I continue to believe that “quantum field theory” is a concept that we physicists don’t do nearly enough to explain to a wider audience. And I’m not going to do it here! But I will link to other people thinking about how to think about quantum field theory.

Over on the Google+, I linked to an informal essay by John Norton, in which he recounts the activities of a workshop on QFT at the Center for the Philosophy of Science at the University of Pittsburgh last October. In Norton’s telling, the important conceptual divide was between those who want to study “axiomatic” QFT on the one hand, and those who want to study “heuristic” QFT on the other. Axiomatic QFT is an attempt to make everything absolutely perfectly mathematically rigorous. It is severely handicapped by the fact that it is nearly impossible to get results in QFT that are both interesting and rigorous. Heuristic QFT, on the other hand, is what the vast majority of working field theorists actually do — putting aside delicate questions of whether series converge and integrals are well defined, and instead leaping forward and attempting to match predictions to the data. Philosophers like things to be well-defined, so it’s not surprising that many of them are sympathetic to the axiomatic QFT program, tangible results be damned.

The question of whether or not the interesting parts of QFT can be made rigorous is a good one, but not one that keeps many physicists awake at night. (more…)

Back in 1814, Pierre-Simon Laplace was mulling over the implications of Newtonian mechanics, and realized something profound. If there were a vast intelligence — since dubbed Laplace’s Demon — that knew the exact state of the universe at any one moment, and knew all the laws of physics, and had arbitrarily large computational capacity, it could both predict the future and reconstruct the past with perfect accuracy. While this is a straightforward consequence of Newton’s theory, it seems to conflict with our intuitive notion of free will. Even if there is no such demon, presumably there is some particular state of the universe, which implies that the future is fixed by the present. What room, then, for free choice? What’s surprising is that we still don’t have a consensus answer to this question. Subsequent developments, most relevantly in the probabilistic nature of predictions in quantum mechanics, have muddied the waters more than clarifying them.

Massimo Pigliucci has written a primer for skeptics of determinism, in part spurred by reading (and taking issue with) Alex Rosenberg’s new book The Atheist’s Guide to Reality, which I mentioned here. And Jerry Coyne responds, mostly to say that none of this amounts to “free will” over and above the laws of physics. (Which is true, even if, as I’ll mention below, quantum indeterminacy can propagate upward to classical behavior.) I wanted to give my own two cents, partly as a physicist and partly as a guy who just can’t resist giving his two cents.

Echoing Massimo’s structure, here are some talking points:

* There are probably many notions of what determinism means, but let’s distinguish two. The crucial thing is that the universe can be divided up into different moments of time. (The division will generally be highly non-unique, but that’s okay.) Then we can call “global determinism” the claim that, if we know the exact state of the whole universe at one time, the future and past are completely determined. But we can also define “local determinism” to be the claim that, if we know the exact state of some part of the universe at one time, the future and past of a certain region of the universe (the “domain of dependence”) is completely determined. Both are reasonable and relevant.

* It makes sense to be interested, as Massimo seems to be, in whether or not the one true correct ultimate set of laws of physics are deterministic or not. (more…)

The question of the day seems to be, “Is the wave function real/physical, or is it merely a way to calculate probabilities?” This issue plays a big role in Tom Banks’s guest post (he’s on the “useful but not real” side), and there is an interesting new paper by Pusey, Barrett, and Rudolph that claims to demonstrate that you can’t simply treat the quantum state as a probability calculator. I haven’t gone through the paper yet, but it’s getting positive reviews. I’m a “realist” myself, as I think the best definition of “real” is “plays a crucial role in a successful model of reality,” and the quantum wave function certainly qualifies.

To help understand the lay of the land, we’re very happy to host this guest post by David Wallace, a philosopher of science at Oxford. David has been one of the leaders in trying to make sense of the many-worlds interpretation of quantum mechanics, in particular the knotty problem of how to get the Born rule (“the wave function squared is the probability”) out of the this formalism. He was also a participant at our recent time conference, and the co-star of one of the videos I posted. He’s a very clear writer, and I think interested parties will get a lot out of reading this.

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Why the quantum state isn’t (straightforwardly) probabilistic

In quantum mechanics, we routinely talk about so-called “superposition states” – both at the microscopic level (“the state of the electron is a superposition of spin-up and spin-down”) and, at least in foundations of physics, at the macroscopic level (“the state of Schrodinger’s cat is a superposition of alive and dead”). Rather a large fraction of the “problem of measurement” is the problem of making sense of these superposition states, and there are basically two views. On the first (“state as physical”), the state of a physical system tells us what that system is actually, physically, like, and from that point of view, Schrodinger’s cat is seriously weird. What does it even mean to say that the cat is both alive and dead? And, if cats can be alive and dead at the same time, how come when we look at them we only see definitely-alive cats or definitely-dead cats? We can try to answer the second question by invoking some mysterious new dynamical process – a “collapse of the wave function” whereby the act of looking at half-alive, half-dead cats magically causes them to jump into alive-cat or dead-cat states – but a physical process which depends for its action on “observations”, “measurements”, even “consciousness”, doesn’t seem scientifically reputable. So people who accept the “state-as-physical” view are generally led either to try to make sense of quantum theory without collapses (that leads you to something like Everett’s many-worlds theory), or to modify or augment quantum theory so as to replace it with something scientifically less problematic.

On the second view, (“state as probability”), Schrodinger’s cat is totally unmysterious. When we say “the state of the cat is half alive, half dead”, on this view we just mean “it has a 50% probability of being alive and a 50% probability of being dead”. And the so-called collapse of the wavefunction just corresponds to us looking and finding out which it is. From this point of view, to say that the cat is in a superposition of alive and dead is no more mysterious than to say that Sean is 50% likely to be in his office and 50% likely to be at a conference.

Now, to be sure, probability is a bit philosophically mysterious. (more…)

Videos from our Setting Time Aright conference are gradually filtering online, courtesy of the Foundational Questions Institute. Perhaps the very first question that should be asked, of course, is whether the subject of the conference actually exists. So we recruited two well-known partisans on this issue to hash things out. Tim Maudlin is a philosopher of science who has argued forcefully that time is real — and furthermore that the arrow of time is an intrinsic part of reality, not just a byproduct of the low-entropy Big Bang. (Crazy talk.) Julian Barbour is a physicist who is well known for arguing that time doesn’t really exist, we can happily eliminate it from all of our equations of physics. (Even crazier.)

So we asked them to go at it, with a twist: here Tim defends the proposition that time doesn’t exist, while Julian argues that it is real. I was not the only one to conclude that these guys were just as good at arguing this side as the one they actually believed.

Well worth watching — both talks are quite brilliant, in very different ways.

I have my answer (“yes, but not by finding meaning `out there’ in the world”), which I hope to write about more soon. In the meantime, listen to a great conversation between philosophers Owen Flanagan and Alex Rosenberg from Philosophy TV. “What there is, and all there is, are bosons and fermions.”

Both discussants have written really good books. Rosenberg recently came out with The Atheist’s Guide to Reality: Enjoying Life Without Illusions, while I very much enjoyed Flanagan’s earlier book The Really Hard Problem: Meaning in a Natural World.

Empirically, of course, naturalists often lead very enjoying and fulfilled lives. Here’s a great profile of newly minted Laureate Brian Schmidt, in his capacity as a cook and winemaker as well as an astronomer. And here’s Bob Kirshner, writing to the NYT from Friendship, Maine, about the meaning of dark energy.

One last thought on all this God/cosmology stuff before moving on.

The crucial moment of our panel discussion occurred when John Haught said that he couldn’t imagine a universe without God. (Without God, the universe couldn’t exist.) It would have been more crucial if I had followed up a bit more, but I didn’t because I suck (and because time was precious).

Believing that something must be true about the world because you can’t imagine otherwise is, five hundred years into the Age of Science, not a recommended strategy for acquiring reliable knowledge. It goes back to the classic conflict of rationalism vs. empiricism. “Rationalism” sounds good — who doesn’t want to be rational? But the idea behind it is that we can reach true conclusions about the world by reason alone. We don’t ever have to leave the comfort of our living room; we can just sit around, sharing some single-malt Scotch and fine cigars, thinking really hard about the universe, and thereby achieve some real understanding. Empiricism, on the other hand, says that we should try to imagine all possible ways the world could be, and then actually go out and look at it to decide which way it really is. Rationalism is traditionally associated with Descartes, Leibniz, and Spinoza, while empiricism is associated with Locke, Berkeley, and Hume — but of course these categories never quite fit perfectly well.

The lure of rationalism is powerful, and it shows up all over the place. Leibniz proclaimed various ways the world must work, such as the Principle of Sufficient Reason. Lee Smolin uses Leibnizian arguments against string theory. Many people, such as Oxford philosopher Richard Swinburne, feel strongly that the world cannot simply be; there must be a reason for its existence. Paul Davies believes that the laws of physics cannot simply be, and require an explanation. William Lane Craig believes that infinity cannot be realized in Nature. Einstein felt that God did not play dice with the universe. At a less lofty level, people see bad things happen and feel the urge to blame someone.

But the intellectual history of the past five centuries has spoken loud and clear: the dream of rationalism is a false one. (more…)

Reading about emergence and reductionism and free will and determinism has led me to finally confront a concept I had vaguely heard about but never really looked into before: downward causation, a term that came to prominence in the 1970′s. (Some other views: here, here, here.) I think it’s a misguided/unhelpful notion, but this is way outside my area and I’m happy to admit that I might be missing something.

Physicists are well aware that there are different vocabularies/models/theories that we can use to describe the same underlying reality. Sometimes you might want to talk about a box of gas as a fluid with pressure and velocity, other times you might want to talk about it in terms of atoms and molecules. Philosophers and psychologists might want to talk about human beings as autonomous agents who do things for reasons, while admitting that they can also be thought of as collections of cells and tissues, or even once again as atoms and molecules. The question is: what is the relationship between these different levels? In fluid mechanics/kinetic theory things are pretty clear, but in the mind/body problem things begin to get murky. (Or at least, there are people who take great pleasure in insisting that they are murky.)

Reductionism notes that some of these descriptions are more complete, and therefore arguably more fundamental, than others. In particular, some descriptions are in terms of entities that are literally smaller than the others; atoms are smaller than neurons, which are smaller than people. The smaller-level descriptions tend to have a wider range of validity; we can imagine answering certain questions in the atomic language that we can’t answer (correctly) in the fluid language, like “what happens if we divide the box in half, and then divide that in half, and so forth a million times?” It therefore seems natural to arrange the descriptions vertically: “lower” levels refer to small-scale descriptions, while “higher” levels refer to macroscopic objects. The claim of reductionism is, depending on who you talk to, that the lower-level description is either “always more complete,” or “capable of deriving the higher-level descriptions,” or “the right way to think about things.”

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A handful of musings about free will have been popping up in my blog reader of late. Jerry Coyne has been discussing the issue with Eric MacDonald in a series of posts (further links therein). Russell Blackford writes a long post that he promises isn’t the post he will eventually write, David Eagleman has an article in the Atlantic, and Zach Weiner also chimes in. So we have a biologist studying theology, an ex-Anglican priest turned agnostic, a philosopher and neuroscientist both of whom write science fiction, and a webcartoonist studying physics. That constitutes a reasonable spectrum of opinion. Still, what discussion of reality is complete without a cosmologist chiming in?

In some ways, asking whether free will exists is a lot like asking whether time really exists. In both cases, it’s different from asking “do unicorns exist?” or “does dark matter exist?” In these examples, we are pretty clear on what the concepts are supposed to denote, and what it would mean for them to actually exist; what’s left is a matter of collecting evidence and judging its value. I take it that this is not what we mean when we ask about the existence of free will.

It’s possible to deny the existence of something while using it all the time. Julian Barbour doesn’t believe time is real, but he is perfectly capable of showing up to a meeting on time. Likewise, people who question the existence of free will don’t have any trouble making choices. (John Searle has joked that people who deny free will, when ordering at a restaurant, should say “just bring me whatever the laws of nature have determined I will get.”) Whatever it is we are asking, it’s not simply a matter of evidence.

When people make use of a concept and simultaneously deny its existence, what they typically mean is that the concept in question is nowhere to be found in some “fundamental” description of reality. Julian Barbour thinks that if we just understood the laws of physics better, “time” would disappear from our vocabulary. Likewise, discussions about the existence of free will often center on whether we really need to include such freedom as an irreducible component of reality, without which our understanding would be fundamentally incomplete.

There are people who do believe in free will in this sense; that we need to invoke a notion of free will as an essential ingredient in reality, over and above the conventional laws of nature. These are libertarians, in the metaphysical sense rather than the political-philosophy sense. They may explicitly believe that conscious creatures are governed by a blob of spirit energy that transcends materialist categories, or they can be more vague about how the free will actually manifests itself. But in either event, they believe that our freedom of choice cannot be reduced to our constituent particles evolving according to the laws of physics.

This version of free will, as anyone who reads the blog will recognize, I don’t buy at all. Within the regime of everyday life, the underlying laws of physics are completely understood. There’s a lot we don’t understand about consciousness, but none of the problems we face rise to the level that we should be tempted to distrust our basic understanding of how the atoms and forces inside our brains work. Note that it’s not really a matter of “determinism”; it’s simply a question of whether there are impersonal laws of nature at all. The fact that quantum mechanics introduces a stochastic component into physical predictions doesn’t open the door for true libertarian free will.

But I also don’t think that “playing a necessary role in every effective description of the world” is a very good way of defining “existence” or “reality.” (more…)

And now for something somewhat different. After I posted my article on “Does the Universe Need God?“, there were a few responses at the Intelligent Design blog Uncommon Descent, including a list of questions by Vincent Torley. Vincent then went the extra mile by inviting me to write a guest post for UD. Not my usual stomping grounds, but I ultimately agreed, precisely for that reason.

Here’s the post, which I’m cross-posting below. This might be controversial, as a lot of people on my side of things will say that there’s little point in engaging with people on the other side. And admittedly, this is a subject where feelings can be pretty entrenched. But you never know — not everyone has their mind made up on every issue, and it’s good to try to explain yourself to unsympathetic audiences on occasion. That’s all I tried to do here — to explain how I think about these things, not necessarily to pick a fight or even persuade any skeptics. I tried pretty hard to be as clear and unpretentious as I can be. (Success is for you to decide.) In a world of shouting and diatribe, I remain optimistic that real communication can occasionally occur! We’ll see how it goes.

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I wanted to thank Vincent Torley and Denyse O’Leary for the opportunity to write a guest blog post, and apologize for how long it’s taken me to do so. I’ve written an article for the forthcoming Blackwell Companion to Science and Christianity, entitled Does the Universe Need God?, in which I argued that the answer is “no.” Vincent posed a list of questions in response. After thinking about it, I decided that my answers would be more clear if I simply wrote a coherent argument, rather than addressing the questions individually.

My goal is to try to explain my own thinking to an audience that is not predisposed to agree. We can roughly break people up into two groups: naturalists such as myself, who think that the best explanation we have for the universe involves physical quantities obeying laws of Nature and nothing else; and those who believe that a better explanation can be found by invoking a powerful being/designer/creator/God. (For the sake of simplicity I’m going to use “God” to refer to this notion, but feel free to substitute the more accurate description of your choice.) Obviously there are many nuances that are being passed over by this simple distinction, but hopefully it will suffice for this moment.

The dispute between these two camps isn’t one where people often change their minds at the drop of an argument. Minds do change, in either direction — but typically after extended periods of reflection, not suddenly in response to a single killer blog post. So persuasion is not my goal here; only explanation. I’ve succeeded if an open-minded person who disagrees with me reads the post and still disagrees, but at least understands why I hold my positions. (After giving an earlier talk, one of the theologians in the audience told me that I had persuaded him — not that God didn’t exist, but that the argument from design wasn’t the way to get to Him. That sort of real-time response is more than one can generally hope for.)

What I want to do is to elaborate on some crucial aspects of how science is done that bear directly on the issues raised by my article and some of the responses to it that I’ve seen. In particular, I want to talk about simplicity, laws, openness, explanation, and clarity. This isn’t supposed to be a comprehensive treatise on the philosophy of science, nor is it especially rigorous, or anything really new — just some thoughts on issues relevant to this conversation.

I will be taking one thing for granted: that what we’re interested in doing here is science. There are many kinds of consideration that may lead people to theism or atheism that have nothing whatsoever to do with science; likewise, one may believe that there are ways of understanding the natural world that go beyond the methods of science. I have nothing to say about that right now; that’s a higher-level discussion. I’m just going to presume that we all agree that we’re trying to be the best scientists we can possibly be, and ask what that means.

With all that throat-clearing out of the way, here’s what I have to say about these five issues.

(more…)

Fabio Gironi recently interviewed me at length for an issue of Speculations, a “Journal of Speculative Realism.” The subject was science and philosophy, which I’ve been known to opine about at some length. But here we’re talking great length indeed. The interview isn’t available separately, but you can download the pdf of the whole issue here (or buy it as a bound copy). My bit starts on page 313. (The rest of the issue is also worth checking out.)

I’m a big believer that academic disciplines should engage in messy interactions, not keep demurely separate from each other. But it’s a tricky business. Just because I’m (purportedly) an expert in one thing doesn’t make me an expert in everything else; on the other hand, it is possible that one area has something to offer another one. So I am in favor of dabbling, but with humility. It’s good for people to have thoughts and opinions about issues outside their immediate expertise, and to offer them in good faith, but it’s bad if they become convinced that experts in other areas are all idiots. So when you find yourself disagreeing with the consensus of expertise in some well-established field, it might very well be because of your superior insight and training, or maybe you’re just missing something. Hopefully in an exchange like this I have something to offer without making too many blunders that would make real experts cringe.

Here’s a sample of the interview.

SC: I would be extremely suspicious of any attempts to judge that the world must ‘necessarily’ be some way rather than any other. I can imagine different worlds—or at least I think I can—so I don’t believe that this is the only possible world. That would also go for any particular feature of the laws this world follows, including their stability. Maybe the laws are constant through time, maybe they are not. (Maybe time is a fundamental concept, maybe it isn’t). We don’t yet know, but it seems clear to me that these are empirical questions, not a priori ones. Because we want to understand the world in terms that are as simple as possible, the idea that the underlying laws are stable is an obvious first guess, but one that must then be tested against the data. Said in a slightly different language: any metaphysical considerations concerning what qualities the world should properly have can be taken seriously and incorporated into Bayesian priors for evaluating theories, but ultimately those theories are judged against experiment. We should listen to the world, not decide ahead of time what it must be.