The Unsettling Illusion of Time

It doesn't feel as though time is passing. Where is time passing? Time remains the central concept in physics that is least understood. There's lots of debates in theology, aren't there, about whether God exists outside of time. I think they're extremely interesting.

Professor Simon Saunders, Emeritus fellow at Merton College, Oxford, is one of the most celebrated philosophers of physics alive, working on what time actually is. I find it almost insane all of the minuti of our present existence with all its craziness and insanity built in from the beginning of the world. No, I cannot believe it. It is not necessary. On this channel, I KJ Mongol interview researchers regarding their theories of reality with rigor and technical depth. Today, we go deep into the philosophy of time and what any of this has to do with the many worlds interpretation. We go over the professor's boldest claim that the violation of Bell's inequalities is not evidence against locality. It's evidence for the many worlds. We even touch on consciousness.

"I am just in my Everettian branch. You are in your Everettian branch. We can coherently treat ourselves as a sort of extended observer."

"Professor, what is time?"

"Ah, yes. The easy questions first. Richard Feynman said something about time. He said this. He said time is how long you have to wait. And you can't really say anything more than that, uh, without getting into trouble." And you know, it goes back to, I forget the name of the philosopher now, that says, "You know time, if I have to, if I, if nobody asks me what it is, I know perfectly well, but as soon as I am asked, then I become deeply puzzled and so forth." So, yeah, I mean, time, I think, probably remains the central concept in physics that is least understood. I think that's probably right to say. So, at some fairly fundamental level, we, I think, do not altogether understand time, but we have very adequate representations of it in physics, and we know how to deal with it in our ordinary lives, but it is extraordinary. I mean, look, but think of it in terms of just your life, whatever you've lived, perhaps only 20, 25 years, heavens, um, long enough. There are many, many hours packed into those years. Consider yourself in any one of those hours. There's time to think and comment and reflect and, you know, uh, look around and so forth. This is you being you. This is you being a person living in an ordinary way over a period of say, an hour. But then there are thousands of such persons on that basis, because there's been thousands of hours where you have lived through those hours. So there's thousands of, call them larger than moments. I don't want to make it just something instantaneous. Uh, there are thousands of times of your life to persons, as it were, considering themselves at the time to be all that they are, that there's nothing more to them. As I speak now, I'm not considering that there's more to me in the future. So I'm perfectly adequate as a person with my past. But then there are thousands of persons with my past going back in time. I'm one of those thousands. And of course, if we make it not an hour, but a minute, we make it not a minute, but a second. If we really compress it, you can have as large a number as you like of previous moments. So what is the status of all of those moments? I mean, how do we kind of deal with that reality, that multiplicity of realities? Because each seems like a reality. You know, you ask me, "What is real?" I can't do better than just point to stuff around us and say, "Well, look, you know, this is what's real. It's Johnson kicking the stone." So, they're all realities, and they all somehow exist. Well, they don't all exist at the same time. They exist with a sequential structure, and more than that, there's a causal structure. And we build and build and build until we've got a quite an elaborate understanding of that multiplicity as something fairly integrated. And yet, there remains something really problematic about it all. It seems that we ought to take all of those momentary selves as equally real. They're separated by intervals of time. They're not all simultaneous with one another. But it seems we must take them to be all somehow real. And this follows in particular from the theory of special relativity. To some extent, also general relativity. But GR brings in, uh, some extra considerations. But in special relativity, we learned that there's no such thing as a global present. And if that's the case, there's no such, if you think reality is a present, there's no such thing as a global three-dimensional reality that isn't somehow, I mean, when I say there's no such thing as a global present, there's a momentary presence centered on me.

"So I can have a momentary reality centered on me, a three-dimensional reality centered on me, but there's no intersubjective objective public space three-dimensional spatial reality because if there were such a thing, that would be a privileged frame of reference, and it's of the essence of relativity theory that there is no such thing. If that's the case, so there's no three-dimensional reality that is global that that exists. So it seems then that the only public reality is the one that takes in all of the presents, all of those different presents. So you, this is the block universe picture. So you have this picture of the histories of people, the histories of objects laid out, as it were. It's static, unchanging. Think of it as some extraordinary block of glass in which all of the myriads of threads weaving through it are each particles, objects, peoples. That then would seem to be the reality. And that reality just doesn't look or feel like time at all. I mean, it looks and feels like space. So, in a certain sense, I mean, that's a quick way of putting it. Time has been spatialized. It's been turned into a dimension similar to space. There are structural differences. There's the light cone structure, you know, absolutely inherent to Minkowski spacetime, spacetime of special relativity. Uh, and with that, um, one partitions straight lines, what curves as well momentarily, but take straight lines as timelike or spacelike. And if timelike, whether present timelike or past timelike. So you have those that light cone structure in the Minkowski spacetime, this block universe, but still, it doesn't feel like time. It doesn't feel as though time is passing. If I ask, where in that block universe is time passing?"

"Mhm."

"What I can do is look at a segment of my, my light, my world line, as it's called. We can look up a little piece of my world line, the little piece that is now, if you want. And there, in that little piece of my world line, you see me talking, you know, discussing this crazy subject of what is time. And, you know, my lips are moving, the sound waves being emitted, you know, my head is changing and so on and so forth. But all of that is, as it were, a static configuration. So in what sense does that capture my sense of the real, of what is of passage? I mean, Stephen Hawking put it in a slightly different way. What breathes fire into the equations? You know, we've got this sort of abstract, representational thing. What makes it come alive? And surely what makes it come alive is time actually passing. That somehow this picture, this representation of space and time, spacetime together, fails to capture the felt experience of time of passage. So this is a very ongoing debate, and I think it's, um, a very fundamental one, and I think it connects fairly closely to the mind-body problem. Um, and it's, I, I think one of the aspects to it that is little attended to is it focuses on the fact that awareness is local in time, and it, it doesn't have to be, and it's not even instantaneously local. It's, you know, the species present, 50 milliseconds, whatever. So, so that that is the localization of awareness. Now, I think we must believe, if we look at other animals, if we look at insects, um, that as you go down in scale, the, the time scale of, of their awareness is, is much shorter. You've only got to look at a small dog and the way that it moves, or cats and so forth, if they're, and it's extraordinary. Birds, it becomes even more pronounced. They are clearly living at an accelerated, a faster rate than we are."

"Yes, except my mom's cat, who seems to be living extremely into the future, plotting my demise."

"Wow. Well, well, of course, maybe that's a special case here. You know, it's quite, um, but so there's something, you know, rather extraordinary about what, what, what it, the localization of awareness. Um, and presumably, it can't get down to extremely small, small time scales. Um, uh, but why is it local at all? Well, you could say, well, because dynamics is local. You know, physics is built on locality. You know, fields are local. So, well, all right. But awareness somehow, [sighs] is underpinned by all of this phenomenology. It could have been underpinned by phenomenology spread out over times, you know, enormously longer than, uh, so perhaps that's the point that to really capture spacetime, to see in it adequate representation of time, it would help if we could see the thought of a creature on an enormously slower time scale than ours as something spread out over hours or days. I mean, I, I struggle. So anyway, so this is, these are the issues that arise with time, and they're especially interesting, um, vis-à-vis other topics in physics, and particularly in many worlds interpretation of quantum mechanics, because there is something absent there too, and that comes in particular with probability. The nature of probability, and that absence of something felt and is missing in the theoretical representation when it comes to probability, is rather parallel to the sense that something is missing in the representation of time. Forget about quantum mechanics altogether."

"Yes."

"You know, so, so it may be that there's something more fundamental going on, which is kind of like what you really is involved in buying into a, uh, a view from nowhere, as it's sometimes called, um, or, um, a, a god's eye view, as it's also called, um, a timeless view, a view that is not in time. And I think there's lots of debates in theology, aren't there, about whether God exists in time or or is somehow outside of time. And I, I, I think those debates are probably extremely interesting, actually, if you really drill down into them. Um, but, um, what quantum mechanics is adding, I think, is actuality is now missing too. It's just as the god's eye view is not temporal. It's not in time. Takes in all the time. Uh, the god's eye view with quantum mechanics is, it doesn't take in the actuality, the particularity that we think is also a part of reality. So, and the way it, it, it, it takes in all of the particularities in the way that the god's eye view takes in all of the moments in time."

"Let me see if I could summarize. So there's reality, then there's physical reality, and most people, most physicists believe physical reality is all all of reality. So let's just go with that for now. There's this physical reality here. Then there's the our models of physical reality, which are something like special relativity, QFT, QM, and so forth. Then there's our experience. So we feel as if we have some unmediated access to this physical reality. It doesn't go through the models. Indeed, we didn't come up with these models till relatively recently, hundreds of years ago. And with time in particular, is it the case? I like to deconstruct. I like to boil something down to its root to find out at the root of all of these problems with time. Is it that we feel as if there's a a privileged now? So that's one of the problems. And then furthermore, that we have a flowingness to time, a directionality to this time. And is it also a third that the past seems fixed and then the future also seems open? And all three of these seem to be in conflict with the models of physical reality. So, so we seem to be accessing this this true reality, whatever that means, and then we have our models of of the reality, which are far more explicated and accurate, and we have this tension between our phenomenal consciousness, our experience, and then the models of it. And I'm just wondering, firstly, is that correct? And then do all of those tensions boil down to those three? Or potent, or maybe even these three collapse to two or something like that? Like, what is at the core here?"

"Yeah. Yeah. Yeah. No, no. Perfect, um, uh, summary of the, as it were, the, the problem situation, because we, we are in this problem situation. We, we are all of us, uh, perfectly at home. [clears throat] There's nothing alien about our world. And yet, when we try to intellectual process of discovery and analysis, um, attempt to construct a more accurate representation of the world, we find that it, it has to somehow nullify the personal perspective. That the way to get the thing off the ground is we've got to remove our personal perspective from the picture. And in a way, that's just fine. I mean, it's a bit like drawing a diagram, you know, you can draw a picture drawing of your garden without saying where you are in the garden, right?"

"I mean, it'd be even more lovely to have, as in the Harry Potter, uh, map where you have the map showing where everybody is when they're moving about. But anyway, so you have a map of your garden, and it does, you don't have to put yourself in it, and that seems fine. We can deal with that. But now think about a period of time and envisage that period of time without putting yourself in it. Then you've got a problem. The point about putting yourself in your garden is you put yourself in it as something local, spatially local. [gasps] The point about putting yourself in a duration of time is that you put yourself in it as something temporarily, temporarily local. Something with a specious presence. [laughter] particular 50 milliseconds, which is now, you know, it, it has that immediacy and urgency. Well, that's fine. So it does the place I'm standing in that has immediacy and urgency. Uh, and, and what is the difficulty? The, the difficulty is to say again, because I can represent and understand or represent or conceive of the three-dimensional space without myself being situated in it, but I struggle to understand the duration of time without me being situated in it. I can only see it as somehow something frozen and lacking temporal characteristics. I, if I, if I imagine putting a little spot on that four-dimensional, it's a four-dimensional structure, but we can represent it as one temporal time and one spatial dimension. So you can do it in two dimensions. So you can't just label a particular point in the time axis and say, "That's now," and that would be like labeling a particular point in the garden and saying, "That's where I am."

"Mhm."

"So you can do that, and as long as you are not, as it were, [laughter] the point is, I can put many other dots there too, which would be myself at different times."

"And the problem then is that it seems arbitrary. It seems that, um, there's something inadequate about the representation, [sighs] the spatial representation of time to capture its sense of flow. So anyway, it's very, um, I mean, it's, it's, it's one of the philosophical debates that, um, has persisted for a very long time, but it became particularly urgent with relativistic physics. That's the point about you. You, as it were, make it no longer even an option to declare there to be a privileged moment of time for the whole universe. There just is no reasonable moment of time that one could pick out in that way, because it would be a preferred frame of reference. I mean, this is all within a special relativistic context. When you go to GR, you also go to the real structure of the universe. And the real structure of the universe includes a microwave background. And the microwave background does very nearly pick out a unique global time."

"Right? The way to do it is you, at each point, you, um, you seek a velocity, um, in which the microwave background is isotropic, and you can knit together all of those points, neighboring points, each moving with velocities such that the, uh, microwave background is isotropic, and that picks out a foliation, the slicing of spacetime. The trouble is, it only does so up to certain approximations."

"What do you mean? You cannot get to precision in this way. You cannot get to a precise momentary time, instantaneous time in this way. Um, or you can make certain conventional assumptions and so on and so forth. So when you go to GR and the general actual structure of the physical universe, then the situation is different. Um, and there is even a way of theoretically changing, uh, this, the, the way the time works in here. But in special relativity, the theoretical way it works is that there's no privileged frame of reference vis-à-vis dynamics. So dynamics, which is all about change, does not acknowledge a preferred frame of reference. It's, it has a symmetry with respect to changes in frames of reference. And that [gasps] symmetry is what destroys the option, as it were, of of having a, a global three-dimensional space. As long as you had that, I mean, Newton, people like this, they thought they had a global three-dimensional space. Whatever the issues are to do with four-dimensionalism and are other times or other moments of time real, these are sort of philosophical questions, because they can work with the physics which is grounded in a universal three-dimensional space, absolute space. And it is this that was lost in the shift to relativity. Yeah."

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"Let me ask you a question, a slightly technical but ill-defined question, and hopefully you can make sense of it. It seems like space doesn't inherit these paradoxes of time. It seems like there's something special about time. Few people talk about paradoxes of some spatial dimension. Yet, in special relativity, we're supposed to be on equal footing between time and space. Okay? So if that's the case, whatever that means, if that's the case, then why can't you take a paradox of time, boost it to space where there is no paradox, solve it in a sense there, and then boost it back?"

"Yeah. So in other words, why doesn't space inherit the problems of time, or why doesn't time inherit the solutions of space?"

"Yeah. Excellent. So I think one aspect, fundamental one, has got to be that we can imagine a space that endures, and that we can visit every part of it as we wish. So that's not really a three, an instantaneous three-dimensional space. It's a space that endures. So it's already got additional structure in there. It's involves space as well as time, this endurance of the parts of the space. But granted that endurance of the parts, and just take the surface of the earth as it rotates around the sun. Um, we there have, um, continuity, and we can track objects on the surface of the earth and so forth. Um, as that is happening, we can visit whichever parts of the earth we wish. Yes."

"And that is lacking in time. So there's the obvious, what time is one-dimensional, space is three-dimensional. You're right that in Minkowski spacetime, there's a sort of fusion of the two, but this aspect remains that if you consider the analog of, um, an object moving through time, it is not possible to visit any point along that trajectory."

"Mhm."

"Whereas if you had a three-dimensional space that endures, you can visit each part of that three-dimensional space as it endures. It's a more complex idea. I mean, what I, what I, what we've just done [laughter] in a way, what we've just discovered is that as soon as you speak about space and time in terms of points, events, instantaneous three-dimensional shapes, whatever, things are reasonably clear. [sighs and gasps] As soon as you introduce things that endure, you've got a problem because you have to say, well, what does that mean for an object to endure, to persist, you know, not to sort of vanish in a puff of smoke? What is involved in that? And the answer is, well, resolve it into a bunch of timelike lines representing the points, all of the pebbles, if you like, of a beach. Consider each of those pebbles and give it a timelike line, a line that changes over, connects together events in spacetime, but is always timelike. So in the vertical part of the light cone, and look at the neighboring pebble, and the neighboring pebble, and the neighboring pebble, and what you've got is a congruence of timelike lines that are all roughly parallel to one another. They don't move about too much. The structure is preserved. So when you've got all of that physically in place, you then have a notion of a three-dimensional space that endures, which is everybody's idea. You know, I think of my garden, you think of your home. It's a space that, you know, doesn't suddenly disappear on you. Um, so, but what is involved thinking about it in terms of the language of four-dimensional events, spacetime points, is that you've got this congruence of timelike lines, all roughly parallel to one another, with many different characteristics that are relatively frozen. They don't change too much. If we're talking about my garden, it doesn't change too much from one day to the next. Of course, there's the change in lighting that comes and goes, and understood in that way, when I revisit and I visit every part of it, it's, it's me always going timelike, but I can move timelike and check out different parts of the big congruence of lines. This kind of needs a graph. A graph or two would be nice here just to depict it. So that's an awful lot that's in place which is behind our intuition that when it comes to spatial structure, we can revisit every part of it, and that when you try to do anything, there's nothing comparable like that that can be done with the timelike line."

"Actually, I'll place a graph on screen so people who are listening, if you have the video version, you can watch, and there'll be a graph of what was just talked about so that you can see the visual."

"Yeah. Yeah. Yeah."

"Okay. So when you say endurance, what precisely are you meaning? So, for instance, there's a rock. I'm, I'm in Toronto right now, and if I look outside my window, I could see a huge rock. Now, I could say, I want to get to that spatial point. So, I'm going to go. But then the question is, did I get to that spatial point when I hit the rock, or should I be in outer space and the Earth is completely past me to be in the same spatial point technically, because the rock was at a certain, is that what you mean, or is something different?"

"No, no, but what you've just done is absolutely, you pointed to two contrasting systems of coordinates. One system of coordinates is just spatiotemporal points, and that can be external to the earth as it orbits around the sun and everything else that is happening. That's fine. So we have a system of spacetime points, and you can refer the question, when I say revisit a point, you, you can express it at the level of a spacetime point, and then the answer is no. You never revisit the same spacetime point. It's like you never step into the same river twice. So you can't revisit the spacetime point. That is not what is meant. What is meant instead is a different system of coordinates. One where it's a set of rigid bodies. Think of it in terms of rigid bodies. Okay. Now, and made out of a mechanical, make it a mechanical system. You can think of it in terms of electric or mechano or something."

"Should I be thinking of these rigid bodies as actual powder instead then? Because if they're extended rigid bodies and we're using special relativity, well, wouldn't we have a contradiction?"

"Oh, sure. We don't, we don't [laughter] there are difficulties making sense of a notion of perfect rigidity in in relativistic, relativistic terms. We don't need perfect rigidity. Approximate rigidity will do just fine."

"I see."

"The point is that the coordinate system thus defined as persisting over time. We can then think of events as being labeled by position with respect to the special parts of this coordinate system. Okay? So we can use the special parts of that coordinate system, uh, to pick out different events at different times, and in particular, as we move around and later and later times, we can visit many different points of that mechanical system of rods. We can revisit all of those points, and in that sense, revisit the spatial points, but these are special points, not meaning spacetime points. That's the difference. Spatial points. It's a hard thing to properly with precision and clarity convey. It's part of why teaching special relativity is not so easy. It's becomes easier, in the sense, in terms of visual diagrams and equations, and you learn how to manage those, but in terms of how to really translate and express these ideas in ordinary words, it's more difficult. That's one of the things that makes philosophy of physics so much fun, and actually also, I think, makes it important to do philosophy of physics, because I think physicists can get away too much with merely relying on the equations."

"Expound on that."

"Yeah. Well, I mean, it's something that I've often found in discussions with physicists. As a lowly philosopher of physics, I can without damage, as it were, admit my ignorance of physics. I can say, 'Oh, I mean, I really, please explain, you know, and I'm really, there's something here going on that I really don't know.' And so the physicist present, um, will will enjoy, um, you know, explaining to someone who is admitting their ignorance. And it's great. But then what happens is the other physicist who's present finds that they don't agree with with the explanation that's being given. And then I, because I'm sort of know some of the game, can point out that, oh, well, that's not quite right, is it? And they find themselves having to discuss things that they mostly didn't need to discuss."

"Why don't you give a concrete example, a recent one? You don't need to name names."

"[laughter] [gasps] Well, [gasps] I mean, I, I think I, I probably, okay. [gasps] Yeah, it would be two people talking about, um, stochastic quantum mechanics, the dynamical collapse theories, and having very different views of how localization works in that context, um, and when pressed, but with, you know, sort of saying, 'Look, I really don't understand it, please explain it.' So then one will attempt to explain it, you see, but then the other person who was present, um, took a very different line, and I can't, I would have to give names, wouldn't I? Not go into..."

"[laughter] [clears throat]"

"But those sound like philosophers of physics. People who talk about stochastical collapse versus dynamical are already philosophers of physics. So what I was wondering is, are there examples of two physicists who don't consider themselves to be doing any philosophical work at all? Maybe they even looked down at philosophy, and then they were exp- one was explaining to you, and then you found the difference between the the other two physicists."

"Yeah. No, absolutely. Both of these guys were physicists. They were not philosophers of physics. These were physicists. These are physicists, and they were talking about certain kinds of diffusion equations and how they relate to certain conceptual questions that I was interested in. I, I could express ignorance as to how the mechanism worked. It's something that we do all the time in ordinary conversations. Very often we express false ignorance, as it were. You know, we, we pretend to be ignorant. Actually, we know quite well what's going on, but can you explain that? You know, and so somebody does, and then they find themselves actually not in their comfort zone. I mean, this is also a part of it, taking yourself out of your comfort zone. I mean, any conversation about the measurement problem in quantum mechanics has this result among ordinary physicists. Um, they, um, are not really prepared to discuss it in a very serious way, usually. Usually, and they typically have not thought very hard about it. So, um, that's an example of of how it's possible to get away without talking about something very fundamental. So it is not an example where I can say, 'Oh, tell me about what happens in quantum mechanics. I don't know anything about it,' because [laughter] I mean, you know, that's, they know me well enough that's why I'm there. I know something about quantum mechanics. So that's not an example I can give, but it's an example of how physicists will not engage in a conversation. Whereas as soon as somebody who is genuinely, you know, without guile, saying, 'Please explain, how does it work?' Um, then they will, interestingly, differ one from the other. Um, actually, I mean, somebody else who made the same point was Steve Weinberg in one of his last essays, I think, on the foundations of quantum theory. It was in the New York Times book review, I think, around 2017, something like that. Um, and he said, 'I'm not as happy as I once was about the foundations of quantum mechanics. I used to think,' and he went on a little bit. He said, 'The trouble is that the experts don't seem to agree,' and, 'and that's a bad sign.' [laughter] [gasps] So there is this failure to agree, but very often one doesn't want to have a, a conflict. You know, 'I disagree with what you are saying about such and such in physics.' If it's a matter where it affects the equations, it becomes a real dispute. This isn't something that can amicably be nodded through. You somehow got a problem there. So if, but if you can keep it at the level of conversation, then one can pretend that it doesn't really matter."

"Kurt, here, note that if you'd rather listen to tow, we're on Spotify, iTunes, everywhere with a podcast catcher. You can just search my name or theories of everything. And also remember to hit subscribe. Earlier you talked about when I asked, 'What is time?' There was a quote from Feynman that it has to do with how long you wait, which to me is more about duration of time, but not what is time itself."

"So unless all investigations into time come down to speaking about duration, then to me, that doesn't answer what is time."

"Absolutely. Now, of course, you could also ask me a counter question like, 'Kurt, what, what do you mean when you ask, what is an NX?' Like, what precisely are you looking for? Are you looking for equations? Are you looking for me to point out something like a cup? I don't know how to respond to something like that. I actually don't know precisely what I mean when I say, 'What is time?' But I'm going to pose that question to you once more. What is time? And how do you think about it?"

"[clears throat]"

"Sir, I can say why, if you're interested, I can say why I'm dissatisfied by duration as a full accounting of time, but I can't tell you what a full account would look like that would satisfy me."

"Well, let me, let me just give an honest answer. What I think time is, I think time is precisely this geometrical structure. Um, and it's not any old geometrical structure. It is one which is threaded together by certain dynamics, and it is represented in this four-dimensional picture, and I think that is the correct characterization of the reality. [sighs and gasps] Um, and I think that its seeming lack of the, where is the fire in the equations, or where is the passage taking you from one moment to the next, um, is precisely a price you've paid for a perspective, a view from nowhere, a god's eye view. Uh, so it's very alien to ordinary sensibility, 'subspecies tonitati' as Spinoza put it, too. It's, it's very difficult to inhabit that view. So there's a tremendous challenge involved, I think, in really trying to grasp what is, uh, spacetime, if you're serious about it. Um, and equally, if you're trying to understand, uh, cosmology, the universe as we understand it, um, it's a very [laughter] I think character-affirming but life-changing experience to properly engage with the enormity of the universe. I mean, the only way that I can put it is in terms of one of many experiences of seeing the night sky. I mean, the night sky, I think, on a properly dark night, which is so rare, but a real starscape is one of the most extraordinary experiences we can have. I hope you've had such an experience, because it's profound. Um, so we can't live with that all the time. We go about our daily lives, you know, we forget about it all and so on and so forth, but it's there, and this is the reality that we live in. Um, something similar happens in the foundations of quantum mechanics. Well, let's get there. So, what does all this talk about time, the problem of time? Time as a parameter versus time as an operator and so on. What does all this have to do with quantum mechanics? And also this talk of quantum mechanics, shouldn't it technically be QFT? Like, why are we always talking about interpretations of QM and not interpretations of QFT? Is at the end of the day, shouldn't it be QFT, or even QG? But those are a variety of questions I, I throw out to you."

"Yeah. No, they're great set of questions."

"Simple ones."

"Yeah. Yeah. No, I think, but, um, I mean, absolutely, and I was very much took that direction in my own early career. I thought, look, if we're going to be serious in doing philosophy of quantum physics, we really ought to be looking at relativistic quantum field theory. So I spent a lot of time studying relativistic quantum field theory. And it's enormously rich and extraordinary and so on and so forth. But actually, I think the most important lesson to come away from it is that, um, physics is scale-relative, and the physics that is adequate at one scale will not be adequate at another. Um, and they are not in tension, these different, uh, scales, and the physics that is going on at them are not in conflict with one another. Um, and in particular, what is of importance to, uh, things like conceptual questions about probability, um, and the problem of measurement in quantum mechanics, um, is all can all be articulated at low energy scales. So that doesn't mean you get rid of relativistic stuff altogether, that you've always got radiation, photons, and that's actually playing a very important role in foundations of quantum mechanics. But it does mean that you can be pretty sure that if you've got an analysis that works in non-relativistic quantum mechanics, that is not too dependent on well, structures that we don't also find in relativistic quantum theory. So as long as we, uh, have that confidence, and we can do those checks to make sure, then we can make do with the language of relativistic, of non-relativistic quantum mechanics reasonably well. And what has also been found is that you can express or probe many profound-looking concepts in quantum gravity [gasps] using elementary concepts of quantum theory like entanglement. There may be great progress to be made, um, not through looking at the standard model [laughter] and, um, you know, high energy phenomenology or structures that are of interest or relevant in high energy regimes. No, we do not learn about quantum gravity so well like that. We better learn about it by applying quantum concepts directly to something like, to recover something like spatiotemporal concepts. I think, and I, I find these, these questions to be absolutely fascinating, but they give further emphasis that the answer isn't to go immediately to relativistic quantum field theory, unless, of course, you're of the view that people are making arguments and claims in the non-relativistic regime that cannot be recovered in relativistic quantum theory. I mean, a good, a good example of that, in fact, is localization of particles. You know, in what sense can particles be localized within regions? Quantum states, can they be localized within regions? Answer, yes. [sighs] Pose the same question in the relativistic case. Can, um, particles be localized? The concept becomes problematic. We don't mean localized in spacetime. That would mean they came into existence and went out of existence. So we mean localized, spatially localized. Can they in relativity be spatially localized? And the answer is, well, not in a covariant way, not in a way that respects the symmetries of relativity theory. So there's, there's something problematic about the localization concept. If you ground too much foundational work in quantum theory on on position, and even the existence of a position operator, because of there is no position operator in relativistic quantum field theory. No covariant position operator. As long as you are, are careful not to rely too heavily on that notion, say, of a position operator in some interpretation or foundational questions in quantum mechanics, um, then you can be fairly confident, I mean, that's, as it were, deals with that potential problem. [clears throat] Other potential problems arise with things like numbers of particles. So you can have interpretations of quantum theory that really drill down into the actual number of particles involved, and if each particle lives in a certain number of degrees of freedom for each particle, let's just imagine one degree of freedom for each. You've got N particles, N degrees of freedom, and that's your physical arena, as it were. And you conclude, oh, well, then you've got a 3N-dimensional configuration space. Um, and a quantum state is assigning a complex number to every point of a 3N-dimensional space. That's what the quantum state is doing for those N particles. And you can then ground an interpretation of quantum theory on from that structure, recover objects in three-dimensional space. Go from this complex field in three-dimensional space down to objects in three-dimensional space. So fine, that's a whole program that you can engage with [laughter] um, and perhaps successfully carry through. Um, but it's not going to be workable if you've got change in particle number, right?"

"And indeed, what then is the arena where particle number is changing? We, we know how to do it in quantum field theory. It's called Fock space. But, um, the people that are doing this kind of game aren't really interested in Hilbert space structure, and Fock space is a Hilbert space structure. They, they're interested in something more attuned to philosophy. You know, that philosophers can get their head around. Okay, we all understand what it is to have a scalar field in three-dimensional space changing over time. So great, you know, we're happy and confident with that. There's lots of interesting philosophical questions to be raised, but never mind, you know, blah, blah, blah. So now looking at quantum mechanics, can we not construe it as just a, now, a complex field in a much higher dimensional space? And this is a way of understanding all of the peculiarities of quantum mechanics. Okay. So, um, but then the question is, how do you carry that through in a situation where particle number is changing over time? I mean, it's, it's the same with pilot wave theory, or de Broglie-Bohm theory. You have a, a reasonably adequate non-relativistic quantum mechanics in terms of hidden variables, local, but they give you the same probability distributions, um, as does non-relativistic quantum mechanics. So this is a great achievement. Fine. Can it extend to the relativistic domain? Work in progress, [laughter] you know, but, but then of course, it's been work in progress for a very long time now. And actually, if you look at what's going on in pilot wave theory, um, what are giving you the guidance equations, or the, the standard guidance equations, we introduce the additional variables, the hidden variables, the point of configuration space as it wanders around, which is the motions of all of the, the hidden variable particles, um, is, is just the integral curve of the probability flux, the probability current, sorry. So it's got a very simple mathematical expression. The probability current is a, is an object you can build up out of the quantum state. Um, and it's like a vector field, and you can look at integral curves of the vector field, and each such integral curve is a de Broglie-Bohm trajectory. Great, brilliant. It's a very natural structure. So you can try to do the same in a relativistic quantum theory where you've got something similar, and the answer is, it completely fails. And one of the major reasons it fails is because of the lack of permanency of particle number and associated with it the so-called negative energy difficulty that is present in relativistic wave equations. But look, I'm, I'm sort of just going a little bit theoretical just to indicate how the examples, you know, where, um, you use non-relativistic theory and you're not doing it in a proper relativistic framework, um, can be, you know, damaging. You can, you can be misled. I think I think one can be, take make wrong choices if you make use of structures that either don't work at all in the relativistic case or work in dramatically opposed to what you want them to do, which is roughly what's happening with you try to do with the pilot wave theory in the relativistic wave equations, what you do with the Schrödinger equation anyway, with all of that. I mean, let me just make a personal note. I spent years on relativistic quantum field theory, and I, at the end of it all, I thought, no, it's just not really of substance in addressing, engaging with the conceptual questions I'm really interested in, which can indeed all be expressed, or mostly at the non-relativistic level, but allow me photons. [clears throat] So that's my answer, sorry, rather long, um, as for why I frame things in terms of quantum mechanics. Um, but another answer is to say quantum mechanics is just shorthand for quantum theory, and it means to include field theory. I subscribe to The Economist. Their science and their AI coverage is among the best I've found anywhere. And I say that as someone who reads plenty of it. I'll give you some examples. They just ran an analysis on how attitudes towards science are changing in American politics and what this means for research and funding in scientific institutions moving forward. This sort of high-quality reporting is fantastic. They even covered how dark energy may be weakening over time. Now, if that holds up, it completely changes our understanding of the universe's fate. If you watch this channel, those are exactly the kinds of questions that we explore every week. I subscribe to The Economist because their science and their AI reporting regularly surprises me with how deep it goes. And they're also, of course, known for global affairs, both political and economic reporting. They are top."

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What are the foundational problems in the philosophy of QFT? Distinct from philosophy of QM.

>> Oh, wow. Well, particle localization, I think that's a very distinctive question and problem and one that I still think is unresolved.

Um, what do you mean?

>> Well, what one does without a covariant position operator? What does it mean not to be able to express that notion of a spatial localization um without privileging a frame of reference? And there's a way of doing it the so-called Newton Vner representation which is it breaks the Lorenzian symmetries. So that otherwise has many this is is a fascinating field. I mean it's a bit mathematical though I I mean is it the way you want to go? I mean I'm happy to make this more.

>> Oh yeah. Just so you know the the directed audience for this are philosophers in physics and math and computer science professors, researchers, academics. So it's quite technical. For instance, you could talk about Coleman Mandulla or Ree Scherers theorem or whatever you like as technically as you like.

>> Okay. I haven't understood that. I don't understand that.

>> Okay. Well, I've been doing a bit of handwaving previously, haven't I? Well, do me give you one example of what happens with Newton vign representation. You find that the vacuum ceases to have the rish leader property. And this is, you know, really very interesting. And the reach leader property itself in the Mikovsky's base vacuum the covariant or invariant vacuum um is is rather extraordinary and you know it is the case it seems that by local operations you can approximate any state that you want in the whole space of states you know it's a sort of what on earth I haven't really expressed it accurately enough so it it is something that I think has led to a great deal of bafflement as to what is quite the right thing to say about it and is connected interestingly with Newton Vner localization as [clears throat] is the way that the complex fields are represented in wave equations the covariant wave equations the complex numbers that are used there have a different role from the complex numbers used in a Hilbert space representation we're used to writing down clang Gordon equation durac equation um uh Yangmill's equations equations for QCD QED and so forth um and um the complex numbers that occur in those they are as it were fixed numbers [laughter] which do not change um in the dynamical evolution of those fields. Those numbers are not the complex numbers used in a helper space representation in terms of particle number and arguably not in any Hbert space representation at all. But what is used in the Hilbert space representation is something else. A decomposition of covariant fields, these covariant fields >> into positive and negative frequency parts. And that deco decomposition then works so that each of those parts uh can function so as to have positive energy and contribute to total number. Now the way that representation works is a non-local operation with respect to the the complex unit that occurred at the covariant field level. The complex numbers in the Hilbert space representation um are non-locally related to the complex numbers used in the covariant fields and it's for this reason that they cannot be a position operator in relativistic quantum field theory. Again, if you do a Newton vignner representation, these things essentially go away. What you do what you do in the Newton VNA representation, if you were to construct fields local using the complex numbers in the Newton vner representation, they would then be non-local fields understood in the standard Loren covariant representations. So I mean this is just a whole dimension of understanding or analysis of well look the major question that stars you in the faces of philosopher physics is you can do [clears throat] statistical mechanics non-relativistic quantum mechanics thermodynamics GR really with absolute you know it's not so hard after two or three years one or two years of graduate work but to do anything serious in relativistic quantum field theory it's much more I relativistic is like you fall off a cliff. [clears throat] So, you know, there's a mathematical depth and profundity to relativistic quantum field theory that we still haven't understood properly. I think the fact that there demonstrabably there does not exist a non-trivial relativistic quantum field theory satisfying the Whiteman axioms.

>> Uhhuh.

>> That's amazing. You know it's amazing that these actions are so constraining that it seems they have an effectively unique solution as free field theory. I mean relativity is partly involved. Relativity and quantum mechanics go together beautifully. I mean the dur equation is I think the greatest work of art in in mathematical [laughter] physics. I don't think that I do find it it's has a cathedral quality of beauty and elegance.

>> Why? [sighs and gasps]

Well, it's I think it's I think it's it's difficult to characterize what is beauty, but I think one feature of it is the way that you understand the equation can be greater or less profound. So, if you understand the equation in terms of various symmetries, it acquires an elegance that no partial differential well some random partial differential equation doesn't remotely convey. It's already in the symbolism and the notation involved as a unification of a whole system of partial differential equations. I mean theory of geos is exactly the same.

>> So how is this related to many worlds?

>> I mean yeah because they are related. I mean because I think that the multiplicity of moments in time we kind of got our heads around it. We understand that there's a sort of systematic way of understanding it that makes sense. uh and we're not there yet with the many worlds. We we have yet to have a way of of managing articulating this multiplicity with the same sort of familiarity um and confidence and competence that we have in dealing with linear times as it were. Look, here's where it exists very much in our ordinary lives. It does exist in terms of probability, ordinary notions of probability. I mean, if you think about all the contingencies that happen in our lives, that's what we're managing managing the many worlds in a in a way, you know, all of those contingencies carry with them all of the contingencies that did not take place.

>> [sighs]

>> The very notion of of contingency, highly contingent is what so much else that is not the case. So we live with that and it's deeply shocking to us. I mean, I think I find it almost [clears throat] insane that from the beginning of the universe, this utterance of mine would be deductively arrived at deterministically [sighs and gasps] along with all of the minuti of our present existence with all its craziness and insanity. that all of that was somehow deterministically built in from the beginning of the of the world or was somehow necessary. No, I cannot believe it. It was not it is not necessary. So this this is one of a multiplicity of possibilities. We think of a multiplicity of possibilities. And that multiplicity of possibilities is just vast and mind-boggling. And mostly we try not to think about it. But we do have to think about it when we're making choices relating to events in the future. Um, which are among the possibilities. And the the choices we have to make are really how seriously do we take these various possibilities. you know, there's a possibility you'll be run over by a bus the moment you step out of your front door. Heaven forbid. So, there's a possibility that you'll win, you know, a fantastic lottery. And I don't know if you're a betting man. Um, and if you do, then you would be enormously fortunate. So, there are these possibilities, >> but what gravity do you give them? What weight do you give them in how you live your daily life? And the answer is we you we're all of us extremely adept in as it were correctly estimating risk. We mostly are really good at avoiding risk. We we most of the time as long as there's not wars happening and crazy stuff going on that and we we very reliably will you know for years after year after year we'll [laughter] we'll manage to negotiate all of the you know potentially disastrous things that can happen in ordinary lives you know so we're very good at handling risk and that goes with having a sort of [sighs and gasps] kind of primitive theory of everything which is common sense reasoning, a sort of an Aristotilian way of uh you know analyzing the world or understanding the world and so forth. And there's notions of causation and propensity and dispositions. There's not quite the notion of probability in there, but we can fairly easily put it in there. And once it's in there, that really then starts to make it all rather clear what's going on. We have to negotiate amongst probabilities all the time. [gasps] And that negotiation from an Everettian point of view is is exactly an understanding of branching structure. And branching structure from an Everettian point of view is the source of the many worlds. It's by virtue of the unitary evolution of the quantum state having this branching structure and with nothing else, no other added input that we take it seriously not just as representing possibilities [sighs] but as representing actuality.

>> Mhm.

>> Of which the actuality that we see the branch that we are in located in is just one.

>> Just a moment a quick question. You say the branch that we are in now in the many worlds in your particular view would it not be more correct to say the branch that you are in that the even the wei here is somehow approximate and it's going to diverge and

>> I I no I well that's a very interesting question um I I think the the right answer to it is it comes back to this issue of is consciousness localized I mean the point is that you and I as we talk are able to exchange signals at the rate of you know a few hundred or probably several thousand bytes actually um a second sorry a minute probably I what what is the data transmission between us it's fairly engaged and it's fairly integral to the conversation that we're having that there is this time for me my thoughts oral impact you hear think you respond I think all that that cooperative effort which is this conversation [laughter] it's true is taking place over several thousand miles um But uh it might as well be understood um as a common observer. As long as we're not going to bring in localized quantum systems that we're looking at, uh we can coherently treat ourselves as a sort of extended observer and [clears throat] as such speak of as inhabiting one everin branch or a common everin branch. This is an interesting question. I I I you it's I mean you know I think probably the quicker and this sharper reply must be you're right I am just in my Everettian branch you are in your Everettian branch but we are correlated and if you look at the branching structure that unfolds mostly we are tightly correlated with one another and in that sense we share a a common uh exchange of ideas and so forth. But if we were to start doing experiments, quantum experiments, you um in the US, me in the UK, um then we would maybe find some interesting from a branching structure point of view, uh some very interesting aspects to it that underly the appearance at least of Bell um non-locality. So this sort of negotiating with contingency. Now in a sense you could say look it's nonsense to suppose this is uniquely somehow comes up with the everchin quantum mechanics. It would arise in pilot wave theory or it would arise in dynamical collapse theory. In any of those series, you equally have to manage possibilities. And the possibilities that we'll come up with will be much the same regardless of which of those approaches to quantum foundations you take. And you know, yeah, sure, they're sort of right. But the point is, if you go to the theoretical point of view and you really look at what are the contingencies involved in the real world [clears throat] using unitary quantum mechanics, you're doing something that from their perspective is either just a mistake like in a dynamical collapse theory, it's just a mistake, or from a pilot wave theoretic point of view, a very incomplete description. So you know in a sense the perspective which just looks at this multiplicity this network of multiplicities and says this is what we ordinarily we we negotiate it in ordinary lives when we really look at it from the perspective of physics. We we we investigate it from the point of view of Everettian quantum theory. And from the point of view of Everettian quantum theory, what one world requirement is doing um is very [laughter] you know it's it's rather like with the block universe where you put in a point and say that's where you are. You're picking out a single one of the branches and you're saying that and only that is unique and all of the others do not exist. and with each branching event that all are cold but one and so forth. So the Everettian is taking seriously this this space um of possibilities as all actual and if that's the case then there is no contingency to reality any longer because all of the particularities exist. What seemed extraordinarily contingent was that this unique particular world as it is now, this instant in time should exist. That seems something really too specific. [gasps] Um the multiplicity in the Everettian quantum theory would return us to something much more. [gasps] It's not that all possible scenarios exist, most not all. [clears throat]

>> [sighs]

>> Um, but that there's a a I mean, if I can just put it in terms of a probability distribution over these possibilities and the ones the really crazy ones have zero probability going to zero or close enough to

>> Yes. But if it's close enough to but non zero, then it does occur.

>> Yeah. Well, um I think it's it's Uh [laughter] it's it's a difficult question. I mean I think it depends partly on how you actually and analyze probability. A recent approach that I've been developing which is analyzing probability quantum probability in terms of frequentism involves looking at uh decompositions of the universal state into uh microates of equal amplitude.

>> Okay.

So with such a decomposition you then examine any proposition or property or projection operator and you can pose the given the quantum state the total quantum state from an everin point of view and you can ask the question um given such a decomposition which diagonalizes that projection operator what fraction give the answer yes to that property what fraction the answer no you can do that with finite decompositions of the total state and if You do do that and you look at projection operators that correspond to things like records of multiple experiments radically in disagreement with the Bourne rule. So take a projection onto that.

>> Mhm.

>> And you can never give it a non-zero probability.

>> Okay.

Or rather, in order to give it a non-zero probability, you've got to these microates, this decomposition of the quantum state into microates has got to uh be so insanely detailed that you're looking at distinctions of difference in amplitude comparable to the amplitude of the very low bulner violating branch state. So look, I mean this is a bit of a complicated thought, but it's a conception probability that makes it you will never you can never as as it were see the real extremes in terms of any finite analysis on any finite analysis probability in terms of a finite expansion of the state. [gasps and sighs] You will not see the very low amplitude branch. It doesn't mean it's not there. This is one of the interesting features of this whole framework.

>> Yes. um there's always going to be a shredding a cat state a state which is in shredding cat state for the projection operator or the property of interest. So the general framework of this we've rather jumped into this haven't we without perhaps a bit of introductory remark but I just wish to give it as an illustration of how the extremely low amplitude scenarios may not have quite the consequence they usually thought to have. I mean look another aspect to it all even if there did exist I mean one takes it as straightforwardly there exists people staring in the face the huge born rule violating statistics and all of their experiments have been have been functioning properly and hasn't been confected is not a putup job and they're staring at that in the very next second with enormously high probability they will see things go on as normal with vulnerable compliant probabilities and so forth but then of course you can play the same game again and again and again. So you can always isolate and I think perhaps another way of answering the question is a little bit like the Boltzman brain scenario which is also a serious problem. I'm not suggesting that therefore one should just forget about it [laughter] but I'm saying that this may be very related actually to Bzman brains but in again in the perspective that I'm offering you will not even see the probability for the Boltzman brain. you will not see the probability for the because it cannot be captured in a finite analysis.

>> It's called finite frequentism.

>> Yes. Well, finite frequentism is um well the theory comes in two or three initially I presented this is a theory which only involved finite decompositions of the state. Um and as such you can approximate the bonal quantity extremely closely and the nature of the approximation is not that with very small probability it differs. No no no no it's just it's it's giving you a number that is very very close [clears throat] to the borne rule quantity. It's completely categorical. It doesn't involve potentialities or propensities or anything like that. It's just the number of these microates that fall within the projector acting on them as value one. But one micro state usually almost always will be indeterminate for that projector will be a shredding a cat state for that projector. One of those microates must be um so what you've really got is you've got a set of microates that give the answer yes to some question. You've got a bunch of microates give the answer no to that question and then you've got one or perhaps more than one. We can come on to that in between the two. The ones that neither give the answer yes or no where there's a superp position of the projector with the one answer and the projector with the other answer.

>> And for the person who's listening and they're wondering, well, what does yes and no correspond to? Does that mean so and so obtains, in other words, is actualized when it's a yes and doesn't happen or doesn't occur if it's a no or what?

>> Yeah. Well, think of it. You're we're trying to give probabilities to properties. These could be properties attached to times. So this could be the property of displaying spin up on a register of an experimental apparatus. So it's just a a physical property in that sense. So the question becomes what is the probability of that property? The question of whether that property actually exists or does not exist. Is it hypothetical? Is it you know we don't have to engage with that at the moment. We're just, you know, assigning probabilities to different properties.

>> Sure.

>> Propositions as well. Properties are in one one correspondence to propositions here. Propositions are often understood as yes, no questions. But all equally can be just assertions. You know, if I assert something to be the case, you can turn it into a question. You know, the sun is shining. You can turn it into a question. Yes or no. So this is very much how um properties, propositions and yes no questions have been handled in foundations of quantum mechanics for you know there's a whole tradition of it the logical algebraic tradition and so forth. Anyway, so um on on this picture, this is a way of assigning probabilities and you've got you've got the total number of microates say n and all. Let m of them say yes to the projector um and then the residue the remainder say no except for one. So what this means is you've got a lower bound um to the probability of that projection given by the number of microates that assigned it the answer yes

>> and you've got an upper bound an upper bound because you've got that bunch of microates that assigned it no and then you've got the one in between a sort of gray zone a no man's land if you like. So what's happened is that rather than having probabilities as rational numbers, we're getting probabilities as small intervals of real numbers, an upper and lower bound. Okay.

So this construction is it's quite an interesting construction. I hadn't come across it before, but it existed before my own work on it. Um so-called imprecise probabilities. I called them interval probability. It's a whole branch, recent branch in probability theory. But the but the point now about the the gray zone the interval you know you've got a lower bound and an upper bound is it's a bit like instead of a point probability being a point it's like a blip.

>> Yeah. [laughter]

So and it can be a bigger blip or a smaller blip. And the bigger the blip then the less informative the probability is because it's bounded by zero and one. And if you got a blip that just takes up the whole of the 01 interval you got no information at all.

>> Yes.

So the point about the very low probability stuff using the B rule is it's always in the gray zone.

>> Huh. Okay.

>> The amplitudes the the precision that would be required to pick it out cannot be increased beyond that gray zone. I mean look it needs a further argument to uh I should

>> well I think at this point what I'll do is I'll place a link to your paper/papers on this topic on screen and then in the description so people can find out more. Now I want to spend the next say 20 minutes or so just going over many worlds and I want to see what attracted you to many worlds. What keeps you there? And then many people not many worlds many people who are listening they think of many worlds as just one theory one interpretation but there are different sects in a sense in many worlds. So there's a Wallace many worlds type interpretation. There's a Saunders type interpretation. There's a Shan Carol type interpretation. And so firstly, what is it that unifies all of these such that they can even be called the same sort of umbrella of interpretation? And is it merely are you seeing it as a delightful place in the sense that it is a has properties you want that you that you wish a theory an interpretation had? or is it merely the best of a bad lot and you're just saying well I mean if my competition is pilot theory I'm going to go with many world like you understand what I'm saying

>> I do I do so um it's very interesting set of questions um perhaps just a comment on the last one um it surprised me the sort of cacophony that emerged over the last 20 years if I may call it that of of very different views it seemed to me because I suppose my own understanding of Everett was worked very well worked out in my mind and appeared to be harmonized beautifully with the development of decarant histories theory. I thought this would be immediately obvious to anybody who studied these ideas. So I really expected durian history based ever interpretation to to be widely in in pursued actually investigated um examined and so forth as to its various conceptual challenges blah blah blah blah. So, but that didn't happen. [laughter] What instead happened is that most people were very distrustful of decoding histories altogether. Quantum history is formalism. It seemed something somewhat alien to ordinary quantum theory and you know blah blah blah blah. So, and instead pursued um their own imaginative reconstructions of Everett's ideas, many of them without regard to decompense theory at all. So I did find this odd but I mean David Wallace and I were you know highly um at one in almost through throughout this period up until when he left Oxford in whatever it was 2014 or something. So there there wasn't much of difference between us. I suppose what differences there were concerned. [sighs and gasps] I mean there were certain issues bound up with the nature of divergence and branching and certain metaphysical questions like that that I was probably more insistent on than was to his liking. But in the meantime, uh there did arise uh these other very articulate proponents of views, pictures of many worlds that involved certain ideas that to my mind are completely antithetical to anything that is in Everett or in the development that came out of Everett. For example, Sean's picture um that wells are in one one correspondence with the spectra discrete spectra of the energy operator. I find that very different from um anything that I have gathered from Everett. I want to take um another point of view that some people have been just interested in which is a sort of a hybrid of Everettian worlds which nevertheless are able to interact with one another and this involves modifications perhaps to quantum formalism and so forth.

>> Sure.

And yet further that there has remained to my mind remarkably and I do find it um surprising a tenacious view that Everett himself was not really committed to many worlds and indeed that there is a different reading of his work that should indeed be an open-end inquiry in its own right. Well, okay, I'm fine with that. I mean, I'm all for opening worry as it were, but I do think um it's fairly obvious that ever indeed grasped the the many worlds aspect to his work even though it's not his favored way of framing it and indeed rightly I think many worlds doesn't quite get at what was special about the ever interpretation. I mean the evident interpretation it was special about multiplicity. So in that sense many worlds but the multiplicity did not have to involve worlds. I mean they they could have involved just trajectories of particles and multiplicity of trajectories arising with the same degrees of freedom. So these would be trajectories as in sequences of localized quantum states um all in a superp position. Okay. So that is aian thinking and you don't have to think of those things as worlds. Right.

>> So, it's just to illustrate how the core of Everett's thinking was was not so much the global cosmological blah blah blah blah.

>> Wait, I'm not sure. Sorry. I'm not sure how I can think of those as not worlds. So, what would be the interpretation? What what else would they be?

>> Well, they're very small worlds. [laughter] No, I I I agree with you. You know, of course, aggregate and have very large numbers of particles involved and then you've got superp positions of them doing very different things and then well, heavens above, aren't these like many worlds and [clears throat] you can blow it up and look at the actual solar system and so on and so forth. I mean, it's quite interesting when you do look at the actual solar system from an everetting point of view. In fact, I think this is a little known truth that the Everett's most detailed model that he gave of um how to rec well of how to interpret the shreddinger equation, the evolving shreddinger equation.

>> Uhhuh.

>> Was in terms of something very close to a model of the solar system. I mean in fact had he had he given he he was he could have done that. He could have put it in terms of a model of the solar system, forgetting about chaotic orbits and moons of planets like Jupiter and and so forth, but just just have four or five very large masses in gravitational interaction and have them in a superp position of well have them initially in localized states and then bring about um a superposition of two of those localized states. And that would involve some very serious collision between um the particles involved. So it would be non-trivial. But what would then develop would be a superposition of motions each of which would be perfectly akin to a classical mechanical system would exactly satisfy exactly would very high degree approximation satisfy classical equations of motion. And Everett more or less indicated that it was in this last section of his thesis. I think it was the last section called supplementary topics if I remember it correctly and he only sketched the idea of how you could um recover classical motions from quantum states. So this is the key to thinking you've got to it's not the degree of freedom that gives you the handle on tracking an object over time. It's quantum states of that degree of freedom that may evolve in such a way and in involve a superposition of others that you can track them over time because they obey approximate equations and you've got very large masses well localized the rules are the rules of Newtonian gravity or Newtonian mechanics and they're very precise in those ways but the rules could be slightly different and you can get out Navia Stokes equations and you know hydrodnamic equations um and Brownian motion and so forth. So you get out these rules for how states behave over time in accordance with these equations. But it's always states in superp positions with other states similar evolving over time obeying definite rules. So that's the Everett interpretation and it it he never put it quite like that and indeed the way he put it in his 1957 paper the rules in question were the measurement protocols you know prepare the instrument measure spin um put the spin into value in memory reset measure again that's a sequence of steps and those are those are the rules you can then analyze the quantum state evolving unitarian in terms of a sequence of states satisfying that protocol the same rule the protocol rule satisfying the protocol rule but differing in as depending whether it's a spin up state or a spin down state so it was sort of there it was present in ever but it it wasn't very vivid I mean to give another example of this and I do want to push it because I do think it's quite central to the ever interpretation

>> if you consider um radiation so so the classical linear theory just like shing equation maximum equations everything's fine so you've got the electromagnetic field and it's evolving over time just like you got the shing equation and the state quantum state evolving over time now imagine what is happening in that electromagnetic field evolving over time when you switch on two torches well what happens is you create excitations in the electromagnetic field in the vicinity of the torches where you switch them on which then propagate through the electromagnetic build okay and they propagate in accordance with well- definfined rules and they gradually spread over time and so forth and it will depend on the medium and various other aspects to the the whole setup as to what exactly happens where and when and what time intervals. Okay. But now imagine somebody comes along and says look what we've got for each instant of time of the electromagnetic field is um we've got a superposition of a beam of light here and a beam of light here and at the next instant we've got a superp position beam of light here beam of light here and so forth. And you end up to think so what we've got in this evolution is the development of a superp position of a beam of light pointing in two different directions at once which is a contradiction.

>> Mhm.

>> Therefore you cannot have this beam of light pointing in two directions at once. It makes no sense. Okay. To which the answer is um no. It's not a beam of light in a superp position pointing in two different directions. It is two beams of light. And I think it would be a madman who would deny that on being pressed. You know, the same goes with mobile phone conversations going on all of the time. What's happening in the electromagnetic field? Well, there's lots of mobile phone conversations taking place all at the same time. And it's not that there's a superposition of a conversation taking all of these different [laughter] words.

>> I see what you're saying.

>> All of the words at the same time. Right.

>> Okay. So you know this sort of indicates what is the Everett interpretation is this preparedness to recognize now looking back at the quantum state this preparedness to recognize the sequence of of stories. I've got a story here and I've got a story here. Um and that amounts to two stories. It's not a single story saying two things at once. No, it's two stories.

>> Okay. So let me ask you about ontology then in your mind is what is ontological or the most ontologically real is it the universal wave function

>> say bosons are not a part of this picture because I know you have some issues with with bosons versus firmians firmians may exist to you is the universal wave function what's ontologically real or what about reduced density matrices are those just as real or or what I say real I mean

>> much like how earlier in the conversation you were saying is If you look around, you would say that the tree is real and that this is real and so on.

>> Right. Right. Right. Yeah. Yeah. I mean, um I think the I think the issue is if we're looking just at low energy quantum mechanics, uh and you've got a reasonably stable system of degrees of freedom particles in question, and you're neglecting things like phonons and you certainly photons. And you then consider what would be the quantum state for that complex system. You've already suppressed a whole lot of stuff. So it will not be an adequate representation of the physics. But you can imagine that maybe this is a world in which I don't know what phonons somehow have been so suppressed and maybe there is no radiation. So it's a sort of pretend world and we've just got that quantum state unitarily evolving. Now, does that quantum state unitarily evolving in that way give us everything that we want? Well, I think the answer is yes. It's an adequate ontology. Um, but it's not a prospicuous one. And because it's not even that adequate because it doesn't account for all of the other bits and pieces. So then you could go to well let's take the standard model and we'll take the quantum state for the entire Hamiltonian as defined by the standard model and we'll look at this representation of the lens group in terms of this and we'll consider that evolving state which will now be I mean even fox base is not really adequate to this the whole thing is too it's over precisified it's made precise in inappropriate ways but anyway you could imagine that somehow being unitarily evolving that is the mistaken thing to say although in practice it's a very difficult thing to really make sense of um and that would be an adequate representation of reality but again it would not be a prospicuous one so what isuous um it comes back to the sound waves or electromagnetic waves you know what is a prospicuous representation is not that I I give you a snapshot of the electromagnetic field at each moment of time that may be adequate for you to you've got reality there, but it's not a prospicuous way of showing you what's going on or getting you to understand what's going on. So, what is needed in that showing an understanding of what is going on in quantum mechanics is the pretty well the whole Q number structure too. You need the quantum operators, you need the algebraic structures, you need the group representation theorem above all. So you need all of that in place in order to really it's not just all in the Hamiltonian. You've got the unarily evolving state and it's just all locked up in the Hamiltonian. It's it's you you are really articulating local structures to whatever discrimination you want depending on what you're interested in doing and you will need all of those Q numbers. One way of framing it is that the the Q number structure um or Q numbers and the whole mathematical technology involved in that is what allows you to give structure to the quantum state. The quantum state has that structure but what expresses it or articulates it is this technology of Q numbers and what I what I find particularly prospicuous about it of course is projection operators and sequences of such so quantum histories but that's also just a coarse graining of you know many different types of of fin and path integral approaches to quantum theory so it it's a fairly structured way of of breaking down pretty well any quantum theory there relativistic quantum field theory as well into something like quantum histories with a measure over quantum histories and the the particular kinds of histories involved and the kind of structure that they have and the probability relationships that are thereby involved and whether decoherence is going on and if it isn't then maybe we got to give up on probability is something involved in the dynamics and so forth so so I know that sounds like a bit of a big fudge I'm saying yes but not really you know yes there's only the quantum state, but no, not really. Because we need all of the other stuff in order to understand the quantum state, in order to express it, in order to write it down, make sense of the representation that we've ended up with. It's a real collaboration.

>> I see.

>> I think philosophers do see some some, you know, just some base level where everything else, you know, it just you've got changes in that the primitive ontology sort of position. you there's some primitive ontology which is clearly stated and a dynamical theory just tells you how it changes over time end of

>> are you an onic structural realist

>> oh yes yeah pretty well yeah I mean sure this structural this structural kind of representation of reality I take has been giving us reality yeah I mean I think it got taken in various ways that you know I don't think were very helpful. I mean one of the aspects to it is what is the role of mathematics okay and as long as you think that oh mathematics is really ultimately something more like set theory and the structure in question is a set theoretic structure [sighs and gasps] then I'm not a structural realist at all. So part of what I took to be important about structural realism um was that it really was the mathematics that came first. I have pages and pages of questions for you regarding many worlds theory. We're running out of time and hopefully in this world in this branch we can speak again just on that topic because even a single question to delve into what precisely something means would would require maybe an hour for just one question and I have a variety of them. So those who are watching and listening if you have further questions feel free to leave them in the comments. We can get to them in the next time. I do have a question which you can tell me has a quick answer or doesn't and then if it doesn't have a quick answer we can answer it next time. So speaking of set theory in ZFC or in ZF sorry sorry the axiom of choice is equivalent to Zor's lema. So I'm wondering in your finite frequentism if equal amplitudes giving way to equal probabilities is that then just another way of saying the borne rule in the same way that axim choice in zf is another way of saying zor lema.

>> Yeah. No I don't think that's right. Um, and partly because I think I would put it in terms of a postulate. And the postulate is very simple. If there's such a thing as physical probability at all, and that's highly contentious. There's lots of people in many world approaches who um who deny that there is any objective probability other than agents, rational agents and so forth. Um, but if you think um that there is anything like physical probability, then let it obey the following postulate. You cannot change X by an action a physically allowed action on Y when Y is disjoint from X and the action preserves disjointness throughout. So it's an extension of a kind of locality principle. It's a bit like the bell locality. You can't change the probability physical probability of X by messing around with Y which is remote. And I'm saying you can't change the probability of X by messing around with Y which is disjoint. Now if you have that postulate then that forces equa amplitude states to have equal probability. So any analysis of probability that gives you probabilities of states if it satisfies that postulate must give you equal probability to equal amplitude states and that's translating disjointness into orthogonality. So that's a way and that's a strict derivation of the born rule from a physical principle but it's a new physical principle and heavens people may find that a step too far but notice that it's a principle that is obeyed by the born rule as long as it's not being supplemented by anything like state collapse dynamical collapse um and as equivalently as long as the actions that are relevant and permitted are unitary. So when it the original posture you cannot change the probability of X by a physical change to Y that is disjoint. It's it's got to be a physical change something you can actually do. Um and if and this is unitary quantum theory all physical changes are unitary then you force that equal amplitude states must have equal probability. I mean as as a mathematical derivation I doubt it's that interesting but I think conceptually it's interesting but well we'll see whether others find that. So yeah it is my hope but look I'm I'm sorry we've we've overran haven't we and I sort of got the sense that we were getting sidetracked in so many interesting ways you know but um but I think that's probably what you do and you did it brilliantly. But I I worry that the result is going to seem really a bit haphazard. You know, visiting one topic after another discursive discussions into reality is another name for this channel. So

>> I should have realized that better. [laughter]

No, it's been lovely. So I do hope for another occasion.

>> So my last question, my last question before you go,

>> okay,

>> is do you have any tension in any of your world views?

>> [snorts]

>> that are rigorous. So let me give two examples to circle back. One was that we have a feeling of a now and we have a feeling of a moment in time moving and a feeling that the future's open not determined and so forth and then we have our more articulated physical models of the world. Okay. Okay. So those two seem to be in tension. Now one way of resolving that tension is to say that the experiential one is more of an illusion. So you just somehow dispense with that in favor of the physical model. And then the other way is to just say, well, I have to live with the tension. I don't know how to resolve which one is more correct. It seems like at least this one is more able to be tested in the lab and get results, but it doesn't mean that my experience is wrong. I don't know how to make them compatible. So that's that's attention. That's living with attention. Hillary Putnham. Now the second example is Hillary Putnham said that he goes to temple on Saturday and he doesn't know how to make his conception of God compatible with his philosophy and his view of science. And he just said that's just a tension that I have. I don't know how to reconcile that. And then he had some platitude verbiage of well I I find this tension productive. I mean I I don't buy that he finds that tension productive. I feel like that's something you have to say when you have attention because you have to make the tension turn it into something positive otherwise you look foolish. But anyhow, do you have any tensions between your world views?

>> Well, absolutely. And they what they what they do is stretch me very far. [laughter] I mean, I do find myself stretched and it's not always pleasant. I mean, it's it's I'm not in my comfort zone as a result. That's how the tension is expressed. Um but I think the stretching is mostly productive and it's not to be somehow alleviated by ignoring one or the other. It is to work with it and it's potentially fruitful. So I mean look you could one it's possible to make progress on very limited things which may yet really illuminate much more greatly. Um and I'd give an example partly as a result I suppose of of of teaching lietsian um metaphysics but um I found liance's methodology a very interesting way of resolving the mind body problem and it was entirely devised to that end. I mean that is why liance devised it. It was the only way he could see of resolving the mind body problem. [sighs] said all of the arguments, Chinese cells, Chinese rum argument, things like that and Lebn [clears throat] saw it all very clearly. So there is a quantum version of the manodology find it extremely interesting. It it is more like it's not [sighs and gasps] quite idealism but

It is a framework of thinking in which representation, or in Lenitian terms, perception is the fundamental, and that really the world is built up out of perceptions. So this is the mandology. Um, and there's a quantum version of it where these are really correlational structures, but I mean, you could call them perceptual structures if you wanted. I mean, it's, I think there may be an appropriate shift of language in that way. And so might it be a way of seeing the physical reality really very differently in quantum mechanical terms? I think, I think there may be some possibilities of that. So that would be an example where if something like that were to really make sense, I think it would be a shifting of a lot of the things that are under stress. Whether, whether it would change the more fundamental of them, I'm not sure. I, I suppose I doubt it. But I was sort of struck by one thing. Bertrand Russell, when he read the monadology, he more or less said, well, look, in accordance with this, there'd be no such thing as absolute simultaneity. Something like that.

Huh?

That was in 1904 or three or something.

Right. Right. So, you said something interesting about stretching.

Actually, I just started training at the gym. So, I have a trainer now. Okay. And his name is Satchet. For, for those who are watching, I recommend him. He's at Goodlife at Young and King in Toronto. Fantastic trainer. Satchet. Anyhow, he trains me. I always tell him, "Kill me." When I go, if I've slept well, I tell him, "Kill me." And he loves to hear that cuz he's just push me hard, push me hard. And then I started to get some pains and they started to be relieved by going to stretch therapy, by getting stretched, like actually getting stretched. I could stretch myself, but sometimes other people stretching me, they can do much more than I can do.

And so just that, just you talking about the productivity of stretching reminded me of a more concrete physical instantiation of this at, at the Goodlife here in Toronto. So [laughter] if that can be good for your body, then the stretching is interesting for the mind. So I like that that use of

tension.

Yeah.

Okay.

Yeah.

Thank you so much.

Well, thank you so much. Really, it's been a great pleasure. Really enjoyed it. I've also got a sense that you had all kinds of really hard questions to ask. [laughter]

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