Some of you may have been following a tiny brouhaha ("kerfuffle" is so overused, don't you think?) that has sprung up around the question of why the universe exists. You can't say we think small around here. First Lawrence Krauss came out with a new book, A Universe From Nothing: Why There Is Something Rather Than Nothing (based in part on a popular YouTube lecture), which addresses this question from the point of view of a modern cosmologist. Then David Albert, speaking as a modern philosopher of science, came out with quite a negative review of the book in the New York Times. And discussion has gone back and forth since then: here's Jerry Coyne (mostly siding with Albert), the Rutgers Philosophy of Cosmology blog (with interesting voices in the comments), a long interview with Krauss in the Atlantic, comments by Massimo Pigliucci, and another response by Krauss on the Scientific American site. I've been meaning to chime in, for personal as well as scientific reasons. I do work on the origin of the universe, after all, and both Lawrence and David are friends of the blog (and of me): Lawrence was our first guest-blogger, and David and I did Bloggingheads dialogues here and here. Executive summary This is going to be kind of long, so here's the upshot. Very roughly, there are two different kinds of questions lurking around the issue of "Why is there something rather than nothing?" One question is, within some framework of physical laws that is flexible enough to allow for the possible existence of either "stuff" or "no stuff" (where "stuff" might include space and time itself), why does the actual manifestation of reality seem to feature all this stuff? The other is, why do we have this particular framework of physical law, or even something called "physical law" at all? Lawrence (again, roughly) addresses the first question, and David cares about the second, and both sides expend a lot of energy insisting that their question is the "right" one rather than just admitting they are different questions. Nothing about modern physics explains why we have these laws rather than some totally different laws, although physicists sometimes talk that way -- a mistake they might be able to avoid if they took philosophers more seriously. Then the discussion quickly degrades into name-calling and point-missing, which is unfortunate because these are smart people who agree about 95% of the interesting issues, and the chance for productive engagement diminishes considerably with each installment. How the universe works Let's talk about the actual way physics works, as we understand it. Ever since Newton, the paradigm for fundamental physics has been the same, and includes three pieces. First, there is the "space of states": basically, a list of all the possible configurations the universe could conceivably be in. Second, there is some particular state representing the universe at some time, typically taken to be the present. Third, there is some rule for saying how the universe evolves with time. You give me the universe now, the laws of physics say what it will become in the future. This way of thinking is just as true for quantum mechanics or general relativity or quantum field theory as it was for Newtonian mechanics or Maxwell's electrodynamics. Quantum mechanics, in particular, is a specific yet very versatile implementation of this scheme. (And quantum field theory is just a particular example of quantum mechanics, not an entirely new way of thinking.) The states are "wave functions," and the collection of every possible wave function for some given system is "Hilbert space." The nice thing about Hilbert space is that it's a very restrictive set of possibilities (because it's a vector space, for you experts); once you tell me how big it is (how many dimensions), you've specified your Hilbert space completely. This is in stark contrast with classical mechanics, where the space of states can get extraordinarily complicated. And then there is a little machine -- "the Hamiltonian" -- that tells you how to evolve from one state to another as time passes. Again, there aren't really that many kinds of Hamiltonians you can have; once you write down a certain list of numbers (the energy eigenvalues, for you pesky experts) you are completely done. We should be open-minded about what form the ultimate laws of physics will take, but almost all modern attempts to get at them take quantum mechanics for granted. That's true for string theory and other approaches to quantum gravity -- they might take very different views of what constitutes "spacetime" or "matter," but very rarely do they muck about with the essentials of quantum mechanics. It's certainly the case for all of the scenarios Lawrence considers in his book. Within this framework, specifying "the laws of physics" is just a matter of picking a Hilbert space (which is just a matter of specifying how big it is) and picking a Hamiltonian. One of the great things about quantum mechanics is how extremely restrictive it is; we don't have a lot of room for creativity in choosing what kinds of laws of physics might exist. It seems like there's a lot of creativity, because Hilbert space can be extremely big and the underlying simplicity of the Hamiltonian can be obscured by our (as subsets of the universe) complicated interactions with the rest of the world, but it's always the same basic recipe. So within that framework, what does it mean to talk about "a universe from nothing"? We still have to distinguish between two possibilities, but at least this two-element list exhausts all of them. Possibility one: time is fundamental The first possibility is that the quantum state of the universe really does evolve in time -- i.e. that the Hamiltonian is not zero, it truly does push the state forward in time. This seems like the generic case (there are more ways to be not-zero than to be zero), and it's certainly the one that we spend time considering in introductory courses when we foist quantum mechanics on fearful undergraduates for the first time. A wonderful and under-appreciated consequence of quantum mechanics is that, if this possibility is right (the universe truly evolves), time cannot truly begin or end -- it goes on forever. Very unlike classical mechanics, where the universe's trajectory through the space of states can bring it smack up against a singularity, at which point time presumably ceases. In QM, every state is just as good as every other state, and the evolution will go happily marching along. So what does this have to do with something vs. nothing? Well, as the quantum state of the universe evolves, it can pass through phases where it looks an awful lot like "nothing," conventionally understood -- i.e. it could look like completely empty space, or like some peculiar non-geometric phase where we wouldn't recognize it as "space" at all. And later, through the relentless influence of the Hamiltonian, it could evolve into something that looks very much like "something," even very much like the universe we see around us today. So if your definition of "nothing" is "emptiness" or "lack of space itself," the laws of quantum mechanics provide a nice way to understand how that nothing can evolve into the marvelous something we find ourselves inside. This is interesting, and important, and worth writing a book about, and it's one of the possibilities Lawrence discusses. Possibility two: time is emergent/approximate The other possibility is that the universe doesn't evolve at all -- the Hamiltonian is zero, and there is some space of possible states, but we just sit there, without a fundamental "passage of time." Now, you might suspect that this is a logical possibility but not a plausible one; after all, don't we see things change around us all the time? But in fact this possibility is the one you immediately bump into if you simply take classical general relativity and try to "quantize" it (i.e., invent the quantum theory that would reduce to GR in the classical limit). We don't know that this is the right thing to do -- Tom Banks, for example, would argue that it's not -- but it's a possibility that is on the table, so we should think about what it would mean if it turns out to be true. We certainly think that we perceive time passing, but maybe time is just emergent rather than fundamental. (I don't like using "illusory" in this context, but others are not so circumspect.) That is, perhaps there is an alternative description of that single, unmoving point in Hilbert space -- a description that looks approximately like "a universe evolving through time," at least for some period of duration. Think of a block of metal sitting on a hot surface, not evolving with time but with a temperature gradient from top to bottom. It might be possible to conceptually divide the block into slices of equal temperature, and then write down an equation for how the state of the block changes from slice to slice, and find that the resulting mathematical formalism looks just like "evolution through time." In this case, unlike the previous one, time could end (or begin), because time was only a useful approximation to begin with, valid in a certain regime. This kind of scenario is exactly what quantum cosmologists like James Hartle, Stephen Hawking, Alex Vilenkin, Andrei Linde and others have in mind when they are talking about the "creation of the universe from nothing." In this kind of picture, there is literally a moment in the history of the universe prior to which there weren't any other moments. There is a boundary of time (presumably at the Big Bang), prior to which there was ... nothing. No stuff, not even a quantum wave function; there was no prior thing, because there is no sensible notion of "prior." This is also interesting, and important, and worth writing a book about, and it's another one of the possibilities Lawrence discusses. Why is there a universe at all? So modern physics has given us these two ideas, both of which are interesting, and both of which resonate with our informal notion of "coming into existence out of nothing" -- one of which is time evolution from empty space (or not-even-space) into a universe bursting with stuff, and the other of which posits time as an approximate notion that comes to an end at some boundary in an abstract space of possibilities. What, then, do we have to complain about? Well, a bit of contemplation should reveal that this kind of reasoning might, if we grant you a certain definition of "nothing," explain how the universe could arise from nothing. But it doesn't, and doesn't even really try to, explain why there is something rather than nothing -- why this particular evolution of the wave function, or why even the apparatus of "wave functions" and "Hamiltonians" is the right way to think about the universe at all. And maybe you don't care about those questions, and nobody would question your right not to care; but if the subtitle of your book is "Why There Is Something Rather Than Nothing," you pretty much forfeit the right to claim you don't care. Do advances in modern physics and cosmology help us address these underlying questions, of why there is something called the universe at all, and why there are things called "the laws of physics," and why those laws seem to take the form of quantum mechanics, and why some particular wave function and Hamiltonian? In a word: no. I don't see how they could. Sometimes physicists pretend that they are addressing these questions, which is too bad, because they are just being lazy and not thinking carefully about the problem. You might hear, for example, claims to the effect that our laws of physics could turn out to be the only conceivable laws, or the simplest possible laws. But that seems manifestly false. Just within the framework of quantum mechanics, there are an infinite number of possible Hilbert spaces, and an infinite number of possibile Hamiltonians, each of which defines a perfectly legitimate set of physical laws. And only one of them can be right, so it's absurd to claim that our laws might be the only possible ones. Invocations of "simplicity" are likewise of no help here. The universe could be just a single point, not evolving in time. Or it could be a single oscillator, rocking back and forth in perpetuity. Those would be very simple. There might turn out to be some definition of "simplicity" under which our laws are the simplest, but there will always be others in which they are not. And in any case, we would then have the question of why the laws are supposed to be simple? Likewise, appeals of the form "maybe all possible laws are real somewhere" fail to address the question. Why are all possible laws real? And sometimes, on the other hand, modern cosmologists talk about different laws of physics in the context of a multiverse, and suggest that we see one set of laws rather than some other set for fundamentally anthropic reasons. But again, that's just being sloppy. We're talking here about the low-energy manifestation of the underlying laws, but those underlying laws are exactly the same everywhere throughout the multiverse. We are still left with the question of there are those deep-down laws that create a multiverse in the first place. The end of explanations All of these are interesting questions to ask, and none of them is addressed by modern physics or cosmology. Or at least, they are interesting questions to "raise," but my own view is that the best answer is to promptly un-ask them. (Note that by now we've reached a purely philosophical issue, not a scientific one.) "Why" questions don't exist in a vacuum; they only make sense within some explanatory context. If we ask "why did the chicken cross the road?", we understand that there are things called roads with certain properties, and things called chickens with various goals and motivations, and things that might be on the other side of the road, or other beneficial aspects of crossing it. It's only within that context that a sensible answer to a "why" question can be offered. But the universe, and the laws of physics, aren't embedded in some bigger context. They are the biggest context that there is, as far as we know. It's okay to admit that a chain of explanations might end somewhere, and that somewhere might be with the universe and the laws it obeys, and the only further explanation might be "that's just the way it is." Or not, of course. We should be good empiricists and be open to the possibility that what we think of as the universe really does exist within some larger context. But then we could presumably re-define that as the universe, and be stuck with the same questions. As long as you admit that there is more than one conceivable way for the universe to be (and I don't see how one could not), there will always be some end of the line for explanations. I could be wrong about that, but an insistence that "the universe must explain itself" or some such thing seems like a completely unsupportable a priori assumption. (Not that anyone in this particular brouhaha seems to be taking such a stance.) Sounds and furies That's all I have to say about the (fun, interesting) substantive questions, but I am not strong enough to resist a couple of remarks on the (tedious but strangely irresistible) procedural questions. First, I think that Lawrence's book makes a lot more sense when viewed as part of the ongoing atheism vs. theism popular debate, rather than as a careful philosophical investigation into a longstanding problem. Note that the afterword was written by Richard Dawkins, and Lawrence had originally asked Christopher Hitchens, before he became too ill -- both of whom, while very smart people, are neither cosmologists nor philosophers. If your real goal is to refute claims that a Creator is a necessary (or even useful) part of a complete cosmological scheme, then the above points about "creation from nothing" are really quite on point. And that point is that the physical universe can perfectly well be self-contained; it doesn't need anything or anyone from outside to get it started, even if it had a "beginning." That doesn't come close to addressing Leibniz's classic question, but there's little doubt that it's a remarkable feature of modern physics with interesting implications for fundamental cosmology. Second, after David's review came out, Lawrence took the regrettable tack of lashing out at "moronic philosophers" and the discipline as a whole, rather than taking the high road and sticking to a substantive discussion of the issues. In the Atlantic interview especially, he takes numerous potshots that are just kind of silly. Like most scientists, Lawrence doesn't get a lot out of the philosophy of science. That's okay; the point of philosophy is not to be "useful" to science, any more than the point of mycology is to be "useful" to fungi. Philosophers of science aren't trying to do science, they are trying to understand how science works, and how it should work, and to tease out the logic and standards underlying scientific argumentation, and to situate scientific knowledge within a broader epistemological context, and a bunch of other things that can be perfectly interesting without pretending to be science itself. And if you're not interested, that's fine. But trying to undermine the legitimacy of the field through a series of wisecracks is kind of lame, and ultimately anti-intellectual -- it represents exactly the kind of unwillingness to engage respectfully with careful scholarship in another discipline that we so rightly deplore when people feel that way about science. It's a shame when smart people who agree about most important things can't disagree about some other things without throwing around insults. We should strive to be better than that.