Daniel and Kelly’s Extraordinary Universe - What is Relational Quantum Mechanics?

Episode Date: November 4, 2021

Daniel talks to Carlo Rovelli about a fascinating alternative way of thinking about the nature of reality. And aliens. Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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We'd love to have them on the podcast. We all know that quantum mechanics can't be quite right. I'm not talking about the counterintuitive, probabilistic aspects of it, that it forced us to accept that the world we live in is fundamentally weirder than we have ever imagined. Probabilities and correlations and uncertainlies, no, those bits are probably right. But there's a problem at the heart of quantum mechanics, one that has baffled physicists and philosophers for nearly a hundred years, and that may take another hundred years to solve.

But some recent ideas may be showing us a path forward, even if it's a stranger path than we imagine. Hi, I'm Daniel. I'm a particle physicist and a professor at you. see Irvine and I'm still confused by quantum mechanics. Confused, but not frustrated. You might say I'm deliciously confused. What could be more delightful than grappling with the deep mysteries of the nature of reality, seeing the truth written down in cold black and white mathematical equations and struggling to gain an intuition to incorporate those alien concepts into our human brains?

After all, that is the deepest goal of physics. And that's the goal. of our podcast, Daniel and Jorge Explain the Universe, a production of I Heart Radio, in which we tackle the biggest and hardest and nastiest and funnest of questions of the universe. The ones that make your brains twist, the ones that

slip away from you just as you thought you had figured them out, the ones that might elude humanity for centuries or forever. We don't shy away from any questions on the podcast, but we seek to approach them and explain our knowledge and our ignorance to you.

My friend and co-host Jorge is on a break, but I have a special treat for you. We are very lucky to have as a guest one of my favorite physicists, one of my favorite writers, and one of my favorite writers about physics. Truly a poet of science communication, today we'll be talking to Carlo Rovelli about some of the problems at the heart of quantum mechanics and explaining a lesser-known but absolutely fascinating alternative version of quantum mechanics. So today on the podcast, we'll be answering the question. What is relational quantum mechanics? So it's my great pleasure today to introduce Professor Carlo Rovelli.

He's a professor of physics in Marseille, and he cut his teeth and made his name for himself developing theories of quantum gravity, mostly loop quantum gravity, if I understand correctly. He also became a household name as the author of the book Seven Brief Lessons on Physics, which sold more than a million copies, and was transatlantic. translated into 41 languages. I've read it and enjoyed it immensely and heartily recommended to you. Today, Professor Rovelli is here to talk to us about his new book, Helgoland, and a fascinating alternative take on the measurement problem in quantum mechanics. Professor Rovelli, welcome to the podcast and thank you for joining us. Thank you very much, Daniel. It's a pleasure and a honor

of being here. Wonderful. Well, I always love talking about quantum mechanics and puzzling over it with other people. I feel like every time I talk about quantum mechanics with somebody else, I think of a new question I've never thought of before or a new angle on it or a new mystery, frankly, like a new corner of my mind that I haven't ever really examined and I get confused. And so it's always fun to figure things out sort of on the fly. So today we wanted to talk about your new book and Helgo Land. And the book essentially lays out for LA audience this idea of relational quantum mechanics, sort of a new interpretation on quantum mechanics. And the first question for you I have is if you could describe for us, what is the problem that relational

quantum mechanics solves like why do we need another quantum mechanics interpretation what is it at the heart of relational quantum mechanics is trying to do one of the difficulties of the problem of quantum mechanics is to say exactly what the problem is so quantum mechanics on the one hand it's extraordinarily successful is used in a lot of our technology is used a lot of our understanding of the world we explain with quantum mechanics the basic of chemistry based of astrophysics, all sort of stuff. And it works. It works fantastically. On the other hand, there is a persisting, mysterious aspect about this theory.

It's a theory that it's sort of formulated differently than previous basic physical theories. And this difference puzzles everybody. And this is where the problems come in. Because the scientists disagree about how to think about quantum mechanics. If you go to a thesis conference and you want to really start a furious discussion, just like drop the question at the banquet, say, oh, by the way, what do you think of quantum mechanics? And 10 minutes later, everybody's screaming against it.

But is this a conference of philosophers or physicists in your mind? No, no, physicists, physicists, physicists. I mean, including the ones who say, there's no problem at all. What are you talking about? And then they say, of course there's a problem. Now, where is the problem? I mean, how can we characterize the problem? Well, sort of all physical theories from, say, Newton or maybe even before, from Descartes

or even from Aristotle, all the way to Maxwell and to Einstein, special activity, general activity, electrodynamics, the theory describes the systems. And the systems of some variables, I don't know, pendulum, it's a physical object, and the variable is the angle with the vertical, and to tell you how this variable is, and a involves given something that you know at the beginning. You know where it is, you know the velocity, and then you have an equation that tell you how the variable changes continuously in time. So, of course, this allows us to make predictions because we can measure the position,

the velocity, then close our eyes, wait two seconds, open again, and the new position, new velocity can be predicted by the theory, because the theory tells us exactly how it moves in time. So that's how Newton physics works or Maxwell theory works, of Einstein theory works. Well, quantum McKayne does not work this way. That's the point. It works in a completely different manner. It does not tell us what happens at the pendulum while he's moving. It only tells us the prediction without saying what is in between. And if you try to reconstruct what is this in between, so the data that you know and the prediction that you make, all devils go

lose, namely every strange things happen and people disagree about what happened in between. In between, you mean that moment when you closed your eyes and you're trying to use physics to predict the future. Exactly. So typically you think about quantum mechanics in this way. I mean, you make a measurement. That's the language which is used. The observer makes a measurement and you make a prediction of what's going to happen at the next measurement sometime later. And the prediction is bingo. It works. I mean, it tells us, exactly. exactly what we see. But if you ask what happened in between, it's confusing. And the reason is confusing, one way of presenting it, one of the many ways of presenting it is the following.

If you have a particle, an electron, or a little ball for that, or an atom or a molecule, and you throw it in some way, then quantum mechanics tell you where it's going to be, or at least probability distribution of what's going to be, because the prediction of quantum mechanics are not exact. That's one of the characteristics of quantum mechanics. But it's telling me There's a certain probability going to be here, a third probability going to be here, turn to be going to be here. But then, in between,

what happened into the mathematics is this particle opens up in a wave. It's everywhere. And with this wave, you compute the different probabilities. So the particles at the same time in many positions in the mathematics. So some people say,

okay, so the particle is in many, it's a wave, it's all over. But then, when you look at the particle, it's not a wave. it's in a single position. So other people say, no, come on. I mean, we look at the particle is always a particle. When you look where it is, it is always in one point.

But when you compute, how it goes from here to here, it's not in one point. And if you assume that it is in one point, you're led to make some kind of mistakes and things like that. So it's capable of making predictions, concrete predictions, or we can compare to measurements. But to make those predictions, you need this intermediate step,

which seems sort of nonsensical and conflicts with our sense of what is real in the universe around us. Exactly so. But the real problem, the thing people are debating with is what does it mean that the theory describe what we measure? That's why it's called a measurement problem in a sense. It's like when we measure something special happens, but we are not special, right? We are just a piece of the universe like any others. So the standard formulation of quantum mechanics, the one you read in textbooks, it's only in terms of the observer and the measurement, which is fine as long as you don't ask what about the physics of the observer,

what about the observer itself, isn't the observer itself a quantum system. So it should be described like quantum mechanics as well. So it should also open up like a wave, but does it? And that's where people start disagree. And some physicists think, yes, we do our waves. multiple copies of Daniel and me and Carlo and some versions of us, some answers are given and some other versions of different answers given, because we're a wave of different configurations ourselves. Other people think, no, come on, that's not a good way of thinking about reality.

And that's where the disagreement starts. So the disagreement is realized in a number of so-called interpretations of quantum mechanics. Ways to make sense about this funny story about the observer and the measurement. And on the market, there are many, but maybe three or four or five, which are dominant and many variants of these, which are pretty different for one another and give a profoundly different picture of reality. And that's the beauty of the story. It's not just a technical thing about using the theory. In fact, it's not a technical thing about using the theory.

Everybody agrees on using the theory. But everybody disagrees on what happens between the measurements. That's one way we're putting it. So I think it's really fascinating that it's so important to us to understand what happens when we're not looking, right? Clearly, physics takes it very important to predict what happens when we look because it's very practical. We need to know how transistors work and whether our airplanes will fly. But when we're not making measurements, where we're not looking, when we're not watching the universe, we still want to know what is real, what is going on.

We want to have like a model in our minds of how the universe works. Are we still doing physics in that case? Or do you think they're forever intertwined? I think they're forever intertwined. I think they're forever intertwined. And I think they should be forever intertwined. Because it's precisely by asking those kind of questions that in the past, physics has made big jumps. Let me make an example or maybe two. One example is, is the Earth the center of the universe? This was a huge debate at the time between Copernicus and Newton. It's a century of debate. And nobody could say that this was not a scientific debate because that's the debate that allowed Newton to do Newton mechanics and allowed Galileo to understand Galileo relativity and this debate on which

Kepler built. Kepler was a strenuous defender of the idea that the Earth is not the center of the universe. The universe is not the center going around the center? But if you think for a moment, is this Earth at the center of universe a scientific question in the sense of something we can test of course not. There's no way of testing. There's no operational meaning. There's no measurement that I can make that can distinguish if the earth is the center of the universe or is not. Because, you know, I mean, I can stay somewhere and see it moving, but what does it mean? If I am moving, I see the center moving. So it's really a non-emperical question. And yet, it's a profoundly scientific question. Why? Because it gives a different way,

depending if you answer one way or the other. If you start by thinking the Earth is the center, everything goes around, or if you start by allowing the Earth to be one of the things that moves, you go to a completely different way of conceptualizing reality, and one which worked very well. The other was bad, was not the good one. So I think this is one of the aspect of science, which I think is often misunderstood, I would say.

Science is not just about, you know, close your eye, make a model, make a mathematics, make a measurement, make a prediction, and that's it. Science is about understanding reality, meaning building up a conceptual structure, a way of thinking reality, how to put things in cases.

One way of putting things in cases is in one case, there's the Earth, and in the other, the celestial bodies, the sun, the moon, the planets, the stars. And then you throw that way, that's not the right characterization. The right way is there are stars, the planets, and then there are satellites to do something different.

And that works really different. And boom, we understand the universe better. So I think there are questions which are not directly empirical, but are scientific. And I think the interpretation of quantum mechanics should turn out something like that. Namely, one way of thinking about this story will turn out to be useful on the long run.

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one thing or the other. And the question is, how do you go from being probabilistic to being sort of classical? And, you know, it seems arbitrary to say, I'm making a measurement now, or I'm not making a measurement. And you can ask questions like, well, when it interacts with other particles, why isn't that a measurement? Or if I'm making a measurement and I'm using a stick, then, you know, the tip of that stick is made of quantum mechanical particles. So why isn't it just interact with my stick the way it interacts with other quantum mechanical particles and maintain its probabilistic nature. And, you know, people think about like, do you have to have a conscious observer involved, but there's all sorts of fun things that you can dig into there and had a fun conversation with

the author of what is real, which people can also dig into if they're interested in more details on what the measurement problem is. And the classical approach to people take, this Copenhagen interpretation seems sort of arbitrary. It says that, you know, the wave function moves by the shortinger equation and it's smooth and continuous. And then boom. all of a sudden, when there's a measurement, everything collapses. But it doesn't tell you what a measurement is, right? And so there's this deep question, still at the heart of the most common interpretation. And listeners to the podcast can also check out our episodes on the many world's interpretation.

But today I want to talk about your idea or this concept of relational quantum mechanics. So how does relational quantum mechanics deal with the measurement problem in a way that's different from this arbitrary, obviously unworkable approach of the Copenhagen interpretation? Yeah, very good. And I think this is a nice way of putting the question. Let's take a concrete case. The simplest quantum mechanical experiment and measurement that we can make, we can take an atom, a radioactive atom, right? It's just a piece of a block of radius, radius, uranium. You waited there, and you put around it some detectors of radiation, you know, the gagular detector, the click when a radiation arrive. You wait for a while, and after a while, one of these clicks, bling, okay, and then you say, okay, the atom has emitted a radiation

and it has been detected by these detectors. This is the facts. Now, how do we describe this in the equations? What do we do when we do quantum mechanics? We pretend that this radiation is some particles that are trapped inside the atoms, and they have a wave that describe them. There's a wave, the wave function describes them.

And in our mathematics, this waves leaks out from the atom, or from the nucleus of the atom, slowly and continuously, and it's all over. And slowly, imagine that this wave continuously leaking out, so there's this continuously wave around, spherical, and go out. So each one of the detectors is touched by these waves goes on. Then, at some point, the measurement happened. As you say, something happens. We go from this quantum wave to a classical factor, which is one single detector that happened.

And this is a measurement. Now, different interpretations, try to fill up this story in different way. Let me just compare to one or two interpretation. One is the many-world interpretation that you mentioned, and the other is the Copenhagen interpretation with a mention. The mini-world, in a sense,

is the most radical idea of taking the way very seriously. The Copenhagen interpretation is the one we study at school, the one that teacher tell you, tell you how to use. So the manual interpretation says that there is this wave that come out and you see only one detector clicking, but why do you see one detector clicking? Because in reality, you yourself are a wave. Okay. So you yourself are a superposition of different you, like the particle in different position is in a superposition of different positions. So it's not true that there is one detector that clicks. All the detectors clicks, but you yourself interacting with

this detector, so split up. In one U that sees this detector clicking, one U that sees that detector clicking, one U that sees that detector clicking, and so on. Okay? So now you have to imagine that the wave contains copies of you, and the reason you've seen one and not the others

is only because you happen to be one copy of you and not the other copies of the other. Now, does this work? Yes, it works. Is it plausible? Well, it sounds very implausible, extremely implausible. The problem is everything you do with quantum mechanics sounds implausible. So many people say, okay, we have to accept this,

that in reality we live in immense waves, that there are copies and copies of ourselves, and that's the many-world interpretation. Can I ask you a question, actually, about that many-world interpretation? There's something about that that's never set right with me, but maybe it's very naive. And that's this. You say that there are many, many copies of me,

and so it breaks the problem of why is this one detector clicked by saying it's not just that one detector, clicked. All of them have clicked just in other versions of the universe. But that doesn't make all the versions of the universe equal or on the same footing because I'm in this one. You know, this is the one that I feel is the truth that I'm making the measurement in. And that's different from the other ones because I'm not in those other ones. And maybe there are other me's in the other ones that say that they're, you know, who I actually am. But this is the one that I'm experiencing. And so it still feels to me like it has a special place. Is that a naive concern or is that something?

that you think many worlds can address? It is a concern because somehow many worlds plays exactly on the fact that there is a contingency in who you are, which is hard to make either informal or in words, and it is left a little bit vague into that. So it is a concern that some philosophers have raised about many worlds. All right. Well, as long as philosophers agree with me, then Amman Salaf, but you know that you can always find a philosopher that they do with you so careful

as a low bar apparently all right so that's the many world's interpretation let's go to the copenhagen which is what you studied at school okay the Copenhagen says well there are special things which are observers okay which are classical namely big a big compared to to the quantum phenomena it's when you don't see quantum phenomena because they're too subtle for you They're not visible to you because you're too heavy. To see quantum phenomena, you have to isolate things. You go to small things.

So there are classical objects. There are many, all the ones around us. And these are classical. So forget quantum mechanics about this. When a quantum system interact with a classical world, bingo, that's a measurement. And this works in practice very well. That's what we do in the laboratory. But in theory, it obviously does not work.

Because what is a classical system? I am many electrons and protons, and each one of them, it's a quantum particle, so each one has a wave, so I have a big wave. I am a quantum system. In a sense, Copenhagen, it works with an approximation, say, well, imagine that your theory is wrong for big things. Forget about it and still use it for small things. And, you know, that's why it appeals to everybody who does not.

want to ask questions, and you can. But if you don't ask questions, you don't get answers. I mean, on those sides, the scientists is about asking questions. There are a lot of scientists who say, well, I don't want to care about that. There were more in the past, in the 16th, in the 70s, in the 80s. I would say the majority of physicists would have said, oh, come on, just use the theory. It works very well. Nowadays, there are less and less people who do that. So I would say the large majority of people agree that there is something to say better here. So let's come to relational quantum mechanics. What does relational quantum mechanics say about that? It says that all systems are quantum and that's the first assumption. Let's see that

we're, as far as we know quantum mechanics, the best actual we have about the world. So up to disconfirm all systems are quantum. So that's already in contradiction with Copenhagen interpretation that says like, I am classical and you are classical. It says everything is made of quantum. Even And classical objects are just like massive quantum objects. This is just what quantum objects look like when they're really big. Exactly. So therefore, there's nothing which is an observer but by itself. Nothing that separates an observer in the sense of quantum mechanics from a piece of stone.

Of course, you know, there are people who have eyes, their machines, the things that store information. But that's irrelevant here. The point is in relational quantum mechanics, that's a suggestion of relational quantum mechanics. But the measurement happens in some sense every time two systems interact, but the actual element of reality, which is realized in the interaction. So when the detector clicks or the particle is here because I see here, or the particle hits another particle and therefore it has a specific position,

that has to be thought not as a, property of the particle by itself, but a relational property of the particle and the system it is interacting with. In relation to the thing doing the measurement. Exactly. In relation to the thing during the measurement, it can be anything. It can be another particle, in a sense. So therefore, the central idea of regional quantum mechanics is, let's be radical in the following sense. We describe the world in terms of systems, particle and lectern, etc. And these have properties, variables that describe them. But these variables do not describe how the system is. It interacts how the system interact with something else. Okay, so for example, we have a particle and it's flying

along and it has a certain wave function for things it might do. And Copenhagen interpretation says it stays probabilistic until something classical interacts with it. Relational quantum mechanics says it can collapse when it interacts with anything, but that collapse is different depending on who is interacting with it. So if it interacts with this particle, it might collapse in this way. If it interacts with somebody else, then might collapse in another way.

In that sense, it's also sort of like a branching of what is real, because what is real now depends on who is doing the measurement. Is that right? That's exactly correct. So the so-called collapse of the way function is by itself, something which is relative to the system against which the particle is interacting.

Which doesn't need to be a macroscopic object, doesn't need to be anything special. It's anything. But the subtlety is the following. Suppose a particle interact with my machine, my detector, okay, and I'm, I'm, that's a distance. And the tooth is not interacting with me. So with respect to me, there is no collapse happening.

So the way the particle and the machine are going to manifest themselves to me later on, if they interact with me, is still computed with quantum mechanics. So if I want to compute what happens with respect to me, I still use the wave function of the particle and the wave function of the machine. Because the machine is quite mechanical. I see.

So for example, I'm here talking to you, so I'm measuring you, I'm collapsing your wave function, but our listeners have not yet heard this podcast. So from their point of view, you and I are still in some uncollapsed quantum state, and only when they hear this, does it collapse into an actual stream of words? So for us, it is collapsed, but for them it's not yet collapsed.

Exactly, sir. I believe that this is coherent. This can be made coherent by going into details. It offers a possible solution of the puzzle quantum theory. This is a relational interpretation. It does require, it has a cost, there's a conceptual cost. Because as you said, a reality is a little bit more subjective, no subjective, relative. I'll come back to the difference between subjective and relative.

So it weakens realism, in a sense, but it's still a realistic interpretation. So I think that this cost is more worthwhile paying for being fruitful for the future of physics than imagining that there are many copies of myself or than not asking the question. Then you come to the subtlety. In your example, you talked about you and me and the listeners. We're all people. okay that's fine but

the key intuition of relation to quantum mechanics is this has nothing to do with people that's the key point and that's a big difference with respect also with Copenhagen in a sense right this is nothing to do with subjectivity nothing to do the subject but the fact that you and I are

are subject of perceptions nothing to do it would also work if you and I were dogs or bananas or particles right exactly the same mathematics work exactly and some of the same story would work. And to make an example of this, because I think that, you know, relative to observer, observable, are often misinterpreted as subjective while it's just relational. For instance, there's a very well-known example of relational notion in physics, which students struggle at the

beginning when they encounter it the first time, because it's very counterintuitive, if you think at the beginning, which is velocity. When we study, Galilean theory or Newton theory we learn that velocity is not absolute, velocity is relative so what is the velocity of the moon?

Well, depend. There is the velocity of the moon with respect to the Earth, the velocity of the moon with respect to the sun, the velocity of moon with respect to the center of the galaxy, the velocity moon with respect to the cosmic microwave background average.

Which one is the true one? Well, none. It's that velocity, it's a notion that pertains to two objects. The moon is something else. It's always velocity, the moon, with respect to something else. We sort of know that, right? Because when we say, don't move, we know that don't move is relatively to something. For instance, if you're in a train and your little daughter is jumping around and you say,

don't move, you don't mean that your little daughters would jump out of the train and don't move with respect to the earth, right? you mean don't move with respect to the train and she understands correctly right and she should move respect to the train and she's still moving very fast

because the train is running so we understand that velocity is a relational notion but this is nothing to do with subjectivity right because if I say that the moon has a velocity with respect to the sun I'm not saying that the sun is a subject

that sees the moon and perceive something is nothing to do with that nothing to do with mind the subjectivity, just physics, pure physics. There is a quantity, which is velocity, which doesn't depend on one thing, it depends on two things, the moon and the sun. And it's the velocity, the relative velocity of the two things, how they move respect to one another. So that's a big philosophical step, right?

You've sort of taken away from the moon the fact that it can have a property called velocity. You say you can't have a property called velocity, only a pair of things can have this property. A single object cannot. and the sort of the mental game I like to play is to imagine like a particle in an empty universe. Can it have a velocity? Well, it has no meaning to have a velocity if you are the only thing in the universe, right? And so I want to also ask you about this concept of cost.

You're talking about the cost of a theory. And so here, is that the cost you mean that you're like now changing philosophically what it means to be real, that no longer can you include in your sort of, you know, category or your index of what it means to be, that you have a certain velocity that you've taken that away from objects? Yes, this is a course. I think you said it very cleanly. It's a deep philosophical point

when we realize that velocity is not a property of an object. You need another object. It's a property of two objects. Relation of quantum mechanics, in a sense, the following suggestion, we understand better quantum mechanics if we just interpret all properties,

all variable in this way, not just velocity any variable that you can think about a system only make sense if you have another system you're saying

this is a variable with respect to this other system and what you mean is that there was an interaction to these two systems and that variable was realized in that interaction namely describe the way one system affected the other one. And can't you use even the same

language because now we talk about velocity we say velocity of the moon as measured by an observer on the sun or is measured by an observer on the earth. Now you can say the location of this particle or the color of this particle as measured by this observer versus as measured by that observer. Exactly. A foot washed up a shoe with some bones in it. They had no idea who it was.

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I'm like, oh my God, it's go time. You actually sent it? Listen to the Good Mom's Bad Choices podcast every Wednesday on the Black Effect Podcast Network. The IHeart Radio app, Apple Podcast, or wherever you go to find your podcast. Now you went strongly in saying, does this mean that reality itself is relative to? I hesitate. Namely, this same reaction could have been raised against Galileo and Newton when they started using velocity in this sense. I think that the reality of the motion of the moon and velocity of

the moon and the sun and things, in spite of what we have understood about relativity to velocity, is still very solid. We still have a realistic interpretation of motion. So I would like to think about relational quantum mechanics

as a realistic picture. So reality is there. I mean, things are there. There's nothing to do with us, of course. We are just a part of nature. It's not that nature is a part of our mind. That's my naturalism, if you want. But the way reality is, is subtle.

According to quantum mechanics, can be described, reality can be described by systems that have properties relative to other systems. So then can you have something which exists by itself? Like imagine, you know, the sort of simple example I mentioned earlier for velocity, a single particle in an empty universe can't have a velocity. Now you've said that everything is relational the way velocity is. Does that mean that a single particle in an empty universe does not exist or cannot have any properties because it would need something else in order to measure it? Can a single particle universe exist?

No, if quantum mechanics is the correct description of the universe. And in fact, that can be said in a besides way. There's a philosopher, which is Mao Dorato, who is elaborated match on that, saying if we take relational quantum mechanics correctly, we cannot talk about the quantum state of the universe. we cannot state of the quantum state of everything. We cannot even talk about the description of the universe because the description of the universe needs to be, if we use this way of thinking about quantum mechanics, a description relatively to something else.

So in the moment in which you're saying the description of universe, you're assuming there is something outside that is looking at the universe. You can, but then in your reality there is something else in addition to what you just called the universe a moment ago. For instance, the famous wave functional universe what is it, is a mathematical object that allows you to predict what you would measure if you are outside the universe and interact with it. But then it's not the universe anymore because there's

something else outside the universe interacting with it. So there's, the wayfunctional universe is meaningless. It's always a way function of something. I was hoping to use that example to lead you down the garden path to admit that that means the universe can't exist, but you just went straight there and said, if this is true, then we don't have a meaningful concept of what the universe is. I mean, isn't that a problem? I mean, if we want a theory of reality that tells us what is happening, doesn't it need to also allow us to think about what reality is? I think that it's a deeper point. One of the things we have understood more and more in modern physics, but also another realm of modern culture, is that we're always part of the story. So,

We see reality from within, not from outside. In completely different parts of our culture, for instance, this was famously an immense issue in anthropology, right? You want to describe a society, when the Europeans went around the world and tried to describe societies objectively, non-European societies, maybe, I don't know, some people in the Amazon.

And then they realized that they were not doing nothing objectively. They were just comparing their own culture with the culture of these people. They were interacting with these people and not being outside a cultural scheme, but doing something enormously interesting, not something that lacked value, but that was not a stepping out from what is being described.

And I think physics, to some extent, has encountered the same set of problems. Namely, we describe the universe from within. We are part of the universe. what physics tell us is if I encounter this, I know what I can expect. And I know what I can expect, what kind of things I have around me. But the idea that physics give a list of all, everything existing with a state, it's first of all totally implausible by itself, right?

I mean, how many stars there are in the universe and how many atoms in each one of these stars. So it's just totally outside of our capacity. But even in principle, what does it mean? I mean, physics is all in the form, if this, then that, right? If I found this pendulum in this way, then it's going to move like that. It's a model, as the philosopher says. So it's always a story about given some initial condition, this is what I expect to happen. Given some data, this is what comes later.

And then it tells what kind of atoms are around. the forces are around. But we shouldn't expect from physics the total novel of the universe. That's what everything is in the universe. That's the total state of the universe. Why it's not as far outside our capacities and probably I would say it's meaningless

because of what we said before. Because that would be an issue that God might have if he or she exists. I mean, seeing from the universe from the outside, not of our business. And I think this makes exactly the point that you were talking about earlier that we want to know the answers these deep philosophical questions because they tell us which questions then make sense to ask, which questions have answers and which questions are even relevant to think about. I think that's some of the most important reason why physicists need to think about philosophy. And I know that you talk about quantum mechanics a lot and you're quite an expert on this.

So I want to try to ask you a question that maybe nobody's asked you before, put you on the spot. And that's this. Have you thought about what it might be? like to talk about physics or quantum mechanics with aliens. Imagine that we meet extraterrestrial physicists, right? And we go and we say to them, oh, wow, you must know the secrets of the universe because you figured out how to travel here from distant stars. Do you think there's any possibility that we would be asking the same questions or that their answers to our questions would make any sense to us? Do you think that the way that we look at the universe is sort of

the way Europeans, you know, looked at other cultures. Do you think that it's deeply imbued with our human bias? Or do you think that we are probing something universal, which should be part of sort of like some galactic physics project? Great question. A bit of both, but more of the first and the second. That's my answer. There is a precise sense in which what we discover in science is just true and universal. I mean, I don't think this can be denied. And people who denied it, I think, have a task way of denying it. In Newtonian physics, in some sense, it's true. It works.

It describes reality. It's, I mean, the fact that everything is made by atoms and there are 90 and so kind of different atoms, that's a fact. It's like when you, you know, when you go down the street and you see that, you know, your three friends are in the cafe, that's a fact. It's true. And I believe that science is a higher level of understanding fact. It is the way humankind organizes an understanding of reality better and better.

But from this, to have an idea that there is a unique, clearly path to the perfect description of reality and we are on it or even that we are close to the end of it, that seems to me unbelievably full of self-pretention and I see no sign that we're close to that. And we have a sort of experiment of that because we know what past centuries scientists thought. And we do know that some of the things that we know would be meaningless for them. It would not answer their questions

really go in a different direction. Which does not mean that they are better than us. We are better than them because we know what they are in some objective sense. And the objective sense is that if Boltzman came alive and if I could have enough time both in me sitting here, I believe that I could slowly arrive to convince him that there are a lot of interesting things he doesn't know.

So I think it's the same with some aliens. If some aliens would come, I wouldn't be surprised if they have a completely different story. And it might be possible that we just don't understand one another because the story is too different. It might be different because they have a different way of perceiving reality, right?

We view reality on the basis of what useful to our Darwinian evolution, not on the basis of some kind, anti-rationalism, or simply, even if they had the same senses, because culture then was going in other directions. But I believe there is communication, right? Cultures communicate. So maybe with this aliens, we could learn to communicate. And we were very surprised of learning completely different perspectives, if they are more advanced than us in some sense. Or maybe they would be surprised to learn something from us, or maybe both. So in other words, I don't think

there is a path to truth and they would know everything we know and more. Maybe they would never figure out quantum mechanics and we teach them quantum mechanics. You're right. It's a relationship we'll send you to meet the aliens and hope they don't eat you. It's a great novel, the dark cloud, the black cloud in which there is this, it's a huge cloud that come toward the sun and somebody realizes that it's actually an intelligent something down there. And somebody realized that it's, and somebody realize that it's going to be a disaster for the earth. This is going to suck the energy of the sun somehow. So the scientists arrived to communicate and just gently convince this cloud to leave us alone.

But then there is one scientist or two, I don't remember, that said, wait a minute, I want to learn everything you know. So the cloud answer, well, it's dangerous. I mean, your little brain is, but they really want it. So the cloud say, okay, if you really want, so I think. is two of them. They sit around the screen and the screen start flickering and they just look at this at the beginning. At the beginning they don't understand but then they start understanding and then

finally they start learning and then they're lost because they're considered fool and they're put in a psychiatric hospital and the body knows what they're I see. So that's the danger of talking physics with aliens is you can see reality so clearly the humans can no longer relate to you. So let me bring it back to one last question

about relational quantum mechanics, what you're proposing here is really a radical departure on a way of seeing the universe, imagining that objects don't contain on their own properties, but that these things are just dependent on pairs, essentially things that are measuring other things. So my question to you is that how could we know if this is true? Is there some experiment we can do that could tell us, look, Copenhagen fails here and we need relational quantum mechanics or we can dispense with the many worlds? Like, is this something which can only ever be able philosophical conversation among physicists motivating our questions about

reality, or is this something we can actually one day put to the test? I don't see any way it could put to the test. There's some interpretation of quantum mechanics that assume that quantum mechanics is actually wrong and so they can be tested because you can do an experiment to see

whether the quantum mechanic is wrong or right. And these are very interesting. And so far, many of these tests have been done and quantum mechanics are always going to be right. And all alternative for the moment have been all eliminated. But the various interpretation like many worlds or the bloody bomb, the pilot wave interpretation, hidden variables or relational quantum mechanics, which I think are the main one, so Q-beams, maybe in one sense. They are not distinguishable in an empirical way. As far as I know,

nobody has come out with an experiment. It distinguishes them. And in a sense, studying them, there's no experiment to make a difference. So how would we know? Well, I think exactly the same way which we finally all agreed that the Earth is not the center of the universe, even if there's no experiment that tell us that the Earth is or is not the center of the universe, because there's no way to measure what the center is in the center of the universe. Namely, the progress of science will work better with one conceptual scheme than the other. I do quantum gravity. The reason I got into the issue of interpretation of quantum mechanics,

And Rationaldomechanic goes back to the 90s, in fact. My first papers on that in the late 90s. And then other people came in and have developed it. And in recent years, there have been a much stronger increase in interest in it. And it's not isolated because there are other people who have very similar ideas. So visual motor mechanics is really part of a little group of interpretation, just similar in some sense, maybe with different emphasis, different tone. I'm thinking of writings by Zilinger, by Charles Lachner, by Richard Haley, and Nader.

There are a number of ways of going in the same direction. So in my own work, in trying to put up a quantum state of gravity, I found that this way of thinking is much more helpful. When you don't have space, you don't have time to locate things, you don't have the observer. This relational way works well. And one reason it works well, this works very well with general activity, where location is relation. Nothing has a position. To be somewhere, it's only meaningful if you're somewhere with respect to something else.

And now the things start staying together well, because to be next to something is the possibility of interacting with something. So you can exchange information with something. That's what we're talking about in physics. This being next to one another and exchanging something, rather than having a big canvas. there and placing things and saying that's what reality is. So I hope that the discussion about the individual quantum media is going ahead. It's not blocked. It's not the same as

30 years ago. And I think that with the new ideas, with the new things being discussed, at some point it will become more and more clear that one way of viewing things it's productive, it works. It doesn't require us to assume things which are absurd, but it does require

us to assume things which are necessary to make sense. of reality of nature. Wonderful. Well, that's a fascinating insight into how we might make steps forward. And I agree with you that it makes sense to unify quantum mechanics with relativity if quantum mechanics itself becomes sort of relativistic, not in the sense of things moving at very high speeds, but in the sense that the objects and the quantities we measure themselves are relational or relative to other objects. So that makes a lot of sense. Well, thanks very much for this fascinating conversation, really stimulating, you know, about quantum mechanics and physics and

relativity and, of course, aliens and philosophy. I want to thank you very much. And I want to point our listeners to your book, Helgo Land, which has just come out recently. And it's a fascinating read on these ideas and how they were developed and people who are interested in digging more deep into them. I encourage you to check them out. So, Carlo, thanks very much for joining us on the program today. Thank you very much. That was great. Thank you. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. Every case that is a cold case that has DNA, right now in a backlog, will be identified in our lifetime.

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