The Quantum Experiment That Defies Logic Exactly 1/12th Of The Time | Dr. Lídia Del Rio

In 2016, this quantum setup started shocking the foundations of physics. I wanted to understand why. You can accurately say, “I have an error one of the time,” but you cannot predict when you get the error. Correct? Yes, I mean yes, yeah. It’s teaching us that the way we’re using quantum mechanics is kind of breaking down at this level. What if you spend 11 hours in a completely logical world, but then, exactly between 11 and 12, logic breaks down? So Bob got one, so Alice got one, so I must have gotten fail. But look at my measurement outcome; I got okay.

This is physicist Dr. Lydia del Rio explaining to me the Fra Rener thought experiment that predicts a strange quantum paradox exactly one-twelfth of the time. What we used to call facts, which are observations, are not absolute. With one observer, quantum mechanics leads to the strangeness of the measurement problem. With two observers, you get the witness-friend paradox, which is highly mysterious but doesn’t break down the theory. That seems to happen when you get to a setup with four observers. Now, all of a sudden, quantum mechanics seems to be playing Russian roulette on itself. This is like physics entering philosophy, and as physicists, we are very unprepared. You cannot have both, like this outside view and know what people are experiencing inside the experiment. I can give you Super Mario on a disc; here’s the game, you know, and then there you have it; that’s the universe. But we both know that the game is to be played and to be in it and to have these different perspectives. Yes, yeah. It’s also one criticism that many of the “shut up and calculate” kind of physicists say of this is like, yeah, you should not be allowed to take the point of view of these little agents inside their boxes; like, just apply the Schrödinger equation from the outside. You see everything evolving like beautiful, but like, but we are the little people inside the boxes.

Yeah, exactly. That doesn’t tell us anything in the end. We are not experiencing the video game from the perspective of the developer outside reading the code; we are inside. The famous quantum physicist John Wheeler once said that physics used to look at the world from behind a thick plate of glass, but that quantum mechanics broke the glass, and that as observers, we had to reach in to accept the fact that we have a participatory role to play in our universe. This conversation is exactly about that. This conversation consists of three parts. In the first part, Dr. Lydia del Rio and I discuss the necessity of modeling observers in quantum mechanics. In the second part, we have a deep dive into the Fra Rener thought experiment. And in the third part, we discuss the meaning and implications of this thought experiment. Now, before we start, I would really want to thank Lydia because she is six months pregnant but still took all the time to explain this profoundly weird quantum experiment to me. I hope you enjoy [Music] it.

Welcome at the Assencia Foundation’s YouTube channel. I’m very thrilled and happy to be sitting here in the Netherlands with Dr. Lydia Del Rio. Lydia, warm welcome in the Netherlands. Thank you for having me. Nice that you’re here. You were sort of in between traveling to Bristol where you also had some work to do, right? Yes, I had a thesis defense there. Yeah, yeah. And um, you are research fellow of the Asencia Foundation and a senior scientist at the University of Zurich where your work focuses on quantum information theory and, in particular, on how to model agents as observers or to model agents um and observers as physical systems. Um, I always like this quote by by John Wheeler where he says that we used to look at the world behind a thick plate of glass, sort of us, the observer safely—he said safely—behind the plate of glass, and that after quantum theory the glass is broken, he said, and we have to reach in. So sort of this border between object is subject, object-subject divide isn’t there anymore. Is that a way to understand it? Do you like that metaphor, or how do you sort of explain it to students who are new to this?

So I like the metaphor. I don’t think it’s only in quantum theory, but maybe quantum theory makes it more unavoidable to to see this uh this issue that we cannot just take ourselves out of the out of the picture. Yeah, I mean, it somehow it’s something that’s very natural in social sciences uh or or even in journalism, right? When I go to interview someone, who am I as a journal, like what is my position going to be? And yeah, and so on, right? Or or like can I ask questions about something that I really care about and like what’s the boundary between activism, internal like all these things, right? Exactly. Uh, and in physics, indeed, in many areas of physics, we we can kind of safely stay behind this glass pane. Um, quantum theory is one example where this is uh really breaking down, and we see that no, there’s problems if we cannot just keep this view, the way to distinguish between observer and observed, sort of that what in quantum theory is called the Heisenberg cut, right? Where you say where does sort of the the the quantum reality become classical in a defined state, where we put that cut, that plate of glass that Wheeler says is smashed, is now is it now completely arbitrary?

Yes. I mean, you can put it wherever you want, and then you’ll get different results depending on on where you put it, right? So one example is, oh, let me look at the hydrogen atom; there’s a proton in the middle and an electron going around it. If I treat the whole thing as classical, then I will not get the right results for the behavior. If I treat all there’s a classical particle going around feeling some electric force and yeah, going around this, then I will not get the right result. If I put the Heisenberg cut just around the electron and say, okay, I’ll trace the electron as a quantum system, but it’s too complicated to also treat like the nucleus of this of this atom as a quantum system, so I just say there’s a quantum particle that feels an electric potential, right? Then I can get pretty good results, quite accurate of like for the behavior of this particle that then match measurements. I could go one step further and say, okay, now I also want to treat the proton; I treat the whole thing as quantum, so these are two quantum particles; then the calculation becomes much more complex, and you know, depending on how deep you go, then you might get some corrections to your previous results, right? So to some extent, there’s some freedom. Mhm. And then what some new results show us is that there could be incompatible ways to put the Heisenberg cut, in which if we do put the Heisenberg cuts in different places, then we can reach contradictory uh conclusions, which would mean like, oh, there’s not like a single experience of a reality for the two of us. This is really like it’s really just not compatible. This is where it really gets cool, and also we’re not there yet experimentally, right? Because now we’re talking about thought experiments in quantum mechanics. Mhm. And I think Eugene Wigner was one of the first physicists to to really bring this up, sort of this where do I put the cut, right? Because yeah, if you how would you explain his his thought experiment?

So the Wigner’s friend experiment. So it’s a it’s kind of going one step forward from Schrödinger’s cat. Mhm. So Schrödinger’s cat: you have a cat in a box; you do some quantum like I don’t know, particle can decay or not; this this generates a chain reaction that ends up in a microscopic change, right? The cat can die or not, but if it’s in the box, if you’re looking from the outside, you still see the cat as being in a superposition, right? Yeah. Wigner is like asking, okay, what about the cat’s perspective? So instead of a cat, now we have a person in a box, uh, let’s call her Alice, and Alice is measuring a a qubit, a small quantum system, an electron spin or a photon polarization, whatever. Um, and so from her subjective experience, she sees one outcome because this is what we do when we go to the lab; we do the experience, see an outcome. From the outside, if I treat like the whole of her lab as an isolated quantum system evolving, there’s no collapse; from his perspective, evolution that happened. So Wigner sees her as being in a superposition of seeing one outcome in the other outcome, right? Yeah. And this seems to be like a little bit maybe a bit incompatible views of of nature, of what’s going on. It already tells us that we were used to conflating observations with facts of the world. So, for example, I measure an electric current, or I measure the height of a tree, and this is a property of the tree, you know. Of course, Einstein’s relativity already challenged this a little bit in saying that you know, a distance that you measure depends at the speed at which you travel, but still like if I have someone else traveling together with me, we’ll see the same. Yeah, Dennis check again. Yeah, and you can like transform this to some property of the of the the tree I’m measuring, right? Still anyway, good. And what quantum mechanics and in particular his experiment tells us is that no, this is not the same. The fact that you observed something, that is a subjective experience, a measured outcome, is not a fact of the world; it’s a it’s a relational thing between you and the world that is not seen like this from other people’s uh perspectives.

What quantum mechanics tells us is, look, so I’m making a measurement; I’m measuring a spin of a of an electron. How how do I do this? Classical example: I do this Stern-Gerlach experiment where there’s an electron coming; I’m put some magnets here; it goes through a magnetic field; then depending on the on the spin of the electron, it goes up or it goes down. If it started with well-defined spin up or spin down, if it starts in a superposition of up and down, what is happening is that now I’m correlating this kind of position degree of freedom, meaning like, you know, the speed of the electron, which direction it goes; I’m correlating this with the spin, right? So now it’s like it’s a superposition of spin up and going up and spin down and going down. Yeah. Okay. Then this electron hits the screen, and we say, oh, we measure where the electron arrived, but in reality what’s happening, I don’t know, maybe there’s some photoelectric effect going on here that then releases some some photons; this is also a quantum process, right? So at here we have a superposition of spin was up, electron went up, excited some atoms here, photons release here, or down, it went down, photons release here; it’s still a superposition; there’s not one single outcome; is like this this thing that could be if I put now a mirror there and there and you know, I have much better control than I actually have, then maybe I could reverse the whole thing and come back to an electron here. Yeah, but sorry, but this to me is like new in a sense that it seems like when when a particle hits the screen in the Stern-Gerlach experiment or in a classical double slit, I always thought that sort of when it hits the screen we’re collapsed and we have a definite outcome. So that’s from your perspective because this is when you perceive it as thing, but what’s happening now, okay, now photons from here and here are coming to your eyes, right? And this is still a quantum process; whatever is happening inside your eyes, these are atoms and molecules interacting; it’s still a quantum process, right? But somehow, and that’s a miracle, we have the subjective experience or seeing just one of these things happening. Yeah. Okay. How does quantum mechanics explain this? It’s actually very nice; it’s saying that you know, this photon is not just getting correlated with with like a few things; there’s like all this environment around it, and as soon as like you have a like even if you have this reversible evolution, as soon as we ignore part of the information, what is called tracing out in quantum mechanics, as soon as I lose track of some of the environment, then what it appears to be a classical mixture; it appears to be one thing or the other with some probability distribution. So uh the perspective we have is a subjective thing; like you say, okay, it’s when the electron hits the screen that that the collapse happens, and what this translates to is like when the electron hits the screen, you know, this kind of information is kind of spread out; there’s interaction with so many um particles in environment that we don’t keep track of; our eyes don’t keep track of, and therefore we see one thing or the other, right? But from God’s view or super-observer’s view, it’s still all uh a reversible process, and there’s not a single outcome; you just see a single outcome; one is just start losing information, and losing information is not an objective thing; it’s a it’s a subjective; losing information is is a is a measurement or…

Yeah, essentially like the the moment when you stop seeing a unitary evolution, you see a single outcome is because you you traced out something; you ignored something because yeah, the world is… I think people listening were not physicists, and people on the street would say, okay, maybe you’re right, but you’re making stuff now so abstract, philosophical, even. How is this practical? I love I love it actually because you’re you say we have to treat you have to look at the whole picture here; you cannot just say exactly what you’re saying; quantum effects are taking place in my brain, so the moment I measure I if I want to have a physical theory that sort of takes account of all of that, I have to take steps back, right? And of course, there’s a limit to what we how many steps we can take back, but but yeah, sort of my initial sort of sort of not critique but just is that it just sounds so abstract; how is that on earth practical? How can we put it to tests? But that is exactly what you are doing with these thought experiments, right? Right. Yes. So there you say, okay, now we’re not going to treat uh a particle hitting a screen uh as an outcome because we’re going to take a step back; we’re going to look at who looked at that particle hitting the screen, and we’re going trying to model that agent, right? That’s right. Okay. And how do you do that? How do you model agents? Okay, so I don’t I’m not trying to like simulate brains or try to model all of you; we just like go to the most minimal minimal thing that we can do. Okay, okay. And this is treating again a measurement as just entanglement between—so I’m sorry for the word; there’s no good I’ve not found a good classical analogy precisely because it’s a new phenomena, right?— uh, but it’s like a measurement is two systems, which is like the system I want to measure and the observer getting correlated. Yeah, and that’s we call entangled. So now I’m entangled with… yeah, so now I’m entangled with this in this thing that could be reversed in principle, but because usually these systems become so big and complex, like nobody can reverse it. Um, okay, so we have and now the question is like, okay, how small can I make the systems and still have this phenomena? And and how small is like essentially two bits, one bit for the system I’m measuring and one bit for say the memory of the observer, where like a notebook where the observer writes uh the outcome. Okay, of course, not classical bit, quantum bit, but still like a four-dimensional system, so very very… Then uh so we need to have a few more things; like agents don’t just observe things, but they make decisions based on their observations, right? You can say if if I look outside, I see clouds or rainy clouds, I’ll bring my umbrella out if not, right? And all your all your decisions you can kind of trace back to an an observation, naively speaking. Okay, so we want to we add a few more qubits, much like a simple computer circuit that you program and see, okay, if I measure X then I do A, if I measure Y then I do B. Mhm. So we can we can still model this like in the same way that you could program a very very simple, you know, four-bit program to do this, you can do the same in with quantum circuits, and the difference is just everything from the outside; everything is kind of a reversible evolution. Yeah, when we talk about sort of just good to understand uh reversibility in quantum mechanics, um, what exactly are we talking about? Okay, maybe we can talk about the simplest experiment: say one electron is measuring another electron spin. Hey, electrons have this property inside called spin. Mhm. Um, and it’s measuring whether it’s like spin up or spin down. In the way this the way we model the measurement as a physical evolution is like these two things are getting correlated in a way that if this was up then this becomes up too, and if this was down this becomes down too, right? Um, and for maybe people with classical computer science background, this is a C-NOT gate, so it’s like a a unitary gate that you can apply. So in practice, we do this by shooting some lasers at them in a way that you know, just correlates one with the other. Okay, just like in knitting, you can take like two threads and you just correlate them, and you can undo this process, and now you have the two threads being separated. So in the same way in quantum mechanics, you can shoot some other lasers at them and get them like do the inverse operation. It doesn’t mean you’re going back in time; like maybe from the perspective of this whole electron, yes, he’d be going back in time, but if I’m modeling a more complex observer, like maybe my my brain is a bit larger than has maybe I don’t know, 10 electrons inside, uh, then I could have some electrons that keep the memory that there was a a measurement, but the outcome of the measurement has been forgotten because I really I reversed this; I I disentangle these two things, and you can really get back to the initial state. In mechanics, in principle, you can go back to the initial state. Yes. And some very uh clickbaity articles call this reversing time; I mean, it’s not time going back; it’s really just just reversing this physical operation. Yeah. I I’ve heard it a couple of times, the sort of reversibility in quantum experiments, and that it’s relevant, and I still don’t really get why it is important, but maybe without going too deep into it, you can explain why we need it sort of as a concept to understand these thought experiments.

Okay. One thing is that it will be an ingredient in the thought experiments that we’ll talk about later. Mh. So we need some observer from the outside to be able to disentangle again um you know the friend inside the lab from the system they observed. This is one thing. The other thing is that a measurement is kind of by definition an irreversible process because you go from this quantum state of the electron spin that could be in a superposition of up and down to just seeing it being up or just seeing it being down, and there’s not like one operation that I could that is not measurement dependent I could to go back to the initial state, right? Unless it’s also you know affecting my memory, and then then you’re looking again from the outside… the such ah so so the measurement is an irreversible thing; the subjective experience of a measurement is an irreversible thing; I cannot go back to not knowing what… so or it would be destructive, right? That’s what I’ve read, sort of because then you have to take a step back; I have to really erase your memory, so it’s a sort of destructive operation to get back to sort of a… yeah. And it’s also like this idea of a collapse, right? Like once I know that uh once I’m following I don’t know which slit the particle is going through, I cannot go back and consider it as as a way of going through the two things; that’s just… yeah. Okay. And we need this for for for better understanding the thought experiments. Okay. And and the idea of like all these thought experiments like also what is reversible or not is a matter of perspective. So from your perspective, you you think you made a measurement; you see a single outcome; it’s your experience; I I believe that you experienced this. And from your experience, it’s an irreversible process, but for me, from the outside, it’s a reversible thing that if I’m powerful enough, I could um you know undo. Okay, that’s that’s interesting. Okay. Um, I think good, maybe now to move on to the whiteboard and you explaining stuff a bit more in detail and also being able to sort of draw stuff. Um, let’s just do that, right? Okay. Yeah. Okay, cool. Lydia, let’s uh let’s go. It’s quantum 101. Okay, starting classically, right? This is still classical one-on-one. Okay, so I’ll tell you about thermodynamics. Um, what what do you think thermodynamics is about? Um, it has to do uh with entropy; that’s what I get, sort of that the universe moves from a low entropic to a high entropic uh state, and somehow miraculously we are sitting here low as a low entropy system; that that’s what I sort of get. That’s what you get from it from thermodynamics, and I think that’s what the idea that today there exists about thermodynamics is a theory that studies the world uh and like phenomena going on in the world, heat and work and entropy flows and so on. It started actually much more operational than this; it started with we want to make trains; we want to make heat engines; we want to take like this resource heat, like I I burn some wood; I have some hot us, and I want to turn this into movement. So there was this idea of like heat is like energy, but energy that that I don’t control, and work is like a train moving, like energy that I can actually control. Yeah, check. Now, given our initial conversation, we can already see that these are subjective uh kind of concepts. And so here is an example of this. So then with time, like although it started with this very kind of practical purposes, then it became more of a theory about yeah, the world, how you know, what thermodynamic process is going on in our cells or or in some natural thing we observe or when we put two materials together, what’s the heat flow like; it looks like a very objective thing from the outside. Yeah. And then Maxwell came up with this thought experiment. Okay, so let’s start with the gas. What is a gas at a certain temperature? Um, it just means like from the beginning people already knew like the gas was made of particles, right? And they moving at kind of random speeds and directions. So I’ll write here like a small arrow if it the particle is moving slowly and a fast arrow if it’s moving, but but we don’t see this, right? So all we know is some some microscopic properties of this gas like the temperature, the pressure, the volume, right? Which all result from this microscopic quantities that we don’t have access to. Yeah. And I smell it equally in the in the room if it’s a gas that I can smell, right? It’s not just local that I only smell. Yes, yes. No, it’s kind of spread out here on the this whole box with gas, right? And one of the laws of thermodynamics, like one of the formulations of the second law, is like you cannot just take heat into work; cannot take energy that you don’t uh control into energy that you control. Normally, you need like a cold gas and a hot gas, and now from this difference in things, then you can build a fridge or you can build whatever, right? Okay. And then Maxwell thought, okay, yes, sure, we cannot see the position and velocity of all these particles, but now imagine that you know there’s no reason why someone couldn’t. So imagine now we have like this demon; he has like microscopic vision; he can actually see all these particles; and the v this demon puts like a little partition here in the middle of the box and as a little gate. Okay. Yeah. And now the demon can kind of control when to open the gate or not, and he opens the gate if there’s a fast particle coming this way or a slow particle going the other way. Yeah. So after a while, what we have, he’s separating the two gases; we have all the fast particles here, lots of kinetic energy, and all the slow particles there. Okay.

So this means that this is, in practice, this is a hot gas, mhm. Because, on average, uh, the particles are faster, and here this is a cold gas. So from a single temperature, now we created like two temperatures, okay. So what does the what does the demon do now? Well, we can, for example, try to extract work from here. So put some system of pulley here, some bucket, and you know, hot gas will try to expand precisely because these are these particles with more energy can expand against this cold gas. So this thing moves, and this goes up, and now we have like we lifted a bucket. This is the same way that like uh dams work for production of of energy, right? You pump this thing up here in the Netherlands, so you're not a fan, lots of pumping up, which usually costs work. Look, I did it for free, but so I created work energy that I can control from from heat. Yeah, the only the only sort of what I would say is that opening and closing the door would take some energy also for him to do, or in principle not, I mean not not to the same uh, yeah, okay. Check. So it's it's very much about sort of he just doing like with minimal work separating those two, and then you can can can extract sort of work from the system, which sort of basically gave you free free energy. Looks like he gave me free energy, right? And now at the end, so I have like, okay, now at the end after this expanded as much as it will expand, I just remove the partition, I put it back in the middle, let the particles mix again, and then I go back here. Yeah, and then I do it again. Perpetual motion, right? Yeah, so yeah, violated the second law of thermodynamics. So what's the? Yeah, and in in sort of entropy talk, sort of the the the initial state is high entropy, and the demon is able to to bring it back to low entropy and go back to high entropy, and and this already tells us that there's some issue with the notion of entropy, right? Because one way to look at it is to say for us the state has high entropy, but for the demon, you know, the dream has perfect knowledge of everything that's going on here, right? For the deage has low entropy, but if you just want to look at it from from the outside and say, okay, why is this not a a reverse like a not a reversible but a perpetual motion engine? Why can I not repeat this forever? Uh, the the solution is kind of bringing the demon inside the theory. So instead of just applying thermal dynamics here, I move kind of the Eisenberg cut of my theory, so I say, no, the theory should treat also this demon as a thermodynamic system. So what's going on in the demon? So he has to store this information somewhere in the brain or memory or mhm computer of which particles are fast and slow, right? So this is like a hard drive that the demon is filling in, right? If he repeats this a few times, then he'll have to erase this hard drive or buy a new hard drive. Then the hard drive is a resource, right? And then um what uh Charles Bennett and Randau showed is that actually the the how much it cost you to format this hard drive again is precisely the work that you gained here, and that we know mathematically that's just physics. Yes, yes, this is the physics. So these things these two things cancel out. So this is an example of like, oh yeah, even in classical physics, sometimes we need to bring the observ inside the theory, otherwise our analysis just just breaks the theory, right? So the theory does not apply just at the scale, and it's you need to really consider yeah, that's that's really interesting when I saw sort of that erasing uh information costs energy, and it's the same energy that is gained in sort of this free energy example. Yeah, so so another way to look at it if you don't want to model the memory explicitly, it's like, no, this is a set of low entropy for the for the demon. Yeah, he's just transforming like the information he knew like he had information and energy here to information and energy here, and then at the end he lost information about where all these particles are now again. If he wants to repeat, then he needs to gain this information again, and this again costs work, right? And if we now make this sort of I like this analogy, I think it's very good, it's very understandable. Um, if we now make it quantum, what would sort of be the sort of the the quantum version of of Maxwell's demon? I mean, one can do the quantum version of Maxwell's demon, uh, and then there's there's funny things like the a quantum demon can actually extract even more work than a classical demon because you can have quantum knowledge of like if if his memory is entangled with this particles, not just classical correlations, then you can even extract more work, okay. Uh, but maybe we can talk again about a quantum measurement that's better, I think. Yeah, cool. Okay, so now let's talk about quantum measurement a little bit, and now let's think again. So I have Alice here, um, and she's trying to measure I don't know the spin of an electron, okay, and this thing is in a superposition of being up and down with different weights which translate to different probabilities of seeing one outcome or the other, okay. Uh, so then let's say now she's inside her lab, so this is Alice subsystem, and now from outside there's someone else, uh, and normally I draw the outside observers with hats, okay. So this is Bula vignner will show up later because we we're building up to a bigger experiment, right? So what is happening here? So from the perspective of Alice, she's making a measurement, getting a single outcome. From the perspective of uh Ursula, mhm, alice is just getting entangled with this thing, okay. There's no single measurement from her perspective. She knows that Alice is a single measurement, but the whole state is a superposition. Yeah, it could really be both uh spin up spin down. Yeah, so I'll write this as a as a little circuit, okay. So we start with like this is the thing she's measuring, your thing, here's uh Alice, okay, and time goes left to right, mhm, and now there's some operation here. So in quantum mechanics this is a control not gate, mhm. It doesn't matter, it's it's something that in principle Ursula can come here and reverse, shoot some lasers at it, reverse the operation, okay, and then we go back to the initial stage, okay. So just so Alice is kind of braiding herself with R. So far this is what's happening from her perspective. From Alice's perspective, what's happening is something different. So if I use squares for reversible operations, then I can use uh I'll just write it here, then I'll erase, then I can use like this for measurements, they're irreversible for irreversible measurements. This is like the traditional symbol or or even like something like this that has a pointer that shows us yeah, what's going on, right? So from Alice this is happening, she's measuring R, it's a reversible thing, she gets a single outcome. From Ursula's perspective, no, there's a reversible operation going on, okay. So now so far so good, I can make it a little bit more complicated. Yeah, so so Ursula can, when we say it's reversible for Ursula, it um because Ursula hasn't yet sort of performed a measurement, hasn't performed anything, right? So yeah, so it could be it could mean two things, right? So one thing it means is that Alice is puts the Eisenberg cut just around this particle like in the Maxima, we're just looking at the box. Yeah, yeah, whereas Ursula is putting her Eisenberg cut outside the whole thing, okay. Yeah, and for now it's just a matter of perspective, I'm looking at it from two different points of view. Later on, yes, ursula will come here and do some other joint operation that does what Alice did, is that okay? Yeah, that's okay. Yeah. Okay, so now we forgot about for a while and let's talk a little bit about reasoning, logical reasoning, very simple, okay. So we add another um thing, we say that Alice, so she's measuring the spin and can get two outcomes, let's call it zero and one, is that okay? And Alice says, okay, she has a second spin here that she'll prepare. Yeah, and Alice says, okay, so if I get my outcome to be zero, she gets an outcome, she prepares a second spin in some stage up. Yeah, if she gets the other outcome, she'll prepare the stage in what is what we call a superposition of being up and down. Check. Yeah, okay. Something you can do in the lab, nothing special about this, depending on her measurement she prepare prepares another particle in a either defined spin up or superposition spin down up at the same time. Yeah, yeah, yeah. Check, right? She sends a second spin to Bob, who's Bob? So Bob is here inside his own love. So now he got this second spin, okay, in some unknown state for him, alice knows what it is, bob doesn't know. Bob measures it again, up or down, two outcomes can be zero or one, yeah, okay. So if he measures this, it could be still both, right? Because the other one if it's this one, it can only be zero, mhm because it's zero is up. Say oh yeah, yeah, okay. So if you do this then B is one for sure, mhm, but this is a superposition of up and down, mhm. Okay, so if you this then B equals Z or B= 1 50/50 probabilities, yeah, from so yeah, yeah, I thought because the the uh if AS is one for Alice, she prepares a sort of super super position, so it could be both up or down. Um, so the only know only way to know for sure, the only certain information you can get from Alice's lab is if you measure a down I'd say, right? Because that's the if you if Bob sees a spin down then he knows for sure it must have been the super super should be zero, sorry. Yeah, check, check. Yeah, this should be zero. Yeah, yeah, this should be zero. Yeah, yeah. Zooms up and okay, yes, exactly. So there you go. So now the game is exactly what you said. So the game is now Bob Bob doesn't know what Alice saw, measured what she saw, he just did this measurement, and now he needs to guess what Alice did, okay. So if if you obtain zero, sorry about that before, so if you obtain zero, he doesn't know, right? It could be in this case or it could be in that case. Yeah, could could be both, could be both, yeah. But if he got one, then for sure he knows that you know the only way for me to get one, Alice must have prepared this thing, therefore Alice must have obtained. Yeah, so if I got spin down, I know for sure it must have been one because that only the super super particle can give me spin down. Yeah, yeah. Okay, so this is the reasoning. So if B equals 1, then A equals uh one. Okay, for a classical analogy, if this was not a spin and this was a coin, we can still do the experiment. So at this point we can still do the experiment. Uh, so Alice flips a coin, if it's heads, she prepares a second coin in heads, mhm. If if it's tails, she prepares a second coin by just flipping it, not looking at it, says to Bob. Bob then looks at the coin, if it's heads, he doesn't know which one it was, but if it was tails, he knows that okay, y Alice must have done tails, right? The same thing, I just added a Yeah, yeah, replace the coin with a quantum measurement so far, which will only be important later, but this is the kind of reasoning that we think is very natural. Yeah, and Bob so knows two things, he gets a particle with a with something he can measure, and he has to know how Alice reasons, right? He knows exactly he knows that yes, yes, we know this. Yes, it's not an adversarial scenario, everyone knows the recipe for what everyone is doing, and now we can think like in this scenario, what is Bob's Eisenberg cut? Well, he's reasoning about Alice's outcomes, so he believes that Alice saw an outcome, so for him Alice is classical, he's outside the Eisenberg cut, but of course these two particles are quantum particles, so like his Eisenberg cut is Yeah, includes two things, yeah. Can I add one more agent because now it's where it gets a a little bit more complicated? Now it gets Yeah, if we put agents outside the labs right, also watching at Alice and Bob, yeah. So I'm building up to the Yeah, froer um Yeah, experiment, which is an extension of Wner's friend that was proposed by Yeah, we have two Wner boxes here basically, right? Uh, yes, we have two Vner boxes. Yes, so Ursula is friends with Alice, and Bob is friends with Vner who is outside here. Oh, okay. Yep. So now after Bob did his measurement, sometime later Vner will measure this whole thing, okay. So let me now draw the rest of the circuit of what's happened here. Yeah, so let's depending on what she saw, so depending on what's on her memory, mhm, she did some operation on this particle S, okay. This is again a logical gauge just with quantum particles. Yeah, yeah, and then Bob measured this particle. Sorry, s is Oh, S is the particle, s is the second particle. Yeah, oh yeah, yeah, that's the particle that Alice prepared and prepared. Yeah, I'm see, yeah, s is sent, yeah, and then Bob measured S, which seen from the outside, if I'm look at like perspective from the outside, from us, it's also some kind of operation, it's also this control gate that can be reversed in principle. Yeah, good. So this is what it is, this is the whole circuit, and now Vner is going to come here and do a big measurement, mhm, on both the lab and and this, which in practice corresponds to something that undoes this measurement and then does some this is really funny, this is a new way for me for looking at it when you put it like this in this logic gate or what's sort of what's the way to say what you're doing here with putting it in putting it like a quantum circuit. I'm designing it like a quantum circuit. Yes, sometimes it's very helpful too. Yeah, makes it sort of at least so far I think I still get it. So if we give names to this operations, yeah, okay. So let's call this uh it's an operation which is a C not, this I call it V, it's a controlled hardarge if you want to know this again, and this is kind of minus one, it's the reverse of these two things, meaning if I do this and I do that, this is the same as uh doing nothing. Ah, yeah, okay. It's the identity, so it's like I go Mhm, you know like you're watching a movie, you go skip forward, skip back, yeah, you end up in the initial state, okay. So Bob did this, uh Vner does this, and then he does a measurement here, okay, which is a different measurement than what Alice and Bob were doing, mh, okay, but it's a measurement such that and now is the part where where if the initial state of this particle had been this, mhm, okay, where if we had like a equals 1, then Alice prepared s in this superposition, which then got kind of entangled with Bob's memory, and I'm trying really hard not to draw any formulas, so let's just say like they're entangled like a bridge, mhm, yeah, and then Vner does his thing, and what the math tells us is that if he does this measurement, then he'll always get some outcome which for the purpose of this experiment they call this outcome, they labeled it fail, huh, fail, like fail, yeah, like succeed or fail, and the the measurement outcome is fail, okay. But we we could call it zero or one, but fail is more Mhm, okay. So this is kind of the without formulas what would happen is this like this entangled stage that that is happening here that we have here at this point, if I does all this and we had started off with the s that Alice prepares, if we had won, yeah, and then he always gets the same outcome, okay, always get that out I guess, okay. If if all of this happened, he will always get that out, alice prepared sort of if Alice prepared this from from Alice's perspective, right? So which means that now Alice, what is Alice's Eisenberg cut? So now she's treating I mean all of this as a quantum system and and R of course, yeah, because he needs to think, oh, I'm Bob is thinks he's making a measurement, but he's just really getting entangled with S, I don't need to think about Bob's outcome, just that they are like in this joint state, this entangled state that if Vner measures he will do he'll always get this outcome fail, okay. So then Alice's chain of reasoning is, okay, then if I get if I start with when I measured this first particle I got one, yeah, I prepare this stage, Wner must get Wner must get fail. Yeah, yeah, that's okay, that's the logic of the experiment. Yeah, okay. So then we have a equals 1 var from Alice perspective. Yeah, yeah, yeah. What does Bob think? Now we go back to Bob's thinking, mhm, just after he got S, if you try to guess what's the outcome of for Wner for Wner, yeah, he thinks, well, if I got one, we'd seen before, then then I know that Alice will get one, and I know that Alice knows that when she gets one, Vner will get fed. It just feels like a circle like Yeah, or at this point it's not a circle yet, it's kind of a chain of like I got this starting to look like this meme, okay, starting to look like this meme, okay. Are they are they communicating here or is this just they don't need to communicate, right? It's just reasoning, right? Them reasoning about sort of what what what other agents will see. Yeah, it's just reasoning about what our agents will see. Um, and the part that you need to take my word for it is that yeah, in the this step Ver will always get fail if he sees this because that's quantum mechanics. Yeah, and the part that's also strange is that you know Bob says himself as classical, but for all this reasoning is taking into consideration how Alice thinks, and Alice is him as a quantum system, right? Sorry again, how how's that the what I mean we're we're going we're doing this chain of reasoning across kind of different cuts of the Eisen different Eisenberg cuts for for different agents, mhm, but this still goes perfectly so far. Okay, no, yeah, it's no paradox at this point. There's no paradox at this point. It's an experiment you can make with you know a small quantum computer with quantum bits instead of observers, and you just run the circuit and and you do a measurement here to see what did Alice get, what did Vner get, and they're always this is true. Good. The problem is that that if you if I put another agent here, Ursula, and she does some measurement in Alice, then this whole thing will break down, okay? Because um let me try first without going to the maths what they found out is that you know it's just funny because this is like a very long complicated way of saying that stuff is still just very logical if you apply the logic you get the same result. Logic, we just made it very complicated with this sort of this quantum setup, but yeah, but okay, we need that we need that because we want to understand why things break down if we add another observer, okay. Check. So what will happen is that here at some point, maybe before even uh before Vner's measurement, Uso will do the same, she will kind of undo Alice's measurement, she'll perform a big complicated measurement here, mhm, right? She'll get some outcome, mhm, which can be again okay or fail, mhm, okay, and the pro and and then she'll reason the same way that if she gets um by just looking at the whole quantum stage she can see that if I if I got okay then Bob got one, okay, she I'm not explaining you this, it's something similar to this, right? So she sees Alice as being in in this superposition, she knows all the things that happen until here, she thinks of Bob as like outside the cut because she wants to reason about his outcome, mhm, and just from the properties of the quantum state she deduces that okay, if I got this outcome, Bob got Bob got one, okay. So now if we chain the whole thing, yeah, because because measuring Alice in his lab gives gives Ursula information about what Alice sent to Bob, so then she knows what Bob sort of got, yeah, in a way. Yes. Yeah. So if we change the whole thing, what Ursula knows is that if she got okay, Bob got one, mhm, she knows that Bob knows that if he got okay, that if he got one, Alice got one, yeah, and she knows that Alice knows that if she got one, Vera got fail. So so far so good, it just looks like okay, if Ursula gets okay, then V gets f, Vner gets f. Now the problem is that if you just look at now we look at it from the outside, we just look at this whole quantum circuit here, so we just look, we put our Eisenber cut like around the two labs, mhm, interesting. Yeah, all these operations happen, and at the end we do these two measurements, there's a probabilities that uh that they're both okay, oh, whereas we just said when one measure is okay, the other one is fail, yeah. So so yes, and you say one 12th of the time, yes, this is something you can you can deduce from the math, you just look at this whole quantum state, you run this circuit, and then you do these measurements, you see 112 of the time okay, they'll both get okay. The problem is like now when they both get okay, Vner asks herself, "So what did you get?" And she's like, "Oh, I got okay." And Vner goes, "Okay, so I know that you got okay, so Bob got one, so Alice got one, so I must have gotten fail, but look at my measurement outcome, I got okay." So you and then he he would say you have to have seen Phil, you have to have a Phil because I have okay, right? But but he's like, no, no, no, look, look here, here it is, I got okay, and there's no and and none of them can then sort of say there's an error, they they know if we compare it to just nodes in a computer network, they know the the the system, the transistors, everything is just the algorithm is okay, the system is okay, it's just a paradox, right? At this point it'd be just a paradox, right? Everyone used very reasonable logic. So what is breaking down? So something you Yeah, what's breaking down here? Logic, I'd say logic is breaking down, so and also with accuracy, one 12th of the times it will happen, that's what the mathematics tells you. Mathematics says 112 of the time they'll get these results, both get okay, and whenever this happens, then always they'll find the contradiction, and you cannot sort of say when so so it's it's almost as if you have sort of you know that you have an error, you can accurately say I have an error one 12th of the times, let's call it an error, but you cannot predict when you get the error. It's correct. I mean, yes, yeah, that's right, yes. Um, so yeah, and and when you get an error, you don't know who's wrong or Right, I mean, I mean Wner is there's no or can we stand outside and say Ursla's right or Wignner is right, or you can say what you want, but this is still your choice, right? Something made you decide which one of them is right, like maybe some some philosophical principle that you have, maybe you think, look, no, it's much more immediate that Vner looks at here and says, okay, so this must be the right thing, whereas going through this chain of reasoning, something must be wrong with that, yeah, but but again, this is a decision you make, yeah, and and just for we will share like the whole experiment with and the papers and stuff, so we have the explanation here, we will share it, I can hardly follow, and I do follow because I've read the paper, but I I'm I'm of course, as I said earlier, I'm very much in it for the philosophy, so I'm just like puzzled here because we have a logic breakdown, a very quite profound paradox here, I always find it hard because I'm just not in that sense.

Analytic enough. I know that how you present it is correct, so and I, I follow some of the steps. I just find it hard to keep it all together in my head, but I, but um, so maybe what I can tell is like each of these inferences. We saw how to do it; like we saw this one, this one, we followed very well. Bob trying to guess Alice's outcome could be done classically; it's a very logical little thing. Like this one and this one are a bit more complex, and I cannot explain it without math. I just have to tell you like, yeah, look, Alice looks at the quantum state that she prepares, knows how it evolves when Bob entangles, and then knows that from here, Vner will always get the same outcome, fail, so the same kind of reasoning is just a little bit more involved to explain uh, without maths. And this is the same, right? So all the individual steps, this is just like applying basic logic and a little bit of quantum mechanics. The problem comes when we start to combine them. And now what are some ways out of this paradox?

Okay, so one is like the many-worlds view. It's like, okay, maybe this is just an expression of many worlds view where like, yes, Vigner sees one outcome, but he also sees the other outcome. Okay, so it's just it's like saying, okay, maybe in your logic system, logicians can come up with whatever logic systems; it's just maths, right? You just put some axioms in, and the machinery gives you some things, and one of the axioms you can put is saying like, no, it's okay to for Vner to know that he gets an outcome and also that he gets the opposite outcome. Yeah, to basically say I'm okay with facts that seem to be in contradiction, but I can account for that because I say it's many worlds interpretations; we're on a different branch of the wave function right now or something like that. Something like that, yes. The problem is then, okay, what, and now you try to apply that kind of logic as you go through the world and try to make decisions, and it's not very, it's not a very useful tool, right? Because it's a logic where like, yeah, I can get hit by a car; I cannot get hit by a car. So I'll just cross the road; I don't know. It's not very operational; it's not very operational, right? No.

Okay, so many worlds would say that's what's happening here, and that's why it seems like a paradox. Yeah, okay, so you know there are many, many worlds; there are more many worlds than there are many-world believers. So, so some people who believe in many worlds on some days might say might say that. Mhm. Uh, so another way to, another way out of this is just saying, no, we're trying to combine all these statements, yeah, but we're combining them kind of across different physical theories in practice. Case like is it as if I was trying to combine, I don't know, we're analyzing some political situation, and you know, there's a chain of people that need to give us insights, but each each of them has such different approaches that at the end like the, the you get just like, I don't know, the things are incompatible. Yeah, because they're applying different forms of reason or logic or so it's not the logic they're applying that's different, but they're modeling the world differently. So Alice is modeling this and this as quantum systems, whereas Ursula is modeling all of Alice's lab, including Alice's memory as a quantum system, right? And Bob is just modeling the two particles as quantum systems. So we, we're having agents with different Heisenberg cuts. Yeah, and clearly something goes wrong when we try to combine logic across these different Heisenberg cuts. Yeah, and what interpretation of quantum mechanics says, yeah, we can expect these paradoxes to arise than if you model differently? Is that I mean most of this kind of objective versions of Copenhagen interpretation, so neo-Copenhagen interpretations say the Heisenberg cut is a tool that we use to separate what we treat as quantum from what we treat as classical, but where you put this cut exactly, you know, whether you include the protons inside or the photons that hit your eyes, like it's kind of flexible, and agents can put it at different points, right? So this would be one option, and then the maybe a more, a more objective Copenhagen thing would be like, no, no, there's limits on where we can put this thing. Maybe like if you're in some kind of multi-agent system, and there's people who consider each other agents, then no Heisenberg cut should include other agents as quantum systems, right? Otherwise, the logic breaks down. Something like this.

Um, there's also, of course, collapse theories which just tell you, no, you cannot put the cut around Alice because there's some objective collapse going here; the systems are too large; there's too much dissipation with some environment or some background field, whatever, and this is not this is not this evolution; this gate is wrong. Okay, and that's just saying, okay, quantum mechanics does not apply at this scale. Yeah, so most physicists at the moment, they think the way around this is by saying, no, there must be some compatibility conditions between Alice and Bob; some rules for when can I combine this reasoning even if we model different systems as quantum or classical, and when can I not? And if we know those rules, we won't get the paradox. This would be the case then, then the solution would be like, in this case, this violates this rules, so you should not be allowed to chain statements like this. Right. Okay, so now we have this extravagant thought experiment that will never be realized with people because this involves extremely complicated measurement, uh, but which teaches something; is teaching us that like the way we're using quantum mechanics is kind of breaking down at this level. Okay, and an analogy to go back is like Maxwell's demon again. So Maxwell's demon is an experiment that again will, I mean, will not be made with a real demon and all the particles of a gas, and again it shows, oh, look, thermodynamics is breaking down. So what solutions are there? And one solution was, okay, we need to bring the demon inside the theory. Another solution would be, no, we keep the demon abstract, but we take the perspective of the demon, and then from the perspective of the demon, it was work all along; we never had heat because they always add information, right? So now what can we do here? And this is related to kind of, you know, this experiment can be formulated as a no-go theorem based in some assumptions, but this the solutions are are this. So one, one option is say quantum mechanics does not apply at this scale; there's some objective collapse going on; this, this is not the right circuit; yeah, you cannot treat someone in a lab in a being in a superposition; there's, there's yeah. Option two is saying, well, maybe it's okay that they get that you can get to different conclusions from different uh, paths because, you know, many worlds; I do see okay and fail. Another thing is uh, like what Cubism says, like quantum theory is a theory that helps me reason and make predictions; helps me reason and make predictions. I always take the perspective, my perspective to do this in a kind of a radical way, which means I should not be considering the points of view of other people at all, at least this is what Cubism would have said 5, 10 years ago, although things are evolving also within that interpretation, and then and finally the other way out is just saying, no, we just cannot chain this statement because we're combining different Heisenberg cuts, and there must, we need to find some kind of rules for compatibility between Heisenberg cuts. Okay, and the problem there is that whenever we find these rules, either they are so strict that, okay, we can't, we don't find a paradox here, like, okay, I can always think of your knowledge if you have exactly the same cut, but then there's many applications where this theory is useless; I can't make predictions, or it's a bit more generous, like, okay, for Heisenberg cuts don't exactly match, but they're a little bit, I can still think of you as an agent and do this reasoning; I know that you know X. Um, and in those cases, we in general we can find a paradox, so we change the setting a little bit, and we find a paradox, right? So there's no kind of easy solution as of now; they all come at a price, right? They all come at a price. Yes. Yeah. Um, and they all are so counterintuitive in how we experience the world. Um, yes, it's the same idea as in Bell's theorem, right? You start with very intuitive assumptions, and you find a contradiction, so you need to let go of one of them, and then none are good. Yeah, yeah. So we have different observers all reasoning quantum mechanically, and we are syncing them up, letting them communicate with each other or let them thinking about each other's thoughts, thinking. Yeah, but I mean there's interactions; we have one preparing a particle and between and then and then we end up with just different stories, different stories about what's what's really the case or what is sort of, yeah, about what happened. That's super weird, right? Yeah. Yes, it's like they, they really experience different realities, right? So we have thought experiments in physics that enter sort of the arena of, of, of metaphysics that it tells us, it doesn't tell us what sort of reality fundamentally is, but it does tell us that certain assumptions cannot be true at the same time, right? If you sort of apply that approach, so the no-go theorem approach and the metaphysical assumptions that are put to the test to this particular thought experiment, the FR, FR thought experiment, what are sort of the underlying assumptions underneath this experiment that cannot all be true at the same time, right? So, so there's many ways to formulate the assumptions; it's like you formulated a set of assumptions, and the experiment is kind of the proof that they cannot all go together. Yeah, right. So in the original paper by Daniela Faraga and Renato Renner, so they had I think three assumptions: one is that quantum mechanics applies at this scale, so I'm not even saying like universally, but that I can put you in a box and model you as a quantum system, like the agents could model each other as quantum systems. Um, I'll just go through the assumptions first, yeah, then we can go ask questions per assumption. Yeah, good thing. The other thing is something about logic, but it's also the way it's phrased; it's like a mix of logic and and something about quantum physics; it's like I cannot conclude one thing and its opposite. If I'm Vigner, I cannot conclude that I'll see the outcome okay and also that I will definitely not see this outcome. Okay. Yeah, just not have that contradiction; not having this contradiction, right? It's something about logic; it's also something about our experience of a measurement, yeah, which sounds super common sensical. Yeah. The other thing is consistency of the theory across different agents, and this in practice what it means is really that I know that Ursula knows that Bob knows that Alice knows X, and I can combine, chain these statements. Yeah, right, which is again how we go through life and how we make decisions and um, in most settings. So the thing that the other assumption that is kind of hidden here among all all of this is like this freedom that agents have to decide where they put the Heisenberg cut, decide where to which systems they model as quantum and which model as classical, and that these other assumptions work together with this, right? I can do this chain of reasoning across agents that have different Heisenberg cuts, but that's that's about it. Yeah. Okay, check. That's an extra nuance on top of this which we can go into, but first to to check them off, right? So um, are these these assumptions to me sound um, very common sensical also for physicists prior to this thought experiment? So uh, if I asked the question differently, was this sort of a uh, a bomb dropped in the foundations of physics? This, this, this thought experiment, it it does seem to me like pretty new, right? Because saying that quantum theory is valid at sort of this scale, at least in what I read, I I thought most physicists think it it is, and that you cannot have these contradictions uh, also um, I think most physicists would say, yes, indeed, if I I I perform a measurement, I'll get a certain outcome, and it cannot have sort of contradictions there, and consistency of the theory when applied by different observers also seems like super common sensical to all the science we do and how we regard all theories of nature. So we have three assumptions that are quite common sensical, and now this thought experiment tells us they cannot all be true. Right. Within the community, there was response to this uh, thought experiment, and you together with Renato Renner wrote a response on that, right? Responding on criticisms. Could you sort of just briefly touch upon how this thought experiment was critiqued and what your response was to it?

Yeah, so so most of the critiques try to put restrictions, questions on when can we apply logic in quantum mechanics, and the very common one is like, no, you should not be allowed to apply logic; I should not be allowed to think about what you think if I know that later as someone like Vigner will come and measure your lab and and destroy this, right? Because in this example, problems start happening when you have like these two super observers measuring the two labs, and there's a few problems with this kind of criticism. Ism, one is that you cannot predict the future, right? You can always change his experiment, put the agents far away, space out their measurements enough or bring them closer such that you know you can put this final destructive measurement by Vigner in the far future. Mhm. And what they're saying is essentially like, oh, I cannot make predictions now if I don't know whether someone will come and measure my brain or measure your brain in 500 years, right? Yeah, yeah, something like this, um, which renders uh, this restriction a bit useless because if I just describe you an experiment, you setting like in the next hour, please make a prediction. And then you ask me, okay, but in two hours will someone come and and measure my lab? Like it's a bit strange to think that this would have an influence, right? That then you can move players apart in a way that this will like start bringing like violate relativity in the sense that like some action from the future is now, so that would also apply to a lot of other different theories, and then also, yeah, so it's a bit sort of, yeah, I don't know how you call that in philosophy, but it's this sort of argument that sort of doesn't really, well, then you have a lot of other problems if this group; we have a lot of other problems. Yes. The other thing is also like, yeah, what is, where is the line? What is classified as a destructive measurement by by Vigner? Because again, measurement is just an evolution; it's just a physical evolution, right? Of a particular type, and we, okay, maybe this experiment has a very specific, very dramatic uh, kind of measurement type of evolution where you see this terrible paradox, but there's must be lots of intermediate cases where it does a measurement that's slightly different or maybe it's a little bit more noisy, and still maybe don't get like this contradiction, but you get some mismatch in probabilities of your predictions, right? You still get something that you wouldn't want, and it's very hard to put this line somewhere, like, does my brain interacting with the environment and kind of dying, does this constitute a measurement? Would this be a measurement of this type? Like, can I not make predictions about whether the sun will die in a few millions of years just because of what will happen to my brain between now and then? All these things are not uh, very practical uh, and and to be maybe a bit more, more precise, there's lots of settings where we have partial information about how you model the world, maybe in in communication scenarios, like you send me a message, and I try to guess uh, something about you based on this message. For example, I I should not need to know like exactly what's your view on the world; what do you model as a quantum system or what not. If I need to know this for everything, if I need to keep track of all this context in in a big chain, then again the theory is not practical, right? I I pay a price in complexity. Yeah, and so now I guess we're very deep into sort of technicalities, of course, because I asked you, so this is on me. I will put the paper in the in below in the description so people can read sort of the criticism and your response to it. Um, I guess the reason why I asked is because I was just wondering if sort of the consensus of the physics community is that this thought experiment is indeed something we need to take quite serious, and I I think it is, right? Because at the we were both in Vienna where Renato and Daniela were sort of both; they were awarded sort of the Paul Arafest award for best paper in foundation of quantum physics for for this paper, which to me signals that, hey, the community is taking this quite serious, right? Yeah, and there's other papers in like like the one by just Brookner, the one by Eric who were also there in uh, in Vienna at this interview that have similar flavor, right? Yeah, exactly, of of being like, oh, what we what we used to call facts which are observations, okay, are not absolute, and they're in fact, they we can find these contradictions between observed facts between uh, between different people. So I think yes, there's quite an awareness in the community of this; it's still a very niche community within uh, within physics. Yeah, and I mean Eric called it; Eric Avocanti calls himself a metaphysical experimentalist, so he he already sort of owns sort of the metaphysics as saying, okay, these experiments enter the arena of metaphysics. Um, there's there's a tension, right? Because as physicists, you're not philosophers, but you guys are doing these thought experiments that do enter sort of metaphysics because we are now talking about sort of just ground assumptions about reality; is it sort of is there an objective reality the same for all observers, or can we formulate theories that apply to this common shared reality stuff like that? And this thought experiment bases us; confronts us that we cannot hold all these assumptions to be true. So then what guidelines do you have as a physicist or or how to navigate these assumptions? Then is it just a matter of preference to say um, I will let go of sort of the the consistency um, or I want to let go that quantum theory doesn't apply at all these scales? How do you navigate that?

A tough question. Uh, sorry, I know. So first of all, like I agree with what you said; this is like physics entering uh, philosophy, and also that as physicists, we are very unprepared for this, like, you know, you do a whole degree in physics without having a single philosophy class or metaphysics or um, or anything, and sometimes it would help just being a bit familiar with the kind of concepts uh, that are treated there. Um, yeah, so how do you navigate these things? So some of these assumptions can be tested, right? Like quantum mechanics applies at these scales; we can test it; it's like testing collapse theories essentially, right? Larger and larger superposition, so that's kind of, yeah, the rest; yeah, experimentalists are working on that, right? Seeing how, how to do, you know, where are we now? Is it sort of at the molecular level, or I think yes, small molecules, I mean, yeah. Okay, so we can keep on doing that test; can get to the level of an observer in in a superposition, yes. Um, the other ones of what putting limitations on logic; again, this is something we can test to some extent because we can say, okay, I'll try this limitation, say, oh, I cannot I cannot think about your thoughts if we have incompatible Heisenberg cuts for some definition of incompatible. I want to test this definition of what compatibility is, and then I can test um, and see like how far I can go or not with this, right? And and and sorry, just to be clear, when we talk about sort of these differences in Heisenberg cuts, it's sort of disagreement on where we put the boundary between quantum and classical, right? Exactly. So yeah, and and and and so we can't say, okay, I'm not sort of; we have to have a sort of an agreement on where the cut is between classical and quantum, yeah? Because you know, if if we do, then then we don't have this kind of paradoxes arising, like this specific kind of paradoxes, of course, I still have, I think, and whatnot, but yes, but not this kind, right? So this is one of the reasons; maybe it's a good time to do a segue of why um, we wrote this paper with Renato and Simon Matis and Marina Galva on called Thought Experiments in a Quantum Computer. So the idea is there is to like model little programs that can be run on a small quantum computer, like the circuit I showed where you really, you need to program all the assumptions that you put in. Mhm. So you need to program how you model each agent uh, as a quantum system, where they put the Heisenberg cut, what kind of logic they use, what interpretation of quantum theory they use, and then you can test this and see if you find a contradiction or not, right? So this is kind of our way of trying to navigate, like answer your question, try to navigate how do you cope with this. Yeah, that that so far is called Quinundrum, right? You're you're working on it; is it sort of working yet? I mean, there's like a prototype version that is on on GitHub; if people can unlock, we'll put it down. Can we put it down in the descriptions? You can, you can. I mean, it's not, it's not amazing; it was it was uh, programmed mostly by a very good master student we had at the time, Simon Matis, and then we Nura and maybe a little bit I refined it a little bit, but it's basically, it's coding this, right? It's coding this, so it's coding this, and then so we now have like thought experiment software sort of to to run these essentially. Yes, and then it spits out whether there's, it tells you what were the predictions of all the agents and if there's a contradiction or not, right? And then you can say, okay, now I want to add one more agent, or I want to change the protocol of what they measure. This is cool stuff because it's it's basically sort of metaphysics software because we now have, you can put in assumptions and play around with it, and you get contradictions and stuff. So this is really where physics and philosophy meets, right? This is cool. Yes. Yeah. And are there sort of already preliminary findings coming out of Quinundrum or working on this that lead you to say, okay, I put my bets on letting go of this assumption, or I think we should keep this assumption about reality? What what are your sort of thoughts there?

No, I mean, the results are kind of what we expected because also what we tested is the things that we we were more familiar with, right? So so ideally, we would have lots of people playing with this and and telling us their insights and improving the software, uh, but you know, of course, we see if you have a contradiction, of course, collapse theories have other problems outside this program, right? Yeah. Uh, it also shows, oh, yes, if you have many worlds, then again, you don't have, if all the if you program the agents to believe in many worlds, then there's no contradiction in the results. Yeah, then you can let go of that one because in a many-worlds scenario, as we said sort of the the fill and if if both have an okay where they should contradict or we have a contradiction where they should should agree in a many-worlds interpretation, that can go because you're just on different, that's it. And there's also like that Alice, when she does the first measurement, Mhm, she thinks of herself or like, oh, yeah, I see this measurement outcome, but I'm actually I am in a superposition of saying this and that, and now I keep track of that, yeah, which is a fair enough assumption, and again, the price you pay here, and this is very clear also when you program it, is complexity. It's like imagine every time you see something, Mhm.

You need to keep track that, oh, I'm seeing this, but there's also—but I'm actually a superposition of seeing this and that and that and that and that and um, talking about sort of these quantum classical divide and and that those the Heisenberg cut. So we have objective uh collapse, which says we we will find sort of this objective point where to put the cut, and that's the same for all observers, right? And we have um many worlds say there is no cut basically, right? Because there the wave function just branches off and there there's not this quantum classical divide. And then we have like these epistemic approaches that do say it's really up to the observer to put the cut, is is that a way, right?

Depending on the epistemic approach you have.

Yeah, I mean many rules also says that does not—many rules does not deny that you have a subjective experience of seeing one thing, but it says for people to—again, I guess good good to just sort—Epistemic approaches—what are we talking about? Epistemic interpretations of quantum theory.

Yeah, this is like quantum theory is about our our knowledge—one maybe one of the most radical examples is cubism—or like quantum theory is like a tool that I use to navigate this world that helps me make predictions uh to win bets about the—Yeah—like outcome measurements yeah of something, and it doesn't tell me what I'm measuring and it doesn't tell me what measurement I will get; it's just me betting in the universe, yeah, which is fair enough, right? We we cannot really say more than that. Uh, but the problem is that you know it's a relatively recent theory; it had not—I don't think it had been like really confronted before this kind of experiments with oh I—it's not just about my knowledge, but now how do I combine this—I need to take into into account the point of view of other people.

Yeah, yeah, right. So for this it has a problem here if you just wanted to apply like the vanilla version of this theory; it would be—Yeah—ver only can reason from his point of view; he should not be allowed to like take this thing lying around like Alice and treat her as as another agent and think of her as an agent and reason through her, right? Which again fair enough, then you don't get a contradiction, but then there's also lots of other experiments you cannot do um like I don't know if you're trying to predict the behavior of someone based on the result of some quantum measurement uh then then like cubism would not be the right tool to do this. So cubism has a hard time to sort of restore a common shared reality in a sense, so it does it does give you a way to to uh explain these contradictions because I say yeah that just different observers using the theory and and you don't combine them—you just don't combine them.

Yeah, just don't combine them; you have your own truth. Uh, but if we want to sort of restore sort of a common reality, it has it has a difficulty. Yeah, and also like you don't go very far if—but what what puzzles me a bit, Lydia, is that when we sat down in Vienna with with Renato um um he only later—I read that in this Quanta magazine article—that that he initially thought that this thought experiment was sort of favoring many worlds uh let's say okay to account for this weird paradox uh this seems to be compatible—most compatible—or with with a many worlds uh interpretation. And then later on when I interviewed him in Vienna when you were present there, right, he was pretty outspoken on the fact that that that he said, no, I really think these epistemic approaches and interpretations are the way forward and not not in a cubist sense perhaps, but but still, so I found that really very interesting. Have you had discussions with him on this or what do you think?

A little bit over the years, but again our opinions change.

Yeah, maybe. But what are your thoughts? So so would you say um can we even say that it's it doesn't favor anything because it just tells us what we cannot cannot be true at the same time but still sort of correct? I mean, it's not even about what cannot be true; I I don't think quantum mechanics as a theory—as a tool—like in the same way that thermodynamics is a tool to help us build trains—quantum mechanics is a tool to help us um you know make predictions about uh a thing going on that we can witness, but uh in that sense I'm more in the epistemic thing.

Mhm. But I also think that you know every theory—it's like this good old incompleteness theorems—every theory at some point if you find like a self-referential problem you'll get to a paradox, right? This also happens—there's some funny examples that I don't remember in in classical mechanics where you find this paradox and then the solution turns out to be like a quantum phenomena um but like even every logical system you make a something that's kind of self-reerence potential and you find and it it's a paradox—it's just a limitation on logic.

Ah, does it—but—and this is in a way it is, right? Because it's kind of quantum mechanics describing itself—like the the observer through this long chain is kind of describing itself as as a quantum system and and that's where—who wrote that—who wrote that paper again—sort of quantum mechanics cannot sort of describe—is this also Renato or this is the title of the original paper by Daniela?

Yeah, exactly—cannot consistently describe how to use itself. So it's just this Godel incompleteness thing, so it points in the end to the incompleteness of quantum mechanics then perhaps, yeah, which is not surprising, right? Because because any theory is is incomplete in this way—any theory that tries to be universal has to has to be incomplete. So um yeah uh Nuran Galva, my co-author, her PhG thesis is exactly on this and comparing these to other types of uh of paradoxes—like even like the simplest example is uh this sentence is false, right? If you try to assign a truth value to this sentence then if it's true then it's false—if it's false then it's true—you know you cannot do this, and this is like a more complicated example of this where you needed a lot of technical ologist to to kind of reach the same conclusion that in the answer of this—just say this this sentence is false—but um but I did—why why I did think that quantum mechan—because also a bit sort of physics history—but I mean Einstein said the theory is not complete, and he came up with a way why he thought so uh the sort of the EPR paper and then but then we had John Stewart Bell putting it to the test and not—he did not prove it is complete, but he did—he definitely did prove that it is not incomplete in the way Einstein thought it was incomplete because Einstein thought of these local hidden variables right that we—and and and—but those are ruled out. So I thought that physics uh was very much sort of on a page that quantity theory is sort of our most our most complete theory of reality—I mean we know it has some problems at at some levels like uh when it enters in conflict with relativity—we only just—I mean it's relatively recent the field of studying what is a reference frame in quantum mechanics—so when you say spin is up—up relative to what—right? It means that like oh we have something else that we—as a big magnet—that this defines direction up—this big magnet is also a quantum system—what happens now if we play with this?

Yeah, right. So I think there's still a lot to explore in quantum mechanics—there are many no-go theorems for incompleteness in um in the sense that you meant like ruling out many classes of hidden variables—This exists more and more—of course all of these theorems bring in other assumptions with them, yeah—but but this is a different type of incompleteness—so it's like the good type of incompleteness—like we can find sentences within this within this theory that cannot be proven true or false.

Mhm. That's it. I mean, does it have—does it have—does it bring problems to us—I guess this is more like does it have practical applications that could be problematic already, right? And this is something that people are starting to explore now. So so can we take this kind of no-go theorems and now find a practical scenario—maybe like instead of four observers you have four little quantum computers—and if they reach this contradictory um this contradictory conclusions like can this result in problems—like can we find a game such that then this really results in problems—can you find a scenario where this uh—we don't know yet—I think this is still active research.

Yes, yeah. But it might—and that's where I think as a storyteller or sort of bit sort of science fiction mind I do think that's very very interesting because if it does—if this would apply—so we we could see this—then we would need four quantum computers—or two—just just Alice and Bob in their box would be a quantum computer—this would be an example.

Yes, you can also think of Vinar—the quantum computers—and Alice and Bob are like subp parts of this quantum computers measuring a little memory that—

Yeah, yeah—and uh but then so then we would have like a node of two quantum computers in the future that we use for stuff and then in one 12th of the cases sort of the logic break down—breaks down—something like this.

Yes, but again it's a fun project for for someone to take in—it's it's also very uh broad project like okay not try to find a problem with with this.

Yeah, but I mean it is sort of the way the work because now it might be still science fiction but thinking about the work of John Stewart Bell, which at that time was also thought of of as just sort of philosophical or metaphysical, but then it led to quantum information and a quantum computer, right? So there—why wouldn't this lead for instance to a breakthrough technology or perhaps like a big fundamental problem in quantum computing right when when we combine quantum computers?

Yeah, this is why we need more funding for fundamental research. So yeah, so for example Bell's tests have lots of applications in cryptography where you want to—because like this lack of hidden variables—like this lack of like nobody could have predicted what this things could be—also tell you that it's a way to for two players to create something truly random between them that they can test that no adversary can can check, right? So it can be used in cryptography um and here we don't know—so we—The answer is that we simply don't know. It makes me worried a little bit that if you try to imbue logic in quantum computers that maybe we'll reach some kind of problem like this and it would be a cool project to investigate how.

Yes, it seems pretty—yeah—it's cool and it's eerie in a sense that it that it breaks down and that we—it would be sort of a um a huge anomaly I think in just the whole paradigm of computation if if it does break down, right?

Yeah, yeah. In Vienna when we when we talked about it with with Ranada, Josh and and Eric in a video—in a previous video—they all said this is like crazy for us too because it was sort of an extra layer of weirdness of craziness in addition to Bell's theorem that already tells us that we cannot have sort of local realism um and sort of freedom of experiment that we cannot hold these assumptions true at the same time. So what is the addition here that makes it extra weird? So so the thing that it adds is like oh look um one if like Bell's theorem does not disprove—observations are facts and facts are universal—we can sell of this in Belthereum like you have these two agents far away they measure their spins—true universal like this—it's a it's a fact of the world, right? Yeah, it might not be local or—but then it's a fact of the world and I mean and one way to check is then go and and ask them—but right—and what this experimental is like no there's no common experience of reality. Okay, I I don't I don't think facts of the world is a very useful expression because we only really have our observations to go by—so like even if I say like okay forget about facts of the world but our observations are consistent, right? Yeah, I look at this apple and it's red and you also see it being red, right? And like um and this is a no—like we can find situations where like we we would have a radically uh incompatible observation of of reality even when we are using the same physical theory—okay we're applying it to different domains but like the facts are really incompatible—right? Like Alice really predicted 100% probability that you'll measure fail and you measured okay, right? So this is the extra layer of strangeness if you're very attached to oh there must be a common reality that we are—like science is a way to get to this common reality, right? Yeah, but one way to look at it is like we only get to this common reality by kind of talking to each other—by correlations—by by measuring the environment—performing measurements and talking—by performing measurements—by like having like the molecules of air escaping your lab and and coming here—right? Correlations is what gives us a common reality—this is currently the way I like to—and would you agree that just sort of this is sort of more on a sort of philosophy note that a common physical reality—we're talking about physical properties that that are common because there could be something underneath it all that still is very objective—observer independent—but it's just not sort of physical or—right? So so there could be something underneath it all, right? So you can write—I can model everyone in this experiment as uh has a quantum system and they have like the global shorting equation evolving this whole thing, right? So yeah, sure we don't deny this, but this is not useful to us, right? Knowing God's view of the universe—evolving purely metaphysics there—but it it's not just this—like it doesn't tell us anything about our experiences—you cannot have—you cannot have both like this outside view and know what what people are experiencing inside the the the experiment, right? If you want to have that—if I want to reason with the things I see in the lab or with the things I see around me—I look at you—like photons hitting my eyes—quantum—this is a quantum process, right? If I want to reason with this then I need to necessarily take take an Eisenberg cut somewhere—right? Take my perspective—Bernardo would say I have to dissociate or sort of there has to be a dissociation—it it seems like it's another way philosophically to talk about sort of a cut between p two personality.

This is fascinating because I remember when we had a pre-discussion of this this um conversation and talking about what it all means you had this metaphor of this this God's view—like I can give you Super Mario on a on a disc—here's the game, you know—and then there there you have it—that's the universe—we both know that that the game is to be played and to be in it and to have these different perspectives and that that's the experience of the game.

Yes, is that—that's what what you're hitting at here, right?

Yes, yeah. It's also one criticism that many of the shut up and calculate kind of physicists say of of this is like yeah you should not be allowed to take the point of view of these little agents inside their boxes—like just apply the shorting equation from the outside—you see everything evolving like beautiful.

Yeah, yeah. But like—but we are the little people inside the box.

Exactly—that doesn't tell us anything in the end.

Yes, yeah, yeah—like we are not experienced the video game from the perspective of the developer outside reading the code—we are inside the game.

Yeah, yeah—this is so interesting. Yeah, and and and also I mean the the the outside view—then how do you say that uh it's not computationally reducible in a sense—you cannot sort of—it has to be played out, right? It it has to sort of render itself out—you cannot—I I had a conversation with a complexity scientist, James Gladfelder, about sort of information and the game of life—you know, that Conway game of life—with with simple rules you get to the very complex fig—you get to like avatar and whole worlds and you just started out with this simple grit with just super simple rules um but I think sort of the the philosophical um takeaway from Conway's game of life is that there's no way where you can get from the initial state to that sort of end complexity that the game will give you if you just render it out—you have to go through that—you have to go through the computations—you could maybe speed that up with a supercomputer—but there's no way—but I I might be misunderstanding this—there's no way algorithmically to to sort of predict—goes from—exactly—you have to go through it, right? Yeah, so I don't know about this particular example, but I think there's many examples of computational problems that have this property that yeah it's already maximally efficient—you cannot predict without kind of iterating it—and the same can be true.

Yeah, but so you basically said you are not sort of—you say okay I'm not a philosopher and and we are as physicists perhaps a little bit unequipped to—we don't have the yet the the right tools to to strongly distinguish between these assumptions—which which one is is the most plausible—but um hearing you talk it does—yeah—so you you do seem to favor more of these epistemic interpretations—so—or I guess—so so I think all all theories are—because of the way we are built—because of the way we interact with the world—all theories are about our interactions, right? Um even in whenever we talk about causality—cause and effect—like even in astronomy say when I say oh there's this gravitational law and this is why the planets move around this—it's not like I'm going around and like changing the planets—I'm running a simulation which can be done—but but I but we do think about it like oh what would happen if this planet wasn't here, right? Which is a kind of intervention—we might not be doing it in in person—but it it's still about our relation—like what what we could do—so I think this applies to—I now think that this applies to physics in general but also like to other physical theories—so in that sense yes—yeah—everything is about our knowledge from our perspective and uh um how how how would it be different if say the the objective collapse is is is true and not this epistemic approach—there is an objective theory about an objective world out there um or the many worlds is true that we are just on different branches of the wave function and there is no collapse and it's basically all all present.

Okay, if the objective collapse is true—that's easier—yeah—check—what would that mean—okay—then then you'd not get a paradox in this situation, right? So this is something we can test eventually like as we superpose larger and larger systems, yeah, and then we can—okay, well at what scale does this objective collapse happen—is it like 5 kilograms—okay—so then uh can a small cat be an observer okay or like I I miniaturize you and now you don't suffer this objective collapse and I run the experiment—I still have the same problem, right?

Yeah. Um or it could be I don't know a collapse in consciousness and whatever when you do a measurement then fine—like then okay you don't have this specific problem—you still have many other questions to answer about your physics and how it interacts with the other branches of physics.

Yeah—what's the biggest one then?

Sorry—I guess you just mentioned—so we—with collapse it has some problems with relativity—being consistent with relativity at some uh for example—

Um yeah—and also needs to justify where is this collapse coming from—like where—why is quantum mechanics breaking down at some scale—

Yes—but still I think that eventually we would find in whatever physical theory I think we'd find the paradox like this—so if many worlds was true then I think again like fine—what does this tell us—what you're going to do with this information—because I still need to use quantum theory from my perspective of seeing this outcome—if I need to keep track of all the branches of the wave function uh it's just—it's not practical—like I cannot do this with finite resources—I'll very quickly run out of of memory—so okay—yes many worlds is true—everything that's possible to happen happens—like so much—also what a boring world it is—world where everything happens is—

Yeah—it—yeah—it only wouldn't be boring if I could switch those branches—but that's—that's unfortunately not—that's not how it works, right? I feel sometimes I like fantasies of divorce dads—what of divorce dad—like people who regret their choices and like okay so what if you know live live with your bad decisions—it's okay—we all do it—no—I have a friend who's a physicist called Johanberg who says that yeah the a world where everything happens is as boring as a world where nothing happens—there's no—

That's one to think about.

Yeah—no reversibility—no no consequences—it's like—

Yeah—which makes me—This is sort of really segue—but free will—do you sort of have in in your—do you believe in free will?

I believe in relative free will in the sense like yeah you cannot predict my actions uh what is the source of free will—I mean is everything I'm made of—if someone knew everything I'm made of—which you know there's many arguments of oh the complexity of this is burn through every hard drive—you you just can't do it—but yeah then they would be able to predict what I do—but does this take away from my experience of thinking I have free will? Like I don't know—we were offline—we're talking about children, right? Yeah—and like when they're hungry you can 100% determine that they'll be cranky—but it's still have their subjective experience of like no no it's my—No—I'm against—I'm reasonable—I'm being reasonable and like and like let them have this experience, right? Uh so I'm kind of at peace with like okay yeah if there's a super observer who can predict all everything I'm going to do like it's okay—it's funny—it's almost as if sort of physics is now sort of sort of confirming this sort of cliche commonly held thing where people say every person has their own reality, right? And somehow you're—and and we all know that's not true—there are sort of objective facts in the world um so I'm a bit puzzled here because we still live in a microscopic world where it just seems to be a a commonly shared world that that stays the same—that is observer independent um is that just because we're all—we're we're then just completely entangled—is that the way to put it?

That's—yeah—I mean one—this experiment was very—it's kind of very fine-tuned—very designed to reach this—Yeah—outcome—like you find this contradiction—but yeah I think most of it is—it's also um yeah it's what allows us to have a shared experience of time passing—that we share systems like the earth—the sun—that we use as clocks—in practice it's like everything that gives us a shared experience of a shared reality that we construct together—it's correlations.

Yeah, yeah—we need these tools to construct it—but that doesn't mean that it's really there outside of us or independent of us.

Yeah, I mean I I—Okay—personally I believe that there is a world that I'm not—like in a box creating—oh—I'm not that smart—you're not a solopsist—what philosophers call solips—no—okay—check uh but but like all we can access is really is our experience of the world—like like what Bernardo says of like we're seeing through stuff—through through a screen—and we interacting with the world uh through a screen—and what gives us like this idea of like oh we interacting with the same thing is really this correlations between us—check—and so in your work as a physicist you are at peace.

With the fact that your work is very much about how that screen works, how that interacts, and and not so much about what it really points to, what that really is, which is still metaphysics in the end, I guess. Whereas I don't know, because some other things we I do in my work is like thinking, okay, what if instead of living in a quantum mechanical universe, we lived in a stranger universe or in a simpler universe? So there's different physical theories that you can make up and now see, okay, how would agents experience the world there? How would this be different? And this maybe gives us some clue of like, you know, that's that's interesting though, because I'd say, okay, we we know we live in a quantum mechanical world, right? We have established that experimentally, at least microscopically, as far as we can tell, it's a very good theory. Yes, and that world is pretty weird, but still also common sensical. How would it be? What would a weirder world look like? What would a simpler world look like if you just sat think about that? So I'm curious here. So I think we'll touch this in some of the other interviews. What I mean by a real world is like one with correlations stronger than quantum entanglement, for example, where you remember we talked about Bell's theorem, how it can be formulated with as a game, but like classically can only win it with like three-quarters probability and like with a quantum strategy you can win it like 80 something percent. Uh, and you could imagine like, oh, what would be like a blackbox view of a world that would make you always win this game? Like correlations like non-locality even stronger than what we have in quantum theory, right? And turns out like that this kind of rules, if you impose this condition like, okay, it needs to be stren quantum in this very specific thing, then it restricts like the dynamics of the just this correlations restrict like the dynamics of systems, like the physics of what kind of physics are possible there, and then it also restricts how we model agents and if you find these paradoxes and so on. Yeah, so this is one one direction I'm super curious there because have you do you have any clue how that would be for an observer in such a weirder world experientially? Uh, this is science fiction, of course, but yeah.

Yeah, it is science fiction, but but we looked into this, so it's a paper with Nura and Vasin, uh, where we looked precisely at this world. And so one thing that is not stable there is the notion of of a system of like, yeah, I treat I treat this thing as a as a system as an observer, but then later on I need to kind of combine it with something else, you know, this whole thing is an observer. So this is it's cool things that we take for granted in quantum mechanics like, oh, I call this a system, which is again it's a choice, it's a choice of boundary that I do, like this choice is not kind of stable in in other it's fluid, or and and it's fluid.

It's fluid. Yes. Yeah, yeah. Would that also mean, because observers are systems in the end, that sort of observerhood is also fluid then? Yeah, in a way it's funny, so your boundary between you and is is just more fluid. And in so in that kind of world, you you can still find this kind of paradoxes. And then we studied also simpler worlds, so like this kind of hidden variable worlds that simulate some of the properties of quantum mechanics, but they're really they are classical worlds, like no all particles have a definite position and momentum, we just don't know, they're just like enforced ignorance. Um, this is, for example, speck and story theory in their model, what would it be like to be an agent in such a world? And then what we find is that it's very hard to then make decisions for agents to like take information and then depending on this information making some decision and taking some action. This is extremely extremely hard.

Why exactly? Why is it hard? Uh, it could be it could have to do with the mechanics of this particular world where like it's kind of all probabilities are 50/50 to some extent, so everything needs to be like you cannot kind of bias your decisions more one way, like a very black and white world. It's a very black and white world, and there you don't find this you don't find this uh paradoxes because it's a classical theory, so you're not expecting, but also you're very limited what you'd be as an agency. Yeah, you can play around with these things. This is just really cool because yeah, that this will be is of course very technical how you construct these worlds, but it's just you you can do this. So how do you do that? Is it just is this is all math? So other people have studied it's a very fruitful I think field of research to try to study to try to understand why is the world quantum mechanical? What we do is we test alternatives. Okay, there's different ways to test alternatives. One very popular one is like, okay, treat everything as black boxes, like I do I maybe there's some electrons in a box and I shoot some lasers, but I just think of like, oh, there's some buttons I push, I get some outcomes. Mhm. I study the probabilities, the frequencies of these things, and now if you put quantum systems inside this this gives you some probabilities, if you put a classical system inside this you get different probabilities, right? Um, but then you can like forget what's inside and just say, okay, what if now I want to see if I get different types of probabilities, so I fix the outcomes, mhm and then I try to kind of reverse engineer what's inside the box. So this is a big field of research like call like generalized probability theories that people it's a way to study it's like one direction to study how how the world could be different from from quantum theory and and what will the uh what's the sort of the the this is maybe sort of um not meant as an insult, but what's the relevance of of coming up with these different worlds for for for physics? How where does it become relevant for for sort of the foundations of physics? So many things, so one would be that maybe we find something that is very close to quantum theory, right, where the probabilities are like almost indistinguishable from what we can see, but which has much more intuitive properties, right? And then is this would be oh interesting, maybe we should study this thing instead, right? Or maybe we try to find or maybe this leads us to try to find an experiment to determine like are we in the quantum mechanical world or are we in this other world, right? Um, another direction of research people do is like quantum theory is such an elegant theory, right? But all the it's derived from four or five axioms that are all mathematical like states are vectors in a Hilbert space, a measurement, and this is it's not very satisfying, right? Because it doesn't tell us much about the nature. So there's ways of directions of research where you just try to find more intuitive axioms that are maybe they're more physical, yeah, and then from here try to derive quantum mechanics, and then you can say oh what happens if I change this a little bit, now I get a different class of theories. Why are we not there? Like why why why is the world like this? So this is kind of the

Yeah, I think this is all just very inspiring sort of as a storyteller and and giving ideas for science fiction, right? We know the science fiction around sort of more many worlds kind of theories that you have these alternative world lines, but then you end up in pretty much the same world as we know, but just different that I can have like a different life, and it's all possible at the same time and can maybe switch those, but it seems to me that the stuff you're working on could lead to even more exotic sort of science fiction. Do you have any any ideas here? Yeah, I think it's it's a very good question. I think we quantum theory has the potential to inspire even more interesting science fiction. This is one of the examples, uh, I don't know in a non-science fiction uh thing I've seen someone almost try to justify an affair with things like, oh, you know, if they don't know then you know, ah, so that's how your physicist how you use this theory, like come on, you cannot control correlations, it's not this, right? Uh, but yeah, I know I there's very rich terrain here for stories that are exactly this, right? So we we had alternative realities or experiences of realities until until we meet and then what happens like do we forget our previous experience or do we? Yeah, it's I think it's something cool to explore that also has not been very explored. I I have a friend who's a physicist, Nicole Younger Halpern, every here well, she did it once, I think she'll do it again, she runs a contest of like short stories inspired by quantum theory. Cool, so like to give us something like yes, so um yeah, I don't know, but I think there's potential here. And because I think many people, of course, at the Assia Foundation, we're about sort of analytic idealism, consciousness being fundamental, and idealism is associated mostly worldwide um with people who are religious or spiritual, right? That that's sort of the idealism we we we know that is sort of just ancient coming from all these traditions, and we approach it analytically. And that limits what we can or cannot say. So if you ask me or Bernardo or does God exist, I'll say yes, but not in the way you commonly think about God as delivered in these ancient texts and stuff. So you have all these limits because you want to approach it analytically, right? M um but I do I do think that quantum quantum mechanics, for instance, inspires especially people who have this inclination who are sort of spiritual to say, ah, see, everything's about consciousness, I can sort of just um manifest my own world if I truly believe in things, and you have to sort of like this whole scene of manifestation, and they all sort of run away with quantum theory, and then of course physicists are annoyed by that. We even had this movie, What the Bleep Do We Know, like which really popularized quantum mechanics, I think it's 20 or 25 years ago, but it just went off the rails because it had these fragments in it basically saying that if you want a Ferrari and just focus on it, you manifest you know, stuff like that. But what's sort of the way how all of this informs you or do you think how it can inform people on a meaning level? Um, can physics tell us anything there? Or

Yeah, so I think it's told me some things. Uh, one thing is that we draw these boundaries around us, we try to classify the world because that's how humans survive, like we find patterns and we classify things like from this point on it counts as a mammal uh before it doesn't, right? But but nature doesn't care about these boundaries, right? Like the Eisenberg cut, I treat these photons as quantum mechanical but not the ones that are entering my eyes um like this idea of like be aware that whenever you try to classify something, whenever you try to draw a boundary that someone else might be drawing it differently, and that these things are not um so so one example, for example, uh when I teach the double slit experiment, I like oh is light a particle is it a wave? Now because I have quite a young like generation of students now, I say there's like people saying that you have to be a man or a woman, like we created these categories based on some biology, but like they're not always useful, nature is more diverse than this. It's like yeah, we said oh we divide the world into particles and waves, but like light is is something else that yeah has properties of both, right? I think physics has really informed me in this, maybe I had this inclination already, right? I mean I'm biased too, and and that's why this is the lesson I see in physics, yeah, but yes, the main thing is really like for me it's like yeah, more and more things are subjective and relational, like yeah, I see my organized chaos, I know exactly where I put this thing um and that this I can I think it has helped me going through life, yes. Yeah, like like there's no universal bad thing like from the perspective of the bad person, they actually uh you know their own interests and views and stuff, and like this I know helps me connect to people when ah yeah, that's like this non-dualist notion from coming from from physics here that you say there's no objective way. Yeah, I yeah, I to me sort of that was it's funny because yeah, we're I was brought up religious and in religion in Christianity, I was brought up was was quite dualistic in a sense, definitely when it comes to good and evil and making that like very very objective, right? It's like God and Satan, and it's like pretty pretty clear divide, and it's transcendent evil and stuff like that, and I still when I'm entering into discussions with friends who are still more on that page know there's like pure evil, I'm much more like no, it's us sort of having to sink and navigate this uh we create this moral landscape as mammals as humans, and that doesn't make it it's it's absolutely real for us. Yes, and right, but in the end if if if physics also teaches us that there's no outside way to to to just objectively say this is the case for everyone, I can map that perfectly well with sort of a non-dualist way of looking also ethically at the world. Mhm. Yes, yes, which is not like a free-for-all, right? It's not like all the options are true, like we have correlations, there's a reason why we consider some things to be bad as a as a decision that we make together, right? Yeah, yeah, it's not like yeah, genocide is okay, it's not notification. Absolutely not. No. Um, so and then and were you brought up religious if I may ask? Uh, so no, not my parents, I mean I was brought up in a Catholic country, but Portugal in I think still recent I can still call it recent years, but but but my parents are not religious, but they're very artistic and cultural inclined uh people, but like I went through a very religious phase when I was six, seven, eight years old. Oh, that's interesting. I was very that's pretty you were devote then? Yeah, yeah, yeah. I was like comparing different versions of the Bible and wow, but then at some point it's like this is not there's all this inconsistencies. It's not it's not just inconsistencies, it's like it's a very limited way of looking at the world, and you look at different religions and learn a little bit from everything and you know um a few a few points. One, I think it's very useful for scientists working on foundation, I mean for everyone internally, but for scientists working on foundations to be aware of their own relationship with faith, religion, and their own mortality, because these things kind of sneak up into your research if and then it's good to be aware of this of like um like I don't know, I'm really afraid of death, I don't want to think about it, and then now suddenly I believe in many worlds in which you know there's always a branch of quantum theory. Are you not me? No, no, I know, but are you so you really think that sort of these psychological mechanisms slip into even I I think they do a bit a little bit that you know I favor some view of the world because of the way you know why was Einstein so upset about quantum mechanics? I mean, I don't don't think it's about his his view with mortality, but you know about some way you brought up and you think about how the world should work that you might not be uh aware of. Uh, yeah, and in in foundations I see many people believing something because they want it to be true, and then if you if you dig in enough like sometimes they're just they're afraid to die, and this maybe explains a bit of the generational. Yeah, that's super it's super interesting. I think it's just being brutally honest with how the human mind if we say modeling the agent in in the system, we have to also model in metaphysical assumptions or sort of psychological mechanisms that just play into how you want the world to be, but then in the end we do we do have this still this sort of this stick that can ground us which we call mathematics then, right? Or still

Yeah, I mean, of course, yes, yes. Yeah, yeah, yeah. But it's about like, oh, you have a no-go theorem now, which which assumptions do you prefer or which ones are you happy to let go of, or which directions you explore from there? Sometimes like all this beliefs we have kind of, but but how if I may ask how does it play in into your view on your being a human and your mortality that does your work in in some sense sort of inform you about this? Is that

Yeah, actually, yeah, I mean this was at university that I learned about what are waves like think of a Mexican wave in a in a football stadium, like people get up and do this and then the wave propagates, right? The matter moves locally, but the wave is actually propagating. Oh yeah, that's nice. Yeah, it's like this is the perfect metaphor for life and death. I'm happy I'm not going anywhere else. It's like okay, I'm here, I do my own thing, I move a little bit, then I die, and I'm part of the big wave, and like the wave propagates, and this doesn't mean like my spirit goes somewhere, but like my actions have consequences in the world, and this this moves on, and I'm okay with this. Like check. Yeah, we lose my point of view when I die, or I will lose my point of view, like other people don't have my point of view to start with, right? But the but the information associated with it or the correlations associated with with it in some sense sort of will will survive you or yeah, like in the sense that I like I remember my grandmother as if she was here, right? And her actions have still an influence in yeah, that's that's in my life, and that's that's it's beautiful. It's beautiful, but it's I'm lucky in in having the kind of brain that is okay with this belief, right? Many people cannot help it, and you know, and you really need something else to comfort you, and and it's just good to be aware of like what tools am I using to comfort myself and like and is this also influencing my work, which is okay, I just should be aware of it all. Yeah, wow, it gets so it gets so subjective, but in a in a sense that how how to put it is I think I think close close closer to truth than than thinking that there's this objective way to to to go about here. I I I agree with you that I also think I'm more inclined thinking that I I play this role that I don't understand yet, and I will might dissolve, but it it has had its function for the whole um yeah, it's what it makes me think is that we're talking about Eisenberg cut and where what's quantum and what's classical, but as persons we also are making this cut, right? This is the cut I place sort of literally here, and that's that's my cut. Yeah, yeah, and I just think that it's all classical, but or say I'm quantum and then you're classical, where how can I know? But um, hearing you um and if there's no objective way to make that cut also sort of feels very human and humbling that I cannot know sort of the larger hole I'm a part of, right? And that the the the gut I make is is very subjective, even though I experience it to be very real. That's what it sort of is telling to me. Yes, yes. Also like the boundary between what is I I mean I is a is a convention, right? That I say, okay, maybe like this feelings in this body, this is where I cut it, but like but but this is a convention, there's been others throughout history, and you know, and it's something that we use for I don't know for Yeah, applying responsibility or for punishing people or rewarding people or for whatever has some fun social function, right? But it's this boundary is in principle like motel and is just where we put it, and it's just super cool that we're that you're working on modeling sort of that all right, we are later this year we will host a conference uh together where you are also involved and co-organizing with us on integrated information theory, which is also a a theory that tries to model the boundaries of consciousness or sort of where a whole sort of a conscious how we can sort of map that, and that I think is pretty cool that we can sort of say something meaningful about how at least people experience being a conscious unit, so to speak. And I'm still puzzled, and this is also science fiction, and I mean uh I spoke to Kristoff Koch, who who says okay when you get to sort of a new maximum integrated information uh so he say if I connect the brains of the two of us, you get to this maximum of new integrated information, there will be a new subject, and it will also that both Lydia and Hans at that moment cease to cease to exist, and you have this one one bigger brain um so that that might be true, but it also might be true that perhaps the bacteria, the cells in my bodies have some form of consciousness that I'm not aware of, and I'm just experiencing this whole larger meta consciousness. These are all sort of the questions that I'm very much open to, and I just think it's cool that we now are finding ways or trying to explore it, right? Yeah, it's cool. Yeah. Okay, Lydia, thanks so much for this conversation, and I really enjoyed it, and I I really like how we touched how we got from quantum computing and these thought experiments to sort of more questions about the meaning of life, so just thank you very much. It's great for your sensia to have you as a research fellow working on all of this, and we have much more content uh also with you um co-interviewing with me people we visited in in the Vienna foundations of physics conference, and we will of course have a conference on IIT later this year, so there's more content with you coming up. And for you watching this, we will put descriptions in the descriptions below links to papers of Lydia and the scientists and physicists she referred to, and please leave questions because uh Lydia and I will be filming more this year, so we can touch and answer uh touch upon your questions and answer try and answer them. Thanks so much for watching.