I first ran into the fine-tuning argument around twenty-five years ago when I read astrophysicist Martin Rees's wonderful book Just Six Numbers, in which he looks at how a handful of fundamental constants -- the gravitational flatness of the universe, the strength of the strong nuclear force, the ratio between the strength of the electromagnetic force and the gravitational force, the number of spatial dimensions, the ratio between the rest mass energy of matter and the gravitational field energy, and the cosmological constant -- have combined to produce the universe around us. One by one, he goes through each of them, and shows that if you changed them -- some by as little as one percent in either direction -- you would have a universe profoundly hostile to life, and (in come cases) one in which matter itself wouldn't be stable.
To a lot of people, this looks very much like someone superpowerful tweaked the dials to just the right settings, so that these constants have values that allow for stable, long-lived stars, complex chemistry, and -- ultimately -- life. We don't yet know any underlying physics from which any of these could be derived; they seem to be, essentially, arbitrary. For some people, this line of reasoning ends with, "ergo... God."
Well, I have two objections, and if you're a long-time reader of Skeptophilia, you probably know what they are. First, there's that awkward little word "yet." We don't yet know if these constants are constrained -- i.e., if their values are required to be what they are by some overarching principle. There may be such a principle that we just haven't discovered, just as the properties of the elements seemed arbitrary until Mendeleev (and Bohr, de Broglie, Pauli, and others) came along and showed that there was an organizing scheme and an underlying set of physical mechanisms that made sense of it all.
My second objection is that of course we live in a universe that has properties that allow life. If the universe didn't have properties that allowed life, we wouldn't be here to ask the question. This formulation, called the Weak Anthropic Principle, more or less devolves into a tautology, a little like being puzzled about why organisms that require oxygenated air to breathe are found only in places that have oxygenated air.
The question, though, is not as facile as I'm perhaps making it sound. There are a great many seemingly arbitrary constants in physics (physicists prefer the term free parameters), such why the fundamental particles in the Standard Model of Particle Physics have the masses they do.
Also unknown is why there are three "generations" of fermions and only one of bosons, why there are four fundamental forces, and why gravitation has (again, thus far) resisted all attempts to incorporate it into a Grand Unified Theory.
Some physicists have attempted to explain this messiness by saying that this is only one universe in a multiverse, and all the other universes have different properties -- in fact, all the possible combinations of parameters exist in a universe somewhere. The problem with this is that it's an explanation that doesn't really explain anything. We have no way of detecting those other universes, so what does it even mean to say they "exist?" In my mind, this is no better than the "God-as-dial-twiddler" model. In fact, it's worse; at least in the latter, there's an entity who cares enough about us to create a relatively hospitable universe for us poor slobs who are stuck inside it.
The reason this comes up is a new paper from physicist McCullen Sandora, that I found out about from Sabine Hossenfelder's physics news YouTube channel. Called "Multiverse Predictions for Habitability: Fundamental Physics and Galactic Habitability," Sandora turns the entire discussion upside-down; instead of looking at the physical free parameters and asking why they are what they are, he asks the question, "What does the presence of life tell us about how the universe had to be?"
Sandora's intriguing conclusion is that "neat" universes -- ones with unified forces, few free parameters, and simple interactions -- are incapable of generating the complexity required for life. Hossenfelder says:
Hossenfelder herself has argued vehemently against using the criteria of "beauty" or "elegance" as the driver to find theoretical frameworks in physics; her excellent book Lost in Math: How Beauty Leads Physics Astray is one long plea to go back to an empirical basis for physics research. (An especially egregious example is the long, expensive, and fruitless quest for supersymmetry, a postulated system that argues the existence of a "supersymmetric partner" for every particle in the Standard Model; a decades-long search has turned up exactly zero of these hypothesized partners.)The surprising result is that the idea that the fundamental forces are unified do badly. Well, at least I found that surprising, but the more I thought about it the more sense it made. You see, a unified theory will in one way or another tie different parameters to each other. Then, if you vary one parameter, you break several others at the same time.
As a consequence, the more strongly different interactions are tied together, the more difficult it becomes to create life. Most universes end up either short lived, empty, or chemically boring. More flexible theories do better. Theories where parameters can vary more independently produce a larger fraction of observer-friendly universes. In other words, once you include the multiverse and selection effects, physics that is slightly messy beats physics that is mathematically elegant.
