Quantum Worlds w/ Sean Carroll

Jan 11

EPISODE #31

Physicist Sean Carroll shares his insights into the many-worlds interpretation of quantum mechanics, how theoretical physics informs our understanding of reality, and what the human mind can comprehend about nature of the universe.

Sean Carroll is a theoretical physicist at the California Institute of Technology. His papers on dark matter and dark energy, the physics of extra dimensions, and alternative theories of gravity have been widely praised. He is also one of the founders of the group blog cosmicvariance.com, named one of the five top science blogs by 'Nature'.

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Luke Robert Mason: You're listening to the FUTURES Podcast with me, Luke Robert Mason.

On this episode I speak to theoretical physicist, Sean Carroll.

"We're building quantum computers. We're doing physics at the nanoscale. We're pushing things around at the level where quantum mechanics really becomes relevant." - Sean Carroll, excerpt from interview.

Sean shared his insights into the many-worlds interpretation of quantum mechanics, how theoretical physics informs our understanding of reality, and what the human mind can comprehend about the nature of the universe.

This episode was recorded in person before the outbreak of coronavirus, while Sean was in London promoting his book, 'Something Deeply Hidden'. You can view the full video version of this conversation at FUTURES podcast dot net.

Luke Robert Mason: Perhaps you can understand quantum mechanics. At least, that's what it feels like when you read this book. It feels like suddenly, maybe, possibly, you can start to begin to understand this weird thing called quantum mechanics. I want to just dive straight in and ask probably the question you've been asked the most on this book tour. In what way is quantum mechanics the deepest and most comprehensive view of reality?

Sean Carroll: It's a great question to ask, and the embarrassing thing is that physicists themselves know that they don't understand quantum mechanics. The best way I can conceptualise it is before quantum mechanics, there was classical mechanics. Isaac Newton came down from the mountains and said, "This is how the world works. It's made of stuff. You can tell me where the stuff is, how fast it's moving, and that's all you need to know. The laws of physics do the rest." There was never any talk about what you could measure or anything like that. You can measure whatever you want. That's how classical mechanics works.

When quantum mechanics comes along in the early 20th century, suddenly it seems that physical systems - mostly little tiny subatomic systems like an individual particle or maybe an atom - behave differently when you're looking at them than when you're not looking at them. In particular, they seem like a wave when you're not looking at them but then you measure an electron and it looks, to you, like a particle. This is completely new. There's no analogue of this. There's no metaphor, similarly, that does justice to the weirdness of this feature. That's what we're still trying to struggle with.

Mason: The thing that makes a wave look like a particle: that's known as the observation effect; observer effect. What is the impact of that and why is it so important?

Carroll: Well these days, we teach our students - I'm at Caltech - if you take an undergraduate quantum mechanics course at Caltech, we say there's something about the wave function. Don't think of the electron as a point. Think of it as a wave that's spread out inside of an atom. There's no such thing as where the electron is because it's spread out in some sort of profile. When you look at it and send the electron through a detector, it leaves a track just like a particle moving in a line. What we teach our students is that when you look at that wave, it collapses. You never see the wave. The wave is what the electron is when you're not looking at it. When you look at it, the wave suddenly localises to some location. You can't predict where it will be. You can see it's more probable that it'll be here than somewhere else, but there's this inherent randomness there.

Worst of all, if someone in the back raises their hand and says, "Well what do you mean look at it? What is the technical definition of measure or observe in this context?", they're told to leave the room. There's no good answer to that in modern physics. This purports to be our deepest understanding of how nature works. If there are ideas like measurement and observation that don't seem like they should be playing a fundamental role in how nature works.

Mason: If I was that annoying student and I stayed in the room, the question I'd probably ask you is, "Does it matter that it's a human eye? If the observer is conscious? If a camera is looking? If a piece of equipment is looking?"

Carroll: What if you're short-sighted? None of these are answered. You're just not supposed to ask those questions. That's fine as long as you're in a regime where it's clear when you're not observing it or when you are. You have an apparatus and you've turned it on, or you haven't turned it on - that much is clear. One of the reasons why people are revisiting the fundamental nature of quantum mechanics is that these days, we're building quantum computers. We're doing physics at the nanoscale. We're pushing things around at the level where quantum mechanics really becomes relevant.

Mason: So I guess then I guess the question is, how do we deal with the fact that what we see isn't really reality?

Carroll: Well people like Einstein and Schrödinger who invented the Schrödinger equation and quantum mechanics - they had the same question. They were like, "What are we supposed to say about these things?" Other people like Niels Bohr and Werner Heisenberg said, "Look, just don't worry about it. Move on. Do your calculations. Build things. Build bombs. Build materials. Build lasers." Quantum mechanics is really, really good at explaining what we see, but it doesn't quite answer those detailed questions about what it means to look at something. The best we can do according to the textbook interpretation is just to not ask that question.

There have been some plucky individualists who've said, "No, we have to do better." They've tried to develop more complete, rigorous, formulated theories of quantum mechanics. These theories are given the name 'interpretations', but they're really not. They're separate, distinct physical theories that often make different experimental predictions. The problem is that all of them fit into the framework of quantum mechanics at the level that we've been able to do the experiments so far. There are different physical theories that could be right. We don't know which of them, if any, are actually right.

Mason: We're going to explore some of those theories in a second. Quantum mechanics has been described as spooky. It's been described as weird. It's been described as mysterious and baffling. I just wonder what's your favourite adjective to describe quantum mechanics?

Carroll: It's a weird thing because the example I'd like to use - or the metaphor I'd like to use - is the fable of 'The Fox and the Grapes'. I don't know if you know this one, but it's one of Aesop's fables. The fox sees a bunch of grapes up there and the grapes look really juicy and yummy. The fox jumps up to get the grapes but the grapes are just out of reach. The fox can't get them. The fox says to himself, "You know what? I never wanted those grapes anyway. They're probably sour." That's how physicists deal with understanding quantum mechanics. It's not just that they don't understand it. They've convinced themselves they never wanted to understand it; that understanding quantum mechanics is somehow bad or wrong. This is the origin of the fact that when you read a quantum mechanics book at a popular level, it's emphasising that it's mysterious and spooky and mystical and impossible to understand.

What I want to do is move entirely in the other direction. I want to say, "Actually, it's physics. It's not magic. It's not a mystery. It's completely understandable. We might not understand it yet, but it's not fundamentally mysterious."

Mason: The thing I struggle with is that even though quantum mechanics is necessary in modern physics and in modern life, how are we able to make use of it without understanding it?

Carroll: Yeah, that's a great question, but then again think about your cell phone. You can use your cell phone, you can take pictures and send email. You can even call people on the phone. You can't - I don't know, maybe you can - but I can't build one from scratch. I don't know what's going on inside the cell phone. There's plenty of examples where you can use something, whether it's a cellphone or a car or whatever, without knowing the details about how it operates. Scientists are supposed to have higher aspirations than that. They really want to understand how things operate. But right now, quantum mechanics is like your cell phone. We can use it without really knowing what's going on inside.

Mason: Is there something oddly, innately human about that? To be able to use something but not understand it.

Carroll: It's a necessary first step. It's not like the founders of quantum mechanics were wrong about quantum mechanics. It's just that they gave up too soon in trying to understand it. We put models together that are kind of kludgy and don't really work, and we hope to build them into lean, mean scientific machines. With quantum mechanics, that programme is never quite complete.

Mason: You spoke about this thing called the wave function, but another part of the quantum theory is the Schrödinger equation. How do those two things work together to give us quantum theory?

Carroll: Well it's very much like in the old days of Newton. You would tell me if there's a ball moving through the air. You'd tell me where it is and how fast it's moving. Newton gives you an equation from which you could predict the entire trajectory. The Schrödinger equation is the equation from which if you give me the wave function, I can predict how it will evolve in time. I can predict how electrons will evolve moving through the universe, and how things interact and become entangled which is an important quantum phenomenon. The Schrödinger equation is the single equation that tells us how all quantum mechanical systems behave.

Mason: With regards to this idea of quantum entanglement that you mentioned briefly, you've got two things happening: superposition and quantum entanglement. Those two are key to understanding this thing called quantum theory. Firstly, what's the difference between those two and why are they so important?

Carroll: Yeah, they're very different. Superposition is only a word because of this weird difference between what the system is when you're not looking and when you are. If it weren't for measurement, you would say the electron is a wave, and you would stop. That would be it. We know how to deal with waves - electromagnetic waves, or light and things like that - we're fine. But then when you look at it, you don't see the wave and you see it located at some position. So what you say to yourself is: Okay, I can relate those two things to one another - what the electron is and what I see - by saying that I will think of this wave as a superposition; a combination of every possible location the electron could be at. To each possible location, I assign a little bit of a number that says what's the probability I'm going to observe it there. To say that an electron is a superposition of different places or different locations that can be observed is just a fancy way of saying that it's a wave.

Entanglement comes in when you have more than one system. Like when you have two electrons or three electrons. In classical mechanics, according to Isaac Newton, if you have two particles you just give me the state of one particle and what it's doing and then you give me the state of the other particle and what it's doing. They're separate in some sense. In quantum mechanics, it turns out that there's not a wave function for particle number one and a wave function for particle number two. There's one wave function for the combined system of both of them. It might be that you don't know what exactly you will observe when you look at particle one or particle two. But you know that when you observe a certain feature of particle one, you instantly know what particle two is doing. That's entanglement. There's a relationship between the possible observational outcomes of all the different subsystems of the universe.

Mason: So firstly on entanglement, is there a possibility that we might actually be able to use this weird quirk of quantum entanglement for the development of technology or quantum computers?

Carroll: Absolutely. It's absolutely at the heart of quantum computers. I thought you were going to ask, "Can we use it to send signals faster than light?"

Mason: Ah, or teleportation.

Carroll: Yeah, or something like that. So the answer is "no" to those things, but no absolutely - there's a lot of richness that gets involved. Not just quantum computers, but quantum encryption. Quantum money is a thing - you want to make money that no-one can duplicate. Quantum entanglement is your boy.

Mason: That sounds a little dangerous. That sounds like the next hype after blockchain or bitcoin. You go one step further with this whole thing around the wave function. You say that the whole world is a wave function. Can you just drill into that a little bit more just to help me to understand?

Carroll: You know, it's a natural road to walk down. Once you realise that when you have two electrons there's still only one wave function, you want to say, "Well okay, what if I have three electrons?" Guess what? There's only one wave function because there's only one wave function for the entire universe. I think Hugh Everett - who as a graduate student invented the many-worlds interpretation of quantum mechanics - was the first to use this phrase. He called it the 'universal wave function'. These days, following Stephen Hawking we call it the 'wave function of the universe'. But still, any version of quantum mechanics has it. There is only one wave function for every physical system that exists.

Mason: So I want to talk about Everett's theory, or the multi-world theory. Really, that's at the crux of your new book, and at the core of something deeply hidden. In the simplest terms possible - and I guess you've just written 350 pages on it, so it's unfair to ask - but what is the many-worlds interpretation of quantum mechanics?

Carroll: Basically the one way to think about it is that Everett looked at the rules of quantum mechanics which say there are wave functions and there's a Schrödinger equation - that kind of makes sense. But then there's these extra rules about measurement and observation. When you measure something the wave function collapses. You can't predict when it will collapse and after it collapses it's in a certain state. Everett said, "What if we just erase all the rules we didn't like? What if we just had a wave function that obeyed the Schrödinger equation?" The secret to doing that in a sensible way is to realise that it's not that the electron is a quantum system, but you the observer are a classical system. You're made of atoms and atoms obey the rules of quantum mechanics. You're part of the wave function of the universe. What he said is that if you just put you into the Schrödinger equation along with the electron, then all the mysteries about measurement and observation disappear. You're a physical system, the electron is a physical system, you interact with each other. Rather than talking nonsense about measurement, what you should do is solve the Schrödinger equation and see what happens to you and the electron. The answer is that you and the electron become entangled. Rather than the electron being all spread out and you seeing it somewhere, there's part of the wave function that says the electron was there and you saw it there. There's another part of the wave function that says the electron was over here and you saw it over here - for all the different possibilities. Everett says, "Just accept that. Just lean in and just believe what the equation is trying to tell you." Rather than saying, "Well, I've never been in a superposition and I don't know what that feels like", you should recognise that all these different possibilities define different worlds. So the worlds were always there inside the wave function. Everett is just saying, "They exist, and that's okay."

Mason: So does that mean it's impossible as a human subject, as a human being, to have an objective view on what reality is?

Carroll: It says that the reality we experience is a tiny infinitesimal slice of the whole shebang, which should be the least surprising thing in the world. That's the history of science since Copernicus, telling us that we are less and less important to the universe than we'd like to think.

Mason: There's a reason why you like many-worlds theory so much. It's simply because it works.

Carroll: It works. It's mathematically far and away the simplest version of quantum mechanics. I get it why people resist it because it seems to imply the existence of much, much more than we see. The reason why you should like it is because it's the simplest explanation of what we do see. The question is: are you up to the task of accepting all the implications even though you don't see them directly? Maybe you shouldn't. Maybe I'm wrong, and maybe there's something new in physics that gets rid of the other worlds. The worlds are there in the wave function and Everett says, "Just accept them, it's okay." Other people are like, "No, I'm going to erase them somehow. I'm going to mess with the laws of physics to get rid of these worlds." I just don't think that's necessary.

Mason: Are there other contenders to the many-worlds theory? Are there other possibilities for what could be happening here?

Carroll: There are. There are two broad classes of alternatives. One is hidden variable theories, sometimes called 'pilot-wave theories'. There, you take the wave-function and the Schrödinger equation, but you add something extra to it. The thing you add to it is roughly a finger that points at one part of the wave function and says, "This is the reality." There's a wave function describing all these worlds and there's also a little pointer that says, "This is the real one, where we live." It kind of makes you feel better about not needing to take seriously all of those other worlds. But someone who likes many-worlds would say, "it's just many-worlds in denial." It's just trying to avoid accepting what the equations would predict.

The other set of contenders actually change the equation, the Schrödinger equation. The Schrödinger equation again unambiguously says, "These worlds come into existence." Other contenders say, "But the Schrödinger equation isn't always obeyed." Occasionally, the wave function sort of gathers itself up and picks out one of the worlds.

Mason: This whole idea of there being many, many worlds - is that similar to the idea of the cosmological multiverse, or is that completely a different thing?

Carroll: It's actually a completely different thing. The words sound relatively similar but I would argue that despite the fact that it's way more mind-bendy and profound, the quantum many worlds are also much more likely to be there. The cosmological multiverse is really just the idea that if you go very, very far away out there in space - so far away that it's beyond what we can actually see, given the speed of light as a speed limit - maybe there are regions of the universe where things are very, very different. The laws of physics could be different, the particles, even the number of dimensions of space could be different. We group regions where things are relatively similar to each other and call that a universe. Maybe there's many different universes stitched together within space. Is that reality or not? I have no idea. Maybe there's good theories that predict that the world is like that and there are other good theories that predict it's not. At the present state of the art, we have no way of knowing.

Many-worlds is much more profound because it means that literally right here in this room as we're speaking, the universe is being duplicated into many, many different copies. Also, there's a lot more evidence for it. It explains data that we've actually collected.

Mason: The other thing that it reminds me of when you say 'many-worlds' is the idea of 'parallel worlds'. Again, is that similar, or is that something completely different?

Carroll: 'Parallel worlds' is just a more vague term, so yes, it's actually more similar in the sense that when people want to know where these worlds are located, there's no answer to that question. They're not located in a place. You should think of space as being part of each universe, not each universe having a location in some big space. The best way to think of all these multiple worlds is as literally simultaneously existing, essentially parallel copies of reality.

Mason: I always thought - correct me if I'm wrong - that parallel worlds had something to do with B-theory of time. B-theory of time is where the past, present and future is happening simultaneously right now. It holds true, to some extent, that everything must be happening in parallel.

Carroll: Well it's a little bit trickier than that and it's a whole other discussion I'd love to have. It's not that the past, present and future are existing simultaneously. It's that they equally well exist. When you use the word 'simultaneous', you're undoing words 'past', 'present', and 'future', right? 'Simultaneous' means the same moment of time. I think that the B-theory simply accepts the equal reality of past, present and future without saying 'at the same time'.

Mason: Do you think that has something to do with human subjectivity, though?

Carroll: Yeah, humans are terrible at this because of course we developed English language long before we developed relativity or quantum mechanics. We nevertheless try to force these crazy physics ideas into the everyday language that we're equipped with, and we come up a little bit short sometimes in understanding the translation.

Mason: So there is simply then some multiverses - they're far away - and many-worlds are here right now, quite possibly happening in this space.

Carroll: Parallel. That's right, parallel.

Mason: Yeah. So are these new worlds created by the observation or do they already pre-exist? That's the tricky thing I'm trying to get my head around.

Carroll: I think it's closer to saying that they already existed, because it's not that you take the universe and literally duplicate it, like putting it into a Xerox copier and making another copy of it. It's more like the universe had a certain thickness and you divided it. It's more like the universe sub-divided and differentiated itself into two bits of it where now it's a slight divergence. It used to be one big, thick universe where everything was the same and now it's two slightly different universes. We're here, and that bit decayed, and here did not.

Mason: It feels like we've almost covered off this idea and the way you talk about it feels so simple and so normal, in many ways. What sort of responses have you found to this book, where what you're saying feels very matter of fact?

Carroll: Yeah, I think that one of the differences between my book and previous ones is that I don't try to highlight the weirdness of quantum mechanics. I try to say, "Look, here's an attempt to make it actually make sense." It might not be the right attempt. It might turn out, 20 years from now, that we learn better. That's fine. Much more important to me is that it makes sense at the end of the day; that we can make sense of it. I think that people are sympathetic to that point of view. I think that the general idea that we should not just take quantum mechanics as already a success and be satisfied with it is catching on. More and more people want to do a little bit better.

Mason: The issue I had is when I closed the book, I went: so what does this mean for me in this present world, right now? I just wonder how are people coming to terms with this idea that there could be a proliferation of other realities?

Carroll: Yeah, you know it's not something which is going to lead to some technological improvement. You're not going to get a multiverse smartphone or anything like that. You're not going to cure malaria. But, I just try to be honest about it and say, "Look, we're human beings. We're curious. We want to understand the fundamental nature of reality." Of all people, physicists should be most invested in understanding the fundamental nature of reality. The prospect that many-worlds helps us do that is its own justification, as far as I'm concerned.

I also try to make the case in the book that certain questions within physics - like reconciling gravity in quantum mechanics - become much easier when you take the many-worlds perspective. I think it is also helpful in the practice of physics, as well as just scratching a curiosity itch, but mostly it's there because we want to understand things better.

Mason: I mean gravity is the elephant in the room, isn't it, really? Can you explain why and how it comes to help understand this tricky thing called gravity that we don't quite understand, but we know at least it's keeping us here on this planet?

Carroll: Right, we know that gravity is there. Einstein gave us a very successful theory of gravity in the form of general relativity where he says that space and time are part of a four-dimensional spacetime. That spacetime is curved, has a life of its own and that curvature manifests itself as gravity. If you think about quantum mechanics which says the electron is not a location in space at a point - it's more spread out - and you say, "How do we fit gravity into that picture?" Well gravity...if the way the universe could be curved, if the way that spacetime could have a geometry is a quantum reality, that means that it's in a superposition of all of the different possible ways that spacetime could be curved. That means it's very hard to even indicate a single point and say, "That's where I saw the electron." Once you have different parts of the universe with different geometries, they don't match up in any simple way. There are more conceptual and technical difficulties in fitting gravity into the framework of quantum mechanics.

Mason: Again when you describe it, it feels so simple. I can't help but go back to that feeling of what that means for me, personally, as a human being. If there're multi-worlds and I'm making different decisions in all these worlds, how does that change how I act personally or ethically in the world that I'm living in right now?

Carroll: It's a good question and I think that I have a favourite answer but actually, I'm happy to admit that we should think about this harder than we have so far. Let's imagine for example that you're a utilitarian. You thought the greatest good for the greatest number, okay. Furthermore, you thought that overall, the existence of human beings was good. It's nice to have human beings around. Sometimes things are sad, but overall it's a net good, okay? You might think that your moral imperative was to go into your basement and branch the wave-function of the universe as quickly as possible, creating all these new universes with a bunch more people in them. That would increase the net utility of the universe, even though nobody outside your basement would have any idea you were doing this.

I think that rests on a mistake. I think that's not a moral imperative because again, the universe has a thickness and you're making it thinner and thinner and thinner. When you duplicate the number of universes but each universe is less thick, so it has less to it, you shouldn't count that as increasing the utility of the universe. If no one in the universe knows you've done anything, you shouldn't get any moral credit for doing that. When you take that attitude seriously, what you conclude is that essentially, up to some bizarre counter examples, you should act within this ever-ready and multiverse exactly the way you think you should act in a universe governed by the rule of quantum mechanics that was given to us by Niels Bohr in the 1920s.

Mason: So I can't argue that I've paid my taxes in another world?

Carroll: Does not help.

Mason: Does not work.

Carroll: No, sorry. I'd love to be able to say that.

Mason: There is still something oddly comforting about the idea of there being multiple worlds. Do you think the reason why this theory is so popular is because there is something innately comforting about the theory?

Carroll: No, because it's not that popular. I think there's something innately disconcerting about the idea that there's all these other copies and they're slightly different, right? I think the crucial, crucial thing that if you want to accept many-worlds and make sense of it is to understand that the worlds are not created equal. Yes, there are other worlds where some version of you - not you, because you're in this world - there's a different person out there, just like identical twins are not the same person even though they're from the same starting point. There's a person that started as you but is now somewhat different that something wonderful happened to, that would be very unlikely by conventional ways of saying - you won the lottery or whatever. It matters that the probability of that is very small in the conventional way of thinking. It's not that you and the other version of you are 50/50 and it could have been either way. The probabilities really make a difference.

Mason: So you don't subscribe to the conspiracy that once we switched on the Hadron Collider, the world started to get weird and we jumped into another world?

Carroll: No. That's compelling for various reasons but yes, I don't think that's fair.

Mason: It's not true. I guess the wonderful thing about reading the book and the wonderful thing about listening to you now is that you sound more like a philosopher than you do a physicist. I just wonder, are philosophers and physicists strange bedfellows, or is there a complementarity they need to help understand these more complicated, weird things about reality?

Carroll: Yeah, I mean there's definitely a difference and most of physics gets along perfectly fine without anything philosophical being involved. If you want to calculate the rate of decay of a certain atomic nucleus, philosophers are not going to be of any help to you. If you want to know how a galaxy formed, just do your calculation. There are certain areas of physics where you bump right up into philosophy problems. Quantum mechanics is an obvious example but also things like the origin of the universe or why the past is different from the future. Many modern philosophers of science got PhDs in physics before switching into philosophy because they realised that the questions they were interested in would never give them a job in a physics department.

Mason: Is there a weird kind of taboo, perhaps, about talking about quantum mechanics in the physics department?

Carroll: There absolutely is. I think it's going away. I like to be optimistic about the rate of change, but there's still plenty of people who, if you say, "I'm interested in the foundations of quantum mechanics." they'll say, "Oh, it's too bad you're not doing physics anymore."

Mason: Why do you think it's such a controversial field?

Carroll: There's a bunch of reasons. There were the battle days when thinking about the foundations of quantum mechanics was something one did after the serious work was done. You came up with what were called 'interpretations' - just a bunch of words like interpreting a novel or a painting or something like that. It didn't seem to have any practical benefit when it came to calculating the cross-section for a certain nuclear reaction or two particles bumping into each other, or anything else that could really help us understand the observations that we made in our telescopes and our particle accelerators. I think that it got a bad reputation for that reason. The fact that these days, we have real physical theories that potentially make very different experimental predictions has sort of gone under the radar. Most physicists don't even know that. That's part of the hope, is that we can spread the word that this is actually really physics that's being done here.

Mason: Is the problem that we just don't see the practical application of this right now?

Carroll: The word 'practical' means something very different to a working physicist than a person on the street. 'Practical' might mean you can calculate the temperature at which a certain substance goes from being solid to liquid. As far as those things are concerned, it's very practical to a physicist and no one else cares. The interpretation or the formulation of quantum mechanics doesn't help that much. It might help, as we said, with gravity, and that would be enough to get physicists very excited about it if that turned out to be true.

Mason: In a funny sort of way, is what you're doing, Sean, is it propaganda? Are you trying to make quantum theory as interesting and exciting to the general public as possible?

Carroll: There's definitely a marketing aspect to it which is only fair because Niels Bohr and Werner Heisenberg etcetera had a marketing campaign in the 20s and 30s against people like Einstein and Schrödinger. Bohr and his friends won the marketing battle. They convinced both physicists and the general public that quantum mechanics was fine, let's move onto other things. I'm trying to counter-program that.

Mason: That's great, but sometimes is there a danger to how the public may misunderstand something like quantum mechanics? I know Schrödinger's cat experiment has entered popular culture and consciousness but it's often been twisted in ways that are not how it originally formulated. I read in the book that it was a joke, originally.

Carroll: Well no one in the popular culture has ever misunderstood anything I've said about quantum mechanics so I hope to do better than that. In fact, I think it's the other way around. I think that a lot of the misunderstandings of quantum mechanics have been allowed to flourish precisely because physicists have not taken seriously trying to get to the bottom of it. That gives room for people to impose their views on it. Maybe my conscious perception is bringing the world into existence just by looking at it, right? I think that once you are able to think more carefully about the foundations of quantum mechanics and go, "Look, honestly, there are some equations and the world obeys the equation." - that's really what it is. There's nothing about you perceiving things or your consciousness or anything that actually will make it more in accordance - the popular view with the physical reality.

Mason: But then you could counter-argue that if you're saying that humans are a quantum system in their own right, surely consciousness in some way, shape or form must come into play. How do we deal with this consciousness problem?

Carroll: I think consciousness comes out of the physical behaviour of the world. I think that consciousness exists - there it is, it's real - but it's not one of the ingredients in the fundamental description of reality. Just like tables and chairs exist but they're not there in the standard model of particle physics. They appear as useful approximations in some macroscopic, observable regime. Consciousness, likewise. It's an emerging phenomenon out of the varying complicated interactions of all these electrons obeying the Schrödinger equation.

Mason: You could say without Sean's consciousness, we wouldn't have the understanding of quantum mechanics. Does it go the other way - that we might not understand human consciousness without quantum mechanics?

Carroll: No, I don't think so. I can say that without the table, I would not have the support for my coffee cup, but that doesn't mean the table plays some fundamental role. Maybe the understanding of this or that bit of quantum mechanics might be different, but consciousness comes after - is the important point, I think. Consciousness is like wetness or tables or anything else that arises in our world. It's very important and we should take it seriously, but it's not bringing things into existence.

Mason: So consciousness doesn't create reality.

Carroll: I don't think so. You knew I was going to say that.

Mason: I knew you were going to say that, but partly because I wanted to talk just a little bit about what's been popularly known as 'quantum woo'. Quantum has kind of captured the imagination of so many and it feels like it really captured the imagination of esoteric individuals or people in the New Age, or New Age spiritualism. I just wonder how you feel this idea of quantum is being distorted or misrepresented, and why has there been this appropriation and then misrepresentation? What is it about quantum that makes it so appealing to those individuals?

Carroll: There is a joke called 'the law of conservation of mystery', that says consciousness is mysterious and quantum mechanics is mysterious and so they must be related somehow. It's just a joke, it's not actually true. When I wrote my book, one of the research projects I did was to go to Amazon dot com and type the word 'quantum' into the search engine, and just look for all the different titles of books involving quantum. Yes, you get quantum therapy, quantum leadership, quantum healing, quantum prayer, quantum touch. No equations in any of these books, it's weird. I think that misunderstanding of quantum mechanics has allowed these to flourish. The way that they flourish is as a wish fulfilment kind of fantasy. The fact that the world obeys the laws of physics has always been a bit of a downer. It limits what we can do. I cannot use my brain power to lift something up across the room because the laws of physics don't let me do that.

Mason: Not in this world.

Carroll: Not in this world, not in the world that obeys the laws of physics as we know them. If you think that the act of observation of a quantum system is a crucially foundational part of the definition of what it means to be a quantum system, then it's a short step to saying observation, perception and consciousness are playing a role in the fundamental laws of physics. Consciousness is bringing that observational outcome into existence. I think that's false, but it's not crazy talk if you don't do a better job.

Mason: So I can't make a decision and branch the waveform?

Carroll: Not by making a decision. It's the other way around. It's the electrons in your brain that are making the decision, not you pushing around the electrons.

Mason: So in your opinion, the world probably isn't magic.

Carroll: The world obeys the Schrödinger equation. That's a little bit of magic in a sense, but it's still obeying the equation.

Mason: You have a highly popular blog and one of the blog posts is the idea that there is no magic. For someone who's so enthusiastic about all the mysteries of the world and trying to solve those mysteries of the world, it just feels almost disappointing in some ways that you don't believe there might be a little bit of magic - something that's still unknown or something that constantly remains mysterious to human beings.

Carroll: That's right. I do have a blog post that I'm very happy with called 'The World is not Magic' for exactly that reason. I think that I like it so much better that way. I like the idea because it would be one thing if we understood the world perfectly. If we had the theory of everything and we had all the information about the world. I think this is never going to happen. We're never going to get all of the information about the world. Where would we fit it? The world is a very big place. There will always be things we don't understand, but in the real world today, there's even features of how the world behaves that we don't understand. There are puzzles and there are mysteries, but they're answerable puzzles and they're solvable mysteries. I think that's far more interesting.

Mason: Do you think that at the end of the day, it's a limitation of human evolution and human biology, and therefore human consciousness that means we're constantly going to have a degree of limit to our understanding?

Carroll: No, I do not. I mean there's a limit to our understanding, literally, because our brains are only so big. There's this thought experiment called 'Laplace's Demon' from Pierre-Simon Laplace in the 1800s. He didn't know about quantum mechanics. He was still thinking in a Newtonian framework, but he said, "If you knew the position and velocity of literally every bit of matter in the universe, this vast intelligence could predict the future and retrodict the past." Laplace knew perfectly well no one will ever have this information. You literally can't store that much information, so it was just a thought experiment. The laws - the dynamical equations that the universe obeys - as far as I can tell, should be perfectly knowable. Detailed information about which world we live in, we might never know. But the rules that that world obeys? I don't see any obstacle to us figuring them out.

Mason: The fact that we're stuck in this human body and stuck with this human brain - doesn't that feel like there's still that limit to understanding the world objectively? We're constantly and always going to be this human subject with human consciousness and therefore, all of our theories about what reality is are going to be human-based.

Carroll: They are, but I do think that we have undergone a tipping point, a phase transition, once we figured out a hypothetical deductive method. You cover a bunch of ideas about how the world could be and then you do experiments and collect data to figure out which one is right, circa Francis Bacon 1600s or something like that. We have just been on a rocketing trajectory of learning more and more about the world at an incredibly rapid rate.

If you think about 100 years ago, we didn't know the universe was expanding. We didn't even know that there were other galaxies outside our own. We didn't know about nuclear physics or anything like that. We didn't know about quarks or neutrinos. The amount that we've learned in 100 years is flabbergasting. 100 years is not a long period of time. If you're saying, "In the last 10 minutes, I haven't learned anything profound about the universe." that's a little bit impatient. I would say not that we're coming up short in our attempts to understand things, but we've been going so fast that it's almost too good to be true.

Mason: I wonder if there's something that's changed culturally. It has been 100 years. It was almost 1900, I think, when these ideas were first proposed. It almost took 100 years for us to be comfortable with the idea of quantum. I just wonder if that's not to do with physics or science, but it's to do with society and culture. When we live in a world that is comfortable with the idea of post-truth - another form of political reality - that there's another world that seems quite normal to the general public.

Carroll: I actually don't think that. I think that's a tempting hypothesis and maybe I'm wrong, so you could test that using data. I think there is definitely a relationship between what's happening in the world and in culture and how scientists think. It could be a stronger relationship and sometimes a weaker relationship.

The huge thing that matters is if you think that we started on the road of quantum mechanics in 1900 but we kind of put it to its final form in the last 19020s. This was a bunch of people - largely German, Danish, French, English - and a few years later, they were at war. Literally, people like Einstein and Schrödinger had to flee their home countries. Whereas in the 20s, these people would just get together over coffee and hash things out. They were left sending letters across the Atlantic ocean to each other. The whole discourse slowed down. The brightest young minds were told, "What we really need are some weapons. We need to build some bombs here." Their attention was distracted away from the fundamental questions in quantum mechanics. I think there's no doubt that what was going on in the world had a big impact on the history of physics in the 20th century.

Mason: Is there a parallel between what was happening in the 1920s and now what's happening in the 2020s? Are we, again - I mean there's faster communication with emails sent across over the world - but are we in this period of pastorally gathering our information on quantum before we make the next step?

Carroll: I think it's a fascinating idea. It seems almost impossible to get an accurate idea when you're living in the moment of how recent changes in culture are affecting how we're thinking scientifically. The fact that there are not just TV shows but social media and podcasts, and physicists can share their papers electronically very, very easily. I don't necessarily need to fly to another country to see a talk - I can see the video of the talk. This is absolutely changing the way that scientific communication happens. Whether or not that's changing the science that's being done, I'd say probably but I'm not going to venture an actual guess as to what specifically is going on there.

Mason: I just wonder, again 'culturally' is not just the way in which we communicate but it's the things that we see, the things that we watch, the things that we're receiving in terms of media. In the same way that the 'Matrix' films of 1999 made people comfortable with the idea that we live in a simulation. 'Avatar' made in 2009 - which was 10 years later, oddly enough - made us comfortable with the idea of biotechnology or inhibiting some form of biotechnology. Pokemon made us comfortable with Kymera. I just wonder, are things like the Marvel Cinematic Universe doing wonders for quantum physicists?

Carroll: I would like to think that. I've been a consultant on some of the Marvel movies, actually. The time travel in Avengers: Endgame, I influenced in interesting ways. I love the Marvel movies. My friend, [Spyridon] "Spiros" Michalakis was the one who introduced the phrase, 'the quantum realm' into the Marvel Universe - he's a physicist at Caltech. They're not documentaries - he didn't disillusion anyone out there. They're not even hard science fiction, but they can nevertheless be inspirational. They can get people thinking. They're not accurate or true, but the original Iron Man movie, I loved. Not because a guy in a cave in the middle of nowhere could build this suit and fly away, but because Tony Stark did act like a scientist. He did experiments and he failed. He spent a lot of time in the lab. The scientific method in some artistic way was on display in that movie.

Mason: You're not saying you're Iron Man, then.

Carroll: I'm not saying that I am Iron Man, I wouldn't do that.

Mason: I want to touch on that a little bit more because it would be silly of me not to. In Endgame, the thing about time travel and the way in which they conceptualise time travel in Avengers: Endgame versus Back to the Future - were you partly responsible for that? That there was going to be this splitting off of different worlds that would be created where Captain America can live quite comfortably within this world and be creating these loops. I just wonder if you can explain some of the work that you did on the Marvel series.

Carroll: I did. So, as a physicist I've worked on time travel - whether it's possible within general relativity and things like that, so I have thoughts on what is good and bad time travel. When I consulted with the Russo brothers and the writers on Endgame, they already knew they wanted to do time travel. They hadn't even filmed the first movie yet - Infinity War - but they knew that it was going to end in disaster and they were going to have to go back in time to fix it, so they wanted to know how that would work. I tried to make the case that the most important thing to me about time travel is that you can visit the past, but you can't change it because it has already happened. In fact, someone in the room when we were having that discussion said, "Do you mean to tell me that Back to the Future is just bullshit?" That line appeared in the movie - Paul Rudd as Ant-Man said exactly that line. We did go into a more elaborate scheme where there were quantum mechanics and branching worlds and different possibilities, but I was advocating for keeping it relatively simple and sticking to one timeline, if at all possible.

I think that in the movie - you know again, it's not a documentary. They're not subject to peer review, for example - but they struck a compromise where they talk about different timelines. The actual action that you see in the movie is compatible with just a single universe, with Captain America going back in time and having a nice love life with Peggy, so that was good.

Mason: Have there been any recent good examples of science fiction that you've seen - either novels or short fiction, or even in the movies - that have really done well in understanding this quantum weirdness, this quantum mechanics.

Carroll: Quantum mechanics is probably the hardest thing to get accurately portrayed in movies or even in novels. Ted Chiang is a very accomplished science fiction author who has a new collection of short stories. Everyone tells me that some of them are about many-worlds quantum mechanics and that they're great. I will confess I've not yet read them, but I'm sure he knows what he's doing.

It's a little bit bittersweet because quantum mechanics gives you this fantastic idea that every time we measure a quantum system with world branches, but then the people in one branch can't talk to the people in the other branch which is why we don't have these ethical implications but also makes it a much less interesting story to tell if you can't talk to each other. If you can't say, "Well I had pizza, what did you have? Is that the right decision?"

Mason: The way in which you talk about this stuff and the sorts of ideas that you're espousing and sharing - when I first read them, they were so surprising to me. I just wonder what was the last thing that surprised you?

Carroll: Here's something. Energy is conserved. This is something that we think is true in the world. In quantum mechanics, is it still true that energy is conserved? Remember, we said that there are different formulations of quantum mechanics. Many-worlds and these spontaneous collapsing theories and these hidden variable theories. In the spontaneous collapsing theories, energy is just not conserved. Actually, that gives you a handle to do experiments looking for evidence of these theories. I got to thinking about many-worlds and I realised that here, the energy that is conserved is spread out over all the worlds. If you had a God's eye view and could see all of the worlds at once, you would think that energy is conserved. But in any one world, it need not look like that. You might be able to do an experiment in a lab here on Earth that would show energy either increasing or decreasing overall because of quantum effects. It's basically a total amount of energy being shared unequally between the different worlds. That was a surprise to me, but I think it might be right.

Mason: What does that mean for our conception of reality?

Carroll: It just means that instead of energy being completely constant, it sort of will vibrate up and down a little bit. It doesn't mean anything for your digestion or your diet or anything like that, but it's an important part of thinking about how reality works.

Mason: What was so lovely in the book is that you map the history of scientific progress. The thing that seemed really obvious to me is that what was key to that scientific progress was generations. New generations of scientists coming in, coming up with new theories, challenging previous generations. I just wonder, what do you hope for the next generation of scientists? What do you hope that they might discover in the next, say, 10 years?

Carroll: There's another joke that says science progresses funeral by funeral, but I think that you're exactly right. This idea that when we teach quantum mechanics to students, there's a bit of a 'then a miracle occurs' step when we do a measurement, is something that teh younger generation is a little bit less willing to shut up and put up with than the older generations were. Partly, this is because of bad news. We haven't had a real revolution in fundamental physics in recent years. We haven't been surprised by new discoveries in our telescopes, or microscopes or whatever. We have theories that fit the data and they've fit the data for decades now, so we can go back and think about the foundations a little bit more carefully. But look, I could be very honest about the fact that when I write a book like 'Something Deeply Hidden', part of the audience is just people who are on the street who are interested in physics. Part of the audience is my professional colleagues who might want to know a bit more about this particular point of view. A big part of the audience is young people who haven't settled into a particular viewpoint on these things yet. I want them - whether or not they agree with my viewpoint - I want them to not sit quietly in their quantum classes. I want a high school student to read my book and know that this set of issues exists, so that when they first are exposed to them at a technical level, they really think as deeply as they can about them.

Mason: I mean, I guess being comfortable with some things being unknowable is almost as human as wanting to find an answer to everything. How has an understanding of quantum mechanics and dealing with these questions - what is knowable and what is unknowable - taught you about what it means to be human.

Carroll: Again, I don't like the word 'unknowable'. I do think that if the laws of physics are knowable then I'll be happy even if I can't know what's going on in all the other universes. But what it tells me about what it means to be human is two things. Number one: we're really tiny. Copernicus said we're not at the centre of the universe. Giordano Bruno and later Edwin Hubble said there are a lot of stars and a lot of galaxies out there. Now, the many-worlds of quantum mechanics say all these other copies of me are out there, that I can't talk to. As science progresses, it discovers a bigger, bigger universe and I stay the same size. Relative to the universe, I seem to be shrinking and that's okay. That's one thing that we learn.

The other thing we learn is in the opposite direction of the idea of unknowability. It's that the very rules that govern not only tables and chairs and planets and stars, but people and the universe itself are knowable. I know that there's no experiment going to be done next year where we learn that people actually can do telekinesis to lift up coffee cups from across the room using just the powers of their minds. We know what the mind is able to do and it still obeys the laws of physics.

What we'd like to know are questions like where did the universe come from? What happened at the Big Bang? There, we truly don't know, but I'm very optimistic that it's knowable and therefore questions about the meaning of the existence of the universe become a little bit more quantitative and we can put some meat on those bones.

Mason: There's still so much we don't know, that soon we may.

Carroll: That's right, yes.

Mason: Sean Carroll, thank you for your time.

Carroll: My pleasure, thanks for having me on.

Mason: Thank you to Sean for sharing his thoughts on the mysteries of quantum physics. You can find out more by purchasing his new book, 'Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime', available now.

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