Craig Callenderinterviewed by Richard Marshall.

If he’d been born with one less ‘l’ his name would have been a spooky resonance with his interest in time. Craig Callender is a groove sensation in metaphysics and the philosophy of science. He wrote the Oxford Handbook on Philosophy of Timelast year and his other books include Introducing Time, Time Reality and Experienceand a co-edited volume Physics Meets Philosophy at the Planck Scale. Like all time lords past present and future he neither stops brooding on time and physics nor eats meat.

3:AM:You’re a philosopher of science, physics and metaphysics. Were you always interested in philosophising? As a child did you brood a lot about the existence of time and what happened before the big bang and stuff like that? Were you into sci-fi and comics and stuff like that? Or was the decision to become a philosopher a bit of a surprise?

Craig Callender:I was supposed to be a printer, so becoming a philosopher was quite a surprise. On the Callender side of my family, my father, grandfather, and great-grandfather were all printers. I could see the pattern, so I assumed that I would follow in their footsteps. Plus, printing was much preferable to the mills in which the other side of the family worked. I kept getting good grades in school, however, and with computers taking over, printing seemed an area doomed to shrink. So like many first-generation college-goers, I assumed I should be a medical doctor and went to school on a pre-med track, far from philosophy.  That didn’t satisfy me intellectually, and soon I found myself bouncing from major to major, either formally or informally (psychology, math, art…). All the while, I did harbor a fondness for philosophy: I had stumbled across Hofstadter's Gödel, Escher, Bachand The Mind’s Eyeand really enjoyed them. But philosophy felt like a guilty pleasure. It took a while to work up the courage to major in it.  

Looking back, I do attribute a lot to the fact that my mom was and is obsessed with sci-fi. Hence as a child and teenager I devoured all her leftovers. This was mostly classic sci-fi, e.g., Asimov, Heinlein, and never authors like Gibson. With these classic works so stuffed with philosophical problems – personal identity, free will, time travel – I can see the seeds planted then of what I do now.

3:AM:One of the big issues you are interested in is the scope of metaphysics. One concern about metaphysics ever since the positivists has been that it is too untethered from empirical testing and science and so it is just too fanciful. Linked to this is the worry that metaphysicians just don’t know enough about scientific theories to be able to provide a metaphysics connected with science. Can you say something about these connected issues and perhaps give examples of the kind of speculation you think is okay and the kind you think too wild, such as mereological speculation or ideas about the fourth dimension?

CC:I think it’s a fool’s errand to try to prejudge inquiries on the basis of being too speculative or not. History is littered with embarrassing judgments stating that such-and-such is too speculative – e.g., Newtonian gravity, quantum non-locality, relative simultaneity - only to see those ideas later vindicated. I want no part of that tradition. Nor do I wish to accuse metaphysicians of not knowing the relevant science –many know much more than I do. The more typical problem, if there is one, is that they know the science reasonably well but just don’t see it as relevant to their concerns.  

That said, let me not shirk from the spirit of the question. You’re right that I do find some contemporary metaphysics misdirected. Are roads that split and then recombine one road or two? Do hands that close into fists introduce new objects into the world? Are bunnies 4D worldines of fur or “wholly present” 3D instances of furriness that persist through time? I just can’t believe that these questions and others like them are about interesting deep facts of the world. By indexing roads, hands and bunnies in the appropriate manner all the problems disappear. I admit that there are some hard problems about dividing verbal disputes from genuine ones (see below). But all the background theory we have is suggesting that these disputes aren’t deep or promising.  

I also worry about the justification of various alleged principles of mereological metaphysics. Suppose one insists that simples (objects with no proper parts) have no spatial extension. Suppose also that our best physics posits fundamental entities that are spatially extended. What should we say? One option would be to stick with the principle and say that it turns out that the fundamental objects aren’t simples, so understood. Then I have no objection, for the work is simply a study of the architecture of our concepts and not necessarily about the world. (Philosophers call this Strawsonian metaphysics, and it’s a kind of anthropological exercise, one that maps the contours of our concepts.) Another option is to say that these spatially extended objects really do have parts, parts that could– in some metaphysical but not physical sense – break apart. Here is where I object.  

Have these sub-entities and the metaphysical modality that goes with them earned their way into our overall system of the world? The entities and modalities posited by physics earned their way in through their empirical power, simplifying and unifying equations, and much more. These mereological principles and attendant modalities haven’t earned their way in. Mereological metaphysicans often think of their subject as almost like geometry, as having to hold but also about the physical world. If so, they need to be reminded of what happened to Euclidean geometry. What intuitively seems necessary might not be.

3:AM:What would you say to the metaphysician who might argue back and say that science often is underdetermined by empirical evidence in the present time but who knows what will happen in the future? The position might be confirmed in some way? There’s some justice in that isn’t there? How do you propose to sort out bogus metaphysics from decent stuff in cases where there is substantive disagreement and undetermination? I guess this argument concerns the danger of being overcautious. If you close down wild ideas from the start then you murder wild ideas that might be just be what are needed to take us out of what seems like an untractable problem at some unknown time in the future. After all, when we look at the metaphysical implications of current physics, for instance, we’re in a very strange place. Isn’t there a danger that worrying too much about the dangers of wild metaphysics just threatens the whole enterprise of discovery and science? Who knows when an idea might not be useful? And if its useless, so what?

CC:I agree. Like I said above, I don’t aspire to the post of grumpy, conservative arbiter of when speculation has gone too far. I don’t want that mantle for all the reasons you say. Stifling wacky ideas may sometimes impede scientific progress. No lover of sci-fi such as myself would want any part of that. I love speculative science and metaphysics. Is spacetime discrete? Is it “gunky”? Is our epoch going to repeat itself? Build the models and let’s see what work they do for us.  

Here is an objection to what I said above. Some philosophers have said that there couldbe empirical tests of what I regard as non-issues. For example, the fundamental entities of physics might someday break apart. Would that harm my position? I don’t think so, for if they break apart into extended objects then the mereologist under consideration still somehow knows, with no evidence apart from intuition, that they could break apart further. It’s these “unearned” modalities that bother me.  

In the case of bunnies, I’ve read and had it suggested to me that there is an empirical fact of whether a 4D stretch of fur and flesh is one bunny or an infinity of instantaneous “wholly present” bunnies. So I imagine hooking up some wires from a device to this fluffy pile, flipping a switch, and the pointer moving to the BUNNY or NOT-BUNNY position. It’s really hard to see how this would work. What variable will covary with the pointer position? Mystery property X? Everything we know suggests that there is no such property, and crucially, that that property won’t do any work for us in explaining anything else.  

It is true that in principle we never know. Laudanand Leplinmake this point in a famous paper in philosophy of science. Take two theories that look different but predict the same empirical claims, say, a theory with non-trivial spacetime curvature versus a theory with a flat spacetime where gravitons mimic the effects of curvature. There may be a temptation to regard them as notational variants, the dispute between the two as merely verbal. But they point out that it’s always possible that we discover some new particles, say X-ons, and X-ons turn out to be sensitive to the difference between the two theories, e.g., maybe X-ons feel curvature but don’t couple to the graviton field. While one may grant this point, I don’t think that it invites a free-for-all. We don’t need to worry that future experiments will distinguish between a bunnyand a lapin. Philosophy has not settled on a good criterion of when a dispute is verbal. Relying on that fact alone to stimulate a debate, however, strikes me as desperate.

3:AM:So that’s about the concern that some metaphysics is too far away from science. But then there’s a worry that science is also getting too untethered and is proposing solutions as facts that are too fanciful as well. So there’s a gulf between empirical science practices and science’s theorising hugely complicated ontologies just to keep current theories elegant. One of the familiar examples is the idea of multiple worlds and the claim that everything that could exist actually does. This seems to me to be a huge ontological commitment that physicists lie to make but it’s untestable and therefore based solely on simplifying the calculations needed to explain what can be tested. Is this something that concerns you? Or are there such things as these other worlds branching out from ours?

CC:Yes, it’s ironic that as metaphysics is getting accused of being too disconnected from science, science itself is positing unobservable structure that would make Leibnizblush.  

I’m not a fan of multiverses. To understand why, we need to get beyond the question of whether it’s okay to posit some physics based on reasons of theoretical elegance. Of course it is! If we didn’t focus on the simpler, more compact, more unified theories of the observable data, we would have way too many theories and no guide to the physical world. Imagine that you plot all the data points you have for your scientific domain on a graph. The number of curves that would fit this data is infinite. We need to cut this down, and the way we do it is by appealing to theoretical virtues.  

The way I think of scientific theorizing is as a kind of optimization problem: can we find the theory that best balances the various theoretical and empirical virtues? I expect many trade-offs, both within and between the theoretical and empirical virtues. How we value the costs and benefits achieved by trade-offs is a bit vague – that’s why we don’t have a simply expressible scientific method. But it’s clear that no theory would accept, at one extreme, a simple list of what happens, or, at the other extreme, a “theory” that narrowed almost nothing down, such as F=ma by itself.  

Given the above fuzziness, it’s no surprise that reasonable people might disagree over whether a theory optimized the theoretical and empirical virtues. To me, it seems clear that the huge hit we take in extra ontology isn’t worth the small gain we get in explanation. As a general point we can additionally notice that theories will always have “unexplained explainers” or brute facts in them. We can puzzle over why they are thus and so. Then we can posit a new theory where that brute fact isn’t so constrained, pull a measure out from Plato’s heaven, impose that measure over all the other ways that brute “could be,” and then argue that mostof those ways lead to worlds like ours. Knowing that in principle this is always an option should make us extra cautious when we wonder whether we’re getting enough for our trade.

3:AM:I guess if a physicist tells us that her theory requires weird ontologies such as multiple worlds then there’s a general feeling that we folk don’t have a right to challenge them. After all, common sense intuitions are pretty rubbish when it comes to truth about the world and so what else have we got? But you seem to think that we have philosophy and that this can be a helpful way of approaching this kind of claim, although of course someone like you are pretty well versed in the science. Is that right?

CC:Before there was physics and before there was philosophy of science, there was of course natural philosophy. When you read Newton, Maxwell, Thomson, Hertz, Boltzmann, and other greats, you would be hard pressed to circle what’s philosophy and what’s physics in these writings. There were just certain questions – e.g. are fields real or are they accounting devices? what explains entropy increase? are space and time real? – and a range of considerations for and against. The answers to these questions would draw on conceptual distinctions and scientific knowledge, however the two were classified, because these are the ingredients needed for an answer.  

Specialization into distinct disciplines obviously has many benefits, but it’s naïve to think something doesn’t get lost too. Fallen into the cracks are a huge assortment of questions like the ones I just mentioned. These questions are deep and significant, but they are largely untouched. One can give sociological explanations for this: little grant money flows to such work in physics, the “linguistic turn” in metaphysics, and more. Whatever the reasons, there are good questions that deserve answers and often answering them will require the skill of a natural philosopher, be they officially a philosopher, physicist, or mathematician.  

I do think that philosophy has something to offer science. The information transfer can be “two-way.” Due to their differences in training, philosophers often look at the same questions physicists do through different lenses, and sometimes this can help. Philosophers have training in logic, philosophy of science, metaphysics, and often history of science; with the distinctions they learn and a Socratic “follow the argument wherever it leads” ethos, they can raise questions about contemporary science that don’t seem to get asked much, e.g. like whether inflation really solves, say, the horizon problem. Though some physicists I know may grimace at what I’m going to say, I do think that philosophers have helped us understand the deep commitments of spacetime theories, especially in the 1970s and 1980s, and in framing and tackling the quantum measurement problem and quantum non-locality in the 1990s.  

Many scientists thought for a very long time that relativity theory vindicated a Machianunderstanding of spacetime. Machianism may rise again, but the lesson philosophers have taught us is that the opposite is the case. Many scientists thought that decoherence by itself solved the quantum measurement problem; now, thanks in part to distinctions made by philosophers, that position is seen as a non-starter. Inasmuch as getting a clear picture of what one’s theory is committed to is important for future progress, one might also see this work as significant for future progress, too.


3:AM:So one of the big issues in science and metaphysics that you have looked hard at is time and implications running from different conceptions of time. There are several philosophically interesting and paradoxical seeming things about time. McTaggart's arguments about the unreality of time is a pretty cool argument. He basically argues that time doesn’t exist because the past doesn’t exist because it’s finished, the future doesn’t exist because it hasn’t happened yet and the present doesn’t exist because to understand the present you have to relativise it to the past and the future. Given that they don’t exist time doesn’t exist. You have recently asked the question whether time is an illusion. So is it? If not, what is it and what isn’t it?

CC:We all seem to possess a kind of proto-theory of time. Let’s call it manifest time. Manifest time says that time has a global, shared present that is metaphysically distinguished. This present carves the world into three, a past, a present and a future. This present flows. It is intrinsically directed. It is independent of the distribution of matter. And so on. No matter how scientifically literate, I bet the reader shares this theory. Think of how important this conception of the world is to you! The way you live your life depends crucially on it. What you take yourself to cause, know, your very freedom and sense of self are all bound up with this conception of the world. Manifest time really is, as the philosopher Mellorcalls it, the time of our lives.

Yet physics tells us that this picture is more or less complete rubbish. I believe that. That is, I don’t think, merely by sifting through the physics, you’ll be able to recover manifest time.  

Does that mean that physical time is inaccurate or incomplete? No, physical time may be all the time we need, fundamentally. However, it may be that by looking at more than physics we can explain why a creature embedded in a physical world such as ours would conceive of the world as we do in terms of manifest time. That is, I think that we can show why manifest time makes sensefor creatures like us. I talk more about this project of getting from physical time to manifest time in the book I’m currently writing. The idea is that physics provides important constraints on any being like us, and these constraints, in concert with our psychological mechanisms, means of communication, macroscopic environments, and senses of selves, all together explain why we conceive of the world in terms of manifest time.  

Is manifest time an illusion? In one sense, yes: manifest time is not accurately representing physical time. But there is a sense in which a world without perceivers lacks colors; only when perceivers are around do colors obtain. (Alternatively, maybe the colors exist as dispositions to look colored if an appropriately configured perceiver is present.) Either way, we don’t regard colors as illusions. Maybe some aspects of manifest time are like this, in which case we wouldn’t judge them as illusions. Embed a creature like us in a world like ours and that triggers a sense of flow, say. Once you break down manifest time into its various components, it becomes a bit tricky whether we ought to say its illusory. What I can say is that I don’t think manifest time maps onto the fundamental picture of time we have from physics.

3:AM:You recently took up arms against a really interesting issue in physics relating to time’s arrow, the one way direction of time. Entropy explains the direction of time. So the universe moves from having little entropy, as with an egg and then becomes very entropic, as with an omelet. But there is nothing in the second law of thermodynamics to explain why the universe starts with low entropy. Now maybe its just a brute fact that there’s nothing to explain. But some physicists believe they need to explain it. So Sean Carroldevelops an idea of a multiverse to explain the low entropy. You make this a parade case of the kind of ontological speculation that is too expensive. Having to posit such a huge untestable ontological commitment to explain something like low entropy at the big bang you just don’t think is worth it. Is that right? Are there other ideas coming from physics that worry you in the same way?

CC:Philosophers are raised reading Socrates, who tried to be a gadfly to conventional wisdom. Here I’m just trying to be a similar kind of pest to a prevailing opinion in physics. Scores and scores of our best physicists say that one of the great unresolved problems of physics is that it doesn’t explain the initial low entropy state of the universe. I want to express some healthy skepticism about this claim.  

At least two thoughts motivate this skepticism. First, suppose we judge the constraint on initial conditions to be lawlike. (I think that there are some powerful arguments for this.) Then all the universes that don’t begin in a low entropy state are, strictly speaking, unphysical and have zero probability. The initial state is then hardly monstrously unlikely (hence demanding explanation), but rather has probability one!

Second, won’t the problem just creep up on you again? The new theory that explains the initial state of the universe will have unexplained explainers in it too. Every theory does. Then presumably one of the great unsolved problems of the new physics will be to explain those unexplained explainers. How deep in unobserved physics do we go before we say enough?

Note that I’m notsaying that physicists shouldn’t posit models that explain the low entropy initial state. Often in the history of science one sees physicists motivated by something they find unnatural in the existing theory, even if the existing theory handles the phenomena pretty well. Einstein’s initial motivation for relativity was like this. He saw in electromagnetism an asymmetry in the theory not reflected in the phenomena - and he bristled at this asymmetry. Others didn’t. Einstein then invented an alternative theory that eventually crushed the original.  

But I think it would have been wrong to say, prior to relativity, that we knewthat the asymmetry between moving magnets and magnets at rest was a deep unsolved problem of physics. No, it was a curiosity of a successful theory that was a clue for developing a new, and as it turns out, better theory. Similarly, I don’t think we know that the initial low entropy is a problem for the standard picture of physics. The problematic aspects of the posit are overstated. Whether it emerges as a clue to something better is something we shall see.      

3:AM:You also disagree with people like Stephen Hawkingabout the fine tuned nature of the universe. This is the idea that had the universe been just a tiny bit different there couldn’t have been life. Is that right? What’s wrong with that kind of thought? Is Hawking just speculating rather than doing science?

CC:I think that most of the fine-tuning arguments one sees are nonsense. The idea is something like: a physical parameter P is finely tuned iff the probability of P taking a value compatible with life is low. Its low probability is then supposed to call for explanation via new physics. Forget the vagueness of what we mean by “life”. That’s a serious problem, but a deeper one is that we typically have no warranted probability metrics to support these judgments. Where are these judgments of unlikeliness coming from?  

Maybe this objection seems like a technicality, but it isn’t. Most of the parameters people talk about being finely tuned are things like force strengths and particles masses. In most cases these are nomic, i.e. lawlike, features. So to get a probability metric over a bunch of nomic parameters, one needs, it seems, a kind of meta-law, a law over laws. That is, one needs a law that tells us the probabilities of P taking on various values. But why would we ever need this, and what possible evidence could we ever muster in favor of one rather than another?  

Suppose someone argued that light going 300,000 km/s was crucial for life. Is the speed finely tuned?Given that the speed of light is a law, no! It hadto be that way. However, that doesn’t stop the determined, for they can invent a theory of theories that includes a probability metric that assigns a 1% chance that the speed of light is 300,000 km/s and then conclude that light’s speed demands explanation.

What possible warrant could there be for that theory of theories and metric? I was never a strict empiricist, but reading this literature makes me want to be. I reach for Hume, the great Scottish philosopher, and one of my favorite passages in the Dialogues Concerning Natural Religion. Hume has the skeptic Philo say:

…the subject in which you [Cleanthes] are engaged exceeds all human reason and inquiry. Can you pretend to show any such similarity between the fabric of a house and the generation of a universe? Have you ever seen Nature in any situation as resembles the first arrangement of the elements? [If so]…cite your experience and deliver your theory.

Now, of course, as I said above, one can use a hunch about what’s natural or not as motivation to develop a theory. I’m not an arbiter of hunches, and the history of science teaches us that scientists have used quite a variety of hunches successfully. But hunches about tuning shouldn’t masquerade as arguments.


3:AM:I guess this leads to the issue about whether you think scientists overstate what their theories actually commit us to? Jerry Fodorquips that the last people he asks when trying to work out the philosophy of biology are biologists. We folk get a lot of physics made simple, and great TV shows with Brian Coxtelling us about the wonders of the universe and so on. You yourself wrote a very cool book introducing us to the notion of time which was zany and full of cartoons and so on. Clearly this stuff is very enjoyable and surely gives us some genuine knowledge. But is there a danger that the scientists might not be philosophically sophisticated enough to handle the scientific knowledge they have?

CC:That doesn’t keep me awake at night. Some scientists are tremendously sophisticated philosophically, and it’s true, some are not. Gosh, look at the interpretational mess Bohrand Heisenbergcontributed to with their more philosophical writings. The treatment of the quantum measurement problem in textbooks, popularizations, and elsewhere is a scandal. Maybe there is something to be said for being too close to a theory to appraise it carefully…I don’t know. But overall I think many scientists are very good at philosophy, e.g. the later Einstein, John Bell, Bob Geroch, John Maynard-Smith. And philosophers certainly can’t get on their high horse when it comes to these matters. Did they do any better on the quantum measurement problem until they started reading Bell?

What worries me most in this neighborhood is, contra Fodor, the separationbetween philosophy of science and science. To go with your Fodor quip, but coming from the other side, one might add Feynman’s famous claim that philosophy of science is as important to scientists as ornithology is to birds. The two quips seek a kind of separation between the two fields that I find unhealthy and unnatural. And both strike me as ironic, for Fodor’s work is often best when connected to what scientists are saying, and Feynman, despite frequent jibes at philosophy, was very philosophical - as anyone who has read him will attest.

Feynman is right, a bird can get bywithout ornithology; however, wouldn’t a bird equipped with ornithology be in a vastly superior position relative to other birds? For answering the sorts of questions I work on, it’s important to talk to scientists and philosophers. One wouldn’t want to take lessons on a foundational issue from an uninformed or unreflective scientist, but neither would one want to do so from that kind of philosopher.

Another worry I have in the neighborhood of your question is whether science is being communicated to the public in a responsible way. On the one hand, we have all these new and great venues for popular science. Look at the top-notch exposition and production values in the latest Brian Greene series on PBS. But on the other hand, a lot of the work today seems so much more sensationalistic than in the past. Compare the brilliant George Gamowbooks that were popular a generation ago (One, Two, Three..Infinity, etc) with current ones and you’ll see what I mean. If the speculative ideas are advertised as such when they come, no problem. But we obviously don’t want a public voting on science policy who believe that physics is only about multiverses and multiverses of multiverses, nor do we want the public thinking string theory is (yet) as secure as relativity.

3:AM:Methodological naturalism claims that philosophy should work in a way that is consistent with scientific methodology. But you don’t actually think there’s a scientific methodology do you? So do we need to revise methodological naturalism or else aren’t we in a danger of the claim to be a naturalist become a little too vague to be useful?

CC:Good question. I don’t think that there is a particularscientific methodology, but I do think that there is a difference – along a spectrum – between scientific and unscientific methodology. Look at all the pseudo-scientific garbage that is being thrown at us daily, e.g. power wrist bands that make you energetic, magnetic therapy wraps, most vitamins, detox therapy. Just going through a CVS gets it thrown in your face.  

What’s the difference between the methodology behind this stuff and behind the theory of the electron? Not always one single feature like Popperthought (falsifiabilty). But as Philip Kitcher, John Dupreand others have stressed, maybe there is a set of criteria that makes something scientific. Each field has particular metrics for how to handle the trade-offs among various criteria, so I think it’s often hard or even practically impossible to articulate a simple criterion.  

Often it’s easy: the tests on wrist power bands aren’t blinded, aren’t independent, aren’t over a statistically significant sample. Sometimes it’s hard: could you tell while it was happening that cold fusion would be bunk but high temperature superconductivity would be a tremendous achievement? Even if hard to state, I don’t think the claim behind methodological naturalism need be contentless. But as you say, it may sometimes be too vague to be useful. If the philosophical case is like the power wrist bands, then I think it is clear enough to use; if it’s trickier, then perhaps not.

3:AM:It always seems puzzling to me that Schrödinger’s catstory is forever being used as an example of something that is supposed to clarify what the quantum universe is like because I can’t make sense of the claim that the cat is dead and isn’t dead until someone observes it. So I don’t believe anyone except Graham Priestcan either because it claims that true contradictions exist. But everyone does. So where am I going wrong?

CC:No, you’re right, Schrödinger’s cat is a problem, not an answer. Schrödinger devised the thought experiment to highlight the trouble of thinking that the whole world behaves as sub-atomic quantum particles do. The key to explaining quantum phenomena is letting particles enter into superpositions, states that compose contrary properties. If everythingis quantum mechanical, however, then in principle everything – cats, tables, you – can superpose. The problem with this isn’t that these states comprise true contradictions. They don’t. The problem is that the states appears to conflict with our ordinary macroscopic experience of determinate objects.  

Why isn’t Schrödinger’s cat a true contradiction? It could be, in a sense. The ‘quantum logic’ program offered something like this picture. But I don’t think that program was successful as a physical theory, and many controversial assumptions go into reading superpositions like that. I prefer instead to just think of a superposition as a third kind of state. There is the state of the cat dead, the state of the cat alive, and the state of the cat in a superposition.  

If you want a picture, imagine a state represented by a unit vector along the x-axis in a plane, a state represented by a unit vector along the y-axis, and then a state represented by a third vector that turns out to be equal to the sum or difference of the other two. Three vectors, three states, zero contradictions. Understood this way, the problem is the empirically motivated one, and we needn’t call on Graham Priest for rescue just yet.

Of course, figuring out why the heck we have determinate macroscopic experiences when we’re in superpositions, if we’re in superpositions, is the million dollar question. Here there are basically three answers: (1) many determinate experiences somehow “emerge” from this superposed state, (2) quantum mechanics is incomplete, and the completion adds determinate non-superposed stuff, or (3) we’re not in Schrödinger cat-like superpositions, for quantum mechanics is modified so that these states collapse in an appropriate way. I have no idea which answer is correct. But I feel that I best understand option (2), so I tend to sympathize with it.  

3:AM:Eric Schwitzgebelhas argued that all metaphysical positions depart company from common-sense at some point. As a leading metaphysician, could you tell us about some of the metaphysical strangeness that you actually think may well be true. Sometimes philosophers give the impression that there’s nothing too amazing about the world beyond some of it being really big or really small, but there are very weird possibilities aren’t there?

CC:I think that the example I work on – tracing the emergence of ‘manifest time’ from its origin in physical time – is a great example. Our manifest impression of time departs quite radically from the fundamental reality, once that is properly understood. But let me describe four other weird possibilities:

1. Flashy Worlds. Suppose that the “flash ontology” for quantum mechanics is true. Invented by John Bell and used to great effect by Roderich Tumulka, the flash ontology is a contender for interpreting the world described by GRW, one of the more promising interpretations of quantum mechanics. According to a flash ontology, the “beables” of the theory – the ontology, the stuff that exists – are distinguished spacetime events correlated with the spontaneous localization of the quantum wavefunction. That’s it – just these flash events and spacetime. If this picture is right, the universe is much more sparse than ordinarily conceived. Tables, chairs and cats are constellations of sparse collections of flashes, not features of some continuously existing stuff. When the wavefunction is evolving and not undergoing a GRW localization, there is simply nothing there – no flashes. If I prepare an electron in the void and send it to you, when you measure you’ll produce a flash, a real thing; but until that point, there was nothing there. We’re used to the idea that tables are mostly empty space filled with matter; but here the matter itself is replaced by mostly empty space too!

2. Flashy Causal Set Worlds. One idea in quantum gravity, associated especially with Rafael Sorkinand Fay Dowker, is that spacetime emerges from a collection of events produced by a probabilistic “sprinkling” process. One can imagine joining Bell’s flash ontology with the causal set picture to eliminate the background continuous spacetime. All there is in such a world would be a sparse collection of flash events. From this sparse collection, tables, chairs, cats and even spacetime would all emerge. The universe would look continuous and filled in when really it is just a bare collection of flashes.

3. No Time. Thanks to relativity we’re used to the idea that there is, fundamentally, no space or time but only spacetime. Many approaches in quantum gravity go a further step. They break spacetime up into space and time again and then get rid of time altogether. Of course, the appearances are recovered. What looks like change and time “reappear” only at certain approximations. But it’s certainly very counterintuitive to think of change as in some sense emergent.

4. Spatial Topology Change. Relativity makes this hard but not impossible. The idea is that the topology of space change with time. Imagine that space is connected and then at some point becomes unconnected – then one has what is sometimes called “trousers” spacetime. Or maybe the universe all of a sudden becomes a big torus – a four-dimensional donut – or a Poincarédodecahedron. There are rigorous constraints on what can happen according to the laws, but some cool possibilities remain open.

3:AM:I guess as a long term Dr Whofan I have to ask you, as our man about time, whether time travel could happen?

CC:The idea is logically possible, even if it could get weird. Here is a quote you’ll like from the guy who “fixes” time travel mistakes in [Charles] Yu’s How to Live Safely in a Science Fictional Universe:

“Other people are just looking for weird. They want to turn their lives into something unrecognizable. I see a lot of men end up as their own uncles. Super-easy to avoid, totally dumb move. See it all the time. No need to go into details, but it obviously involves a time machine and you know what with you know who.”

Weird doesn’t mean impossible. If it did then TV-for-dogs wouldn’t exist.  

Maybe time travel is physically possible. I don’t have any special insight here. General relativity allows many models that allow time travel. So in that sense, sure, why not? When you start adding conditions, however, it becomes harder and harder to find such models. You want the Tardis to create the spacetime path on which Dr Who goes back? Then that means you want a time machine, and it’s much harder to have a spacetime with a time machine in it than just a spacetime with some paths that might take you backward in time. You want a “realistic” matter-energy distribution? Then it’s harder still. You want the spacetime to have certain “reasonable” features, like no holes or weird topological identifications? Then harder still. I don’t think these conditions necessarily shut the window of possibility shut. Hardness doesn’t necessarily mean unlikely. Still, the opening starts to look depressingly small, however, and maybe that’s telling us something.  

Whether future physics opens the window wider I can’t say. Perhaps it will vindicate the Doctor when he says that “People assume that time is a strict progression of cause to effect, but actually… it's more like a big ball of wibbly wobbly... time-y wimey... stuff.”

3:AM:Finally, do you find reading non-physics and non-philosophy can help you get your ideas in order? So are there science fiction books – or films – for example, that have been influential for your thinking? Do you have favourites?

CC:To write a fictional story taking place in a universe unlike ours in some respects takes enormous imagination. The character is embedded in a world, a world containing time travel, time-reversed galaxies, or Euclidean signature (Greg Egan’s Orthogonal) and the author must really envision what challenges would face such a creature, what they would find strange or natural, and so on. In the book project I’m currently engaged in, I’m trying to do something similar, imagining why creatures like us embedded in a relativistic universe would come up with a conception of time as we do. So in some ways it’s the same kind of project, and in some ways its approached the same way as I imagine someone writes sci-fi. Given the similarities, reading the sci-fi probably helps. I just hope that my story turns out non-fictional.

Do I have favorites? Sure. For thinking about time and free will, it’s hard to beat the films Twelve Monkeysor La Jetéeor the short story 'Hundred Thousand Light Year Diary'by Greg Egan. For puzzles about personal identity, I also like Egan’s 'Learning to Be Me'.

3:AM:And finally finally, can you give the sassy bunch of metaphysicians at 3:AMyour top five books on the subject that you think will help us get a firm grip on this weird and wonderful domain of thought?

CC:Let me narrow the focus from metaphysics in general to metaphysical issues arising from modern physics. Then, in no particular order, some accessible and excellent books are
Time and Space, Barry Dainton
The Metaphysics Within Physics, Tim Maudlin
Quantum Mechanics and Experience, David Albert
An Introduction to the Philosophy of Physics, Marc Lange
Real Time II, D.H. Mellor


Richard Marshallis still biding his time.