This is the fourth and final part of a series discussing the Fine Tuning argument (FTA). The outline is here.

Mundane multiverses

A “multiverse” is a set of multiple sub-universes, which together comprise a single super-universe. The idea is that the universe that we know is a single sub-universe, and there are multiple other universes like ours. So we can imagine another sub-universe where everything is the same, except that ever coin flip comes up on the opposite side.  Or a sub-universe where everything is the same, but we’re all evil and have goatees. Or another sub-universe where

Typically, when physicists talk about parallel sub-universes, what they mean are non-interacting sub-universes. So, you can’t ever talk to the evil goatee’d version of yourself. Although, people sure like to imagine that sort of thing in sci-fi.  So let’s talk about the kind of parallel universe that we could, in principle, interact with. What if I told you that this kind of parallel universe is one we already know exists?

To travel to a parallel universe, just hop in a space ship, and travel 4 lightyears over to Proxima Centauri. You will find a universe exactly like ours, except that the sun is different, and the planets are different, and the constellations are different.

When people talk about multiverses, they often think of the feverish dreams of theoretical physicists. Well, sure, there are all sorts of multiverse theories. But I’m here to tell you that the basic idea of a multiverse is just mundane.  It’s so mundane that you didn’t realize that you already believe in it. If you travel to another star, that’s practically a parallel universe. If you insist that parallel universes need to be non-interacting, then let’s just consider a star that’s even further away. Consider a star that’s so far away that there isn’t enough time in the universe for light to travel between us.  And now consider a star that’s twice as far. Three times as far. And so on. There are infinite parallel universes.¹

This is what Max Tegmark calls the Level 1 multiverse. You’re welcome to read that article, and decide that the other kinds of multiverses at Levels 2, 3, and 4 are too wacky for you.  But it doesn’t matter. For purposes of discussing the FTA, Level 1 is plenty sufficient.

Mundane anthropic reasoning

The FTA refers to several parameters of the universe, usually the fine structure constant, the cosmological constant, the energy density of the universe, or the energy released by hydrogen fusion. One parameter that people do not talk about, is the distance of between our planet and our sun. If the planet is too far away, it’s too cold, and if it’s too close, it’s too hot. The sweet spot in the middle is called the “circumstellar habitable zone“, and there’s a significant amount of research on the subject.

Perhaps we don’t talk about habitable zones in the context of the FTA because we can easily (well, not that easily) see other planets at various distances from other stars. If Earth hadn’t been the right distance from its own Sun, life would have simply appeared on some other planet around some other star. The distance from the Sun is not a relevant parameter, because we live in a “multiverse” where this parameter adopts different values in different locations.

Here, we’re using the (weak)² anthropic principle. If life is only possible in certain parts of the universe/multiverse, then it’s not surprising that that’s where we live. By “not surprising” I mean that it can’t be used as evidence for anything. If we do not care about habitable zones in relation to the FTA, this implies that we already accept the weak anthropic principle.

Now let us consider the hypothesis that one of the parameters of the universe changes very slowly across space.  If this were true, then we would have a mundane Level 1 multiverse, where other universes don’t just have different planets and stars, but also have different fundamental constants. If this hypothesis were true, then it wouldn’t make sense to talk about the FTA in relation to the fundamental constants of the universe.  It is precisely the same situation as with the circumstellar habitable zone.

If only certain parameters allow for the possibility of life, then it is not surprising that we live in a part of the multiverse where the parameters are just right.  And so by applying the anthropic principle, we can defeat the FTA.

The Fine-Tuning Argument in cosmology

But hold on.  When I said that the parameters of the universe change slowly over space, I didn’t offer any proof of that hypothesis.  Indeed, this hypothesis has been frequently considered by cosmologists, and they try to test the idea by, for instance looking to see if one part of the sky looks systematically different from another part of the sky.  As far as I know, none of these tests have come up positive.  That doesn’t necessarily rule out the hypothesis–maybe the parameters of the universe just change really slowly.  But still, not too promising.

One of the best arguments for a universe that changes slowly over space is, you guessed it, the Fine-Tuning Argument.

That’s right, the Fine-Tuning Argument can be applied in physics. All you do is swap out the God hypothesis for some sort of multiverse hypothesis. It works fine. In fact, it works better than it ever did for God, since we’re usually talking about a very specific multiverse hypothesis, one that generates specific probabilistic predictions.

Although, funny thing, most people still don’t consider the fine-tuning argument for a multiverse to be particularly compelling. The physicist’s perspective is: yeah you can wave around your fancy anthropic principle, we can argue back and forth over Boltzmann brains,³ but maybe that’s all bullshit. Where’s the experimental evidence? The evidence, y’ know, the evidence!

To give an example of this, let’s talk about the cosmological constant problem. The cosmological constant is about 10^-122, a very small number that is nonetheless not quite zero. According to standard quantum field theory, the magnitude of the cosmological constant should be about 1. Although maybe not exactly 1, maybe it’s anywhere between -1 and 1. People regularly use the cosmological constant problem as part of the FTA for God. However, physicists also use the cosmological constant problem to argue for for alternative theories of physics.

From the physicist’s perspective, there are two solutions: either come up with a theory that predicts a much much smaller value for the cosmological constant (e.g. light front quantization, don’t ask me what that means), or try positing a multiverse where the cosmological constant is different in different subuniverses.  The latter approach is quite interesting, and endorsed by some well-respected physicists.  You can, in fact, use this approach to predict the most likely values of the cosmological constant.  But in the end, I feel the theory is missing a certain something. Hard evidence, perhaps.

The God hypothesis, as an explanation for fine-tuning, is missing more than just hard evidence.  It’s missing all the rigor and specificity.

The takeaway is that the Fine-Tuning argument is reasonable enough in its structure, and similar arguments occasionally get used in cosmology.  However, as far as cosmology arguments go, fine-tuning-type arguments are not the most compelling tool in scientist’s toolbox.  And the god hypothesis is missing a lot of the details and math needed for it to even live up to those not-very-compelling cosmology arguments.  I give the Fine-Tuning argument some credit for being a decent effort, but it’s still not a winner.

1. This assumes the universe is infinite in extent. Note that when we talk about the size of the universe, usually what we mean is the size of the observable universe. The observable universe is just the part of the universe that is near enough that light could have travelled between us.  As far as we know, the unobserved universe is infinite in extent. At least, that’s the default hypothesis. (return)

2. There’s a distinction between the “weak” and “strong” anthropic principle. The strong version says that we should not be surprised that we live in a universe/multiverse that allows life. The weak version says that if the universe/multiverse allows life, we should not be surprised that we live in a particular time and location where life is allowed. If you accept the strong anthropic principle, that immediately defeats the Fine-Tuning argument.  But I only use the weak version, because I find the strong version to be sketchy.  The problem is that if you accept the strong anthropic principle, then it seems like you’d have to say the flatness problem and the cosmological constant problem are not really problems and that physicists who try to find solutions are wasting their time.

In the course of my research, I found that influential authors Barrow and Tipler have their own idiosyncratic interpretations of the “weak” and “strong” anthropic principles.  I consider their interpretations to be unworthy of discussion.

By the way, whenever I talk about the FTA, a lot of people like to quote some Douglas Adams at me.  While I love Douglas Adams, I think maybe the quote only applies here if you take the strong anthropic principle. At least, it’s unclear.  (return)

3. Boltzmann brains are a common limitation of arguments that use anthropic reasoning.  If a theory predicts that there are few places where intelligent life arises, anthropic reasoning means that’s not a problem.  But if the theory predicts that intelligent life is radically unlikely, eventually we have problems.  After all, there is some small chance that entropy will temporarily decrease, and that random particle collisions will spontaneously coalesce into a brain (a “Boltzmann brain”) that merely believes itself to have memories and senses.  If a theory predicts that intelligent life is even less likely than Boltzmann brains, then we have problems. (return)