulate in different manners. Bianca said, well, I have a fundamental theory of the world. This postulates a number of things. These are the things that I want to call existing within this theory, which is perhaps not so horrendously far away from what Don says, what we give a certain respect to. I'm Italian. That makes me think of the mafia immediately. Steve said the basic construct from which we derive the rest. We were talking about Naximander, the archaic, the thing from which we postulate. So I would suggest that there is an agreement about how to use existence, and use it in this sense. So let's use it in this sense here. Not in any strong metaphysically charged sense. Not in the sense, does this exist, of course it exists, but it's made by atoms. Fine, let's forget about that. So let's not talk about the existence of, sorry, of that. But go down to the set of object, notion, ideas, whatever, out of which we want to, we think we might provisionally be able to reconstruct the world as better as we can. So the question is, what are these now? And some of you gave an answer already, but already I've heard some disagreeing answers. So let me go, let me list the thing you have mentioned here, and see whether you all agree or not. That it seems not, whether they exist or not. So in the order I have mentioned, you didn't, you're strong, you're very radical, so you didn't want to measure anything. Okay, you want to add something before we go into that? I'm going to just illustrate how I think about this stuff. So right now, I'm an Everettian, and when I do quantum mechanics, I take the wave function very seriously, and I actually love the Weinberg quote. It's what you're saying. I'm happy to tell you what I respect without using the word exist, but, so I take it seriously. I take the whole thing seriously. If in the future, someone comes up with an alternative theory that excites me, and so far that's just shown no sign of happening. We all agree on that. So there's full agreement on that. Then I'll work with that theory. If I was excited about it, I'd work with that. Okay, so let's not talk about what we'll think tomorrow. Let's talk about today, at the very provisional, we are mortal beings limited in time and space. We only know very little about the world. At the level of what we know today, what is the best set of things out of which we can reconstruct the rest? Okay? So let me ask each of you whether you think that the following object is part of it or not. The first thing that... Part of it. Our... What we work with. Okay. So the first thing that was suggested by Bianca is a quantum space-time. Let me start from Steve, because Steve may have... So I want a yes or no, or a short, or I don't know, or a maybe, or a short comment, but short. Quantum space-time, does it exist or not? I'm not yet sure how to define it, but as a quantum mechanician, I would start with Hilbert space. So is it sort of no? Is it sort of no? You said label states. Well, no, I think that there probably... Well, there could well be some notion of quantum space-time that comes out of, say, an appropriate Hilbert space with additional structure, but I haven't yet understood how that works. Right. So you're more radical. Sorry, the state that exists. Well, or the space of states of quantum gravity. Space of states. And then, you know, in some sense that could give you a notion of quantum space-time and then an emergent notion of classical space-time in an approximation. Clear. Janan, it's not really your world, but do you think this... Pantos, I have to ask also you. Yes. I'm just going to say that I'm waiting for y'all to sort it out. Okay, your excuse for that. I'm waiting for you all to sort it out. Raphael, I'm curious about the answer, especially because you talked about existence and non-existence in a way which seems to me contradicted dramatically what Steve was saying. Now, your entire talk was based on, say, on picture of space-time with a fixed background structure given forever, which might be dynamical, of course, but not quantum. So nothing in what you said included the possibility of superposition of different causal structure, which Steve seemed to imply that could be there. And on this, I would be on the side of Steve entirely. So does quantum space-time as opposite of classical space-time exist? No, I don't disagree with Steve at all. We're just digging around in different corners. So I think we all agree that we need a theory that can incorporate both classical general relativity and quantum mechanics. We both, I think, don't know whether that theory is still going to be based on the principles of quantum mechanics. Everybody seems to agree that GR has to go out the window. Quantum mechanics, it's harder to come up with modifications, but I think that's a big question. No, we're digging in different corners. I was discussing a very limited setting in which I'm assuming that we have a classical space-time that's emerging that obviously would be only a tiny, tiny corner of that theory. But that's a corner we know well, and it seems to me that you want to understand as much as you can about how that corner is going to emerge. So what I was suggesting is that my bet would be that what's going to come out of a quantum gravity theory in that particular limit is a description not of a global universe that includes the ability to compute all sorts of correlations that nobody can measure, but a more economical description of a smaller, causally connected region. But that doesn't mean at all. I mean, there's only one thing I could give a talk about, and of course I agree that, for example, quantum fluctuations even make uncertain my claim that there's sort of objective decoherence as degrees of freedom leave the causal patch. Well, that's only true up to corrections, perhaps of order e to the minus the area surrounding the causal patch, which measure the likelihood that you have a huge quantum fluctuation that brings about a recurrence, for example, that could make things unhappen. So yes, there are lots of questions that I simply didn't talk about. There's no disagreement that I can think of with anything Steve said. Although I'm extremely nervous about stopping my existence when I leave your causal patch. So it seems that even though I said this seven times, my causal patch doesn't really come into this. So could I summarize that by saying that the way you presented it is not within the global picture of quantum gravity that Steve was referring to, or I was referring to in my talk the first day. But in some approximation, nevertheless, you would expect that some version of that would still hold in a full quantum gravitational theory where space-time is quantum and nevertheless you don't want to talk to speak. Some extension of what you say, you would expect to make sense? Yeah. So I mean what's clear is that a successful quantum theory of gravity, among the things it has to be able to do is reproduce cosmology as we see it, which is a pretty classical space-time. And I think it makes sense to ask how it does that. So an analogous example is these holographic relations between surface areas and information content. These are things that we can recognize as being correct but they seem coincidental. We can formulate them only in a regime where we can make sense of classical geometry, which is a very limited regime from the point of view of quantum gravity. But we might then expect that we're learning something about the structure of the underlying theory that will help us get to where we want to go, which is of course the complete theory that does include the quantum regime. Yes. Yes, probably emergent. Bianca, you already said that you think that quantum space-time is in the basics of the theory you expect we have now, or we should have now. Yes, so let me clarify. I wanted to emphasize that one needs to include the gravitational field and that one can compute at some point amplitudes for processes involving telling you something about space-time geometry, that this has to be included. And this of course could come from starting from a Hilbert space and the wave function. So Steve maybe was more saying what is more fundamental. So that's where I'm not fixed yet, that the Hilbert space would be kind of the final fundamental structure. I mean for me, I think it's a mathematical structure which we use now, which has a long history coming from quantum mechanics. Does it allow us to be local enough in some sense for being practical? I'm not sure. So I like Rafael's point for trying to be more local. And the point I don't like is this apparent classical background, but maybe one can work on that. I agree with Bianca for the record. Laura? I agree with Steve. It's assuming that the quantum theory is more fundamental than general relativity, than you'd expect the Hilbert space. With what we know so far, perhaps in the future... The question is not space-time, it's quantum space-time, whatever it is. So that's why I vote for Hilbert space being more fundamental than a quantum space-time. It's not clear at all at this stage where there is such a thing as quantum space-time. We have worked on quantum phones, for example, that replace. All we know from our models in quantum cosmology is that space-time is emergent, so you don't need a quantum space-time. I can, of course, see and empathize with alternative point of views where a quantum space-time is desirable. But at this stage, in whatever we do, whether we calculate a scattering amplitude or a gravitational path integral, the Hilbert space is the more fundamental entity. Thank you. Don? Yeah, I think I agree. So I'd say strictly the answer is no. I don't believe in quantum space-time because I do not believe in space-time as fundamental. It's, you know, maybe emergent or something. So whereas I think I do believe, at least for the moment, tentatively I strongly believe in quantum theory, probably Hilbert space, but I wouldn't be too unhappy with a C-star algebra, you know, enlargement. Good. So quantum space-time, you put it apart. Let me pick another thing in which I thought there's disagreement. Time. And please don't take much time on that. Short. Shall we start the other direction this time? So yes or no, and no more than five seconds. Not believing in space-time includes not believing in time as well. So no time. Laura. I make a distinction between a global parameter of fundamental time and a local error of time. So the local error of time is associated with space-time, and that's emergent. But the fundamental time parameter, if you like, in the multiverse, it's fundamental, and it exists. So yes, some version of time. Yes. A no and a yes so far. Yes? No. Fantastic. Bianca's clear no. Yes, but most likely emergent. No, no, but the question, sure. Did you say fundamental? You're wanting fundamental? Right. I mean, we all agree that time exists, right? I mean, we only have a certain amount of time. So I don't want to say the time in which we live, the time in the approximation in which we live certainly exists. We'll agree, I think. The question is that in the fundamental theory, is there a time or not? You mean like an external global? I doubt it. Fantastic. No, like... Raphael. Well, if you look at how space-time breakdown near a singularity, say inside a black hole or at the Big Bang, there doesn't seem to be anything there that singles out time and protects it from, you know, stopping to make sense. But I think that's a really interesting question, whether there might be something that singles out time. That would, I think, make a big difference to the likelihood of quantum mechanics surviving. I think I'm willing to believe no. But I'm also going to say that I think all of the interesting questions about time are about how it emerges with all of the structures that we recognize. I agree. Stephen. In the fundamental sense, not necessarily. Oh, boy, there's more agreement than I expect. there is a sort of consensus that it is emergent in some sense. Not full consensus, but there's a majority. Sure. Since you are the dissenting voice here. If time is emergent, we can't ask what was there before space-time emerged. We are forbidden from asking that question. We have a fantastic answer from St. Augustine. I think you all know, right? What was happening? What was God doing before time started? And the answer is he was preparing hell for those asking too deep questions. No, no. I must correct that. That's what a lot of people erroneously think. OK, so let me give the full story. He mentioned this joke and he says it, I will say jokingly, He says the wag will answer maybe God was creating hell. And Augustine says, no, it's a serious question that deserves a serious answer. And then he said, roughly the time has created thing and therefore there's nothing before time was created. I'm sorry. I apologize. Augustine must not be attributed to saying he was creating hell. He also said in his confession he knows what it is but he can't explain it. So, back to my point. There seems to be some consensus on quantum theory being more fundamental. I can't see how we can do that without having a time parameter. The time parameter is built into the quantum theory so that that's one of the reasons why I was willing to take time as fundamental. And the second reason is it puts a curtain on our scientific curiosity and inquiry. We can't ask what was there before if there is no before. Okay. So let me... May I... Sure. Give a counterpoint to that? Sure. Briefly, I would say if we're going to describe things in terms of quantum theory we probably need to think about a more general formulation of quantum theory that doesn't necessarily input time. There will be some parameter there. I don't think the fundamental time is not in the normal... I think we're not agreeing on... They're not going to converge on that. There was more conjugation than expected. Soon we'll go to audience questions but let me... Since this was a bit controversial let me say something. Let me take one non-controversial and I only want a yes and no. No more than that. God. It was mentioned as the only existing thing by one of the people in the panel. So yes and no is God one of the fundamental thing in terms of which he is he does we can understand the world. I take the fifth. We're in Canada. You don't have the fifth. No. It's not in any theory that predicts observations. Anders? No. Bianca? No in my definition of existence as being a structure in quantum gravity in the series quantum gravity. Laura? I take the fifth. Don? Yes. We are not representative of the majority of the people in the world. So I'm not sure if this is a question or statement but this question of existence has bugged me for a long time and it still bugs me and one of the things that bugs me is things like if you imagine an M.C. Escher painting right? An M.C. Escher painting you know you can't physically build that but in some sense it exists because it's there it's on paper or like the transporter in Star Trek right? We know we can't build one but it's there. So the only answer I could come up with was that there's kind of two types of existence that there's a physical existence and then some kind of what I call a representational existence. I mean if I can imagine it in my head and my head's part of the universe then doesn't it exist in some way? So I don't know if anyone wants to chime in on that but this is a problem I haven't solved and I'm very interested Well the agreement here was that there aren't two types of existence there are many. The sense in which a number exists the sense in which Pinocchio exists the sense in which you exist the sense in which an atom exists Why do you say writing about this God exists the sign exists maybe there's different types of existence. Yes, yes, yes. The question was not whether God exists the question was whether God exists within the definition of existence that people arbitrarily chose here. So I just say your life will be more fun if you don't worry about that. Enjoy the enjoy Star Trek And you say you're not definitional? Okay. Okay, so this sorry for changing the tone. I mean this is not really going to be a philosophical question this is going to be a much more technical question but related to the technical content we had in the talks this morning. So it's a question probably for Raphael and maybe for Steve also but Raphael you you proposed your vision of what is real as partly as a way to get around the black hole information paradox and I just want to ask this question because I don't know if I'll ever have any other opportunity to learn at this conference but the answer you seem to give seems to be very similar to Susskind's original answer you know like 20, 30 years ago and recently there's been a lot of people seem to be worried that that's not a sufficient answer so I want to understand why you think that your causal thingamajig patch is a sufficient answer to to that problem. Yeah it's a necessary condition for the information to come out not sufficient. So what does that mean? Well it's if you assume that the information comes out right then then you get these two copies now that's not enough to assure that the information comes out so what what you know was shown a few years ago by Polchinski and Merolf and collaborators is that there appears to be a much more drastic necessity for you know deviation from the equivalence principle at at the horizon of an old black hole but the argument for that kicks in sufficiently late that you can't just say that the copy inside the black hole can't be reached so if you if you fall in with the star for example that there should not be a firewall preventing you from entering but I agree that there's an interesting interplay between these questions. This is a topic in which everybody here has a lot to say so I prefer to skip this because we can go into a discussion about that and many people have things to say here. So in the realm of philosophy one of my favorite questions and open problems is to deal with non-physical existence. In each of your own frameworks is existence and physicality are they synonyms? Is there a gap between them? And if so how do you characterize the gap? What is it? So put it another way one of the things I did not ask is how do you put it consciousness thoughts are these part of existence according to the definition that you started from? Yeah it is there. That is one of the things that were mentioned. So who want to answer? I would also suggest that we go rapid everybody yes or no about existence of non-physical entities. Well again for the purpose of answering the question does something exist? I'm taking this more basic level notion of what exists. Yes consciousness in some sense exists in an emergent sense it seems to me self-evident but at the more basic level I don't see how it is part of the fundamental description. So consciousness exists yes absolutely is it physical? I genuinely don't know. I bet with Steve it's clearly there and since I believe in a physical explanation of phenomena it must be an emergent phenomenon. very complex I don't know how long it will take us to understand it. Yes emergent from physics. Yes as an emergent thing. Yes. Don? Oh it's yes but not a logical consequence of present physics but as a physicist I would like to subsume it and say when we do have a theory that includes consciousness it will be part of what I would call physics. Ken? I also have a short question I would like to have yes or no answers from. If you don't think time exists on a fundamental level extrapolate to the first emergent level. My question is does the future and the past exist at the same level or is one somehow more real than the other? I think that depends on the state you're in. Our state? The state of the universe. In our state it looks like time may well cease to exist in the far past in the sort of Planckian era. In the future if it's really just asymptotic time exists in future exist in don't see too much difference there. The same. I don't have anything useful to say. The same, yes. I don't see a difference, especially if you believe in a wave function of the universe that would contain both past and future. Laura? The same except for the era of time. So the five minutes in the future won't be as useful as five minutes in the past. The same, except that we know much more about a minute ago than we do about a minute in the future. I'm wondering if you'd please try to elaborate just a little bit on what you mean by fundamental. What do you mean by fundamental? Well, there's sort of an arrow we can get derived things from more fundamental things. And we see that in physics over and over again, where we can derive certain constructs from more basic constructs or more fundamental constructs. I think there's two cows, but not here. So, I mean, the philosopher's answer, which I think is right in this case, is there are different kinds of orderings. There's an epistemic ordering and there's an ontological ordering. And in the ontological ordering, that's the ordering that physics takes. You start from whatever is physically fundamental in the epistemic ordering. You start with the stuff that you know first, that's the most immediate epistemic context, contact with. There's this question about whether one of those is more, there is some overarching order in which one of those is fundamental. And I just don't think there is such an overarching order. I think you take your starting point and you define your ordering and then you can talk about what's more fundamental in that order. I would say that anything is fundamental if it's sort of a minimum set of axioms of your theory or ingredients from which you could in principle derive the rest. But I think what's really interesting is that there are more and more hints that that may not be unique. So there may be different ways of setting up your theory and working your way out from that. For example, duality suggests that there are systems that you can think of in completely different ways in different regimes. Condensed matter physicists hate this idea that the particle physicists are somehow more fundamental because they deal with the short distance stuff. But it is a fact that RG flow goes in one direction. I can have a low energy thing that emerges from different high energy physics. If I know the high energy physics, I know what's going to happen. But again, the viewpoint that there is such a clear separation is being undermined. In a conversation with Jenan yesterday, I said fundamental is a word that is a tool to communicate how I'm thinking about a theory in my mind and what priority I'm giving to what piece of the theory. Maybe in a different construction I'll use it differently, but it's a way of communicating how I'm building a theoretical idea, not more. Yeah, so I guess I agree that fundamental is basically the set of things in a hopefully economical theory, and I agree that there could be different theories describing the same thing, and so you have different notions of fundamental, and another example, maybe quantum mechanics, there are different ways of reconstructing quantum mechanics from different sets of axioms, and so basically whether you see information or whatever may be more fundamental might not have a final answer in this sense. So, our job as scientists is to discover, not invent, and that assumes that there is something out there waiting to be discovered, so as a working notion, we don't think of exact definitions of the terminology, but as a working notion of what's fundamental, is a building block of nature that's always been there, and which we are trying to discover and understand rather than something which emerges or is derived. Don, maybe you have a different answer. Yeah, well, I'm a little bit torn, but I like today's thing that there's a difference between the epistemological and the ontological. On the ontological level, I would take that God is the fundamental thing, and everything follows from his nature, but of course, at the epistemological level, I know much better about the laws of physics, and therefore it seems easier to derive things from the laws of physics, and of course, most fundamentally, I know my own present conscious perception, and, you know, I regard it as sort of a fundamental result of whatever the fundamental laws of nature. I don't think it's at all a result of present laws of physics, but I would hope that there are some future laws of physics of which it would be a result. Thank you. So we're approaching to the end. Last questions. So, everybody who is there. Tommaso. Okay. At some point during the discussion here, the meta question emerged of whether or not asking what exists is sensible or not is a reasonable question. I think it is a reasonable question. Especially if you can conceive of a simple answer. I'm not saying that you can prove the answer, but at least you can conceive it. Now I have a super simple answer. The only thing that exists in my point of view is a discrete version of space-time, not quantum, and algorithmic. So it evolves algorithmically. Everything else, including the wave function, time, God, our consciousness, our conscious perception emerges. And that's what Max Tegmark calls human baggage. Thank you. Bill. Yeah. I guess basically what I wanted to do was to agree with Andy and phrase it slightly differently. I think something exists if it is necessary to explain the time development, the motion, et cetera, of other things that I already know exist. Now then you say, well, where does this change start? Descartes answered that whatever 400 years ago. And so starting from that, I can then build up all the other things that I want to say exist just by looking at how they develop and how they develop and what I need to do explain that development. Thank you, Bill. Can I say a quick comment? Sure. There's a useful distinction between stuff that we find and stuff that we introduce. And I think one can be completely deflationary about the stuff that we find. Everything that we find exists and there's questions about what it means for those various things to exist. And then there's stuff that we introduce in the context of explaining it. And there I think there's the kinds of strict discipline that Raphael was talking about become operative. We're very close to the end and I think we're very fortunate of having a very wise man giving us his last thought, Jeremy. So that sort of vetoes the bad joke. That's the wisest thing of all. No, well, I wanted really to pick up on this idea of the word fundamental that has recently, in the last few minutes, been discussed, with the idea that there is some minimal set with which we can explain some collection of evidence or phenomena. I mean, Bill just mentioned this and also Raphael earlier. I think it is a great question, isn't it, whether we have a right to expect that there would be one such or if there are more than one, whether there should be some kind of cognitively tractable correspondences, as in dualities, and I think intuition is pushed in different directions in different cases. We're all familiar with the case Bianca just mentioned about different formulations of what we happily regard as a single theory or different axiomatizations, and we don't see any need to select one. We go back and forth happily. Notational variance between different theories, if you like. Then there are cases where we feel we do need to quotient. Philosophers' standard case of this is the Einstein whole argument where you're anxious that you shouldn't be stuck with all these isomorphic representations. You need to quotient the theory to get some favoured formulation. So I invite the panel of FQXI to consider the whole issue of dualities and quotient. Wonderful. So I use my authority to not allow the discussion about dualities because I don't believe in dualities personally, but I think it was a fantastic discussion. There was a remarkable amount of agreement which I didn't expect so much, both about how to use or not use the notion of existence and also quite a lot with crucial disagreement. But I think the disagreement were very indicative of where the interesting problems are in a sense. So I just want to thank everybody and it's noon. It's time to stop. Bye-bye. APPLAUSE