Theories of Everything with Curt Jaimungal - Renato Renner: Quantum Mechanics Contradicts Itself (and He Proved It)

Episode Date: March 30, 2026

SPONSORS: - Sign up for Claude today at https://Claude.ai/theoriesofeverything and checkout Claude Pro — which includes access to all of the features mentioned in today's episode - Accelerate your efficiency. Sign up for your one-dollar-per-month trial today at http://shopify.com/theories - As a listener of TOE you can get a special 35% off discount to The Economist and all it has to offer! Visit https://www.economist.com/toe Quantum theory may be the most successful theory in history — and Renato Renner has proved it can't consistently describe itself. This is not a philosophical objection. It's a theorem. From there it spirals into black holes, reference frames, and why some of his students refused to continue working on the subject. This one is a quiet storm, blending the foundations of physics with something uncomfortably personal: the question of what you are. FOLLOW: - Spotify: https://open.spotify.com/show/4gL14b92xAErofYQA7bU4e - Substack: https://curtjaimungal.substack.com/subscribe - Twitter: https://twitter.com/TOEwithCurt - Discord Invite: https://discord.com/invite/kBcnfNVwqs - Crypto: https://commerce.coinbase.com/checkout/de803625-87d3-4300-ab6d-85d4258834a9 - PayPal: https://www.paypal.com/donate?hosted_button_id=XUBHNMFXUX5S4 TIMESTAMPS: - 00:00:00 - Quantum Theory's Internal Contradiction - 00:05:51 - Recursive Consistency Checks - 00:11:00 - Wigner’s Friend Paradox - 00:18:11 - Global vs. Local Inconsistency - 00:25:04 - The Inhabitant’s Perspective - 00:31:12 - Modeling Multi-Agent Reasoning - 00:44:25 - Three Fundamental Assumptions - 00:56:36 - Quantum Reference Frames - 01:06:14 - Black Hole Information Paradox - 01:15:06 - Operationalizing Hawking Radiation - 01:24:48 - Realizability of Thought Experiments - 01:41:16 - Emotional Physics: Many Worlds - 01:50:03 - The Limits of Probability - 02:01:06 - Operationalizing the Measurement Problem - 02:15:41 - Generalized Probabilistic Theories - 02:29:16 - Quantum-Gravity Correspondence - 02:40:10 - The Source of Disagreement - 02:56:54 - Physics as Communication LINKS MENTIONED: - Renato's Papers: https://scholar.google.com/citations?user=OEBtlWgAAAAJ - Renato's Lecture at Helgoland [Lecture]: https://youtu.be/NxIyldEZzZI - Against Probability [Paper]: https://arxiv.org/abs/2601.18872 - Testing Quantum Theory with Thought Experiments [Paper]: https://arxiv.org/abs/2106.05314 - Quantum Advantage in Cryptography [Paper]: https://arxiv.org/abs/2206.04078 - Quantum Theory Cannot Consistently Describe the Use of Itself [Paper]: https://arxiv.org/abs/1604.07422 - Security of Quantum Key Distribution [Paper]: https://arxiv.org/abs/quant-ph/0512258 - Thought Experiments in a Quantum Computer [Paper]: https://arxiv.org/abs/2209.06236 - The Black Hole Information Puzzle [Paper]: https://arxiv.org/abs/2110.14653 - Quanundrum Software: https://github.com/jangnur/Quanundrum - Wigner's Friend: https://en.wikipedia.org/wiki/Wigner's_friend - Reimagining of Schrödinger's Cat [Article]: https://www.scientificamerican.com/article/reimagining-of-schroedingers-cat-breaks-quantum-mechanics-mdash-and-stumps-physicists1/ - Collapse Theory [Paper]: http://www.psiquadrat.de/downloads/grw86.pdf - Entropy of Hawking Radiation [Paper]: https://arxiv.org/abs/2006.06872 - QBism [Paper]: https://arxiv.org/abs/1003.5209 - Respecting One's Fellow [Paper]: https://arxiv.org/abs/2008.03572 - Why Do Humans Reason? [Paper]: https://www.dan.sperber.fr/wp-content/uploads/2009/10/MercierSperberWhydohumansreason.pdf - Quantum Mechanical Rules for Observed Observers [Paper]: https://www.nature.com/articles/s41467-024-47170-2 - Game Theory [Book]: https://amazon.com/dp/0262061414?tag=toe08-20 - Tim Maudlin [TOE]: https://youtu.be/fU1bs5o3nss - Jacob Barandes [TOE]: https://youtu.be/gEK4-XtMwro - Sean Carroll [TOE]: https://youtu.be/9AoRxtYZrZo - David Wallace [TOE]: https://youtu.be/4MjNuJK5RzM - David Bessis [TOE]: https://youtu.be/GHGi_XDqKNw More links at https://curtjaimungal.substack.com Guests do not pay to appear. #science Learn more about your ad choices. Visit megaphone.fm/adchoices

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It's time to turn those what-ifs into with Shopify today. Sign up for your $1 per month trial at Shopify.com slash tow. That's Shopify.com slash T-O-E. 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. It 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 its heads. I see its tails.

That's Professor Renato Renner of E.T.H. 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, Kurchi-Mungle, 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 connected 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. I don't see it as a constraint on what we can technically measure because we can individually measure position very precisely and momentary. 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 this raises the question that when we cannot know everything about the world,

can we actually 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 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 in momenta of all the particles in the world, and then we can make predictions and so on. And now when quantum theory tells us that apparently there is a construct,

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 could, for example, let's say, interact with an experimentalist.

I'm not sure you are not doing experiments, 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 do some checks there. And then 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 interact.

of 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, 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, let's say, again, within quantum zero, in that case. So in other words, 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 how he's doing

experiments. 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 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 Schrodinger 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, it 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 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 it would say thermodynamics applies to everything. 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 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 thermodynamics system. And if you assume that termodynamics 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. If you develop any new theory of the world, I would always say this is a good consistency check whether the series could. 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 this work that you were mentioning is to somehow apply this consistency check to quantum theory. So we were asking, is quantum theory able to describe a quantum physicist who is applying the theory to some physical system? So that's the question.

Now what we found is that 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? 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.

Now one would expect intuitively that your description of the experiment of the friend. This could be a prediction. Let's say 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 I come to the conclusion that you actually indeed predict that 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 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 this experimental friend of yours, I find the spin will actually be down. That could be, because I just apply quantum zero 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, I can maybe explain what they mean, but they're very artificial setups.

But in these artificial setups, we get contradictions. So in other words, if I 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 analysis. I first analyze the system of interest myself. This 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 why are 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, 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 consistency condition is kind of that if you are very, so if I come to conclusion using my analysis of your process of doing physics. So I analyze how you do the calculation and so on. And I come to conclusion that your calculation will have to result that with

absolute certainty the spin measurement will show that the spin is up. In that case, 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 at least not to the conclusion that with certainty it will be spin down. So that's kind of a minimal consistency. 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. And 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 you take a look at that coin,

the coin says heads, that's in your head. Okay, so the head says in your head. Then I'm thinking, okay, what is Professor Renner 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, whatever it means. So there's something that's ultimate about it,

that's final. There's no wiggle room here about just predictors and knowledge and so forth. I need you to spell that out about the difference between just making an arbitrary prediction versus the ideal reason or with ideal rationality or what have you, making a prediction. Yes, I think this is a very important point to kind of make the distinction between,

in uncertainty or incomplete knowledge or contradictory knowledge. So indeed, if I here flip a coin and 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 the 50% or 0.5 probability up or heads and the point 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 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. 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 if 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 flipped, you come to the conclusion that it's heads, you are absolutely certain it's 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 SOT 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 sort of 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 tales

and you say, okay, then you believe it's tails, so now you're also certain it's tales. 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 cannot do an update. You will just run into a contradiction. When you try to apply base 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 with 100% accuracy. We're in classical mechanics, so we have no Heisenberg.

a certain thing. 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 so-and-so. There are a couple escape routes that you can take, which we'll get to. But this is the

classical case. It's just to build some intuition for the quantum case. Yes, yes. I would completely agree with what you said. That's a very good summary of the type of contradiction. Yes. So indeed, I mean, 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 sort of 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 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 sort 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 flaws. 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 sort experiment, I give you the full information and the correct information about the world when the experiment starts. Okay, 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 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, 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 demon who is perfectly applying whatever theory 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, we 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,

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never sycophantic, unlike some other models, and it doesn't just hand me the answers. The way that I prompted it is that it helps me think through problems. Ready to tackle bigger problems? Get started with Claude today at clod.a.ai slash theories of everything. That's Claude.a.com slash theories of everything 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 sort experiment that I'm kind of proposing as really just the test purely of the theory, not of, let's say, the experiment. It's a sort 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 the SOTExperient 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 doing that, then I cannot really compare 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. 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. So 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 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 that. 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 this. In the example I had before, I applied it directly to an experimentalist and make a prediction about the experiment list. That's one way of reasoning. And the other way of reasoning is I applied 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 this, I would call such a, 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, but 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 I think it's really important to make this distinction

between a sort of 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 kind of do consistency checks, assuming that 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 where the different uses of the theory lead to consistent results. And so that's, in a sense, what you are doing with quantum theory now in this sort 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 offers too many ways to come to a conclusion and we cannot decide which one is the right one. Let me ask you something funny. 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 actually a core point of the whole question, because you could say this shouldn't happen.

And this brings in, let's say, another 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 would necessarily have to describe ourselves how we are applying the theory. This would lead to a cycle. Like if I describe the whole world, I necessarily describe myself. And 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-worlder or a boomian, 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. 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, a very basic ingredient in the, let's say, approach I took is that at the end we want 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 it's very disputed. So 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 being, then you are again part of the universe, that leads to troubles. So the question I'm asking is really, can I do physics? And if I'm asking this question, 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 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, 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 if 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 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-daily life situation, that one of the physicists is kind of going to be measured by another physicist in a very strange basis in not a very strange basis,

not just in the, let's say, I mean a measurement would be, I just ask your question, that's also a measurement, but I could do a very complicated measurement like a Schrodinger-Cat-type measurement. And if I, if someone did such a measurement on you, then that, 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, that part of the experiment in a sense. And so by using such building blocks in this sort 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 just a view that we usually have in daily life. Like if you talk about the experiment of your friend, you will not describe the whole world, you just describe the that one. And so you have a different perspective from, for example, from me who decided to, let's say, to describe you as a physical system. And if you are 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, you 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

set up some operations out on on them and so on. When I'm wrestling with a guest's argument about, say, the hard problem with 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 interalia, technical philosophy, mathematical, 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 prompted it is that it helps me think through problems. Ready to tackle bigger problems? Get started with Claude today at clod.a.ai slash theories of everything. That's clod.a.i slash theories of everything and check out Claude Pro. which includes access to all of the features mentioned in today's episode. What is the minimum amount of observers required for this no-go theorem or whatever you want to call it? That's a very good question, and unfortunately I don't have an answer. I would say certainly three are needed, but 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 the way we try. So, yeah, this is also important. So it's, in a sense, 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, so because there's often a criticism saying that, oh, at the end, I actually describe myself indirectly by this. describing the other agent. But actually to sort out this criticism, we actually wrote the software, together with one of my collaborators,

Lydia Del Rio and Nurea Nurek Alieva, we once 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 the software kind of proves we can describe it by not running into a loop. and it's actually even publicly available

we put this 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 convince people that it works by having this kind of automatic reasoning if you like so

the software allows you to specify what the agents need to describe and then you see that the description is not running in to 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 so just to be clear, if you in the experiment you would describe me, 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 not 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 way function that hasn't been localized? Yes.

So we tried in this experiment to actually avoid talking about states when we phrase the final statement or the, final claim. Of course, 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 the entire experiment is a bit like you can maybe compare it to Bell's theorem, where one 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 sort experiment.

So in the analysis, of course, we use quantum theory and have to involve, involve, I mean, that involves talking about states, or you can also take the Heisenberg picture. Then you talk about operators, if you like. But at the end, the objects the agents are talking about our 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 he would say, let's start with a spin 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 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 phrased in terms of the predictions. So the agents have this initial state they know and then they just calculate a prediction

which is of the type I 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 there will 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 because 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 gods 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 what you're saying is, let's imagine you're a rook.

Maybe if I may say something about that. So I wouldn't even phrase it as a criticism. I would say, let's say someone believes there is an outside view and that this view makes sense, like the players view, the chess players view. I wouldn't object. I would say, okay, you can take this, have this opinion that there is this outside view, but there should at least also exist. inside view from the, let's say, chess or whatever, there are these different characters on the chess, but from the king's perspective.

Because the king is one of the parts of the world. But they say, so 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, 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 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 a physicist? 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 U.S. e-commerce from all birds to gym sharks 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 slash tow. That's Shopify.com T-O-E. 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 chess board, 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 ponds 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 serum. 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 seems a queen or spell it out? Yes, I think that's one way to put it. I could say, indeed, I see the bishop and I'm modeling what the bishop is. Okay, I don't know what the characters were. Let's say I'm a rock, I'm seeing a bishop, and I'm asking, does the bishop see a, what figures are there else?

Pawns, rog's knights, kings, queens. Oh, yes, a knight. I know the German turns. 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 rock sees the bishop, the bishop now claims there is a knight in front of it, him, let's say. And it's important that you do not directly see the knight, or is that okay as well? Then I could say, 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 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 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 is 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.

that's just the bishop has uncertain incomplete knowledge. But if I know the bishop is certain the knight is in front of him, but I actually see knight is not in front of him, then that's a contradiction. And so this is the type of contradiction we get. So I think in this case the analogy works very well because it's just about like two type of statements being part of the world. So all these pieces on the checkport.

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 that turns out not to be the case now 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 point? Yes. Actually, there are three that we state. And of course, assumptions can always be grouped or kind of subdivided. So this number is not so important in a sense. But I think it's a useful group or a useful kind of fine-graining that we have. So one assumption is that quantum theory, the way we use it, is,

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 series universal in the sense that

I can apply to any system I like, maybe except 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 office chair. Quantum zero applies to everything. So that's the first part. But then there's another type of universality, namely that everyone is allowed to apply quantum

zero from his or her perspective. And that's another universality. Because in principle, it could be that 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 we are really allowed to apply the theory. But that's kind of what is implicit to many worlds or booming mechanics because they somehow say,

we need to have this external perspective. So in a sense that this second type of universe, is actually something some people would deny and they would say, no, that's not true. We are not eligible to apply quantum theory as being part of the world. So the assumption that goes into the theorem is this universality assumption in this two-fold way that I just explained. And that's maybe the most important assumption.

I think this is also the assumption that was mostly misunderstood because 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. We used them, because for me it was very natural, but it wasn't really emphasized. But I think

this is a 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 our selves in superposition. That makes us not value.

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 second important assumption is this consistency assumption. So we have to ask or make an assumption about what do we expect to be true when you describe something and I describe the same thing, in what sense should it match? And here one could make very different assumption. 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 states would deny it. 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 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 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 series, 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 see, a part, maybe the no-go theorem is maybe interesting, but I think the more relevant part of the proposal is actually to propose that test of a theory. So I'm saying 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 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, communication in a sense is a very important part of doing science. 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 sorts and predictions and now I'm verifying them. And this is also in this like the consistency should even hold there. So for example

I could just see my former self as a different agent and then ask for this type of consistency. and ask myself if I know at that time I did a correct calculation and so 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 relevant for any communication including the communication of our former selves to us

I see that as communication I mean it's a very powerful communication because it just stores stuff in our brain and communicate via that. But it's a form of communication. From an information's rhetoric viewpoint, I'm constantly sending messages into my future and read them later. And so 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 space time and could not explore the universe. So I have to communicate between different experiments. 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 physicists who lived on this planet shouldn't matter. It's all communication. And so if I, for example, only read just a book in quantum theory, that's communication from earlier physicists to me.

So we are actually doing, I mean, communication is so inherently in our scientific development that it's very hard for me to think about what science even means without communication, even to phrase a sort, like to have a theory means I can write it down and read it later. And if that's impossible, then I wouldn't really call that science. So for me, this is almost a 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, there are also quantum systems. Yes.

Okay, and then consistency, which is what we talked about for 40 minutes or so. And then the third was what? Yes, then was a third we didn't talk about yet. This is actually one that is so in a sense natural that it's often not even mentioned. but it's that if I make a prediction and then and I make another prediction, there shouldn't be contradictory. So in this other assumption, the consistency assumption, I should maybe have been a bit more careful.

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 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 I will also be sure the spin is going to be up. So that's, strictly speaking, the assumption C. So 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 third assumption is now saying that this should not contradict what I calculated before.

So maybe 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 the fourth assumption that you're allowed to foliate the world into a now moment and then have time steps forward that everyone agree on? Yes. So there are actually, if you ask the question, is this now the complete set of assumption? 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 serum?

Like, for example, the logical reasoning means if I know A holds and I know B, holds, then also the conjunction A and B is true. Yes. We need that, but of course we need that to even phrase, let's say, the sort experiment. We need language and so on. So in a sense, it is an assumption, but without this assumption, I couldn't even like start phrasing the experiment. Yes, yes.

And if that assumption goes well, then every opponent theory goes as well. Right, yes. So that's just to say that there are certainly more assumptions. I'm not claiming they are not more, 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 time is kind of, one can see it as part of the assumption of standard quantum theory applies. But it is an assumption. So I would just see it as part of this assumption that we apply quantum zero because quantum zero tells us there is a unitary that brings us from time to time. But it's correct that this is an assumption.

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, interalia, 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 larger problems? Sign up for Claude today and get 50% off Claude Pro when you use my link, clod.a.ai, slash theories of everything, all one word.

why I'm super excited to speak with you is that you take an interesting escape route 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 what the heck does this have to do with gravity at all? Yes so gravity wasn't now coming in

but actually I should tell you that there is a third way to escape the paradigm 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, many world or some years ago. I know. And I changed by going to becoming more a patient type of person. And also about this soot experiment, I mean, this sort experiment, has actually still an impact 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 have no satisfactory way to deal with it. It really troubles me that there is this problem, this contradiction. And so that's why 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 one of the 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 up. 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 you 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 this consistent communication.

And then clearly we don't want to have contradiction. So all these assumptions I really want to keep. But now, actually our statement, I would really call it the no-go serum, says the following. It says, if we carry out this experiment with several agents, so I didn't really describe the experiment, 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 he performed this experiment, 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 this is not problematic or these assumptions are unproblematic is that we can not even start analyzing the experiment because it's not executable. And so far I was always assuming clearly we can principle execute this experiment. It's very hard to do it because we need as I said before, we need to do something that I call a Schrodinger-Cat-like measurement. So it would be like Schrodinger-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 process. So in the Schrodinger-Cat example, the Schrodinger cut is kind of poisoned, and this whole process of being poisoned has to be reversed, reverted 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, let's say soots, which also involves gravity, let me now suspect

that this is actually wrong. We cannot carry out such experiments. and maybe that's it's not obvious why I'm saying that but there is actually a kind of circle in the argument or a loop and the loop goes as follows so maybe I'm sure you know

about the recent developments in this research that is concerned with quantum reference frames so 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 intuitively, the reference needs to be larger

than the system that I measure in. 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, and the gravity considerations kind of tell me that not only spin requires a reference, but everything that could in principle be measured requires a reference. Maybe 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 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 measuring process and it's a bit more uncertain because in order to measure the spin, I have to let the reference and the spin interact with each other and this will kind of 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 the sort of experiment that we have, then we have kind of a circle of, in a sense, measurements or places where we need a reference. So we have four, or let's say just for simplicity, we have only three parties or three layers 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 bulk. 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 you are now in this kind of Wigner'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 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 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 can show that the experiment is not executable because of these 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 kind of do a Schrodinger cat experiment, 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 already that is difficult but now if I do a second measurement on that universe that measurement it needs to be even bigger 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 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 the 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 and twice weekly long format shows.

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Head over to Economist.com slash T-O-E to subscribe. That's Economist.com slash T-O-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 this is also not obvious. But so by large, I meant kind of the Hilbert space dimension.

So I meant it as a quantum information series that 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. But there is actually a connection, as you know, between size in that sense and, for example, size of a black hole. 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 one can associate an entropy to a black hole and that kind of 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 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 one kind of utter work that I did completely independently up-prior of these quantum mechanics sort of experiment was

an information theoretic analysis of this 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 famous thing like black hole surf as a mirror that Patrick Hayden and John Preskill once proposed that if you throw a notebook into a black hole and then you, you collect the radiation from the black hole, you can kind of retrieve the content of the notebook. But the radiation will also look kind of completely scrambled. And so one can now ask what, 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, 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 work that I did with one of my former students, Jinzao Wang, is that actually both views can kind of coexist. And 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 just a source. Let's suppose there is nothing in the universe. It's completely empty, but for some reason there is a source. that 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 one now asks the question, is to 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, let's say this process continues and it emits further spin particles, then of course we can compare them and say now relative to the first spin particle, the next one is up and so on. And now 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, right now you're talking about one black hole that just emitted one bit of hawking information?

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 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 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 99 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 thousands 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 nine hundred,

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 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 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 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 just something new like a new spin that is created in a universe where there was nothing that breaks symmetry but then if I kind of use many black holes

many others as a reference, then a new black hole that I create and let, or like I collect radiation, that radiation will, with respect to the others, have a lot of structure. So in a sense, I can use some of the stuff 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, no, I said. 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-goes here. But, okay, one could also call them inconsistency theorems. Because, okay, technically what they are is just there's a set of assumptions. Like in our case, quantum series 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 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 devalied. So in that sense it's a no-go serum. There are assumptions that cannot simultaneously hold. So to be compatible with what I said before I should say a force assumption is it can actually carry out the experiment. But these four assumptions are then kind of incompatible with each other. And of course 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 the spin is down. I give it a different status. I just say the other three assumptions imply that this assumption, 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 goes in a room is very neutral in terms. of priority. It just says there's no priority to one of some 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 Bail's CRM, 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 is Sean Carroll, because Sean Carroll had a talk about how God is not a good hypothesis. And he said, look, I can write down a theory where there's just a single electron in the universe. Right here, I wrote it down. I, 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. Okay. Pedantic sounds a bit negative, I would say. But indeed, in that sense, I'm very pedantic.

But I want to be operational. I will call it being operational. I see. Many of the problems we have in, let's say, many of the discussions that arise in physics, are because there are 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, I mean, with the spin is now an example, if I have a universe or like this black hole example,

this is an example telling us, look, as long as we don't specify how we measure what is the reference, we actually get contradictory statements because in whatever you do, we'll 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 it's not. So I want to be explicit and say, look, if you do the experiment, we have to explicitly make the system, the system we measure interact with the reference. If we don't know 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 of Jinzhao 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 doesn't have to be modeled. So I'm not claiming that we should really model all the detail. I mean, I'm a quantum information series, and quantum information series really love abstraction. So we want to get rid of everything that is unimportant. So in that sense, I don't want to be,

pedantic in saying, look, if I have a qubit, I don't want to be forced to say which atom is it that realizes the qubit this is a superconducting qubit or is it a trapped I want to be on that abstract level and say, I don't tell you my statements will anyway be true independently of what the 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 we 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 we run into troubles, like with this sort 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 mistake, at least for this type of experiments. We need them. And so it's indeed a matter of taste or of, yeah, this is kind of scientific 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 reference frames are so important that we cannot leave them out. They play such a fundamental role in our understanding of space time that dropping them will just not, 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 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 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. 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 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're in the same space time. And so this

will lead to some warm holes and stuff. So there are other difficulties that arise that I think are very important to take into account. So I think for me this is very strong, I have a very strong feeling that one needs to be patented 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 the guess that it's the reference frame. So I'm not claiming this will be the right answer, but at the moment it's the approach I'm pursuing. Most of my best ideas don't happen during interview, they come spontaneously, most of the time in the shower actually or while I'm walking. Until I had plod, I would frequently lose them because by the time I write down half of it, it's gone. I tried voice capture before, like Google, home, and it just cuts me off in the middle. It's so frustrating. Most of my ideas aren't these 10-second sound bites. They're ponderous. They're long-winded, and I wind around.

They're discursive. They're five minutes long. Apple notes, even Google keep. The transcription there is horrible. But Plawd lets me talk for as long as I want and there's no interruptions. It's accurate capture. It organizes everything into clear summaries, key takeaways, action items. I can even come back later and say, hey, what was that thread I was talking about regarding consciousness and information? In fact, this episode itself has a plot summary below and I'm using it right now over here. My personal workflow is that I have their auto flow feature enabled, so it sends me an email anytime I take a note. Look, the fact that I can just press it and it turns on, like right now it's starting to record, without a delay is extremely underrated.

This, by the way, is the note pro, and then this is the note pin. I have both. Over 1.5 million people use Plod around the world. If your work depends on conversations or the ideas that come after them, it's worth checking out. That's plod.aI-T-O-E. Use code TOE for 10% off at checkout. 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 think it's actually operationalism that's a particular philosophical strain called operationalism which I imagine you're not an operational list. Yes. So, I mean, I have my, let's say, personal list of requirements to a good sort experiment.

And one very important entry on that is that I want to be able to do the experiment in principle. So there should not be a fundamental reason that excludes the sort experiment. And now in order to answer the question whether it's really feasible in principle, This is a question I can never answer definitely unless the experiment is actually done. Usually the SOT experiments are of course about stuff we cannot easily realize, but my hope, 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 physicists.

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 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 like, let's say, a 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 is 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, in that case, it's no longer a good sort experiment. Maybe we learned something by proposing it and finding this fundamental reason, but then it's kind of settled in a sense.

So for me, a good SOT experiment is something where I know how in principle I would do it. And otherwise it's under spec not well specified. It's the same with, let's say, Maxwell Demon and all these other SOT experiments. For me, they are 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. I mean, I'm talking about SOTE experiments in physics.

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, 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 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 there is at the end of fundamental reason that limits the experiment, 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 sort 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 a fundamental reason, not a technological reason, really something that we know can never be overcome. Then I think that's exactly what helps us making progress. So in a sense, it's then maybe, okay, maybe I shouldn't say a thought experiment

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, carried out, then it's maybe also a successful SOT 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 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 I learned that this is relevant, which is then a lesson I can learn from the SOT experiment, or it actually can be built. And then it's also, of course,

interesting 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 space time 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 symmetrical. odd type of observer, but I don't know how...

I would need to think more about that. So help me, Sparrow. Okay, now I see what you're aiming at. So I think, okay, actually an experiment should also be, in a sense, robust. So what I mean by that 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 this is not, let's say, convince. rinsing 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 both sides. And now one could have asked, is this actually a good, so at that time it was a salt experiment. And now one could say, okay, maybe it's not a good experiment because we cannot precisely do the measurements that we want to do. There will certainly be always some fluctuations, we will have imprecisions 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 with the different observer, this weakness-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, 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 still want to get to the conclusion. And so now for Einstein's elevator, I would also say, so even if we 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 we don't want to use them. So I think we have somehow a robustness requirement and that robustness requirement is important.

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

So maybe I should say, yeah, maybe to still comment on that, I see it in the following way. So we are testing not the world at the end with a SOT experiment, but a theory. So when I say an experiment should be realizable, I wanted to be realizable according to. to the principles of that theory. So let's say, with the SOT experiment we discussed at the beginning, I kind of want to test quantum theory.

Now, what does it mean to be realizable? It means that within that theory, there is no reason that excludes it. So this is a priori 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 series, I want to be able to exclude candidate series that have no chance to describe the world. Because that's what we want. We want to exclude all the series that are not good candidates. And now I 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, then it's not a good sort experiment to test that theory in a sense. So 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 sort experiments within that theory and check whether everything works out.

And if it doesn't, then the theory 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. So I'm a physicist. 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. if 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 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 something I can, so this work I can carry out independently of any experimentalist.

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 like until I see a contradiction. And if I see a contradiction, I'm unhappy if the series asks for a new proposal. And so that's how I understand quantum theory. It's a playground for me as a theorist, and now I play within it. I invent salt experiments and check whether they're possible in this theory.

And now I found that there is a problem with them. Okay. So what does Frau Hecker, if I'm pronouncing that correctly, who also thought of this thought of experiment, That's correct, yes. Great, great. What does she think about your interpretation? Okay, so actually she left science after her PhD and we have, she kind of observes, she observes

the whole development from the outside. So she reads all the, like, public articles that are written, so there are sometimes even some press articles in the press about it and she, but she's no longer very close to the subject that works in software engineering did you so disturb her with your metaphysical insight I hope not

no I think this was I mean it's a good point because I had some other students who said that they would not psychological for psychological reasons not be able to work on such projects this would disturb them too much wait wait that is super interesting I want to hear more about

because it's It's at the end about, look, these projects are about like really the fundamental question of what are agents in a sense. So what are we? Yes. You know, at the time when I was still a many-worlder, 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 we are now experiencing both outcomes, that kind of has an impact on what we are doing, or at least for me, it was the, I mean, I cannot give you a practical example in impact, but it's kind of worrying to know that

or to think this is the true story. And so now I'm now in a kind of in this view that I currently have, 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 something must be wrong with that view that there is something here in the world. And then suddenly everything falls apart. All this like kind of the whole model one had of the world has to be revised.

And then something people take this very seriously and say, look, if everything has to be revised, if things are not out there, there's just, everything is just kind of in my mind, which is, for example, suggested by these epistemic views, then one can ask really fundamental questions about what's really now the goal of our lives if there's nothing out there. And so interestingly, some of my collaborators say they kind of don't. link that question to real life. So they completely separate that. They work on that and have opinions about how it is, but somehow they disconnected from their daily life. But there

are others who say this is intrinsically connected and they're still happy with that, but there are others that connected and find it very stressful that this is connected because it constantly tells them that, 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 the 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 I think actually almost any decision, not only what interpretation we are admitting is at the end, kind of an emotional decision. Because let's say if we ask ourselves, what are rational arguments? So I mean, we of course try to be rational, but the rationalities 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 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 Schrodinger equation. Right. And that's not giving much insight into what happens in quantum theory. Now, I was very kind of unhappy with that, but I didn't know why, because I didn't know anything else. I just learned it and I thought, okay, that's a bit like electro dynamics. I just now have a new system of equations. It's no longer the Maxwell equation.

It's now the Schrodinger equation and I learned how to solve it in different potentials and so on. And then at some point I started looking into quantum information and quantum computing. I mean, this was not very popular at the time. This was around the year 2000 when almost no one was working on the subject, at least here at DTH Zurich, where I studied. And so, nonetheless, 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 learned about these papers about many worlds. And then suddenly things started to make sense. sense of me and I realized, okay, 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 revealing thing that I was immediately convinced that must be the right to you 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 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 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 under the change. 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 I was just convinced because of that and I thought that must be the right thing. Then I started to, of course, investigate that more and actually paradoxically the more I talk

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 were thinking about many words for a long time, they will certainly be able to give convincing answers. But then 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 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. So I just got, again, a kind of bad emotional feeling about these explanations

and go more and more unhappy. And then I had another kind of encounter with cubism, and 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 many worlds. And then this suddenly resolved again, and then I was kind of almost completely convinced Cubist. And now the problem is that this exotic experiment for Daniela Frauchiger puts that again in question because cubism doesn't have a good answer

about what is this assumption about the consistency between agents. So cubism is kind of a single ancient theory. I mean, okay, Cubists would now deny that they want to, they say it can be applied by many agents, 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 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. So I think there's a paper about 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 more negative emotional feeling that it doesn't feel like the right answer although I cannot at any point 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 sort experiments or 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? I'm now in no man's land. So I don't see any of the interpretations that's currently existing is 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 this sort experiment that I have if Daniela Frauchiger is the communication between agents or how they incorporate knowledge of other agents. And this is just not defined in a sense.

So the series don't tell me what to do. So Cubism 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. And that's just missing. And it's not obvious how to add that rule. And therefore, it's kind of more incompleteness of these interpretations. And there's no obvious completion, so to speak. And in more recent work I now did, I guess you mentioned that, 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 knowledge in terms of probabilities. I mean, you asked before, what is the kind of role of emotions? And 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 Cuba. 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 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 once, then one gets the emotions from there. But I think in my experience was often first a very vague feeling, an emotional feeling of how it should be. And then this was driving me to think more deeply about the subject. Right. And often the emotions were 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 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. So 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, they're various sorts of rationality,

that they have of 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 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. Yes. Actually, I had an experience a bit more than a year ago.

We had a conference for the birthday, or on the occasion of the birthday of the Bell CRM here in Switzerland. And we had team Motlin as one of the participants, and he got very emotional. So I was always feeling that a fist fight would start at the end of a talk. And then I saw to myself, Bell could be very proud that he managed to just write down a serum about which people get so emotional. That's kind of the best thing that can happen to you if you invent a serum. a serum that makes people fight and be emotional about. Because that really means the serum has a significance.

Like most serums are in the sense boring, one could say. I mean, I can prove something like I could maybe take a random calculation or an integral and try to solve it and say there's a serum, the solution is that. That's completely unemotional. But if I manage to write down a statement that is true in the sense of a serum but also makes people actually shout at each other. 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. And I was really thinking like Bell managed to do so. Yeah, no one gets decapitated over NOM's theorem about supersymmetry or Coleman Mendoulos theorem. Yes, right. Yes. Exactly. So I think emotions are important for scientific. progress. And of course, 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 good question. So I think it's like the emotions are most or these like psychological impact is kind of still, I would say, a bit limited to science itself. So I think if I really ask myself, did I make 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, emotions towards science. So I try to still separate private life from the profession.

So I do lots of sports, for example, and that helps. Like if you are frustrated about some scientific thing, then this helps me kind of distance myself from that. So I think I try to get some distance 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 separate that from that. I think if I would only think of physics, this would really have probably a, let's say, if I would fully take it seriously what quantum theory tells me, I would have. I wouldn't know how to kind of act on that in my real life. Interesting. At the moment, I'm anyway confused about what quantum theory is because, as I said,

this no-go theorem 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 make my life depend on that. So Feynman has a phrase that no one truly understands quantum mechanics. I tend to think that 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 Rafael Bousso just spoke to Brian Green 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 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 these cryptograms, cryptographic things where one has to ask very precise questions, they don't depend on the solution of the measurement problem at all. So it's only true that the stuff that was done could be done without solving the measurement problem. But it's very important to not forget that this is a very fundamental question that may prevent us from understanding things that are beyond the current reach 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. 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 quantum system is a small system and we are outside. And 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 there are little things in a lab and if you go to that lab, we all agree that it's 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 have the same perspective.

And I think in these situations, the measurement problem has an operational impact. Because for me, the measurement problem is really 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 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 the sense that it's only relevant if introduced 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, philosophical question that will not have an operational impact. But this is not true for the measurement problem because we have these sought 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 it's clearly relevant. So I think it's just the technology wasn't for the last hundred years on the level where it would have an impact. It's a bit like comparable. They do, let's say, electro dynamics before we had relativity theory. As long as we can only do experiments with low velocities, no one who cares 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 also if you now think about, experiments like we had talked about this black hole information paradox that's an experiment where we apply quantum theory to a very big system and there we are clearly

making mistakes if you are not aware of like the meaning of the quantum state in this context because we have if you talk about the black hole then the black hole is a quantum system and if you are now doing sort experiment 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 have 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 recursive use of the theory.

So 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 soot experiments that black hole people do. Now, without having solved 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 we need to be able to know what, or we don't know what happens there without having solved the measurement problem. And so that's another reason why I think.

So even now, because we are now talking about these experiments, experiments, they are discussed. We need 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 are now in very good conditions to make progress, because with like this insights that come from quantum information, general and also from gravity. If we combine them, then I think we can really maybe see 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 anyway cannot solve it. Because I think it's actually something we should solve and if we solve it, or we need to solve it to lift

quantum zero on 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 CEO looks like in this case. And so for me, the measurement problem is linked to that. Because 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 pilgrimers. 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 C-U-R-T-J-I-M-G-A-L.org. Kirt-Jemongel.org. Polychronicon, I have 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 may be. isn't the full story and quantum mechanics. Is there overlap between you and him there? Or is it more just at a gestural level?

Yeah. I think there's no... So he had a different... Yeah, I think there's not so much overlap between these two approaches. So our argument is basically a criticism on series that, right, to use probabilities to represent quantum states.

So you know, for example, in quantum basinism or cubism, 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 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 the idea of some of the theories, like of cubism, but also of the so-called GPPs, 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 say, okay, we are more, why should we kind of use this density operator formalism? That's very special to how quantum series, let's just more generally talk about the list of probabilities. And that, to me, that was also something I found a few years ago an incredibly fruitful idea because it allowed us to kind of think about generalization of quantum theory. So I think what was really progressed 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 belline quality 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 zero. One could ask question, why is exactly the Bell violation as large as it is in quantum zero? Why is it not larger?

Why is it not smaller? And so this generalized probabilistic series helped us answering these questions. And you need, 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.

We can do much more general things. And I think this was an extremely fruitful idea. There was actually very much in favor of exploring that. But then you see, there were often like this approach. about things I was in favor, like many worlds. I kind of realized after thinking more about that and discussing it with, actually here at ETH with my PhD students, 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 same. 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 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

the quantum state buy a lot, let's say go from one quantum state to the other, and I look at the list of probabilities, it may be that they change arbitrary little. So this means, like, in a sense, the distance between, so in quantum mechanics we have a certain notion of

how distant are two different quantum states. So, for example, I have the state pointing in that direction, in that they are very distant because I can distinguish them, but then 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 lose the, so that's what we said, we kind of lose the topology in the sense of the quantum state. Things that are close together in terms of quantum states are not closed, or that are not close together in terms

of quantum states are close together as probabilities. And so 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 he 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. 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 very general probabilistic theories that are beyond quantum theory. I even was working on these proofs. I'm kind of even talking about my own work that is now put in question, because we now found the link between probabilities and quantum states is not robust. It can be that, or it now turns out that if we prove security in this probabilistic framework, it doesn't necessarily imply security of the cryptographic theme in a quantum world. So, and this is really relevant because we thought we were actually more general and better in a sense, because we kind of had the claim that security holds

independently of assuming that quantum theory holds. 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 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 in other words, cryptographic proofs we did in this generalized probabilistic framework are actually not applicable to the quantum world. And this was quite shocking for me because we did these proofs 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, let's say, so it's part of my program to find kind of a replacement or a resolution of this thought experiment with Daniela Frauchiger. And because, as I said, I was initially convinced that cubism would be a good way. I tried to now see what needs to be changed. And so cubism, 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 cubism to be subjective to represent knowledge in states is a very good idea. It's for me the most promising idea. I know, I mean, it's similar, on that level, similar also to relational quantum series. 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 cubism, 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 final result of any quantum theoretic treatment is a prediction about the measurement outcome. But now in this sort 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 kind of a bit unusable in this sense. So probabilities really make sense if I can say, like if I talk about the coin, for example, that we have before, I can say there's a probability of landing on heads and tails and this reflects my knowledge of what I will see. But if I, if I can can no longer talk about the outcomes or if the outcomes are not the real thing. So that's what all these SOTExperments 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 recursant. 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. 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 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 a 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 notion of knowledge should be able to talk about knowledge. Why is that problematic? Why can't you have probabilities of probabilities of probabilities, et cetera?

Yeah, because, 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 problem comes more from, let's say, the way, let's say, if you want to build it axiomatically in a sense, which is important for understanding cubism. Then you start with kind of saying something like probabilities are about, are a way to 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.

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 the 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 cubism or no? This is just a problem more generally. Oh, this is a general. So I would say whenever you want to apply a theory, in a way like

or let's say 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 I am kind of 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, okay, let me make an example where you see that it's really hard to come up with a good way of dealing with these probabilities. So let's suppose 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. Yeah. So let's suppose you now, 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, 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 your knowledge that it trains tomorrow is 10%. That's a simple case. So if you tell me that, then I'm certain, so I have a probability one assignment, that your probability assignment to the weather is 10%.

But I also have my own probability assignment of 50%. But 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 number should I, after you told me assigned to the, to the weather tomorrow?

I could say, okay, 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

you have some other knowledge and I just ignore that or there's something in between but there is no clear way how to do that. 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, there are different probabilities. And now it could be more complicated. 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. Now the 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 these betting games. But the betting games now don't work well if the bets are again about probabilities, because 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 concepts of probabilities, where we have multiple users,

or not necessarily multiple users, but probabilities about, so we have knowledge about knowledge and how 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 nothing analog to, for example, the base updating rule. The base 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.

Yes. And there's also no unique answer. So there's something missing there in a sense. And so that's something that I think we need to develop. So in a sense, I'm going now back to a classical series. So the whole thing started with me investigating quantum theory and agents talking about other agents. But if you really take this now back even to the classical realm of classical probability series, it's not clear that there is a solution there in a sense.

Because usually people don't see it as a multi-user theory. So patient probability is always about me making predictions, but not me making predictions based on other people's predictions. And there are no contradictions there, but still there's no clear rule. So there's something to be done there. And of course, I mean, 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 don't think there is a separation. So for me, 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 better 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 collapse theory? No, no. No. I mean, there's just no experimental evidence for it. So I also have no strong.

So this is again one of these emotional things. For me, a collapse here 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 a collapse mechanism. And this, of course, doesn't exclude. the idea that, for example, on larger scales that we couldn't test, there could be a collapse. But maybe if I try to explore the reasons, I have this negative emotional feeling against collapse, is that it kind of sounds very artificial.

It sounds to me like we want to desperately reintroduce classicality because we were used to it. And so we want to return to this familiar concept. But there is no, let's say, natural real. or physical nice concept that would lead to that. Some physical concepts just sound so natural that when you hear them, they feel right. It's like when you first learn relativity and you suddenly see like space time is like that. It's just sounds much better than whatever you heard before, before you were in contact with relativity. That's at least my impression.

So I immediately felt like that's nice, that's so elegant, that cannot be wrong. And so with collapse series, it's a bit the opposite. I think we have a very nice series. It's like having a relativity. Someone would tell us, no, we have to go back to the Newtonian picture or to the 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 or Lorentzian variant.

And there are some constructions that, probabilistic constructions, that try to do that. but most are not. And so it really feels like returning to an old concept. But as I said, this is really an emotional arguing. And maybe I should, 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 I'm trying out is kind of just connected to my feeling of that's probably right. But it's before I have any rational arguments. It's maybe just an intuition. But maybe I should really call it an emotional feeling. because intuition sounds like something I could explain why. Whereas there 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 a relativistic environment, but why is that bad? Why should we adhere to being Lawrence invariant? Of course, we 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 gravity. So you have a lecture

which I'll place on screen of Helgeland 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 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 think it goes both ways. And of course the examples I would have told you would have involved black holes. But I think even if you are like away from these extreme cases, I mean, we can come up with, let's say, stuff that is related.

So let me try to make an example. So I mean, this is maybe a quite high-level example. But let's take the double-slit experiment from quantum zero. 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 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 particle has no answer. Now, an analogous thing is kind of the whole argument. which is not the black hole, but the whole. Einstein's whole argument where you just say, let's take space time and just kind of virtually cut a hole in the sense that we think of a region that is kind of bounded. And let's just change space, the solution of the Einstein equations,

which determine how space time is curved within that region. and consider another solution, which is however diphtheromorphic 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 designated point that I kind of designate by giving a coordinate?

Maybe I could, like in the double slit experiment, have two points 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 I can take one of the solutions and then, as I said, 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 usual resolution or Einstein's resolution was it's not even a well-defined physical question through which point the particle went. Of course it's important here

at a point, it's just a coordinate point, it's not implemented by a physical object. This is like in the double slit experiment I said I don't measure through which slit it goes and if I don't measure then I cannot tell the question through which it went

doesn't have an answer. In general 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 theory,

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. The measurement device tell me it went through this lit or that. Now again, in the whole argument, in Einstein's whole argument, I can say 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 of the detector the particle passes has a definitive answer. So in other words, in both series we had the situation that, if you ignore the measurement, if you just don't put the measurement device, if there's just some abstractly defined place. So in relativity, it's just by coordinates,

but also in quantum series, it's just two slits, which are in a sense abstract, in a sense, 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 series tell us, it's very important that we don't just abstract, talk about the location of a particle.

If you want to actually give it the meaning, we have to put the device there. And I find it quite striking. 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 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 a location of a particle,

we need to put a measurement there. need to make it operational. This is the very same lesson that we learn from very different series. And I find this really surprising that two extremely different series, on completely different scales where they developed, sometimes tell us precisely the same lesson that we didn't learn before, like classical mechanics or electronics, didn't teach us this.

This were really new lessons learned actually very painfully by Einstein. when he developed this argument, but also in quantum theory, they were learned by thinking about these interference experiments and double-fleet experiments with all the variants, with the delayed choice and so on. So 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 each of, so if we just understood one of them, we could have

kind of maybe learned about the other. Of course, these things were developed independently in a sense. I'm now linking them. But there are other maybe places where we are ahead. And I think, for example, these Wegener's Friend experiment, or the type of effects 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 type 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 the finite time, so we can only see stuff that is not behind the horizon. and in quantum theory we learned it in a different way

so they're 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 ceiling. 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 series need to deal with combining knowledge of different agents in a sense. So in relativity, we somehow do that by saying there are the different patches of space time that can be in principle analyzed by different agents. There's, for example, one agent that is behind the horizon of another, so that agent sees far 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 to say in these soot experiments that we discussed, we have different observers, they want to somehow combine their knowledge. And so in both series, we have kind of have a similar challenge. How do we consistently combine knowledge?

And so I'm really convinced that here we can learn a lot, because in both series we hit problems, like in quantum theory, these are these beginners friend experiments and experiments we discussed, where we have inconsistency. And in the case of, gravity, we have these

kind of firewall paradoxes, but they, as you probably know, they're even without black holes. It 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 suit experiments with Danila Frauchiger, this is kind of the core, I would say, new thing in a sense that we take the agents seriously as users of the Syrian, we therefore have to combine the knowledge of different agents. And we simply have no established rule for that.

And in relativity, this is often just done ad hoc. When people analyze the firewall 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 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 that we can have as physicists is very parallel in these two series. Professor, we began this conversation about inconsistencies between, physicist 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, there are two truths, in which case we still shouldn't disagree that much, so we should converge on two truths. Or maybe there are a 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 diver, 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 reasons, reasoning. Yeah, it's a very interesting question that I ask myself a lot, because when my paper with Daniela Frauch 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

maybe some angry reactions, but most of them were, I mean, there were disagreements, but in a constructive way. And actually, all my knowledge about the different interpretations of quantum theory that existed at the time and still exist now, or were developed also over the time, has profited a lot from talking to these people who had very different viewpoints. So I found, I mean, that's maybe one part, like the disagreement first was very fruitful in the sense of having very deep discussion trying to identify the source of the disagreement. But I think I was also asking myself, why is it that it's so hard to kind of come to an agreement because I would 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'm a disagreement, so whenever I'm I kind of found that, 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 this assumption can be something very general that we usually don't talk about, like from whose do we do physics?

I mean, I talked about that before, like we could have an outside view or you from the inside of the universe. And then you would say, okay, obviously that's an assumption whether I'm looking at physics from the outside or from the inside. But as long as, like, if you talk about quantum,

you're 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 series 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. It's kind of inherent in now,

thinking but we just don't talk about it and so 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 it said of course a theory like is something that has to be separate from it's kind of something that's applied from the outside and so I guess we should try to 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 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 for example, even the fact that we are ourselves's 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 don't mention it. And so 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 in the case of the black hole information paradox, my conclusion was both parties were kind of right, those who said that 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 both could be right in a sense.

this is also true for other like paradoxes we have in physics like there is these um there are these questions about um like the um Maxwell demon where people have different opinions but then 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 who have very good reasons to believe or to say what they say. And so I think the task is really not too early just try to defend one an opinion, but to try to understand what assumptions do they really make deeply in their sort 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 I can probably not rationally argue against. I had this experience with my colleague Nikola Shiza 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 sort 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 to both of us what we were kind of assuming. So one had to kind of force it to say,

you necessarily are a collapse serious if you assume that. So it was not that he started being a collapse series. It was more like 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 series or a collapse series a good description. I think it's not.

And that's why we disagree. And we couldn't really argue why a collapse theory 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 reject the collapse theory. So in a sense, that was for me the ultimate point of reaching a consensus despite disagreeing.

It's like 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 collapses 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 measurement outcomes. So for him, a measurement outcome has to be real. And that's just also an emotion. He's so convinced that a measurement outcome has to be real that he's ready to bite

the planet and say, okay, there has to be a collapse. Ah, okay. Now, my emotion is the opposite. I would say, okay, 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, 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 reasoning is completely rational. and legitimate once this assumption is made. I see. 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 whatnot, but then you have to kind of give a value to the different assumptions. 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 have the statement that the measurement 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 language to the different towers. And for me, something are harder to give up and some are easier to give up. and I think it's completely normal that different people

put these weights differently and I think the rational thing is to have no-go service that tell us if you make this assumption, then the other is invalid or must be invalid, but no-go service will never tell you which ones you have to give up. It will only tell us that you cannot have

all of them and you will have to pay a price and the price tax are different from people to people. And I think this is, on that level, it's then getting in a sense uninteresting, but I think 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 if people are ready to listen to each other. Of course, your thing actually contributes to that. I have to

say I prepared, myself for this interview by watching a few of the interviews. And I found it very insightful to actually really be able to digest, like, because it goes 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 preferences? And for me, this was actually an interesting 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 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 up well actually something that I like about this whole conversation yours and other peoples 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

tacit knowledge. They give other forms of knowledge that aren't explicit. So I was speaking with David Bessus, and he said the textbook should be more, he's a mathematician, 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. And 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 that's valuable, not necessarily in the book. Yes, I think this is a very important additional concern. communication channel at least. I think one requires both. I think the reason thing forces oneself to be precise and kind of put things in a way that one thinks they

survive the next or eternity in a sense. Whereas 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 this is kind of important information, also for the discussions usually.

If I know someone never changed this mind or someone changed this mind, I will talk to the person differently because I see different chances or I see different ways to kind of make the conversation fruitful in a sense. And so I think it's very valuable to have that format as well. So I really appreciate. It's also fun to listen to the two. Great, great. I hope so. Yes, it is. 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 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 meta-critique or all logic, is classical logic the correct way, is pericistent, et cetera. That's fine. All 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 a 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 good. groundless, so baseless, so subjective, it's just values. But it can't just, like, how do we then come to a consensus or a conclusion or make progress? Help me understand that. So I guess at the end, so I had this example with Nikola Shiza thinking of the collapse, Siri and me denying collapse. I mean, in this particular case, the emotions are important, or the taste or let's say, I could say the choice of approach is important because we don't yet have an experiment to decide, but we need to decide how to invest our resources.

So for example, if I'm a collapse series that would invest more resources in trying to find that collapse mechanism, whereas as I'm now 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 these emotions are very important to kind of allocate resources in research. 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 kind of direct our efforts. And maybe we are sometimes wrong or not, but of Of course, we need to try to invest our resources as effectively as we can. It's like when you maybe build a company, a startup, and you have to decide what product could be, could work or could be profitable,

you probably have that beginning a feeling and then you do it and you're either successful or not. And of course, those people who have a good feeling of what could be successful 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 it will be 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,

is in principle very valuable. And the emotions about making assumptions and so on is all necessary because we don't yet have these experiments and we need to 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 a few hundred years or I don't know when we will have decided on many of the questions that currently look like, 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 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 datum telling you? So is it an outcome per se, or is it just updating our knowledge about someone else's knowledge? How does an experiment even distinguish between that? Yeah, so I think if the question is at the end a question that I need to, 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 this difference or in sorting out the difference I would say okay 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 I really have this viewpoint that the relevant questions

the ones we should really fight about or have invest effort in solving them are questions that have an experimentally verifiable answer or an operationally verifiable answer because otherwise the question may not even 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 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 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 difference may just not exist.

And so we don't need to try to find an example. 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 I would call them a kind of purely philosophical question. And if someone can give me a definition of what real means that is different from having intersubjective agreement and different in a 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. Yes. So 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 you talk about physics questions, then my, let's say, requirement would be that the question is only a physics question if someone can tell me an in-principle experiment that one could carry out to decide the question. And so, yeah, I would, as a physicist, I 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 could. 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 and one can find 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 maybe let me put it very careful 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 like our scientific approaches. So I think it's 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. 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, I think this is something that I'm interested in because it's a very immediate feeling that is there, like I feel conscious,

but there I'm doubtful whether there is kind of an experiment we could do that would give us insights on 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 science protection. I've just seen many words

saying that I think consciousness is so little at least by me and the stuff I've heard about it that I'm not confident 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 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 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. that concept, I mean, the fact that I can talk and I'm interested in has to do with my consciousness. So in the sense, it's very deeply self-recursive, whereas science is usually like we have what I call before a Heisenberg cut. You have the person who applies the theory and the object, we apply the theory too. And so in that sense, it's kind of a third person that we have the object that is observed, it, we have the person that observes it and the 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 it really eludes

by my way of putting things. Yeah, of course I think it eludes almost everyone even though it's in some respect the most intimate aspect of them, of their lives. It is, yes, yes, yes. So when it comes to not defining consciousness, 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 can ask what 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 in saying,

well, the reason why I'm saying it's 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. Yes, I see. Yes.

But it may, it may not. I don't know. 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. The 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 there's two forks.

Is it today's physics? Is physics defined by today's physics? most people say, no, physics isn't complete, 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, 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 hand-wavy, 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. Yeah, I think that's right.

But maybe before we were talking about communication, and so I could say there is a kind of minimum let's say operational thing that is there in physics namely I can in for example teach students in physics and then they can make predictions that are again maybe the leads to more knowledge that they can communicate to others so physics is 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 physical law is just a compressed description of the world. But for me, it's this additional aspect.

It's a compressed description, but also one that can be communicated to other physicists, which could just be my future self, for example. And 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, for example, it would be very hard to communicate our physics to an alien who has no, let's say, shared reference with us, not even language or anything. So to make it communicatable is actually a non-trivial requirement in a sense. And then I could say what, again, 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, kind of to survive in the world, to 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, very operational, where I don't need to ask the question, is this now physics law or not? In the 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 an as efficient as possible way. And so that's different from saying physics is just about making predictions.

That's often what it's said, what operationalists say. But I'm kind of an information series. So for me it's very important that. we are also kind of taking into account that there's this information. That's right. 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 turn this around and say physics is about communication.

It's about communicating experiences, the description of the world that helps our future selves and our future our future physicists to navigate in the world and make better use of it. Then it's not about verifying it experimentally, it's just something we do in order to kind of make our life's more convenient and all that. And of course, that doesn't really capture the aspect that 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 yeah, that's maybe the non-operational aspect. Again, the emotional aspect of me trying to justify why I am, 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 started doing physics.

Humanity started to do physics because we want to use the patterns that we see in the world, represent them and communicate them among us. And I think we should put more focus on this communication aspect, on this information theoretic aspect, which is largely neglected. So usually we don't talk about how we communicate physics, in the sense. And that there are fundamental limitations to that. I think this can maybe help us making sense of certain things you don't yet understand. Professor, thank you for spending so long with me.

I hope it was... Thank you for the many questions. 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,

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