Can we make sense of why the universe is this way, rather than some other way? Or will the ultimate laws simply be the end of all our explanations? Is there something about our universe that a deeper explanation could latch onto, something noteworthy or rare or clever or preposterous?
The fine-tuning of the universe for life is certainly a surprising fact, if the audiences I’ve addressed on the topic are any indication. As presented in Chapter 8 of “A Fortunate Universe” (AFU), fine-tuning gives us as a way of comparing naturalism (the physical universe is all that exists) and theism (God exists).
- Naturalism gives us no reason to expect physical reality to be this way, rather than some other way, especially with regards to its ultimate laws.
- Theism naturally explains (and thus, makes more probable) ultimate laws of nature that permit the existence of moral agents, such as intelligent life forms.
- The laws and constants of nature as we know them are fine-tuned – vanishingly few will produce intelligent life.
- Thus, the probability of a life-permitting universe is much greater on theism than naturalism.
The problem with this argument, as I pointed out in AFU, is that we don’t have the ultimate laws of nature (premises 1 and 2). Why should we care that the laws as we know them (premise 3) are fine-tuned?
Checking our Expectations
In science, and in everyday life, we often ask: if this idea were true, what would I expect? A geologist thinks: if this canyon was formed by erosion, would I expect such steep cliffs? A parent thinks: if my child had really cleaned their room, would I expect them to panic when I say “I’m coming in to check”?
Naturalists argue against the existence of God in this way. If we are an accidental product of blind nature, then we would expect to exist in a boring, typical, insignificant part of the universe – and here we are, third rock from an average star. We’d expect good and bad things to happen, with no rhyme or reason – and life is profoundly unfair. We’d expect natural forces to run the universe with no exceptions – and miracle claims are rare and dubious. There’s nothing special about our circumstances. So says the naturalist.
These claims are, of course, debatable. They appeal to intuition – not that there’s anything wrong with that, but we would like to do better. We would like to know: if naturalism were true, what kind of universe would be expected to exist? Would it be like this universe? This leads us to The Big Question.
Alas, the Big Question is too big. We can’t handle the set of all possible physical universes. It would involve (at least) every possible mathematical law of nature, even ones we haven’t thought of yet. We seem to have two options.
Option 1 – Abandon the project: we have no idea what kind of universe to expect on naturalism. I think this is a very bad idea for the naturalist. They would have to abandon most arguments for naturalism – they can’t say whether, on naturalism, they expect life to be insignificant, unfair or even orderly enough to do science (i.e. for the universe’s laws to be discoverable).
Option 2 – Find a smaller, answerable question: we try to find a question that reflects the big question, and that we can handle. We need a subset of the set of all possible physical universes.
Looking at the deepest laws of nature that we know, a promising candidate emerges. There are constants in the ultimate laws of nature, such as the strength of the electromagnetic force and the mass of the Higgs boson, and also the freedom to choose the initial conditions of the universe. We can thus pose The Little Question.
The Little Question is reasonable:
- It has been addressed by physicists for the last 40 years, but not for the purposes of testing naturalism or promoting theism. They were just exploring the consequences of the laws of nature.
- With some degree of confidence, we can calculate what the universe would be like with different constants and initial conditions.
- It is systematic. There is a well-defined set of possible constants and initial conditions. We aren’t just checking every possibility that we can think of.
- It reflects the best physics we have, rather than indefinitely postponing the Big Question until physics is finished.
- The probabilities we need are provided by Bayesian theory testing (or model selection). More details in this paper of mine.
- If anything, it is biased in favour of finding universes like ours. Like searching for bears starting at a place where bears were recently sighted, we are looking at other universes starting near our universe.
And the answer to the Little Question is: no. An emphatic, lingering, melodic nnnoooooo with a chuckle in the middle.
This isn’t an argument for the existence of God, yet. To do that, we’d have to answer the question: what kind of universe would we expect God to create, if God existed? That’s a topic for another day. Read AFU for a summary, and Richard Swinburne’s “The Existence of God” for a detailed argument. But, at least, we should have made the naturalist rather uncomfortable.
What about the Multiverse?
Here’s an interesting approach. What if there were a vast ensemble of universes, with varying constants and local initial conditions, of which our universe was just one? Then, the right conditions for life are likely turn up somewhere, and of course we could only observe a universe in which the right conditions prevailed.
Here’s the important question. Can we use the multiverse to attack the Big Question? Can we use physical models of a multiverse to pose a “Little Question 2.0”, which better approximates the Big Question? Can we we systematically explore an unbiased set of possible universes/multiverses?
No, for several reasons.
- There is no standard multiverse model whose parameters we can vary. We have a menagerie of models.
- The ultimate laws of nature as we know them (the standard model of particle physics, general relativity, the standard model of cosmology) were proposed to explain the data of our universe. Multiverse models were proposed with at least one eye on solving fine-tuning problems. We have studied bespoke, proof-of-concept, cherry-picked examples of multiverse models. These are not enough to tell us what we would expect of a typical multiverse. Multiverse models that rely on cosmic inflation illustrate my point. Such models must begin by proposing the existence and properties of the inflaton field, and must add by hand all the physics of varying constants.
- We have trouble handling probabilities within a given multiverse model – there is a sprawling, unkempt literature on “the measure problem”, for example. So there are dim prospects for anyone who wants to assign probabilities across broad classes of models.
In short, the multiverse – even if it exists – is irrelevant to addressing the Big Question. It might turn out that, in a systematic set of all multiverses, our universe is still not what the naturalist would expect. But we don’t know. No one knows the identity of the field that causes inflation, what its properties are, what is the possible range of those properties, what is the likely initial state of the field, what the mechanism is that varies the other constants (such as the mass of the electron and the strength of electromagnetism) across the multiverse, or what the distribution of that variation is – and that’s just for models of the multiverse that depend on inflation. Those are supposed to be the best models.
If a convincing, natural, elegant, glorious multiverse model appears tomorrow, then we’ll pose a “Little Question 2.0” properly and try to answer it. Until then, the multiverse isn’t appealing to evidence we have; it’s appealing to a theory we don’t have.