IRA FLATOW: This is Science Friday. I’m Ira Flatow.

You think you understand how the world works, but do you really, really understand it? I mean, what would you say were the central ideas that shape our understanding of the universe? And how complete are those ideas?

Dr. Sean Carroll is the Homewood Professor of Natural Philosophy at Johns Hopkins, in Baltimore, and author of The Biggest Ideas In The Universe: Space, Time, and Motion, and he argues real physics shouldn’t be just the realm of PhDs and grad students. It should be understandable to all of us so we can all wrap our heads around this stuff.

Sean, welcome back to Science Friday.

SEAN CARROLL: Thanks very much for having me, Ira.

IRA FLATOW: Nice to have you. How did you decide what big ideas should be included and what should be left out?

SEAN CARROLL: I think it’s pretty obvious as soon as you open the book, the big difference is that there are equations in this book. And Stephen Hawking famously told us that every equation cuts your sales in half, but–

IRA FLATOW: [LAUGHS]

SEAN CARROLL: –my attitude is that everyone knows what 2 plus 2 equals 4 means. And that’s an equation. It’s not that hard. And it’s just a matter of degree to get up to the harder equations. So we go through all of classical mechanics, a la Isaac Newton, and then relativity, special relativity, the twin paradox, things like that, all the way up to general relativity and black holes. So you really know at the end of the book what a black hole is and why they’re predicted by Einstein’s theory.

IRA FLATOW: So how is your book different? Because as you say, Stephen Hawking has written about this and other physicists have written about this. What was your idea about how your book would be different?

SEAN CARROLL: I thought that there was a gap between popular level treatments of physics– which I myself am a big fan of and have both read and written throughout my life– and then there’s textbook treatments of physics that assume that you’re going to take years of courses and that you’re really dedicated to it. And if someone wanted that little bit of quantitative understanding but didn’t want to do all the problem sets, they didn’t want to spend years and years getting there, then there is not a lot of resources for that person.

So I don’t think it should take you years to understand things like general relativity. So that’s why I thought that this was a gap in the literature.

IRA FLATOW: I know that I’ve had to read this stuff over and over and over again over my career just to understand it.

SEAN CARROLL: Yeah. I mean, it’s not easy. Because when we don’t use the equations, we tell stories. We use metaphors. We use analogies. And those are great. But they’re never exact, right? They’re never capturing exactly what is there. So people talk about how curvature of space and time warps things. And then they want to say, well, was time running at a different speed close to the Big Bang?

And if you look at the equation, the answer is easy. It’s no. It was not running at a different speed. But if you don’t look at the equations, you kind of got to trust people. And that’s never quite as satisfying.

IRA FLATOW: Yeah. You say in the introduction to the book– My dream is to live in a world where most people have informed ideas and passionate opinions about modern physics. Do you think that’s possible to get to a place where people might discuss the latest theories as deeply as they analyze last night’s baseball game?

SEAN CARROLL: Well, there are people out there who do have passionate opinions about these things, and they don’t always have the understanding to back it up. So I both want those opinions to be more widely shared and for them to be more informed, so that the discussions are more interesting. I think that physics should have a similar status in society as history or economics or movies or whatever, things that we go and argue about at a very, very passionate level.

IRA FLATOW: Do you think, then, that physics as it is taught to people is just too oversimplified?

SEAN CARROLL: No, I don’t think it’s too oversimplified. I think there’s absolutely a place for the simplifications. And like I say, I do it myself. But it’s not the only thing. So it’s not that I’m saying that other attempts are wrong or bad or inadequate. But there should be a richer ecosystem. We have enough books and online resources out there that we should be catering to all different levels of interest.

I imagine my own 16-year-old self would have loved books like this, where you could go a little bit deeper than one more picture of a bowling ball and a rubber sheet, trying to explain general relativity.

IRA FLATOW: You start off with things that people might remember from high school, with mass and acceleration. Do you believe that that is such a central principle, that if you understand that and how that is working, that might be enough for a lot of people?

SEAN CARROLL: Well, I really do believe that everyone will have a different level into which they want to dive, right? And I think some people are going to be just in love with the first half of the book, where we’re talking about the basics of classical mechanics, Newtonian physics, just appreciating the fact that momentum is conserved. That if you have an object out there in outer space, where there’s no friction or air resistance or whatever, it keeps moving at a constant velocity.

That’s a really deep thing that it took hundreds and hundreds of years for scientists to figure out. So appreciating that at a deep level is very, very important and is an accomplishment.

And then there’s going to be other people who want to get to the tensor calculus at the end of the book, where you’re like, what does it mean that Einstein says space and time are curved by matter and energy? What does that literally, exactly, precisely mean? You can get it here.

IRA FLATOW: As you get further and even deeper into the book, that chapter or those ideas that deal with time, I think that confuses so many of us. I mean, we think we know what time is, right? But then we have scientists saying, no, your conception of time is all wrong.

SEAN CARROLL: Well, it’s a tricky one. And it’s not as if there is something out there in the universe called time and we’re just trying to figure out what it is. There’s a word called time, and we’re trying to figure out how to best deploy it in understanding the universe. And it turns out, post-Einstein, that what you used to refer to as time means different things in different circumstances.

There’s the time that the universe feels. There’s the time that individual people within the universe feel. And there’s different aspects to them all.

IRA FLATOW: Yeah. And what exactly is time? Einstein wrote– people like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion.

SEAN CARROLL: Well, and that’s why I wanted to not just do a dry rehearsal of all the equations and concepts of physics, but also talk a little bit– take seriously– the philosophical questions that they raise and sometimes that really impacted how we think about the physical ideas.

So what is time? Is the universe eternal? Are all different moments of time equally real? Or is only the present moment real? These are hard questions to answer definitively, but the reader will at least come away knowing what the possibilities are.

IRA FLATOW: Let’s get into the philosophical side, then. I know you just started a new position at Johns Hopkins. It sounds like it spans in between physics and philosophy. Would that be an accurate take?

SEAN CARROLL: Yeah, I think that’s exactly accurate. I mean, one of the great things about this position is that I got to invent the name for it. And I named myself a professor of natural philosophy, harkening back to the time of Galileo and Newton and their friends, who didn’t distinguish between science and philosophy. It was all the same to them. It was all a single endeavor, trying to understand the natural world as well as we possibly can.

So people who would be interested in things like this include not only philosophers and physicists, but the right kind of mathematician or neuroscientist or biologist, who’s really just saying, what are the fundamental principles by which nature works?

IRA FLATOW: Yeah.

SEAN CARROLL: That’s a set of questions that it requires both philosophical and scientific tools to try to answer it.

IRA FLATOW: That’s good to hear. Because I’ve always thought that one of the mistakes we make in high school, for example, is that we teach students to be like scientists. We give them those inclined planes. We do chemistry with stuff they’re never going to use, right– well, for 90% of them?

SEAN CARROLL: Yeah.

IRA FLATOW: But we never teach them how to appreciate science or where it fits in. And it sounds like that’s what you’re trying to do.

SEAN CARROLL: Well, that is absolutely part of it– both understanding the context, the bigger picture, where science fits in, but also just trying to do better science. One of the things that you realize when you dig into science and philosophy at the same time is that they have complementary skill sets. Scientists– bless their hearts– they’re just not very patient.

They often know what the right answer is because they’ve done the experiment. And so if you ask them, well, why is that the answer, they’ll give you some very half-baked reason that just doesn’t hold up logically. But they know what the right answer is, so that’s good enough for the work that they’re doing.

And I think that if you want to go beyond our current understanding, then sometimes you have to be a little bit more patient, a little bit more careful. That’s what philosophers are really good at.

IRA FLATOW: Yeah. So what kinds of books, outside of the normal realm, do you tell your students to read if they want to understand philosophy and science?

SEAN CARROLL: Well, there are a group of people who work in what are called the foundations of physics or the foundations of biology. And these are people who really are fundamentally interested in science but, in a way, using a methodology maybe that will never get them hired as professors in physics departments or biology departments, so they become philosophy professors. There are people like David Wallace and David Albert and Jennan Ismail– my new colleague here at Johns Hopkins– who are really analyzing the basic structures of the world in a very careful philosophical way.

And so it’s actually, I think, a beginning of a Renaissance for this kind of work. That’s why it’s very exciting for me to be here. And we’re starting up a little forum on natural philosophy, and we hope to inspire others to take up this way of thinking about the universe.

IRA FLATOW: Natural philosophy, isn’t that what they used to call science before it was science?

SEAN CARROLL: Yeah. It’s philosophy, but philosophy in dialogue with the natural world. That’s what science is.

IRA FLATOW: So if there is this shared space between physics and philosophy, are there things that we think of as nebulous ideas in philosophy that might be testable and provable scientifically?

SEAN CARROLL: Oh, yeah. When people talk about the foundations of physics, for example– let’s take quantum mechanics as an example– we’ve been arguing over the foundations of quantum mechanics since the 1920s, when quantum mechanics came on the scene. Einstein and Bohr famously had a series of debates about exactly this.

And generations change and times change, and the physicists of today are less interested in those most fundamental questions about the nature of quantum mechanics. Philosophers are interested in them. And they’ve developed, in concert with the physicists, different ideas have been developed that literally do have different experimental implications.

So again, there’s not a bright line between the thinking that philosophers do and the experimenting that physicists do. It’s all people trying to understand the universe.

IRA FLATOW: This is Science Friday, from WNYC Studios, talking with Dr. Sean Carroll, author of the new book, The Biggest Ideas in the Universe: Space, Time, and Motion.

I was always impressed by Richard Feynman’s idea that if you really want to appreciate nature– and he used to talk about this as a flower– I can appreciate a flower more than an artist because I know how a flower works. I know what makes the insides of it work. Are there parts of physics that you can teach your students or get them to appreciate nature more by knowing the physics or the biology behind it?

SEAN CARROLL: Well, absolutely. And furthermore, there’s ways to make them appreciate the physics more by understanding the philosophy behind it. I’m teaching right now at Hopkins a seminar on topics in the philosophy of physics. And we were talking about the arrow of time, the difference between the past and future and how it relates to entropy and things like that. And some of my students, who are physics majors, came up to me and said, my physics professor told me this and this and this. And I’m like, yeah, that was not very good. We can do better now in this class.

IRA FLATOW: Are they shocked to hear that physics professors and physicists can be bogus?

SEAN CARROLL: I don’t think so. I mean, maybe. It depends on the individual. But I think that at this point, once you’re a couple of years into your undergraduate career, you don’t think that your professors are infallible, right? You’ve seen that there’s a couple of mistakes they make along the way. We’re all human. That’s to be understood.

And you can be a really, really good scientist without understanding any philosophy whatsoever or without even being especially careful about the foundational aspects of your field. But if you wanted to be careful, then you’re going to be disappointed when you go back and listen to some of the things your professors have told you.

IRA FLATOW: Now, do you think that science, a lot of science, starts out as philosophy that is put forth, and then scientists say, hmm, that may be a good idea, let’s look into that?

SEAN CARROLL: Well, I think that what happens is that, when problems become better defined, they will often move from the purview of philosophers into the purview of physicists. Philosophers really spend a lot of their time struggling with problems that are just not very clear what the right way to ask them is, much less what the answer is.

You talk, for example, about consciousness. And David Chalmers famously formulated the hard problem of consciousness– how is it that we have a first person feeling for what it is like to experience something? And that’s hard to answer at a scientific level. But as science advances and as philosophy advances, we get more and more ideas about how to do exactly that.

IRA FLATOW: Do you get into the philosophy and the physics of when there was nothing and how you could get something out of nothing?

SEAN CARROLL: We’re going to. I wrote a paper about that– Why is There Something Rather Than Nothing? And I’m hoping to get to that in the class. But there’s so many good things. Who knows how far we’re going to get?

IRA FLATOW: Yeah. Well, what other good things? Can you give me a list of other good things that you’d like to talk about?

SEAN CARROLL: Well, I’ve narrowed– for the course that I’m teaching– I’ve narrowed it down to three big topics. One is, like I said, the arrow of time, its relationship to entropy and disorderliness and the Second Law of Thermodynamics, but also its relationship to cosmology and the universe as a whole. Which leads into the second topic, which is the multiverse, the anthropic principle, questions that arise when you have many, many observers in a universe. How do you make predictions? How do you compare it against data? How do you use that in a good scientific way?

And then, finally, the classic, obvious thing about the foundations of quantum mechanics– what really happens when you make a measurement in quantum mechanics? Are there many worlds? And once again, how do you deal with the fact that there are many worlds, and we might be living in one of many, many copies of the reality that we experience?

IRA FLATOW: Do you get into, at all, whether we’ll actually ever be able to understand the universe?

SEAN CARROLL: Not really. I think that there’s a whole set of questions in the philosophy of science that are these kinds of meta-questions, right? Epistemology and metaphysics questions– why is science possible at all? What is the best way to do science? How do we formulate hypotheses? How do we change theories from one to the other? How do we update our beliefs on the basis of evidence?

These are all great questions, and there should be a whole other course on them. And there are courses on exactly those questions.

IRA FLATOW: And so what’s your next book going to be about? Are you thinking about that– taking this idea one step further, or turning your courses into a book idea?

SEAN CARROLL: There’s so many books that need to be written. But happily, I have my immediate future planned out for me. Because the current book, The Biggest Ideas in the Universe: Space, Time, and Motion, is volume one of a three-volume set. So there are two volumes forthcoming.

IRA FLATOW: Oh, is that, right?

SEAN CARROLL: Yes, that’s right.

IRA FLATOW: All right. Well, we’ll look forward to it, and look forward to having you back. It’s always a pleasure.

SEAN CARROLL: It’s always a pleasure to be here, Ira. Thanks for having me.

IRA FLATOW: Sean Carroll, Homewood Professor of Natural Philosophy at Johns Hopkins University, author of The Biggest Ideas In The Universe: Space, Time, and Motion. You can read an excerpt of the book on our website, sciencefriday.com/physicsideas. And you can tune into his podcast, Mindscape, to hear some more of those ideas.