Sean carroll is one of the world's leading popular science writers whose bestselling books include The Big Picture: On the Origins of Life, Meaning, and the Universe Itself and Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime. His latest book is The Biggest Ideas in the Universe: Quanta and Fields.

In this interview, Carroll explains how our understanding of the world around us has developed over time, how much stranger the world is than our "commonsense" perceptions of it would indicate, and how laypeople with no background in the physical sciences can begin to grasp and appreciate the deepest hidden truths about the Universe. Carroll is an eloquent spokesperson for the view that a godless universe can still be a poetic and beautiful one and that there may be more connections between the political and the physical than first appears.

Nathan J. Robinson 

It's possible to go through life, as many people do, in almost complete and total ignorance of physics, despite every single thing in your life depending on and being determined by it. And so when I told one of my colleagues that I was interviewing a professor of physics or natural philosophy—we can discuss what that means—they said, why would you talk to such a person on your Current Affairs program? At the beginning of The Biggest Ideas in the Universe, you say everyday people should have a grasp of, should get to understand, and should know more things. So how do you take the familiar world around us to make it strange—take the mundane and make it weird and puzzling—to make people want to learn more? 

Sean Carroll

It's a great question. I think it's different for everybody. We should be thinking about this more systematically than we do as professional scientists and academics. And also, I don't think that every person needs to care deeply about physics. I suspect that they want to. I suspect that people actually think this stuff is interesting, and that interest kind of gets beaten out of them over the course of their secondary school and higher education experiences because we turn it into a list of facts you're supposed to memorize and tests you're supposed to take and things like that.

So I would say two things about sparking the imagination. One is connecting some of these crazy, big ideas to our everyday lives. They actually do matter. In my first book, From Eternity to Here, I talk about the arrow of time and the fact that we remember the past, but not the future. We can scramble an egg, but we can't unscramble it. These are things that are very familiar to us. The past and the future are different from each other. So why is that true? Ultimately, it's because of conditions at the Big Bang, 13.8 billion years ago. The universe set us up for this kind of experience, and we're still living with the ramifications of that. It's in your kitchen every day, which I think is kind of awesome.

 And the other thing is to emphasize the fact that science is not a list of results. It's not even a list of theoretical speculations. It's a process. It's a process of learning about the world by being open to it, by hypothesizing all the different ideas that we can and then collecting data and winnowing down to the hypotheses that work. That's the scientific method, but it has vast applications. That's a good way to go through life about a whole bunch of issues, not just theoretical physics.

Robinson

Friend of the magazine Noam Chomsky often says that the beginning of scientific inquiry is the willingness to be puzzled by things that you're not naturally puzzled by, and the way he talks about that is language. And he says that his inquiries began with the question, why can we use language? Where does that come from? And isn't that strange? Isn't that weird? No other animals can do this. Why can we? And then trying to get to the bottom of something that is taken for granted. Now, you mentioned there scrambling eggs. So tell us a little bit more about this willingness to be puzzled, this taking of the seemingly obvious and making it very non-obvious or making it interesting or making it a kind of challenge to think about.

Carroll

Yes, I think a willingness to be puzzled is absolutely crucial, but then also a willingness to improve upon one's puzzlement. That is to say, to resolve one's puzzlement by being open-minded and looking at the alternatives. I think there are two easy places to live. One is thinking everything, and the other is thinking nothing. And science forces us to live in the in-between where we say, there are some things we know with pretty good confidence, there are other things we don't know, and how are we going to decide we're going to go out and look at it? We're not going to just use the force of reason. As great as it is, it's not enough to tell us what is going on. And that initial puzzlement that can come from anywhere. It can come from scrambling eggs, and sometimes it sneaks up on us, like the idea that you can scramble an egg but not unscramble it. That's a puzzle. Why? Why does it go in that direction of time, but not the other? Nobody asked this question 200 years ago, it was just taken to be woven into the fabric of reality. And therefore it wasn't something that we questioned, where we said, well, could it have been otherwise? It was people like Pierre-Simon Laplace and others who pointed out that our best understanding of the underlying laws of nature doesn't have a directionality to time. It works equally well forward and backwards. So in fact, there's a puzzle here we didn't even know that we should be puzzled about. So it's this ongoing dialogue between our ideas, our experiences, and everything that really drives us to ask these wonderful questions.

Robinson

So, why does time have a direction? It turns out to be an incredibly difficult and complicated question.

Carroll 

Super important, yes. And before, like in Aristotle's time, it would have been like, well, it just does, what do you want from me? 

Robinson

I think everyone gets handed down the image of Newton supposedly watching the apple fall from the tree. But then, when you start to think a little bit, you go, well, actually, if you're sitting watching an apple fall from a tree, there is a very profound question, which is, what the hell is happening here? If you're living hundreds and hundreds of years ago, that turns out to be a very fascinating and difficult thing. And the deeper you go, the more layers you unravel.

You talk a lot about the history of the development of our understanding of the world and how it has gotten more and more complicated. The simple has become very complicated.

Carroll

Well, it's become complicated in some ways because we know more. It’s a great feature of physics. I like to say that physics is hard because physics is easy. 

Robinson

Explain that. 

Carroll

The studies that we do in physics are literally about ripping the universe apart into its simplest pieces and analyzing them to death. And because we take this strategy, like an apple falling from a tree, how hard can that be? We can do it. We can get the right answer. We can't get the right answer to, how does the apple grow? We can make some advances there, but we don't have a perfect, pristine, predictive theory of that. It's too complicated. But the apple falling from the tree we can get exactly right. And because of that, when you get it exactly right, guess what? There are differential equations involved. There's calculus involved and things like that, and suddenly, exactly because we understand it so well, it seems a little intimidating.

Robinson

But perhaps you could take us back to some point in time at which we didn't understand some feature of the universe that we came to understand. I've always thought that a better way to introduce science to people, rather than the list of results and facts, is to study the history of how we came to know things because it's totally fascinating. Give us an example of something that may seem obvious that someone puzzled over, and then we finally figured out how it worked.

Carroll 

You're just asking for trouble here because I could talk for hours if we just stick to Newton, for example. It was the Age of Reason. It was the moment in history when people were switching from a mode of trying to think about God's thoughts and write them down to poking at the universe. Francis Bacon sat down with his friends and invented the scientific method, and they founded the Royal Society, and also the good people of London invented the idea of the coffee shop, which was crucially important for all these scientists to get together and talk over coffee.

At one point, talking over coffee, there was this group of Christopher Wren, the famous architect, Robert Hooke, a famous physicist who invented Hooke's law about springs, and Edmond Halley, the guy who discovered the comet and later became famous. And so they are friends and they had helped found the Royal Society. And they said, we're trying to understand if the law of gravity works in a certain way—can we recover all of the results of Galileo and Kepler and Brahe and people like that? Can we show that the planets should move in a certain way, in ellipses in particular? And they realized none of them were really up to the task mathematically. They had these intuitions, but there were some calculational chops that they didn't quite possess. And they said, we know who is good at this, Isaac Newton, but we all agree he's a jerk, and we're kind of afraid to talk to him. And so they kind of bullied Halley, who was the youngest guy there, to take the trip up to Cambridge.

Halley knocked on Newton's door and said, so we have this question: if there was a certain law of gravity, how would the planets move? And Newton's like, well, they move in ellipses. And Halley is like, Well, wait, how do you know that? And Newton says, Oh, I calculated it, I just didn't tell anybody. And then Halley cajoles him into writing something up, and then eventually it becomes the Principia Mathematica, the most important book in the history of humankind. He realized that this could be extended to a whole bunch of other things. It's that human side that is amazing. The apple story was one hundred percent fabricated by Newton to make him look better later in life. So, we scientists look at the universe, but we look at it in a very human way.

Robinson

Now, Newton, to make this discovery as I understand it, did have to invent calculus to find the answer, which is kind of a crazy thing to just go, the math that I need has not been invented, so I need to invent the correct math. We can't all invent a new form of math, but you tell us not to be intimidated by things like calculus, that these things can be understood by many of us who are frightened, who are driven away, who see an equation and react to it like a vampire to garlic. You help us achieve that confidence.

Carroll

Yes, I think it's a very interesting phenomenon, and it goes back to the way that we are all intrinsically interested in this stuff, but it gets beaten out of us over time. Math is the quintessential example of something that just becomes scary to people. Even if they're good enough in high school to push around the symbols and do well in the tests, they don't love it a lot of the time. And so later in life, if they can get away without thinking about that, they will. As a result, when it comes to explaining science to people, we have popular books and podcasts and whatever where it's all stories and words and metaphors. It's great, and I do it myself, and I think it's very important. Or you can say, I'm going to become a professional physicist and buy a textbook and go through years of work, and there's a gap in between. The point about the professional physicist is that they need to use those equations. They need to manipulate them and solve them and explain how something is going. But there's an idea that you can just understand the equations without being able to use them in the way a professional physicist does. I can drive my car, but I couldn't build it. I can't even repair it. And I think that it's possible to give people who are not professional physicists or mathematicians a better understanding of science by showing them the equations and simply explaining what they mean. They're very graspable if you just learn to speak that language a little bit.

Robinson

Let me ask you, as we look across the history of the development of our understanding of the way that the universe works, how did the universe become weirder the more we learn about it?

Carroll

Well, it's not a surprise in retrospect. Maybe it even wasn't a surprise at the time, but the progress of science and, in particular, the progress of our technology, allows us to look at the universe in more and more in-depth ways, whether it's very small distances with microscopes and particle accelerators, very large distances in cosmology and astronomy, or just extreme conditions overall, these are all things that we don't experience in our everyday life. Thomas Nagel tells us we don't know what it's like to be a bat. We certainly don't know what it's like to be an electron. So you shouldn't be surprised that when we learn about how electrons work, it's unfamiliar to our everyday intuition. But the surprising and good news is that we can work at it. We can understand it. It's not easy, it's not immediate, it's not intuitive. But we can train that intuition. That's what professional physicists do all the time. 

Robinson

What are some of the ideas that you think people are really trapped in? One of the things I think that comes up a couple of times in The Biggest Ideas in the Universe is being trapped with the idea of absolute time. It’s really, really hard to break out of that. 

Carroll 

There are a lot of examples. The idea that time and space are two different things: that's just the most intuitively firm and obvious thing in the world. And Einstein comes along—in fact, it wasn't quite Einstein. It's another interesting history story. Einstein in 1905 puts the finishing touches on what we now know as the special theory of relativity. But he doesn't come out and say, space and time are unified into one thing. It was Hermann Minkowski, who had been Einstein's professor at university, who was a mathematician. [...] He said the right way to think about relativity is to unify space and time together. And Einstein said, that's just mathematicians making things hard for me again, and he didn't think it was very useful, but then eventually he came around to thinking that way.

And again, it's one of those things where once you really immerse yourself into it, you realize, oh, okay—me, as an observer, I have a personal view of what is happening at any one moment of time in the universe. But the lesson of Einstein is that someone else moving in a different way, moving close to the speed of light compared to me, will have a different way of slicing up space time into space and time. It's very hard to wrap your mind around, but you can get through life without worrying about it too much.

I think that questions like determinism are actually a little bit closer to human issues. There's this boiling debate in certain corners of the internet about free will and determinism, and the state of sophistication of that debate is not very high, in part because physicists aren't always very clear about saying what they mean. So, the universe that we live in is not deterministic. You cannot predict certain things about like when a nucleus is going to decay, or something like that. 

Robinson

Do you have free will? 

Carroll

That's a philosophical question on the side of which I come down saying yes, because I'm a compatibilist. I would say that even if things had been deterministic—this is why it's very frustrating for me. I'm a compatibilist. I think determinism and free will are compatible. But as a physicist, we don't even have determinism. So, what is it that you're actually using against free will here?

Robinson 

InThe Big Picture, one of the most clarifying things you discuss is levels of understanding. You mentioned earlier that we understand what's happening when the apple falls from the tree, but we don't understand things at certain other levels. I want to read a quote from you. 

“In our daily environment, we have a complete inventory of the particles and forces and interactions that are strong enough to have any noticeable effect on anything. That's a tremendous intellectual achievement.”

My question for you is, then, why can't we predict the outcome of a presidential election?

Carroll

If you knew the wave function of the universe, why aren't you rich?, as my cosmology colleagues sometimes say. Well, the answer is, you know the laws, but you don't know the configuration of all the atoms in the universe. Up to the fact that there is some quantum indeterminacy there, which is probably not relevant for presidential elections, the idea is that if you truly knew exactly the physical state of an isolated system, then you could use the laws of physics to explain how it will evolve in time. That's the thought experiment of Laplace’s demon—Pierre-Simon Laplace, who we already mentioned—but it was never meant to be taken seriously. Laplace never thought that you could be his demon. That's ridiculous. We have wildly incomplete information about the physical world. We have tiny little amounts of information.

One of the amazing things, as you just said, is that we know the laws at the lower level. Another amazing thing is, you don't need to know the laws at the lower level to understand a lot about what happens at the higher level. Presidential elections are hard, but trajectories of apples falling from trees are easy. We can land a rocket on the moon without knowing the exact atomic structure of the rocket. The rocket has a center of mass, and that's good enough for us. That's the layers that the world appears in. That's the issue of emergence. And volume three of The Biggest Ideas in the Universe is subtitled Complexity and Emergence. So I'm literally writing about those issues right now.

Robinson

We mentioned that you are a professor of natural philosophy. Now that's interesting because I don't think there are that many people with the title professor of natural philosophy anymore. It sounds like a very 18th century kind of term. It sounds like political economy, the merger of two kinds of things. I assume you [take that title] deliberately; I assume you deliberately chose to revive that category, that name for the kind of inquiry you conduct. So, let me ask you, why?

Carroll

It’s a great question, and I did, in fact, choose. That's one of the nice things about this job that I have at Johns Hopkins. They let me choose my title, and effectively, my job a joint appointment in the physics department and philosophy department, which is fine, but it sounds kind of boring. So I chose to be a professor of natural philosophy because that's what Isaac Newton was. He was a professor of natural philosophy. The idea back then was they didn't have separate subfields, like physics or even science. It was all philosophy. And there was theological philosophy, where you thought about the mind of God, but then there was also natural philosophy, where you thought about the natural world. The joke among philosophers is, as soon as a subfield of philosophy starts making progress, it breaks off and becomes its own discipline.

So it looks like philosophy itself never makes progress. But Isaac Newton was a philosopher, and I think that in that breaking off of different subfields in the course of academic history, we have lost something. We have lost the idea that there are interesting questions at the boundary of science and philosophy, and it's not just physics and philosophy—biology and philosophy, computer science and philosophy, etc. These all have huge questions. What is consciousness? What is natural selection? And so I got this job title, and then a colleague of mine here at Johns Hopkins, Jenann Ismael, who is a philosopher of physics, and I founded something we call the natural philosophy forum here at Johns Hopkins, and we're trying to revive this term. So we're having in May a big conference where philosophers of mind and philosophers of physics are going to come together, and they'll meet people who are actual neuroscientists and physicists, and we have economists and systems thinkers and complexity scientists and linguists. It's great because we're going to take these questions seriously, and I think that it'll be to the benefit of both sides of that connection. 

Robinson 

A person can have a career as a physicist without ever taking a philosophy course, and a person can have a career in philosophy without ever taking a physics course. And each one of these people that you might meet might say, I don't understand why I have any use whatsoever for the other subject. Tell me a little bit more about what you think happens when these disciplines start to speak to each other.

Carroll

A hundred percent. If you're a moral or political philosopher, and you're trying to understand the application of John Rawls original position to economies that are not quite equilibrated yet, or something like that, physics is not going to help you. Knowing the Standard Model of particle physics is of no help. Likewise, if you're studying the Standard Model of particle physics and you just want to calculate the interaction rate of two protons or something like that, what is the probability of making a Higgs boson? Philosophy is not going to help you. That's fine. The point is not that philosophy is helpful in every physics question. The point is that there are some physics questions which are absolutely contained within philosophy.

When we built the Large Hadron Collider in Geneva, this massive experiment that came online in the early 2000s and discovered the Higgs boson in 2012, it cost about $10 billion overall. And it did discover the Higgs boson, and that was great. But a huge motivation for building that machine was that the properties we thought the Higgs boson had seemed unnatural to us, and so we thought that not only would we discover the Higgs boson, but we would also discover other new things in physics that would help us explain the unnatural properties that the Higgs boson had. So number one, we have not yet found anything else, sadly. But number two, that is clearly a philosophical claim that this mass of the Higgs boson is unnatural. What does that mean? Unnatural? What is your expectation about what the laws of physics should be? Here's a question that is squarely at the overlap of physics and philosophy, and we spent ten billion dollars to study it. So clearly it must be important.

Robinson

Well, you say that people studying John Rawls might have no use for physics, but it is the case that in the book The Big Picture, you cover both. In your big picture of the universe, there are questions about morality, and if we are having a discussion about political philosophy and someone argues that the ideal status that derived from the Divine Order handed down by the deity, whether the universe is created by a deity, what we know and what we don't know becomes relevant. So I feel like you may have undersold how much the moral and political philosophers can get from a deeper engagement with the sciences.

Carroll 

A hundred percent. Fair enough. You caught me. I was trying to come up with an example in real time of where philosophy doesn't rely on physics, but there is a very important dependence there. You're absolutely right. In fact, I try to emphasize this to my friends in science and philosophy, that if there's any one thing we have all done—I don't mean like people who build lasers and things like that. They've done important things, they actually matter. But for people who think about the origin of the universe, etc., what have we done for the average person on the street? What have we discovered that is truly relevant to their lives? And the answer is that God doesn't exist. That's the thing that we have figured out. And that's relevant to, you say, political and moral philosophy. It's relevant to consciousness studies or anything like that, theories of retribution versus rehabilitation, all sorts of things.

So there's absolutely a background ontological playground in which we are moving that physics plays a huge role in informing. My point was just that once you accept that and move on to the more nitty-gritty detailed questions, just like with the particle physicist trying to calculate a scattering amplitude. Well, doing that doesn't require philosophy, but understanding why you're doing that requires philosophy. So the boundary is pretty darn permeable. There are a lot of connections back and forth.

Robinson 

One of the reasons I wanted to talk to you is that, at Current Affairs, we write a lot about war, great power competition, economic inequality, and all these kinds of things. And one of the nice things about studying the universe, as Carl Sagan so beautifully put it in Pale Blue Dot, is the way that pulling it back gives you this incredible alteration of perspective. It doesn't dissolve human conflict. You call it poetic naturalism, the kind of overall philosophy of inquiry that you apply to every problem. It's a calming way of being.

Carroll  

Well, there are absolutely sources of existential anxiety in what we have learned about the natural world. There's a famous paper that was written in the 1970s by Freeman Dyson, who is a well-known physicist. He asked the question, if the universe lasts forever, can life last forever, in principle? And he answered, yes, it can. Because he figured out a way to say that we can think our thoughts by using up a certain amount of energy, but by just using up less and less energy per thought, even in an infinite universe that is sort of getting colder and colder, we can think an infinite number of thoughts. And that was very comforting from a certain point of view. But guess what? It turns out not to be relevant, because since then, we've discovered that the universe is not only expanding, but accelerating, and that means that entropy is increasing in a way that we will eventually hit maximum entropy. The universe will be as disorderly as it possibly can be, and at that point there is no more chance to have more thoughts. There is an upper bound in our current theories on how long life can possibly exist in the universe. That's sad. I admit that's sad. I don't like it to be true, but I think it is true on the basis of what we know about cosmology.

So I'm not simply trying to tell comforting stories about the world, but I also think that this view where ultimately it's the laws of physics that matter that push us around, that we are these emergent, higher level configurations with vastly incomplete information about the world, leads us to a picture where we are agents, making decisions, coming up with purposes and goals in our lives, attaching meaningfulness to actions we take. It's not out there in the universe, but it is inside us. It's comforting, but it's also true. It's just as true as the finitude of our future life expectancies.

Robinson  

Of course, there's the wonderful scene in Annie Hall where Woody Allen's character as a child is taken to the doctor because he's paralyzed by thoughts that the universe is expanding. She's like, why does it matter to you? Why does it matter to you? And he’s like, well, all life will disappear.

Carroll 

But Brooklyn is not expanding, he learned. Cold comfort.

Robinson  

In The Big Picture, you understand where the kind of religious impulse comes from. And you talk about the religious story of the universe as a miracle created by God in his unique act of love, which culminated in the appearance of human beings on Earth who are unions of soul and body, who appreciate and return God's love. And you say you could see why someone would believe this, but you actually think that naturalist inquiry, or empiricist inquiry, into the world leaves us at a place where we have an equally amazing and awe-inspiring story that can actually bring us, if not comfort, some degree of terror, but that we can live with that.

Carroll  

Yes, and it didn't have to, a priori. I guess I want to emphasize this: you could imagine an alternative world in which the religious picture was just better in terms of making us feel good about ourselves. I'm actually participating in a debate at the Oxford Union in a few days where the resolution is “this house regrets the death of God,” and I'm on the “no, it doesn't” side. But we could have, I get that. It's hard. It's kind of a terribly worded resolution because you're not sure whether we're arguing about whether God exists, or whether we're sad that God doesn't exist, or whether we're sad that people realize that God doesn't exist. But I do sincerely believe that’s the way the world actually is. Maybe it's because we are such bounded, finite creatures that we can't understand everything. Laplace’s demon knows everything that's going to happen in the future. He or she doesn't ever get excited or sad or disappointed, but we're subject to all that stuff, and for better or for worse, that makes us who we are, and that's absolutely part of the natural understanding of things.

Robinson  

Well, actually, I wanted to ask you about this because you say, maybe we're these bounded, finite creatures who can't understand. But in The Big Picture, you say, 

“Everything we've experienced about the universe suggests that it is intelligible—if we try hard enough, we can come to understand that. There's much that we still don't know about how reality works, but there's a great deal we figured out. Mysteries abound, but there's no reason to worry or hope that any of them are unsolvable.” 

Now, one of the things that Chomsky also has argued is—I think he has called himself a mysterian—the idea that, because human beings are not angels, because we're animals, we have no more hope of coming to a complete understanding than a pig has a hope of ever understanding chess. The pig could think for a million years, and it will never understand language and art. We’re bounded in that same way where we're never going to know most reality because we're basically like little ants.

Carroll 

You're welcome to think that. It's absolutely possible and conceivable that that's true. But I would point out two things. Number one, there has been a phase transition. Good for the pigs, but there is a difference in how human beings think compared to pigs. And Chomsky himself should know that that difference has something to do with the symbolic manipulation of language. [...] It is at least as plausible to me that we can think in principle as well as it is possible to think that there is no higher level that we will ever bounce onto. We can certainly be more accurate and be faster and process information more cleanly, but it's not qualitatively a different kind of thing. And the other thing to say is, empirically, the evidence we have seems to be entirely on the other side.

A hundred years ago, we didn't know the universe was expanding, and we didn't know there were other galaxies. I can only use that example for a few more years, because 1927 was when we discovered these things. But I'm going to milk it for the next couple of years, like the amount that we have learned in the last 100 years that we didn't know, and especially if we say the last 500 years compared to all of human history, the rate at which we've been figuring things out has been astonishingly fast. If anything, the empirical suggestion is, oh, my goodness, we human beings are kind of awesome at this, aren't we? We're going to keep keep going.

Robinson  

It's incredible. This is why I think that the best way into science is to study the history of science. You talked a lot about Einstein and appreciating what he managed to do as a clerk in a patent office with the technology of 1905. He's living in 1905, a time that we think of as quite primitive, or at least there's not a desktop computer around. There's barely even a car.

Carroll 

Maybe that was for the best.

Robinson  

What human beings have managed to uncover about the world around them in an incredibly short period of overlapping lifespans. There are still a few people around who were born in 1927.

Carroll  

A few years ago, I was at the Santa Fe Institute, a wonderful research institute that studies complexity and things like that. They hosted a workshop on how we are going to deal with bumping up against the limits of knowledge. It was a great workshop. I had a lot of fun and participated, but only halfway through did it dawn on me, wait a minute, you people think that we're close to the limits of knowledge. And some of them are like, yes, we're very close, and no, I don't think we're anywhere near that way. Different subfields of academic inquiry make progress at different rates, and so if yours is in a slow point, then you can feel frustrated, but oh my god, the rate of change overall, the rate of improvement and understanding continues to amaze me.

Robinson  

Your latest books discuss the biggest ideas in the universe, so perhaps one place to conclude would be to ask, what are the biggest remaining mysteries in the universe? What is the frontier of the unknown today, here in 2025?

Carroll  

It makes a big difference that the frontier is too big. There are many, many different subfrontiers that play a role. In what I grew up doing professionally—theoretical particle physics, gravity, cosmology—there's a very canonical list of unsolved problems. How to quantize gravity, that is to say, how to reconcile the ideas from 1915 of Einstein's general relativity with the ideas of 1925 of Heisenberg and Schrödinger's quantum mechanics. That's a huge unsolved problem. What is the Dark Matter and the Dark Energy that constitute 95 percent of the stuff in the universe? What happened at the Big Bang? Why is there more matter than antimatter? There's a whole list of very sensible problems, but these are all elementary physics problems.

There's a whole other frontier of complex systems problems. What is the origin of complexity in the universe? After all, if the story of the universe evolving is just entropy increasing and things becoming ever more disorderly, how did things like you and I ever come into existence? Were we likely? Are we a happy accident? And when I was your age, this was a philosophy problem. Nowadays, we're building telescopes that are looking at other worlds around other stars, and we're going to need to know what to look for. So I think that all of this understanding, the marvelous emergent, higher level structure of the universe, is where a lot of the action will be at in the near term future.

Robinson  

Of all the mind-blowing things, how does a Big Bang produce a human being? is the strangest one to me. And the fact that we think we're alone in the universe, but also that's nuts because why am I the thing in the vast universe? I'm on the only planet? There's this one place where this happens? That's bizarre.

Carroll  

It is. And I think that one sign of intellectual maturity is saying, maybe we are, maybe we're not, maybe we are. This is science again. This is the hypothetico-deductive method. We need to be able to say we don't know that. It might seem absurd to us that this little planet we live on has the only higher level intelligent life in a cosmos where we have a trillion galaxies with a trillion stars each with a couple of planets each. But maybe we just don't know. Alternatively, it's also possible that there's life everywhere out there. That's going to be one of those we have to actually go and look.

Robinson  

The probability of being alone seems quite low given the size.

Carroll  

That's what you would say if you were alone. That's just, if we weren't here, we wouldn't be having the conversation. So that's a precondition.

Robinson  

I do understand why, if we were alone, people might be tempted to believe in God. Then in that case, we are very special.

Carroll  

In that case, we're very special. But the thing is, we can only observe part of the universe. It's a very big part—a trillion galaxies, like I said—but there could be a trillion times that many galaxies out there. It could be there's lots of life in the universe, but it's literally just too far away for us to talk to. So again, it's a plea for open mindedness about these questions that we are not yet able to definitively answer.

Robinson  

Not to drag the conversation to aliens and black holes, which I'm sure is what everyone else wants to ask you about. 

Carroll

It could be worse.

Robinson

It's like, can we talk about aliens instead of fields?

Carroll  

I do tell a story that there was a formative moment in my life when I was in the audience for a popular talk on cosmology by Martin Rees, who's a famous British theoretical astrophysicist, and he was talking about galaxies and so forth. And then in the middle of his talk, he starts talking about aliens; he starts talking about intelligent life on other planets. And I'm sitting in the audience. I'm already a cosmology professor myself, and I'm thinking, no, that's cheap and tawdry—he's clearly playing to the gallery, and he should be intellectually more serious than that. But the audience loved it, because they don't care that we academics draw this distinction between “studying galaxies: good”, “studying extraterrestrial life: bad.” And I think he was right. I think that all of these things do blend together, and we should talk about them and admit things we don't know and hope we discover some more things that we could.

Robinson  

Yes. I think any level of interest is a good place to start, and then you will hook people, and they will eventually want to know the equations. 

Final question: Are there any dumb questions? 

Carroll  

There are dumb questions, but there are no questions that we can't turn into an intelligent question. 

Transcript edited by Patrick Farnsworth.