Sean Carrollinterviewed by Richard Marshall.

[Photo: Ken Weingart]

Sean Carroll is the uber-chillin' philosophical physicist who investigates how the preposterous universe works at a deep level, who thinks spats between physics and philosophy are silly, who thinks a wise philosopher will always be willing to learn from discoveries of science, who asks how we are to live if there is no God, who is comfortable with naturalism and physicalism, who thinks emergentism central, that freewill is a crucial part of our best higher-level vocabulary, that there aren't multiple levels of reality, which is quantum based not relativity based, is a cheerful realist, disagrees with Tim Maudlin about wave functions and Craig Callender about multiverses, worries about pseudo-scientific ideas and that the notion of 'domains of applicability' is lamentably under-appreciated. Stellar!

3am:You’re a physicist with philosophical interests and skill. How did this begin?

Sean Carroll:My own interests in physics and philosophycertainly stem from a common origin – I’m curious about how the world works at a deep level. I got interested in physics at a fairly young age, reading books from the local public library about relativity and particle physics. I didn’t discover philosophy in any serious way until I went to college. It was a good Catholic school (Villanova), at which every arts & sciences major was required to take three semesters of philosophy (as well as three semesters of religious studies, which could be fairly philosophical if you took the right courses). I really enjoyed it and ended up getting a philosophy minor. As a grad student in astrophysics at Harvard, I sat in on courses with John Rawls and Robert Nozick. Rawls in particular was a great person to talk to, although we almost never discussed philosophy because he had so many questions about physics and cosmology.

3:AM:I thought we’d start by getting your overview of the situation as you see it regarding the relationship between physics and philosophy. There have been some high profile and rather bad tempered disagreements recently between the two camps – I’m thinking of the Kraus vs Albertrecently which led to an invitation for Albert to share a platform with Krauss at an event being pulled, and Hawking and Mlodinow who start off their book ‘The Grand Design’ by announcing the death of philosophy – so I wondered if there was any general points that such cases helped illustrate for you about the relationship, in particular, whether there is some truth in the thought that physics has such an elevated status in the general culture reflected (as reflected in both popular culture eg The Big Bang Theoryand funding eg it gets LHCmachines built) that it feels itself impervious to criticism?

SC:From inside physics, it hardly seems like we are impervious to criticism! Funding is being cut, our ability to do big projects is running up against problems of finance and international cooperation, and it’s a struggle to explain the importance of increasingly abstract basic research. Much of this feeling is a matter of historical context, of course; fifty years ago physicists were at the top of the heap, a position that is increasingly occupied by biologists (or maybe economists?). But anyone paying attention can tell that there is still immense public interest in discoveries like dark energy and the Higgs boson, and a great deal of respect for physics as a profession.

The public spat between physics and philosophy is just silly, more a matter of selling books or being lazy than any principled intellectual position. Most physicists know very little about philosophy, which is hardly surprising; most experts in any one academic field don’t know very much about many other fields. This ignorance manifests itself in a couple of ways. First, a lot of scientists are quite comfortable with simplistic philosophy of science. This usually doesn’t matter, but there are cases where good philosophy has something to offer, and scientists rarely put in the work necessary to understand what that good philosophy has to say. Second, scientists tend to think of philosophy as a service discipline – what good does it do for my practice of science? The answer is almost always “no good at all,” which they then translate into thinking that philosophy has no real purpose. The truth is that almost all scientific work can proceed quite happily without philosophy – you can be very good at driving a car without knowing how an engine works. But when it’s important, philosophy very important indeed.

Very few philosophers, by contrast, are going to accuse science of being worthless. Nevertheless, it’s no surprise that there are problems of appreciation and understanding flowing in that direction as well. The only remedy, if one is interested in finding one, is constant interaction and communication.

My own default position is that respectable people in other academic fields probably have something interesting to say, even if I don’t immediately understand it. Not always true, of course – there are pockets of nonsense within every discipline. But the less I understand about the basics of some field, the less likely I am to start declaring it to be useless and antiquated.

[Cartoon from here]

3:AM:I guess the key issue from the philosophers about the two cases I mentioned above was that the scientists were discussing philosophical issues but weren’t doing it well. Krauss gave the impression that he was answering the philosophical question ‘why is there something rather than nothing?’ but he wasn’t was he? A fall-back position seems to have been taken – I noticed something like this came up in your ‘Moving Naturalism Forward’ conference – that says that the issue he was addressing was a better one than the philosophers! So what do you think – does physics say anything helpful about the philosopher’s question – which I take to be a logical objection to the idea that something can create itself out of nothing pace Aquinas - and if it doesn’t does that mean that we should start wondering about the relationship of physics to logic?

SC:I’m not sure if one version of the “something from nothing” question is better than any other version, and I’m not even sure there is a single thing we would agree on to call “the philosophers’ question”! But I do know that disentangling these kinds of issues – taking an ill-posed question that nevertheless is based on something real and interesting and figuring out the ways in which it might be translated into something meaningful – is exactly where good philosophy can be helpful. There’s no question that a philosopher who has thought about this issue will have an enormously more nuanced and comprehensive understanding of what it means than your typical working cosmologist.

On the other hand, there’s also very little question that any serious philosophical approach to the question needs to be informed by the best physics to which we have access. The idea that there is a “logical” objection to the creation of something from nothing clearly hinges crucially on one’s conception of what “something” and “nothing” are, and that’s something (as it were) that physics can usefully inform. Not that physics can simply come in and definitively answer it – as per usual, there will be different possible definitions, with correspondingly different answers to the original question.

There’s an important point here worth emphasizing. Science has an enormous advantage over other disciplines when it comes to making progress: namely, the direct confrontation with data forces scientists to be more imaginative (and flexible) than they might otherwise bother to be. As a result, scientists often end up with theories that are extremely surprising from the point of view of everyday intuition. A philosopher might come up with a seemingly valid a prioriargument for some conclusion, only to have that conclusion overthrown by later scientific advances. In retrospect, we will see that there was something wrong about the original argument. But the point is that seeing such wrongness can be really hard if all we have to lean on is our ability to reason. Science has data in addition to reason, which is the best cure for sloppy thinking. So in principle it might be possible for a very rigorous metaphysician to be so careful that everything they say is both true and useful; in practice, we human beings are not so smart, and a wise philosopher will always be willing to learn things from the discoveries of science.

3:AM:Your ‘Moving Naturalism Forward’ conference brought together a super-powered bunch of naturalist philosophers and scientists to discuss a naturalistic world view. Before discussing aspects of it, can you say what made you think this would be a good idea for a conference? Were there highlights for you?

SC:In the contemporary intellectual climate, especially in the U.S., there has been a great deal of argumentation between atheists/naturalists on one hand and religious believers on the other. Which is fine as far as it goes, but within the set of folks who are already comfortable with naturalism, it doesn’t really help answer all of the crucially important questions that the naturalist position bequeaths to us. (Free will, emergence, meaning, morality, you can easily think of them yourself.) And we disagree in serious/interesting way about the answers to these questions!

So I thought it would be useful to bring together a group of people from a variety of disciplines who already agreed on the basic tenets of naturalism, and have a wide-ranging discussion that didn’t involve bashing religion or defending ourselves against it. In some sense, “Does God exist?” is the easy question; the hard question is, “Given that God isn’t here to give us instructions, how are we going to live our lives?”

I think most of the participants would agree that we didn’t answer our questions in any definitive way, but it was incredibly useful to hear the variety of thoughtful opinions among a group of smart people who agree on the basic ontology. We wanted to keep the discussions small and informal, with formal presentations limited to an absolute minimum, which is without question the best kind of workshop you can have. But we also made a bit more effort than usual in recording the sessions – hiring a videographer who knew what he was doing and had worked with similar groups before, making sure there were enough cameras and microphones to get good-quality recordings. So even though the group itself was small, the results are available to anyone who is interested.

3:AM:I was interested to see ‘mad dog naturalist’ Alex Rosenberg’sposition being regarded as provocative by most of the assembled where perhaps I might have expected his austere brand of naturalism to have been acceptable. Were you surprised by this?

SC:Not at all surprised. Alex is a fantastic person to have a meeting like that, because he is absolutely committed to an unflinching acceptance of the consequences of his worldview, which in this case means tossing out all sorts of common-sense everyday phenomena as “illusions.” That gets right to the heart of the challenge to the modern naturalist: given that the world really is just a quantum state evolving in Hilbert space (or whatever physics ends up telling us that it is), what is the status of tables and chairs, baseball and democracy, beauty and moral responsibility? Everyone in the room agreed that the fundamental-physics picture gives a correct way of talking about the world; but is it the only way, and if not, what are the relationships between the different ways of talking?

3:AM:Much of the conference seemed to be engaged with discussing the Sellarsian issue of how to reconcile the manifest image and the special sciences with physics. The discussions seemed balanced between a commitment to physicalism that nevertheless worried about reductionism, on the one hand, and a denial of any sort of strong emergentism that denies physicalism. Where do you stand on this issue?

SC:I think that’s right: the participants seemed very comfortable not only with naturalism (“only the natural world exists”), but also with the slightly stronger position of physicalism (“the natural world is no more than the physical world”). Someone like David Chalmers would describe himself as a naturalist but not a physicalist – he believes in strong emergence, the idea that there are behaviors of macroscopic objects that cannot (even in principle) be understood in terms of their component parts. At the end of the day, I don’t think anyone in our room was sympathetic to that kind of approach.

I think emergence is absolutely central to how naturalists should think about the world, and how we should find room for higher-level concepts from tables to free will in a way compatible with the scientific image. But “weak” emergence, not strong emergence. That is simply the idea that there are multiple theories/languages/vocabularies/ontologies that we can use to usefully describe the world, each appropriate at different levels of coarse-graining and precision. I always return to the example of thermodynamics (fluids, energy, pressure, entropy) and kinetic theory (collections of atoms and molecules with individual positions and momenta). Here we have two ways of talking, each perfectly valid within a domain of applicability, but with the domain of one theory (thermodynamics) living strictly inside the domain of the other (kinetic theory). Crucially, the “emergent” higher-level theory can exhibit features that you might naively think are ruled out by the lower-level rules; in particular, thermodynamics famously has an arrow of timedefined by the Second Law (entropy increases in isolated systems), whereas the microscopic rules of the lower-level theory are completely time-symmetric and arrowless.

I think this example serves as a paradigm for how we can connect the manifest image to the scientific image. Sure, there’s nothing like “free will” anywhere to be found in the ultimate laws of physics. But that’s not the only question to ask; at the higher-level description, we should ask whether our best emergent theory of human beings includes the idea that they are (in the right circumstances) rational decision-making agents with freedom of action. Until we come up with a better description of human beings, I’m perfectly happy to say that free will is “real.” It’s not to be found in the most fundamental ontology, but it’s not incompatible with it either; it’s simply a crucial part of our best higher-level vocabulary.

Morality, meaning, and other value-centered issues are a different question, of course. When it comes to scientific vocabularies, which aim merely to describe the world, it seems as an empirical fact that even higher-level descriptions are essentially fixed by the data. (There isn’t one viewpoint under which species evolve according to natural selection, and another one according to which they were created intact, both of which are equally valid at the same time.) But value questions are not simply answered by the demands of finding a useful description of the phenomena. That’s okay; it simply means that we have to bring in criteria that fall outside the domain of science when we go about deciding what is moral, what is beautiful, and so on. Human beings are not blank slates, and they have many such criteria ready to hand. So I don’t think that morality (for example) is objective or derivable from science, but I do think it’s a sensible subject for human beings to discuss and use in making sense of their lives.

3:AM:Should we agree with James Ladyman just think of reality in terms of different levels of ontological objects without saying that any level is more fundamental than others, or is there a fundamental level? So, for example, isn’t there a problem around causality, which seems to suppose a level of reality that presupposes a separation between space and time, and a fundamental physics which has space/time together?

SC:I shouldn’t speak too definitively about Ladyman and Ross’s attempts to naturalize metaphysics, as I haven’t read their book (although it’s sitting on my desk – does that count?). At a purely metaphysical level, it’s conceivable that there would be multiple levels of ontology without any one being fundamental. (In physics, for example, we have examples of “dualities” that give two different ways of talking about precisely the same underlying physical system, but with different domains of applicability.) However, as an empirical matter nature doesn’t seem to work out that way. The domains of applicability of economics or evolutionary biology seem to be proper subsets of the domain of applicability of fundamental physics. (I once tried to convince physicists to use the term “elementary physics” rather than “fundamental physics,” as it seems a bit more self-effacing, but that never caught on.)

I don’t think there’s a “problem” around causality, although there are of course many interesting “issues” about causality that are worth figuring out. My own ontological views are, I suppose, more dramatically eliminatavist than even Alex Rosenberg’s – I don’t even believe in spacetime! (Here and elsewhere “believe in” should be interpreted as “currently think this is what will end up being the best available explanation, subject to revision as more data and better ideas arrive.”) Our best theories of reality are based on quantum mechanics, not on relativity, and I think that eventually we will understand spacetime as an emergent semiclassical property, not at all fundamental. Timeis very crucial in quantum mechanics, but space is only an approximate notion.

I’m also an Everettian when it comes to quantum mechanics, which means I think the underlying reality is described by perfectly reversible and deterministic differential equations. So questions about “causality” as understood in the metaphysics context should be related to fundamental physics by exploring how intuitive notions of cause and effect can arise in a deterministic, reversible theory (subject to a past hypothesis responsible for the arrow of time).

3:AM:Is the ultimate language of physics – and reality – maths? Does this mean that actually it is impossible to discuss reality in natural languages?

SC:I don’t know. It seems to be the case that mathematics provides the best way we have of talking about fundamental physics, and I think the smart money would bet that this will continue to be true, but I don’t think it’s an a prioridemand. Of course it’s always possible to translate any mathematical statement into natural language, although the translation will become increasingly cumbersome as we demand a greater level of fidelity to the math itself.

3:AM:And doesn’t maths escape any naturalistic world view and if so doesn’t that make the success of physics mysterious – or the success of maths mysterious?

SC:I don’t really know what it would mean for math to “escape” a naturalistic world view. (Maybe a certain strongly Platonistic notion of math, to which I feel no temptation to subscribe.) And – not to be difficult! – I really don’t see what’s “mysterious” about it. To say something is mysterious implies that we had some pre-existing expectation that this state of affairs violates. How does one expect the world should be described at a fundamental level, if not in logical/mathematical terms? Not that such language is obviously right, but I don’t see why we should expect anything different in particular.

3:AM:Are the theories of physics best understood as just useful instruments for solving practical conceptual problems we have invented rather than being discoveries? I guess this is the issue of realism vs anti-realism regarding science.

SC:I’m a cheerful realist, as I suspect most scientists are. The world is what it is, and it does what it does, and we’re trying to figure out what those things are. I’m not even sure instrumentalism is coherent. If you don’t believe there is a real world, how can you believe in practical problems to be solved?

3:AM:So philosophers like Tim Maudlinwonder what physics actually describes. We seem to know how to use the theories to engineer things without understanding how to interpret the theories. He wonders about the relationship between the maths structures we use and the world we live in and so he asks - can the world be only a wave-function? He doesn’t think it makes sense to say it can and this indicates a fundamental problem in physics in that no one really understands what quantum physics is about. Is he right?

SC:I don’t think he’s right (but maybe I’m wrong). Tim and I are actually in the midst of a long email exchange about these issues, which we hope to put online somewhere when we’re done. I think we both agree that physics should be the basis for metaphysics, and that what the world “really is” comes down to what we learn from our best physical description of it. But I am quite happy with accepting the current view that the world is fundamentally a quantum state, and he feels this is too far removed from the reality of everyday experience (hopefully that’s an adequate paraphrase).

So yeah – I think the world is nothing but a wave function. (Until we come up with something better.)

3:AM:Craig Callenderpushed back against your thoughts about a multiverse. And Maudlin’s questions about the wave-function links up with your idea too doesn’t it? So how do you defend the multiverse? Where are the philosophers going wrong?

SC:I don’t think this is a physicists vs. philosophers conflict. Plenty of philosophers are comfortable with the multiverse, while plenty of physicists are repelled by it, and vice-versa. My current view is that the conditions we see within the part of the universe we can observe – especially the low-entropy past – are extremely puzzling unless we are part of a much larger multiverse, in which case we at least have a puncher’s chance of explaining them “naturally.” But it’s a matter of developing a compelling model that actually does that, which we haven’t yet achieved.

So I’m actually a moderate when it comes to the multiverse. There are folks who reject the idea out of hand, based either on simplistic philosophy of science (“things I can’t see shouldn’t be part of science”) or worries about ontological extravagance (“that sure is a lot of universes you need to invoke to explain a relatively paltry amount of data”). The first objection is just wrong, and the second is based on a misunderstanding; we should judge ontological extravagance by the number of ideas/concepts in a theory, not by the volume of space or number of entities it predicts. The multiverse is compelling to some physicists because it is a prediction of a very simple set of ideas (especially inflationary cosmology), and we should take the predictions of our models seriously. On the other hand, there are certainly multiverse triumphalists, who think it has already solved all of our problems. I don’t think we have anything close to a final picture of how the multiverse would work or how we could reliably use it to make predictions within the part of spacetime we see. But I’m hopeful we can get there.

3:AM:Maudlin and Callender are examples of top philosophers engaging with physics with excellent credentials in both philosophy and physics. But, for example, Thomas Nagel’s ‘Mind and Cosmos’ argues for teleology, of a universe unfolding a ‘cosmic predisposition.’ This seems to be an example of a top philosopher not knowing his physics! Interestingly, Nagel has been probably most harshly treated by fellow philosophers. Were the philosophers right to attack the book, and how much of the vociferous reaction was to do with fears of the religious lobby in the US? Is this lobby distorting science?

SC:Rather than the “religious lobby,” I think it’s useful to think about the whole spectrum of pseudo-scientific ideas (creationism, climate denialism, the anti-vaccination movement) that stem from ultimately non-epistemic concerns, whether they be religion or politics or personal empowerment. No doubt, the experience of fighting against these ideas has inculcated a bad habit in some scientists and philosophers: to ask of a new idea not simply “Is this true?”, but also “How might this be used against us in the court of public opinion?” But when we get down to brass tacks, most scientists and philosophers care about what is true more than they care about anything else.

And Nagel’s book, sadly, isn’t anywhere near true. He shows a rather willful indifference to what we know about biology, neuroscience, and physics. Because of this, ultimately his book is just not that interesting. But the questions that drive it are certainly worth thinking about, so hopefully it will spur further discussion.

3:AM:You’ve recently weighed in on the issue about Templeton Foundation funding. There’s a split on this issue in philosophical circles – Jason Stanleysupports an anti-Templeton position and Tim Maudlin is baffled by such a reaction. Your worry is that fundamentally the Foundation is out to make it seem as if science and religion are compatible when they aren’t isn’t it? Is there really no room for supernatural, unknown forces?

SC:There is plenty of room for supernatural, unknown forces. There just isn’t any need for them, credible evidence that they exist, or justification for taking them seriously in this day and age.

The elevator-pitch version of my objection to Templeton is that, as you say, they are committed to spreading the idea that science and religion are gradually reconciling with each other. Not only is this idea completely false, it strikes at the heart of the real-world issues on which philosophers and fundamental physicists can potentially have the most impact. When I try to come up with a better cosmological scenario to help explain the low entropy near the Big Bang, some members of the general public might think it’s interesting, but a much larger number are utterly unaffected. But every one of those people has some (likely implicit) view of how the world works, and the implications of that view are necessarily crucial.

Is there life after death? Is morality handed down by God? Does human life have intrinsic meaning and purpose? These are questions where what we have learned over the last few centuries from science and philosophy has an enormous impact, and it’s not in the direction that the Templeton Foundation would like people to believe. There is no life after death, morality is invented by human beings rather than handed down from outside, and any meaning or purpose to human life is something that we need to give it rather than something we get from the universe itself. These are important ideas to everyone, and we should be doing everything in our power to make them better understood. Templeton is not helping.

3:AM:Finally, are there any new ideas or facts coming out of physics now that will leave us all here at 3ammagazine in a state of mind boggled shock that will require us to revolutionize previously held views?

SC:Taking seriously this idea of “domains of applicability” of scientific theories, I think it is dramatically under-appreciated that we already have a theory (the Standard Model of particle physics plus general relativity) whose domain of applicability includes all of everyday experience. We will not be discovering any new fundamental forces or particles that are relevant to ordinary human life; we have the basic rules of that realm figured out. (Which isn’t to say we’re anywhere close to understanding how those basic rules are manifested in complicated real-world situations.)

But reality is much larger than the realm of our everyday experience, and we’re very far from having the whole world figured out. Obviously we don’t understand dark matter, dark energy, the Big Bang, quantum gravity, etc. We don’t even have a consensus on what really happens during the process of a quantum measurement. My own guess is that the most dramatic potential for new ideas lies at the intersection of quantum theory and cosmology. I previously confessed to having fondness for the multiverse, but we honestly don’t have a compelling model of it as yet. It’s absolutely conceivable that the whole multiverse idea is dramatically on the wrong track, and the truth is going to look completely different once we understand how space and time emerge from quantum mechanics. Even better and more exciting would be if we find that our current view of quantum mechanics is completely wrong and has to be replaced by something deeply different – I should only be so lucky.

3:AM:And finally finally, which five books would you recommend to us (other than your own which we’ll be dashing away to read straight after this) that will help us go further into your world?

SC:Alex Rosenberg, The Atheist’s Guide to Reality– I think we can salvage more of the manifest image than Rosenberg does, but this is a good starting point for any aspiring naturalist.

Owen Flanagan, The Problem of the Soul– An open-minded naturalistic attempt to understand what it means to be human.

Peter Hoffman, Life’s Ratchet– A physicist travels the road from fundamental laws, to entropy and Maxwell’s Demon, all the way up to life.

David Wallace, The Emergent Multiverse– The best current exposition of the Everettian (Many-Worlds) approach to quantum mechanics.

Michael Lockwood, The Labyrinth of Time– An overview of how time works, from Boltzmann and Einstein to modern cosmology, from a philosophical perspective.

Leonard Susskind, The Black Hole War– Ostensibly about a longstanding disagreement with Stephen Hawking, actually a masterful introduction to how a modern physicist thinks about fundamental reality.

(Yes I know that’s six books. Electrons are cheap, right?)

Richard Marshallis still biding his time.