Why does measuring a quantum object change it?

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Speaker 1: Once upon a time, physicists seemed to have the world figured out almost There were one or two loose threads to be tidied up, but a solid idea about how the universe worked had emerged, and it made sense. At a particle level, physicist thought the universe worked basically the same way it did at the human level and at the planetary level little balls moving around, orbiting each other, occasionally even careening off of each other. How wonderful and symmetric and symbol it seemed that the same concepts worked from the very small to the very large. What a deep, satisfying truth about the universe we had revealed that explained almost everything other than a few pesky experimental results, And then those remaining loose threads unraveled the whole picture. To explain those weird experiments, we needed to break that symmetry and come up with a new vision for how the microscopic world works. At the smallest level, the world wasn't made of little spinning balls, but of something new and quite alien, quantum objects that followed a weird set of rules that gave headaches to even the geniuses of the twentieth century. So then, what is real. Is the world made of stuff that moves through space like basketballs and planets? Or does it follow some crazy new, weird quantum rules. And if the quantum rules are real, does that mean that the universe doesn't make any sense deep down? Hi? I'm Daniel. I'm a particle physicist and I'm desperate to know what is real about the universe. And with me today is a special guest astrophysicist Adam Becker, and an expert on reality, given that he's written a wonderful book about quantum mechanics titled What Is Real? Adam, Welcome to the program. Say hello, Hi, Yeah, thanks for having me Daniel. This is a lot of fun. Well, I'm especially pleased to have you on today because when listeners right in and ask me what they can read to get a better understanding of the thorny problems of quantum mechanics, I always recommend your book. So thanks for joining us and for helping us unravel a little bit about what we know and what we don't know about the universe. Well that's great. I'm really glad to hear that you're plugging my book And welcome everybody to the podcast. Daniel and Jorhigg explain in the universe where we try to do just that, embrace our knowledge and admit our ignorance. We do a deep dive into what things really mean and admit when words and science are being used to describe our lack of knowledge rather than our actual understanding. We're here to explain all of it to you, and on today's program, we're gonna attempt to do something very hard, to tackle one of the trickiest topics in modern physics, the meaning of quantum mechanics. But our goal is to find the solution today, but instead to help guide you through the current situation, the status of our knowledge and ignorance, to show you where the tricky bits are, why they have been so persistently tricky, and what the various ideas are for making progress. I mean, we know that quantum mechanics works right, the math is correct, it can predict the results of experiments with incredible precision. It seems like our universe is quantum mechanical. But we want to know what that means about the universe. We want to know what is real to me. That's what physics is is explaining the unknown in terms of the known. Is grappling with it and getting our intuition around it. So my first question to you, Adam, is do you think that's possible? Do we even have the tools, the mental tools to explain and understand something this alien, to understand what is real? I mean, hopefully, because that's the short answer. The longer answer is, you know, so far we seem to be doing a pretty okay job. I am certainly open to the possibility that the human mind is just not capable of comprehending some basic features of reality. On the other hand, you know, throughout the history of science, we've got a pretty good track record of you know, banging our heads against a problem and eventually coming up with a pretty good theory of what's going on in a pretty good mathematical, you know, description to go along with that theory. Quantum mechanics, of course, is deeply strange, but there's no rule against reality being strange. Right. In fact, it be kind of weird if the world we're not weird, be disappointing, yeah, exactly. Yeah, And also, come on, have you been here? Have you seen this place? Of course it's weird, but you're toutings are the history of our accomplishments, and those are wonderful, but a lot of those are limited to sort of the the narrow regime in which our intuition works, right, because we grew up experiencing at things, These are familiar questions we have. Why does the thing roll down the hill? What are these stars moving around us? Does that necessarily mean it's possible to like extrapolate to other weird realms where just have not grown up with any sort of natural experience. Yeah, I mean not necessarily. It's possible that we'll hit some sort of limit. I'm just not convinced that quantum mechanics represents that limit. I mean, first of all, we have this phenomenally accurate and precise theory in quantum mechanics and quantum physics more generally, you know, arguably the most accurate scientific theory ever in terms of, you know, how well it matches experiment. And it's not as if we've looked at the theory and thrown up our hands and said, you know, this is impossible to understand. We have, you know, mathematical tools, but there's no way that the human mind can comprehend the reality behind the math. If anything, it's the opposite. We have too many ideas about how to understand the reality behind the math. So that doesn't sound like a failure of human intuition to me, at least not yet. You know. Yeah, I'm totally open to the idea that we'll get there at some point. All right, Well, you sound like a supporter of quantum mechanics. In fact, it sounds like you're a shill for Big Quantum. You know. I don't think anyone has ever accused me of being a shill for a big quantum before. But I can't say that you're wrong. I'm just not exactly a paid shill. There's no money in it. Oh you're not getting your checks? Oh man? Yeah, well I not from big Quantum, just from the publisher. Well, we can't tagle all of qualum mechanics in a single episode, so today I want to focus on one issue in particular, which is I think at the heart of it all and sometimes known as the measurement problem. So on today's program, we're asking the question what makes the wave function collapse? What is measurement in quantum mechanics? And Adam, before we dig into it, we do something fun on the program, which is that I ask our listeners to answer this question just to get a sense for like what out there are people thinking, How much of a grass to people have on this question. It helps orient us to know where to aim our answer. So I have for us some clips from listeners, and if you they're listening, would like to participate for future episodes to answer really hard physics questions with no preparation. Please write to me two questions at Daniel and Jorge dot com. It's a lot more fun than it's outs. Here's what our listeners had to say. Okay, I have I've been learning about wave functions and quantum physics and I think I know that's a really important part of measuring something, and I have heard about it collapsing, but I'm not sure what makes the collapse. I'm going to guess that what makes a wave function collapse is when um something's measured. I would say, once the energy in the wave is reduced to a certain point, the wave can no longer support itself and it collapses. I'm just going to guess here, probably something that intervenes from outside so on outside interaction. Alright, So those the answers from our awesome listeners Adam, what do you think about those speculations. I mean, they mostly sound like the kinds of things that you would see in a quantum mechanics textbook actually, which is to say kind of fake. Right. There's a lot of talking there about like, well, I'm not sure something about measurement, but I don't really know what that means, and like that's really the heart of the problem, right, Like nobody really knows what we mean by measurement or by observation. So we like have a word for it, we have a phrase for but we don't really know exactly what that means is happening deep down. Yeah, that's exactly right, all right, So I think maybe we should start just by orienting ourselves, just by getting sort of like using the same vocabulary and making sure everybody out there knows what we're talking about when we say the wave function, Can you give us a short definition of what the wave function is and why it's so important to this question. I mean, there's a sense in which the question of what the wave function is kind of what's at the heart of all this too. But yeah, I can take a stab of that's so in sort of classical physics, the physics of Isaac Newton, the physics of everyday life, the physics of you know, billiard balls and car crashes. The way that we describe where things are. We usually use three numbers to describe where something is, or at least where the center of mass of something is. You know, we say, okay, well, here's where it is in the X, y and z. Here's how high off the ground it is. Here's how far in front of me it is, and here's how far it is off to one side. And that's all you really need to describe where something is, because we live in a three dimensional world exactly right. But in quantum mechanics, if you want to do the same thing, if you want to take all the information we have about where something is, like if you want to talk about where an electron is, you need more than three numbers. You actually need on infinity of numbers spread out across all of space. And that's the wave function. So the numbers are higher in some spots and lower in others. And usually what we say the wave function represents is that it's to do with the probability of finding the electron in a particular spot when we look, which makes it sound like the wave function isn't really a thing in nature, and it's just a lot more about our information. Yeah, so hold on it. Let me ask you about that, because you're making an analogy to a classical particle where we're talking about like where it actually is, and you're saying that you need more information to talk about where an electron is. But now you're describing the wave function in terms not of where the object is or what it is, but about our measurements of it already, sort of like we've already sort of snuck into the electron. That's exactly right. I mean, there's there are I wasn't kidding when I said that, you know, the question of what the wave function is it's sort of part of the controversy here and part of what's at stake. We'll see if we get past this one question and the whole thing exactly. Yeah. I'm not convinced succeed, but yeah, I mean the reason I say, oh, this is sort of the quantum analog of you know, the three numbers describing where something is in Newton's physics is that, you know, it sort of plays the same role in the same sorts of physical laws in quantum physics. So the question of whether the wave function is a thing out in the world or if it's just something about our information about stuff out in the world is you know, a live question and a subject of active debate. But it's not just as simple as saying, oh, it's just our information about what's out there. It's just that in the most simple way imaginable because wave functions, I mean, like the name sort of implies the wave functions. Wave they behave in ways that we actually tend to associate with physical objects. They undulate smoothly, the numbers change smoothly from one place to another, they change over time, and they can even perform some of the same tricks that waves perform. You can send them through a narrow slit and they'll defract outward on the other side. They can interfere with themselves and cause you know, patterns of light and darkness on a photographic plate, that sort of thing. So we call it a wave function because mathematically it's described by similar equations to describe other kind of classical waves were familiar with, that's right, yeah, And it also performs some of the same tricks as classical waves that were familiar with physically, so you know, we don't tend to think of our information about a thing as doing that. And it doesn't mean that it can't. It just means that if we want an account of wave functions in terms of our information about the world, as opposed to saying, you know, the wave functions actually a wave that's out there, we need to do some extra work. So you're saying the wave function tells us how to predict the outcomes of our experiments, like if I'm going to find the electron over here, or if I'm gonna find it over there, or if I'm gonna find it's been up or going to find it's been down. It tells me what I'm likely to observe, but we're not sure whether it's like actually real and distinct separate from our ability to measure it and interact with it. We don't know sort of philosophically, whether we can take that step. Yeah, yeah, I mean, there are theorems that suggests that there's something like the way function out in the world, but it's not. You know, this is an open question and has been since the beginning of quantum physics almost addrey years ago. And I think that's sort of one of the most fascinating points. I mean, you talk about classical physics, there's no distinction there between the thing exists and we measure it to exist there. It's just like those two things are just part of the same idea. You see the ball there because it is there, right, and it would be there if you weren't here to look at it as all this kind of stuff. So I love these discoveries and physics that make us sort of crack open two ideas we thought were so naturally entangled, so like deeply connected, and now we realize there's a possibility for these two things to be actually separate and distinct. Observing something is there doesn't mean that it is there some deeper sense. It's like totally crazy. Yeah, no, it's it's absolutely true. And one of the strange things about wave functions and measurement is just the idea that, you know, measurement and observation are coming into what's supposed to be a fundamental theory at all, because you know, measurement is not a well defined thing. What do we mean when we say measurement? Right? And I think that a key concept we need to tackle before we get into measurement is this question of superposition, right, because quantum mechanical objects can do something that like our baseballs can't do, which is that they can for a long time, have two possible outcomes and maintain those two possibilities simultaneously. Right, So the wave function can incorporate various possible outcomes inside of itself. Yeah, now that that's exactly right. If you send uh ping pong ball into a maze, it's just gonna go down one path in that maze and you can watch it take one path. But if you send an electron or a photon or some other you know, tinique wantum object into you know, some sort of maze, like send a photon into a set of mirrors and prisms on an optical bench, you know, in the basement of a physics department somewhere. It's not going to take just one path. Quantum mechanics says, no, it's it's going to take a superposition of all of those paths. It's going to go down two or three or four different paths at the same time, or as wave function will. But then when we measure it at the end, right, we say, all right, I'm going to put a device in here. I'm going to ask the question, which one did it actually go down? I can't measure all those paths, right, I get one answer. That's right, you get one answer. If you put a detector on one of the paths, then you'll get an answer saying you know, yes it went down this path or no it didn't. But if instead you set up your optical bench, your maze, whatever you want to call it, or your double slit experiment so that there's an interference pattern on the other side, you will get interference. So when you're not asking the question of which way the photon went and when it went into your setup, it sort of goes down all of those paths and then interferes with itself. But if you ask weight which way did you go, well, then it says, oh, no, I went this way, and then it doesn't interfere with itself. And so we know that the wave function does this superposition thing having two possible outcomes at the same time because we see the interference, that's proof of superposition. But then when we try to measure the photon, the superposition sneaks away. And for you, is that like the clearest encapsulation of this question, this measurement problem, like how the wave function goes from having like various quantum mechanical possibilities for the paths of a photon to basically picking one. How the universe like ends up rolling that die choosing this path or that path or the other path. I think it's a very clear example of that. The way that I usually talk about it is a little different, you know, I I say, Okay, this is one example of the measurement problem, But in general, the measurement problem is more about that sort of undulating property of the wave functions that I was talking about before. Right, we have this very nice equation, the shroding Er equation. It's a law of physics. Right, we think of it as a law of nature, and that describes how wave functions wave. Right, it says that they always wave sort of smoothly and evenly, and you know, they don't make any abrupt transitions. But sometimes the shroding Er equation is temporarily suspended. What you can't do that you can just pause the laws of physics. Sometimes it says, if someone just hits pause on the shrodding Er equation and instead you have to use this other thing called the Born rule, which says, okay, take the wave function and look at all the different possibilities. It gives you for the way that your experiment is going to turn out, and that gives you the probabilities for the different outcomes. That's what the wave function does. When you use the Born rule, And then whichever one you actually serve, cut out all the other parts of the wave function, that's the only one that's left. All the others just instantly go to zero and then violates the shorting your equation, and that violates the shroting your equation. The shrodding your equation doesn't have anything in it like that. And then when you stop looking, that's what happens when you make a measurement. And then when you stop looking, the shorting your equation applies again. And so then the question is, okay, well, so if you have these two different rules and they contradict each other, then first of all, why And second we better be really, really really clear about when to use one and when to use the other. And this is the collapse of the wave function. And so the usual answer to the question when do we use one and when do we use the other? When does the wave function collapse. The usual answer that is, when we make a measurement. And so now the fact that the word measurement is kind of vague goes from you know, a little troublesome or annoying to a really serious problem. Right, So that's a great encapsulation of the question we're facing. Like, we have this beautiful equation, the shortening your equation that tells us how this wave function moves and changes and squishes through the world, and we verified it to zillions of degrees of accuracy, except that, as you say, sometimes it seems to be ignored and sometimes it doesn't seem to apply. And so we have this question of like how a wave function goes from this smooth under lending property to like collapsing into just a point when we make a measurement, what that measurement even means. So I want to dig into that in much more detail, but first let's take a quick break. All right, we're back and we're talking to ask ify is this Adam Becker, author of What is real about the nature of the quantum a function and what happens to it when we look at it? And I have this quote that I love from Bell who says either the way function as given by the Shorting your equation is everything, or it's not right. What do you make of that? I see what he's getting that there, and I think he's kind of right. I mean, there's got to be something more that tells us, Okay, when do we apply the shorting our equation and when don't we because otherwise, you know, uh, And I think Bell said something along these lines as well. We're stuck in this situation where these laws of physics that are supposed to be fundamental or just kind of hopelessly vague. So we have to take the Shortinger equation and the wave function, all of which seems wonderful and perfect and beautiful, and we love it, and we need to add something else. We need to say. Plus, there's this other mechanism that makes things collapse. And this, I think is where people disagree how to describe this weird combination of ideas and sort of a holistic concept. And so first let's talk about maybe the most mainstream way to a hack, this the most common, the one that people most read about in their textbooks. And then I think a lot of our listeners referred to which is basically the Copenhagen interpretation, where you add something to the shorting your equation that makes the wave function collapse when you look at it. So how does that actually happen? Like, what do you add to your system, you know, mathematically to make that happen. You can't just have like a thing that says and by the way, if there's a measurement, I mean, doesn't everything have to be like you know, written down in some sort of equation. The thing is it is just kind of added on. They do basically just say, yeah, use the shorting your equation, except when a measurement occurs, and what a measurement occurs, use this other thing instead. Use this collapse roule called the Born rule, named after Backs Born, you know, one of the architects of quantum physics. And it's just got a completely different mathematical form and it's said, you know, it says, okay, you know, like I said before, it lets you use the wave function to calculate probabilities, and then says, and then the rest of the wave function other than the part that you saw collapse. So if you're looking for an electron somewhere and you've got the wave function of the electron, which is, you know, this sort of nice smooth thing that's maybe got three or four different peaks. You look for the electron, you find it near one of the peaks. The Born rule says, okay, well, then what happens to the electron's wave function is you know, now it just has one extremely narrow peak exactly where you found it, and it goes to zero everywhere else. And then when you stop looking the shroudding, your equation just applies again, which is kind of incredible. And it requires us to sort of insert our selves into the equation, right, it needs like the measurement part is when we are interacting with this thing. And I think the most common question is what counts as a measurement? If I put a particle detector in there but don't turn it on, does it count? If I turn it on but don't connect it to my computer? Does it count? If I hook it all up but then don't look at the computer screen? Does it count as a measurement? And what's special about my part goal detector? Isn't that particle constantly interacting with other things? Doesn't that count as a measurement? So can we tackle it by thinking about so the quantum particles we're using to make the measurement, you know, because we don't just like look at an electron, right, we like bounce a photon off of it or something like that. Yeah, yeah, I mean the Copenhagen interpretation sort of lives or dies on on what you mean by measurement, right, And so it is tempting to say, okay, well, maybe we can get around this problem by saying that, you know, the measurement devices itself also quantum. But the problem is that if you use the shrouding your equation, you know, wave functions and the mathematical machinery of quantum mechanics to describe the measurement device as well, then all that happens when you make a measurement is you go from your measured system in a superposition and you're measuring device sort of ready to make a measurement, to a state where both the measured system and the measuring device are in a superposition. And so you know, if you wanted to say, okay, did the electron go left or did it go right, and then you use your measuring device to answer that question, then the shrodding your equation says at the end of that measurement. What you're gonna end up with is a superposition of the electron went left and the measuring device says left, and the electron went right and the measuring device says right. So the measuring device has become sort of part of the experiment exactly. Yeah, And so that's not gonna get you out of it. Eventually, you have to, according to the Copenhagen interpretation, you have to say, well, then a measurement occurs, and so the shrodding your equation doesn't apply anymore, and we have to use the collapse rule, the born rule, and so you really need to get comfortable with whatever it is that a measurement means, which the Copenhagen interpretation is kind of famously vague about. So let's break that down a little bit more. So. We have our electron and we probe it with the tip of some very very narrow tool, and the tip is just like a single particle that's interacting with the electron. Now we might say, okay, the tools touching the electron, so it's being measured, it should collapse. It should just collapse right there. But somebody else coming in and looking to the experiment. The same way could say no, no no, no, I think the system is the electron plus the tip of the tool, and the rest of the tool is the measuring device. So the tip of the tool plus the electron, that's your experiment. That's what you're probing. The measurement happens when like the next particle over reads off the information from the tip of the tool, And somebody else could come along and say no, no, no, I think the original electron plus the first two things on the tip of the tool are part of the experiment, and the rest of it is the measurement. And you can just basically do this forever, right all the way down the tool, including as many particles as you like, or even including the experiment or him or herself. Yeah, I mean, quantum mechanics says this weird propensity to just sort of generate these sorts of thought experiments like the one that you're talking about that sounds like something out of like ancient Greek philosophical dialogue or something Zeno's quantum experiment exactly. Yeah, I mean there even is a Zene experiment, though not that one. But yeah, the problem here is, you know, it'd be easy to say, when a measurement occurs. If we could say, well, there's a quantum world, and then there's a non quantum world, and when a non quantum thing interacts with a quantum thing, that's a measurement thing. Is nobody believes that anymore because all non quantum things are made of quantum things, right exactly. Yeah, we believe that the world is made of everything in the world is made of molecules, which are made of atoms, which are made of sub atomic particles, all of which are subject to the loss of quantum mechanics. And quantum mechanics doesn't have a real size limit. I mean, certainly there are you know, sizes at which some of its features are more obvious than others. But in principle, there's no limit to the size of a system that you can use quantum mechanics to describe. And we believe that the whole world is made of particles subject to the laws of quantum physics. So yeah, the question becomes if there's no border between the quantum world in the non quantum world, because the whole world is quantum, then when does a measurement occur? Is it when the measurement device touches the object. Well, in that case, does that mean that quantum mechanics doesn't apply to the measurement device? Okay, Well, maybe it's when somebody looks at the measurement device. Maybe it's when I look at the measurement device. Okay, But does that mean that quantum mechanics doesn't apply to me. I'm made of quantum stuff, you know. I'm made of cells, which are made of molecules, which are made of sub atomic particles and so on. You know, there's no reason to think that quantum mechanics doesn't apply to all of the stuff in my body and brain. In fact, every time we've checked on something like that, we've found that, you know, every biochemical process is fully explained by quantum mechanics. All right, but there is one wrinkle there, right, Like you are different from you know, the ascilloscope or the probe using to touch the electron. We think in that you are conscious, right, you are a self aware, you're living, thinking, breathing person. I don't know about the ascilloscope. I didn't invite the acilloscope to be a guest on the podcast to speak up for itself. But you know, I think a lot of people. Imagine that that might be the moment when the measurement happens, when the information like goes through the tool and up the computer and into your brain and it's like known by a conscious observer. What about that? Why can't we use that as a distinction. Well, so there's a few issues there, right. One is, you know, consciousness is something that we don't understand. Well, what do we mean by consciousness? You know, you were sort of getting at that before when you were bringing in this thought experiment of you know, okay, well, when I look, is that the measurement? No? Well, then when you come in the room and you look at me, is that the measurement. This was put together by this famous physicist Eugene Vigner, and it's called the Vigner's friend experiment. And I'm conscious, right, my friend is conscious. So maybe the measurement happens where I look, or is it when my friend looks at me? Or you know, does it have to be a human? What if we put a chimp in there? Right? What about a dog or a crow? Right? Crows are really smart. Hold on, let's break down the Vigner's friend experiment, because I think this is where it described having like essentially, the idea is, you are doing this experiment, you are measuring this electron. You're using a tool, and you do the evaluation and you get a number. Now I don't know the answer yet, and so in some sense I can look at you and say, well, you are a part of my tool. Adam, I haven't asked you yet what happened, So I don't know the answer. So you are still in a quantum superposition of the electron went left and the electron went right. So in that sense, like a conscious person can be in a quantum state, Right, you can have like the wave function of Adam Becker having two different possibilities. Yeah, I mean that's a problem, right, because that that's the sort of thing that feels like you could lead to an infinite regress, right you know, Okay, but then you're in a superposition until your friend talks to you, and then we start having problems like okay, but then, why does everybody agree about the outcome of the experiment? And right, what was it like to be in a superposition? What did it feels What was it like to be in a superposition? Exactly? What does it feel like? I don't think that I've ever been in two places at once, so that I've ever believed that an experiment I felt like I needed to be into businesses but exactly and never achieved. Yeah, And I certainly don't think that I've ever like, you know, looked at the outcome of an experiment and thought, oh, that went both of the two mutually contradictory ways that it could go. You know, the electron went left and it also went right. And I saw the measurement device say left and also say right at the same time. That's not an experience I remember having. And that kind of rules out this notion of consciousness being the threshold, because you can put a conscious observer into your experiment and still have the wave function not collapse until after the conscious observer reports their results. Right, Just at a conscious observer observing something doesn't necessarily make the wave function collapse, isn't that right? Yeah? With an asterisk tell us about the asterisk. Yeah, So, I mean you can set up a system where you've got a quantum wave function and superposition, and you can sort of verify that it's in a superposition, and that verification does not itself lead to the collapse of the wave function. Wait, how do you do that? How do you verify that? Is that by observing interference? Yeah, that's by observing interference. You can see that there's interference. But what you can't do, at least what we don't have the ability to do right now, is put a human in there as part of the system that is exhibiting interference. Right, I see, I can't see the various modes of Adam interfering with themselves exactly. Yeah, but there are other reasons to think that consciousness is probably not what's going on here, or if it is, you need a really good account of how that's possible. Right, Because one of the things that we want to be able to use quantum physics to do, and you know, as a cosmologists by training, this is near and dear to my heart. We want to use quantum physics to describe the very early universe. Who want to do quantum cosmology, and you know that means talking about things like the wave function of universe. And indeed, you know we talk about that when we talk about things like, you know, patterns in the cosmic microwave background radiation, the oldest light in the universe, and echo of the Big Bang. We see the imprint of quantum mechanics in the sky. And now I am going to almost directly quote John Bell. Was the wave function of the universe waiting for billions and billions of years for you know, a paramesium to arrive and collapse the wave function? Or did it need a better qualified observer, you know, someone with a PhD. The first PhD collapse, the universe's waiting exactly. Yeah, I don't think that that's how that works. I don't think that conscious beings are necessary in order for quantum mechanics to work. So if the wave function collapses, and we've really know the wave function is real and exists outside the mind of humans. But if it is real and it does collapse, it seems like it must have been collapsing for billions of years before we showed up. That's right. And if it doesn't collapse, we need to figure out why it looks like it does seem important And I think that you mentioned something really interesting about how we know that asymmetries do exist, right, Like, this question is important not just for philosophers, but for like quantum cosmologists. How do you go from the beginning of a universe where you assume I have a symmetric wave function because anything else would be bonkers, and somehow get asymmetries, right, how do you get a universe that's not exactly the same in every direction? Where do those things come from? Those come from quantum fluctuations, which are quantum collapses, right, Yeah, that's right. Quantum fluctuations happen when you might have equal probabilities to go left or right, but like one of them is chosen. The universe does roll a die. And so the fact that we exist here and not a billion light years to the left is evidence that quantum mechanics does fluctuate, that there are these collapses, that these things are real somehow, Yeah, exactly, or at least that's something like collapse or something that in tates collapse or gives the appearance of collapse happens all right, So it seems like this Copenhagen interpretation is pretty problematic, right, Like there's a basic fundamental unknown in it, like what this even means by measurement. We need measurement to get the way function to collapse, or to get it to look like it collapses, but we don't even really know what that means and what triggers it. So how could this possibly be the most mainstream, core idea in the most fundamental concept in physics. That's a great question, and it unfortunately has a very simple answer. The answer is when people ask these sorts of questions, they were just sort of waved away. It's more complex than that. I wrote a whole book about how this happened. Before I did, I used to say, well, I could write a book about it, and then it turns out that I made good onto that way. Function did collapse into a book, Yes it did. But yeah, the answer is, you know, people ask, well, what do we mean by measurement? We have to get more specific about this. How could this possly be this vague and ill defined and contradictory And the answer that was usually given was it works, so don't worry about it. This was sort of summed up famously by the physicist David Merman as shut up and calculate. That's basically like, don't worry about what it means. It works, it predicts our experiments. Who cares. Yeah, don't pay any attention to the man behind the curtain, just you know, do the calculations, and you'll be able to build most of the technology that the modern world is based on, which is you know, absolutely true. You know you don't need to answer this question in order to do things like design semiconductor transistors and build computer chips and lasers and l ed s and you know, nuclear power and all of the other incredible and you know, awesome in the most literal sense technologies that quantum physics has enabled over the last century. But science and physics is not just about delivering technological improvements for humanity, right, Like, I love that all our listeners can hear this podcast on various devices enabled by quantum mechanics. But I didn't go into physics to like make a better iPhone. I went into physics to understand the universe? Right, So doesn't that really fly in the face of sort of the core mission of the entire discipline to gain some understanding. I completely agree with you, but apparently not everyone does. Alright, Well, I'm glad we agree on that, and I want to dig into some other possibilities, some other ways people have attacked this problem, some crazy, totally different ways of thinking about what might be real about the universe. But first, let's take another break. All right, we're back and we're talking to Adam Becker about what is real and if anything is real and if the universe can possibly make sense at all, and are we even here? And we've been talking about the wave function and how the Copenhagen the mainstream interpretation of quantum mechanics has a basic problem in it and that it can't define what a measurement is. But a measurement is essential to making the theory work, so it can't be the only idea that's out there. Surely people have thought of some other approaches. And so if you're a listener to Sean Carroll, for example, you probably heard him as a big proponent of this ever already in many worlds interpretation. So how does a many worlds interpretation, How does manufacturing billions and billions of branching universes solve this measurement problem? Or does it? That's a great question I want to say before I answer that this is something that I get a lot of email and people asking me about it. Back when you know, in person, back when you know we could do things in person. For whatever reason, a lot of people came away from my book thinking that I agree with Sean Carroll, and I like Sean Shan is a great guy. Yeah, but I think that his position is a reasonable one. But I don't think that it's the only reasonable one. And I think that I am not here as a shill of big many worlds, just a shill of big quantum. Many world is already big. You don't say big in front. Yeah that's true. Now, many worlds, I think is pretty clearly the next most popular interpretation after this sort of jumble. That is the Copenhagen interpretation. All right, to breakdown for us, what is the many worlds interpretation? So the many world's interpretation gets out of the measurement problem by saying, oh, yeah, collapse doesn't happen. It just sort of looks like it does. It takes the sort of infectious property of superposition, where you know, if a measurement device measures something that's in superposition, then it will go into superposition. It takes that property and sort of embraces it wholeheartedly and turns it into a strength. It says, okay, well, collapse never happens. The Born rule is, you know, not a fundamental thing in nature that describes what's actually happening in the world. It's just something we need to make predictions. And in reality, the Schrodinger equation or you know, the relativistic extensions thereof always apply, and so wave functions always just evolved smoothly, which means that when you make a measurement of an electron or something in a superposition. You know, the classic example here is Schrodinger's cat. You set it up with a quantum device where you know, if it goes one way, the cat lives, and if it goes the other way, the cat dies. So when you open the box, you find a cat that is both dead and alive. It's in a superposition of dead and alive. And then you know, in the process of opening the box, you enter into a superposition yourself of seeing a dead cat and seeing a living cat. And then someone else comes in the room and they enter a superposition of you know, seeing you crying over this dead cat and seeing you, you know, playing with the living cat and so on, and this just sort of spreads out into the universe. So you're saying, the way function never actually collapses and picks one of these branches. It just keep branching, and we are only in one of those branches, which is why it looks like it collapses sort of. The question of where we are in the theory is definitely something that's contentious, But I think that what Sean would say, Well, I don't want to put words into Sean's mouth. If I put on my many world's hat and say, Okay, I'm gonna pretend to be a real advocate of many worlds, I would say, no, we're in both branches. But once we split, a quantum process called decoherence ensures that the two branches, you know, once they involve large objects made of many quantum particles, can't interfere or communicate with each other in any way a very very quickly, and so we split. And so you know, if you ask one of the copies of you that is in either branch, hey, how many cats do you see? How many outcomes do you see for this cat? Both of them would say, oh, I only see one outcome. They just disagree about what that outcome is. And so this is why it's called many worlds, because it imagines that multiple versions of the universe are existing decoherently, that they've decohere from each other, and they're all out there. They're real, but they're sort of like not accessible to us anymore because we're only in one branch of the way of function. Yeah, we're this copy of ourselves is only in this branch, but there are other near identical copies in all of the other branches. And so I was trying to lead you down the garden path there, I think to my major objection to the many worlds interpretation, which is like, why are we in this one? Because, as you say, like there's a copy of me in all of those universes that have observed every possible outcome of the Schrodinger's box experiment. But still I'm this one, and I know that in the other universe, the other me thinks that it's that one, and that's fine, But why is it in that universe? And I'm in this universe? Like there is still something special about this universe because I'm in it, right, Well, you know, your copy in the other universe where the cat died would say the same thing. I know, but it also has a reasonable objection, Right, there is still something different about this universe because I'm in it, And there's something different about that universe because it's in that one. It seems to me like does not really avoid this problem because it still takes one universe to be special in some sense, because this is the only one that I'm experiencing anyway. What are your concerns about the many world interpretation. Well, the classic concern about it is that you still actually need the Born rule, right, because quantum mechanics doesn't issue forth certain predictions. It doesn't say this is you know, definitely what's going to happen. It says that sometimes, But most of the time, the predictions of quantum mechanics come in the form of probabilities that says, well, there's a twenty percent chance that this is going to happen, in a thirty percent chance that's gonna happen, a fifty percent chance that that's going to happen, and you know, those are the possible outcomes. But in a world where the shroting Your equation always applies, well, the shroting Your equation has no probability in it. The Shrotinger equation is actually like Newton's physics, and that it's completely deterministic. You know, when you ask, uh, many worlds person, well, what's going to happen? When you open the box. They'd say, well, with one d percent probability, I will split into two copies, one of which we'll see a living cat and the other we'll see a dead cat. That's definitely what's going to happen. And so then you know the next question, which is, you know, what's the probability that you're going to see a living cat because I don't want to get a cat. The answer to that has to be, well, you need to use the Born rule. You need to use this collapse rule, because that collapse rule is how we get predictions out of quantum physics. It's what makes it so phenomenally accurate and powerful. And so you need to figure out, Okay, how do we introduce probability into a theory in which literally anything that can happen does happen. That's a little thorny and one way of going about it. And this is actually a position that I know for sure Sean holds. So now I am gonna like do my best Sean Carroll impression. Sean says that, well, the probability comes from basically exactly your concern that we don't know where in this branching multiverse of worlds we are located, and so we have uncertainty about where we are, and that uncertainty is where the probabilities come from. When we do the experiment and we see an outcome, what we're actually doing is learning where in the many worlds we are. And then you can look at the structure of this multiverse and sort of derive the Born rule from it. You can say, okay, well, this is the right way to answer questions about probabilities. So the problem with the many worlds interpretation, then, is that it keeps the wave functions sort of too long. The wave function itself doesn't actually make predictions, as you say, it can be used to make predictions, but if you just keep the wave function going forever, then how do you actually predict the outcome of an experiment? All right, that's fascinating. Yeah, you need to answer that. And I want to be clear the many advocates of many worlds generally have answers to that, although I don't think that there's consensus around one single answer to that among all of the many advocates of you know, this kind of interpretation. They're aware of this problem and they've addressed it, and they have answers to it. So the question isn't you know how do you answer it? The question is does one or several of the answers that have been provided work? And that is an open question. Awesome, And so before we wrap up, I want to touch on a couple of other ideas. So those totally different directions people are taking about attacking this deep question about the meaning of the wave function and what measurement means. And one of my favorites is this hidden variable theory or the categories of theories called hidden variable theories, because when I was learning about quantum mechanics, I remember thinking like, well, sure, but how do we know it's not like just actually determined by something we're not aware of? You know? It feels like maybe our lack of information, this uncertainty in the universe doesn't just come from inherent uncertainty, but it just comes from like our not seeing the full picture. So maybe there's like something going on behind the scenes that's controlling everything. How do we know that's not the case? How do we know that it's not just like more to the universe that makes it actually deterministic. That's a great question, and you're a really good company there as I'm sure you know. You know, Albert Einstein basically had exactly the same question. You know, it's a myth that he never accepted quantum physics. He knew that it worked, and he he fully accepted that it worked. He just thought it couldn't be the whole story for basically the same reason that you're giving. And in particular, he was very unhappy about the idea that things depended on observation and that you couldn't talk about what was happening when you weren't looking. He thought that this was, you know, as you were saying, just kind of avoiding the whole point of science. Science is about figuring out what's in the world and how it works. And he was also really concerned about the possibility introduced in quantum physics for what he called spooky action at a distance, these long distance connections between objects, and that is actually directly related, as it turns out, to the answer to your question. You know, the question, is there's something going on that we don't know about some you know, hidden properties what we usually call hidden variables of these particles that you know to termine their behavior, and we just don't know what they are. And that's where the uncertainty comes from. The answer to that that we know is yes, it's possible, but you have to pay a price. The price is spooky action at a distance, and that was proven by experiments that were done to test a theorem by John Bell. So we can have a super deterministic universe, but it can't be local. We can't also have like everything we determined about what's happening right here. So yeah, I gotta be like really pedantic and nitpicky. There is actually a class of interpretations called super determinism, and that's something else. But yes, we can have a deterministic universe. We can have hidden variables that determine everything that's going on. But the price we have to pay is that, you know, things that happen right here can instantaneously influenced stuff that happens arbitrarily far away, and do so in a way that provably can't be used to send anything information or material faster than the speed of light. All right, So that's super deterministic with a lower case as and two words, Yes, what's super determinism single where where the capital as is that superman is determining the outcome of all these quantum experiments. Super Determinism is the idea of like a hardcore clockwork universe. It's this idea that, oh, we can explain the outcome of these Bell experiments without sacrificing locality and without you know, without sacrificing the idea that you know, instantaneous action at a distance can't happen. We can also do it without sacrificing the idea of determinism. But to do that instead, we have to say, oh, at the beginning of time, in the Big Bang, a whole bunch of really fine grained information was encoded into every particle in the universe about the outcomes of those experiments, those Bell experiments that would be conducted, you know, thirteen point eight billion years later um, that would arrange for them to turn out in a way that would trick us into thinking that, you know, the universe was nonlocal. So we do these experiments and they seem to suggest the universe is nonlocal, but that's just because they've been cleverly arranged fourteen billion years ago to look that way. Yeah, that's super determinism. Yeah, that's super nuts. Yeah, I'm not super sympathetic to supers, but you know, all these ideas are kind of crazy. All of them have things you might object to. I guess. In the end, the question I have for you is are we going to figure this out? Or how could we figure this out? Are there experiments we can do to figure out which of these things are real? Or is it just going to rely on philosophers smoking bananappeals and organism in their minds? Yeah? I know this is a great question. So the answer depends on what you mean by is there an experiment that we can do? Right? So, if you're asking, is there an experiment that we can do to figure out which of these different interpretations of quantum physics is the right way to think about quantum physics right now? The answer to that is no, because they all give the same or almost all give almost all the same outcomes for all experiments provably. I mean, there are some proposals for solving the measurement problem that aren't just new interpretations but are actually completely different theories. There's a class of theories called objective collapse theories that modify the shrouding R equation. Those can be tested, so yes, some of them can just be directly tested, and those tests are sort of ongoing. But for most of these, like many worlds, interpretation or the best known and most developed of the hidden ables interpretations called Bonian mechanics or depoid bone theory or pilot wave theory, has a few different names, which is, you know, nonlocal. And they think that that's like a good thing about the theory, and that's a whole other story. I'm not saying that to disparage them. I understand why they say that, and I think that that's also a reasonable position. But the point is those two and you know, many of the other interpretations of quantum mechanics are just that their interpretations, they all provably give rise to exactly the same outcomes in all of these experiments. And so if you ask them, well, what's the experimental evidence, they'd say, all experimental evidence for quantum physics, you know, is experimental evidence for this interpretation as well. So in that sense, no, there's no experimental way to distinguish between them. Doesn't that mean we just haven't been clever enough. I mean, if we have seven theories that all fit the data, it just means we need to come up with a more clever experiment that can distinguish between those seven theories right. Otherwise you have to accept the possibility that there could be multiple theories of physics that work perfectly to describe our universe, in which case, like the whole project of physics of coming up with a unique idea to describe the universe and then interpreting what that idea means is sort of cast into doubt. I have some good news and some bad news. Let's start with the bad news. The bad news is all of these theories provably give rise to exactly the same mathematics and provably you know, spit out the same results for the same experiments. And in fact, you can even prove that given the mathematical structure of any scientific theory you could ever devise, there is an infinite number of interpretations that you could come up with to you know, explain what's going on in that theory. So that does sound bad from the perspective of you know, physics and the project of trying to understand what's going on in the world. The good news is that's not at actually how physics works. You know, we don't sit down and say, okay, well, there's an infinite space of possible theories for the mathematical structure that we devised, and so now we need to try to narrow that infinite space. That's not how we do physics. How we do physics and how we do science more generally, as we say, okay, well, look, we have these ideas about how nature works. And these ideas come from a variety of places. They come from the results of experiments. They come from older theories that we have that worked and now seem to be breaking down. They come from new theoretical ideas that we've been kicking around because we like them. And they come from you know, preconceived cultural and social norms, and you know, mythology and and storytelling and whatnot. You know, just ideas that we have about the world. And all of that goes into the process of judgment that is made when new theories are developed and choices are made about how to interpret those theories. It sounds like you're talking about a neta level of measurement where we're like measuring the ability and experiment to satisfy our need to understand the universe exactly. Yeah, that's not wrong. But the thing is that's actually good news when it comes to the question of interpreting quantum physics and finding out which, if any of these, is the best way to think about quantum physics, because we know we're not done. You know, the one thing aside from the success of quantum physics that I think that you could get everyone in you know, the world of quantum foundations, in the world of physics more generally to agree upon, is that we are not done in our search for the fundamental laws of the universe. We know, if nothing else, that we have not found a way to get our best theory of gravity general relativity to work with our best theories of you know, quantum physics, you know, quantum field theory and the standard model of particle physics. And we also know that that standard model is not just missing gravity, but is also missing things like dark energy and dark matter. So we know that we're not done, and so that means we're still on the hunt for new theories. And one of the things that goes into the mix when coming up with new theories is the interpretations of old theories. So the way we think about quantum physics now can influence the hunt for the next theory that will go beyond our current understanding of physics, and it also goes backwards. If and when we come up with that theory, it will probably suggest to us an experiment that can be conducted that would distinguish between some or all of the existing options on the table for interpreting quantum mechanics. Yeah. I have faith in the future of experimental physics to get us out of this jam by coming up with a clever new experiment. Yeah, we should all right. Well, that's wonderful. Thank you very much, Adam. I think it's been a delightful journey through the problems and possible solutions to the questions at the heart of quantum mechanics. And I'm glad that there's still a lot of work to do because us quantum physicists will still have a job. So thanks Adam very much for joining us and for explaining these things so clearly. Before we go, do you want to tell our listeners about any upcoming projects you have or at places they can find you other than your excellent book What Is Real? First, I just want to encourage people go find my book if you like hearing about these things. I like talking about them, but I like writing about them even more. And there's you know, three hundred pages of it available wherever fine books are sold. If you want to find me on Twitter or really almost anywhere else, my Twitter name and online handle is freelance Astro, so i'm you know, on most social media under that name, and my website is freelance astro dot com and you can find links to my latest work there. And yeah, aside from that, I am working on another book about science and Silicon Valley, but it will not be out for another couple of years, still in the very early stages. But yeah, if this sort of thing is interesting to you, please have a look at my book. All right, Well, thanks again for coming on, and I hope your next book collapses into a very readable pile just like the first one. Thank you best of luck with that, and thanks again for coming on. Thanks thanks for having me. This was a lot of fun, and thanks to all your listeners for coming along on another ride of curiosity where we investigate what the universe actually means and try to explain it all to you. Tune in next time. Thanks for listening, and remember that Daniel and Jorge explained the universe. Is a production of I Heart Radio or more podcast for my Heart Radio, visit the I Heart radio, app, Apple podcasts, or wherever you listen to your favorite shows. No.