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Who's in the Video
Dr. Sean Carroll is Homewood Professor of Natural Philosophy — in effect, a joint appointment between physics and philosophy — at Johns Hopkins University in Baltimore, and fractal faculty at[…]

Do you have free will? Is our Universe the only one, or do we live in one of many? And what does Einstein’s theory of relativity really say about the nature of reality?

These are some of the big questions that theoretical physicist Sean Carroll tackles in this Big Think video.

In 2022, Carroll published The Biggest Ideas in the Universe: Space, Time, and Motion, a book that achieves that rare feat of detailing the inner workings of scientific concepts in a way that’s accessible but not overly simplistic.

SEAN CARROLL: I'm Sean Carroll. I'm a theoretical physicist and philosopher at Johns Hopkins University, and I'm the author of "The Biggest Ideas in the Universe: Space, Time, and Motion."

NARRATOR: What is the multiverse, and why is it important to us?

CARROLL: There's no question the multiverse is having a kind of moment in popular culture. You know, Marvel movies have been leaning into it, "Dr. Strange in the Multiverse of Madness," various "Spider-Man" movies. But also a movie called "Everything Everywhere All At Once" centers the multiverse as its idea, and of course, on TV, we have "Rick and Morty" and various other things. So it's clear that popular culture, the stories that we tell to entertain ourselves and to make us think, are using this idea of a multiverse to make us think in slightly different ways. You might think it's the fault of physics, because physicists have been talking about a multiverse for a few decades now- both in cosmology and in quantum mechanics- but in fact, as often happens in Hollywood and elsewhere, you ignore what the scientists say, and you do your own kind of thing. And in retrospect, the kind of multiverse that is being used in Hollywood is more like the philosophical idea: of the set of all possible worlds, that you can imagine literally anything else happening. And the difference is, of course, that Hollywood lends itself the ability to talk between the multiverses, so that you can go and interact or even fight with a version of yourself who made different decisions early on in your life. It's clear why this would be so interesting to people because we've all made decisions, we've all wondered what life would be like if they were a little bit different, so the ability to actually confront a version of ourselves who made those different decisions is sort of very juicy narrative material. In some sense, the multiverse is a coping mechanism, right? We see terrible things happening around us, we think of ways that things could have been better, right? If an election had gone a different way, if our team had gone a different way, if we had not gotten hurt, if we had asked that person out for a date. And this sort of makes us think, "Well, maybe we're in the wrong universe, we're in the wrong timeline." The idea of a timeline has to do with the fact that there are different things that could have happened, different choices you could have made, and you can imagine that they're almost real, all those different possibilities. And when things don't go your way, or when the world doesn't conform to your hopes and dreams, you might say, "Well, I'm not in the right timeline. There's another world out there. That's the one I wanna be in."

NARRATOR: What is the physicist's version of the multiverse?

CARROLL: I think it's really important, when we're thinking about physicists' versions of the multiverse, to realize physicists never start out by saying, "Hey, wouldn't it be cool if there were a multiverse?" It is always the place they're dragged to, kicking and screaming, because they're trying to explain what we do observe. You know, a big criticism of multiverse ideas is that you can't observe the multiverse. You can't falsify it, you can't test it, etc.- but what you can do is observe what happens in ours. And what you want to do as a scientist is come up with a theory, which is a kind of a story, a theory that accounts for what we see, for the data that we can have access to. And some theories, some very, very simple, very easy to write down theories, like the 'Inflationary Theory of Cosmology' or the 'Many-World's Theory in Quantum Mechanics.' They both explain what we see in our universe and unambiguously predict the existence of other universes. So at that point, you say, "Well, do I like the explanation so much that I accept, the existence of these other universes?" Or "Do I dislike it so much that I throw it out and try something different?" Both camps exist for all of these puzzles the physicists are facing right now. Storytellers have long imagined alternate realities, right? I mean both in the future, science fiction has imagined all sorts of different possibilities. And even toward the past, there's a long tradition of alternate history storytelling. The slight difference that maybe is inspired by scientific ideas, is treating all of the different possibilities as real at the same time, right? When you're thinking about an alternate history, you're just saying, "I'm imagining how things could have gone differently." Now science comes along- physics, my own field- and says, "You know, it's possible. There literally are other universes out there- places where things are different." And that definitely feeds into our imaginations a little bit. But there's different ways in which science can lead us to the idea of a multiverse. In cosmology- which is maybe the most famous one up until recently- there's literally just parts of our universe, that are so far away, where conditions can be radically different; they can either be kind of like us but with different details or even different laws of physics, different particles and forces and the whole shebang. And then we can start asking cosmic questions about why we find ourselves in this particular kind of universe. There's an entirely different conception, called "The Many-Worlds of Quantum Mechanics," which is both in some sense more realistic and easy to bring into reality, but also more mind-blowing. It just says, whenever you have a quantum mechanical system- which spoiler alert, all systems are quantum mechanical at heart- but whenever you measure it in a particular way, you can get different possible measurement outcomes. This is something we've understood for a hundred years now. The question is: what happens to the alternative measurement outcomes that you didn't observe? So when you look at the position of an elementary particle, it is here, the quantum mechanic says, "But you could have seen it there or there or there." What happened to those other possibilities? The idea of the many-worlds of quantum mechanics is, they're out there. Every possible measurement outcome of a quantum system becomes true just in a different universe. This is an idea that was put forward in the 1950s by Hugh Everett- physicists kind of poo-pooed it. But these days, after thinking a lot about how quantum mechanics works, it's becoming increasingly popular. So this is literally a parallel universe. This isn't someplace very, very far away. This is a simultaneously existing reality, where the outcome of a quantum mechanical experiment, turned out differently to ours. And if big, noticeable things in the world depend on the outcome of that experiment, you could wind up in a very different looking universe.

NARRATOR: Is every possible world real?

CARROLL: I think that it's important that most scientific versions of the multiverse are not simply saying, "Everything happens!" Literally every possibility becomes true, somewhere. Rather, there are equations- and what the theory says is: 'What the equations predict, is what happens.' So if you have a particle, an elementary particle like an electron and you're gonna observe it's location or it's spin or something like that, the equations will tell you that certain outcomes are possible, certain outcomes are not. An electron is never gonna turn into a proton. They have different electrical charges that will literally never happen. But the electron could be spinning either clockwise or counterclockwise, and the many-world's version of quantum mechanics says, both of those will generically come true in different universes. So it's a limited set of things that can happen and it's all based on what the equations are telling you. It's not like I'm choosing to live in the universe where the electron is spinning clockwise- you don't get that choice, the equations pick that for you. In the cosmological multiverse, again, we were dragged kicking and screaming into this because we look at our universe, we see it's very smooth on large scales, the same number of galaxies and so forth in distant regions of the Universe and we say, "Why?" Why was it like that? If you just run the numbers, if you say, "I'm gonna pick a universe out of a hat randomly," it would not look anything like that; there's no reason for the universe to be so uniform. So we invented this theory called "Inflationary Cosmology," where the Universe at early times is stretched. It's like pulling the corners of a bedsheet and you're removing all the wrinkles, right? So you end up with a universe that is very, very smooth and even. The problem or the benefit, depending on how you look at it, is that inflation keeps on happening elsewhere, creating more different kinds of universe. So that's a version of a multiverse that is utterly different than the quantum mechanical one. The quantum mechanical one literally happens here in the room when you're doing an experiment, measuring a system. The cosmological one happens very far away, trillions of trillions of light years away in an invisible unobservable parts of the universe. And then there are much more metaphysical, philosophical versions where you do imagine every possible world that you can conceive of. And then we argue about whether or not those worlds are real or not. Most people think that they're not, but there are, there's a lucky minority out there who's holding out for the reality of literally every possible world.

NARRATOR: Why should we trust the many-worlds of quantum mechanics?

CARROLL: I honestly think the best reason to think that the quantum mechanical many-worlds are real, is that it's the simplest explanation, for the puzzles of quantum mechanics for what we call, "The Measurement Problem of Quantum Mechanics." You know, quantum mechanics came into its modern form about a century ago in the 1920s, and it's a replacement for classical mechanics. And in classical mechanics, everything has a position, everything has a velocity, and you can measure those properties as accurately and precisely as you want. There's nothing special about the act of measurement: In quantum mechanics there apparently is. We cannot predict the exact outcome of any measurement we're gonna do. All we can predict is the probability of getting different things. And we can't even define exactly what you mean by a measurement in the traditional age-old version of quantum mechanics. So in the old version, what we call the "Copenhagen interpretation of quantum mechanics," people said something like, "Look, I'm a human being, I'm classical. I have a position, I have a velocity. I'm not gonna think of myself quantum-mechanically, but I'm gonna think of this little electron that I'm observing as a quantum mechanical system." And then it became a sort of song and dance about how the classical world of people interacts with the quantum world of particles and so forth. The big breakthrough from whoever it was, human beings are part of the quantum world also. They also have a wave function, a quantum-mechanical state. They exist in a super position of different possibilities. So if you can think of the electron, as having before you look at it, a little bit turning clockwise, a little bit turning counterclockwise, then whatever it says is, "A human being can be in the state." "I saw the electron turning clockwise," or "I saw the electron turning counterclockwise." And what he showed using math, was that the Schrödinger equation, the famous equation that governs the dynamics of quantum systems, predicts that's exactly what happens when a human being looks at an electron that is spinning, there will be part of the quantum state of the multiverse, which say, "The electron was spinning clockwise and the human being saw it spinning clockwise." We call that a 'world.' There's a whole nother part, saying, "The electron was spinning counterclockwise, that's what the human being saw," and never says, "That's a completely different world." So the reason to accept it is, it's just what the equations say. Every other version of quantum mechanics has to say, "The worlds would've been there, but we're gonna remove them somehow." And it's awkward, you don't know how to do it, and there's no special reason to do it. So I think it's just simplest to accept that the other worlds are probably there, even if we'll never know for sure. It provides a really nice, elegant, simple explanation for what we observe in our world here.

NARRATOR: How many worlds are there?

CARROLL: The thing about the many-worlds of quantum mechanics is people always wanna know how many worlds are there, and how often are they created? And the sad fact is we don't know the answer to that. We're not even sure if there is an answer to something like that because the number of quantum mechanical worlds could be infinite, and that's a mind-boggling kind of possibility, right? But it's infinite in the same way that the number of real numbers between zero and one is infinite. You can still say, "Well, the numbers between zero and a half are twice as many as the numbers between a half and three quarters, okay?" Likewise, even if there's an infinite number of multiverses, of universes in the multiverse, you can still say, "Well, the probability of a universe looking a certain way, has a certain number that we can attach to it." And so, how often the splitting occurs is just not defined. If you think there's an infinite number, it's a continual process. It's happening all the time. But we don't know if it's an infinite number. Maybe it's a finite number, in which case you could attach some value to how many universes there are. It's gonna be a big number. The numbers we're talking about are something like 10 to the power, 10 to the power 100 or 120, okay? A ridiculously large number that you cannot even contemplate. So these universes, in that case, are being brought into existence all the time. One example I like to give is that in a single, average human body, there are about 5,000 radioactive decays per second. Don't worry about that, there's a lot more particles in your body than there are 5,000 radioactive decays, but it's a noticeable thing. And if those decays interact with the rest of the world, which you would typically expect them to do, you have instantly branched the Universe into the universe where this nucleus decayed, and the one where it didn't. So that's not 5,000 new universes, that's 5,000 times the Universe splits in two every second. So that's two to the power of 5,000 new universes being created just because of you every second. The honest answer is to say again, is that we don't know, but whatever the answer is for the number of universes being created, it's a really, really big number.

NARRATOR: How does personal identity in the multiverse work?

CARROLL: Philosophers have long wondered about the question of personal identity through time. Am I now the same person that I was a year ago, or when I was five years old? Clearly there's a relationship there, but also clearly I'm not exactly the same person I was, when I was five years old. I'm pretty different in very important ways. The idea of a multiverse, complicates things in pretty obvious ways- especially if you're not just thinking about conceptual other universes- but you say, "Because of quantum mechanics, there really are different futures of my past self," okay? So then you have to ask, "Well, if I could have seen the electron spinning clockwise or counterclockwise, and I saw it spinning counterclockwise, what is my relationship to the person who saw it spinning clockwise? Are they me just in a different universe or are they a separate person?" I think the clear answer here is, there's a relationship, but they're a different person. It's very much like identical twins: You have one fertilized egg, there's at one point in time, a single cell that is one identity, right? One entity there in the Universe. But it splits into two different people, and no one is tempted to say that two identical twins are two versions of the same person. No one is tempted to say, "Well, I can kill one identical twin, cause the other one is still around," right? They're separate people with their own existence, their own dignity. I think that's how we should think about different versions of ourselves in the multiverse. They might share a past, but once they've diverged, once they're in a universe of their own, they're now separate people. It's not weird or impossible to contemplate, it's just a slightly more sophisticated version of our notion of personal identity updated for the multiverse.

NARRATOR: Do our decisions create different universes?

CARROLL: Human beings love to put themselves at the center of every story. So when you start talking about the multiverse and different ways things could have gone, they instantly start thinking, "Oh, if I had made a different decision, things would've turned out differently." And that's fine if you're just being philosophical and thinking about the space of all possible ways the world could have gone. But if you're thinking like a physicist, you solve the equations; it has literally nothing to do with human beings making decisions. If you think about the cosmological multiverse, the other universes are literally billions of light years away. They have nothing to do with you and your choices throughout the day. The quantum mechanics multiverse is a little bit of a different story because it does happen sort of everywhere. There's things going on that create two parallel realities, but the things that are going on, are not human beings making decisions. They're subatomic particles being measured in some quantum mechanical way. If anything, it's the quantum measurements that force you to make a decision, not your decisions forcing different universes to come into existence. After all, you are a body made of a whole bunch of quantum mechanical particles: electrons and protons and neutrons. If you choose to describe yourself that way, there are different versions of you branching the universe, and then you can say, "In this universe it appears that this decision was made," but the universes aren't branching because of any choice or decision that you made. You know, it's actually slightly strange to me to hear people complain that the multiverse renders our decisions meaningless in some way. You know, before quantum mechanics ever came along, the theory that we thought was right was classical mechanics and classical mechanics is perfectly deterministic. If you knew the state of the Universe completely at any one moment in time, everything that happens in the future is a hundred percent determined. Once quantum mechanics comes along, you say, "Well, you can't predict everything," but we don't know whether the underlying laws of physics are deterministic or not. They could be truly random or like in the many-worlds theory, they're deterministic but we don't know what universe we're in, so for us, for all intents and purposes, it's random. The thing is, either way, there are laws of physics. The point is not whether or not the laws of physics are deterministic or not. The point is not whether or not there's only one universe or many. The point is, do we human beings obey the laws of physics? And if we do, then we do what the equations say, and if we don't, I have no idea what's going on. I have no idea how human beings can fail to obey the laws of physics, but that is something you can at least imagine. I have no trouble, accepting both that I as a human being obey the laws of physics and that my life has meaning, because I don't know what the position and velocity of every single atom in my body is. At my level, the level that I can describe the world, make choices for myself: I am an agent with responsibility for what I do. Even in the quantum mechanical multiverse, I can say, "I'm gonna turn left or turn right." That's a meaningful decision. There might be another version of me that did the other thing, but maybe the probability of being that person was .0000001, so who cares? I think that whether or not you believe in the multiverse, it doesn't really affect how you go through your life. It's not gonna get you out of responsibility for anything bad you did, nor will it take away the credit you get for anything good that you might do.

NARRATOR: Why are we drawn to the multiverse, and how does technology propel it?

CARROLL: I do think that we're increasingly thinking of how things could have been different, and technology is helping us along the way. We're sort of peeking into alternate realities that didn't come true. Certainly, TV and movies help us with this, right? Either a dating show or reality show, right? Or on the internet, you know, a dating app where we can think about dating different people, and where it shifts from a useful function, a dating app, to voyeurism, is people who just look at profiles in the dating app without any intention of dating anybody- they just wanna imagine what it would be like. I recently was hunting for a new house, so there's plenty of websites where you can look at different houses like Zillow. Apparently, there's a whole genre of 'Zillow porn,' which is not actually pornography, it's just getting kicks out of looking at houses you can't afford on Zillow, and imagining what it would be like to live there. And this is something, again, that we just couldn't do in the same way. Technology is letting us see what we're missing, in some ways, and I don't know if that's psychologically good or bad, but it definitely leads us to imagine alternative universes in a more vivid way than was previously possible. I absolutely believe we can imagine thinking about the multiverse as a useful psychological tool or personal tool, right? Visualization exercises have been part of psychology for a long time. Imagine yourself in a different set of circumstances. And we know, one result of modern neuroscience, is that the human brain, the human mind is not just software running on a computer. Our minds, who we are, are embedded in our bodies. And so when we physically imagine ourselves embodied in different set of circumstances, we think about that possibility differently. And the multiverse is sort of a nudge in that direction- and maybe you can argue that technology these days is making that more possible with virtual realities, with alternate ways of thinking about ourselves, augmented reality- just wearing a headset in the world that we're in- boosting it up a notch a little bit. These could be both positive and negative psychological tools if we use them in the right way, by imagining the way things could have gone better, and then saying, "Okay, what do I need to do, to increase the chances that next time, it actually will go better?" One of the nice things about the multiverse idea is we can imagine undoing the decisions we made, or at least seeing what it was like. You know, almost every multiverse movie is gonna have the moral of the story at the end that you did everything right, don't be jealous of those other universes- you're doing fine. Even in "Everything Everywhere All at Once," that was directed and written by the Daniels, who I interviewed for my podcast, you know, they literally say in the movie that Evelyn, the protagonist, is the worst version of herself in all the different multiverses, but still she's doing okay. She can recover that universe that she's in. And I think it's a little bit of wishful thinking, to imagine ourselves doing a little bit better in some other universe, but it's psychologically not healthy to push that too far, because there are some decisions we can't undo. This is why we talked about the serenity prayer, right? The ability to recognize what we can change, what we can't, and accept the things that we can't as well as to be able to tell the difference, okay? You can imagine in a multiverse, having made all sorts of different decisions, but in fact there's no such thing as time travel in the real world. You cannot actually go back and remake the decisions differently. It's fine to imagine all sorts of possibilities, but at the end of the day, we have to live and affect the universe that we're in. You know, psychologists talk about different orientations that people have, vis-a-vis time. You might have heard of the marshmallow test, right? Little kid has been put a marshmallow in front of them and they say, "I'm gonna go outta the room. If you cannot eat the marshmallow, you'll get two marshmallows when I come back," and different kids either eat the marshmallow or don't. And the claim is, that you can predict this kid's future trajectory, on the basis of what they did on the marshmallow test because the way that they think about it is, some people are just intrinsically right about now, like the present moment. "I just wanna do what gives me pleasure right here." Some people are past-oriented, they're constantly dwelling on, you know, their memories both good and bad. They're living in the past. We've all met people like this- some of us are- and others are future-oriented. Others are like, "Well, whatever happened in the past, what is ever happening right now, what I'm gonna do is save for the future, plan for the future, thinking about making future me as happy as possible." I'm not gonna comment on the relative psychological benefits of these different orientations to time, but you can clearly see how sometimes you'll get in a rut, sometimes the orientation you have will lead you to make a mistake, right? A past-oriented person can get lost in dwelling in the mistakes they made, the important decisions that define their lives, and forget about the possibilities in the future. The future-oriented person can forget about having fun right now. They're constantly saving for a day that may never come. I think that really_ let's put it this way- I like to think, maybe I'm being a physicist here more than a human being, but I like to think that by contemplating all of these different possibilities, past, present, and future, we can put things in perspective a little bit. We can think about how, "Yeah, there was that moment when things went terribly wrong. Either maybe I did something wrong or there was an unforeseen event that I couldn't have controlled. But you know what? The causal influence I have on the world only extends toward the future. The choices I can make right now, will have an impact that I will feel down the road, but I cannot make a choice right now that undoes what happened in the past." I think this is a truth about physics and cosmology and the world, and, psychologically, it's a very important principle to keep in mind.

NARRATOR: What is time?

CARROLL: The question of what time is, I think is easier than sometimes people make it out to be. Saint Augustine of all people wrote a famous essay on the meaning of time and he says, "I know what it is until you ask me what it is. And then I have no idea what time really is." But I think about how we use time when we actually talk about it. If you say, meet me at 7:00 PM, no one panics, no one says like, "Oh my God, what are you talking about with these esoteric concepts about 7 p.m." We all know what to do operationally. And time, in some sense, is just a label, on different events in the Universe. The Universe happens over and over again, at different things we call 'moments' and time helps us tell the difference between one moment and another. So what time is, I don't think is the problem. The issue, the real puzzles, come about when we talk about the properties that time has: we have a past, we have a present, we have a future. How are they different? Are we moving through it? We have memories of the past, but we have no memories of the future, why is that? Where does that asymmetry come from? Why are we all born young? Why do we all inevitably age? Why do we think that we can affect the future but not the past? Could we possibly travel back into it? Anyway, there's a lot of questions about the nature of time that are really confusing and many of them we don't know the answer to, but what time is, I don't think is one of them. The idea that time has an "arrow," is so deeply ingrained in how we think about time, that it really requires a bit of mental discipline to say, "Well, time could exist without an arrow, right?" We could have time even if there were no difference between past and future. And one way of thinking about that is, there is no intrinsic "arrow of space," but there's still space, okay? We live in a three-dimensional world- up, down, left, right, forward, backward- at the level of the fundamental laws of physics, there's no special direction in space. And how you perceive that is: Imagine you're an astronaut, you're flying around in your little spacesuit. There wouldn't be any difference between any direction you could look. There's no experiment you could do in physics that would point out a direction in the Universe, but space still exists. Likewise, time would still exist even if there wasn't an arrow. But here on Earth we do have an arrow of space. If I pick up a coffee cup and let it go, it will always fall down. There's clearly a distinction between up and down. No one is tempted to think that the arrow of space, at least no one in this century, is tempted to think that that's a fundamental feature of the Universe. We all know why things fall down. It's not because downness is embedded in the laws of physics. It's because we live in the vicinity of an influential object: the Earth. And we can even imagine it wasn't there. We can put ourselves in an alternative universe, where we're floating in outer space. Here on the ground, there's an arrow of space, but it's clearly because of the Earth, not because of the fundamental nature of reality. The "arrow of time" is exactly the same way. We, in our everyday lives, perceive an arrow of time because we live in the aftermath of an influential event: The Big Bang. That Big Bang set up the Universe for its subsequent evolution, where about 14 billion years after we're gonna go for many, many, many billions of years toward the future- so the Universe is still young in some sense. We're still very much feeling that influence, of the Big Bang. We're nowhere near relaxing to the ultimate state the Universe will be in. But that doesn't mean the arrow of time is a feature of the fundamental laws of physics. It's just a feature of our local neighborhood. A mere 14 billion years after the Big Bang. Lexicographers will tell you that time, the word T-I-M-E, is the most used noun in the English language. We can't get through the day that talking about time all the time. There you go, there's an example, okay? And part of it we take for granted, we think that there's certain things about time that are just natural, like the fact that we can affect the future, but not the past. Like the fact that we're all a year older now than we were a year ago. And so one of the most noticeable features of time, is that it has a direction, right? That there's a difference between the past and future. Sometimes we think about this as just an intrinsic feature of reality: Like the past already happened, it's in the books, the future is up for grabs, it hasn't happened yet, and the present is where we live. But then along comes physics, along come Isaac Newton and Albert Einstein and all these great brains, and they think about how to describe what happens in the world. And they give us equations, they give us theories of physics. And what people notice about our best theories of physics, is that those theories do not distinguish between the past and the future. They do not have any intrinsic difference between yesterday and tomorrow. They're all created equal in some sense. There is no deep down in the laws of physics distinction, between moving toward the past and moving toward the future. But in our everyday lives, nothing is more obvious than the distinction between the past and the future. So why is that? And the answer is, the world of our experience, the macroscopic world in which we find ourselves, is a little bit different than the microscopic world of fundamental physics. If you have just a few moving parts in a system, it doesn't need to be microscopic, think about the Earth going around the Sun, or a pendulum swinging back and forth. These systems don't have what is called an arrow of time. If I make a movie of a pendulum swinging back and forth, and I play it to you backwards, you don't know that it's going back and forth, it's still going back and forth. You don't know that I played it backwards- there's no directionality there to time. But if I throw a rock into a pond and I make a movie of that, if I play that backwards, you know. It's obviously reversed because rocks do not get thrown out of ponds randomly. This arrow of time, this directionality, is a purely macroscopic big world phenomenon that comes about by having many moving parts. And that gets us into a realm of the concept of 'entropy.' Entropy is what physicists talk about when they talk about how messy, how disorganized, how random a system is when things are nice and neat and tidy, they are low entropy; when they're all messy and all over the place, they're high entropy. And there's a natural tendency of things in the Universe to go from low entropy to high entropy. This is called the 'second law of thermodynamics,' a very important feature of the macroscopic world, but it's not there in the fundamental laws of physics. It's just there in the macro world. And why is it there? Why is it that entropy tends to increase over time? Well, for one thing, there are many more ways to be high entropy, than to be low entropy. There are many more ways for your room to be a mess, than for it to be cleaned up. So if you have a system that you're not directing toward a nice, organized, low-entropy state, it will naturally tend to move toward higher entropy. The real question is: 'Why was the world ever low entropy to begin with?' If there are many more ways to be high entropy, why do we see orderly things around us at all? Why was the world lower entropy yesterday than it is today? And the only and the correct answer is: 'Because it was even lower entropy the day before yesterday.' And why was the universe even lower entropy the day before yesterday? Because it was even lower entropy the day before that. And this chain of reasoning goes back 14 billion years, to the Big Bang, to the origin of our observable universe- in a hot, dense state- which looks kind of disorganized, but if you run the numbers, it's actually a very special, very delicate, very unusual state for the Universe to begin in, a very low-entropy state and the Universe has been increasing in entropy ever since. Now that's all well-established stuff. No one agrees with that. Here's the radical claim that I think is true, but we haven't quite internalized yet: The only reason why in our everyday lives, we think of the past and future as different, is because entropy was lower in the past and will be higher in the future. And that's ultimately because it was low near the Big Bang. The Universe was orderly 14 billion years ago. So the reason why, as you scramble your eggs, you will see eggs breaking and turning into scrambled eggs, but never scrambled eggs reforming into an unbroken egg, is ultimately because of what was happening at the Big Bang 14 billion years ago. People don't think of it that way. Everyone knew that we were all getting older, that we all had memories of the past and none of the future. They knew that thousands of years ago- they didn't know it was because the Universe was orderly near the Big Bang. Now we know, and we're still struggling a little bit to quite come to terms with what all that means.

NARRATOR: What is the 'past hypothesis?'

CARROLL: We've known about entropy and the second law of thermodynamics since the middle of the 19th century. In fact, it's a very down-to-earth thing. People were trying to build better steam engines at the time. It was the industrial revolution, and they kept finding that no matter what they did, they were losing some energy. There was no perfect cycle that could drive their steam engine. And ultimately, they realized that a law of nature was at work. There was no way to make a perfectly efficient steam engine. There's always some loss, some dissipation. And we now interpret that as an increase of entropy over time. And so people put together the laws of thermodynamics. The first law is that the total energy is conserved. The second law is that entropy, however, is increasing. So if you think about it as useful energy and useless energy, there's a finite amount of useful energy in the Universe and it's constantly being turned into useless energy as we burn our fuel, as we go through and increase the entropy of the Universe. So that was pretty well-established. Entropy increases with time, but then if you go through the math, you can derive the fact that this could happen. This was the great insight from Ludwig Boltzmann, who was an Austrian physicist. He said, "You don't need to pause it as a separate law of nature, that entropy increases. I can explain why entropy increases if you believe that we're made of atoms." So we're talking about the 1870s, and physicists at the time didn't believe the Universe was made of atoms- the chemists did. The chemists had caught on cause they saw the reactions happening with certain frequencies and they said, "Ah, discreet units are rearranging themselves." The physicists were more skeptical. Boltzmann and some of his friends said, "If you believe in atoms, I can explain why entropy increases." There's just more ways for the Universe to be high entropy than to be low entropy. Great, but it only explains why the entropy of the Universe will be higher tomorrow than it is today. It does not explain why the Universe was lower entropy in the past. And because we're very bad about reasoning about time, it's taken us decades of work to really narrow down why it is that entropy was lower in the past- and the explanation is not completely satisfying, to be honest. The explanation is the following: It started out really, really low near the Big Bang; entropy of the Universe just started out low. We don't know why, it just did: And this is called the 'Past Hypothesis' by philosophers. David Albert, who's a philosopher of physics, gave it this name. It was sort of name-checked by other people as an idea but not really highlighted- like people like Richard Feynman and Arthur Reddington, even Boltzmann himself thought about it. So the idea was in the air but it wasn't written down as an important part of our understanding of the rules of thermodynamics, until pretty recently, until the past couple decades. So now we say, 'If you know that the world is made of atoms, and you know what entropy is in terms of rearranging all those atoms, and you know the past hypothesis that the entropy of the Universe started really low, then you can explain everything that happened after that.' If you believe this dramatic claim, that all of the reason why the past and future had any differences between them is because entropy is increasing, then that also goes for our feeling that time is flowing or that we are flowing through time. People talk about it, cultures actually talk about it in different ways, but the idea that we're moving through time, that we move from yesterday to today to tomorrow- where does that come from? And it's a great question because it clearly involves both psychology, it's a feeling, right? But also physics because it's a different feeling about the past than about the future. And all of those differences come from entropy increasing. So why is it that we feel that time is flowing if really, from the physics point of view, there's no difference between one moment of time and another? Well, the answer is: At any one moment of time, the human brain is sort of reaching out to other moments of time, not physically reaching out, but it's carrying inside itself a version of what it thought happened a little bit ago, and what it predicts will happen a moment next. The human brain in some sense, all brains- all animal brains, maybe even plant nervous systems if you want to go that far- are trying to understand and predict their environment. They have information coming in and information only comes in from the past. And also, if you're an animal anyway, you're gonna act, right? You're gonna behave in some way. So what the brain does is predict what it will do next. And at every moment of time it remembers what it just did, and it also compares what it thought was gonna happen. You know, when you put your hand on a surface, you thought it was gonna be cool or hot or whatever, and then it is one of those or another, and you're remembering the difference between what you predicted and what actually happened. And at every moment in time you're updating, you're going, "Ah, I thought this was gonna happen, it was gonna be this. That informs what I think will happen next." And the brain is just a sort of prediction and updating machine. And that whole process has a directionality in time because all of your information is from the past. It's an old Mitch Hedberg joke, he says, "His friend shows him a picture and says, 'This is a picture of me when I was younger.' 'Aren't all pictures of you when you were younger?'" Yes. All of the photographs, all the memories we have are of the past that gives us an imbalance to how we flow through time- and that leads to that psychological feeling we all have.

NARRATOR: What is 'Laplace's Demon' and do we have human agency?

CARROLL: You know, after Isaac Newton invented the rules of classical physics, it wasn't for quite a while until Pierre-Simon Laplace, around the year 1800, he really understood the implications of what Newton had done. He really noticed that this is the clockwork, deterministic Universe. And the way that he emphasized this, to make it vivid, was a thought experiment: He said, "Imagine a vast intellect, an intellect that was so vast, so good, that it could perceive the location and the motion of every single thing in the Universe." Every atom as we would now say- although he didn't know about atoms, but every particle, every photon, every bit of the Universe- and this vast intellect was so smart that it could actually solve the laws of physics from that starting point and it would project it into the future. Laplace realized that that same vast intellect could also, 'retrodict' what happened to the past. The information about what is happening at any one moment in time in the Universe, if you are this vast intellect- which subsequent generations dubbed 'Laplace's Demon'- if you knew all that, the future, the present and the past are equally visible to you because the fundamental classical laws of physics are deterministic and tell you exactly what's going to happen. Ever since then, people have worried about this. They're thinking, "Well, if everything is determined, then I don't have any free will. What can I do? How can I get through the day?" The answer is: You are not Laplace's Demon. Laplace knew perfectly well that we're not gonna come close to being Laplace's Demon. If you take the amount of information in a grain of sand, you would need, I forget the exact number, but it's something like a million, billion laptop computers to contain the information contained in the atoms and the molecules in that grain of sand. It's never going to happen. And those computers wouldn't have the computing power to predict what would happen next anyway. We human beings are finite. We are computationally limited and bounded. But the good news is: that's okay. We can still get through the day, we can still throw a baseball and catch it. We have the ability to have vastly incomplete information about the Universe, and yet do a pretty good, although not perfect, job of predicting what will happen next. So thought experiments about what life would be like if we were Laplace's Demon are fun to do, but irrelevant to human life. The question of what human beings can do is a very different one. And there are interesting, subtle, complicated, sophisticated answers to that, that lie at the intersection of psychology, neuroscience, computation theory, information theory and physics. You have to keep emphasizing the difference between the real life of a human being, and the imaginary life of all the particles in the Universe obeying the laws of physics. If we were Laplace's Demon, if we did have the computational and informational capacity to predict everything that would happen, it would be hard to imagine that life would have meaning- because you know everything is gonna happen. There's no choices that are gonna be involved, but the real world isn't that- none of us is like that and in fact, not only do we have incomplete information about the world, but it's very hard to think about how we interact with the world, given that when we do, the world interacts back on us and then we think about it, we interact back on the world. There's a feedback cycle here. There's a give-and-take between us and the world. So given that we have incomplete information, and given that we're embedded in the world and that we have an effect on it, the right way to conceptualize things is that we are agents, we are not collections of atoms and molecules. We are, but that's not the right way to think about it, if you're a human being trying to make sense of the world. We are agents that can make decisions, and our decisions have an impact on the world. And it goes back to this idea of the space of all possible worlds; the space of all possible things that can happen. We human beings, in a way that might be unique- this is unclear among people who are studying other species- we have an ability to reason 'counterfactually,' to think not about just what will happen, but of various things that could happen, and then pick the one that we think is a good one. And since we don't know the positions and velocities of every molecule in the Universe, we can't say what would happen, just given the laws of physics. What we have to say is, "Given the choices I make, what is the future that I'm going to help bring about?" So like it or not, the world that we really know and live in is one where our choices matter. That's where meaning comes from: from recognizing that in the real world, of the knowledge that we have and our computational boundedness, we have some responsibility for bringing about what is going to happen next.

NARRATOR: Why is entropy essential to living?

CARROLL: There's a way of talking about human life and entropy which I think is misguided, which is that we should think about life- you know, literally living, being a biological organism taking in food and everything- as a fight against increasing entropy. I think that's wrong. I think that we owe life to the fact that entropy is increasing because what would it mean if entropy were not increasing? It would mean that nothing is happening. Nothing interesting is taking place. Without entropy increasing, there's no memory of the past, without entropy increasing, there's no causal effect that we have on the future. You'd just be in what we call 'thermal equilibrium.' Everything would be the same everywhere. It would be the maximally boring universe. But what we do have as a scientific question is: 'Why do complicated complex structures come into existence at all?' It's clear that they need increasing entropy to exist because if entropy were already maxed out, there would be no complexity. But that doesn't mean they have to come into existence. Think about a very simple example of entropy increasing: If you have a famous example that Boltzmann talked about where he opens a bottle of perfume and the perfume is all in a little bottle, it's in a big room, you open it, and it all floats through the room- the entropy of the perfume increases. But if you think about it, when the perfume is all in the bottle, it's very simple. That's a very simple configuration. No perfume outside, all the perfume inside. Once it's all spread through the room, it's also very simple. It went from low entropy to high entropy, but it went from simple to simple. It's in between. It's the journey from the simple, low-entropy starting point, to the simple, high-entropy ending point, that there's a large space of possibilities where things can be intricate. There's more perfume here, over there, there can be swirls of eddie and eddies caused by the motion of the wind in the room and so forth. The Universe is just like that. Our universe started out simple and low entropy. In the future, the stars will die, the black holes will evaporate. It'll be dark, empty, and again simple, but high entropy. So the Universe goes from simple and low entropy to simple and high entropy. And it's in between, that things like us, complicated, intricate systems that feed off of the increasing entropy of the Universe can and do come into existence. We don't know the whole story there; I think this is a very fun, active scientific research area. Why did complex structures like living beings come into existence in exactly the way we did? What is the role of information? What is the down-to-earth chemistry that is going on here? What is the geology that is going on here? Could it happen on other planets? Very interesting questions, but one thing I do know, is that if entropy weren't increasing along the way, none of it would've come to pass.

NARRATOR: Why are there complex structures in the Universe?

CARROLL: I think that once you learn about the second law of thermodynamics, it can be a little depressing. You've been told that there's a fundamental feature of reality that disorder increases, that things just wind down, right? That batteries run out, that cream and coffee mix together, that ice cubes melt. It's kind of a depressing view of the future. And then when you think about it just a little bit more, that depression becomes a little bit sharper because you're saying, "Well, how did all of this interesting-ness come about in the first place?" If the whole thing that reality does is just move closer and closer to complete disorder, then how did something as exquisitely organized as a human being come about? And there's a sort of cheap and wrong version of this which says, you know, 'Here on Earth the biosphere somehow came into existence even though the early Earth had no life on it, doesn't that violate the second law of thermodynamics?' That's obviously not true because the second law, refers to closed systems, systems that are not interacting with the outside world. The Earth is not a closed system. We get light from the Sun, we radiate it back out into space, so it just doesn't apply there. But all that says is that it's okay for the entropy of the biosphere to either go up or down. It doesn't say why it was in fact the case, that we went from no life on the surface of the Earth to the rich environment that we find around us today.

NARRATOR: Do complex structures require design?

CARROLL: As human beings, we definitely have this experience of creating something very complicated, right? We can build things and in fact we get the idea that when you see complicated things, it's cause someone built them. That's a very familiar thing for us to think. So William Paley was a British theologian who built this idea into an argument for the existence of God. He says, "When you walk around the beach and you stumble on a grain of sand or a rock or something like that, you're not surprised," right? You don't think that that requires any explanation. Those are the kinds of things that'll just naturally happen. But if you stumble across a pocket watch, which is like a fancy, high-tech thing, at the time, you would say, "Clearly this has been designed." And the reason for saying that is, all of the pieces of the pocket watch have a purpose. They work together. It's as if they've been arranged for reasons rather than simply randomly. And that makes perfect sense cause you know that the pocket watch was in fact designed. It was built by human beings. But then Paley says, "We have another example of very intricate structures which seem to be designed where the different sub-parts of the thing have reasons for existing in working together, namely us, or other animals or other living beings." So this was pre-Darwin of course, and he said, "How can you possibly account for the exquisite design of biological organisms unless they were put together for a purpose by a higher power." Which he identified as God.

NARRATOR: What is the effect of increasing entropy?

CARROLL: This idea that there's this tension between the organization of things in the Universe and the natural evolution of things, became a little bit sharper over the course of the 1800s because that's when we put together, this idea that entropy increases all the time. Entropy is a way of talking about the disorderliness, the disorganization, right? The randomness of a certain physical system. Originally, the very first time that entropy was introduced into physics, it was more about dissipated heat cause they were trying to build better steam engines. Later in the 1800s, they realized that if you understand atoms and the fundamental constituents of the things around us, you can quantify what we mean by entropy. It's the number of ways you can rearrange the constituents of a system so that it looks the same macroscopically. And then you understand why entropy tends to go up. Things tend to go from orderly to disorderly, just because there are many more ways to be disorderly. This is the second law of thermodynamics. This is a deep down law of nature, and it seems to have the consequence, the implication, that what happens over time is that the Universe becomes more disorderly. And if you just extrapolate that, if you say the Universe is a closed system, which we think it is, so the entropy of the Universe increases. What does this mean? What does this imply? It implies what we call "the Heat death of the Universe." That all the stuff you see: the engines, the burning stars, the living beings, these all represent systems that are increasing the overall entropy of the Universe. And if you think, which is probably true, that there's a maximum entropy you can reach, a maximum level of disorderliness and chaos, then eventually we will get there and all the interesting-ness in the Universe will be gone. The Universe will reach what we call an 'equilibrium,' a state of maximum chaos and nothing interesting happening anymore.

NARRATOR: What is the difference between entropy and complexity?

CARROLL: One of the issues with really feeling through the implications of the second law of thermodynamics is that people tend to mix up, simple versus complex and orderly versus disorderly. The truth is: these are two different axes, two different ways of thinking about something. The increase of entropy says that we go from orderly to disorderly, but it says nothing about simple versus complex. Or maybe, let's put it this way: What it says about simple versus complex is more complex than you might think. When you're very, very low entropy, when you're as orderly as you can be, things will tend to be simple. Think about all the molecules of air in a box are all in one tiny little corner. That's a very unlikely, very low entropy configuration. It's also very simple cause they're all like pressed up against each other in the corner. Likewise, a high-entropy configuration tends to be simple once again. There are many ways for the individual atoms to be organized. So they're spread evenly throughout the room, but it's still very simple. They're all very evenly spread throughout the room. In order for complexity to arise at all, it seemingly insists that we have a medium-entropy configuration- you're balancing enough room for the different atoms and molecules to arrange themselves in interesting ways, without just being equally spread throughout the whole system. Now this raises a complicated, sophisticated, and quantitative question. If you go from simple and low entropy, to simple and high entropy, and in between you could be complex, will you be complex? The answer is: "maybe, some of the time," it depends on the details. Think about cream mixing into coffee: If you stir cream into coffee, you can get a complex, fractal pattern along the way, right? Swirls, things that look like galaxies and the cream mixing into the coffee. But you can also just put the cream very gently on top of the coffee and it will dissolve smoothly and uniformly until it's all mixed together. So this is a new scientific question that we are facing right now: 'What is the journey from low entropy to high entropy like? And how is it affected by the laws of physics so that, the actual path it takes leads to complex structures?' So, we can exist as exquisitely organized human beings as part of the journey from the Universe, from low entropy to high entropy. But why we exist, was it likely? Was it inevitable? Was it very, very lucky? That's something we don't know yet.

NARRATOR: What is emergence?

CARROLL: One of the great features of the world, that really helps us understand it and grasp it is that the world appears to us in layers. What I mean by that is you can sort of look at the world and perceive it at different levels of focus. We human beings, we walk through the world and you say, "What is the world made of?" Well, there's people, there's chairs and tables, and there's plants and animals and the floor and the building and whatever. There's a set of objects that exist in the world. And then if you ask a scientist, a particle physicist, "What is the world made of?" They might say, "Ah, the world is made of elementary particles. It's made of electrons and protons and neutrons and forces acting on them." So who is right? Are there people and animals and plants in the world? Or is the world really made of electrons and protons and neutrons? Did realizing that there were atoms and particles in the world make us wrong about the existence of people and animals and plants? No, it did not. They're both true at the same time. And this is a kind of puzzling feature of reality because when we talk about the world at the level of human beings, of objects in our macroscopic world, we are clearly not giving the whole story. We're not giving the details of every single atom and molecule that make up this stuff. We have incomplete information about the world. When you look at a box of gas or a cup of liquid, you don't know the positions and velocities of every atom inside. But the good news is you don't need to either. You still know that if you put an ice cube in a glass of warm water, the ice cube will melt. You can make that prediction reliably, even with vastly incomplete information. And this is what we call the phenomenon of 'emergence.' The idea that there's more than one way of talking about how the Universe works. There's a way that might be the most fundamental, maybe that has to do with atoms and particles and quantum mechanics, but then there's higher-level ways, emergent levels in the macroscopic reality that we perceive every day, that still tell us something true and useful about the world, even though our information about it is radically incomplete.

NARRATOR: Is life a struggle against entropy?

CARROLL: A lot of times you'll hear people talk about life as a struggle against increasing entropy. That's crazy. That's like saying, "My life is a struggle against eating food." I think that eating food is really important. In fact, it literally is a transfer of low entropy into high entropy. Think about-my favorite example is the Sun. Okay, what does the Sun do for us here on Earth? And if you ask people, "What good is the Sun for biology and the ecosystem here on Earth?" We get energy from the sun, right? In the form of sunlight. It's true, but it's not the whole story. It's not even the important part of the story. The important part is: we get sunlight and most of it is in the visible wavelengths of light, right? So, we get a certain set of photons with certain energies and then we absorb them. And what do we do? We convert them into higher entropy forms of energy. The Earth radiates into space, more or less exactly the same amount of energy as it gets from the Sun. But, it radiates it in a much higher entropy form. For every 1 photon of energy we get from the Sun, we radiate 20 photons of light back into the Universe with on average one-twentieth of the energy each, but the total has 20 times the entropy. What happened? We got fuel from the Sun, we got usable energy, and then we degraded it. We turned it into a higher entropy form. And we need to do that. This is called "metabolism." This is called maintaining our lives. All of that low entropy energy we get from the Sun is captured by animals and plants, it's used to make sugars and other fuels. It's what keeps us going. The way to think about it is we are riding a wave, from low entropy to high entropy. And you shouldn't-just because the wave will eventually crash upon the shore-you shouldn't say, "Ah, we're fighting against the wave." No, no, no. We're riding the wave, that is what let's us exist. That is what gives us the ability to think about the past, to affect the future. All of the things that we think of as, part of being a living being are possible because we live in a world of increasing entropy.

NARRATOR: Why is physics such a difficult field to study?

CARROLL: I always tell my students that physics or astronomy- these are the right sciences to go into if you have a short-attention span. And the reason why is because physics really makes progress by simplifying the world, right? I mean maybe all the way down to elementary particles. But even when we think about the Universe as a whole, the modern cosmologist simplifies the Universe enormously. They don't think about every individual galaxy or a star. They would never get off the ground, right? You just say, "Well, maybe the Universe is more or less uniform everywhere." That's an incredible, dramatic simplification. The good news is it works. You can get very, very far by making that simplification, thinking about what it means, and then putting in the complications later. This is a legacy that we were handed by Galileo of all people. Galileo knew, he read in Aristotle, that if you had two objects, a heavy one and a light one, and you let them go, the heavier one falls down faster. And guess what? That's true. If you actually do it, that will be what happens. Galileo says, "Yeah, but that's cause of air resistance." If you imagine ignoring air resistance, a book and a feather will fall down at exactly the same rate. He couldn't do that. Galileo couldn't actually do an experiment to ignore air resistance. We need to fly to the moon and drop two things and then you actually see the experiment is right. The Apollo astronauts verified that Galileo was right, but the method of ignoring the complication and putting it in later, that works for whole wide swaths of physics- and that's why physics is able to make such enormous progress. When it comes to biology, when it comes to sociology or psychology, when it comes to the emergence of higher levels out of the simple microscopic world, that methodology doesn't always work anymore. If you say: "Imagine a person and forget about all their interactions with other people," well you know, okay, you can try to do that. But it turns out that those interactions with other people are actually crucially important. When it comes to the macroscopic world around us, the world that we see every day, the complications really matter, but we know that it has to be compatible somehow with the simplicity of the microscopic world. So, you can tell those two stories independently. You can just be a psychologist or be a biologist, and that's fine. Or you could be a physicist, and that's also fine. What I'm interested in is reconciling the different layers of reality, explaining how they can be consistent with each other. It's one thing to say you have biological evolution, and that leads to the complicated ecosystem with different kinds of species and different niches. How about: 'Why is that compatible with the laws of physics?' 'How does that arise out of the evolution of the Universe?' It clearly must be. And in somehow tying them together, maybe we will learn something interesting about both.

NARRATOR: What are the origins of life here on Earth?

CARROLL: One of the difficulties in figuring out specifically how life here on Earth came into being, is that it's not just a random sort of complex system. It's a very specific thing. And life as we know it now, involves different aspects, all of which are important. You need replication, right? You need Darwinian evolution. We have DNA, we have a genome that gets replicated, not perfectly, but pretty well. You need compartmentalization. Every cell has a cell wall so you can separate the living cell from the rest of the world. And also you need metabolism, right? You need that fuel, you need that low-entropy energy, that we can use to keep ourselves going and then expel to the world in a higher entropy form. So which came first? This is why explaining the beginning of complex structures is always hard because they all seem to depend on each other. There is a replication-first camp in the origin of life studies literature that says, "Look, clearly the genetic information is necessary to call it life. That must have come first and it must have hooked up to an energy source." But there's also a metabolism-first camp that says, "Look, it doesn't matter if you have information sitting there, if it's not going someplace, if it's not doing anything, if it's not moving around and metabolizing, you can't call it life." And the nice thing to me about the metabolism-first point of view is that you can kind of see how it might arise out of purely physical, non-biological processes. Because remember, to maintain its orderliness and its complexity, living beings need to increase the entropy of the Universe. They need to feed off of low-entropy energy- and that's a very standard thing outside the realm of life. When you strike a match and light something on fire, you're increasing the entropy of the Universe. The thing is that that's pretty easy. The tinder that you had or the crumpled newspapers, they were ready to light up. Sometimes you have a chemical configuration where, it could go to a higher entropy configuration, but there's no direct route to get there. So maybe it'll just sit there, maybe it will never increase in entropy, but maybe under the right circumstances, there's a sequence of chemical reactions with the right circumstances, the right environments, the right catalysts and enzymes to make it all happen. And maybe that chain of interactions and chemical processes, is the beginning of life. So this was an idea that a number of biologists and geologists had. And on the basis of it, they made predictions. They said, "Life is not gonna form in some warm pond." This was Darwin's idea, that maybe you just put all the stuff together, all the ingredients and eventually a little bug crawls out, okay? Or a little bacterium. And they said, "Look, that's never gonna happen because there's no increasing entropy there." What you need is just the right biochemical, geological arrangement of things, to have these long, sophisticated, complex reactions happen that can then be captured into the first living organism. And they thought about, what kind of conditions would they be? And they thought that it might be a warm hydrothermal vent, under the oceans, right? With certain chemical balances- and I'm a physicist, I don't know the details and I shouldn't say anything- but the point is, these kinds of hydrothermal events were not known, at the time. They had not yet been discovered, but a prediction was made. These must exist in order for life to have come into existence. And after the prediction was made, we found them. Famously, the "Lost City" formation at the bottom of the Atlantic Ocean, was found by submersibles going down near the bottom of the ocean floor. They found exactly the kind of geochemical conditions that have been hypothesized as possible places where life could have formed. Does that mean it's right? No, but it's a little bit of evidence that it could be on the right track. The origin of life I think, personally, is one of the most important unanswered scientific questions, but it's one that we're absolutely making progress on right now.

NARRATOR: How many things had to go right for us to exist?

CARROLL: The thing about life coming into existence is it required a lot of things to go right- but it was less unlikely than you might think. A lot of times people try to like run numbers and give you some scary looking calculation, that seems to be indicating that it was wildly unlikely for life ever to come into existence. And of course, that depends on the assumptions that you're making. A lot of times what they're really doing is thinking, "Well, if you just have random configurations, if you have a random number generator, how likely would it be that you would print out Shakespeare or something like that," right? A random letter generator. And it's very, very unlikely- although it'd happen if you waited long enough. The nice thing is, in the real world, we didn't have a random number generator. We started with a very, very orderly configuration. That's what was given to us by the low entropy of the Big Bang. So it's a different question you should be asking: It's not in all of the possible arrangements of stuff, which of them would've looked like life, it's in all the different paths you could take from the low-entropy beginning of the Universe to the high-entropy future, which of them would pass through, something as complicated as a living being? I don't know how to do that calculation either, but I know that it's much more likely to get complex structures that way, than just to have a random number generator spitting things out. Of course, there's a whole other question, which is, 'What kind of processes are allowed in the Universe?' What processes do the laws of physics let you imagine? Why do we have atoms and molecules at all? We seem to live in a Universe where the very delicate laws of physics that give you the mass of the electron and the proton and the neutron and the strength of all the forces that let them interact, are such that they allow for the existence of complex structures. Is that easy? Is that surprising? Is that something that requires an explanation? I think that naively, as of right now, it does seem to require an explanation, and we don't have one. The idea of a multiverse is a very plausible explanation- like maybe in some universes these complex structures can come into existence, in some they can't. Of course, if that's true, we will only ever find ourselves in the places where we can find ourselves- that's not a great discovery. But if there aren't other universes out there, how likely is it that something like us would've come into existence? I don't know. I think this is a great question for scientists and philosophers to be thinking about.

NARRATOR: If this isn't God's design we're seeing, what is it?

CARROLL: William Paley wasn't crazy when he said, you know, "I look at the complexity and the arrangement, the sophistication of a living being, and I attribute it to a designer." Okay? Which he then identified as God. It makes sense; it's a paradigm that we know how it works. People really do build pocket watches, maybe God built human beings. To me, the history of science is a series of discoveries that make these seemingly inexplicable things make a lot more sense. Now that we know there's genetic information, it mutates, we can explain by a natural selection the existence of all these different biological species, that part has been explained. We have the next bigger picture question about why the Universe allows for these kinds of processes. We don't know the answer to that yet, but my feeling is it will be explained. Furthermore, I think that's cheating a little bit, what Paley did, and I think it's worth sort of being good scientists about this because we're very often faced with a puzzling circumstance, in our lives as well as in science, right? And we say, "Well, why is this this way?" And we come up with different explanations and the way that we think about it is, "Well this would be the case if this theory of the world had been correct, but that's not the final story. That's not the end of how you should be thinking about this. If you have two different theories of the world: God exists and God doesn't exist, for example, you have to think about all the predictions that would follow from that. Do we live in the kind of cosmos, that we would expect to live in if God had created it? Versus do we live in the kind of cosmos that we'd expect to live in if it were just the laws of physics chugging away? You know, the people thousands of years ago, who first started thinking about this stuff, they did think that there was a divine watchmaker. They didn't have watches, but you know what I mean, world maker. And they also put themselves here on Earth, more or less, at the center of that cosmos because we human beings had an important role to play. We know better now. We live on a planet surrounding a star, which is one of several hundred-billion stars in our galaxy- and there are hundreds of billions of galaxies in the observable Universe. We human beings are not that important. There's a lot more stuff out there in the Universe than you would imagine, if you thought that we had been part of the reason why the Universe came into existence. So to me, there's a lot more that fits well, that fits nicely, that is explicable in a universe that just obeys the laws of physics. Even if we don't know the answer to all the questions. Of course, saying that undoes some of the nice things that had been done by God's existence- like giving us a purpose, a meaning to our existence because God tells us what the purpose is. I am also someone who takes very seriously the fact, that was emphasized by Friedrich Nietzche and many other people, that when you take God out of the story, we have a lot of work to do to decide, 'Is there a purpose to our lives?' How should we lead them? If there's not a reason for our existence given to us from the outside, is it possible for us to create that purpose? I think the answer is "yes." In fact, I think that's what we do all the time, since we human beings invented God: that was just a roundabout way of giving ourselves purpose. I think we're still now doing the same thing that human beings have always done; we're doing it in a slightly more open-eyed way, in a way that is much more compatible with the fundamental nature of reality.

NARRATOR: What are the different viewpoints on free will?

CARROLL: You know, I have a personal view that discussions of free will are some of the most boring discussions that there can be at the intersection of philosophy and human life. It's not because the topic is boring, but it's because people very quickly start arguing about the definitions of words. What do you mean by "free will?" So, I've often made this suggestion to people who want to talk to me about free will, that I'm happy to do it, as long as we don't use the phrase "Free Will," as long as we actually say what we mean, we can make more progress. No one ever agrees with that suggestion that I make because they like talking about free will even though they don't agree on the definition. So, there are two big idea definitions that are a little bit different from each other and it's worth distinguishing: There's what we call "Libertarian Free Will," and Libertarian free will is that: "Somehow, I am not beholden to the laws of nature. I as a human being, I am a law in to myself, Immanuel Kant and other people, put it in these terms, they said, "There's no way of thinking of a human being, as somehow a collection of physical things obeying the laws of physics. There is something that is inescapably human that cannot be reduced to an understanding of ourselves as just mindless pieces conglomerated together to make something with a mind." So Libertarian free will is truly an ability to make choices and do things in the world that cannot even in principle in any way be explained by stuff obeying the laws of physics. Now I would say, no modern scientist believes in that- to say "no" is an exaggeration. There's probably some who do, but the overwhelming majority of scientists take seriously the idea that we know what we are physically: we are collections of atoms, molecules, etc., that in principle obey the laws of physics. In practice, we don't have the information to predict what a human being will do, but in principle it's there, okay? So Libertarian free will is something you can imagine. There are people who believe in it, but it's not sort of the locus of discussion in the modern world, so much, anymore. The question is: 'If you are not believer in Libertarian free will, is there anything left?' And so this is where we enter into the idea of "Compatibilist Free Will" because there's a set of people who say, "Look, I'm just a collection of atoms and molecules. I obey the laws of physics and therefore there is no free will. Therefore, there's no sense in thinking of myself as an agent, making decisions, having responsibility for anything, anything like that." And there's another school of thought that comes in and says, "Wait a minute-surely you can't be serious. You can't be telling me that just because I'm made of atoms and molecules obeying the laws of physics, that I can't talk about myself as a person making choices." In fact, and I'm in this camp, so I'm not gonna be fair. I'm gonna try to be correct, but I'm not gonna be completely unbiased when I compare the different points of view. Compatibilism says that we can still talk about human beings as agents making choices, while also agreeing that we don't violate the laws of physics. And to say, "Well, how are those two things compatible?" That's a perfectly fair question. And the answer is emergence. The answer is layers of reality. The answer is there are different ways of talking about the world that are compatible with each other, but very, very different. And in fact I would say, that every person who denies the existence of free will still goes through their life trying to convince other people to act in certain ways, puzzling over choices they could make, etc. The compatibilist just takes those things seriously and says, "Therefore I should talk about people as agents making choices." And then the thing is, and we call that free will, and people say, "Well, you shouldn't call that free will. You're redefining it." There I truly don't care- if you don't wanna call that free will, be my guest. What all I care about is, I am allowed to talk about human beings as agents making choices and responsible for their actions in exactly that way. It's very often that in the conversations about free will, you find people who believe in free will contrasted with "determinists," who just think the laws of physics are gonna tell us what happens in the world. To me, this is one of the biggest mistakes we can make. Not that you should be determinist or not, but that there is some relationship between determinism versus non-determinism and free will versus non-free will. Those are two separate questions. Determinism is a statement about how the laws of physics work. It goes back to Pierre-Simon Laplace explicating the implications of classical mechanics a la Isaac Newton. He says, "If you knew the position and velocity of everything in the world, the equations of classical physics, deterministically predict what will happen next." There's no randomness, there's nothing that is unknown or probabilistic or stochastic- you know exactly what's gonna happen in the future. Quantum mechanics comes along in the 1920s and says, "But the world doesn't work that way. The world of our observation, the world in which we live, you can't predict exactly what will happen. The laws of physics are not up to the task." Now there are subtleties, if you're a 'Many-worlds person' like I am, the multiverse is deterministic, but still in our world, the evolution is clearly not deterministic, okay? That's just how the laws of physics actually are. None of that has anything to do with free will. Quantum mechanics allows for non-deterministic laws in the sense that it says, "From this initial condition, you can't predict what you will observe next, you can only predict the probabilities." So what? As far as free will is concerned, it's not that my volition brings the electron into spinning clockwise or spinning counterclockwise- the laws of physics still do that. If what you are worried about is free will versus the laws of physics, it doesn't matter at all, whether those laws are deterministic or indeterministic. What matters is, are there laws? That's what matters. If we are beings that obey the laws of physics, if those laws of physics are a little bit stochastic, that doesn't give us any more free will than we had before. And in fact, the laws of physics are a little bit stochastic, so that's the world that you should try to explain. But the compatibilist position, is not one that denies determinism or indeterminism, it doesn't care. What it's saying is that you can both be a law abiding thing in the Universe- a physical system subject to the laws of nature- and it makes sense to talk about you as an agent making choices because you're talking about a different level of description, a higher-level, emergent kind of phenomenon. So I really think that once I start talking to someone about compatibilist free will, and they start talking about determinism, I know they've missed the point.

NARRATOR: How do our feelings fit into the molecular world?

CARROLL: There's a way that we have a thinking about human beings that works at the human being-level, right? I have not only an idea of who I am, but I have an idea of who you are. I have idea of what other people are like, what they're thinking. I have a theory of the mental state and the emotional states of other human beings. This is part of how I describe the world. And where you get into a little bit of trouble or at least a little bit of tension with the underlying laws of physics is, you say, "Okay, I'm describing myself in terms of: I have some knowledge, but I also have some preferences. I have some desires, I have some values, I have some feelings, I have some emotions. And you're telling me that I'm also, a collection of neurons, right? Or cells, and they obey certain principles of biology. And then you're telling me that those cells in those neurons are made of atoms in particles and they obey the laws of physics. And once I get to the level of neurons, much less the level of atoms and molecules, there's no feelings there. There's no desires, there's no emotions, there's not even any sort of memories or anything like that. So how can I say that I really have emotions or feelings and desires if I also think that I'm a collection of neurons or a collection of cells, or a collection of atoms and molecules." And I think that that whole discussion, is just a category mistake- you're mixing vocabularies. The whole idea of a multilayered emergent universe, is there are different ways of talking about reality, and they're all equally valid, or at least they're all valid in their own regimes, but they're separately valid. So, when I open the closet door in the morning and say, "Should I wear the blue shirt or the red shirt?" It doesn't help me to say, "Well, I'm gonna do whatever my atoms want me to do." My atoms have no wants. There is something that I have that is a want- and the fact that I'm made of atoms, doesn't make that go away. I'm sitting on a chair- 200 years ago, I would've said The chair is made of wood. Today, I can say it's made of atoms and molecules. Does that mean it's not made of wood anymore? No, it's still also made of wood! That's just a different way of talking about it. So when you attribute desires, wants, feelings, choices to people at the level of people that's a hundred percent valid and it is compatible with describing them as collections of atoms where there are no feelings, wants, desires or choices.

NARRATOR: Are there objections to the Compatablist worldview?

CARROLL: I think that the best objection to my own view about free will-the compatablist view, the emergent view- is that it's a little loosey-goosey in some sense, right? I'm saying I can talk about human beings, a level of human beings, or I can talk about atoms and so forth. Well, where do I draw the line? I mean, really what the compatablist needs to be able to say is, "Our best way of talking about human beings, given the information we actually have about human beings- so not given the information about all their atoms or neurons, but the actual information we see talking to them, discussing things with them- is as agents making decisions." Okay? So the non-compatablist could say, "Well, what if someday we have a better theory? What if someday I'm able to read your microexpressions on your face and exactly and reliably predict what you will do next? Would you still have free will?" And my answer is, "maybe not." My answer is: I can imagine in principle, getting so good at predicting how human beings actually behave on the basis of truly accessible information, that the concept of free will no longer becomes helpful or necessary. That's just an improvement in our scientific understanding. Do I think that actually will ever happen? No. For lots of good reasons, human beings are incredibly complex. There's chaotic dynamics, there's quantum fluctuations, there's a whole bunch of reasons why we can improve our picture of human beings without giving up. Why we should expect to never give up on the picture of human beings as agents making decisions. Still, we're pushed in this direction by the fact that we can build artificial attempts at trying to be a human being, trying to be an intelligent agent, trying to be something conscious. We can write a computer program that mimics human behavior. An AI has become really, really good at mimicking human behavior. It has not become conscious, when you try to show that the AI is just mimicking and not truly understanding the world, it's not that hard. All of the modern AI programs, are very easy to see, they don't truly have understanding of the world. But, how long will that last? I don't know. Could we be approaching a threshold, a change where we truly have artificial programs that- for all intents and purposes- we can't do any better to talk about them than we do about human beings, as agents with desires making choices? I honestly don't know. I think that's fascinating. I think that there is a big, noticeable gap between a human being making choices, and a collection of atoms where I predict what they will do next. But nowadays we're exploring the space of possibilities in between those different extremes- and that will hopefully help us think about it more clearly and maybe we'll get a better vocabulary for discussing not only human beings, but artificial intelligences as well. I do think that the existence of computers and the ubiquity of computers everywhere has sort of tainted our way of thinking about thinking; about thinking about what it means to be a thinking thing because computers think in some sense, right? I mean, they add numbers together just like we do, but they're way better at it. Like the simplest, in the old days we would have, you know, wrist watches with calculators on them, and they're much better at multiplying numbers together than the best human being is- but they're dumber at everything else. And this sort of distorts how we think what it means to be intelligent. You know, Go and Chess are two of the games that human beings invented, and we hold them up as pinnacles of intelligent thought- and now computers are way better at those games than we are, and we kind of feel bad about it. But the truth is, those games are tailor-made to be things that computers can be good at. They're finite, there's a rigid set of possible rules. There's only discrete number of possibilities- it's a very big number of possibilities- but you can brute force your way into figuring out what to do, and human beings don't think in the same way. A human Grandmaster at chess does not think about the chessboard in the same way that an AI does. A Grandmaster will think about big configurations in a holistic way. If you suddenly told a Chess Grandmaster, "We're gonna play chess except that every other move, the rook will move diagonally instead of horizontally and vertically." The chess master would, you know, think about it and probably make some elementary mistakes, but still be able to play a pretty good game. A computer would have no clue what to do; all it's done is run simulations with the actual rules. If you change the rules to something else, it doesn't know what to do. And that doesn't make the computers better or worse than human beings. It's a different way of thinking. We human beings, through our biology, are not trained to add numbers together or play chess- but we're trained to get a picture of the world very quickly and react to it in this kind of holistic way with vastly incomplete sets of information compared to a perfect reasoner or a perfect observer of the world. The miracle of the world is, that the incomplete information we can get about it still gives us a handle on what's going on, how to react to it. We can predict the future a little bit. We can predict the tides very accurately, the weather-not quite so much- the stock market, not very well at all. But those different levels of accuracy are important. It gives us the impression, which is correct at the level of human beings, that we can affect the world in which we live; that because we do this thing rather than that thing, we don't know which one we're gonna do, but if we did something, the consequences would be different. We can put it in the following way, because a lot of people who are anti-free will, the way they phrase it is, "I will believe that there's free will, if there's a way that I could have acted differently." And what they have in mind is: they are collections of particles or you know, whatever physical system that is obeying the laws of physics, and in fact, they could not have acted differently because the laws of physics are the laws of physics. But the reality is, you could have acted differently depending on what? Given what information? Given the actual information you know about yourself, you could have acted differently because the information you have about yourself, it's wildly incomplete; it's compatible with all sorts of different microscopic arrangements of what's going on in your brain and your body. That incomplete information is why we're not perfect reasoners about the future. Why we have decisions to make and why those decisions have consequences- 'cause that's the world in which we live, we are not microscopic, we're real world, that matters. We should take that seriously because we can. We can do things, we can talk about what the consequences are, and we can attach rightness or wrongness to them. That's what makes it all- in the end of the day- matter.