EDIT: This post has been edited to better reflect Luke Barnes’ position.
Recently, Richard Carrier has re-opened his public discussion with Luke Barnes on the fine-tuning argument, in a blog post (for Barnes’ reply, see his own blog post). Their discussion is quite tedious and personal, but at the heart of things I think Carrier is right: fine-tuning is evidence against God, for the reasons Carrier champions. I’ll explain.
I think Luke Barnes, who supports the fine-tuning argument, uses a somewhat misleading terminology. What I take “fine tuning” to be, and what I think most physicists do, is the empirical finding that we live in a universe with laws of nature such that if the constants in these laws were to be altered slightly, then the laws would describe a (different) universe where life cannot evolve or survive. The constants in this sense seem to be “fine tuned” to produce life. Let us call this fact about the laws of nature of the universe we actually live in “FT”.
The fine-tuning argument is then the argument that this fact indicates that God exists. Now in judging whether God exists or not, we are contrasting two hypotheses. It does the atheists injustice to say that they simply don’t believe God exists; rather, they believe the world is natural. Barnes usefully provides a way to characterize this view: the atheists believe that all that exists is Lagrangian, meaning that it is described by one particular uniform, local, set of laws of nature. Let us call this hypothesis “N”, for “Natural”. The question is then whether the data that fine-tuning holds (FN), supports the God hypothesis (G) or naturalism (N). The real question here is whether the data is more likely under G or N.
I think Barnes confuses fine-tuning with a slightly but importantly different mathematical fact. This fact is that in the space of all possible natural (Lagrangian) universes, the ones bearing life are exceedingly rare. This is because life, as we know it, is a very complex phenomena, and its evolution even more so. In order to “build” such a thing from the very simple, local, uniform building blocks that a Lagrangian provides, you need to get things just right. Thus, the Lagrangians that support life are very sparsely distributed among all possible Lagrangians, and even small deviations from them (changing one constant by a bit) will mean a universe that isn’t life-bearing. (At least, that seems to be plausible; there is no way to actually calculate any of that.) Notice that this is a logical fact about the nature of Lagrangians (i.e. of natural hypothetical universes). It makes no sense asking what is the likelihood that we will observe it or what is the likelihood of something given it, just like it makes no sense to ask what is the likelihood that we will observe “1+1=2” or to ask what is the likelihood of something “given” that “1+1=2” (since “1+1=2” is true regardless of what else we consider “given”).
Now, what Carrier essentially argues is that we should be very careful to distinguish the finding that there is Life (let’s call it “L”) from the finding that there is fine-tuning (FT). He rightly claims, and Barnes agrees, that given that there is life in the universe, and given that naturalism holds, the probability that we will find fine-tuning is 1; P(FT|L,N)=1. This is because the few hypothetical Lagrangians that do support life are fine-tuned. In contrast, given that there is life and that the God hypothesis is true, the probability of fine-tuning is lower than 1, since God could have created life without fine-tuning; P(FT|L,G)<1. It follows from this that the evidence of fine-tuning supports atheism – the fact that we find ourselves in a fine-tuned universe lowers the probability of God. I think in this Carrier is right.
Theists in contrast often argue that if we just consider fine-tuning on its own, then it is more likely under theism than under atheism. This is because the probability of fine-tuning given atheism is very low, since the probability of life under atheism is very low, since most Lagrangians don’t support life; P(FT|N) is low. In contrast, the probability of fine-tuning under God is supposedly fairly high, since God wants to create life and he might as well do it with uniform laws; so P(FT|G) is high since P(L|G)=1.
This argument is problematic in that fine-tuning is a separate fact from life, and is only relevant in those universes that have life. So we can’t just write P(FT|N). We have to take each piece of data on its own to maintain clarity. We have to write P(FT|L,N) – and similarly P(FT|L,G). And Carrier is still right – the new data FN is certain under naturalism, so that P(FT|L,N)=P(L|N), whereas it is less than certain under the God hypothesis so that P(FT|L,G)<P(L|G), so that the new information FT actually lowers the probability of the God hypothesis.
Now, this isn’t quite Barnes’ argument. Barnes instead essentially argues that since life is rare within the space of all natural (Lagrangian), the fact that we find it in our universe indicates that the process that chose which Lagrangian to instantiate was highly-biased towards choosing life-bearing Lagrangians. Implicitly, of course, this implies God chose which Lagrangian to instantiate.
Note that this amounts to what I will call the “argument from life”, namely that life is much more likely under God than under naturalism; P(L|G)=1 whereas P(L|N) is very small. And this is exactly the same place where the more usual theist argument leaves us – having established that Carrier is right that the finding that our universe is fine-tuned (FT|L) supports N, we are still left with the question of whether L does. So – how can the atheist reply to the argument from life? Well, he has two replies to this.
First, one can note that the specific God hypothesis the theist is working with is already carefully selected to fit the data that there is life. There are lots of other gods we can think of, that won’t create life. So the fact that P(L|G)=1 isn’t really saying much. To be fair, we should really consider all possible gods, and there is really no way to calculate the likelihood of life under that theory but it stands to reason it would be very low too. Or, if we decide to limit ourselves to just the life-permitting gods, then we might as well limit ourselves just to the life-permitting natural universes, and then P(L|N)=1 too.
In Barnes’ variant of the argument, this amounts to saying that even though a natural Lagrangian-selecting process that chooses a life-bearing Lagrangian seems unlikely, a divine ones that does so is also unlikely. (Technically, the atheist here doesn’t accept that there is a process that chooses the Lagrangian; rather, there simply is a particular Lagrangian. So what Barnes’ really shows is that if you believe in Naturalism is that life is surprising (assuming that Lagrangian space is indeed sparse as is assumed); and the atheist replies that it’s surprising if you believe in God, too.)
This objection is closely related to the fact that one can’t really do rational probabilistic analysis unless one knows beforehand how to divide the landscape of possibilities. The answers you get from a probabilistic analysis, especially one involving infinities such as the values of the constants in the Lagrangians or the possible types of deities, will depend on how you divide the infinite range of possibilities up. This is part of the reason why I said above that we can’t really calculate how common are life-bearing naturalistic universes among all naturalistic universes.
Secondly, one can object to Barnes’ (or the more usual) argument by declining Barnes’ characterization of naturalism. I said above he effectively defines it as maintaining that there is one Lagrangian – implying that there is one uniform, local, simple set of laws of nature. But quantum physics seems to suggest otherwise. A big part of our understanding of the laws of nature that we have involves the idea that some of the “constants” in our laws didn’t start that way, but rather had a range of possible values and “froze” at the values we see (this is called “spontaneous symmetry breaking”). This occurs in a quantum theory, and one of the leading interpretations of quantum theory – the leading one in quantum cosmology, I think; this is the Many World Interpretation – is that whenever there are multiple possibilities, all of them are realized, each in a separate parallel universe. Thus, instead of reality consisting of the laws of nature we have in our universe, with their current values of the constants, contemporary physics suggests that reality actually consists of a multiverse with numerous parallel universes, each with their own “constants” of nature.
This is hardly well-established science; it’s just an interpretation of current science (although one I tend to believe in, for reasons unrelated to the fine-tuning argument). If one adopts something like this view, then, one is led to define naturalism not as there existing one Lagrangian but rather a plethora of Lagrangians describing parallel universes, perhaps even an infinite variety of all possible Lagrangians. In such a multiverse, the probability of there being a Lagrangian universe with life in it is 1; P(L|N)=1.
We have therefore reached the stage where both under naturalism (in the multiverse sense) and under theism the probability of life is 1; so the argument from life fails.
Now the question becomes – which is more likely, the multiverse or God? That’s yet another argument to be had, but I’ll simply note that I think the multiverse is strongly suggested by well-established physics, whereas God is a childish, anthropomorphic (in the “mind of a human”, not “body of a human”, sense), metaphysically incoherent (when the so-called “theologian’s God” is meant), and is in short a highly unlikely hypothesis. At any rate, this question bears little relation to the question of whether fine-tuning implies that God exists – which, as I argued above, it does not.