Daniel and Kelly’s Extraordinary Universe - The Many-Worlds Interpretation of Quantum Mechanics

Episode Date: November 25, 2021

Daniel talks to Sean Carroll about the quantum multiverse, and whether it is real Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

This is an I-Heart podcast. I'm Dr. Scott Barry Kaufman, host of the psychology podcast. Here's a clip from an upcoming conversation about how to be a better you. When you think about emotion regulation, you're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome. Avoidance is easier. Ignoring is easier. Denials easier.

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or wherever you get your podcast. It's important that we just reassure people that they're not alone, and there is help out there. The Good Stuff Podcast Season 2 takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick

as they bring you to the front lines of One Tribe's mission. One Tribe, save my life twice. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the Iheart Radio app, Apple Podcasts, or wherever you get your podcast. What if your idea of how the universe works is just wrong? I mean, you live in a world that seems. to make sense of you. That seems to follow rules that you're familiar with. But what if that's just wrong? Could reality what's actually out there beyond our brains and our senses? Could it be something

so strange and bizarre that we would hardly recognize it? Could it be dramatically different from the glimpses we get through our senses and experiments? There's a vital clue that might just point us in that direction, something that has puzzled physicists and philosophers for nearly 100 years and that may take another 100 years to solve. Solving it might require us to swallow a picture of reality that is mind-bendingly strange to our little human brains. Hi, I'm Daniel. I'm a particle physicist, and I'm drawn to the possibility that the universe might be very different from the way we imagine it. What is the goal of physics anyway, if not to reveal the true nature of reality to us? We build mathematical stories in our minds

and apply them to our experiences, but why? Immediately, it's because we want to predict what will happen when we throw a stone or jump a river. But going deep, it gives us the chance to ask questions about how the universe works. If our mathematical story describes the universe, then we can look at that math and ask why the universe seems to follow that and what it all means. And sometimes the universe out there seems to insist on a mathematical story that we find very weird, shocking almost. And that is the goal of physics, not just to give us the power to throw rocks and jump rivers, but to reveal the truth. truth. And the most exciting moments are when the truth and our intuition clash dramatically.

When the universe says to us, no-uh, your ideas about the universe are just wrong. And that's the goal of this podcast, Daniel and Jorge explained the universe, a production of IHeart Radio, in which we tackle the biggest and hardest and nastiest and funniest and funnest questions of the universe. The ones that make your brain 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 today. We are very lucky to have as a guest one of my

favorite physicists and one of my favorite writers about physics. Today we'll be talking to Professor Sean Carroll about some of the problems at the heart of quantum mechanics and a potential solution. So today in the podcast, we'll be answering the question. What is the many worlds interpretation of quantum mechanics? So it's my great pleasure to introduce Professor Sean Carroll. He's a theoretical physicist at Caltech, and he's known for his work on cosmology, general relativity, and the foundations of quantum mechanics. He's also the author of several widely acclaimed and widely read books, including something deeply hidden and the big picture, and is the host of the podcast Minescape, which might actually be nerdier than this

podcast. Today, Sean is here to talk to us about the many worlds interpretation of quantum mechanics and the measurement problem in quantum mechanics. Sean, welcome to the podcast. Thanks very much for having me here. Wonderful to have you. So I want to dive right in. And before we talk about what the many worlds interpretation is, I want to get your view on what problem it solves. like, why do we need so many interpretations of quantum mechanics? What problem is it that they are trying to address? I think there's actually two problems. I mean, this is the right question because are we just wasting our time?

Honestly, it's not a lot of time compared to other physicists, thinking about other things. The foundations of quantum mechanics is a minority pursuit. But I think there are two problems, and they're such looming large problems, and quantum mechanics is so important to modern physics that I do wish we were spending more time on them. So in quantum mechanics, I'll try to give my briefest version of quantum mechanics. mechanics that we can. We talk about objects in the universe, whether there's an electron or whatever, in a different way than we talked about them in classical mechanics, in Isaac Newton's view of the universe. In Newton's view, we would have a position, a location in space for a

particle, and we would also have a velocity, and if you knew the positions and velocities of everything in the universe, in principle, you could predict what would happen, and you could measure what would happen as much as you want. In quantum mechanics, we say, no, no, no. That's not how we describe reality. There's something called the wave function, which for a single particle like an electron is just very wave-like, basically at every point in space that

has a value. But positions and velocities are not properties of the electron anymore. There are things we can observe about it. And the wave function tells us the probability that we'll get different answers if we observe, like the position

or the momentum. The momentum is just the mass times the velocity. So this raises two big questions. One is, what is the wave function? Is it supposed to be the real world? Is it that somehow we're not measuring the real world exactly when we do our measurements, but it really is described by this weird thing

called the wave function that we don't have direct access to? Or is it just part of the world and there's other extra variables in addition to the wave function, hidden variables we sometimes call them? Or does the wave function have nothing to do with the real world? that it's just a way of predicting the experimental outcomes,

and the real world is something much more definite than that. So all three of these options are very much on the table. This is what I call the reality problem. Like, what is the real world? Is it the wave function or something weirder? Something else, I should say, not necessarily weirder. The other problem with that brief version of quantum mechanics I just gave you is that it involves the word measure or observe, right?

No other fundamental theory of physics uses those words at all. They just assume that you can measure whatever you want. But in quantum mechanics, it seems to be the case that you need separate rules for describing systems when you're not measuring them and describing them when you are measuring them. And so this raises what we call the measurement problem, which is, what's up with that? Which includes, like, what do you mean measure? What is the definition of the measurement? Does it need to be a conscious creature?

Could it be a robot or a video camera? What if you just measure it badly? Does the same thing happen? when does it happen? How quickly does it happen? Why is there even a separate set of rules? So a whole bunch of questions get swept under the rug of the measurement problem in quantum mechanics. And I think both these are really big problems. If we want to think that quantum mechanics is the right theory of how reality works, we need to know what reality is and we need to know why this measurement process plays such a special role.

And you make a really interesting distinction there. You say an electron, we can observe these properties of it or we can observe these quantities, but it no longer has these properties, that it's velocity's position are not like aspects of the electron. You've like separated the electron from these things we can learn about it. Yeah, and actually in doing that, I've already cheated. I've already sort of slipped into my favorite way of thinking about quantum mechanics because there are people who would say that the wave function is just a way to predict the outcomes of what you measure.

And there really is something called the position, something called the velocity. We just don't know how to predict what it's going to be until we measure. them. Whereas someone like myself is a wave function realist. My point of view is, look, every version of quantum mechanics uses something like the wave function, okay? Either the wave function or something completely equivalent to it. So the simplest, most minimal version of quantum mechanics would only use the wave function, right? Like not as a route to get to somewhere else or as part of the story, but the whole story. Like, why not imagine that the wave function is actually what the world is. And when that's true, in that perspective, it becomes the case that things like

positions and velocities are not features of the wave function. They are possible experimental outcomes. And that's the biggest conceptual hurdle here, because we all look at things and we think they have positions and we think they have velocities. And if you're this wave function realist kind of version of quantum mechanics, no longer is that true. So let's move from positions and velocities to something that's more binary because I think it's easier to think about. Let's talk about the electron and it's spin. Like maybe it's spin up or maybe it's spin down. So then what's the problem with the orthodox, the Copenhagen approach to quantum mechanics where you say,

I have a wave function that describes the probabilities of this electron being spin up or spin down. And then when I make a measurement, when I poke it with my finger, the universe rolls the die and says, okay, well, you had a 60% chance of being spin up, so you got spin up. Or nope, you got spin down this time. What's the problem with taking that approach? Well, you already said when I measure the spin or when I poke it. I want something more definite than that. If what I'm talking about here is a really fundamental theory of physics,

I should not be able to rely on weasel words about poking and measuring, or at least I should give a super duper rigorous definition of what exactly that is. And the originators of quantum mechanics in its conventional textbook form, the Copenhagen interpretation, resolutely refused to do this. The strategy they adopted was to say that observers like you and me just aren't subject to the rules of quantum mechanics. We are classical. We are as if quantum mechanics never happened, okay?

You and I. And this is perfectly compatible with our everyday experience of the world. But then they say, but individual particles or atoms obey the rules of quantum mechanics. And you come along and say, well, but I'm made of atoms. How can it be that my atoms obey? quantum mechanics and I obey classical mechanics. And so the real problem with this sort of conventional textbook version of quantum mechanics is that it's just not a definite physical theory.

It's not even something you can compare to other things. It assumes that we're in a regime where this division between quantum and classical is good enough to get us by. And I think that when it comes to fundamental physics, we need to do better than that. And for example, if I'm poking something with my finger, you could say, well, I'm classical, so it should collapse the wave function. But then you can imagine the very, very tip of my finger is just a quantum particle. And that shouldn't collapse the wave function.

And so at what point is that wave function collapsing happening? Is it two layers of quantum particles? Is it 10? Is it when it gets to my neuron? As you said, there's no good answer to that. Yeah, exactly right. And what if you miss the particle or what if you like graze it? All of these questions you can ask are just, you're told you're not allowed to ask them

in the conventional way of thinking about quantum mechanics. Well, I mean, I am a professional particle physicist. I think I know how to poke a particle when I want to, but I won't take umbrage at your example. So then what is the solution offered by the many world interpretation? How does that solve this problem? So many worlds came about from a graduate student, Hugh Everett. So this is always what you should aspire to do as a graduate student,

overthrow the fundamental nature of reality. And interestingly, Everett was working with John Wheeler, who was a very famous physicist, who was an acolyte of Nealz-Bor, the grandfather of the Copenhagen interpretation. And Wheeler gave him the following thesis problem, quantized gravity. And this turns out to be very hard, quantizing gravity. We use the word quantized as a verb to turn an existing classical theory into a quantum theory. And what happens with gravity, gravity we understand classically pretty well

in a theory called general relativity, given to us by Einstein. And the point is the reason why it becomes a problem for quantum mechanics is both technical, Like when you try to quantize gravity, you run into infinities and other things you don't like. But there's also conceptual problems. Everett said, look, in the Copenhagen view of quantum mechanics, it's crucial that I have the quantum system I'm looking at and the outside observer, poking it. But if I'm quantizing the universe, then I don't have an outside observer. I have the whole universe. I should include all the possible observers in there.

So he started thinking about that. He said, what happens if we just include the quantum state of observers as well? I don't know why it took 20 years for people to guess this, but it's a very natural place to go. What happens if you let the observer be part of the wave function rather than treating them differently? So consider that electron that we had where you could get either spin up or spin down, right? And consider the following possibility. Since you're a particle physicist, we're going to assume that you're pretty good at measuring the spin of the electron. And what that means is, if the electron was absolutely 100% spin up,

we're going to grant you that you would always measure it to be spin up. So that means that you, as a physical system, would evolve into a state where your brain says, I measured it spin up. And likewise for spin down. If the electron was 100% spin down, we're going to grant you that you would say, yep, I measured that to be spin down. Now you're saying that I can do something which is unfamiliar to me,

which is I can be in a quantum state. I can have a superposition of having measured one thing and the other thing. Well, I haven't said that yet. I'm about to say that. What I was so far saying is if the electron was 100% spin up and you measured its spin, you would find it to be spin up. You're not in a superposition of anything, right? And likewise, if it's 100% spin down, you'd be spin down.

Let's assume that. Let's assume we want to be getting the right answer when the answer is definite and known already, okay? I mean, that's the least that we can ask. I'm a reliable measuring device so far. Yeah, exactly. But then if you are a quantum system, that's all you need to know. Because quantum mechanics, in the technical jargon, it's linear.

So what that means is if the electron is definitely spin up, you always get spin up. If it's definitely spin down, you always get spin down. Then when it's in a combination, when it's in a superposition of both, we know what you're going to evolve into. The wave function of you plus the electron will evolve into part of it, where the electron is spin up and you measured it spin up and a part where the electron is spin down

and you measured it spin down. So this is taking advantage of the quantum mechanical feature of entanglement that you don't separately say well here's the wave function for you here's the wave function for the electron, etc. There's only one wave function for everything and in fact Everett referred to his own theory

not as many worlds but as the theory of the universal wave function. And so everyone agrees with this by the way. Everyone agrees that if you treat you as a quantum system and you measure that spin, you evolve into an entangled superposition, part of which says the electron was spin up and that's what you saw, likewise for spin down.

Everett's only move is to say, and that's okay. There's nothing wrong with that. So the immediate visceral response is that can't be right because I've measured electrons before and I've never felt like I was in a superposition. And Everett says, that's fine

because you've misidentified yourself in the wave function. You think that you're this combination of having measured spin up and having measured spin down, but that's not right because you're entangled with the electron. There's two parts of the wave function, one of which is very consistent. The electron was spin up and you measure it to be spin up. The other part is also very consistent.

The electron has been down and you measure it to be spin down. And Everett points out that in the future evolution of the wave function, these two parts of the wave function will never interfere or interact with each other ever again. They have no influence on what each other are doing. If I poke, as you say, if I change or alter what's going on in part of the wave function where the spin was up, let's say, the part where spin is down doesn't know.

It is not influenced by that. So it is as if these two parts of the wave function are now describing separate worlds. And the crucial thing to keep in mind whether or not you like many worlds is that Everett didn't put in a bunch of worlds. All he said was that, we're going to take wave functions seriously and include observers in them.

The world's come along for free. Once you believe that the electron can be in a superposition of spin-up and spin-down, you've got to be able to believe that observers can be in the superposition of, I measured spin-up and I measured spin-down. I see. So he avoids this distinction between quantum observers, which don't collapse the wave function, and classical observers, which do collapse it by saying everything is quantum,

nothing ever collapses the wave function. The wave function is the universe. It just keeps going. But you experience one outcome rather than the other because you are no longer every part of the wave function. You are part of the wave function that experience spin up or you are part of the way function that experience spin down. Is that a fair summary?

That is precisely right, actually. That's an excellent summary. So I think that if we're being fair, we should all agree on the pros and cons of this point of view. The pro is it's just quantum. mechanics already taken at face value. There's a wave function for everything. Like you said, everything is quantum, and it obeys one single equation, the Schrodinger

equation, or some equivalent version thereof. You don't need new variables, you don't need new dynamical laws, you don't need any extra stuff. So at the level of writing down the theory, it's the simplest possible version of quantum mechanics. But of course, the cons are, at the level of coming to terms with it, it's the biggest possible imaginative leap, right? Because we're saying that every single time we measure the spin of an electron,

a new world is created. And, you know, you got to give the skeptics a fair nod to say, yeah, like, that's a lot to buy. And we proponents of it will say, but you already bought it when you bought quantum mechanics. Like you sneakily were already there. We're just putting your face in it. Well, what do you think that moment was like for Everett

when he followed this line of thinking and then had this perhaps moment of understanding where he realizes, hold on a second. Maybe the universe has all these different layers and is much vaster and much more complex than we imagine. What was that moment like? Did he ever write about that moment of epiphany or realization or understanding? As far as I know, he didn't directly write about that, no.

But he wrote a lot. In fact, Wheeler, who is his advisor, was trying to pretend. Wheeler himself was in a superposition of Everett has done something radical and interesting and Everett is just going along with the conventional Copenhagen interpretation because Wheeler didn't want to annoy his own mentor, Niels Bohr.

So he had Everett both visit Copenhagen, literally, and Boers people visit Princeton, where they were living themselves, and letters went back and forth. So there is a lot of writing about this. And the one thing I will say is that, you know, again, as working physicists, both of us, some physicists are just lucky sometimes, right? You're in the right place at the right time. You get either the right experimental data or you get the right idea.

and good for you, and you get credit for that. But we do inevitably separate that out from how good you are, how smart you are, how brilliant you are, right? Like, I mean, there's brilliant people who just never were in the right place at their right time, and there's some people who got lucky. And if you didn't know any better, which I didn't when I first started thinking about this, Hugh Everett would be the classic example of someone who got lucky, right? Someone who's just in the right place at the right time.

He only had one idea. He left physics after graduate school and moved on to other things. But you read what he wrote about this stuff and you realize, oh, no, actually he was brilliant. He completely understood what he was doing. And this is what I mean by being brilliant because very often, like someone will have an idea in theoretical physics and someone else super duper smart

will understand and appreciate the implications of that idea and spell it all out. And Everett did both. He really appreciated exactly what he was saying and in these letters going back and forth to the giants of quantum mechanics back in Europe Everett more than held his own. In fact, he kind of ran rings around him.

So I don't know how he felt when he first came up with the idea, but I do give him credit for really thinking through the implications of that idea. Wonderful. Well, I want to talk more about the implications of many worlds and what it means, but first, let's take a quick brick. Hey, Suss, what if I could promise you you never had to listen to a condescending finance, bro, tell you how to make a make a question. manage your money again. Welcome to Brown Ambition. This is the hard part when you pay down those credit

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We're Family Secrets. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're back and we are talking about the mind-blowing idea that maybe the universe is more than just what we see, that there are many universes out there, part of this quantum multiverse, where the universe is constantly splitting based on the universe. various possibilities of what could happen every time quantum particles interact. And I think that the question that probably most listeners have is probably a question you hear a lot is, are these other worlds real? And in what sense are they real? Like, is this a calculational tool to help us understand our experience? Or are those worlds like in some sense really out there? Yeah, I think they're real. You know, this gets into deep philosophical questions about what you mean by real right away. But, you know, here's how I think about it. If we have our best explanation for what we

observe, and that explanation takes the form of some physical theory, and that physical theory predicts the existence of stuff we don't observe, then we take that stuff seriously until we have a better physical theory, right? So, I mean, we often discover new things in the universe by doing this, whether it's, you know, new planets or dark matter or whatever. And so if you take Everett's version of quantum mechanics seriously, he solves both the reality problem and the measurement problem. The answer to the reality problem is

the wave function directly describes reality. The answer to the measurement problem is when a quantum mechanical system becomes entangled with big macroscopic things, that's what counts as a measurement. And a prediction of his resolution to these two problems is that these other worlds are real. So if you

want to get the benefit of his solutions, but you find the other worlds distasteful, that's okay, but then you have to come up with a better theory, and you have to come up with a disappearing world's theory where you get rid of the other worlds. And you can do that. People have done that. I mean, I'm saying it in a sort of facetious voice, but it is, in fact, an ongoing research program to do exactly that. And the

problem is, there's a couple problems. One is, it is inevitably more complicated, right? I mean, you're adding something to a very clean, crisp formalism to get rid of parts of it that you find distasteful. And Number two, it's hard to make it work. Everett is very plug and play, and especially when you go from a quantum mechanical theory of particles to a theory of fields, and then to a theory of quantum gravity, which was his initial motivation, ever adding in quantum mechanics is perfectly happy doing any one of those.

Whereas when you try to mess with it, adding more variables or adding more rules or whatever, you kind of find you have to mess with it in different ways for every version of the theory. and who knows what will end up being. So both from a philosophical point of view and I think from a physicist point of view, the simplicity and success of the Everett interpretation speaks to not working so hard to get rid of the other worlds.

So I think that you're responding to sort of an implied criticism in my question, which is about the nature of what is real, which is totally reasonable. And, you know, I think that a lot of people argue that if you can't measure it directly, if you can't interact with it, then it's not real physically, that it can only be real philosophically. And I think that your response is probably, if it's required by your theory and your theory is

the only one you have that describes the what you can observe, then what's required by your theory but not observed is still real. Is that fair? Yeah, I think that's exactly right. And honestly, if all you wanted to do was to say, I believe everything that Everett says, but I don't believe the other worlds are real, you know, knock yourself out. It's a free country, right? Like, I don't know what you get from that. I think if you face up to that perspective, you're going to have to change

the physics. You're going to have to change the equations to literally get rid of those other worlds and you're going to get in trouble doing that. But if it's just kind of an attitude, like, I don't care about the other worlds because I'm not in them, then that's fine, whatever. I do think, you know, one thing just to put on the table that maybe we'll get back to

later is that it's not just philosophy. You know, I really do think that one of the reasons why we still struggle to understand quantum gravity, for example, as a field of theoretical physics, is exactly because we have not struggled hard enough to understand quantum mechanics. And I think that rather than sort of putting the heads in the sand

and denying the existence of these other worlds, if you do take the formalism seriously, it provides new angles of fruitful approach to long-standing problems in physics. So, you know, it's a reason to not give in to your first input, pulse to be worried about all those other worlds. Well, we were chatting with Carlo Rovelli a few weeks ago, and he said that every interpretation of quantum mechanics has a cost.

And I think that a lot of people would see, you know, these infinite other universes as maybe like a cost of the many worlds interpretation. But to me, it's exactly what I got into physics to do, is to blow my mind and shake up my intuition about the universe. I don't get into physics to have the universe say, yeah, Daniel, what you thought about the universe? That's basically it. I'm hoping to peel back a layer of reality and see something shocking, which at first is difficult

to accept because it's different from my intuition, but that eventually guided by mathematics, I can develop some new intuition and be like, wow, the universe is different from what I imagined and it works in this incredibly beautiful way. So to me, it's not a cost. It's a feature. It's the goal of digging into quantum mechanics. But my question is, you just said that we've been hesitant to dig into the foundations of quantum mechanics. Why do you think that is? Why do you think that for such an important problem at the core of modern physics that progress has been so slow. Why haven't we taken this question more seriously? I think it's a large number of reasons.

And this is a very, very good question also that I've talked about and other people have talked about, but it's sort of a more sociology, psychology, history question, right? So it's a little unshakier ground here, so forgive me. But, you know, part of it is just that we don't know how to answer it, that if there are different competing versions of quantum mechanics that are well-deflicts, find physical theories. So, you know, pilot wave theories and I guess Carlos relational quantum mechanics, et cetera, there are various spontaneous collapse models on the market. And these are real physical theories. The problem with Copenhagen, this is not a real physical theory. It just

doesn't answer certain questions about what actually happens. But once you like put on your big boy pants and actually make a theory, then you can make predictions with that theory and you can try to experimentally test them. So I think that for a long time it was just thought to be not very fruitful to think about these ideas because we didn't know how to get any experimental data about them. And the other aspect is,

you know, we had other things going on. Physicists are very good at, you know, pushing forward in directions they can make progress on. So we're talking about the 30s, 40s, 50s, right? Like, there were particles

and nuclei to understand. There were bombs to build. There were superconductors to construct and quantum field theories to invent and renormal. And, you know, it goes on and on, right? So there's plenty of work to do that you could connect directly to experimental progress.

So it was kind of okay to push the foundations of quantum mechanics into the background. That's sort of like saying, you know, my debit card still works, so I don't need to check my balance because probably everything is fine. That's right. I mean, that is part of it. But also, you know, it's very common advice when you have an enormous task in front of you to first do the parts you can do rather than fretting about the parts you can't do. I think that what has changed recently is number one.

technology has grown to the point where this idea of a division between the classical world and the quantum world as part of the fundamental description of reality has just become increasingly untenable, right? We can make much larger quantum systems than we could back in the 30s that are in superpositions. And we need to deal with the reality of entanglement and so forth when we build quantum computers and things like that. And the other is that this, you know, enormous progress we made on understanding particle physics and field theory, has slowed in the past few decades. And we're in this weird position

where we built these amazingly successful theories that fit all the data, but we know they're not the final answer, right? Because gravity is not included because there are these naturalness problems, etc., etc. So one strategy is just to stubbornly bull forward using the same tools we've used before, but another strategy is to take a step back

and say, okay, maybe we have to think fundamentally in a different way about these questions to make progress on them. and thinking about the foundations of quantum mechanics plays into that strategy. So I guess it's time to check the balance, huh? And we've got to figure out what's happening down there in the basement. We keep getting turned down at the ATM, so yeah.

So then digging deep into what it means, this many world's interpretation, the many worlds interpretation says essentially that none of these universes are special or different, that these branching aspects of the way of function, and it sort of avoids the collapse question that way. But I can't get around what you said earlier, which is that you're redefining what it means,

to be me, right? Because this universe does feel special to me. I mean, I'm in this one. It's the only one that I can interact with. Is that sort of a naive objection to the many worlds interpretation to say that this one must somehow be different? Can you swat that away by just saying, well, the other Daniels also think that they're the only one who interacts with the universe? Pretty much, yes, that is exactly how I will swipe it away. The relevant anecdote here that Everidians love this anecdote, so I will just share it. It is actually about Ludwig Wittgenstein, the philosopher. So one day, one of his former students, Elizabeth Anscombe, was also an extremely accomplished philosopher in her own right. She comes across Wittgenstein, you know, standing in the yard at Cambridge, like looking at the sun.

And Wittgenstein was famously a little idiosyncratic. So she says, what is going on? And he says, you know, why is it that people were reluctant to believe that the earth rotated rather than believing that the sun moved around the earth? And Anskine says, well, it just looks like the sun moves around the earth. earth, right? And Wittgenstein says, well, what would it have looked like if the earth rotated? So the point being that the question you should be asking is not start with an impression, I see the sun moving, and then construct a theory that fits most closely with that immediate impression. The strategy should be construct theories and then ask what observers within those

theories would observe. And if it's consistent with what we observe, then it works. So the point is that in many worlds, if there's you right here and then you go and do some experiments at CERN and you observe some particles, the prediction is there are now many, many branches in which the specific pattern of particles in a collision are different in every single branch and the version of you has now seen different things. And all of those versions of you exist and they've seen different things and they all think that they're special because they exist and the other ones are kind of dubious. But there's no pointer that says this is the real, real one, right? There are other versions of quantum mechanics that try to do that,

to try to say, like, this is the real branch and all the other ones are fake. But it would still be true. We, without that pointer that says this one is real, that all of the different versions of you on the different branches would feel equally real. So the experimental, empirical prediction of this theory is exactly what we observe in the world. And I think that should be the criterion for saying whether or not it's an adequate explanation. It is interesting. it does feel somehow like a bit of slate of hand. Like you've taken the fuzziness of defining a classical object in terms of quantum mechanical particles.

And you sort of transformed it into like, well, I'm just going to redefine what you are. You aren't who you thought you were. You're just an element of this quantum wave function instead of being like the holistic version of you. It feels to me like, you know, when you make this step, you have this interpretation of quantum mechanics, you need to say one, what the wave function is, it's real, it's the universe. but also something about like the correspondence between the quantum state of the universe

and your experience of it as an observer. Yeah, no, that's 100% true. And so, again, if we're honest about the pros and cons, the physics of ever-diting quantum mechanics is as simple as it can be, but the philosophy requires some new moves. And I'm 100% on board with people who say, I just can't accept those moves.

Like it's too much. least say it this way, you know, if we think we don't agree on what the correct version of quantum mechanics is, and each of us has our credence for saying, well, it's probably this, but unlikely that, it's perfectly fair for one of the ingredients that goes into your choice of your personal credence to say, this redefinition of who I am is just so dramatic that I'm going to be skeptical of it. Maybe it's true, but I'm going to be a little dubious until I'm forced into it. I think that's okay. And so I think of this, I'm completely acknowledging that the philosophical leaps required by many worlds are substantial. And in other versions, they're just not

there. They don't bother me as much. Like, you know, I think that in physics, we very often come across better understandings of the world, including of ourselves, right? Like, we might have thought back in the day that we were a spirit animating a fleshy machine that housed us and now we think otherwise. I think that's okay. As long as again, as long as the model that we're building, once we understand it, turns out to be completely compatible with the world we observe, then I'm on board. Well, then let's talk about how to use many worlds interpretation as a functioning theory of quantum mechanics, because something that is a bit slippery for me is that in the

Copenhagen interpretation, I know what a probability means. I'm going to do an experiment, and I'm either going to get spin up or spin down. I can look at the wave function. I can say, well, project it against both of these possible outcomes. those give me the probabilities. They just use the born rule. And whether or not I'm a quantum or classical observer is sort of separate from that. But in many worlds, everything happens. And so I can no longer say like, well, the probability of this happening is 40% or 60% because they both happen in some universe. How do you define probability if everything that can't happen

is going to happen? Yeah, I think this is the right question to ask. Like I said, there are objections to many worlds that are not very good objections, that are just misunderstandings. But there are also puzzles or problems or things we got to address that arise only in many worlds that weren't there before. So I gave Everett credit for solving the reality problem and the measurement problem. An equal number of new problems arise. And the reason why I think that's okay is because I can see the solutions for these problems pretty clearly, even if we don't have them completely spelled out. One problem is just, and maybe we'll get to this if you want to, but it's the structure problem, which is, why does the world look so classical to us

if it's really this big quantum wave function? And there's a lot of details in that. And the other one, like you said, is the probability problem. The benefit of Everett is that the underlying equations are lean and mean and austere. So there's no room to say, okay, the wave function evolves, and there's a rule that says the probability for getting a measurement is given by the wave function square. There's no room to add extra rules like that.

So what you need to do is derive these rules. And there are different strategies for doing it. And to be clear, the fact that the probability is given by the wave function squared rather than just by the wave function or by the wave function cubed or the logarithm or whatever, that's not the problem. Of course it's going to be given by the wave function squared. Because the set of numbers, which are given by the wave function squared, are the unique set of numbers that are all non-negative and they add up to one

and they're conserved over time. that's what you want out of a probability. And it's just Pythagoras' theorem, right? The hypotenuse squared is the other two sides squared. That's why you take all the different amplitudes in the wave function squared, add them up, and get one. So that's not the tricky part. The tricky part is, like you said,

why are there probabilities at all? Because it's a completely deterministic theory. And different people have their angles on that. I think that I've solved it, along with my collaborator, Chip Sabins, because, well, we borrowed an idea from someone else I should give credit to who's Lev Weidman, But here's the idea. When you measure that spin,

so there's some amplitude saying the spin is up, there's some amplitude saying the spin is down, and you measure it. And like you say, with probability one, there is now a version of view on the branch where the spin was up and a version of you that is on the branch where the spin is down. But if you be a good physicist and you do all the details carefully, you can say, well, you know,

Everett purportedly explains to me when these measurement occurs. It's just an entanglement process. I can calculate when it happens. And the answer is, it happens incredibly quickly. The time scale for the branching to happen is shorter than the lifetime of the Higgs boson for those particle physicists out there, right? Like less than 10 to the minus 20 seconds.

And so your brain doesn't work that fast. You can't actually know which branch you're on as quickly as the branching happens. So what that means is inevitably, when the wave function does branch, there's a period of time when there are two causes. of you who those, the world is not identical, but those copies of you are identical, okay?

And neither one of those knows which branch it's on. So even if it knows the entire wave function of the universe, there is something about itself that neither one of those copies of you knows, namely which branch it's on. This is called self-locating uncertainty or indexical uncertainty. And in those cases, you know, there's some fact about the world, but you don't know it, what do you do as a good Bayesian reasoner, reasoner, as a good, modern, rational person, you assign credences.

You assign non-negative numbers that add up to one, right, that act like probabilities. So it's a subjective probability. I don't know which branch I'm on, but I'm going to assign a credence. And you might say, well, what do I care about the wave function? I'm just going to assign credences that are 50-50, right? Because there's two options. I'm going to sign equal credence. Turns out that doesn't work.

It's inconsistent because you can then branch the wave function again, depending on whether the spin was up or spin was down. and you have a different number of branches, and now you have to assign like one-third, one-third, one-third, one-third. So the first guy's probability changes, even though nothing happened in his world. So that's kind of inconsistent over time. Assigning the born-rule probabilities,

giving the probability the credence, the subjective probability assigned by the wave function squared, is the uniquely consistent thing you can do in this situation. So, number one, there are inevitably uncertainties, and number two, the uniquely rational way to assign credences to them is the born rule. So then the uncertainties reflect more like our ignorance rather than some fundamental property of the universe.

That's right. And people like me would go so far as to say that's always what you mean by probability. I mean, there's something that happens in the world, but we don't know, so we assign different credences to it. It's a subjectivist, Bayesian version of probability. I see. So then does many worlds interpretation require Bayesian probability

and rule out frequentist probability? It certainly comports way more comfortably with Bayesian notions of probability. So you don't need to be as extremist as I am and think that all probabilities are fundamentally subjective to be an Everettian, but it doesn't hurt. It helps you sleep better at night. Let's put it that way. Let's take a quick brick. Hey, sis, what if I could promise you you never had to listen to a condescending finance, bro, tell you how to manage your money again. Welcome to Brown Ambition. This is the hard part when you pay down. on those credit cards, if you haven't gotten to the bottom of why you were racking up credit

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There's a lot of love. that flows through this place and it's sincere. Now it's a personal mission. I don't have to go to any more funerals, you know. I got blown up on a React mission. I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff.

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And so then are these sort of like philosophical explorations the only way that we can make progress on these questions of the quantum foundations? I mean, if we have two interpretations of quantum mechanics, many worlds and relational, for example, and they both are, you know, actual physical theories unlike Copenhagen, and they both describe everything that we see as observers, then are we just forced to make philosophical choices between them? You know, as individuals, are there no experiments we can do to help us resolve this question? Well, I think there's two answers to that one. One is that sometimes there are experiments that help us distinguish between them. You know, Roger Penrose has been pushing an idea where there are objective collapses of the wave function, where the wave function violates the Schrodinger equation and really does collapse. Other people, there's a famous theory called the GRW theory, Gerardi, Ramini, and Weber,

who have a similar theory with different equations attached to it. And these are experimentally testable, and tests are going on, right? So we're actually doing them. Theories like hidden variable theories, bo-mean mechanics, I think they should be experimentally distinguishable from ever-ready in quantum mechanics, but the proponents of those theories say they're not. So I'm a little suspicious about that, but I haven't actually, I can't put a good proposal on the table

for how to experimentally distinguish them, but I still don't quite believe the standard lore in that case. For things like Ravelli's relational quantum mechanics, understand it well enough to say. I suspect that it will turn out to be fundamentally equivalent to one of the other approaches. Either it's just the wave function and it's Everett with the many worlds or there's got to be some hidden variables or is epistemic. I think it's closest to what we call an epistemic approach. Epistemic approaches, I haven't even mentioned those yet. Those are the ones where they

really say the wave function is just not reality. In Penrose's approach or GRW or hidden variables, the wave function is part of reality. but its dynamics are a little bit different than in Everett, whereas in a truly epistemic approach, the wave function is just a calculational tool, and reality is something very, very different. And that's fine, but then what is reality? And I'm pretty sure, at least my strong belief

from the current state of the answers people give me when I ask them, there's no good theory of what reality actually is in these models. And they're like, wait till later, we'll figure that out. And in principle, that's okay. We can't ask that every theory, answer every question as soon as it's invented, but it's also perfectly fair to be skeptical of those theories while those questions still linger out there. But okay, but the other answer to your question,

sorry that it's taken me so long to get to it, is the proof of the pudding is in the tasting. And if I can make progress on other puzzles in physics by starting from an Everettian perspective and taking it seriously, Whereas my friends who are being epistemic or hidden variability or whatever don't make that progress, then by the rules of physics, I win. And vice versa, right? If the people who are fundamentally pilot wave theories or epistemic people make progress that I don't, then they win. That's perfectly fair. So in this situation where we're not sure what the right answer is, then by all means, let people do research in their favorite areas.

And whoever actually discovers something interesting will get the credit. I like this idea of measuring competing theories by how much progress you can. make sort of theoretically or philosophically to show that it's like a, you know, a functioning working fertile intellectual playground. But you raised this interesting question of, you know, what reality is. And I want to come back to something you mentioned earlier, which is why the universe, if it is quantum, if the universe is a wave function and there's quantum particles and everything's governed by the Schrodinger equation, why it doesn't feel that way to us? You know, why we have this emergent experience, which is so drastically not quantum. Is that something

we can ever grapple with or is it just like other emergent phenomena like asking like why is there ice cream at some points in the universe and not other points? I think there's actually again two aspects to this. There's sort of the philosophical aspect and the physical aspect. The philosophical aspect I don't have much to say about which is just in a world like ours. Why is it that the idea of a self, the idea of an agent, the idea of a conscious creature is attached to just one branch of the wave function at a time. I already mentioned the fact that, you know, fundamentally the different branches don't interact with each other. So if you tried to say, well, I'm going to treat reality to me as two of the branches, not any of the others. We ignore the others. I'm going to

take these two. Well, I think that someone would say, yeah, but you have two things that have literally no impact on each other. The analogy I use in my book is, what if there were a ghost world? What if there's a world that it was sort of, you know, the same shape as the earth and in the same physical location as the earth and space, and there were people on it and they talked to each other, but there was zero interaction through any force of nature or any other kind of influence between us and the people on ghost world. It just doesn't make sense to call them part of the same reality, right? I mean, they're two worlds for all intents and purposes, certainly. So that's the sort of philosophical move. I think that we don't have, or at least I'm not aware

of, a once and for all definition of what a world is and how you should divide up reality in that way, but that's the rough idea, you know, a set of things that interact with each other is a world. But the other interesting question is, you know, I started that by saying in a world like ours, with laws of physics like ours, but okay, what are the features of our laws of physics that allow for each individual branch of the wave function to be mostly classical? You were pretty good at predicting the positions of planets in the sky and eclipses and so forth using Newtonian mechanics long before quantum mechanics came on the scene, right?

So why is classical mechanics a good limit of quantum mechanics, especially given that you're saying there's all these other worlds out there, right? And that's a trickier question. And I think that we're just beginning to make progress on it. And the answer has things to do with ideas like entanglement and decoherence and locality. But fundamentally, you know, that's still a research level problem. Well, I hope that we make some progress on it in the future. Now I want to take a slight turn and ask you a little bit more of a personal question.

You mentioned earlier that not only are you, you know, talking out there in the public about science, but you're actually a practicing physicist. And, you know, in my experience, most people take one of two paths. They're either a practicing scientist or they are a science communicator. You know, I don't know that, for example, Bill Nye or Neil DeGrasse Tyson are still publishing papers. But you have kept your feet in both worlds. Has it been a challenge for you to remain part of the scientific community and maintain that credibility while also being a public intellectual? Yeah, there's sort of two aspects to the challenge. One is it's a lot of work to do all these things to write papers

and to advise grad students while also having a podcast and writing books and so forth. But you know what, look, to be honest, it's not that much work. And I can compartmentalize it pretty easily. And it's fun for me. Like, I get to do these things that all are individually very fun. And my personality is such that I like being able to switch gears and do different things at different times. I would get frustrated if I did it do the same,

exact kind of thing every day. So this is a good way to do that. The other aspect of the challenge is, like you say, the word credibility. Like, what about how other people think of you? And yeah, that's absolutely a challenge because, of course, two things. Number one, public outreach and communication is itself undervalued and or devalued, depending on who you're talking to. Like, it's considered to be a waste of good brain CPU cycles that you could be using doing research, right? And doing research is what really matters. And number two, like you also imply, there's this idea that even if you think that outreach and communication are valuable, it's very difficult to imagine being productive in both spheres of both doing research and doing those kinds of things.

There are famous counter examples, Carl Sagan, Stephen Hawking, et cetera, Stephen Weinberg, who recently passed away. But they're so rare that people almost don't take them seriously, And especially, like, there's the feeling that you should first become a super successful researcher, and then you'd be allowed to do a little bit of writing books and things like that. I mean, Stephen Hawking invented black hole radiation long before he wrote a brief history of time and so forth. And Carl Sagan, on the other hand, never got elected to the National Academy of Sciences

because people thought that he spent too much time doing outreach work. So that's a challenge, but, you know, I'm too old to worry about that these days. It has impacted my career in very tangible, definite ways. But I'm still having fun doing what I want to do. So there you go. Well, that's great to hear. I experience a little bit of that also when I talk to people in my, you know, card-carrying particle physics world. And they ask me, oh, are you still doing research now that you're doing outreach?

And so to remind them, it is possible to do more than one thing. So what's interesting, let me just put it this way. Because I like this analogy, physicists are not so narrow-minded that they won't allow their colleagues. to do anything else. Like if you are a ski jumper or a professional unicyclist or whatever, or not professional, but amateur unicyclist in your spare time,

other physicists would think, oh, that's cookie and fun, good for you, and you're probably also still doing research. But there's something about writing books and giving talks and making videos and so forth that is different than writing unicycle or being a ski jumper

because that's the kind of thing that people think, well, you should be using that effort doing research, right? like your unicycling is just a different kind of thing. So we don't think that takes away from your research. An enormous number of professional physicists are rock climbers and mountain climbers, right? You probably know many yourself, for example.

That's fine. But if instead of going rock climbing, you write a book, that's actually accounts against you. And it's just a weird thing in my mind, but it's a very definite syndrome. That is strange. And, you know, it's interesting to explore these career paths. And I wonder, is this sort of the path you, envision for yourself. Like, if you could go back in time and describe your life now to fresh-faced assistant professor, Sean Carroll, one year into your gig at University of Chicago,

how do you think that Sean would react? Well, the specific twists and turns of my career were not what I predicted or wanted, but the ending point is pretty close, you know. I always wanted to be a broader intellectual contributor than just a narrow research physicist. You know, since I was a kid I wanted that and that was always the plan in some sense and I was charmingly naive about how happy academia would be to receive such a plan but it was always the plan and I do believe that the right way to do that is to first become an expert at something right like you don't start out as an expert in everything you know you better get good at your research and your PhD project and then sort of branch out after that but you can't predict all of the the ins and outs but maybe I would have

done things differently if I had a crystal ball and could predict the future. A few tweaks here and there would have helped. But, you know, I think I can still hold my head high about the choices that I made along the way. Well, what advice would you give to young folks now who are excited about science communication? In my experience, also, I followed a fairly narrow path and got tenure as an experimental particle physicist before trying to branch out to outreach other types of things. But I see graduate students in my own group doing outreach. And I wonder, you know, if I should advise them. Look, the field is not friendly to this kind of breath at this age. Wait till you get tenure or if I'm telling them not to be their authentic intellectual selves and I should

encourage them and the field with it to grow and accept this kind of activity. What would be your advice or what is your advice to your students when they try to emulate the full breadth of your activities? It's enormously good and important question and the answer is not obvious, but I can always fall back on the maxim that I should tell the truth. So I can tell the truth about factual statements about the world without necessarily coloring them by normative statements. So rather than saying, don't do that until you get tenure, I can say, look, it's great that you like doing a lot of different things, outreach, and so forth. I have one recent student of mine who is actually super duper successful hot property in the job market, wrote a musical and performed

it at Caltech while he was in graduate school. So he did okay. But the point is that it's hard to become a professional academic, right? The numbers game is bad. I'm at Caltech and I tell my students that if you're at a place like Caltech or Harvard or whatever and you get a PhD, maybe one in four of you will eventually be tenured professors

in that field. I don't know the exact numbers and it will depend a lot on subfields and where you get your PhD and so forth but the numbers are against you, certainly. And the thing you can say with a good amount of confidence is that all else being fixed, doing

outreachy things will lower the percentage chance that you will someday become tenured academic. Now, you might still decide to do it, and that's great, but I want you to go with your eyes open, right? Like, I sometimes get in trouble by being a little bit too candid with my students because a lot of my colleagues are like, no, they're young and impressionable.

We have to, like, juice them up about physics. And I'm like, well, yeah, but a lot of them are not going to end up being physicists. And I think that I love the idea of going to grad school. I love the idea of getting a PhD. And I think it's intrinsically worthwhile thing. And I'm not going to discourage anyone from doing that because it's a matter of intellectual growth and so forth. I think the current system where you then have to do

somewhere between five and ten years of postdoc afterward is not ideal. But, you know, if you want to get a PhD, then I think that's the best thing in the world. And then you should know what the chances are and what the different aspects are that affect your chances along the way. And then you make your own decisions. Well, I agree with that.

And I hope that other folks out there can see that is possible to be an academic and to do scientific communication and outreach, and that encourages the community to accept that and to broaden our concept of what a healthy physicist is. You're allowed to also be a unicycler or to talk about science to your friends and neighbors on the Internet. All right, we've taken enough of your time. Thanks very much for joining us and for telling us about your vision of quantum mechanics and explain to us all the crazy, mind-blowing ideas involved in the many world's interpretation. It's been a pleasure.

It's very fortunate that we had to think about these things. So thanks for having me on. All right. Thanks very much. 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. Hi, it's Honey German, and I'm back with season two of my podcast. Grasias, come again. We got you when it comes to the latest in music and entertainment with interviews with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition. I haven't audition in, like, over 25 years.

Oh, wow. That's a real G-talk right there. Oh, yeah. We'll talk about all that's viral and trending, with a little bit of cheeseman and a whole lot of laughs. And, of course, the great Vibra. you've come to expect. Listen to the new season of Dashes Come Again on the I-HeartRadio app, Apple Podcasts, or wherever you get your podcast. Do we really need another podcast with a condescending finance brof trying to tell us how to spend our own money?

No thank you. Instead, check out Brown Ambition. Each week, I, your host, Mandy Money, gives you real talk, real advice with a heavy dose of I feel uses. Like on Fridays when I take your questions for the BAQA. you're trying to invest for your future, navigate a toxic workplace, I got you. Listen to Brown Ambition on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. It's important that we just reassure people that they're not alone and there is help out there. The Good Stuff podcast Season 2 takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick as they bring you to the front lines of one tribe's mission one tribe save my life twice welcome to season two

of the good stuff listen to the good stuff podcast on the iHeart radio app apple podcasts or wherever you get your podcast this is an iHeart podcast