Modern Wisdom - #126 - Sean Carroll - The Problem With Quantum Mechanics

Episode Date: December 12, 2019

Sean Carroll is a theoretical physicist, podcaster and author. Quantum physics is complicated. The quantum world does not operate like the one we see around us, yet our human experience of life emerges from this strange universe. Today we learn how it can be the case that something so alien can give rise to something so familiar. Extra Stuff: Buy Sean's Book Something Deeply Hidden - https://amzn.to/2rsRcL2 Follow Sean on Twitter - https://twitter.com/seanmcarroll Check out everything I recommend from books to products - https://www.amazon.co.uk/shop/modernwisdom - Get in touch. Join the discussion with me and other like minded listeners in the episode comments on the MW YouTube Channel or message me... Instagram: https://www.instagram.com/chriswillx Twitter: https://www.twitter.com/chriswillx YouTube: https://www.youtube.com/ModernWisdomPodcast Email: https://www.chriswillx.com/contact Learn more about your ad choices. Visit megaphone.fm/adchoices

Oh yes, hello humans, welcome back to Modern Wisdom. My guest today is Professor Sean Carroll, New York Times best selling author of the big picture. And today we are talking all things physics, quantum mechanics, gravity, space time, all the easy to understand topics that you want to talk about at a dinner party. However, Sean is incredibly lucid, super articulate. And if I can understand it, you definitely can too. In other news, I've got some episodes coming up with Coon Smet's behavioral economist talking about time and how we make our decisions relating to time and money. I've got Naval Ravakant's brother, Kamal talking about his new book,

Love Yourself, Like Your Life Depends On It. That is going to be absolutely amazing, and I cannot wait to sit down with him. Ryan Fisher from CrossFit Chalk. Who else? Michaela Peterson is gonna come on in the new year. Oli Marchon from Marchon Athletic will be joining me, so it is a stacked few months coming up.

But for now we're going to learn about what quantum physics is and how it all works and how essentially the world that you see around you is totally not what's going on. So yeah prepare for your mind to be blown. Please welcome Sean Carroll. Ladies and gentlemen, welcome back. I'm joined by Sean Carroll, theoretical physicist, especially in quantum mechanics, gravity and cosmology, research professor at the Department of Physics, the California Institute of Technology, and also the man behind the Minescape Podcast.

Sean, welcome to the show. Thanks very much for having me. Super excited to have you on. Been a little while since we've delved into physics on the show, so we're gonna have to get over a little bit of inertia, some audience and my inertia as well. And we'll kick start everything back off again, right? A inertia is part of physics. That's okay. We understand it.

You are the man if there was ever a man to get over some inertia. It's you right there. And before we before we even start talking about physics, mindscape podcast that you it's not been going super long, right? But you've had some insane guests, like Max, Tagmark, Seth McFarlane, the guy that, the family guy, creating the voice of Brian and Stewie Griffin, that's insane.

Yeah. Yeah, no, it's been great. It's been like a year and a half. I've been very, very lucky in people saying it. Not everyone says yes, but I always wanted it to be one of the things where I wanted to do the podcast was just so that I could talk to intelligent people about things other than physics.

And I've gotten some great guests and all over the spectrum. So yeah, it's been fun. Is that informing the direction of any sort of more writing that you're looking at? Are you tempted to branch out into anything after having these conversations? I've always been tempted, you know, and I always do. So, you know, my previous book, The Big Picture, was very broad, a lot of philosophy, a lot of biology was in there. And my last book was Pure Physics. So the next book is not going to be Pure physics again. I want to keep it, keep it mixing it up. I got you. So tell us about your last book, something deeply hidden. Why'd you write it?

Well, it's a book about quantum mechanics and the world does not need another book about quantum mechanics for its own sake. There's lots of books out there of varying degrees of usefulness. One of the things I did for the book, you can see the results right in chapter one, is I went to Amazon and typed in the word quantum into the search bar and looked at all of the books that had quantum in the title, quantum healing and quantum leadership and quantum yoga. And it just goes on. And none of them have the Schrodinger equation or any differential equations at all in them. So I didn't wanna just be negative,

but I think that a lot of even the good books on quantum mechanics have this attitude of saying, like quantum mechanics is really, really bizarre, it's mysterious, we'll never understand it. Look at how bizarre it is. And I wanted to kind of be the opposite. I wanted to kind of say, look, this is just science. It's perfectly understandable.

Even if we don't understand it yet, there's no reason to think we can't understand it. And even though I have a favorite way of understanding it, what I really care about is the fact that it can be understood. What does it mean for something to not be understood but be understandable? Well, you know, we think that there's plenty of puzzles in science, right?

There are things we don't yet know. We don't know exactly how the moon came to be, right? Was it something colliding with the Earth? Or was it sort of the mutual formation of the moon in the Earth at the same time? But no one thinks this is impossible to understand, right? I mean, we'll get there. It's just a matter of, you know, getting the evidence and doing the theory, et cetera. And I think that quantum mechanics has this weird, unique status where it was put together in the 1920s with sort of a wall around it saying, don't ask questions about the deep things going

on here. What I like to say is physicists are extremely good at using quantum mechanics without understanding it. It's exactly like, I'm good at using my smartphone to send text messages or take pictures, but I couldn't build one, right? And that's how quantum mechanics is for physicists. And I don't think that it needs to be. And I think that this is a huge, huge, wrong turn that physics took. And we're trying to sort of correct the course right about now. So what is it about quantum mechanics that makes it so slippery?

It's that in the rules that we developed in the 1920s and we still teach to our undergraduates today, quantum mechanics is unique in the following way. There's a set of rules for what physical systems are and how they evolve, just like every other theory of physics, when you're not looking at them. And then there's another set of rules that apply when you look at something, when you measure it, when you observe it, and no other theory of physics has anything like that. I mean, there's general relativity, Maxwell's equations,

for electromagnetism, Newtonian mechanics, like there's no set Maxwell's equations for electromagnetic, Newtonian mechanics. Like, there's no set of rules that say what you see and what happens to a system when you measure it. And so why does quantum mechanics seem to need these extra rules? And this is what's called the measurement problem of quantum mechanics. And it's just been hanging around unanswered since the 1920s. Could you break down the measurement problem? Could you explain it with some experimental examples? Sure. You know, what we teach our students is that if you take an electron, for example,

right, you imagine the standard picture of an atom that we've all seen that has a little nucleus at the center and electrons are orbiting around it. So we know that can't be right, because electrons orbiting like that would give off light, would lose energy, would spiral in, and all atoms would collapse into a point if that picture were correct. So they came up with the idea,

well, maybe the electron is not a particle. Maybe it's a wave, maybe it's spread out, and the wave sort of has a minimum wavelength, just like when you pluck a vial in the string or something like that, there's different allowed wavelengths. And that's why the electron can't collapse because it's actually spread out. That's a great idea. And then we have an equation, the Schrodinger equation, given to us by Irwin Schrodinger, which says how the electron behaves, etc, except that when we look at the electron, when we shoot an electron through a detector, it doesn't look like a big fuzzy wave,

like an electromagnetic wave would look like. It looks like there's a track, like there's a trajectory, like it's a particle, like it's located somewhere. So that seems to imply, just from on the one hand, we need the electron to be a wave so that atoms don't collapse, but on the other hand, we need the electron to be a wave so that atoms don't collapse, but on the other hand, when we look at it, it looks like a particle that we need two separate sets of rules.

And in what's called the Copenhagen interpretation, the standard interpretation of quantum mechanics put together in the 1920s, they sort of just leaned into that. They said, yes, there's a separate set of rules for what happens when you measure a quantum system. But what they didn't do is tell you what it means to measure a quantum system. Like does it have to be a person? Could it be a video camera? Could it be a cat?

You know, does it require a router? It's someone got a ruler out. Yeah, how quickly does it happen? What if I just glance at it and don't measure it very accurately? Does that count? The corner of my eye. Yeah. Yeah.

What we teach our students is that when you measure the system, its wave function dramatically changes. It collapses, as we say. It was all spread out. And then it collapses to being at one point. That's where you see it. And you can't even predict where it's going to collapse to. You can only say the probability of getting

different measurement outcomes. So the questions of what counts as a measurement, when does it happen, how quickly does it happen, why is it a probability rather than a definite outcome? All of these are bundled together in what you call the measurement problem of quantum mechanics. How much of that is an experimental lists problem, and how much of that is a theoretician's problem? Zero of it is an experimentalist's problem. I mean, experimentalists have done the experiments. I mean, it was an experimentalist problem in the 1920s,

but those experiments have been done. There's extra experiments to be done, but the fact that when we look at the quantum systems, they behave differently than we don't look at them. It's not because the experiments are very good. Experiments are buying. It's because the theoretical prediction is wildly at odds with what you observe if you don't think hard about what's going on.

So there's no, in your view, is there any more work to be done from the theoreticians at this point, or is it that the, sorry, from the experimentalist at this point, or is it that the theoreticians kind of need to get the chalk on the blackboard and do a little bit? Yeah, I would say the theorists are way behind right now because they stopped thinking about this in the 1930s. You know, people like Einstein and Schrödinger, so not exactly light weights in the history of physics. They were outraged by the state of quantum mechanics. They're like, we're not done. Come on. Obviously, we need to think harder. We need to do better than what we have now. But people like Neils, Boer, and Berner, Heisenberg and their friends said, no, no, no, we're basically done. Let's put this to work and, you know, do particle physics and do nuclear physics and do condensed

matter physics and so forth. And as a result, for whatever reasons, you can, there's a long list of reasons as interesting topic to talk about. The physics community went on the side of Bore and Heisenberg against Einstein and Schrodinger and they said, let's just not think about this very hard. They said this very explicitly. When people started to try to think about it, they got kicked out or ignored or abandoned or whatever. I would say, yeah, the theorists are way behind right now. We're just not anywhere near where we should be

in terms of thinking a few things like, you know, what counts as a measurement. There are different ways to experimentally distinguish different theories. How do you get the classical world of the quantum world, all these questions? Is that one of the reasons that there is a discrimination towards the classical world, that the real world applications appear to be the classical world is emerging out of the quantum world.

We know enough to then be able to build a rocket or make a building, work or create an iPhone, and it's like, well, we'll look at that at some point in the future. Is that why? Well, a lot of it is, you know, the story that we were told back in the 1920s is, you're an observer. The thing you're observing quantum mechanically is that particle, okay? There may be a couple of particles, an atom or an electron or something like that, a photon, and you're big and the particle small. And forget about in between. Like you're so big compared to the particle that you're basically classical, right? So you can have

a classical observer, according to this story, which I think is a bad story, but this is what we tell ourselves. The observer can be treated classically. The system that they're looking at is treated quantum mechanically. And you can get away with that as long as you were, you know, in the 20s or 30s and just observing particles. These days, where our technology is moving forward to the extent where we can create quantum mechanical effects in much bigger systems. So the dividing line between quantum and classical

is not nearly as clear as it used to be. And so that's part of what I mean if you want to build a quantum computer for example or all these other quantum technologies, you actually have to take quantum mechanics seriously in a way that you didn't have to if all you want to do is bump two particles into each other and see what happened at the end of the day. Where does quantum mechanics become classical mechanics? Is there a line? I would like to know this question. So there's a whole nother long, you know, you don't get me started, is what I want to say. We are so bad at quantum mechanics that one of the ways that we're bad is that we take

the classical world for granted. So the classical world just to be clear is the world that you're used to, right? There are tables and chairs and baseballs and soccer balls and they move in trajectories and you can look at them and see where they are. What you need to know is where they are and how fast they're moving and you can predict everything else from that. The quantum world, as we learned to describe it in the 1920s, is a completely different place.

You know, there are no positions or velocities. That's just not like we don't know what they are. That's not what exists. What exists is this wave of entangled particles all throughout the universe. It's an entirely different way of talking. And you might think that there is this huge challenge of starting with this abstract wave

that quantum mechanics talks about and explaining how it becomes to look like a classical world with baseballs and tables and stuff like that. But basically we cheat. We say, well, we know that the world does look classical at the end of the day, so we're going to put in that answer into all of our analysis. I'm going to treat me and the planets as classical and not worry about anything until we get down to elementary particles. So this question of how you go from just a purely quantum description to the classical world.

I have opinions about it, but it's one of the things we haven't explored as theorists nearly as well as we should have. Some of the very basic questions are completely unanswered at the current state of the art. It's like, you know, when you go to a theme park, and you must be this tall to ride the ride, it's like, you must be this big to be classical mechanics, like where's that, how high is that from? It's like, come from sign, that's right. Is it strange, again, coming from someone

who has no idea about physics? To me, it would seem natural for physics to be constant, for there to be rules and laws that govern everything, and for there to not be some sort of resolution at which, if you look below that, in terms of how closely you look at something that those laws change. Is that surprising? Well, I think it's not the situation we're really faced with. It's quantum mechanics, it's not something that becomes true when you look at small systems. That is what we were telling ourselves back in the 1920s, but again, that's kind of nonsense. Quantum mechanics is true at all scales.

You and I are perfectly quantum mechanical and there's a limit where things become big enough that you don't need quantum mechanics to describe what's going on. There's a good approximation given to us by classical mechanics. So it's not that it would be indeed very, very weird if the rules of nature were different for big things and small things. And I'm sure we'll get to at some point in the conversation, the many worlds formulation of quantum mechanics, which was invented by Hugh Everett, who was

a graduate student in the 50s. And this is exactly one of the things that he complained about. He was writing letters to Neil's board, et cetera, saying, why in the world can you treat the observer as classical? We're made of atoms and things like that. We're made of things that obey the laws of quantum mechanics. Why don't we? We should take that seriously. And that's exactly what he showed how we could do. So just before we get on to that, I'm writing thinking that there was a few people in your book that you quote as having been dissuaded from working on quantum mechanics, you self-included,

right? Could you tell us some stories about that? I had Sabina Hassan Felderon talking about politics in physics and for the listeners who didn't catch that episode it's a while ago it's back in the 30s I think with the number 30 not in the 1930s. Not the 1930s, that would be good. Sick podcast. And I totally didn't understand just how political and subject to social bias and reputation and all of that stuff that the world of physics was. And the synopsis that Sabina sort of came up with that she finished was, well, if nothing

else today you have learned that politicians are physicists are people too. And I was like, yeah, yeah. So could you talk us through some of the politics of quantum physics? Yeah, you know, physicists are people too. And there's a lot of politics and reputation. All that stuff is very true. I don't like to use the word politics in this context because it has ramifications like it has connotations elsewhere.

And furthermore, you know, it's very hard to get this right because especially in theoretical physics, where we're saying, well, we know that our current theories are pretty good, but not perfect, we want to get to better theories. Everyone has a different opinion about which theory might be better. We don't know the right answer yet. And so if someone's best favorite theories,

not the one that is accepted by other people, they're gonna say, ah, those people just don't appreciate. You know, like theory, they're just being political and driven by, you know, groupthink and whatever. And maybe that's true or maybe your theory's just not as good. Right? So it's, I'm literally, I'm not saying that rhetorically, maybe your theory is better, maybe it's not, and that's very, very hard to pick that out in an unbiased way.

Especially in a situation like we're in right now, or like we have been in quantum mechanics for many decades, when the experiments don't clearly distinguish between one way of doing things and another way. So putting aside like politics and bias and stuff like that, it's certainly true that the course of physics is heavily influenced by influential people, right? It's someone who is a brand new graduate student and has a good idea, an idea that is not in accord with the prevailing standards of the field is going to have a much harder time

than a famous Nobel Prize winner who everyone respects. And that's not crazy because there are a lot more people who are young graduate students who have really bad ideas, right? It's not just by being unknown, doesn't make you right. So there's a filter, you know, physicists have a way of dealing with new ideas and just like everyone else in the world, they're like, they pay attention to different ideas, depending on where they're coming from and how plausible they sound and how well articulated they are, and so forth. Having said all of that, I do think that there, as I've said, there was this

massive abandonment of the right set of questions to ask in the whole 20th century by physicists. Part of it was, so there's so many different things going on. Part of it was just World War II, happened, right? I mean, think about when this is and think about who it was. A lot of these smart people were Germans. A lot of them were Jewish. So in the 1920s, you know, Niels Bohr and Albert Einstein or Urban Schrodinger and Werner Heisenberg

could get together in the same room and talk about these things. Ten years later, they're all in different continents, right, or different countries, and they can't travel back and forth, and they didn't have the internet, so just the pace of progress was enormously slow down. The focus of physicists shifted to very practical things, right, building bombs and stuff like that, and then even if they did weren't practical, things like understanding particle physics and quantum field theory, and all these other very pressing questions.

So quantum mechanics measurement problem kinds of questions were put on the back burner. And for that matter, it's not clear how to make progress on these questions. If you have a question about particle physics, you can take two particles and smash them together and do the experiment and see what happens. For the interpretations of quantum mechanics, as they used to be called, it wasn't clear how you would ever know what the right answer was. And Baudi is to entry

quite high. Well, it's the state of the art was so bad that the ideas that were being thrown around The state of the art was so bad that the ideas that were being thrown around did sound more like literary interpretations than good old physical theories. The state of the art has gotten a lot better. Now we're actually talking about real, distinct, well-defined, rigorous physical theories that we can distinguish between. But in the 40s and 50s, that just wasn't the case. It's a shame. It seems to me a surprise as well that Einstein's side of the fence didn't win. Yeah, exactly.

So that is a surprise you might think that Einstein has a plan. You would have been like the, you know, the, the deon say wilder of the big heavyweight champion of the world when it comes to like directing the physics community. But he was also kind of a loner, right? He didn't collaborate a lot. He didn't have students very much. He moved to the Institute for RANDSTUDI in Princeton where he didn't teach classes, where someone like Neils IV was endlessly gregarious.

Worked with everyone, had an institute of his own where he invited everyone to hang out and so forth and would badge with them and hector them into doing things. So there's a personality. I think this goes to Sabina's point. They're just the fact that Neel's Borin Albert Einstein were very different personalities affected the course of physics. And it became so bad that at some point the major journal within the physics community just stopped looking at papers on the foundations of quantum mechanics. David Bome, who was one of the best people working on this stuff, was hounded by the House on American Activities Committee for being a

communist in the 1950s and was eventually he had to flee to Brazil. being a communist in the 1950s and was eventually, he had to flee to Brazil. I mean, he had letters of recommendation from Einstein and Oppenheimer and he couldn't get a job in the United States, right? John Bell, who was a brilliant theorist who worked at CERN, was a particle physicist by day and worked on the foundations of quantum mechanics at night

and didn't tell anyone that he was a complete, you know, next-door office, didn't didn't tell anyone that he was completely next door office didn't know that's what he was doing. And now his theorem is very famous and we all know about it. Even today I tell the story in my book, I do a bunch of different things. Some foundations of quantum mechanics,

some gravity and particle physics and stuff like that. And I'm told that when we're doing our grant renewals, don't talk about quantum mechanics, talk about cosmology or gravity. That's the serious stuff. That's what's going to get us funded. That's the sexy stuff. It's interesting. You've got someone like Albert Einstein that still needs to play the game. I think I was having a discussion yesterday with someone on Instagram live. And we were talking about exactly that, that you can have the most virtuous, beautiful message in the world, which is full of integrity and adding value. But you still got to play the game to some degree.

And, you know, if Albert Einstein perhaps fell a foul of that a little bit, and he's, you know, one of the, if not the best known physicist of all time, I think there's probably a lesson to be learned that. I don't know. It's probably a difficult pill to swallow, isn't it? Someone who's got so much talent and, and, and had raw ability and understanding, with like was just not playing the social game correctly. It's a, interesting, really interesting point. It's, it's part of it. And the other part is the other part is Einstein had a vision for what the

theory should be like. He could say over and over again, I think the theory should be like this, but he never actually came up with a theory that was like this. In fact, this famous theorem of John Bell, Bell's theorem that we now think is famous. Basically says that what Einstein was trying to go for is impossible to get. So, you know, Einstein could hector his colleagues in the community, but he didn't offer any useful alternative at the time. So, the alternatives that we're faced with now

are all crazy from Einstein's point of view. They either involve non-locality with spooky action at a distance, or branching into multiple worlds, or dramatic unpredictability, or something that I would not have liked. And so, I think that's, if he had come up with a killer theory that would replace the conventional wisdom, things might have been very different. Was he working on that before he passed, if that was part of the work that was unfinished? Well, I think he, I don't think that was a major focus of his, but, you know,

he did in 1935, published a famous paper called the EPR paper, Einstein, Padulski, and Rosen, where he basically emphasized the importance of what we now call quantum entanglement. And I think it was Ferdinger. Uh, Ferdinger and Einstein wrote letters back and forth all the time because they were on the same side. And Einstein helped Schrodinger come up with Schrodinger's cat example, and Schrodinger gave the word entanglement to Einstein's theoretical prediction. But it wasn't an alternative theory. He was just saying, here is the craziness of quantum mechanics.

You can't possibly believe this. I mean, that was also Ferdinand's point, was Ferdinand's cat. It wasn't like, wow, look at how crazy quantum mechanics is. It's like, you can't possibly believe this, can you? And the answer tends to be, yeah, we should believe it. So we've touched on three different things that I've got in my notes that I want to talk about. One of them being how spooky and how the uncertainty within quantum theory allows spookiness and spirituality to be kind of crowbarding. Another one of them is the many worlds. So let's, where do we want

to go first? Let's do, what do you think? Many worlds or should we, should we work on the spookiness? Well, let me explain in tanglement a little bit, because that will help us understand many worlds. And again, this is Einstein's point in his 1935 paper. So I like to describe it using a the Higgs boson, which Einstein didn't know anything about, right? So elementary particles, like electrons and Higs, bosons and photons can spin. And they have a feature that they're always spinning

by the same amount. They don't speed up or slow down. But an electron, for example, can spin clockwise or counterclockwise. The haigs boson doesn't spin at all. It's spinless, spin zero. So the haigs boson can decay. It can, in fact, it does decay very, very quickly

into, let's say, an electron and a positron. Positron is the anti-electron. And we know the total amount of spin in the universe is constant. It's not being created or destroyed, angular momentum. It's not being created or destroyed. So how in the world can a spin zero particle decay into two particles that have spin?

The answer is they better be spinning in opposite directions, so it cancels out. Okay, yep. Yep. And that makes perfect sense. Sorry to inject that. How do you know it's spin zero? Sorry, how do you know that the total amount of spin in the universe is the same?

Oh, how do you know it's the same? I mean, it's one of the famous conservation laws of physics. So energy is conserved, momentum is conserved, angular momentum is also conserved, and spin is a form of angular momentum. Even down at the quantum level, there's no special rule down there. Nope, there's no special rule down there.

It's an absolute, you know, quantum mechanics doesn't say anything goes. It doesn't say anything can happen or even everything has a probability of happening. There are some things that just don't happen in quantum mechanics. Or do you happen? Or all of them in all of these? There's also things that do happen, but you know there's something that don't happen. So a spit zero particle converting into a spin one half particle is not ever going to happen.

Go ahead. It's just going to happen. Cool. So when, so it's, so the higs splits into into two particles an electron and a positron. Oh, sorry, that's my phone. That's a good ringtone. Well, yeah, it's a tune. Yeah, Medesky Martin Wood for anyone interested out there in podcast land. So we say the electron can be spin up or spin down. And quantum mechanics says that the, in fact, when you haven't yet looked at it, the wave function, as we say, the electron, is a superposition of spin down and spin up. It's neither one or the other. It's some combination of both. So there can be, for example, if it's an even superposition, there's a 50-50 chance of observing it's spin up or observing it's spin down.

Likewise, for the positron, going the other way, it can be spin down or spin up and it can be 50-50 chance. And what you know is they're oppositely oriented, so their total spins add up to zero. But you don't know whether it's electron up and positron down or electron down and positron up, or in fact, what it will be is some superposition of both. So that's right there, that's entanglement because it's saying that I don't know whether the electron is up or down, what I measure it,

but I know that if I measure it, the positron will be the opposite. And what this is telling you is that in quantum mechanics, you don't have a quantum state away function for the electron and then separately have a quantum state, a wave function for the electron, and then separately have a quantum state a wave function for the positron, you have one wave function for both at the same time. And it says there's a probability that one is up, one is down, and it's probably that one is down, one is up,

and there's no probability they're both in the same way. Could there not be the case that one is up and one is down? It's not a case of them being either way, and when you observe, it's not a case of them being either way, and when you observe it just happens to be that. It could be, but the quantum mechanics makes a prediction, right? It makes a prediction for when you see the particles come out of Higgs boson decay, there be 50, 50 up and down for the electron, 50, 50 up and down for the positron.

Okay, so it's not a case that it is one of those two and you just look at it and it is the one that it is, it's a case that it could be either of them and the observation causes it to choose. Well, you know, this is exactly the question that Einstein was worrying about. So he makes the point that I can make these two particles. He was using different examples, but the underlying point is the same. I could take one of them and just let it fly off to another star system, light years away, right? And then I could measure my particle here. And the entanglement doesn't fade away or get less and less as the particles become separated.

So if you take quantum mechanics seriously, says Einstein, you're telling me that when I measure my particle here to be spin up or spin down, instantly light years away, the other particle changes to be, obviously, oriented. That's his spooky action at a distance. That's what he's like, how does it know for light years away at Alpha Centauri that I just measured the spin right here. And he's like, and again, you know, his thing is, surely you don't believe. So he thinks Einstein is convinced that there was some fact of the matter about when you were going to measure this electron,

we're going to get spin up or spin down. And therefore, there's some fact of the matter for the other one too, that there's something, something deeply hidden, right, that would predict with 100% probability what you're going to eventually observe. And what John Bell proved is that that can't happen unless there's some other kind of spooky action at a distance all along. So the spooky action at a distance is just absolutely part of quantum mechanics like it or not. Hmm. I'm alright in thinking as well that that's the measurement of one versus another that was four light years away would happen instantaneously. So that would be the idea, yeah, that's the direction, which also would, I mean, does

that break the maximum speed of something? Is it actually a speed that something's happening at the same time? That's a great question. The answer seems to be that it violates the spirit of the speed limit, not the level of the ball. The energy of the spirit of it. This might be true, but it's unfair and we really shouldn't sanction it. The thing is, when you measure the particle here, you know that if you get spin up,

that's gonna be spin down. If you get spin down, that's gonna be spin up. So the state, the way that we describe the particle over there does change instantly right away. Outside of your light cone, right? Outside of your light cone, faster than the speed of light. The thing is the person over there doesn't

know what answer you got until until the very last horse their concern. It's still 50 50. So you can't actually use it to send any information. This the speed limit the speed of light so. Well, you know, Einstein invented that. So of course, he was very offended by this. So it's like that the quantum mechanics was somehow sneaking around the law somehow. He didn't like that at all. Yeah, that is funny. Okay. So we've got entanglement. Yeah. And now let's go many worlds. Good. So Hugh Everett in the 1950s, looks upon quantum mechanics and just says, oh my God, this is not good enough. And he says, basically what you've forgotten two things. One is the observer, as we've already said, is a quantum mechanical system themselves.

So they're not really classical. So when you observe, let's say when you observe at a single electron, what does it mean to do an observation? Well, if the electron has a wave function and can be in a superposition of different things, then in principle, you can have a wave function too and you could be in a superposition. And the other thing that he says everyone forgot

is entanglement. So not only can you have your own wave function, but in fact, really, there's one wave function that describes you and the electron at the same time. And he says, forget about all these dumb rules, about measurement and observation. What if we didn't have those rules? What if we just had the equation, the Schrodinger equation, which says, what happens when you, the quantum system that is you, interacts with the electron that's in a superposition

of spin-up and spin-down. He says, the answer is 100% perfectly clear. You and the electron evolve into an entangled superposition, where there's one part of the entanglement which says the electron was spin-up and I observed it to be spin-up. There's another part of the entanglement which says the electron was spin-down and I observed it to be spin up. And there's another part of the entanglement which says the electron was spin down and I observed it to be spin down. And everyone agrees that that's what Schrodinger's equation predicts. That's a nice thing about having equations. They predict things and they're not really up for debate. That's what it predicts. But where they are upset is nobody in history has ever said, you know, I observed that electron and now I feel like I'm in a superposition of having observed it spin up and spin down. And Everett says, I know why?

Because there are now two copies of you. There's one copy that observed it spin up and one copy that observed it spin down. And what happens to one of these copies of you will not affect or be influenced by or interact with the other copy ever again. They go their own ways. It's as if they are separate worlds. So the many worlds interpretation of quantum mechanics did not come about because you ever said what quantum mechanics needs is an infinite number of extra worlds. It came about because he said what quantum mechanics needs is getting rid of all these dumb rules about measurement and probability and collapse. Just take seriously what the equations are trying to tell you, and it says that you will branch into separate worlds, and all you have to do is deal

with it. If you're willing to deal with it, all of your problems go away. In other words, if you think that an electron can be in a super position of spin up and spin down, then you should be able to believe that you can be in a super position and you should be able to believe that the universe can be in a super position of all different things. And if you treat those different parts of the super position as different worlds, you solve all the problems of quantum mechanics. When you say interact with or observe the electron, what does that mean? Does that mean like how high of a fidelity are we looking at this? Is it every plank length amount of time that happens of every single movement and every

single potential movement. How does that work? Do you know what's going on? Yeah, no, it's a very specific criterion. When a quantum mechanical system that is in a superposition of different possibilities becomes entangled with the wider outside world, the wave function branches. So there's an electron sitting in every atom in your body right now, but mostly they're not becoming entangled with anything. They're just staying in their atoms, right? That's just what they're doing. But when you take an electron and remove it from an atom and then send it through a magnetic field, it'll be deflected one way

if it's been up, be deflected another way if it's been down, and then you let it hit a screen, and you observe a dot, either there or there. And that process, observe a dot, it angles the electron with the wider world, because there's now a dot on the screen. Okay, and that's world branches. So it only happens during specific sorts of experiments, or is this happening when I sit down in a much air? It happens when quantum systems become entangled with the outside world. So it's happening all the time in your body right now because there's a certain number of radioactive nuclei in your body. There's about 5,000 radioactive decays per second in a typical human body. And so that radioactive decay means that some nucleus emitted a particle and that

particle bumped into things and became entangled with the outside world. So, 5,000 times a second are actually much more than that, but at least that many times, you're branching the wave function just by sitting there in your chair. Multiplyed by however many particles there are in the universe that are also potentially doing this. Yes, per second. Again, most, you know, like a typical photon or whatever, it just spreads out throughout the universe. It doesn't become entangled anything until it hits a telescope or a planet or something like that.

So, you know, radioactive elements are rare, you know, in the universe. So particles can become, can go a long way without being entangled with anything. But they often are also. So both things are happening. Yeah. How how is the two worlds that exist at the same time? How does it how do I not just see another chair or another Atom or like there be an extra little bit on the edge of the chair. What's going on that? Well, because they have truly become separate separate. And this is the process, which in quantum mechanics, we call decoherence. I like to think of Schrodinger's cat, right?

So in Schrodinger's cat, there's the box, and there's the quantum event, which with 5050 probability, either in Schrodinger's telling either kills the cat or keeps it alive. And in my book, I made it put the cat to sleep or let it say awake, because it can just be sleeping gas. You had no reason to kill the cat. But the point is that in the conventional telling, the cat is literally neither awake or asleep. It is in a superposition of both until you open the box and you observe it.

And again, Schrodinger is saying, surely you don't believe that. Surely you don't believe that the state of the cat dramatically changed. So here's what a modern Eberidian would say. If the cat's asleep, lying on the floor of the box versus being awake, it's up and walking around trying to get out of the box, all of the stuff in the box,

all the air molecules, all the photons of light and so forth, may or may not bump into the cat, depending on whether it's lying on the floor or walking around. So what that means is that these two different parts of the wave function of the cat, awake and asleep, interact with the environment around it in different ways. And that means the cat becomes entangled with the environment around it, like it or not. So the universe branches into two different copies long before you open the box. To get to the answer to your question, because there's so many photons hitting the cat or

not hitting it, and because one will get absorbed, if it hits the cat or just go right on by, if it doesn't, those two parts of the wave function of the universe, cat awake and cat asleep become completely separated from each other. So you, if it's you in one branch with the cat awake, you can't see any result from the branch of the wave function where the cat is asleep. The particles in that branch of the wave function just don't interact with you. They're perpendicular to you literally in the space of all possible wave functions. It's so challenging to wrap your head around for someone. I recommend buying my book. That's a solution linked in the show notes below, ladies and gentlemen, follow it through that.

You know, we're trying to use, and this must be, I don't know whether it's challenging for you or whether it adds or removes difficulty for you, but throughout this conversation, you're having to use classical systems as analogies to describe the quantum systems you're talking about, plucking a violin string, or all of these things. It's so funny and interesting that we have to about plucking a violin string or all of these things.

It's so funny and interesting that we have to analogize classical physics to describe the quantum world, isn't it? Yeah, it is, but you know what? We're people. That's okay. There's two levels of this. One is, of course, as we're having this conversation, I'm not just gonna write down the equations and point at them and see, say, any questions, right? You know, we're gonna talk about analogies and metaphors and pictures and visualizations

and things like that. And that's very natural. And in some sense, it's an important part of doing science because we have, you know have what Wilfred Sellers called the manifest image of the world, the view that we all carry around that there are people and planets and cats and dogs and purposes and motions and things like that. And then we have the scientific image of the world, which is the better and better view of what's going on given to us by science.

And we have to match them somehow, right? We have to say like what part of the everyday world maps on to this part of the formalism that we're talking about. So all that's, you know, well and good for doing the science. It's inevitable. The other level, which is a little bit more problematic, is that even the world's best professional physicists still kind of think classically. And what that means is usually when we invent a new quantum mechanical theory to explain some particles or quantum gravity or something like that, we start with a classical system and we quantize it.

There's a whole procedure that you can look up in textbooks called how to quantize classical physics. And I think that leads us down a bad road because the world doesn't work that way. The world doesn't start classically and then quantize things. The world is quantum and classical physics is a good limit, a good limiting case, approximation that's valid in certain circumstances. So thinking about it that way is a little bit of a shift of perspective, which I personally think is going to be extremely important when it comes to solving some of the more difficult questions that we're faced with in modern physics. I get you. So moving on to the spooky stuff. I'm sure that a lot of the people that are listening,

and you know, you said yourself, you do your Amazon search, you're thinking about what I'm going to call my new book, it's all about quantum, quantum yoga, quantum leadership, quantum leadership, quantum accounting. I can probably do some quantum, quantum accounting. But yeah, it's, it's used to crowbar all sorts of stuff in, but one of the things is kind of more spooky, spiritual stuff into reality. Yeah, why do you think that is? Well, I think there's two things going on. One is wishful thinking, right? Quantum mechanics seems mysterious

and there's a lot of other things that seem mysterious, so maybe they're related somehow. And you know, other theories of physics are put to bad uses also. So it's just sort of a bit of slopping as the people necessarily inevitably give into. But the other thing is that physicists have done a bad job of trying to understand quantum mechanics and for a long time, they've made it seem like the role of a human observer is somehow crucial to explaining what happens in quantum mechanics, someone measuring things, right? Someone actually looking at systems, and that lets you insinuate that somehow you are bringing the world into existence just by looking at it. And that is very close to saying that you influence the world,

that the ways in which you look at the world and the ways in which you interact with it can somehow change the world out there. So not only do you bring the world into existence by looking at it, but you can choose what kind of world to bring into existence. Now, none of that is anything to do with quantum mechanics. That's all just crazy talk, wishful thinking, woo-woo, kind of world to bring into existence. Now, none of that is anything to do with quantum mechanics. That's all just crazy talk, wishful thinking, woo-woo, kind of nonsense. But the physicists should share a part of the blame for letting people talk that way, because they've not been at all clear about what quantum mechanics really says. People talk about manifesting

realities, and I think they use the measurement problem or that I think they would call it like the observer effect or whatever, right? To explain that stuff. There was another conversation I was having recently about online coaches, so online fitness and diet and nutrition coaches. And I was asking these guys, I was saying, why is the industry filled with so much kind of misinformation and so many charlatans and people like that?

And they said that it's because of a lack of obvious causality between what happens and then the results on the other side of it. And where you have this vacuum of a lack of information, it just allows people to just throw speculation in there, right? And it's like, this sounds like a plausible narrative. And, you know, if there is anything that I hope

that the listeners can take away from today's conversation, it is that plausible narratives are probably not the world in which quantum physics operate. Like it's not because there's a good guy and a bad guy. There's like it's not that the Higgs boson was like the referee or something like that. Like we love to personify these sorts of stories, right? Because it brings them onto our level.

It brings them onto the level of a social being. But that's not what's happening. Yeah, no, it's absolutely right. And as David Albert, who's a philosopher of physics, once said, when it comes to trying to understand the deepest mysteries of nature, if they don't make you uncomfortable, you're not doing it right. Because, of course, the world's most fundamental level

is something very, very different than we experience in our everyday lives. So this is one of the things that is an important consideration when you compare many worlds to other possible interpretations of quantum mechanics. Many worlds is the simplest. As the fewest equations, the fewest ideas, it generates what you see in a very natural way. But it's very far away from what we observe, right? It's so different in structure and language

than the world of our everyday experience that it is perfectly legitimate to say, I just don't believe that this very, very simple lean-in-mean theory really gets us to the world we see. So other alternatives to many worlds generally bring in a lot more extra stuff that is somehow latching on to the classical world that we experience, whether it's extra hidden variables or ways that particles behave or whatever. And it's not crazy to think that that might be the way to go, that there might be something extra other than the

pure bare bones form Lism of quantum mechanics that gives us this classical world we see around us. But the, I would, for people like me, the many worlds version is just so simple and so compelling that it's worth taking that extra effort to map it on to the world we see. So we actually see it as sort of a feature, not a bug that many worlds is so alien and so different because that's what the nature should be like at its deepest level. Hmm. So when people say to kind of close the door to hell

or hopefully that is the observer effect and the measurement problem for, you know, people that are reading the secret and manifesting good energies and bad energies and stuff like that. Without observers in the universe, would everything continue just as it is? Yeah, absolutely. I mean, not only that, but a theory like many worlds or the respectable alternatives to it, hidden variable theories, dynamical collapse theories, the whole bunch of different alternatives, the word observer plays no role. You know, that was a useful approximation when we didn't understand quantum mechanics very well. What we

talk about now are the actual equations telling us what's happening. When a big system with many degrees of freedom interacts and becomes entangled with a small system that's in a superposition, etc. So it was just the bad old days when the idea of observers or experiences were thought to be in any way related to quantum mechanics. And that's just being grandfathered in. Well, not everyone agrees, right? Progress is slow, especially when there's not a killer experiment to show everyone

they must get on the bandwagon. So progress will happen, I think. But there's still people out there trying to make connections between quantum mechanics and consciousness or something like that. And it's much less common than it used to be. It's dying out, but there's still some remnants out there. Yeah. But I remember reading, I might have even heard you say it yourself, talking about how if we knew the position and the velocity of every particle that we could work time back infinitely,

we could work out why everything had been, is that correct? Classical mechanics that would have been true. This is a famous thought experiment from Pierre Simone Laplace around the year 1800. So Isaac Newton gave us the rules of classical mechanics in the 1600s, but it took some time, even though classical mechanics we think of now is pretty trivial compared to quantum mechanics, it still took some time for it to sink in. This idea that left to its own devices, an object will keep moving in a straight line at a constant velocity, is actually pretty radical when you think about it.

No one has ever seen that. Seat things that you don't push on them, they come to a rest, right? As Aristotle would have predicted. So it did take some time for people to catch on the Newton's way of thinking. And Laplace is one of the first people to really get it at a deep level in his bones. And he pointed out that Newton gave us a set of laws,

set of laws of motion, equations of motion, with the property that if you tell me the position and velocity you tell me the position and velocity of everything in the universe, and I have infinite calculational capacity, the future and the past of the universe are now set. They are completely, deterministically predicted by those laws and by that current configuration of stuff. The quantum version of this depends on whether or not, depends on what your favorite version

quantum mechanics is, which we don't agree on. So for many worlds person, there is a version of this, which says that the wave function of all the branches, all the worlds at once, evolves completely deterministically. So it evolves in the way a Laplace would have fought. So you can wind it backward or forward, but you need more than just what the world is doing in your branch right now. You need what's going on in all the branches of all the different parts of the wave function

of the universe. OK. You need a fabric of computing. You're not going to get any one of these things. This is purely a product. That's not even a spider to trying to do it. Wow, yeah. So to round up, can you tell us what you're working on

at the moment or what's been sort of occupying your time right now? Well, this question that we talked about about really taking quantum mechanics seriously and trying to derive the classical world from that rather than putting it in, sneaking it in by hand, is really beginning to seem more and more important to me. So I'm working on that both sort of at the most fundamental level,

just saying, you know, okay, quantum mechanics, how do we go from quantum mechanics to a classical world at all? And then more specifically, we have this longstanding worry about gravity. Einstein explained that gravity is the curvature of spacetime. That's Einstein's general theory of relativity. And ever since then, we've been struggling to reconcile gravity with quantum mechanics. So my conviction is that one of the things holding us back has been this insistence on starting with a classical theory and quantizing it.

So you can start with general relativity and try to quantize it. It doesn't work. So now people start with something like string theory and try to quantize that. I think that maybe you'll get some insight by just starting quantum from the start. And if you understand in general how the classical world can come out of the quantum world, then a classical world featuring curved space of the quantum world, then a classical world featuring curved space time and gravity and things like that should hopefully be part of it.

How easy is it for you to describe why quantum theory and gravity don't agree? Ah, it's not easy, but I like to say there's sort of two different sets of puzzles, and either one of them alone would probably be enough But there's two of them one is just that there's art what we call technical puzzles, right? If you take X plus Y and X is a finite number and why is a finite number and you add them together and get an infinite number Something has gone terribly wrong, right? So This happens when you start with a classical theory and try to quantize it, often things that should be finite blow up. You get infinities and Richard Feynman and others

back in the 50s, one Nobel Prize is showing how to tame these infinities in quantum field theory. We've not been able to tame them in general relativity. And so that's the biggest, single, nice thing about super string theory is that these infinities go away. Okay. So there are technical problems that string theory seems to help us with. It, at the cost of raising an entirely new technical problem, namely that string theory really only makes sense if space time is 10 dimensional. And we look around and space time looks like it's 4-dimensional, 3-dimensions of space, 1-dimension of time. So, string theorists need to be very, very clever about getting rid or hiding those extra

6-dimensions, and that's another technical problem we haven't quite solved yet. There's also the conceptual problems. So, when you talk about electromagnetism or particles, other things that we typically do in quantum mechanics, you can at least say things like, okay, I don't know where the electron is, it has a wave function, but I have two particles that are going to come in, they both have their wave functions, they're going to interact when they're overlapping with the same point in space, right? That's when two things overlap.

That's the two things overlap. That's the principle of locality. That's what Einstein was very, very protective of. He wanted locality to be very, very important. But here's the thing, in quantum gravity, you're gonna take space time and let it be curved, and you're gonna quantize the whole kit and caboodle, then just like an electron can be in a superposition of spin up and spin down,

the geometry of spacetime itself should be able to be in a superposition of all different kinds of geometries. And when that happens, you can't even point to a location in space and say, here, because in these different parts of the superposition and these different geometries of space time, there's no way to associate one point in one space time with another point in the same space time with a different geometry.

The idea of a fixed point in space seems to have no meaningfulness in quantum gravity. So what are we going to do about that? How are we going to make sense of the idea that fields and particles only interact when they're located in the same place? When there's no such thing as being located in the same place in quantum gravity. So that's a conceptual problem. It's not like things blew up when we got infinity. It's just like, what do you do with this? Like, what's going on? How do we make sense of this? And that's the kind of thing people are still struggling with.

To finish up, one thing that I've always been thinking about is, to me, again, as a total newb to physics, it feels like theories should reduce down to something that's very simple. I don't know why. It just seems like there should be universal rules which don't require an awful lot of complexity to get back to. Right. Do you think that as we roll forward over several hundred thousand years

that we are going to see these theories become more complex or more simple? Well, I think I'm pretty optimistic that things will become more simple. That's certainly been the way physics has been going for a very long time. Many worlds is very, very simple, just like general relativity is very simple. They're both very alien to us. They're not our everyday experience, so they seem difficult, but no one denies that writing the theory down is really, really simple.

The algorithm is very, very short. There's no guarantee that it continues like that. And of course, like we said, there's a whole extra work to be done at mapping these fundamental theories to the everyday life. I don't think that the theory of psychology is ever going to become simple. I think that there's some ineluctable complexity about a human being that's not going to go away. But the fundamental laws should be very simple. That might not be true. It's an empirical question. We'll have to find out. But I would definitely say that's the way to bet.

I hope so. For the listeners who want to find out a little bit more, why should they head short? I have a website preposterousuniverse.com where you can see the books I've written, a bunch of videos, and of course, I have my own podcast called Mindscape where I'm talking to all these cool people. Not mostly about physics, some physicists do peak on, but today I released an episode with a philosopher and I've had musicians and poker players and biologists and it's a lot of

been a lot of fun for me. That's awesome. Also, you're pretty prolific on Twitter as well, right? Twitter, yeah Sean M. Carroll and again that's a link for my homepage. But yeah, I really like Twitter. It's for better or for worse. I used to be a prolific blogger on my website and I still do that sometimes but it's easier to get my point across in a quick tweet and then move on to do real work rather than write a respectable blog post. So I'm letting the site down, but I do try to say some interesting things on Twitter, yeah. I like it. I like it. If you're a physicist who's able to get his message across in less than 360 characters or whatever it is, I think. Yeah, you're doing a good job.

Ladies and gentlemen, it's been absolutely pleasure. You know what to do? The links to all of Sean's books plus his website and his Twitter and everything else will be linked in the show notes below. If you've got any questions, comments or feedback, get out me at Chris Wellx on all social media, leave a comment in the YouTube channel or just Hassel Sean on Twitter and he'll give you a reply. Maybe if he's... Maybe, he's a line. Never know. Sean, thank you so much for your time.

It's been great. Thanks very much, Chris. you