This Physicist Says There Are Millions of You — Sam Kuypers

Is time real? What is time? Each moment of time is equally real. Past exists. The future exists. When we measure it, what we’re doing is getting entangled with the particle. We see one version of it, and another version of us is another version. You’re finding yourself in these different universes. The whole of physical reality looks a bit like a collection of worlds. Future in the past already exists. You’re just in a slice. We are using a common sense idea of what a measurement is, but really we ought to have a theory of what a measurement is in quantum mechanics. This then lead people to naturally think that everything is deterministic.

We are here in a beautiful room in Oxford, and speaking of Oxford, my understanding is you worked with David Deutsch, sort of like the father of quantum computing. How is it to work with him? Like, how does the scientist like him get to all these results when, you know, there are like thousands of other scientists who don’t have all the achievements he has had in his life?

I can’t tell you how he did what he did. All I can say is that it was very nice working with him, and I think the main thing important is to have the right ideas about certain topics. My view of the history of quantum information theory is partly that a bunch of people were interested in taking quantum theory seriously, and that includes David, but it also includes people like Shilpa Sark and Charlie Bennett, who were also interested in kind of a pure view of common theory, as I put it, or maybe many worlds view of it. And although it’s, I think, it’s less explicit with them, it still leads to certain conclusions in the foundations of physics and also in cryptography and a bunch of other topics, where there are ramifications to taking quantum theory as the literal description of the world. And one of them is that if you have classical theories, such as classical cryptographic theories, and you think that classical physics is actually wrong, well, then you might want to revise what you know about cryptography in light of quantum mechanics and stuff like that.

Yeah, pops up, and I think that is also true of David’s work.

Yeah, a lot of it is, I guess, the general scientific values, things like being curious, things like being critical of the theories that we have.

Yeah, trying to push them as far as we can, stuff like that. I think one of the problems he has been encountering is that a lot of the scientists, like in quantum mechanics, who are focused, you know, mechanics, are not necessarily willing to just relearn everything from scratch through this new, like, framework that is like Heisenberg mechanics. And so, yeah, there’s little growth there as a result, and it’s basically like one of the only ones who is like actively in it. So, yeah.

So the funny thing is that it’s actually quite an old framework, and people are not willing to—

Exactly, yeah. It’s kind of an old framework, and people are not willing to—

Exactly. Ironic.

Yeah, I mean, it was, it was invented by Heisenberg.

Yes, when quantum theory was invented.

Yeah, almost 100 years ago now. I think 100 years ago, actually. And with that, I also feel like there are other reasons people do not take it on board. I mean, I agree wholeheartedly with Charles, and we, yeah, are in agree. So the topic that this is about in particular is something called locality. And local realism is the view that, roughly speaking, there is a real world, and you can divide it into parts, and when you do something to one part, you can’t affect another.

Yeah, if you want to, there’s no spooky action at a distance.

Exactly, yeah. So if I, if I have something here, if I have a cup of water or something, yeah, and it’s spatially separated from another item, then doing something to the water should not affect whatever else, yes, there is that I’m considering, unless they’re interacting directly at a particular point, like at the same location, say, or through something mediating the interaction. Then those are cases in which you can interacting, they can affect each other. If that’s not the case, they shouldn’t affect each other. And quantum theory, it’s generally thought that does happen. So, but I can explain the general setup if you want.

Yeah, let’s go.

Yeah, so, for example, there are cases where two systems are what we call entangled, and that means the following. You can have a quantum particle with a certain property, say we call spin, which is a little bit like having a magnetic moment, but it’s slightly more subtle, but that’s all right. So the particle can have spin up and down. So it can be found in one of these two states. It can also be found in which case some of the time when you look at it has value up, and some of the time when you look at it has value down, seemingly at random. And now it can be the case that there’s another particle with the same property, up and down, which is correlated to that of the other one. So we have two particles. Each of them separately can have the property of having spin up or having spin down.

So they’re kind of like linked in a way.

Yeah, it seems like. So it seems like they’re linked, but that is, that already seems to suggest that there could be such a thing as protection. So the thing which can happen is that when it couldn’t, it can be the case that the system, so both particles, are in a superposition such that when you measure particle one, let’s call it particle one, let’s go the other one particle two, if you measure particle one having spin up, then the other particle also has spin up. And if you measure particle one with spin down, the other particle also has spin down, and vice versa. So if you measure the second particle and spin up, the first particle has been up, etc. And that seems very bizarre because the outcomes appear to be random. So if you perform a measurement here, you, so I have a particle, you have the other particle, the entangled pair. We go very far away from each other, and then we perform measurements.

Yeah.

Now I’m, I measure my particle, I see it up, and I, now it seems like I instantly know that you also have up, because that is what entanglement tells us.

So it’s as if there was faster than light communication in a way.

Exactly, yeah. It seems like. So it’s not communication because I can’t message anything to you.

Yes.

But there does seem to be some kind of influence which is superluminal.

Yeah, and that’s very bizarre. I mean, one of the reasons it’s bizarre is that means, so in, it seems to be instantaneous description that we have. That means that in certain frames, it’s going backwards in time. So we know that in special relativity, simultaneity is not absolute. If something is instantaneous in one frame, then there’s another frame which the effect propagates backwards in time, which doesn’t seem physical at all.

Yes.

So this is an issue. And now there is actually, in many worlds, actually offers quite a nice solution to this, which we can get into as well. But maybe, I don’t know, maybe we should first talk about what many worlds is in particular. But so this is the, this is one of the issues, yeah, that I and Charles and other people like, yeah, and David also have worked on. We’re in the minority in holding the opinion that actual spooky action at a distance, so this thing that we just described, is not normal.

Yeah, not something that actually happens.

Yeah, I think, I feel like even most people do not really see it as an issue anymore. So most physicists are just accepted and just buying the bulletin and say, well, that is just what con theory tells us. It is what it is.

Yeah.

So I think that’s part of the reason why Charles and other than I also have experience like slightly harder. Paul also, by the way. Paul, yeah, I forgot to mention him, but he had some very important work.

Yes.

And I feel like that’s why we have a somewhat hard time arguing with people that actually we should, this is an issue and we should find a solution.

Yes.

And then we think we have a solution. So it’s kind of twofold. Whereas usually you can argue with people, well, there is a problem and this is what we think the solution is. And here we have to say, well, it’s not just that we have a solution. We have, first of all, we have to convince you that there is an issue here.

Yes. Yeah, people are just, are not willing to accept it in the first place.

Yeah, I think that, as I said, I think they bite the bullet, and they’ve just accepted the world. That’s what one of these tells us about the world is very counterintuitive. Yeah, I mean, so be it. Yeah, and, but I think that’s not quite satisfying.

Before we get into many worlds, my understanding is, so you were previously researcher first at Oxford, then at the University of Montreal, which is where we met.

Yes.

Now your researcher for Conjecture Institute. What does that entail?

It basically means I do the same kinds of things, but I work for Conjecture Institute. So I’m still writing papers. I have a couple of papers in the pipeline.

Nice.

And my day-to-day life looks very similar, except I now work for a Conjecture Institute, which is very exciting.

And what is Conjecture Institute?

So Conjecture Institute is a private foundation which sponsors people, including you and me.

Yeah, you’re another.

Yeah, I mean, at least it sponsors me and you. I know that you are also a fellow. They try to fund neglected topics in physics and other areas of research and thought. I’m mainly there for the physics, and so there’s a bunch of topics I care about that they care about as well, such as many worlds, such as locality, these kinds of things. And it’s a very exciting next step in my research career. So it’s fun. I enjoy it.

Let’s get into many worlds.

So, cool, yeah.

What is many worlds? Like, many worlds, like this is actually in jail. Like, what is even this war like?

Yeah, this is a weird thing. Yeah, it’s a very interesting thing to think about. Some of the assumptions which are not problematic, which are widely accepted, and we think should just be part of the theories it’s properly understood.

Things like what we just mentioned, systems can be in superposition of different states, they can be entangled with other systems. There’s a few others, like the way they evolve should be of a particular type, should be what is prescribed by the Schrodinger equation, for instance. And so those are all fine, those are all commonly accepted. And then there is one axiom, one assumption, which is both ill-formulated and problematic, namely the measurement postulate, which basically says that when you look at a system, which is in a superposition– - Let’s clarify, what is a system? - Well, it could really be anything. I mean, we can make it more, so it– - It could be like a particle in a box, for example. - It could be atoms, yes, a particle in a box, could be photons, could be things like that.

So usually, these systems are small, so we’re considering small stuff. - Yeah, quantum, yeah, microscopic, yeah. - Yeah, so, although I always think that is, the theory is usually presented as being about small things and it’s actually not. I used to be into solid state physics, which is physics that tries to describe, for example, how metals work and how– - Yeah, materials, yeah. - Yeah, exactly, how insulators, a whole range of materials, function and behave and what their properties are. And that whole area of physics is completely dominated by quantum mechanics. Classical physics doesn’t really play much of a role in that study, and those are macroscopic materials.

Like, we couldn’t have stable objects if it wasn’t for quantum mechanics. We know from classical physics that atoms, classical atoms are not stable. So if atoms aren’t stable and things are supposed to be made out of atoms, they’ll think you can’t have a classical description of a table consisting of atoms. That is, in any way, stable. They should collapse immediately into like the nucleus or something. To clarify, we mean specifically in this case, things like maybe an atom, maybe electrons, maybe photons. It could be in a box. It could be experimenting with them in some laboratory setting. - Yeah. - And they can have this property that they are in a superposition.

So maybe the electron is in a box somewhere and it’s in two places at once. In the box. So it’s like roughly either here or there in the box. And now the measurement posture tells us that if we were to look at the box, you would never see two electrons because it’s just one electron. You only see one of them. - Yes. - And you see one of them with some probability determined by the exact configuration the system’s in. It could be 50/50. It’s always a simplifying assumption. It could also not be 50/50. But again, it’s just for simplicity. Sounds fair enough.

Sounds like, okay, well, that could be an assumption. And the first thing that I find annoying is that we don’t really tell people what is this measurement. A measurement is a thing which we know from common sense what that’s supposed to be. It’s like, if I look at a thing in a box, that’s a measurement. - Yeah. It’s not a preservation. Yeah. - Yeah. It’s not something that the theory tells us. - Yeah. - That is to say, we are using a common sense idea of what a measurement is. But really, we ought to have a theory of what a measurement is in quantum mechanics.

And that’s not the case. So we have kind of an undefined term within the axiomatic system that is quantum mechanics. - Yeah. Like, for example, it’s not defined. Is it when it reaches the eyes?

Or is it in the brain? Does it work if I use a camera? It’s also a form of observation. - Yes. - Yeah, that’s very unclear. Yeah. - Yeah, exactly. Yeah. So there’s a bunch of things which it doesn’t specify. - Yeah. - And so that should ring some alarm bells. And the other thing that’s kind of strange about it is that this is a kind of dynamic. So the system, so this is called collapse. So we measure a system, and it is in a superposition. It’s set to collapse into one of these two states, or it could be multiple states, but it collapses to a singular state.

And… That appears to be irreversible. - Yeah. - And that is not compatible with, for example, the kind of evolution that the Schrodinger picture, or the Schrodinger equation prescribes. So there’s already a bunch of issues, even just in trying to say, trying to formalize what the axiom is supposed to say. The many worlds interpretation or explanation says instead that collapse doesn’t happen and that we can derive all the results that we typically get out through this axiom, just from the typical kind of evolution that the Schrodinger equation prescribes. So we’re essentially able to explain more with less. We have fewer assumptions, but we can explain the same things.

And also we have a richer picture of what’s actually going on in the world. And what that actually entails in, for example, the setting we were just talking about, is that when there’s these two possibilities, so the electron… The electron is in two places at once. Then when we measure it, what we’re doing is getting entangled with the particle so that we see one version of it. And another version of us sees another version. So version one of me sees electrons here, another version of me sees electrons over there. And they don’t interact with each other generally. So it appears as if the other version has disappeared from view.

But that’s all something which comes out of the Schrodinger equation essentially. - What collapse says is, so we have this particle in the box. It’s maybe in like two states, like it’s in a superposition of two states. So it’s kind of like in this mix of states, like it’s unclear which is which, but there’s like a, you know, as if you were to like, you know, mix several liquids, for example, like it’s sort of like in a mix of both states. And when we, in the collapse approach, like the traditional way of thinking about it, when we look at it, the particle will collapse into either of those states.

So states like A and B, for example, and then, and that’s why we see only A or B, even throwing the equations, you would see A and B. - Yes. - And then what many worlds says is when I, when I look at the particle, I get entangled with it. So I get, you know, connected. And that leads me to see in one branch, we’re getting through the branches, the states, the particle in state A, and in the other branch, in particle B, and many worlds says both of those are equally real. Is that correct? - Yeah, they’re equally real. And so one of the benefits of this is that we don’t have to assume different kinds of dynamics when we’re performing a measurement.

We also don’t have to assume what a measurement is. We can derive a model of what a measurement is from the theory itself, so that we don’t have these undefined words that we’re using in setting up the theory. And it also kind of makes sense of what it even means for the particle to be in a superposition in the first place, because in the conventional view, it’s kind of unclear. It’s not clear what it means for the particle to be in superposition. It just means it’s either here or there kind of potentially. - Yes, and there’s a probability and it’s, yeah. - Yeah, so it’s vague.

Whereas in many worlds too, it just means, well, there is, the particle really is in two places at once and it’s behaving exactly as if it is actually here and actually there at the same time. And that is true for things which get entangled with it as well. So it gives a more explanatory framework for understanding how quantum theory actually works. So there’s both the formal benefits and then at the level of the explanation, I feel it’s much more intuitive. So those would be some of the reasons why I like theory. - We have those two branches now. And what we’re seeing is that those are equally real.

What does this actually imply? Like, where are we getting to? So there are many worlds then? - Yes. - Kind of like bringing it up, but. - Yeah, it’s in the name. You’ve got the name, right?

Yes, so there are different worlds in which, for example, as I’ve said, it could be small things which initiate the branching. So I see particle being here instead of there. And consequently, that kind of quantum effect gets amplified. And now I have a record of where in my mind, I have a record of where the particle was, as does the environment from then on. It could be that this information is somehow leaked to the environment. So maybe to make it a little bit artificial, but let’s say that I have decided if I see the particle in state of being here, then I’m gonna cook some dinner.

And if I see the particle here, well, then I’m gonna read a book. And then after a little while, you’ll see that the kitchen is messy in one branch and it’s not in the other. And so these small differences in the state of an initial, like just a particle, just a microscopic particle, within that case has been amplified up to cause a mess in the kitchen, in one branch and not the other. That kind of thing is constantly going on. So there’s constantly things are in superposition, like through natural processes. And then they happen to get measured, where measured really in this case, could mean just information about where the particle is, gets encoded in the environment, gets encoded in me, in the mind, or in other things.

And from then on, these kind of differentiating effects percolate throughout the environment. So there’s more and more records are in the environment of where the particle was. And that means that more and more of the world is different, one branch than it was in the other one. So you get these branches, which are initially different only in small ways, and then they become different in larger ways. And because of that, it’s harder and harder for them to reinterfere, to influence each other again. - Yeah, so it is a good way to think about it, that it’s sort of like a bubble growing of entanglements.

And so let’s say, you have the particle in the box. So I’m the observer in the laboratory. I’m looking at the particle. I get entangled with it because the information reaches my eyes and my brain. And then just all of me. And then maybe I will open the door and the information will reach the neighborhood around it, and then eventually the whole world. And so, but there is a separate version of me that saw state B, like the particle in state B, and that for which the same happens. - Yes. - And so eventually that’s the process we call decoherence, where eventually we’re getting split.

And those branches are becoming more and more separate universes, which is, you know, what we’ll call multiverse sometimes. - Yeah. Yeah, so the whole of physical reality looks a bit like a collection of worlds that we know from classical physics. - Yes. - They have other properties as well. So for example, one of the things we haven’t mentioned yet is that particles and other quantum systems can interfere. And that’s one of the primary ways that we know about quantum mechanics. - Yes. Yes. - Interference effects happen in, for example, the double slit, which is this famous set up where you have two slits with the backdrop being a wall or some detector, and you send a particle through one at a time, could be photon.

And then when you detect where the photon is on the screen, after having repeated this experiment in series, so you send one through and two at a time, and you detect it, and then you send another one through, and then you detect it again, then you find that there’s an interference pattern, which wouldn’t be there if you closed either of the two slits. - But what do you mean with an interference pattern? - It means that there are places on the screen where many photons arrive and places where barely any or none arrive. And they kind of, they come in fringes. So it’s like, you know, a lot, less, a lot.

And then again, much fewer or none, a lot. And that keeps going on. And that pattern is called an interference pattern because the… What’s happening is that, for example, in the case of the photon, if you send it through the two slits, it actually goes through both. - Yes. - And we’ll get to why that’s the case. It goes through both. And then there’s an instance of the photon going through the left one and one through the right one, which then end up affecting each other. So they end up interfering, which just means they affect each other just as they might. It’s a little bit similar to a classical way of theory where if you have waves, one of the things that can happen is they can interact and then amplify each other.

Or if you have a trough and a peak, then they could cancel. And that’s what’s going on at the screen. So some versions of the photon are amplifying each other and some versions are interacting to cancel each other. And as I said, if you do that many times, each time you’re sending a single photon through, then you get this interference pattern. - Yes. - And the kind of the bizarre thing is if you close up, close either of these two slits, then it disappears. So that indicates that something must be going through the other slit to interact with the thing that goes through the one that isn’t closed, so to give the pattern.

Otherwise you would still see the pattern if only one slit were open, which we don’t. Again, the many worlds explanation is there’s two versions of the photon. There’s only ever one photon, but the photon can consist of multiple instances or versions or however you want to call it. And they can affect each other. They can interfere. So we know from seeing the pattern that the photon was in two places at once. It took two paths at once. And that is generally, that’s one of the main ways that we know about quantum effects is that we see interference happening in what are called generally interference experiments, of which this is an instance. - This effectively means that there are billions if not an infinity of versions of you and I in these different worlds.

Where are these worlds? Like, wait, are they somewhere? Like, can I go there? Like, can I visit? - So, firstly, there’s not gonna be infinitely many. There are reasons to believe that there is a bound on how many different universes there can be. - Interesting. - That comes from, it’s the so-called Bekenstein bound, and it gives an argument for why in any finite region there is an upper bound to the amount of universes that can be there. - Is that due to the number of interactions possible? - It is due to the amount of information you can store in a system. So this is maybe a bit technical, but the way to think about it is the different universes, it should be the case that they do not interfere with each other.

They could in principle, but we generally want them to evolve independently. For that to be the case, they have to be orthogonal, which is a technical term which I don’t think I can explain. But we generally want the universes to be orthogonal to one another. - Separate in a way. - Yes. - Yes, very distinct. - Yes, yeah, they don’t have overlap. And the number of orthogonal states that you can have in a finite region of space is determined by the maximum entropy in that region, and the maximum entropy is bounded. - Yes, so there’s a limit capacity of how much you can store, essentially. - Yeah, exactly, yeah.

So, Bekenstein’s bound gives an upper bound, and that is a little bit technical. I think that’s the most technical thing we’ve said so far, but the main idea is there is some bound which determines things like how much information you can store in a system, and also how many universes there can be. - Where are these universes? - An enlarged space that we call Hilbert space. - Yeah. - And in some sense, the universe is, it kind of depends on our views of space and time, but in some sense, they are here in space. - Yes. - But they are in different points in Hilbert space.

So Hilbert space is this larger space in which quantum states live. - But it’s a mathematical space, right? Like, we’re not talking about like, necessarily like physical space, like going to the moon or something, yeah. - Yeah, no, it’s a mathematical space, and it’s not the same thing as string theory having multiple dimensions or anything. - Yes. - Everyone who does quantum physics is in agreement that there is this thing called Hilbert space in which the states of a quantum system live. I think the best answer I can give is they live in Hilbert space. Another way of saying it is that they exist in some sense, they are just as real as we are, because we also live in Hilbert space, but they’re just not where we are, otherwise we would see them. - So there’s no way to interact with them?

Like once they’re like orthogonal, as you said, once they’re like distinct, like once the branch is distinct, there’s no way to interact with them, right? - So maybe we should say that we have all these different terms and they basically mean the same thing. So we have the terms branches and universes and things like that. - Universes, worlds, it’s all the same thing. - Yeah, it’s all basically just parts of a quantum system that don’t really interact with other parts anymore and evolve as if the other parts aren’t there. And sometimes also we have the connotation that we mean, they are obeying semi-classical operations of motion.

They look like they’re classical systems effectively, is what we might also mean, but at any rate, so just to clarify some terminology, it’s always the case that they can interact in principle, which is very hard. - It’s increasingly unlikely. - Yeah, it’s increasingly difficult essentially, yeah. Like if, for example, in the photon experiment, you’re reinterfering a single photon and that’s quite a manageable task. It’s much more difficult to make a couple of photons interfere with one another. So they would interfere. Well, maybe that’s not the best example. So let’s say that the photon becomes entangled with the environment somehow. Then it’s not just the photon that we need to reinterfere, we also need to reinterfere the thing that the photon is now entangled with. - Yes, so we need to undo essentially our changes in a way. - Yeah, and the reason for that is, roughly, that for interference to happen, the photon really should have taken both paths.

So one instance of the photon should take one path and the other one should take the other one. When the photon becomes entangled with the environment, it essentially even traces of where it was. And that means that there’s now a definite answer in that world where the photon was actually at, which is information that you can’t have if the photon literally took both paths. You can’t say, well, I know that the photon took this one, but also it was in both. - Yes. - That’s logically incoherent. So instead, the photon shouldn’t have any information. I think it’s sometimes called which-where information. That means that either you erase that information before the interference happens, or the thing that was entangled with the photon also interferes with the other version of itself so that information gets erased.

But somehow you need to get rid of that. And it’s hard because of course the photon and other parts are constantly leaving ever more traces in their environment of where they were. So that means you have like an exponentially cascading number of things to undo before you can re-interfere the thing again. - One of the, I guess one of the criticisms, I guess you could say, or like the common objections, let’s say, of many worlds is this idea of energy. Because, well, if you’re telling me that we’re now going to have all these parallel, quote unquote, universes, where does the energy come from?

So we’re just like duplicating each time? Like, how is this actually possible? - Yeah, so the funny thing is that intuitively that seems very strange. And actually, if you look at the mathematics, it’s much simpler. - Yes. - So in the case of it, so people would expect that somehow many worlds violates energy conservation. And in fact, I think generally is a measurement which does, because in… In the many worlds picture, you only ever have unitary evolution and that preserves the energy of a system. - What is unitary evolution? - So we only ever have things like Schrodinger type evolution. - So like linear, clean line. - Yeah, reversible, all of the good stuff. - Yeah. - We like this, we like this. - It’s simple, it’s beautiful. - Yes, and that preserves the energy of a system. - Yes. - And measurement does not have that, and so you’re not guaranteed to conserve the energy. - Yes. - Anyway, so there’s one note.

The way I view it, and I think lots of people who are into many worlds view as well, is that this view of splitting the universe is not right. So it’s not like you are splitting the universe. It’s more like every system already is a collection of instances, all of which are completely identical initially, as kind of like a slap of the multiverse. And then they, some of them become distinct. So like maybe first it’s this many, and then some of them, you know. - So we’re constantly like slicing the same block of energy in a way. - Yes. - And so, yeah, so it remains constant across all of those branches, but the branches themselves may have a smaller slice. - Yeah, exactly, yeah.

So it’s not like you’re making mass. - Yes. - It’s not like, oh, you, something happens, something entangled, something entangles with another thing, and now there’s more mass because there’s more universes. It’s more like there was already a certain amount of them, which we can have a general measure of. - They were just identical, and now we’re splitting them. - Exactly, now they’re, yeah. So splitting, or they’re branching. - Yeah, slicing, yeah. - All of that just means some portion of them now do one thing and some portion are different and do another. - Yeah. - So we’re not creating any mass or energy in the process. In fact, that is exactly what’s conserved. - A lot of your research is about time.

Is time real? Does time actually exist? What is time? - So some ways of viewing time or some notions of time don’t exist and others do. And so I always like to say time doesn’t exist. I mean, it’s a phrase that Julian Barber and others have used and he wrote a very famous book, which I highly recommend called The Convention of Time. And the argument in that book is that our general conception of how time works is unnecessary. And to explain what that means, the convention of view is something like there is this parameter, this number which represents time. Maybe if you’re thinking very classically, then it’s the same everywhere in the universe.

Of course, that can’t be true because of special relativity, but never mind that. And it’s kind of like God’s clock. Like God has a clock. - It’s like his wristwatch and like, yeah. - And we cannot see it, but we’re invoking it because things are changing. So things are changing and that means that it’s a different time now. The actual things that we use to measure time, like your watch, for example, and other things, like the solar system is a thing, which in some sense is a clock. And the different positions of the planets in the solar system tell us what time of year it is.

Maybe the seasons are different, things like that. Those are things internal to the universe which tell us what time is. Whereas the kind of Newtonian view of time is slightly different. It’s that, as I said, it’s more ethereal. There is some notion of time. - Across everything. - Across everything. It’s not something that you can necessarily see, but it’s there. - And it’s fixed everywhere. - It’s fixed everywhere, yeah. - It’s the same clock for wherever you go. - Yes. Again, that aspect of it was already no longer tenable, with Einstein. - Yes, relatively, yeah. - But in some sense, we don’t need to concern ourselves with that so much.

The main thing that’s weird is just, it’s not something that’s observable within the universe. We’re just saying that there is something that’s changing. So this unobservable property of the universe is changing. And because that thing is changing, things around us are changing too. So people are getting older, but that’s because time’s passed, things like that. And as I already kind of alluded to, the thing that we actually use both in everyday life and in science, and I think also just nature more generally uses it in some sense, is time that is like internal to the universe. So clocks are a way of seeing that some change has happened.

So there’s some kind of periodic process in a clock and you’re counting how often that process has happened. And that tells you, okay, so the clock says X numbers of periods have elapsed. I know now that other systems have changed as a result as well, because they’re constantly undergoing their own physical processes. Like as your clock is ticking, your wristwatch, the earth is also moving around the sun. And we know that a certain number of ticks on your wristwatch corresponds to a certain distance traveled through space by the earth, things like that. And it turns out even classical physics, as Julian showed, that you can replace this ethereal notion of time, this kind of like, okay, the backdrop against which things happen, which is itself, this is not really clearly physical.

You can replace that notion of time, which is time as it exists internal to the universe. Julian’s way of doing it is very pretty. What he does is he says, you assume you just have celestial objects and they are obeying Newton’s equations of motion. So the kind of gravity making them interact and things like that. And then adding a simple assumption that the energy of the system is equal to zero. So it’s conserved and it’s equal to zero. It turns out that their positions at any particular time are unique. So you can kind of use their positions. - To figure out what the time is. - There exist counterparts of this in quantum mechanics, which are very nice.

And I work on the quantum counterparts in particular. So I think both in Julian’s view and in my view, as you rephrase that, I think both in Julian’s model, although not on Julian’s view, you get a kind of many-worlds picture again, because to really get rid of time is not quite enough to say that time’s internal, because you also, if you still have to say, well, these different configurations are somehow coming about and ceasing to be, then you need this external number again, which is telling you what the state of the system is. So instead, to really get rid of time, one thing you can say is well, all these configurations of the celestial objects, say, exist together.

They all exist. And the passage of time is just the change of these celestial objects relative to you. But there is no external time to take into account. So they already exist. The future and the past already exist. You’re just in a slice and you can see what time it is, because the configurations are unique. - The way we can measure time is by observing the change in the world. - Yes. - And there’s no fixed time. And so each of our internal clocks in a way, like– - Well, so that doesn’t quite follow. So again, that becomes more dependent on what specific models that you’re talking about.

So in Julian’s model, time is fixed everywhere, just by the position of all the celestial objects. - Yes. - So in that sense, again, it’s the same everywhere. - Yes. - And, but the main thing about it is that it’s internal. It’s like you have this internal label of what the time is, because you can look at the celestial objects and say, okay, well, these are their positions. And you can kind of, you can glue them back together, knowing the laws of physics, you know how to line them up, the sequence. And it’s that understanding that allows you to replace this unphysical idea of time with internal labels of what time it is inside the universe.

So it’s kind of like, we want to get rid of this weird background structure that we can’t observe. Like the theory already tells us, we can’t observe this ethereal notion of time, this absolute time, as Newton called it. So instead, we should just look at the things that we can observe, like for example, your clock or the positions of celestial bodies. That is both much more based in observation, and much more satisfying theoretically. And it leads again to something like a many worlds view. So in particular, we talk mainly about Julian’s classical setup, which I like a lot. And there is a counterpart to this in quantum mechanics, where it’s literally many worlds.

So you assume that you’re in the universe, you have the description of the universe or the multiverse. And you say, well, it’s completely stationary. We want to get rid of this idea that things are changing relative to the external time parameter, relative to the absolute time. So let’s just assume everything is stationary. And now let’s carve the world up into something we’re going to call a clock and the rest of the universe. And they’re not interacting, it’s an important assumption. And then what you say is, well, these two systems are entangled with one another. So that, for example, when the clock says it’s 3:00 PM, then the rest of the universe is doing a particular thing.

And then when the clock says it’s 3:05, then the rest of the universe looks slightly different. And when it’s 3:10, then the rest of the universe looks slightly different again. In such a way that if you keep conditioning on the clock, the rest of the universe evolves according to the laws of motion that we know. - Yes. - And then again, you have time coming from a time. So you have a notion of time that comes from a timeless description in that case. And it’s a many worlds view because the different universes are now just the different moments in time and they all exist together in a kind of block in what are sometimes called the block universe. - There’s a clock. - Yeah, so you assume that there is a clock.

The model assumes there is such a thing as a clock. - Is this like a theoretical clock or is this actually like a real clock? - Yeah, so, okay. So one of the things that I am quite happy with having done is, so this model is due to Page and Woodhurst. - Yes. This is an old model by the way, right?

This is like many decades ago. I think in the ’70s. - I think in the ’80s. - Oh, ’80s, ’80s. Okay, yeah. - And a very nice model. One of the assumptions that went into it is, as you’re writing the point out, it’s kind of strange is that there’s no interactions between the clock and the rest of the universe. So it kind of feels, if you again, ‘cause now, I was saying, well, you can look at your watch, but this is not a clock you can look at. - Yes. - And since then, so I think what has happened is that initially, people kind of were very critical of this model.

It went out of vogue and now it’s in vogue again. And people worked on it a lot. One of the things they’ve done is shown that even when there’s interactions, you can still get these, let’s call them Page-Woodhurst states. So you can still get timeless descriptions, even when there are certain kinds of interactions, not general kinds of interactions, but certain kinds of interactions between the clock and the rest of the universe. And building on that, Simona Rijovec and I showed that, in particular, you can measure the clock without disturbing the model. So you still have the timeless view emerging from it.

So you still have different universes being different times and the clock giving you the equations of motion that were used to all the good stuff. But the only thing that’s different is that, you can actually observe it. So you can say, okay, well, I can see, for example, - It won’t affect the system. - So you, yeah, you won’t disturb it in such a way that the clock no longer functions as a clock. - Okay. - I think that’s the technical way of putting it or the technically correct way of putting it anyway. So that means that, for example, according to this extended model, your wristwatch is really a clock in this sense.

So you can look at it and you provide it that you don’t destroy it. You don’t do certain things to it that completely ruin it in its function as a clock. You can observe it and measure the time. Yeah, that’s one of the ways in which the model has been advanced. So it’s becoming ever more physical, I would say, and we have an ever better understanding of how these clocks should work. So I think it’s fair to say that your wristwatch counts as a clock in this sense. - So time isn’t just like a sequence of these universes or like, how do we best like recap it?

Like if I had to explain this to my mom or something, who is not a physicist, what is the best way to like phrase this, to think about time then from this like quantum mechanical perspective? - I think the best way to view it is that in this view, each moment of time is equally real. And so the past exists, the future exists, they’re not coming into existence. I think that the typical view is they come into existence and then they cease to exist instantly. And there’s this species present, which is constantly moving forward. That’s not what’s happening according to this understanding.

What’s happening instead is you’re finding yourself in these different universes, but they already, like the universes are all there. If we were to say, what was the state of the universe like five minutes ago?

Well, then there is a world that exists, which is the state of the universe five minutes ago, where we were talking. And in some sense, we’re still talking. We’re still, us of five minutes ago are still having that conversation five minutes ago, if that makes sense. - So it’s basically all happening at the same time. - Yes, so yeah, it’s hard to find a good terminology for this ‘cause at the same time, it’s kind of a misnomer, but it’s kind of like maybe an analogy that is helpful is, so perhaps this will go on YouTube, perhaps this will go on a podcasting app.

When you’re listening to it, you’re hearing, you’re hearing a particular moment from the interview, but the moments are already all there. Like you can go to any part in the interview and the recording is already completely, it’s already been recorded. It’s not like our voices are coming in and out of existence. They are already completely present in the recording of the conversation or like with books. I mean, a story already exists before you read it. You’re just on page 10 and you’re gonna go to page 11, but page 10 still exists. And you can reread it and the same things will happen again.

And that’s kind of the picture of the universe that this gives you. - What is happening in the future already exists. What is happening in the past already exists. So how do we then think of free will and your free agency in that context? - The future might already exist, but we can’t predict it. We can’t predict from the current state of the universe what the future will entail. - And I’m guessing you can affect in which branch you end up in. Is that right? - In some sense. If you have to explain why the story unfolds as it does, you have to account for people’s motives, their actions, their ideas.

And even though the book is already there in full, the reason why, for example, a hero defeats a villain might still be because of the various things he did throughout the book. So his actions had consequences, even in the deterministic universe. If every time you watch the movie, it had a completely different outcome, that would not really be congruous or something with this idea that his actions have results, that he’s doing something in the world. So we can have explanations of people and stories and why their actions were good and why they affected the world in certain ways. And I think the same is true for us, even though we live in a deterministic universe. - Actions have consequences and they do still need to be logical.

If I jump from the fifth floor, to the grounds, I will get hurt, right? There’s no universe where I won’t get hurt. Like I’m very few. - Yeah, it’s a bit like, okay, if we think that free will doesn’t exist and we just lie down on the couch forever, well then we won’t do anything. So, you know, lying down on the couch had results, predictable results, maybe even. And whereas undertaking certain actions will have different results. And we know that in advance, we have a choice to sit on the couch or not to sit on the couch. And our actions still have consequences, even if it’s determined.

I mean, kind of because it’s deterministic, our actions have consequences. I mean, I don’t have a full theory of free will, but this is roughly how I think about it. - Yeah, and it’s an active area of research, right? - Yeah, it’s not something I work on. But if you were to ask me as a physicist, how do you think you can incorporate it with that?

It would be something like this. - Yes, that makes sense. Perfect, well, thank you so much for your time. This was a pleasure. And let’s do this again soon. Sounds good. Thanks for having me. Sounds great. Thank you.