Quantum Mechanics Contradicts Itself (and He Proved It)

It really troubles me that there is this problem, this contradiction. I had some students who said for psychological reasons not be able to work on such projects. This would disturb them too much. Most physicists apply quantum theory and never turn it on themselves. It turns out there are scenarios where quantum mechanics leads you to contradictions when applied to quantum observers.

"You're absolutely certain it's heads. I see it's tails."

That's Professor Ronaldo Rener of ETH Zurich telling me that it turns out one of these things must break. Number one, quantum theory applies to everything, including observers. That seems natural. Number two, that measurements give single outcomes. And number three, that reasoning stays consistent. All of these are already assumed to be true. We think that the large world is made up of the small and thus quantum theory applies to everything else. Yet, we don't know which one of these is wrong, and getting rid of any is just as unsettling.

On this channel, I, Kurt Jaungle, interview researchers regarding their theories of reality with rigor and technical depth. Today, we discuss why this drove the professor from many worlds through to what he calls no man's land. We connect it back to the black hole information paradox, to the limits of probability, and to why some of his students refuse to continue because these projects are about the fundamental question of what you are. Professor, you've proved that quantum theory can't consistently describe itself. What does that actually mean?

"So maybe I should first explain what type of question we are really trying to answer here. So what we have learned from quantum theory, or what at least I think I've learned from quantum theory, is that there is a constraint on what we can know about the world. So this kind of starts already with the Heisenberg uncertainty principle, which tells us we cannot really know the position and momentum at the same time. But this is really a constraint on the knowledge. It's I don't see it as a constraint on what we can technically measure because we can individually measure position very precisely and momentum very precisely, but somehow we cannot have the knowledge of both at the same time. So quantum theory somehow very substantially restricts what we can know about the world in a sense. And now, um, this raises the question that, um, when we cannot know everything about the world, can we actually do still do physics in the way we thought we do physics? Or, in other words, we can say that usually when we did classical mechanics or electrodynamics and so on, we kind of assumed that in principle we can know everything about the physical system. Of course, we knew we will not know all the positions of the planets and every atom in the world to to an arbitrary precision, but we always thought in principle it can be known. So there would in principle be a very powerful physicist who would know all the positions and momenta of all the particles in the world and then we can make predictions and so on. And now, um, when quantum theory tells us that apparently there's a constraint on knowledge, we can turn this into the question: is there a constraint on doing physics in a sense? And so what does this now mean for me? It means for me that if I want to, or how can I answer the question whether we can do physics in our world? I could now say a physicist is itself a physical system. So I could, for example, try to describe you doing physics. So you, you could, for example, let's say, um, interact with an experimentalist. I'm not sure you, you are not doing experiments yourself, I guess."

"No, except this right now."

"So, let's suppose you're interacting with an experimentalist, your experimentalist friend. And so this experimentalist friend tells you about an experiment, and maybe you go to the lab and and do some checks there, and then, um, you do calculations, let's say, and make a prediction for what he's going to measure. Now, what I could do is to kind of analyze how you are doing that. So I kind of take you as a physical system that interacts with the experimentalist, who is also a physical system. So I see all you're doing and all your experimentalist friend is doing as a huge physical system, and then once I do that, I can see how information flows. For example, I can see and I can describe that now using quantum theory. And when the experimental friend of yours tells you that the experimental setup looks like this, then some information flows to you, to your brain, and you're picking that up, and you know what is the experiment, and then you will do a calculation, and then maybe you tell him a prediction. So some information will flow back from you to the friend. And now, as I said, quantum theory tells us there are constraints on how much information can flow or how much information we can have. So I can now ask the question: is there a constraint of how well you can kind of predict the experiment of your friend, just by the fact that everything has to be modeled within, um, let's say, again, within quantum theory in that case. So, in other words, I, I don't just consider how you apply quantum theory, but I consider you as a physical system that kind of is part of the world and therefore also described by quantum theory. And I check whether this is now in a sense consistent with how you see the world. So we have now two perspectives, because you would describe, you would from your perspective describe how your friend does an experiment. You make predictions. That's your perspective, and I take the perspective of describing how you describe the the f, um, how he's do he's doing experiments. So I'm kind of on a second level, in a sense, I'm describing how you are doing physics. Now, if you assume that quantum theory is a universal theory, then clearly it should be able to describe not only the experiment itself, but also the experimenter and you, how you interact with the experiment, and how you make predictions about what your experimentalist friend does. So, in a sense, I'm now, um, saying that if quantum theory is universal, it should also be able to describe how we are doing physics. So the very process of doing physics, and by this I mean to get information about an experiment, make calculations, solve the Schrödinger equation, if you like, calculate probabilities, and so on, that's itself a process, and I describe this process now also within quantum theory, and surprisingly, this usually hasn't been done. So, in a sense, you would say this is a very natural question: if you have a theory of the world, then because we are part of the world and we are physicists, the theory should also be able to describe how we are actually using the theory and doing physics. So it's in a self, in some way, a recursion. So we are kind of asking the theory to be able to describe how users of the theory, um, do calculations and so on."

"But that's necessary for a universal theory. So I think in principle, this would already be necessary in a sense for, let's say, thermodynamics, because you could say thermodynamics applies to everything. It also applies, it should also apply to us as physicists. So I could actually take an outside perspective in a sense and say, I now describe using thermodynamics how you are using thermodynamics to describe your heating or so, or your your air conditioning or whatever. So you are applying thermodynamics, but I'm applying thermodynamics to describe how you are applying thermodynamics, and I model you as a thermodynamic system. And if you assume that thermodynamics is universal, this should work. And this type of question has very rarely been asked. And could say the thing that comes closest to that is actually Maxwell's demon, because there we kind of describe another being operating on a thermal system. And now, if we return to quantum theory, I can ask this very same question with quantum theory. But I could in principle ask this question with any theory. So if you develop any new theory of the world, I would always say this is a good consistency check whether the theory is good. If you claim that the theory applies to everything in the world, then in particular, it should apply to the physicists who are using the theory. So it's a new type of consistency check. And so what we did in in this work that you were, um, mentioning, um, is to somehow apply this consistency check to quantum theory. So we were asking, is quantum theory able to describe a quantum physicist applying this, who is applying the theory to some physical system? So that's the question. Now, what we found is that, um, maybe I could say it in two stages. What we first found is that there's something that is kind of undefined if you just apply quantum theory, because quantum theory is usually just applied from the perspective of one physicist. And so we have to ask the question: what are even the consistency conditions that we want to impose? So, for example, let me come back to the example I made before. You are describing the experiment of your friend. Now I'm describing you and how you are describing the experiment of the friend."

"Okay."

"Now, one would expect intuitively that your description of the experiment of the friend, I mean, this could be a prediction. Let's say the the friend measures a spin particle, and you predict the outcome of that measurement will be spin up. That could be your prediction. Now, I'm analyzing how you get to that prediction. And let's suppose, um, I come to the conclusion that, um, uh, that you actually indeed predict that this, um, this spin is up, and I also predict that the system that you're talking about is in spin up, then I would say this this looks consistent, because I've now kind of described what you're describing, and everything is kind of consistent. But now, what if it happens that in my analysis of you, I find that you think the spin is up, but actually, when I analyze the system myself, of of this experimental friend of yours, I find the spin will actually be down? That could be because I just apply quantum theory to a larger system. And now it turns out that for certain very involved setups, which are far from standard, these so-called Wigner's friend setups, that can maybe explain what they mean, but they're very artificial setups, but in these artificial setups, um, we get contradictions. So, in other words, um, if I app, if I, um, analyze how you are doing physics, I find that you get a result which is not consistent with what I get if I directly analyze the system that you are analyzing. So I kind of do two analyses. I first analyze the system of interest myself, could be this experiment that your friend is doing. I just analyze it myself, and then I analyze how you are analyzing it, and I now check whether these two things are consistent. So one is kind of like a direct analysis, and the other is kind of via you. I just predict what you are going to predict. And now the prediction of what you are going to predict about the system should hopefully match what I'm directly predicting about the system. And that's a kind of consistency condition. And we first need to ask ourselves: what type of consistency conditions do we even require? Because we could just say, um, maybe we don't require anything. We already learned that quantum theory is kind of strange. So let's just not even assume, or let's not hope that there is any consistency there, and then there is no problem. So what we did in this work is to kind of have a kind of minimal consistency condition. And the minimal condition, consistency condition is kind of that if you are very, so if I come to the conclusion using my analysis of your process of doing physics. So I, I analyze how you do the calculation and so on, and I come to conclusion that your calculation will, um, have the result that with absolute certainty the spin measurement will show, and that the spin is up in that case, um, if I now directly analyze what the spin measurement should do, I should also come to the conclusion that it's up, or I should not, I should at least not come to the conclusion that with certainty it will be spin down. So that's kind of a minimal consistency. So it should never be the case that you are certain that the spin is up, and I know that you're certain it's up, whereas I'm certain the spin is actually down. That would be a contradiction. So what we are doing is to say, let's ask for this minimal consistency. Let's just say this should not be violated, this consistency condition."

"Just a moment. I want to make it clear because people who are listening may think, well, if you flip a coin, >> okay, and and you take a look at that coin, the coin says heads. That's in your head. Okay? So, the heads is in your head. And then I'm thinking, okay, what is Professor Rener >> thinking? What's in his head? And obviously, I have incomplete information. So, I could say it's 50% heads, 50% tails. Obviously, I can make a prediction and just say I'm going to say it's tails. I was incorrect. But there's no inconsistency in the laws of physics there. So you're thinking about something that's extremely ideal and taking quantum theory quote unquote seriously. People like to use this term and seriously meaning whatever it means. So >> there's something that's ultimate about it that's final. There's there's no wiggle room here about just predictors and knowledge and so forth. I I need you to spell that out about the difference between just making an arbitrary prediction versus the ideal reasoner with ideal rationality or what have you making a prediction."

"Mhm. Yes. I think this is a very important point to kind of make the distinction between uncertainty or incomplete knowledge or contradictory knowledge. So indeed, if I here flip a coin and, um, I could do that. I have one here. So I flip it. I have the outcome. And now I could say I look at the outcome. So I have certain knowledge about this outcome. And I could now ask you: what's your knowledge about the outcome?"

"And I guess you would say it's probably with 50% or 0.5 probability up or heads and with five tails."

"Now, this knowledge is not the same knowledge that I have, because I know for sure it's tails. I just saw it here. So that's, um, in that case, I would say you had incomplete knowledge, and this knowledge is not the same knowledge that I have, and that's however not a contradiction. Um, obviously, I mean, that would be very strange. So why is it not a contradiction? If I later tell you that it's actually tails, you would say this was perfectly consistent with you just not knowing whether it was heads or tails. But let's suppose you were actually, for some reason, because you probably analyzed the way this coin was was flipped, you come to the conclusion that its heads, you are absolutely certain its heads, and you correctly applied the theory and everything. But I see its tails, then you would say there is somewhere a contradiction. And this is the type of contradiction we found in this thought experiment that we were analyzing. So it's really not just incomplete knowledge, it's contradictory knowledge. And contradictory knowledge, I would define it as there is no kind of additional information that I could give you that after the update would make you have the same knowledge, because before, in this experiment, I could have done that. I could just, or I did it. I told you it's tails, and then you do a knowledge update, which is like you hear me telling its tails, and you say, okay, then you believe it's tails. So now you're also certain it's tails. So you just updated your knowledge and got to the same conclusion as I came. But if you were certain it's heads, and I tell you, no, it's tails, then you, you cannot do an update. You will just run into a contradiction. When you try to apply Bayes' rule, you will basically divide by zero and get nothing sensible out of that."

"Okay. I like this classical analog. I think it makes it much more clear. So suppose the person listening can model the coin 100 with 100% accuracy. We're in classical mechanics, so we have no Heisenberg uncertainty. And then they calculate that the coin landed tails. And then you see it as landing heads and you report to them, no, it actually landed heads. They would think, I must have modeled it incorrectly. But you're saying, no, no, no, let's assume you modeled it 100% correctly then. And we're going to get to your three criteria soon. But then you could say, well, either there's something wrong with me, which we've removed by your datum. You're saying like, no, no, no, there's nothing wrong with you. You've used math correctly and classical mechanics correctly, or there's something wrong with classical mechanics, or there's something wrong with with so and so. There are a couple of escape routes that you can take, which we'll get to, but this is a classical case. It's just to build some intuition for the quantum cases."

"Yes. Yes. I would completely agree with what you said. That's a a very good summary of the type of contradiction. Yes. So indeed, I mean, you, you mentioned like, we assume it's very precisely that experiment, and there's no error. And because we are in a sense talking about the thought experiment, we can just do this idealization. We can say, let's suppose all the agents that are involved, so I call these agents the different physicists, they just have a priori perfect information about the system. So let's say before the experiment actually starts, before I flip the coin, everyone gets the very same data about how the world is, what's the state. And then, um, we start the experiment, and I flip the coin and don't show you the outcome, but you calculate it and so on. But I can make these idealized assumptions in a thought experiment and then ask: does it come out well? Because at the end, it's a statement about the theory, not about the actual, let's say, experiment with flows. I'm just telling you, if I feed into the theory all the information I have, what does it give me? And so by construction of the thought experiment, I give you the full information and the correct information about the world when the experiment starts."

"Would it be accurate to say that the classical analog would be something like, suppose Laplace's demon exists? Then what if Laplace's demon is wrong in its prediction? Then you would say, well, how does that even make sense? If you're supposing Laplace's demon exists, almost by the criteria of it existing, of what Laplace's demon is, it has to be right. So would it be that you're doing something similar to that before the quantum case?"

"Yes, I would say this is another way to put it, the way you put it. But I would, the question is, how would I define Laplace's demon? I would, I mean, if I define it as it would really make the correct predictions, um, then this probably wouldn't make sense, because then by definition, it's correct. Yeah. But if I define it to be a demon that applies the theory that we are trying to analyze absolutely correctly, then I think the analogy works, and that's what we are doing. So we are not assuming quantum theory is correct. We're just assuming all these agents are applying the theory perfectly. So it would be like a Laplace's demon demon who is perfectly applying whatever theory it is that we want to test, let's say classical mechanics or classical theory. But I'm not demanding that this is actually the correct description of the world, because that's what we want to test. Kind of want to test whether the theory is a reasonable theory. So it's kind of a perfect demon, in a sense, with respect to the theory that we are testing."

"When I'm wrestling with a guest's argument about say, the hard problem of consciousness or quantum foundations, I refuse to let even a scintilla of confusion remain unexamined. Claude is my thinking partner here. Actually, they just released something major, which is Claude Opus 4.6, a state-of-the-art model. Claude is the AI for minds that don't stop at good enough. It's the collaborator that actually understands your entire workflow, thinks with you, not for you. Whether you're debugging code at midnight or strategizing your next business move, Claude extends your thinking to tackle problems that matter to you. I use Claude actually live right here during this interview with Eva Miranda. That's actually a feature called artifacts, and none of the other LLM providers have something that even comes close to rivaling it. Claude handles, inter alia, technical philosophy, mathematical rigor, and deep research synthesis all without producing slovenly reasoning. The responses are decorous, precise, well-structured, never sycophantic, unlike some other models. And it doesn't just hand me the answers. The way that I've prompted it is that it helps me think through problems. Ready to tackle bigger problems? Get started with Claude today at claude.ai/theoriesofeverything. That's claude.ai/theoriesofeverything and check out Claude Pro, which includes access to all of the features mentioned in today's episode."

"Spell out the difference between being inconsistent at a local level, so from your own point of view, versus at a global level, and which one does your theorem apply to?"

"So maybe one should understand the thought experiment that, um, I'm kind of proposing as really just a test purely of the theory, not of, let's say, the experiment. It's a thought experiment which is based on the assumption that the theory is correct, and I'm not comparing it to the actual world. I'm kind of asking the question: is it a possible description of the world? So I'm, the thought experiment is not really making, in a sense, the assumption that the world is correctly described by quantum theory. It's just making the assumption that quantum theory is a possible consistent description of the world. And if I'm on, if I'm doing that, then I cannot really compare it to the experiment. So I cannot have a direct contradiction to an experimental outcome, because I'm just not testing the actual world, in a sense. So I'm just testing the consistency of the theory. So how can I then check a consistency of the theory? I can say, generally, a theory is consistent if different ways of reasoning within the theory lead to contradictory outcomes. So this is very general, I would say. I could have just a theory that is not even physics. Let's say some mathematical theory, and I want to, let's say, prove a statement, and maybe the theory would be contradictory if there is one way to prove that the statement is actually true, and there is another kind of proof that I find within my set of axioms that shows, um, the statement is actually false. Then I have done two arguments which lead to contradictory outcomes, and this is the type of contradiction we get. So we have quantum theory. Quantum theory can now be used in different ways. And in this case, it's kind of one way to apply it is in the example I had before. I apply it directly to an experimentalist and make a prediction about the experimentalist. That's one way of reasoning. And the other way of reasoning is I apply it to you and study how you make a prediction about the experimentalist. That's a more indirect way of reasoning. But it should also be an allowed way of reasoning. And if these two ways of reasoning contradict each other, then there is, um, this I would call, um, such an, let's say, global contradiction, in a sense, because I cannot even tell which one was now right. I just have two different ways of coming to a conclusion, and the conclusions are opposite. This tells me something is wrong with the theory. I cannot see a contradiction to an experiment because I didn't do an experiment. I'm just theoretically analyzing the consistency of the theory, in a sense. So, um, I think it's really important to make this, um, distinction between a thought experiment where I just think about the possible description of nature, and I accept the fact that for the moment I cannot test it, and then the only thing I can do is kind of do consistency checks assuming that, um, this is a possible description of the world, but I will not test it against the actual experiments in the world, but I can still test whether different uses of the theory lead to consistent results. And so that's, in a sense, what we are doing with quantum theory now in this thought experiment. And so necessarily, the contradiction is kind of a contradiction between two different ways of using the theory. But if both ways are allowed ways to use the theory, then of course we are in trouble, because then we have a theory that somehow has too many, um, offers too many ways to come to a conclusion, and it, we cannot decide which one is the right one."

"Let me ask you something funny."

"Mhm."

"How do you see quantum mechanics that's different than your colleagues?"

"Okay. Yes. So that's a very good question in this context, because I could say, if I analyze how you are analyzing a system, how could I ever come to a contradiction with me directly analyzing the system? That's, um, actually a core point of, um, of the whole question, because you could say this shouldn't happen. And this brings in, let's say, another, um, question of what do we actually do when we use quantum theory? And what we are doing is we always cut out the part of the world which we are actually describing with the theory. We are never describing the entire world. And this is necessarily so, because if we did describe the whole world, we, we would necessarily have to describe ourselves, how we are applying the theory. And this would, um, lead to a cycle. Like, if, if I describe the whole world, I necessarily describe myself. And we, I mean, this may somehow be possible, but at least within quantum theory, we don't have the tools to do that. Quantum theory doesn't tell us how to describe a system that is myself, in a sense, because I would have to automatically describe the reasoning process. And so this leads to a terrible recursion that is just not dealt with in quantum theory. So I would say the minimal, or the maximal part of the world we can describe is kind of everything without me. I cannot describe anything larger than that using quantum theory. Now, this is actually a statement some physicists would disagree with, because if you are a many-worlds or a Bohmian, you would say we can actually take an outside viewpoint and describe everything. And in a sense, they are correct, because they would somehow assume they are outside, because by saying we take an outside viewpoint, they kind of suggest there is another super observer or something outside who does all the calculations and everything, like a godlike being. Yeah."

"And if you say, okay, physics can just be applied from kind of this God's perspective, that would be okay. But my, let's say, the a very basic ingredient in in the, let's say, approach I took is that at the end, we want, um, to have the minimal assumption that we as inhabitants of this world can do physics, and that not only God can do physics, which for me is a very natural assumption, but, um, it's, it's very disputed. So I, I have this assumption that I want to be able to do physics, but I'm myself part of the world. And if I make this assumption that physics should be done by, it should be possible for us to do physics, and not only for an external to the universe being, and it's not even clear what that means, because if you're a being, then you are again part of the universe. That that leads to troubles. Well, the question I'm asking is really: can I do physics? And if I'm asking this question, I, I have to be kind of modest and say, I can describe everything except myself using the current theory. Maybe there is some future theory that allows us to do recursive reasoning. But current quantum theory doesn't even talk about that. It just assumes we are like the description itself, the the physicist is kind of external. That's an implicit assumption in a sense, because otherwise you couldn't even write down the state or anything of the system, because the state will constantly change as you write it down and so on. So this now means that, um, I necess, so if I describe everything except me in the world, and you also take a maximal perspective and describe everything except you, then we still have different perspectives, because you didn't describe yourself, and I didn't describe myself, but I described you as a physical system, and you described me. And this is the type of setup where we find the contradictions. So we find the contradiction in scenarios where there are agents who are only, who are not describing the same part of the world, and they cannot describe the same part because they are themselves part of the experiment. So one important ingredient to the experiment is that at some point, there is a measurement done on one of the physicists who are taking part in the experiment. That's what I meant when I said before that this is a very special situation, a very non-day-to-day situation, that one of the physicists is kind of going to be measured by another physicist in a very strange basis, in not just in the, let's say, I mean, a measurement would be I just ask you a question, that's also a measurement, but I could do a very complicated measurement, like a Schrödinger cat type measurement. And if I, if someone did such an a measurement on, then, um, that, um, or he has to describe you as a quantum system. But if you are also a physicist taking part in the experiment, you will not be able to yourself describe that that part of the experiment, in a sense. And so by, by, um, using such, um, building blocks in this thought experiment, so measurements of agents, I make it impossible for the agents to describe everything. So each agent has kind of a restricted view, and the view is kind of, um, just the view, um, that we usually have in in daily life, like if I, you talk about the experiment of of your friend, you will not describe the whole world, you just describe that one, and so you have a different perspective from, for example, from me, who who decided to, let's say, to describe you as a physical system. And if you're now asked to have the same perspective as me, you would answer, no, this is not possible, I cannot describe myself. So in a sense, we are constructing a situation where it's inherently impossible for the agents to all have the same view, because they are somehow themselves part of this whole experimental setup. Some operations are done on them, and so on."

"What is the minimum amount of observers required for this no-go theorem, or or whatever you want to call it?"

"That's a a very good question, and unfortunately, I don't have an answer. I would say certainly three are needed, but in in the experiment we found we need actually four, and I'm not sure I can reduce it to three."

"Okay. So is it important that my modeling of you has your modeling of me in it, or is it enough that my modeling of you has my modeling of you monitoring person three, monitoring person four, but doesn't have to come back to me?"

"Yes. So, um, and the way we try, so yeah, this is also important. So it's, um, in a sense, um, we try to avoid exactly such recursions, because then we would again be back in that situation. We also, so the experiment is set up in a way that the individual, let's say, uses of quantum theory are uses that you could do without having to describe yourself. And it's not obvious that this works. So we had to play a long time with arranging these agents in such a way that it works."

"Perfect. Okay."

"And, yeah, at the end, it's, um, so because there's often a criticism saying that, oh, at the end, I actually describe myself indirectly by describing the other agent. But, um, actually, to sort out this criticism, we actually wrote the software to, together with one of my collaborators, Lilia Del Rio and Nuran Aliva, we once, um, kind of decided to write the software that actually implements the experiment. And if we would have such a recursion, we couldn't actually program it. So this software kind of proves we can describe it by not running into a loop."

"And, um, it's actually even publicly available. It's, we put this, um, at some point on the archive. So if anyone is interested to play with the software, but this is a question that always came up, and so we try to kind of convince people that it works by having this kind of automatic reasoning, if you like. So the software, um, allows you to specify what the agents need to describe, and then you see that the description is not running into a loop."

"Okay. There is, however, some loop going on, which we may talk about when you ask me about how I think we can resolve it, but I don't, it's not the kind of description. So it's not that, um, so just to be clear, if you, in the experiment, you would describe me, um, you don't need to describe me how I'm describing you. You could just describe me on a, maybe only parts that are not, or only the parts of me that are not involved in describing you, for example."

"Okay. Now, is it important that the state in your experiments that the other person >> measures is a definite state, or can I give my prediction of what you see as a probability distribution or amplitudes, or just a wave function that hasn't been localized?"

"Mhm. Yes. So, um, we tried in this experiment to actually avoid talking about states when we, um, phrase the final statement or the final claim. Of course, we, we talk about states in the analysis, and the reason is that there are very different views on what it means to say that we have this state. So there are people who think of the state, of course, as knowledge, and others who think of them as something real. And so, um, the entire experiment is a bit like, you can maybe compare it to Bell's theorem, where where could say, at the end, the Bell inequality doesn't talk about states, it just talks about correlations that you find and expectation values. And this is a bit similar in this thought experiment. So in the analysis, of course, we use quantum theory and have to evolve, involve, I mean, that involves talking about states, or you can also take the Heisenberg picture and then you talk about operators, if you like. But, um, at the end, the objects the agents are talking about are predictions about outcomes. So you could, the only, let's say, thing we need is to say that to realize the experiment, we have some initial state that is known to everyone. But because that's a state that is kind of known to everyone, the question of whether it's epistemic or ontic, and all these questions don't arise, because it's then just assumed to be common knowledge, and then everything is the same. So it's as if you would say, let's, let's start with a spin, um, pointing in the up direction, then we kind of know what that means operationally. So that's unproblematic. But as soon as I'm going to use state as knowledge, I'm entering kind of a a terror that is a bit ambiguous, because people may have different views on what that is. So what we try to do is to say, everything can at the end be, um, phrased in terms of the predictions. So the agents, um, have this initial state they know, and then they just calculate a prediction, which is of the type, I'm have this probability that this outcome will occur, and this is not phrased as states. So we try to kind of not give importance to the meaning of states, and therefore talk as little as possible about quantum states in this experiment. People should know that I'll be placing the links to your work and the full articles in the description, and they'll also be on screen as well."

"So for the physicist who's not a quantum physicist, >> who maybe is just a relativist, or maybe they're a materials engineer or something like that, >> I want you to give an analogy that I'm forcing on you right now, which won't work, 'cause analogies tend to not work, but it can give a flavor. So, chess is often used as a way of thinking of physics, and the pieces are like particles, and then they have rules and so forth. >> And then you're saying that many physicists like to say, 'Okay, well, let's just take a look at the board.' And you're saying you're taking the perspective of the player, which is the god's eye view, but in physics, everything is physics. It's as if everything is the board. So, you can't jump out and take a player's view. >> Okay. Firstly, that's your critique about God's eye views. Then then what you're saying is, let's imagine you're a rook. >> You are. And if I may say something about that. So I wouldn't even phrase it as a criticism. I would say, even if you, if an, let's say, someone believes there is an outside you, and that this view makes sense, like the player's view, the chess player's view. I wouldn't necessarily, I wouldn't object. They would say, okay, that's, you can take this, have this opinion that there is this outside view, but there should at least also exist an inside view from the, let's say, chess, um, or whatever there are these different, um, characters on the chess, but from the king's perspective, because the king is is one of the >> um, parts of the world. But, um, this, so I'm, I'm kind of asking that this inside perspective also exists. And I'm not necessarily excluding the exterior perspective. I think it's not necessary because we don't have this exterior perspective. But I think whatever your view is about whether this God's view exists, God's viewpoint exists or not, at least the inside viewpoint, our view also exists. So I'm just against denying the existence of that. And I want a, so my requirement on a physical theory is that it's usable by us in a sense. >> Okay, perfect. >> And if it's also usable from the outside, that's fine. So I'm not, um, trying to exclude that viewpoint. I would only object if someone says the inside viewpoint is not a legitimate viewpoint. Then I would say, okay, why are you physicists? You're part of the world. So by denying that as a valid viewpoint, you basically say you're not a valid physicist, >> right? I remember the doubt before launching this podcast. What if no one listens? What if I'm wasting my time? If you've ever felt that way about starting a business, Shopify is the partner that turns uncertainty into momentum. They power millions of businesses and 10% of all US e-commerce. From Allbirds to Gymshark to brands just getting started. No straggler left behind. Shopify's AI tool writes your product descriptions for you. It enhances your photography. It builds you a stunning store from hundreds of templates. Forget about the dormative haze of bouncing between separate platforms. Shopify puts inventory, payments, and analytics under one roof with the propriety of a true commerce expert. Their award-winning 24/7 support means you're never alone. And that iconic purple Shop Pay button. It's the backbone of their checkout. The best converting on the planet. Turning abandoned carts into actual sales. It's time to turn those what-ifs into with Shopify today. Sign up for your $1 per month trial at shopify.com/toe. That's shopify.com/toe.

Okay. So, most physicists are physicalists in that they think that the world is ultimately physics, and every other fact, whether it's about consciousness or about the economy or biology, it's entailed by the physics. Okay? If you believe that, then you, who is speaking as a physicist, are a physical system. We can even idealize you to something that makes no mistakes. In your paper, you have a computer that does quantum mechanics perfectly.

"Yes."

"Okay. So, in this analogy, going back to the chessboard, you're a piece on the board. You're a mystery piece. I was going to call you a rook before, but as you mentioned, you can't actually fully know yourself. Even in regular physics, you have some recursion issues. So, you're a mystery piece >> on some part of the board. And then you look out and you see pawns, and you look to your left, you see a bishop, and so forth, and you see other pieces. Then you model, well, what is that other rook doing? What is it seeing? Okay, so from this analogy, what is your inconsistency theorem?"

"I think one could call it a no-go theorem. That's fine, because we make some assumptions and say they don't fit together. They are not all at the same time valid."

"Okay. So from this analogy, what are you saying? Are you saying that you make a model of the bishop, the bishop makes a model of this and that, and then you're saying that bishop, you're seeing a king, but the bishop actually sees a queen? Spell it out."

"Yes. I think that's that's one one way to put it. I could say indeed, I, I see the bishop, and I'm modeling what the bishop is, or, okay, I don't know what what the characters were. Let's say I'm a rook. I'm seeing a bishop, and I'm, I'm asking, does the bishop see a, um, what, what figures are there else?"

"Rooks, knights, kings, queens."

"Oh yes, a knight. I, I know the German terms. That's why I'm not so familiar with the English characters. I actually play chess, but I never translated them to English. Okay. So, okay, let's say the rook is the bishop. The bishop, um, now, um, claims there is a knight in front of it, him. Let's say that's."

"And it's important that you do not directly see the knight, or or is that okay?"

"So then I could say, I, so I know the bishop sees a knight, and then I could also ask myself, do I also see the knight there where the bishop claims it is? And I don't see it there, and then that's a contradiction."

"Ah, interesting. Okay. So in the sense, I, I make on the one hand a direct statement about what I see, and then I also make a statement about what the bishop sees, and the two statements don't match. And they don't only match in this, um, way as we discussed with the coin, that one is more uncertain, because that could also be, that would not be problematic. I could say, oh, the bishop doesn't know whether there's a knight or not in front of him. >> Got it? >> And I know there is one in front of the bishop, but that's not a contradiction. And that's just the bishop has uncertain, um, incomplete knowledge. But if the, if I know the bishop is certain the knight is in front of him, but I actually see the knight is not in front of him, then that's a contradiction. And so this is the type of contradiction we get. Yeah. So I think in this case, the analogy works very well, because it's just about, like, two types of statements being part of the world. So there are all these pieces on the chessboard. So we are them. So we are one of them, and I can make either a statement directly of what I see, or I can make a statement about what someone else sees. And of course, I kind of expect these to be compatible with each other. And, um, that turns out not to be the case now in, in, in very special situations. Okay. And now, the assumptions that go into this theorem, any theorem has assumptions. So what are the three, and which route do you take as being the escape one?"

"Yes, actually, there is, um, yeah, there are three that we state, and of course, assumptions can always be grouped or, um, kind of subdivided. So this number is not so important in a sense, but, yeah, let me, I think it's a useful group or useful kind of fine-graining that we have. So one assumption is, um, the, that quantum theory, the way we use it, is, um, correct, let's say, the textbook quantum theory. Some people now subdivide that into several assumptions, and it indeed has several parts, because it has at least two aspects, because what we are saying is that in a sense, quantum theory is universal, but universal can be understood in two different ways. I could say quantum theory is universal in the sense that I can apply to any system I like, maybe except for to myself, that's fine, so I don't want to require that, of course, but that's one type of universality. I can take whatever I like, my cup of tea, my, um, my office chair, quantum theory applies to everything. So that's, um, the first part. But then there's another type of universality, namely that everyone is allowed to apply quantum theory from his or her perspective. And that's another universality, because in principle, it could be that, um, there is only the external hypothetical external observer."

Who is allowed to apply quantum theory? And that would exclude, that would precisely be this thing that I think would be very strange to say, "Okay, we, as inhabitants of the world, are not allowed to apply quantum theory. We have to be external to the universe and only then are we really allowed to apply the theory." But that's kind of what is implicit to many worlds or whom in mechanics because they somehow say we need to have this external perspective.

So, in a sense, that this second type of universality is actually something some people would deny. And they would say, "No, that's not true. We, we are not eligible to apply quantum theory as being part of the ver." So the assumption that goes into the first assumption that goes into into the theorem is this universality assumption in this two-fold way that I just explained. And that's maybe the most important assumption. And I think this is also the assumption that was mostly misunderstood because, and maybe that's also my fault, in the original paper, I was not even myself aware that these two sides of universality are so important. I used them because for me it was very natural, but it wasn't really emphasized. But I think this is an import, very important point. In particular, the second type of universality: we can apply quantum theory. And if I talk to people, I often hear people opposing to that and say, "No, we are not legitimate observers because we are ourselves in superposition." And that makes us not valid, um, users of quantum theory, which I find a very strange criticism because we are actually, we want to apply the theory. Okay.

So that's the first assumption. The other, um, the second important assumption is this consistency assumption. So we have to ask, um, or make an assumption about what do we expect, um, to be true when you describe something and I describe the same thing. What, in what sense should it match? And here, one could make very different assumptions. A very strong assumption would be that if you tell me the state of a system is this particular wave function phi, then I should also think it's the same wave function. That would be a very strong assumption, and everyone who thinks epistemically of the quantum state would deny it. And so we make a much weaker assumption. We say that if you make a statement of the form that you are certain that this particular measurement outcome will occur, so like you are certain the spin will be up, or you are certain if you look in front of you there will be a night. So that's a kind of type of statement, then, um, we are requiring that it shouldn't be the case that if I directly check that, or if I make a direct calculation or prediction, that I come to the opposite conclusion. So if I ask, "What's your conclusion?" it shouldn't be opposite to mine. So you should, if you say it's been up, I shouldn't be certain that it's been down.

Okay. And that's what the whole earlier part of the conversation was about, was about this contradiction.

Right. Yes. Yes. So that's kind of the new criterion. That's what I think was never tested before. So when people proposed theories, they made all types of consistency checks. But this check whether two observers who are using the theory are compatible in that sense was not something that was usually tested, in a sense. So I think this is kind of, let's say, a part, maybe the no-go theorem is, is maybe interesting. But I think the more relevant part of the proposal is actually to propose that test of a theory. So say, "This should be, this test should be applied to any proposal of a theory. Does it lead to this type of contradiction when we apply it? Can we consistently apply it in that sense?"

Yes. Okay.

So, can I really, um, make sure that someone else who applies it comes to the same conclusion as I do, or at least not to contradictory conclusions?

Okay. Now, do you also see that as a necessary component as to what a scientific theory is? Because science has intersubjectivity?

Yes, I think so. So, for me, um, communication in a sense is a very important part of doing science. Um, of course, one could in principle think of like me being alone in the world and doing science. And you could say, "What would be wrong about this?" But actually, there is even communication involved there because I, kind of, my younger me communicates to me now, like I did some experiments in the past or maybe some thoughts and predictions, and now I'm verifying them. And this is also in this, um, like the consistency should even hold there. So, for example, I could just see my, my former self as a different agent and then ask for this type of consistency. And ask myself if I, I know at that time I did a correct calculation and thought it's like this, but now I do the calculation directly now, and I come to the opposite conclusion. That shouldn't happen. So this consistency requirement, I think, is is relevant for any communication, including the communication of our former selves to us. I see that as communication. I mean, it's a very, um, powerful communication because we just store stuff in our brain and communicate via that. But it's a form of communication if, like, from an informationic viewpoint, I'm constantly sending messages into my future and read them later. And so, in in that sense, communication is, if I take that as part of communication, then communication is unavoidable to do science because otherwise, I would just be a point in in spacetime and could not explore the universe. So I have to communicate between different experimental, um, let's say, instances that I'm performing. And whether I'm performing them now, or whether it was my former self, or some colleague, or some former physicist who lived on this planet, shouldn't matter. It's all communication. And so if I, for example, only read a just a book in quantum theory, that's communication from from earlier physicists to me. So we are actually doing, I mean, communication is so inherent in our scientific development that it's very hard for me to think about what science even means without communication. Even to phrase a thought, like to have a theory means I can write it down and read it later. And if that's impossible, then, um, I wouldn't really call that science. So for me, this is almost the defining part of what science should be, that it's possible to phrase things, to communicate it to others, or at least to myself in the future.

Okay. Universality. Quantum theory applies everywhere. Meaning that even what we think of as large classical objects, elephants and robots, or whatever, they're...

They're also quantum systems.

Okay. And then consistency, which is what we talked about for 40 minutes or so. Then the third was what?

Yes. Then there was a third we didn't talk about yet. This is, um, actually one that is so, um, in a sense natural that it's often not even mentioned, but it's that if I make a prediction, um, then, and I make another prediction, there shouldn't be contradictory. So, you see, in this other assumption, the consistency assumption, I should maybe have been a bit more careful and, and so actually, maybe I should phrase it in the following way. The consistency assumption is somehow split into an assumption of how to combine knowledge and into an assumption that there should not be a contradiction. So, let me explain that. So the first part is how to combine knowledge. This is basically just to say that if I'm certain that you are certain that the spin is up, then I can take this also as, um, a certainty for me that the spin is up. So, for example, if you tell me that you have made a calculation and everyone is reliable, I know you're a perfect physicist, and you tell me that measurement will lead to spin up, then, um, I will also be sure the spin is going to be up. So that's strictly speaking the assumption C. So the assumption C doesn't talk about contradiction. It just tells me that if you are certain about something, I can also be certain. And now, the, the third assumption is now saying that this should not contradict what I calculated before. So maybe I, I calculated already that the spin is going to be down, and now I have two different predictions. And the third assumption, which we call kind of the single outcome assumption, tells me that if I come to the conclusion that the spin is certainly up, and I also come to the conclusion that it's certainly down, then that's a problem. So it's kind of the definition of what we mean by a contradiction, that we have two opposite predictions with certainty.

Is a fourth assumption that you're allowed to foliate the world into a now moment and then have time steps forward that everyone agree on?

Mhm. Yeah. So, um, there are actually, if you ask the question, are, is this now the complete set of assumptions? Then the answer is no, there are more assumptions. And, for example, one assumption, I mean, even before the one you mentioned, is we can apply logical reasoning. And you could ask, why didn't I even phrase this assumption in the no-go theorem? Like, I mean, for example, the logical reasoning means if I know A holds and I know B holds, then also to the conjunction A and B is true. Like, yes.

We need that. But of course, we need that to even phrase, let's say, the, the thought experiment. We need language and so on. So, in a sense, that's an, it is an assumption. But without this assumption, I couldn't even like start phrasing the experiment.

Yes. And if that assumption goes well, then every opponent theory goes as well.

Right. Yes. So that's, um, just to say that there are certainly more assumptions. I'm not claiming they're well. But there are assumptions that are so basic that we are kind of making them just before we even start doing science. Now, the assumption about, let's say, the foliation in, in time is kind of, one can see it as part of the assumption that standard quantum theory applies. But it is an assumption. So I would just see it as part of this assumption that we apply quantum theory because quantum theory tells us there is a unitary that brings us from time to time. But it's correct that this is an assumption.

Why I'm super excited to speak with you is that you take an interesting escape route.

Mhm.

You take that quantum theory is not universal.

And I want to know more about that. And then my question, which I'm going to get to later, so you can feel free to weave this in.

So what the heck does this have to do with gravity at all?

Yes. So gravity wasn't now coming in. But actually, I, for, I should tell you that there is a third way to escape the paradox. Um, which we didn't talk about. And this is actually now my favorite way to escape.

Okay.

So, I mean, you, so far, okay, to be honest, I'm actually changing my mind over time. I have to admit that I was even, to be honest, a many-worlder some years ago. I know. And I changed my mind to becoming more a patient type person. And, um, also, um, what about this thought experiment? I mean, this thought experiment, um, has actually still, um, an impact on on my daily life in the sense, not only that I get many emails every day, but also that I constantly think about what's the solution. So for me, to be honest, I, I have no satisfactory way to deal with it. It really troubles me that there is this problem, this contradiction. And, um, so that's why I, I'm thinking constantly about that. Makes you change your mind because at some point you have some insights, or you talk to people and think, "This is a good argument." And, um, one of the reasons, more recent insights for me, was that I realized that maybe all these assumptions that I mentioned, they are so reasonable, we shouldn't give them... Well, I mean, of course, you can say, "Okay, maybe quantum theory, we haven't tested it to any precision. Maybe there's something wrong." But we have really no indication that quantum theory doesn't apply to certain systems. And also, clearly, we should be able to apply it. Also, this consistency or the combining knowledge assumption is really a minimal thing that we can even talk because otherwise, it doesn't make sense to talk to each other. If I tell you things and you just hear them, but you cannot really make sense of the content, that's what it would mean if we give up this assumption about, um, this, um, consistent communication. And then, um, clearly, we don't want to have contradictions. So all these assumptions, I really want to keep. But now, and actually, our statement, this, I would really call it a no-go, um, says the following: It says, if we carry out this experiment, um, with the several agents, so I didn't really describe the experiment, but the experiment goes along like someone prepares a spin, sends it to someone else, and then reasons what is the outcome of a measurement and so on. So we describe an experiment and say, "If we perform this experiment and then analyze it using these assumptions, we will run into a contradiction." But now, who tells us that we can actually carry out the experiment? It could just be that the reason why, um, this is not problematic, or these assumptions are unproblematic, is that we cannot even start analyzing the experiment because it's not executable. And so, so far, I was always assuming, clearly, we can, in principle, execute this experiment. It's very hard to do it because we need to, as I said before, we need to do something that I call a Schrödinger cut-like measurement. So it would be like shooting a cat being in a superposition, and I measure in a basis, in a measurement basis that directly tells me there is this superposition. So the way I would have to do that is basically undo the whole, um, process. So in the shooting cat example, the, the shooting cat is kind of poisoned in, and this whole process of being poisoned has to be reverted, um, during the measurement process, which is of course incredibly difficult to do. Now, so far, I was always thinking this is just a technical difficulty, and there's nothing fundamental about that. And now, my more recent, um, let's say, thoughts, which also involves gravity, um, let me now suspect that this is actually wrong. We cannot carry out such experiments. And, um, maybe that's, it's not obvious why I'm saying that, but there is actually a kind of circle in in the argument or a loop.

And the loop goes as follows. So maybe I, I'm sure you, you know about the recent developments in this, like research that is concerned with quantum reference frames. People are asking the question, if I'm, for example, measuring an orientation in space, like a spin particle, what resources do I need to do this measurement? And clearly, I need some reference. I need to know what is up and what is down. And now, if I do a more, if I analyze a more complicated system, not only a spin, I need a larger reference frame because the, intuitively, the reference needs to be larger than the system that I'm measuring. So if I only have a spin as my reference, then my measurement of another spin particle will be very uncertain. So I need something larger than a spin to measure a spin. Now, um, the gravity considerations, um, kind of tell me that not only spin requires a reference, but everything that could in principle be measured requires a reference. We can later talk about why this is the case, but let's just assume every degree of freedom that we measure requires a reference. So, for example, I mean, it's obvious if I, if I measure, like, two spins, I need a larger reference than if I measure only one spin, because the reference gets kind of involved in the measurement process, and it's a bit more uncertain because I, in order to measure the spin, I have to let the reference and the spin interact with each other, and this will kind of, um, slightly degrade the reference.

Okay. So let's now assume that whenever I measure a system of a certain size, let's say L, I need a reference that is larger than L. Now, if you analyze, um, the Schrödinger experiment that we have, then we have kind of a circle of, in a sense, measurements or or places where we need a reference. So we have four part, or let's say, just for simplicity, we have only three parties or three players or agents in this experiment. We actually have four, but let's call them Alice, Bob, and Charlie. Now, let's suppose Alice needs to do an operation on Bob. Um, then Alice needs a reference that is larger than Bob, because otherwise she cannot really do a measurement on Bob as a quantum measurement on Bob as a system. Now, let's suppose Bob needs to make a measurement on Charlie. Then he needs a reference that is larger than Charlie. But if now Charlie needs to make an operation on Alice, he needs a reference that is larger than Alice. But if, if you are now in this kind of beginner's friend type setup where you do measurements on the entire agent, you need to also measure or apply the measurement to its reference frame. So the reference frame is kind of part of the agent, which is also natural. Like, if I see a spin up, this is relative to me as a reference. So if now Alice has to serve as a reference for measuring Bob, Bob has to serve as a reference for measuring Charlie, and Charlie has to serve as a reference for me, um, for measuring Alice, then we have kind of a loop. And if everyone has to be larger than the other, then this somehow doesn't work. So we run into a problem with this, with kind of the size of the reference. Each one has to be larger than the other. And this is impossible. So this is just a suspicion I have now. So I don't yet have a full argument that really convincingly says why these references have to be larger. So I'm still struggling with this idea. But my current hope is that I, that I can show that the experiment is not executable because of this requirements of the reference frames. So that each part needs a reference frame that is larger than the other. And this will make it impossible to do it. So maybe just to give you an idea, to to kind of do a Schrödinger cut experiment to to undo that measurement would require me to control all the 10 to the 23 or even 25 atoms that the cat consists of. And that would require that I need an enormously large reference frame, maybe as big as the universe or so. And so you see already that is difficult. But now if I do a second measurement on that universe, that data measurement, it needs to be even bigger.

Yes.

And so at the end, if I have a loop, this is clearly impossible. So that's one suspicion that I have, or that's currently the route. I'm trying to see whether I can prove that, um, this will lead to an impossibility. And the conclusion would then be, actually, there is no contradiction between the assumptions that I just mentioned. But the problem is, we simply cannot build these experiments where the contradiction would arise. So the contradiction doesn't arise because the experiments where they would arise cannot be built for fundamental reasons. That's a new route.

The Economist covers math, physics, philosophy, and AI in a manner that shows how different countries perceive developments and how they impact markets. They recently published a piece on China's new neutrino detector. They cover extending life via mitochondrial transplants, creating an entirely new field of medicine. But it's also not just science. They analyze culture. They analyze finance, economics, business, international affairs across every region. I'm particularly liking their new Insider feature. It was just launched this month. It gives you, it gives me a front-row access to The Economist's internal editorial debates where senior editors argue through the news with world leaders and policy makers in twice-weekly long-format shows. Basically, an extremely high-quality podcast. Something else you should know about is that if you go to their app, they not only have daily articles, but they also have long-form podcasts with their editors and writers. This is also available online. Whether it's scientific innovation or shifting global politics, The Economist provides comprehensive coverage beyond headlines. As a toll listener, you get a special discount. Head over to economist.com/e to subscribe. That's economist.com/e for your discount.

Now, large can mean large in many respects. Do you mean large in mass, large in volume, large that it persists longer in time? And even if so, whatever I just said, that still doesn't have to do with gravity. If you're dealing with quantum mechanics, you can have large systems of large masses, large composite systems. I don't see where gravity enters.

Yes, I didn't say it. So you, this is also not obvious, but, um, so by large, I meant kind of the Hilbert space dimension. So I meanted, or as in quantum information series, I would say in terms of the number of qubits of a system, which is just a different way of saying the Hilbert space dimension.

Got it.

Up to a logarithm. Now, indeed, it's not obvious how this fits to gravity. Um, but there is actually a connection, as you know, between size in that sense and, for example, size of a black hole. So you can, you know, a black hole has a certain information content, so to speak. Of course, that now depends again on the view one has on quantum gravity, and there are different views, but, um, one can associate an entropy to a black hole, and that kind of con, corresponds to how much information is contained in there. And so if I now talk about the size, you can say this is ultimately, like, the size I mean, the size in the way I talked about, um, is kind of related to the size of a black hole that I could build by putting all that information as close, or by, kind of, collapsing that information into a black hole. Now, why is this relevant, or why do I want to make this connection with reference frames? So what, um, one kind of other work that I did completely independently of theory of, of this, um, quantum mechanic thought experiment, was an informationic analysis of the, um, of, um, this, um, experiment where you throw information into a black hole and then try to retrieve it from the Hawking radiation that is emitted from the black hole. So this is often discussed in the context of the information paradox. So this is this, um, famous thing like black holes serve as a mirror that, um, Patrick Hayden and John Preskill once proposed that if, if you throw a notebook into a black hole and then you collect the radiation from the black hole, you can kind of retrieve the content of the notebook.

Mhm.

But the radiation will also look kind of completely scrambled. And, um, so one can now ask, um, what, um, I mean, there are contradictory views on what actually happens, and there's no consensus about that. So some people would say, indeed, one can retrieve the notebook, and others would say, "No, then, um, this is just thermal radiation coming out of the black hole, you will have no chance to retrieve the notebook." So what we found in this, um, work that I did with one of my former students, Jin Xu Wang, is that actually both views can kind of coexist, and they're just the different conclusions come from the fact that people are implicitly assuming different reference frames. So you could think of those who say there is just thermal radiation coming from the black hole, they implicitly assume there is no reference with respect to which you can make sense of the information emitted by the black hole. So let me make a very simple example. Let's suppose I have, um, just a source. Let's suppose there is nothing in the universe up here. It's completely empty. But for some reason, um, there is a source that, um, puts new stuff into the universe. Let's say some particle, let's say a spin particle. Now, let's suppose the first spin particle arrives, and well, now asks the question, "Is the spin up or down?" Then you would say, "Okay, that's not even defined because there's nothing else in the universe with respect to which I could define whether it's up or down."

Okay.

But now, um, let's say this process continues and it emits further, um, uh, further spin particles. Then, of course, you can compare them and say, "Now, um, relative to the first spin particle, the next one is up," and so on. And now, um, this is an analogy of how one could think of the Hawking radiation of a black hole. Let's suppose you just create one single black hole and it radiates Hawking radiation. Then this first radiation would be like the first spin that was just created. So the radiation is there, but we don't have a reference with respect to which it makes sense.

Sorry, just now, right now you're talking about one black hole that just emitted one bit of Hawking information.

Yeah. Yeah. Let's say it even emits more, or, yeah. You could say, let's say even the first Hawking radiation quantum that would be emitted. It will basically have, um, a direction. Yeah. You could even imagine, let's say, we built the black hole in this otherwise empty universe, and then the first spin particle comes out of the black hole. We wouldn't even be able to say what that means, that it's been partly, it's up or down. But let's now suppose we build, like, we do an experiment where we, in parallel, build identical black holes, let's say thousands. And now we could check whether, like, we could kind of take 999 of them and take the spin particle that comes out, the first spin particle that comes out of each of them, and put them together into a large, like, stick. Let's say they're all spin particles, they all have a small direction. If I put them together, it's kind of a larger arrow, if you like. And now I could say, the thousandth black hole, the spin that comes out there could now be compared to that stick that I have, that now serves as a reference. So I kind of take 900, like, the first ones just serve as creating the reference, and then I say, "Now let's check whether the direction now makes sense."

Yes. And so what we, what we actually did calculations and found that indeed, if you create many black holes in parallel, we could kind of regard, take, let's say, if you have many or a sufficiently large number, we could use them as a reference, and with respect to that reference, the other black holes would emit very ordered radiation. It no longer looks random. But if I just take one black hole and don't have this reference, then it looks completely random. So, in other words, a black hole kind of creates new type of information, like the spin, in a universe where there was never a spin before. So let's say, if I have a universe without any direction, and I create the first spin, this just doesn't make sense. Direction doesn't make sense. But if I have many of them, I can start to make sense of direction because I now have many spins that were emitted, and I can use them as a reference for the other things. So, in other words, um, what we found is like the type of information or the stuff that comes out of a black hole. If I just had one single black hole in the universe and nothing else, would look completely random. It's like a, um, just something new that, like a new spin that is created in a universe where there was nothing that breaks, um, symmetry. But then, um, if I, kind of, use many black holes, many others as a reference, then a new black hole that I create and let, um, emit, or like, I collect radiation, that radiation will, with respect to the others, have a lot of structure. So, in a sense, um, I can use some of the stuff to as a reference for the article. Like in the spin example. Actually, something that I was going to bring up. I said it was a no-go theorem. Then I corrected myself and said it was something different. It was almost like an inconsistency result. And then you said, "No, no, I see it as a no-go theorem."

Actually, I still see it not as a no-go per se. I still see it as an example of an inconsistency theorem. And then I was thinking, the black hole information paradox is another type of inconsistency theorem. And then when you said that you found a way out of your inconsistency theorem, I was wondering if it could be applied to other inconsistency theorems like the black hole information paradox, and it seems like you have already.

Yes, that's what we did. But I would have called both of them no-go theorems. But, okay, one could also call them inconsistency theorems because what, okay, technically, what they are is just there's a set of assumptions, like in our case, quantum theory is universal, we can communicate, or we can make sense of other, like, if you know something, I can adapt that knowledge, and there's no contradiction, um, between my predictions. So these are three assumptions, and I just say these three assumptions, if applied to an experiment, are not at the same time, cannot at the same time be valid. So in that sense, it's a no-go theorem. There are assumptions that cannot simultaneously hold. So, to be compatible with what I said before, I should say a fourth assumption is we can actually carry out the experiment.

Right.

But these four assumptions are then kind of incompatible with each other. And of course, I, I could kind of always give priority to one and for example, say this assumption that tells me there is no contradiction, like I shouldn't at the same time say the spin is up and and the spin is down. I give it a different status. I just say the other three assumptions imply that what, that this assumption, um, this non-contradiction assumption is wrong. So, in other words, the other three assumptions imply there is a contradiction, and then it looks like an inconsistency theorem, but then it's like a different ordering of the assumptions, in a sense. So I would say that no-go is very neutral in in terms of priority. It just says there's no priority to one of assumptions. Just take them as a set of possible assumptions, but they cannot all be true. You have to give up at least one of them. Like in Bell's theorem, you have like locality, you have free choice, and one of them must be wrong.

Now, wouldn't a physicist retort, for instance, as Sean Carroll may say something like, "Look, if you're going to be so pedantic that we can't even write down a black hole emitting a particle with a spin because we'll have to pre-think something extremely practical like how do we measure what is up or down?" Then I, as Sean Carroll, because Sean Carroll had a talk about how God is not a good hypothesis. Then he said, "Look, I can write down a theory where there's just a single electron in the universe right here. Wrote it down. I, as Sean Carroll, cannot write down that theory according to you because I would need to have something that measures how do I know what the charge of the electron is." So if you're going to be so pedantic, then that completely limits any thought experiment or any ideal toy scenario.

Mhm. Okay. Um, I'm pedantic sounds a bit negative. I would say, but indeed, in in that sense, I'm very pedantic. So I, but I want to be operational. I would call it being operational. I want to, I see that many of the problems we have in, let's say, many of the discussions that arise in physics are because they're discussions about concepts that are not well-defined in the sense that we don't know what they would mean if we actually did the experiment. So for, I mean, this with the spin is now an example. If I have a universe or like this black hole example, um, this is an example telling us, "Look, as long as we don't specify, um, how we measure, what is the reference, we actually get contradictory statements." Because in whatever you do, you will either implicitly assume we have a reference or you don't. And as long as you don't talk about it, we don't know what assumptions you make. So it's no surprise that we get different conclusions because some people now implicitly assume there was this reference, some did it's not. So I want to be explicit and say, "Look, if you do the experiment, we have to explicitly make the measured, the system we measure, interact with the reference." If, if we don't know, um, the nature of the reference, whether it even exists or whether it's not there, then we will just come to different conclusions. So I think many of the conflicts can be resolved in that way. So that was also the message of my paper with Jin Sao Wang, that neither of the attitudes is right or wrong. They just were not specific enough to make the question even decidable. And I want physics questions to be decidable. And decidable for me means, in principle, I could do an experiment that decides it. But if I want to make that claim that there is in principle experiment, then I should try to model everything that is potentially relevant. And of course, the challenge is to find out what is relevant and what is just, let's say, technology that is not, doesn't have to be modeled. So I'm, I'm not claiming that we should really model all the detail. I mean, I'm a quantum information theorist, and quantum information theorists really love abstraction. So you want to get rid of everything that is unimportant. So, um, in that sense, I don't want to be pedantic in saying, "Look, if I have a qubit, I don't want to, um, be forced to, um, say which atom is it that realizes the qubit, is it a superconducting qubit or is it a, a trapped ion." I want to be on that abstract level and say, "I don't tell you, my statements will anyway be true independently of what atom I'm using." So I want to leave away unimportant stuff. But I think in quantum information, we were maybe a victim of our own success. I think quantum information has been enormously successful with this abstraction. We could develop quantum information concepts, quantum computing on this very abstract level, and everything worked. But maybe we went now too far and said, "Okay, we completely got rid of the notion of space and time and reference frames and so on," and maybe have to rethink that and reintroduce the concept where they may be relevant. And so my own lesson, let's say, from working in quantum information and now getting into gravity, is that the abstraction went maybe too far. And an abstraction where we just abstractly think about the spin without actually talking about the reference that we need to measure the spin is not a good abstraction to answer certain questions. Of course, it always depends on the question. But if you run into troubles like with this Schrödinger experiment, we have to ask, "Are we on the right level of abstraction? Did we abstract away certain things that are really essential?" And so my feeling is now that abstracting away reference frames was a was a mistake, at least for this type of experiments. We need them. And so it's indeed a matter of taste or of phys, of, yeah, this is kind of scientific, um, maybe approach, which of the elements we consider as unimportant that we can safely leave them away in our idealized models, and which things were actually important and we were wrong to just leave them out. And my feeling is really that, um, reference frames are so important that we cannot leave them out. They play such a fundamental role in our understanding of spacetime that, um, dropping them will just not, um, or a theory that doesn't consider them will not give us the right answers. And so I think in the case of the black hole information paradox, I really see that as an example where, um, there are two opinions based because people make different implicit assumptions about stuff that is not explicitly modeled, like the measurement of the Hawking radiation. They just talk very abstractly about the Hawking radiation. It's, it's some quantum state. But if I measure it, I need a reference, and this reference has to come from somewhere. I should also, I also need to know what's the unitary that this creates the radiation. People usually say, "Okay, there's some unitary going on." But in principle, we would need to do quantum process tomography. So we would need many black holes to do, um, actually the tomography because you cannot do tomography on one single system. And that's another issue that if you create many black holes, they will actually be correlated because they are in the same spacetime, and so this is, um, will lead to some wormholes and stuff. So there are other difficulties that arise that, um, I think are very important to take into account. So my, I think for me, this is very strong. I have a very strong feeling that one needs to be pedantic in certain aspects, like including reference frames, and that we will otherwise miss something, or we have missed something. Otherwise, of course, I don't know what it is that is missing, and so this is just a guess that it's reference frames. So I'm not claiming this will be the right answer, but at the moment, it's the approach I'm pursuing.

So, wait, what is the specific claim? Is it that a thought experiment is not meaningful if it can't in principle be realized, or is it that the thought experiment, what the thought experiment is about, doesn't exist or something, because something can be not meaningful but also...

Exist.

I mean...

Unless one wants to go anti-realist and say that...

All that exists is just somehow operational. I don't, I don't think it's actually operationalism. That's a particular philosophical strain called operationalism, which I imagine you're not an operationalist.

Yes. So I mean, I have my, let's say, personal, um, list of requirements to a good thought experiment, and one very important entry on that is that, um, I want to be able to do the experiment in principle. So there should not be a fundamental reason that excludes the thought experiment. And now, in order to answer the question whether it's really feasible in principle, um, or this is a question I can never answer definitely unless the experiment is actually done. And usually, the thought experiments are of course about stuff we cannot easily realize. But my hope in, let's say, for the specific experiment that we were talking about with the agents who contradict themselves, is that this is something we could actually do in the not so far future. If, for example, we have quantum computers who would actually emulate the physicist because instead of having a physicist doing the reasoning, I could just have a computer doing the reasoning. And it has to be a quantum computer because it has to be a fully controlled system. And a, a quantum computer is a system where we control all degrees of freedom completely. So there, I really have, like, it's important to me that I have this, um, like, let's say, scheme in mind that in principle I could build the experiment and then ask myself, "What goes potentially wrong?" And then I would maybe find that, as I said, maybe there's a missing, or there's a problem with the sizes of these quantum computers. But if someone tells me a fundamental reason why the experiment cannot be done, then I would kind of say, "Yeah, in that case, it's no longer a good thought experiment where you learned something by proposing it and finding this fundamental reason, but then it's kind of, um, settled in a sense." So for me, a good thought experiment is something where I know how in principle I would do it, and otherwise it's under, not well-specified. And it's the same with, let's say, Maxwell's demon and all these other thought experiments. If, for me, they're interesting because in principle they can be built, or now we, I mean, there are claims that they are built. We are not yet completely there. So that's an important point for me. In, I mean, I'm talking about thought experiments in physics.

Yes. Yes.

Of course, if I were a mathematician, I would have different requirements, but I want to learn something about physics. So, it's just a thought experiment because we cannot do it in practice. You don't have the technological means. And as a theorist, um, that's for me a very important tool to make progress. Now, if something is only meaningful if we can in principle realize it, well, then thoughts about, well, what if the some constant of nature was so and so? There's no machine that even in principle can change one of the constants in, say, the Standard Model, or many fine-tuning arguments or anthropic reasonings are about this. If we're going to limit our thought experiments to what we have somehow in principle access to, I imagine that greatly limits.

Yeah. So, and I think if, if there is at the fundamental reason that limits the experiment, then I think then we have learned something. So what I want really is to, let's say, if you have some entropic reason or some other reason why we cannot carry out an experiment, then I think this is exactly what helps us making progress in physics. So in a sense, if, let's say, someone proposes a thought experiment which seems realistic, and then someone else comes and say, "Look, this cannot be built for that thermodynamic reason or for that other reason," and it's an a fundamental reason, not a technological reason, really something that we know can never be overcome, then I think that's exactly what what helps us making progress. So in a sense, it's then maybe, okay, maybe I shouldn't say a thought experiment is is only good if it can be carried out. If it was proposed and we find an interesting reason why it cannot be carried out, then it's maybe also a successful thought experiment because it told us something about physics. But I think it should have this. So I want to have this requirement that in principle I know, I think I know how to build it, and then maybe I learn that I forgot some other thing that is needed to build it, and, um, I learn that this is relevant, which is then a lesson I can learn from the thought experiment, or it actually can be built, and then it's also of course interesting, like for some experiments like Maxwell's demon and so on. So what about Einstein's person falling in an elevator? Because that's not exactly realizable. You would be able to tell the difference because the elevator has some mass to it and warps the spacetime in some different manner, and you also do. So even technically Einstein's thought experiment, which led him to GR, isn't in principle realizable by an observer, unless it's an extremely symmetric odd type of observer. But I, I don't know how. I would need to think more about that. So help me spell out.

Yes. Okay. Now I see what you're aiming at. So I think, okay, um, actually an experiment should also be in a sense robust. So what I mean by that, um, is that if I do an experiment and the experiment has to be very fine-tuned in order to actually give the right results, then, um, this is not, let's say, convincing for me. So, let me maybe make an example. Let's say at the time before we could control individual quantum systems, we couldn't carry out the Bell experiment, of course, because it really requires control of the individual systems on on both sides. And now, um, one could have asked, "Is this actually good?" So at that time, it was a thought experiment, and now, um, one could say, "Okay, maybe it's not a good experiment because we cannot precisely do the measurements that we want to do. They, there will certainly be always some fluctuations. We will have precisions and so on." But the important point of the Bell experiment was that it is robust. So even if our experimental devices deviate from this ideal setup, we will still get to the contradiction. And this is, by the way, also true for this experiment, um, that, with with the different observers, this friend type experiments, because the contradiction is kind of robust. So if you don't precisely follow the experimental procedure, but you have some error that is below a certain threshold, you still reach the contradiction. And so in that sense, um, I, one, a good thought experiment should allow some imperfections in a sense, and I don't want to model these imperfections. I just want to say, whatever these imperfections are, I, I still want to get to the conclusion. And so now for Einstein's elevator, I would also...

say so even if you don't have this perfect situation. It's a good enough approximation that we can still draw all the conclusions. And of course, if there were some if there are some conclusions that really require on this being a perfect elevator, then um we don't want to um use them. So I think we have somehow a robustness requirement and um that robustness requirement is important, I think.

Okay, I need to think more about the robustness requirement. But let me let me linger just a tad longer on this this physical realizability of thought experiments. So to me that means either that it's physically realizable with our current laws.

Mhm.

Or to some sort of metaphysical possibility or logic it's logically possible. So I assume you mean physically realizable with whatever current laws, not somehow metaphysically doesn't contradict the laws of logic or something like that. If it's this where it's our current laws, then aren't many thought experiments designed to push the boundaries of current laws to expose the inadequacies. So Einstein chasing a light beam for instance, I wouldn't say it was designed to show limitations of some electrodynamics, but

you could see it as showing attention with demanding consistency with Newtonian physics. And if you just said that well, it's not actually realizable, then it may have neutered the reasoning that led to special relativity.

Yeah. Um, so maybe I should say, yeah, maybe um to still comment on that. I see it in the following way. So we are testing not the world at the end with a thought experiment, but a theory. So when I say an experiment should be realizable, I want it to be realizable according to the principles of that theory. So if, let's say, I'm with the salt experiment we discussed at the beginning, I kind of want to check test quantum theory. Now, what would it what does it mean to be realizable? It means that within that theory, there is no reason that excludes it. So this is obviously not a statement about the world in a sense. So I think you have a candidate theory of the world, and I want to, so as a, as a theorist, I want to be able to exclude candidate theories that have no chance to describe the world, because that's that's what we want. We want to exclude all the theories that are not good candidates. And now, can say I just take now the theory as a correct description of the world and I ask myself, within the boundaries of that theory, can the experiment be carried out? So when I ask, can the experiment be carried out, it's really a statement relative to that theory, and if the theory has reasons why it cannot be carried out, um, then I don't need, then it's not a good thought experiment to test that theory in a sense. So it, it's really not about the world in a sense. It's about excluding theories. So like an experimentalist tries to exclude theories by doing an actual experiment. I try to exclude theories by actually doing thought experiments within that theory and check whether everything works out. And if it doesn't, then the theory had has kind of proved itself to be problematic. So I don't, in principle, I'm in that sense like a mathematician, although of course, I'm ultimately interested in physics. So I'm a physicist. I'm, I'm really interested in the world, but the type of work I'm doing is in a sense, you could understand it from a completely mathematical perspective. Someone gives me a possible description of how the world could be, which is a theory, and now I'm just exploring that theory by, um, inventing thought experiments within that theory and asking myself, what happens according to that theory, not according to the world, according to that theory, and so whether even the question is the experiment possible or not, is now a question within that theory. So the theory should actually exclude experiments that lead to contradictions, in a sense, a good theory, and if it doesn't, then it's a bad theory. So if the theory allows me to do an experiment that at the end leads to a contradictory conclusion, then there's something wrong with the theory. So in that sense, it's, it's something I can, so this work I can carry out independently of any experimentalist. I, I'm just working, it's like a playground. Someone gives me a theory and I play within this theory, and I play as long as I, I like until I see a contradiction, and if I see a contradiction, I, I'm unhappy with the theory and ask for a new proposal. And so that's how I understand quantum theory. It's a playground for me as a theorist. Then now I play within it. I invent experiments and check whether they're possible in, in this theory. And now I found that there is a problem with them.

Okay. So what does Fraiger, if I'm pronouncing that correctly, who also thought of this thought experiment?

Oh, it's correct. Yes.

Great. Great. What does she think about your interpretation?

Um, okay. So, actually, she left science after her PhD, and we have, um, she kind of observes, she, she observes the whole development from the outside. So she reads all the, um, like public articles that are written. So there are sometimes even some press articles in the press about it, and she, um, but she's no longer very close to the subject. She now works in, in software engineering.

Did you so disturb her with your your metaphysical insights?

I hope not. No, I, I think this was, um, I mean, it's a good point because I had some other students who said that they would not, psychologically, for psychological reasons, not be able to work on such projects. It would disturb them too much.

Wait, wait, wait. That is super interesting.

I want to hear more about.

Because it's, it's, it's at the end about, look, these projects are about, like, really the fundamental question of what are, in a sense. So what are we?

Yes.

And, you know, when, at the time when I was still a many-worlder, um, of course, the kind of implications that it would have on us, even on decisions we are taking, are kind of quite dramatic. I mean, if we know we make a decision and and we are now experiencing both outcomes, that kind of has an impact on on what we are doing, or at least, um, for me, it was the, I mean, I don't, I cannot give you a practical example, an impact, but it's kind of worrying to to know that or to think this is the true story. And so now, um, I'm now in a kind of in this, um, view that I currently have where there are no absolute facts. This is again, in some way, worrying because of course, people come with the idea there is something absolute out there in the world, and then they start working on this and they suddenly realize, look, something must be wrong with that view that there is something here in the world, and then suddenly everything falls apart. All this, like, um, kind of the whole model one had of the world has to be revised. And then some people take this very seriously and say, look, if if everything has to be revised, if things are not out there, there's just, um, everything is just kind of, um, in my mind, which is, for example, suggested by these epistemic views. Then, um, one can ask really fundamental questions about what, what's really now the goal of our lives if, um, if there's nothing out there. And so, interestingly, some of of my collaborators say they kind of don't link that question to real life. So they completely separate that. They, they work on that and have opinions about how it is, but somehow they disconnect it from their daily life. But there are others who say they, they, um, this is intrinsically connected and they're still happy with that. But there are others that connected and and find it very stressful that this is connected because it constantly tells them that, like, or reminds them that it's not clear how all that makes sense that they see. So I think it is very fundamental and very emotional, in a sense. So, okay, you also said, I think in Quantum Magazine many years ago, that one's choice of interpretation of quantum mechanics is very emotional. So I want to know what you meant by that.

Yeah. So, um, I think actually almost any decision, not only what interpretation we we are meeting, is at the end, um, kind of an emotional decision because, let's say, if you ask ourselves what are rational arguments. So I mean, we of course try to be rational, but the rationality always hinges on certain, let's say, goals that we set or assumptions that we took, and these assumptions or goals cannot by themselves be kind of supported by rational reasoning. So, for example, if someone asks me why am I now convinced at the moment, or why was I convinced at the time that there is many worlds, and if I now think back, why was was I convinced then? I think it was actually an experience, in a sense, that I learned quantum mechanics in a very standard quantum mechanics course, and the very standard quantum mechanics course, now in retrospect, are in my opinion, not good quantum mechanics courses because you basically learn how to solve the Schrödinger equation.

Right.

And, um, that's not giving much insight into what happens in quantum theory. Now, um, I, I was very, kind of unhappy with that, but I didn't know why I, because I didn't know anything else. So I just learned it, and I thought, okay, that's a bit like electrodynamics. I just now have a new system of equations, it's no longer the Maxwell equations, it's now the Schrödinger equation, and I learned how to solve it in different potentials and so on. And then at some point, I started, um, looking into quantum information and quantum computing. I mean, this was not very popular at the time. This was in, in the year 2000, around the year 2000, when, yeah, almost no one was working on the subject, at least here at ETH Zurich, where I studied. And so, nonetheless, I, I started to read these papers for a seminar that was organized by a professor here. And then I suddenly realized there are these concepts like superposition, entanglement, and I couldn't really make sense of that. I couldn't also match it to what I learned in the quantum mechanics course. But then at some point, I, I learned about these papers about many worlds, and then suddenly things started to make sense to me, and I realized, okay, oh, I can just see the whole world as a big wave function, and then the entanglement makes sense because I'm just part of that, and suddenly everything cleared up.

And this was such a a revealing thing that I was immediately convinced that must be the right view because it was so much better than what I've seen before. So I was really kind of extremely happy when I realized that, and I didn't, um, doubt that this must be correct because it was just such a change from what I've seen before. So before, it was just some equation that, and I couldn't really see what's going on, and then suddenly everything cleared up. So that was, in a sense, a very emotional moment, and because I was so, the change was so positive, from not understanding things that don't make sense to suddenly something that kind of fit together. I could fit in how measurements work and so on. Then, um, I was just convinced because of that, and so that must be the right thing. Then I, I started to, of course, investigate that more, and actually, paradoxically, the more I talked to people who are actually proponents of many worlds, the more I got doubts about the correctness because I had some initial questions that I couldn't answer, like, what does probability mean in many worlds? And I thought those people who are, were thinking about many worlds for a long time, they will certainly be able to give convincing answers, but then in, in talk, while talking to them, I realized there are no good answers. So I got more and more disappointed about that. But it was again, like, I couldn't say there was this argument. It's just a feeling that you, you somehow have the impression these are not convincing answers, but it's not necessarily a rational thing. I mean, there were arguments that just didn't match what I kind of was hoping to hear. I was kind of hoping there's some natural thing behind, but the arguments seem to be very fine-tuned on, we do stuff in this and this basis, and there's so I just got again, a kind of bad emotional feeling about these explanations, and I got more and more unhappy, and then I had another, um, kind of encounter with QBism, and I, this cleared up all these thoughts. Suddenly these probabilities made sense, and it was again, like, a very positive moment for me, and I couldn't necessarily say there was again some really rational thought process leading to that. It was just that in total, things made more sense. It somehow resolved stuff that before were really stuck, and I couldn't, so I was stuck with many, like, concepts like probabilities and so on, and the basis choice in, in, in many worlds. And then this suddenly resolved again, and then I was kind of almost completely convinced QBist. And now the problem is that, um, this thought experiment with Daniela puts that again in question because QBism doesn't have a good answer on about what is this assumption about the consistency between agents. So QBism is kind of a single agent theory. I mean, okay, QBists would now deny that they want to, they say it can be applied by many agents, but, but what is missing is there is no rule of how the agents would actually combine their knowledge, and that's somehow needed. I need to somehow be able to say that if you have some knowledge about something and you communicate that to me, how do I incorporate that knowledge into my knowledge? And there's simply no rule. So I couldn't even say it's wrong, but there is no rule. And they have also not been able to, um, find a convincing rule in my opinion. I mean, there were some papers by Chris Fuchs and others where they tried to come up with such consistency rules or, um, um, so I think, um, there's a paper about, um, respecting, it's, it's the title is something like respecting one's fellows. So it's really about, I have fellow physicists, and I want to include their knowledge. But here, there's something that, like, again, this, um, more negative emotional feeling that it does, it doesn't feel like the right answer. Although I cannot at at point to the to the flaw. It's just the rules that they introduce feel very ad hoc, and they don't really resolve at the moment, or they don't really match the the thought experiments, um, or I, I can't really fully resolve them with these rules. So it's again an emotional state that tells me it's not the right thing.

So what have you landed on then interpretation-wise?

Um, I'm now in no man's land in the sense. So I, I don't see any of the interpretations that's currently existing as satisfactory. And the reason is mainly because these interpretations simply don't talk about the situation of having more than one agent. I mean, they don't exclude it, but they don't tell me how to deal with the situation. And what is very important in the thought experiment that I have with Daniela Fraiger is the communication between agents or how they incorporate knowledge of other agents. And this is just not defined in a sense. So the theories don't tell me what to do. So QBism tells me how to update knowledge if I make a measurement, but it doesn't tell me how to update knowledge if another agent tells me what his knowledge or her knowledge is. And that's just missing, and it's not obvious how to add that rule. And therefore, um, it's kind of more incompleteness of these interpretations, and it's, there's no obvious completion, so to speak. And in more recent work, I now did, I guess you mentioned that, and against probability, I try to kind of identify what the problem is, that maybe it's fundamentally wrong to try to capture things in, like, a knowledge in terms of probabilities. I mean, you asked before, what, why, what is the kind of role of emotions? And you see, somehow, if I'm honest, it's kind of often like the opposite way that one thinks it should be. There's first an emotion that I think, look, something is wrong with these views, and then I try to find rational arguments to support the emotions. And like, somehow it feels wrong, like now for me, there's something wrong about QBism, and I now try to find out what it is that makes me feel that it's wrong. And now the current status is that it's really to do with the probabilities that are too, a too restricted concept to capture quantum states. And, okay, I'm saying that because one would think there should first be a rational argument, and then one can maybe decide on one, then one gets the emotions from there. But I think in, in my experience, it was often first a very vague feeling, an emotional feeling of how it should be, and then I was, this was driving me to think more deeply about the subject.

Right? And, um, often the emotions were, um, then kind of turned out to be, in a sense, not supported by these rational arguments, which actually helped me to change the emotions. But sometimes there were emotions that were really pointing to the the problem. And so, in a sense, it's like one follows that taste that comes from these emotions and then tries to find out where they come from. In a sense, in that sense, being a physicist for me is an emotional experience.

Yes. Actually, there's something that many people who think of themselves as being extremely rational, whatever that means. There are various sorts of rationality that they have. We need to be completely unbiased and just follow the evidence and logically update and so forth. But one that contradicts how they operate as as people, even in their most ideal times. And number two, there's some evidence from Mercer and I forget who else his colleague is, that says that actually what you want are people who are motivated for various reasons, emotional or otherwise, because they will come up with arguments that you, as the unmotivated person, would never have come up with, because it takes an extreme amount of work to come up with certain arguments. And you have to have some impetus to get there. And then now you just have a team of people who are just all arguing with one another with extremely refined arguments. And then that, as a whole, is good for science. So it's not that you want completely unbiased people. You want people who are communicating with each other and fighting with swords that are extremely refined.

Mhm. Yes. Actually, I had an experience a bit more than a year ago. We had a conference to the birth for the birthday or on the occasion of the birthday of John Bell, here in Switzerland. And, um, we had, um, Tim Maudlin as one of the, um, participants, and he got very emotional. So it was always, I was always fearing that a fist fight would start at the end of a talk. And then I, I saw to myself, Bell could be very proud that he managed to just write down a theorem about which people get so emotional. That's kind of the best thing that can happen to you if you, if you invent a theorem. A theorem that makes people fight and and be emotional about, because that really means the theorem has a significance. Like most theorems are, in a sense, boring, one could say. I mean, I can prove something like, I could maybe take a random, um, calculation or an integral and try to solve it and say, there's a theorem, the solution is that. That's completely unemotional. But if, if I manage to write down a statement that is true in the sense of a theorem, but also makes people actually shout at each other.

Yes.

Not, I'm not trying to imply that this is a good way of communicating, but still, it tells you that there's something important to the theorem. I was really thinking like Bell managed to do so.

Yeah. No one no one gets decapitated over Nom's theorem about supersymmetry or Coleman's theory.

Yes. Right. Yes. Mhm. Exactly. So I think emotions are important for scientific progress, and of course, if I see that also with students here, like a PhD is a very long time that you work on some subject, and without having an intrinsic motivation, which necessarily comes from emotions, you will never manage to survive that time with all the ups and downs.

So let me ask you this about your own psychological stability and your identity. How has quantum physics, your research into quantum physics, affected it?

Okay. So, yeah, that's a a good question. So, I think it's like the emotions are most, or these, like, um, psychological impact is, is kind of still, I would say, a bit limited to science itself. So I think if I really, um, ask myself, did I make decisions, other decisions in my life, or my, let's say, general well-being, how much is it affected by the actual scientific insights? And I think I don't see a very direct impact. It's more about the general, let's say, um, emotions towards science. So I try to still separate, like, private life from, um, from the profession. So I do lots of sports, for example, and that helps, like, if, if you are like frustrated about some scientific thing, then, um, this helps me kind of distance myself from that. So I think I, I try to, um, get some distance from from these emotions by doing stuff like doing a lot of, let's say, biking and so in the forests here in Switzerland, which helps me just, um, separate that from from that. I think if I would only think of physics, this would really have probably, let's say, if I, if I would fully take it seriously what quantum theory tells me, I wouldn't know how to kind of act on that in my real life. Interesting. Okay. At the moment, I'm anyway confused about what quantum theory is, because as I said, this no-go doesn't yet have a resolution for me. And so, I have to kind of accept the situation that I'm kind of clueless of what nature tries to tell us, and therefore I cannot really, um, make my life depend on that. So Feynman has a phrase that no one truly understands quantum mechanics. I tend to think that I almost a modus tollens, or yeah, modus tollens of that, that if you haven't been mentally destabilized, then you haven't understood quantum mechanics. But anyhow, that means that you have to think quantum mechanics is about something and view it more than just a formalism. I'm sure many of your colleagues would say that all this is just philosophy jargon. The measurement problem is not a true problem. I think Raphael Bousso just spoke to Brian Greene about that. That look, there's nothing in quantum physics that hasn't been done because we haven't solved the measurement problem. Like, you point, you tell me how it's relevant. What do you say to that?

Yeah, I, I think this is a, I think one should take the measurement problem very seriously. So I think it's not, I mean, it's true that all the technological developments we had, building quantum computers and even, like, um, like these cryptographic things where one has to ask very precise questions, um, they don't depend on on the solution of the measurement problem at all. So it's certainly true that the stuff that was done could be done without solving the measurement problem. But I think it's very important to not forget that this is a very fundamental question that may prevent us from understanding things that, um, are beyond the current reach of of quantum experiments. And what I mean by that concretely is, as soon as we can apply quantum experiments to agents. So that's what we discussed before, and that may be very soon because the agent could be a computer. Um, we actually don't know what happens, and there, it's really an operational question. So I would say, so far, the measurement problem is kind of avoided because we all look at the quantum system from the same perspective. We look at it from the outside. The, the quantum system is a small system that, and we are outside. And I, you asked before about why are there different perspectives? Currently, with the current technology, there is no reason to take different perspectives because we all have the same perspective on quantum experiments because they are little things in a lab, and if you go to that lab, we all agree that it's, um, this little spin that we are talking about. But once we have experiments that are really on the scale of the agents, then this is different. So then you can no longer, all we can no longer all have the same perspective. And I think in these situations, the, the measurement problem has an operational impact because for me, the measurement problem is really, um, linked to these type of contradictions that we were talking about. And so, let's suppose I have a network of computers, and the computers are kind of acting as agents. So this means the computers are programmed with quantum theory, and they apply quantum theory to reason about the world. And of course, these computers are themselves subject to quantum theory. So they are objects that we are describing with quantum theory. So they can describe each other using quantum theory. So the question whether such a network of computers would actually work properly is an operationally relevant question in the future, and I think not having solved the measurement problem will, um, will will lead to problems. We will not be able to predict the behavior of such a network. So, in other words, the measurement problem is for me not a kind of philosophical problem in, in the sense that it's only relevant if you introduce some terms about what is reality, but in practice, it's never relevant. So sometimes there are these problems where we introduce a concept like, there is a real thing, but we can never see it or never do an operation on it. So then one could say this is, in a sense, a purely philosophical question that will not have an operational impact. But this is not true for the measurement problem because we have these thought experiments with observers and, like, also the Wigner's friend experiment, and we will be able to carry them out using quantum computers, and then, then it's clearly relevant. So I think it's, it's just the technology, the technology wasn't the last for the last 100 years on the level where it would have an impact. It's a bit like comparable. You could, let's say, do electrodynamics before we had, um, relativity theory. Say, as long as you can only do experiments with low velocities, no one should care about these thought experiments Einstein did. But clearly, these things are now relevant. And I think the same is true for the measurement problem. This is just at the moment not relevant because we are not technologically in that regime. But it is, and, and also if you now think about, um, experiments like we had talked about this, um, black hole information paradox, that's an experiment where we apply quantum theory to a very big system, and there, um, we, we are clearly making mistakes if you are not aware of, like, the meaning of the quantum state in this context because we have, um, if you talk about the black hole, then the black hole is a quantum system, and if you are now doing thought experiments where an observer Alice falls into the black hole, she will become part of that quantum system, and then we are exactly in that situation that we had before. Alice can now no longer describe the black hole as a quantum system because the quantum system includes herself. So that would be a, a recursive use of the theory. So the diff, the perspective of Alice is now fundamentally different from a perspective of another observer who would stay outside. So let's suppose Bob stays outside, Alice falls into the black hole. That's the usual type of thought experiments that black hole people do. Now, um, without having solved the the measurement problem, we are in trouble because what does it now mean if Bob applies a measurement to the black hole? He applies a measurement to Alice because she's part of the black hole. So, um, we need to be able, what happens to know what, or we don't know what happens there without having solved the measurement problem. And so that's, um, another reason why I think so, even now, because we are now talking about these experiments, they are discussed, we need to to have more insights into that, and we cannot just neglect it. And people often say, look, we didn't solve it for 100 years, why should I try to solve it? It's almost clear we cannot make progress, let's just try to solve all the other problems and ignore the measurement problem. I think this is a wrong attitude. I think we have now very good, we are now in very good conditions to make progress because with, um, like this insights that come from quantum information in general, and also from gravity, and if we combine them, then I think we can really maybe see new, um, new aspects of this that may give us hints on how to resolve things. So I'm a bit frustrated about the fact that people ignore the measurement problem and just say, look, either it's no problem, or, um, we anyway cannot solve it, because I think it's actually something we, we should solve, and if we solve it, we, or we need to solve it to to lift quantum to the level where we can apply to agents, and we will be able to apply to agents. So it's just a matter of time, and so we better are prepared for that and try to think about how the theory looks like in this case. And so for me, the measurement problem is linked to that, because the measurement problem is really about what happens if you are an agent, or you measure an agent, what happens to that, how should we describe that?

Hi everyone, hope you're enjoying today's episode. If you're hungry for deeper dives into physics, AI, consciousness, philosophy, along with my personal reflections, you'll find it all on my Substack. Subscribers get first access to new episodes, new posts, as well behind-the-scenes insights, and the chance to be a part of a thriving community of like-minded pilgrims. By joining, you'll directly be supporting my work, and helping keep these conversations at the cutting edge. So, click the link on screen here, hit subscribe, and let's keep pushing the boundaries of knowledge together. Thank you, and enjoy the show. Just so you know, if you're listening, it's cur ji mu n g- a l.org. org kurtjong.org Poly Chronicon. I, I probably butchered that name, but I'll place the link to his article on screen here from Nature.

He said that the Born rule itself breaks down when observers are observed somehow.

So you have a paper that just came out in 2026. His was from two years ago. You have "Against Probability" and how probability maybe isn't the full story in quantum mechanics. Is there overlap between you and him there, or is it more just at a gestural level?

Yeah, I, yeah. No, I, I think there's no that, so he had a different, um, yeah, I think there's not so much overlap between these two approaches. Um, so our, um, argument is basically a criticism on theories that try to use probabilities to represent quantum states. So you know, for example, in quantum Bayesianism or QBism, the idea is that you would say a probability or a quantum state is just a compressed way of talking about probabilities of predictions. So if I have knowledge, so my, so the assumption is kind of the type of knowledge I have is of the following sort, that for any possible measurement I could apply, let's say I have a quantum system, a spin, and I know if I apply up-down measurement, the probability of having up is 0.7. If I do a left-right measurement, it's 0.9. And so I compress all this information in a quantum state, and I can understand the quantum state as a compressed way of representing all the probabilities of all the measurements I could carry out. And so now, um, the idea of of some of the theories like of QBism, but also of the so-called GPTs, which stands for Generalized Probabilistic Theories, is that instead of actually writing,

a quantum state down, we just

think of these lists of probabilities. So we just, um, say, okay, we are more, why should we kind of use this density operator formalism that's very special to how quantum theories? Let's just more generally talk about the list of probabilities. And that, to me, that was also something I found, um, a few years ago, an incredibly fruitful idea because it allowed us to kind of think about generalization of quantum theory. So see, I think what was really progress, in my opinion, in foundations was when people came up with these general probabilistic frameworks because they allowed us to see a larger space of possible theories where quantum theory is just a special case in a sense. So, for example, there are theories where you have stronger violation of the Bell inequality or weaker ones, and you could capture all of them in a general framework, and that's very useful because this tells you what is special about quantum theory. One could ask the question, why is exactly the Bell violation as large as it is in quantum theory? Why is it not larger? Why is it not smaller? And so this generalized probabilistic helped us answering these questions. And you need, um, of course, if you want to generalize quantum theory, you cannot talk about the quantum state. You need something more general because the quantum state is very much bound on this Hilbert space formalism that we have in quantum theory. And so the idea was really to say, let's just represent the states in terms of probabilities, and then we are not bound to use a Hilbert space. It can do much more general things, and I think this was an extremely fruitful idea that was actually very much, um, in favor of exploring that. But then, um, you see, there were often like disappointments about things I was, was in favor of, like many worlds. I kind of realized after thinking more about that and discussing it with, um, actually here at ETH with my PhD students, um, that there are problems with this representation, namely that these probabilities don't capture everything. And so one could ask, how can it be that they don't capture everything? Because the probabilities are just somehow in one-to-one relation to the state. So if I know the probabilities of all possible measurement outcomes, I also know the quantum state. It's kind of a one-to-one relation. I can, instead of writing down the quantum state, write down the probabilities, and I can switch between the two.

But let's say the, the problem that we found is, I mean, very roughly speaking, that this relation between the quantum state and the list of probabilities is, in a sense, not robust. So if I change, um, the probab, the quantum state by a lot, let's say I go from one quantum state to the other, and I look at the list of probabilities, it may be that they change arbitrarily little. So this means, like, in a sense, the, um, the distance between, so I want, I have in quantum mechanics, we have a certain notion of how distant are two different quantum states. So, for example, I have a state pointing in that direction, and that they are very distant because I can distinguish them. But then in on the level of the probability representation, the lists of all the probabilities look very similar, they are very close to each other. So I kind of, um, lose the, in, so that's what we said, we kind of lose the topology, in a sense, of the quantum state. Things that are close together in terms of quantum states are not close, or that are not close together in terms of quantum states are close together as probabilities. Is and so the, let's say, the high-level insight was that if you talk about the theory like quantum mechanics, it's not only important to kind of talk about the states, but also about the structure of state space, what does it mean that two states are close to each other? Because this also has to be represented well if we generalize it to something else. And so what we found is that probabilities don't capture that well. So probabilities don't capture well when two quantum states are close to each other. And so that's kind of the insight that we had, and this has actually important practical consequences. It, for example, means, I mean, the real practical consequences, for example, the following, there are proofs that we can do quantum cryptography based on, in, in very general crypto, probabilistic theories that are beyond quantum theory. I even was working on these proofs. I'm,

even talking about my own work that is now put in question because, because the link, because we now found the link between probabilities and and quantum states is is not robust. It, um, it can be that, or it now turns out that if we prove security in this probabilistic framework, it doesn't necessarily imply security, um, of the cryptographic scheme in a quantum world. So, and this is this is really relevant because we thought we were actually more general and and better in a sense because we, we kind of had the claim that security holds independently of assuming that quantum theory holds. There were security proofs were done in a very general framework, but now it turns out that if we go back to quantum theory, to the special case of quantum theory, we lost something. We actually didn't have the right, um, notion of closeness. So if something is very close to being perfectly secure, it's not necessarily close to be perfectly secure on the quantum level, which means it's actually not really secure.

So this, so in other words, cryptographic proofs we did in this generalized probabilistic framework are actually not applicable to the to the quantum world. And this was quite shocking for me because we did these proofs and and had these claims that we can do proofs independently of assuming quantum theory. So that's one of the impacts of that. So it's actually a real-world impact in a sense. I should just say this is a completely different aspect of, um, let's say, quantum foundations. So it's part of my program to find, kind of, a replacement or a resolution of this, um, thought experiment with Daniela Fraiger. And because, as I said, I was initially kind of convinced that QBism would be a good way, um, I tried to now see what needs to be changed. And so QBism, as I said, is very strongly based on representing knowledge in terms of probabilities. And now, because of this recent work, "Against Probabilities," I'm kind of still convinced that the idea of QBism to be subjective, to to represent knowledge in states, is a very good idea. It's for me the most, um, promising idea. And I, I know, I mean, it's similar on that level, similar also to relational quantum theory. So it's kind of everything is relative to me as an agent. But in order to represent that knowledge, probability distributions will not be the right concept. We need to somehow generalize that. We need to find something that is better than representing knowledge in terms of probabilities. And there is actually a reason for that. So what's the intuition? The reason is the following. Probabilities are good if you know what your knowledge is about. And in QBism, you would always say knowledge is about the possible outcomes of a measurement. And I think most people would say quantum theory is really about predicting measurements. So the, the final result of any quantum theoretic treatment is a prediction about the measurement outcome. But now, in this thought experiments, we kind of learned that even outcomes of measurements are not something absolute. They're themselves just knowledge. So in a sense, if I say probabilities are about outcomes, this doesn't really make so much sense if I say outcomes are themselves again just knowledge, because then the probabilities that capture knowledge are probabilities about knowledge, and so that makes the concept, um, kind of a bit unusable in a sense. So probabilities really make sense if I can say, like, if I talk about the coin, for example, that we had before, I can say there's a probability of it landing on heads and tails, and I, this reflects my knowledge of what I will see. But if, if I can no longer talk about the outcomes, or if, if the outcomes are not the real thing, so that's what, what all these thought experiments suggest, not even outcomes of measurements are absolute. Then we have to ask the question, what are the probabilities about? What is our knowledge about? If it's not about the outcome, is there something else the knowledge is about? And so my suspicion is it's kind of a recursion. The knowledge is itself about knowledge. So if I make a prediction, I kind of make a prediction of my knowledge once I've seen the outcome of the experiment, which is a different thing than saying it's about the outcome of the experiment. It's, so a prediction about my knowledge about the outcome of the experiment is not the same as my prediction of the outcome. And this sounds a bit nitpicking, but it's a, it's fundamentally a different thing. In one case, I'm making a prediction about knowledge, and in the other case, I make a prediction about an actual outcome. And so if I now say probabilities are capturing knowledge, then in this language, I would say a prediction should therefore be a prediction, or a probability should be a probability about probabilities. So if I make a prediction about knowledge, it's probability about probabilities.

But this somehow seems to not work well. So we need a new object of knowledge that can kind of be self-recursive. So the knowledge, so the, the notion of knowledge should be able to talk about knowledge in.

Why is that problematic? Why can't you have probabilities of probabilities of probabilities, etc.?

Yeah, because, um, yeah, it's, I mean, a priori, one could try that. So there's not an obvious reason why this is problematic. I think the, the problem, um, comes more from, let's say, um, the way, let's say, if you, if you want to build it axiomatically, in a sense, which is important for understanding QBism, then you start with kind of saying something like probabilities are about, or the way, what does a probability mean? It's how much money I bet on a certain outcome of an experiment, and then I can build up the whole framework based on these axioms or so. I just want to be a rational agent making bets. Now, in this development, I really talk about bets that are realized and give me money, in a sense. But if the probabilities are themselves about probabilities, this building this up in terms of bets does no longer work. And in order to give the probabilities a meaning in terms of knowledge, I need that. So, in a sense, purely technically, I can kind of define probabilities of probabilities in a way that looks useful, or that sounds kind of mathematically consistent, but I cannot give it the meaning I wanted to give in, like, in terms of betting games, or I cannot really give an operational meaning to the numbers. And I think this is important if you really want to use the concept.

But this is just a problem with QBism, or no, this is just a problem more generally.

Oh, this is a general. So I would say, um, whenever you want to apply a theory, in a way like, or let's say, um, okay, I would say it's a general problem whenever I have a multi-agent scenario, and if I interpret quantum states as states of knowledge, because if I have a multi-agent scenario, I want to have knowledge about someone else's knowledge.

I see.

And so then I necessarily have, whatever, however I am, um, kind of, um, model knowledge, the object has to kind of talk about itself. So, like, in this case, the probability has to talk about the probability someone else has. And you see, maybe the problem there is like with the interpretation is the following. If I, there's, let's suppose you have some, um, okay, let me make an example where you see that it's really hard to to come up with a good way of dealing with these probabilities. So let's suppose, um, you have some knowledge about the weather tomorrow in your place. I don't know where you are, but let's suppose,

Toronto.

Oh, okay. Yes. So let's suppose you now, um, I have, let's say, Toronto is far away from Zurich. So let's say if I need to make a prediction about the weather in Toronto, I would, um, my knowledge would be something very uninformative, and maybe I would say something like, it rains tomorrow with 50% probability.

Sure.

And now let's

Suppose you tell me that actually, um, your knowledge that it rains tomorrow is 10%. That's a simple case. So, I'm, if you tell me that, then I'm certain. So I have a probability one assignment that you probability assignment to the weather is 10%.

But I also have my own probability assignment of 50%. But, and now I want to combine this somehow. Maybe someone else, I may even like this 50% may have come from somewhere else. And so how do I now combine this? So what's the, what number should I, after you told me, assign to the, to the weather tomorrow? I could say, okay, I completely ignore my previous probability assignment and now also say it's 10%. But that just means I completely ignore my previous knowledge. So it's not really a knowledge update.

Or I just stick with my previous knowledge. I say, okay, I just knew it's 50% and I still know that despite the fact I mean, I just, you have some other knowledge and I just ignore that. But, or there's something in between, but there is no clear way how to do that. So even so, this tells you probability theory doesn't come with a rule of how to deal with this case that someone else tells me his probability. So I would say we have the problem that even in abstract probability, there's no way to actually deal operationally with the fact that the probability, um, there are different probabilities. And now it could be more complicated, like maybe you didn't even tell me what your probability assignment is, but I just have a guess of what it is. So I would maybe say with 50% probability you're telling me that the weather will be 10% raining, and with 20% you're telling me it's 90% raining, and with another 30% you're telling me it's 50% rain. I have a probability about your probability assignments, and now in order to have a full theory, I would need to turn that into a new probability that is my, let's say, updated probability. And this is simply not present in the probability calculus. And now in order to find such a rule, we need some, we need to link it to the, these betting games. But the betting games now don't work well if the bets are again about probabilities because there are ultimately, there's no payout in a sense. So we need to really start from scratch and ask ourselves, can we have a more general theory of bets where we at the end find some more general concept of probabilities where we have multiple user or not necessarily multiple users, but probabilities about, so we have knowledge about knowledge. And how do we, do we capture that? But you see, there are problems with the current theory. So it's just, and the problem is just that there is no rule present. So there's no, nothing analogous to, for example, the Bayes updating rule. The Bayes updating rule tells me if I see an outcome, how do I update my probability? But it doesn't tell me if you tell me your probability assignment, how do I update my probability assignment? This is just not included in the probability calculus.

And there's also no unique answer. So there's something missing there in a sense.

And so, and that's something that I think we need to develop. So in a sense, I'm going back to a classical. So the whole thing started with me investigating quantum theory and, um, agents talking about other agents. But if you really take this now back even to the classical realm of classical probability theories, it's not clear that there is a solution there in a sense because, um, usually people don't see it as a, a multi-user theory. So Bayesian probability is always about me making predictions, but not me making predictions based on other people's predictions in a sense.

And there, there are no contradictions there, but still, there's no clear rule. So it's, um, there's something to be done there.

And of course, there, I mean, I, I shouldn't claim there's nothing out there. I mean, there's lots of stuff like game theory and so on, but it's not obvious how this can be used now in the context of quantum theory. Speaking of classical theories, have you landed on a separation like a classical-quantum world separation, or do you still see the classical world as just many quantum, large quantum, or composite quantum?

Yeah, I, I don't see, think there is a separation. So for me, the, the classical world is just something, I mean, it's not even necessarily large. It's just relative to your knowledge. So I would say whether something is classical is not a property of the object that I'm talking about. It's just a property of my knowledge about the object.

So you're not endorsing a, a collapse theory.

No, no, no. I mean, there's just no experimental evidence for it. So I'm, I'm also have no strong. So this is again, one of these emotional things for me. A collapse theory is emotionally wrong. But of course, rationally, the only thing I can really say for sure is that with the technology so far, we didn't find the collapse mechanism. And this of course doesn't exclude the idea that, for example, on on larger scales that we couldn't test, there could be a collapse. But maybe if I try to explore the reasons why I have this negative emotional feeling against collapse is that it's kind of sounds very artificial. It sounds to me like, so we want to desperately reintroduce classicality because we were used to it, and so we want to return to this familiar concept. But there's no, let's say, natural reason or physical nice concept that would lead to that, because some physical concepts just sound so natural that when you hear them, they feel right. It's like when, when you first learn relativity and you suddenly see like spacetime is like that. This just sounds much better than whatever you heard before, before you were in contact with relativity. That's what was at least my impression. So I immediately felt like that's nice. That's so elegant. That cannot be wrong. And so with collapse theory, it's a bit the opposite. I think we have a very nice theory. It's like having relativity that someone would tell us, no, we have to go back to the, um, Newtonian picture or to the, um, kind of Euclidean space and time is separate from it. And indeed, actually, this is not only an analogy. I think most collapse models are not relativistically, um, or Lorentz invariant. And there are some constructions that probabilistic constructions that try to do that, but most are not. And so, um, it really feels like returning to an old concept. But as I said, this is really emotional arguing. And, um, maybe I should, um, yeah, to be honest, I cannot be neutral there in the sense of, let's just be rational and try it, because to try it, I need to make decisions on how much resources I invest in trying it. And by resources, I even mean my own time. And at some point, I have to decide what to invest time in. And this can only be an emotional decision because before I investigated it, I don't know what's the right approach. So to kind of decide which approach I find promising and and trying out is kind of a just connected to my feeling of that's probably right. But it's, it's before I have any rational arguments. It's very, just an intuition. But it's, maybe I should really call it an emotional feeling because intuition sounds like something I could explain why. Whereas there, it's, it's really even on the level where I cannot even fully explain it. So I tried to explain it. I told you it's not relativistically invariant. But why is that bad? Why, why should we adhere to being Lorentz invariant? Of course, you could say that's nice, but nice is again something emotional in a sense.

Now, okay, we can speak about some pre-linguistic intuition here about quantum and and gravity. So, you have a lecture which I'll place on screen of Helgoland from 2025. And I watched it and most of the time when someone says that quantum mechanics can tell us something about general relativity, so first of all, people will go from the direction of quantum mechanics to GR. They rarely go and say something like GR can tell us something about quantum mechanics. And anyhow, and any of the times when they're making a regime of quantum mechanics informing us about gravity, it's almost always about black holes.

And the page curve or BPS states or firewalls or something like that. So my question to you is, do you intuit it that quantum mechanics can tell us something about GR, and GR can tell us something about quantum mechanics outside of the purview of black holes?

Outside of the black hole. Okay. Yeah. I, I think, um, it goes both ways. And, um, of course, the examples I would have told you would have involved black holes, but, um, I think even if we are, um, like away from these extreme cases, I mean, we can, um, come up with, let's say, stuff that is related. So let me make, try to make an example. So, um, I mean, this is maybe a quite high-level example, but let's take the double-slit experiment from quantum theory. What's the lesson there? The lesson is, unless I measure through which slit the particle goes, the question through which it goes has no definitive answer. I mean, that's the standard thing in, in that we learn in quantum theory. There is just no answer. It's not that we don't know it. The question through which the part, through which slit the particles has no answer. Now, an, and an analogous thing is kind of the whole argument which is not the black hole, but the whole, um, Einstein's whole argument where you just say, let's take space-time and just, um, kind of virtually cut a hole in the sense that we think of a region that is kind of, um, um, kind of bounded. And let's just change space, the solution of the Einstein equations which determine how spacetime is curved, um, within that region. And, um, consider another solution which is however diffeomorphic to the original one, so it's actually an equally valid solution. And now I could ask a question like, did a particle that entered this whole region pass through a particular, um, designated point that I kind of designate by giving a coordinate? Maybe I could, like in the double-slit experiment, have two points, um, that are defined by two different coordinates, and I ask, does the particle pass through this point or through that point? And now what general relativity tells us is that this also doesn't have an answer because the two sol, I can, um, take one of the solutions and then, as I said, kind of within this whole region, change the solution so that it looks like the solution where the particle went through the other part, through the other point. And it's also a valid solution. So both solutions are equally valid. And the, the usual resolution or Einstein's resolution was, it's not a well, it's not even a well-defined physical question through which point the particle went. Of course, it's important here that the point is just a coordinate point. It's not, um, implemented by a physical object. This is like, um, in the double-slit experiment, I said I don't measure through which, um, slit it goes. And if I don't measure, then I cannot tell the question through which it went doesn't have an answer. And in relativity, I could say if I have two coordinate points, the question through which one the particle went has no answer. It's not that I don't know it, it simply doesn't have an answer. But now, similarly to the double-slit experiment in quantum, if I put the detector there in two slits and I measure whether the particle goes through, the question has an answer. And I mean, it's a measurable outcome that the measurement device will tell me through this slit or that. Now, again, in the, in, in the hole argument, in Einstein's hole argument, I can say, if, if these two dots, points that I marked are not just coordinate points, but they're actual detectors, material detectors there, so the points are now defined by where the detectors are, then the question through which, um, of the detector the particle passes has a definitive answer. So, in other words, in both theories, we had the situation that if you ignore the measurement, if you just don't put the measurement device, if you say there's just some abstractly defined place, so in, in relativity, it's just by coordinates, but also in quantum, it's just two slits which are in a sense abstract, in a sense, and I don't do an operation there, then the question where the particle is has no answer. But if I do, if I measure it, then it has an answer. So both theories tell us it's very important that we don't just abstractly talk about the location of a particle. If you want to actually give it the meaning, we have to put the device there. So, and I find it quite striking. I mean, of course, I now told the story in a way that it really matches, and of course, one could have many arguments and say it's, it's kind of for different reasons, but my point is indeed, there are very different reasons, but the lesson is very similar. So the lesson that if you want to talk about, um, a location of a particle, we need to put a measurement there, we need to make it operational. This is the very same lesson that we learn from very different theories. So, and I find this really surprising that two extremely different theories on completely different scales were developed, sometimes tell us precisely the same lesson that we didn't learn before in the old, like classical mechanics or electronics didn't teach us this. These were, were really new lessons learned, actually very, um, probably painfully by Einstein when he developed this argument, but also in quantum theory were learned by thinking about these interference experiments and double-slit experiments with all the variants, with the delayed choice and so on. So there, there's a lot of literature on both of them. And somehow one can phrase them in a way that one can say they somehow have the same underlying message. And this is just an example now without the black hole where I think, um, each of, so if we just understood one of them, we could have kind of maybe, um, learned about the other. Of course, these things were developed independently in a sense, they're now linking them. But there are other, maybe places where we are ahead. And I think, for example, um, these weak measurement experiments or the type of experiment we thought about before, these were not yet considered in gravity. And I think we can learn from that. And maybe the firewall experiment comes close to these considerations, but they're different types of experiments. So it's again, the analogy is that in a black, I mean, there it's a black hole, but one can say more generally in general relativity, observers have different perspectives necessarily because there is a finite horizon. Even without the black hole, I just see the stuff that can potentially reach me. And in quantum theory, we discussed before that if we take observers to be themselves quantum systems, then they also have different perspectives because they cannot describe themselves. So in both theories, we have fundamentally a limitation on the part of the world that an observer can describe. And this wasn't the case in classical mechanics. I think in classical mechanics, there's no reason why we cannot describe everything as one big thing virtually from the outside. But somehow general relativity tells us that information propagates with finite time. So we can only see stuff that is not behind the horizon. And in quantum theory, we, we learned it in a different way. So there, the more technical way to say is that we always have to put the Heisenberg cut, and everything that is beyond the Heisenberg cut is no longer described by the theory. So both theories share the feature that if we make the observer explicit or the agent, they only see part of the world. They never see the whole world. And so we necessarily in both theories, theories need to deal with combining knowledge of different agents in a sense. So in, in relativity, we somehow do that by saying there are the different patches of spacetime that can be in principle analyzed by different agents. Like, I mean, there's, for example, one agent that is behind the horizon of another. So that agent sees part behind the horizon of the other, and so they describe different things. But at the end, we want to combine the knowledge and have a larger picture. In quantum theory, we want to do the same. We want, in these thought experiments that we discussed, we have different observers, they want to, to somehow combine their knowledge. And so in both theories, we have kind of a similar challenge. How do we consistently combine knowledge? And so I'm, I'm really convinced that here we can learn a lot because in both theories, we, we hit problems like in quantum theory, these are these beginner's friend experiments and experiments we discussed where we have inconsistency. And in the case of, um, gravity, we have these kind of firewall paradoxes. But they, as you probably know, they even work without black holes. They can take other horizons like cosmological horizons. So we have in both cases problems or a lack of understanding how to combine views of different agents. And I would say in my thought experiments with Daniel, this is kind of the core, I would say, new thing in a sense that we, we take the agent seriously as users of the theory. We therefore have to combine the, the knowledge of different agents. And we simply have no established rule for that. And in relativity, we, we this is often just done ad hoc for when people analyze the fire paradox, for example, they just somehow assume that the agents can somehow combine stuff, like one agent meets the other and then they exchange knowledge by talking to each other. But it's just assumed that they, like if one agent tells the other, this is the quantum state, then the other one takes this as the right quantum state. And of course, as you learned in quantum mechanics, this is very problematic. And so we even learned that even for classical outcomes, it's problematic. And so I think this knowledge, what is problematic, and there are limits of knowledge which that we can have as physicists, is is very parallel in these two theories.

Professor, we began this conversation about inconsistencies between physicists in a sense. I'd like to also end on that, but differently than just fundamental laws. So as you speak to your colleagues, as I speak to people on this channel, it's even called theories of everything, not theory of everything. There are various different approaches and invariably when I speak to scientist A about something they disagree with researcher B. Invariably, then the question is, well, look, if we're all rational people, why the heck is there so much disagreement? Possible answers to this are, this is a trivial question. We're different people. We have different predilections, different biases.

So of course we're going to disagree. We have access to different data. We don't apply reasoning as well as other people. So some people have higher IQs or lesser IQs or whatever you want to say. Presumably, there's one truth. So either we could say that maybe there's not one truth or two truths, in which case we still shouldn't disagree that much. We should converge on two truths.

Or maybe there are plethora of truths. In which case then it comes down to, well, is physics even a subset of science? Then there's some problem there with even defining physics.

Consistently, perhaps the overall question I have is, why the heck do we disagree so much? Not the fact that we should disagree, that's like a normative claim, maybe that's great, oh, it's diverse, science is diverse, physics is diverse, blah, blah, blah, whatever slogans one wants to say. It doesn't matter. Why do we disagree so much even when we have access to the same data and we agree broadly with one another's reasonings?

Yeah. Yeah. It's a very interesting question that I ask myself a lot because when my paper with Daniela Fra came out, I got a lot of criticism and had lots of discussions and maybe I should first say I found these discussions generally extremely useful to learn about other perspectives. So there were sometimes, um, maybe some angry reactions, but most of them were, um, I mean, there were disagreements, but in a constructive way. And actually, all my, like my knowledge about the different interpretations of quantum theory that existed at the time and still exist now, or or were developed also over the time, has profited a lot from talking to these people who had very different viewpoints. So I, I found, I mean, that's maybe one part, like the, the disagreement first was very fruitful in the sense of having very deep discussions, trying to identify, um, the source of the disagreement. But I think I was also asking myself, um, why is it that it's so hard to kind of, um, come to an agreement? Because one say, okay, maybe one first has a disagreement, then one just identifies the point where one has a disagreement. And my lesson from that was that there are. So whenever I kind of found that, yeah, the, so there were many success stories in the sense that I had long discussions with people, but after the long discussion, we actually found an agreement in the sense that we could identify an assumption that we were both making implicitly and we never talked about, but this was kind of at the end the ultimate source of our disagreement. And and this assumption can be something very, um, general that we usually don't talk about, like from whose perspective do we do physics? And it, I mean, I talked about that before, like we could have an outside view or a view from the inside of the universe. And then you would say, okay, obviously that's an assumption whether I, I'm looking at physics from the outside or from the inside. But as long as, like if you talk about quantum theory in the standard way, like just technically, like I apply this rule and that rule, you never actually make that explicit. You never, this is never a theme that you talk about from whose perspective you're actually applying the theory. So I had implicitly always assumed, of course, I'm applying the theory, and some of my colleagues were implicitly always assuming, of course, a theory is applied from the outside, that the world is there and the theory stands kind of over everything from, like it's kind of something that is not itself something that is bound to physics. And so that's something that is often not even like identified as an assumption, kind of inherent in our thinking, but we just don't talk about it. And so it, it sometimes took us a very, very long time to get to the point where I said, oh, this is where we actually think differently. I just make this assumption that, of course, I'm part of the world. And this guy of said, of course, a theory like is, is something that has to, um, be separate from the, it's kind of something that's applied from the outside. And so I guess, um, we should try to, um, think much harder about the assumptions that we make, and that would avoid many of the disagreements. And I think the problem is really the way physics is taught sometimes, because we are forced, we are talking a lot about the technical content, which is of course important, so I need to at the end be able to solve equations, but we talk extremely little about just the underlying general assumptions that go into physics. And, um, for example, even the fact that we are ourselves physical systems, obviously, we are, no one would deny that, but it's an assumption that enters. And for example, in my work, this was an important assumption. And others just don't want to talk about that assumption or or don't mention it. And so, um, I think,

one should, so my lesson is really trying to find these assumptions that we usually not talk about helps a lot in avoiding the disagreement. And I told you before that, um, in the case of the black hole information paradox, my conclusion was both parties were kind of right. Those who said that, um, the radiation is thermal, and the other who said, no, it's actually, you can retrieve the notebook. And my conclusion was, okay, they were both right. They were just making different assumptions about the reference they have available. And then, um, both could be right in a sense. And this is also true for other, like paradoxes we have in physics, like there is these, um, there are these questions about, like the, um, Maxwell's demon, where people have different opinions, but then, um, one can also often see, like those who claim the Maxwell demon exists and those who say it doesn't exist, just make somewhere different assumptions, but they are kind of both right. So my approach is to find, kind of the way to, I mean, I think these are mostly, I mean, all of them probably are mostly very intelligent people, have very good reasons to believe, um, or to to say what they say. And so I think the task is really not to, too early, just try to defend one an opinion, but to try to understand what assumptions do they really make deeply in their, um, in their, um, thought process. And I think one can learn a lot from that, because then one can just see these different opinions, these disagreements as actually sourced by different basic assumptions. And here is again this emotional thing that comes in, what basic assumptions are good assumptions is something that one can probably not rationally argue against. I had this experience with, um, my colleague Nikolisa from Geneva, who is a quantum physicist, who is actually a collapse theorist. And it took us actually quite some time to find out that our disagreement is just, is there an underlying collapse or not? I mean, in retrospect, it was obvious, but we talked about technical stuff, about all these, like different thought experiments and that.

Interesting. You would think that that would be one of the first that would be identified.

Yes. But it was of course not so clear, maybe, um, to both of us what we were kind of assuming. So one had to kind of force it to say, you, you necessarily are a collapse theorist if you assume that. So it was not that he started being a collapse theorist, it was more like he, one had to realize this was the underlying thing. But then we had a debate once, a kind of public debate, and the debate was at the end completely boring because we said, look, we have different opinions. He thinks he's a collapse theorist, and or collapse theory is a good description. I think it's not. And that's why we disagree. And we couldn't really argue why a collapse is bad or good because there's no experimental evidence for or against it. And we kind of agreed that this was the source of our disagreement. And so there was nothing more to be said, except to say, let's await the experiment that will either confirm or, um, reject the collapse theory. So in a sense, that was for me the ultimate point of reaching a consensus despite disagreeing. It's like, it, we boiled it down to a point where it's purely taste or, as I would say, emotion. So he's just somehow emotionally convinced that collapse is the right thing. And I'm emotionally convinced that it, like the, I mean, the reason why he thinks so is that he doesn't want to give up other things like realist, um, measurement outcomes. So for him, a measurement has to be real. And that's just also an emotion. He's so convinced that the measurement outcome has to be real that he's ready to bite the bullet and say, okay, there has to be a collapse.

Now my emotion is the opposite. I would say, okay, um, I can live with the fact that the measurement outcome is not real. And for me, the benefit is that I don't have to introduce a collapse. But which one is better? That's, um, yeah, I don't think there is a rational way of arguing one of us is more right. We just make different assumptions. And I think both have their place. And his, his reasoning is completely rational and legitimate once this assumption is made.

Okay. So these assumptions are like towers in a city, and you're willing to give up some, and some are willing to give up others. But part of giving one of them up is that to blow up that tower, you give collateral damage to something else that they consider valuable as well. So that's why it's not always just about this single tower, and you think this single tower is not so important, and this person says, yes, but doing so, you sacrifice this and that.

Exactly. Yes. I see. And at the end, it's a discussion about what you want to sacrifice and what not. But then you have to kind of give a value to the different assumptions. How, how valuable is it for you to keep it? And what value do you give to assumption, how important is it to you to not have a collapse, or how valuable is it to you to have the statement that the measurement, the outcome is real, or how valuable is it that there is locality, or how valuable is it to have free choice? And all that. And these values, I think, are a deeply emotional thing. So, for example, whether I want the world to be such that there is a free choice, it's really something I either can accept or not. And I weigh it relative to the other assumptions, or in your planage to the different towers. And for me, something are harder to give up, and some are, um, easier to give up. And I, I think it's completely, um, normal that different people find put these weights differently. And I think the rational thing is to, to have no-go theories that tell us if you make this assumption, then the other is it invalid or must be invalid. But no-go theories will never tell you which ones you have to give up. It will only tell us, tell that they cannot, you cannot have all of them, and you will have to pay a price. And the price tags are different from people to people. And I think this is on that level, it's, it's then getting in a sense uninteresting. But, um, I think the, the important thing is to reach that point, to really get to the point where I'm going to say, okay, now it's really purely a matter of taste about the assumptions. And I think that point is reachable, in my opinion, opinion, if people are ready to listen to each other. Of course, your, your thing actually contributes to that. I have to say, in prep, I, I prepared myself for this interview by watching a few of the interviews, and I, I found it very insightful to actually really be able to digest, like, because it goes, um, like through many examples and so on. And I think this really helps understanding what are the intrinsic feelings of these people, why do they actually have these particular, um, preferences? And, um, for me, this, this was actually an interesting, um, experience to, kind of, it's different from reading a paper because it's kind of see the emotions more in people, what is important to them, which doesn't really get across in the papers. And I think that's again, not separable from the rest of the scientific endeavor, to, to see what drives people to go in a certain direction, and what do they find very valuable, and what is for them impossible to give. Well, actually, something that I like about this whole conversation, yours and and other people's as well, but especially yours, is that separate from the paper, the paper is just a finished product, and there's many tangents that one takes to get from point A to B, but those tangents aren't in the paper most of the time. It wouldn't fit the length. It's also digressive. It's discursive, etc. But they give, they give tacit knowledge, they give other forms of knowledge that aren't explicit. So I was speaking with David Besses, and he said the textbook should be more, that he's a mathematician, and he said that the math textbook should be more like a reference manual, and that lectures are actually primary. And I said, well, people self-study all the time. He said, yes, but in lectures, just from body language and little anecdotes that the professor gives, they teach you about how to think and how not to think, or how they think and how they think you should not think.

And all of that's valuable, not necessarily in the in the book. So.

Yes, I think this is a very important additional communication channel, at least. I mean, I think it, one requires both. I think the written thing forces oneself to be precise and and, um, kind of put things in a way that one thinks they survive the next, um, or eternity in a sense. Whereas, um, here, when I talk, we can also, like I can talk about new developments that are maybe not put in stone, or new ideas. I can tell you that I changed my mind over the time. I would never write this into a paper. There's simply no place for that in a sense. And I guess, um, this, this is kind of important information also for the discussions. Usually, if I know someone never changed his mind, or someone changed his mind, I will, um, talk to the person differently because I see different chances or, or I, I see different ways to kind of get make the conversation fruitful in a sense. And so I think it's, it's very valuable to have that format as well. So I really appreciate it's also fun to listen to the interview.

Great. Great. I hope so.

Yes. Now, professor, I know I've kept you for three hours, but you said something that I just have to ask you about. Look, if ultimately it's going to come down to something that's that's so personal, preference, taste, values, like you mentioned, and assuming that we're being rational and that we're following logic and so forth, and we could even critique our own logic, is classical logic the correct way, is paraconsistent, etc. That's fine. Okay, all of those are games we can play, but that ultimately we're going to find that it's some hidden assumption that once uncovered has underneath that something like a value or a preference or taste. And furthermore, that these assumptions are what builds our theories and data. So experiments can't on their own decide because data itself is quote unquote theory-laden. I'm sure you've heard this term.

Yes. Okay. Then in some sense, there's something that is just that seems so groundless, so baseless, so so subjective. It's just values, but it can't just like how do we then come to a consensus or conclusion or make progress? Help me understand that.

Mhm. So I guess at the end, so I had this example with Nikolasha thinking of the collapse theory and me, um, denying collapse. I mean, in this particular case, the, the emotions are important, or the taste, or the, let's say, could say the, um, choice of approach is important because we don't yet have an experiment to decide, but we need to, um, decide how to invest our resources. So, for example, if I, if I'm a collapse theorist, I would invest more resources in trying to find, um, that collapse mechanism. Whereas, um, as I'm now, um, thinking of like observers being very relevant, the knowledge being very relevant, I invest more resources and time in finding a good theory of describing knowledge. So it's these emotions are very important to kind of allocate resources in research and, um, but ultimately, I think an experiment will, for example, decide whether there is a collapse or not. So I think these emotions are kind of important to, um, kind of direct our efforts and maybe we are sometimes wrong, not, but of course, we need to try to invest our resources as effectively as we can. It's like when you maybe build a have a build a company, a startup, and you have to decide what product could be, um, could work or could be profitable. You probably have a feeling, and then you do it, and you're either successful or not. And of course, those people who have, um, a good feeling of what could be successful will, um, will be more quickly successful. And I think in science, these emotions will maybe have an impact on how fast we will reach the goal because we will invest in the right direction. But I think ultimately, um, it will, it will be, um, based on experiment. So some of these things that are currently just feelings, could it be like this or that will at some point be decided in physics? Because for me, physics is still all about questions that have an answer in the world that is in principle verifiable. And the emotions about making assumptions and so on, it's all, um, necessary because we don't yet have these experiments and we need to have make some working assumptions. We need to have the working assumption. How likely is it to have a collapse? How likely are we looking at this? So I think it's not a contradiction. So I think ultimately, in, in a few, um, hundred years or, I don't know, when we will have decided on many of the questions that currently look like purely emotional things about the interpretations and so on, and it will be clear what is the right thing. So I think the emotions are more like a precursor to direct our efforts, and, um, not the ultimate answer at the end. So I think all the questions that have physical meaning will have an answer that is experimentally testable.

But in your case, even when you're questioning, well, what the heck is the data telling you? So is it an outcome per se, or is it just updating our knowledge about someone else's knowledge? Or how does an experiment even distinguish between that?

Yeah. So I, I think if the question is at the end, um, a question that, um, I need to, I, I have to admit that it cannot be experimentally tested, then I would say we can, or let's suppose I would have a disagreement with someone on such a question, and then we would find out experimentally it actually makes no difference, then I would kind of no longer be interested in in this difference or in sorting out the difference. I would say, okay, actually, actually we mean at the end experimentally the same. We just use different concepts to talk about it. So I mean, this is maybe a very operationalist viewpoint that I'm describing, but, um, I, I really have this viewpoint that the relevant questions, the ones we should really fight about or or have, um, invest effort in in solving them, are questions that have an experimentally verifiable answer or an operationally verifiable answer. Because otherwise, the question may not even, um, actually be a question. So, let me maybe put it like this. I mean, there are some, if I have a question like, is an outcome of a measurement real, or do we just only have intersubjective agreement among all of us? Now, you could say, if there's really always a complete agreement among all of us what the outcome is, there is no difference, operational difference between saying that and saying it's, it's real, because no one could then even define what's the difference between real and just having an agreement between all of us. But we have this conception that there is somehow more to it if it's real. But then I would ask that someone could really define it in a way that makes it experimentally distinct. And if he cannot define it in that way, I would say, okay, he actually meant the same thing. He just used a different term that, um, um, yeah, it's just a different term for the same thing. So I think we have this vague feeling sometimes that there is something different between calling something real and just something where we all agree on. And I would say in this particular case, this may, this difference may just not exist. And so we, we don't need to try to find an, or there is clearly also no experiment, but then it's also not a relevant thing to have a dispute about. So this is the thing that would call then a kind of purely philosophical question. And if someone can give me a definition of what real means that is different from having an intersubjective agreement and different in the sense that it has an operational consequence, then it's again testable. So in that sense, I'm really operationalist. I think the questions that we are asking in physics should have a testable outcome, and otherwise they are not physics questions in a sense. And, um, yes, so I'm, I'm much, I mean, okay, there are questions I find interesting which are somehow beyond physics, like what is consciousness, for example. I'm not sure we can answer this in physics. I'm very interested in that. The answer to that question, I have no clue on how to address it. But let's say if we talk about physics questions, then my, um, let's say requirement would be that the question is only a physics question if someone can tell an in principle experiment that one could carry out to decide the question. And so, yeah, I would, as a physicist, focus on these type of questions. But I don't mean that the others are not also interesting. I just, yeah, I cannot talk about them. I wish I. Good. So, I think the consciousness question is a very important one, and I wish someone could tell me something interesting about consciousness, but I haven't heard anything that I found insightful about that so far. I mean, I listen to some of the conversations, um, on one can find with people who talk about it, but, yeah, for me, this doesn't really answer the question.

Just a moment. You said something super interesting. You said that it may be the case that consciousness is outside of physics, or that physics can't answer questions about consciousness. That sounds to me like you're open to the idea that physics is not all there is, if consciousness is somehow in reality and physics was supposed to describe all of reality. But

Yeah, or maybe let me put it very carefully. Or I would put it like that. Consciousness is a concept that I don't even have a clue of how to define it. It's very strange. I have a very clear feeling of what consciousness is because I feel I'm conscious. But I have so little grasp of the concept that I cannot even decide for myself whether it's in which realm of science I should put this. It's maybe some concept that is beyond the way, like it's not captured by the way we, like our scientific approaches. So I think it's in a, in that sense, not necessarily in physics, but I'm just saying that because I have, maybe you should just understand it as a statement. I have so little grasp about this concept that I cannot even tell you whether it's physics. Um, if, if I mean by physics, there is some, as I said before, some experiment I could do. So I would say in the case of consciousness, um, I think this is something that, um, I'm interested in because it's a very immediate feeling that is a, is there, like I, I feel conscious. But there, um, I'm doubtful whether there is kind of an experiment we could do that would give us insights on conscious, consciousness or decide on a question about how consciousness is. I think to understand consciousness, we need maybe a very different way of even thinking about concepts in, in science, potential.

Ah, okay. So maybe I've just seen many words saying that I think consciousness is so little unders, at least by me, and and the stuff I've heard about it, that I, um, I'm not confident in in assigning it to any of the scientific disciplines that we have.

Now, would that be because consciousness is a first-person phenomenon, and science is all about third-person phenomena?

Yeah, maybe that's a good way to put it. So I, when we talked before, I was always emphasizing the point that quantum theory cannot be applied to itself, which is also a kind of recursion. And maybe consciousness is exactly in the area where, um, we have, kind of, a concept that talks about itself. I mean, obviously, I'm doing that if I'm now talking about consciousness, I'm somehow talking about the concept itself, using that concept. I mean, the fact that I can talk and I'm interested in has to do with my consciousness. So in a sense, it's, it's very deeply self-recursive, whereas science is usually like, we have what I called before, a Heisenberg cut. We had like the person who applies the theory and the object we apply the theory to. And so in, in that sense, it's kind of third-person that we, we have the, the object that is observed, and we have the person that observes it. And, um, consciousness can only be observed by myself or my own consciousness. But I feel very uncomfortable talking about consciousness because I really feel I cannot really say much many sensible things about it because, um, it really eludes my, my way of putting things.

Yeah, of

course. I think it eludes almost everyone, even though it's in some respects the most intimate aspect of them, of their life.

>> It is. Yes. Yes. Yes.

>> So when it comes to not defining consciousness, there's, there are a couple approaches. So, one could say, "Well, consciousness is what it's like to be something." I don't personally like that definition because then you could ask, "What do you mean by what it's like to be so and so?" And then they may say that so and so has an experience. And then you ask, "Well, what does experience mean?" And then it comes back to consciousness. So, it's somehow a circular definition.

>> But then I'm critiquing myself and saying, "Well, the reason why I'm saying it's, it's a poor definition because it's a circular definition actually betrays my metaphysics." So what I mean to say is, if you were trying to define you, or anyone was trying to define consciousness, and I kept saying, "What does that mean? What does that mean?" And I don't stop until someone gives me something that's outside of consciousness, something physical, say.

>> Then what that means is that I already think consciousness must come from something physical.

>> Mhm. Yes. I see. Yes.

>> But it may, it may, it may not. I don't know. I'm, I, I have no clue. And then also, if we're going to apply this criteria of it must have a well-defined definition in order for it to be a thing. That same critique could also be said about physics itself. I'm sure you've heard of Hempel's dilemma, which is, well, what does physics even mean? Can you empirically test what physics means as physics per se? What's the experiments in your lab to test what physics means? Okay, we don't have that. So, okay. So even by one's own verificationist criteria, it seems to fail. But physics as a discipline, does it dissolve? No, we still somehow know what it means. Okay, but what does it mean? Okay, well, we can't test it in the lab.

>> Okay, then, then there's two forks. Is it today's physics? Is physics defined by today's physics? Most people say no, physics is incomplete. We still don't have QG.

>> Okay. So then is physics some future ideal physics? But then Hempel says that's ill-defined because if we're saying that it's going to be whatever the future physicists come up with, that's also ill-defined. Furthermore, future physics may actually include somehow irreducibly mental consciousness-based phenomena. We don't actually know. I mean, some >> there's some interpretations of quantum mechanics like that. I don't buy into them, but I'm saying we don't actually know. The only point is to say that if we're going to say consciousness is so handwavy, we don't know what it means. We don't have a grasp as to what it means. I'm real. I'm studying physics. But then the same critique could also be said to the definition of physics per se.

>> Mhm. Yeah, I think that's right. But um, maybe before we were talking about communication and so I could say there is a kind of minimum, let's say operational thing that um, is there in physics, namely I can inform, I can, for example, teach students in physics and then they can make predictions that are again, um, maybe um, will lead to more knowledge that they can communicate to others. So physics is, I, I could kind of operationally understand physics as a way to compress our knowledge of the world into and make it communicatable. So often people say a physical law is just a compressed description of the world. But for me, it's this additional aspect. It's compressed description, but also one that can be communicated to other physicists, which, as I say, could just be my future self, for example. And um, I think this puts an additional kind of constraint on what physics is because in principle, it could be something compressible but not communicatable. And actually, the reason for that is that if I communicate something to someone else, I need a common reference, in a sense. So we could, for example, it would be very hard to communicate our physics to an alien who has no, um, let's say, shared reference with us, not even language or anything. So to make it communicatable is, is actually a non-trivial requirement, in a sense. And then I could say, what again, if I think very operationally, what's the purpose of physics? It's kind of to, one of the purposes is to orient ourselves in the world, to make use of stuff we see in the world, um, kind of to survive in the world, to um, build stuff, to make our lives better. And for that, the operational thing is we need to be able to communicate our description of the world to others. So I could take this very operational viewpoint and say physics is just a very sophisticated way to mutually communicate knowledge about the world to other physicists. And so, okay, that's just a possible definition that is, in a sense, um, very operational, where I don't need to ask the question, is this now a physics law or not? So, in a sense, it's also a broad definition, even if I just wouldn't know about physical laws, but just tell people, look, I saw this falling down and wouldn't know about this law of gravitation or so, that would also be physics. It's just a less developed physics, but I could kind of see it in that way. It's just a way for us agents to communicate about what we see in the world in a, in a, as efficient as possible way. And not so, that's different from saying like physics is just about making predictions. That's often what is said, what operationalists say, but I'm kind of an information theorist. So for me, it's very important um, that >> we also kind of taking into account that there's this information. So that's where we started. I said physics actually limits the information we can have and that we can communicate.

>> And so I think we can now take this around and say physics is about communication. It's about communicating experiences, descriptions of the world that helps our future um, selves and our future um, our future physicists to navigate in the world and make um, better use of it. And then it's, it's not about verifying it experimentally. It's just some, something we do in order to kind of make, make our lives more convenient and and all that. And of course, that doesn't really capture the aspect that if I really asking, if you're asking why am I doing physics, I don't necessarily want to make the world better. I mean, I would want to do that if I could, but I'm just curious how the world works. That doesn't fit into that definition. But um, yeah, that's maybe the non-operational aspect. Again, the emotional aspect of me trying to um, justify why or yeah, I cannot really justify why I'm doing physics apart from this emotional aspect. I'm curious how the world works. But operationally, I think we, we started doing physics. Humanity started to do physics because um, we want to use the patterns that we see in the world, represent them um, and communicate them among us. And I think we should put more focus also on this communication aspect, on this informationic aspect, which is largely neglected. So usually we don't talk about how we communicate physics, in a sense, and that there are limit, fundamental limitations to that. I think this, this can maybe help us making sense of certain things we don't yet understand.

>> Professor, thank you for spending so long with me. I hope it was >> Yeah, thank you for the many questions and and for the very interesting thoughts. It was a pleasure.

>> Take care, sir. Hi there, Kurt here. If you'd like more content from Theories of Everything and the very best listening experience, then be sure to check out my Substack at curtjongle.org. org. Some of the top perks are that every week you get brand new episodes ahead of time. You also get bonus written content exclusively for our members. That's cur t a i m u n g a l.org. You can also just search my name and the word Substack on Google. Since I started that Substack, it somehow already became number two in the science category. Now, Substack, for those who are unfamiliar, is like a newsletter, one that's beautifully formatted. There's zero spam. This is the best place to follow the content of this channel that isn't anywhere else. It's not on YouTube. It's not on Patreon. It's exclusive to the Substack. It's free. There are ways for you to support me on Substack if you want, and you'll get special bonuses if you do. Several people ask me like, "Hey Kurt, you've spoken to so many people in the field of theoretical physics, of philosophy, of consciousness. What are your thoughts, man?" Well, while I remain impartial in interviews, this Substack is a way to peer into my present deliberations on these topics, and it's the perfect way to support me directly. kurtjongle.org or or search Kurtj Mungle Substack on Google. Oh, and I've received several messages, emails, and comments from professors and researchers saying that they recommend Theories of Everything to their students. That's fantastic. If you're a professor or a lecturer or what have you and there's a particular standout episode that students can benefit from or your friends, please do share. And of course, a huge thank you to our advertising sponsor, The Economist. Visit economist.com/toe to get a massive discount on their annual subscription. I subscribe to The Economist and you'll love it as well. Toe is actually the only podcast that they currently partner with, so it's a huge honor for me. And for you, you're getting an exclusive discount. That's economist.com/toe. And finally, you should know this podcast is on iTunes, it's on Spotify, it's on all the audio platforms. All you have to do is type in Theories of Everything and you'll find it. I know my last name is complicated, so maybe you don't want to type in Jaungle, but you can type in Theories of Everything and you'll find it. Personally, I gain from re-watching lectures and podcasts. I also read in the comment that toll listeners also gain from replaying. So, how about instead you relisten on one of those platforms like iTunes, Spotify, Google Podcasts, whatever podcast catcher you use, I'm there with you. Thank you for listening.