Well, I’m probably not the best person to explain this as I’m not a proponent (I’m not saying it is wrong, just that I’m not sure if it is and am very uneasy with it to say the least). There are a few related ideas here. First, there is the way in which physics (and most sciences) have progressed in general. We delve deeper and deeper into the nature of reality, probing its structure, and determining how more and more basic elements are governed by simpler laws until we quite literally can’t go any deeper. This was reductionism. It worked really well for a while, but then we found that at the lowest levels of reality things don’t get simpler or more basic. Our best theories require us to throw in parameters derived by observation (not theory).
Enter the anthropic principle. It turns out that if these parameters were slightly altered (for some, even by unbelievably small amounts) we, life in general, and/or the universe couldn’t exist. Even the highly theoretical, mathematically derived attempts at a unified explanation of everything (like string theories) are amazingly complicated (adding to, rather than simplifying, the current model of particle physics). Because we haven’t had any success figuring out how we might conceivably be able to explain the parameters via theory, many physicists have argued that we would be better served to take the fact of our existence as the ultimate foundations from which we should make predictions. In other words, while the parameters have to be thrown in ad hoc if we stick to the reductionist, first principles approach, they are perfectly explained as well as predictable (in at least one case, actually predicted) by noting that if they differed, we wouldn’t be around to observe them.
From another perspective, we can look at the usual arguments offered for a Copernican view of the universe (there’s nothing special about Earth or us). For example, the universe has existed for billions of years and is enormously vast. Only it turns out that the universe HAD to be billions of years old for life to be able to exist at all, and it had to be enormously vast (these properties also allow a small window in which we could make the discoveries in physics we have, such as the recent observation of gravitational waves or the CMB that provided such clear empirical support for the big bang theory). We needed a long, long time for the universe to spread out, cool off, produce the basic elements required for life, solar systems with the right distributions, etc. In a very real sense, we are in an extremely special space in the universe on an extraordinarily unique planet at the perfect cosmic time.
There’s another side to the fine-tuning problem other than that many of the finely-tuned parameters are not predicted or explained by theory but derived from observation: why does it seem like so many slight changes would make complex life impossible? There is no reason that the universe couldn’t have been different than it is (we can imagine a sort of “fitness landscape” for the universe itself in which various possible universes had e.g., different fundamental forces, greater or fewer fundamental constants, etc.). Not only do so many slight changes to any single parameter in this space of possible universes result in a universe in which we couldn’t exist, for a universe to allow complex life you need all the parameters working together (like a system of equations). We would like, in true Copernican form, to suppose that there is nothing particularly special about the fundamental forces or fundamental constants or the properties of particles and so on, in that we would like to suppose that a slight change or even many changes to these wouldn’t change much. Life, for example, may not have arisen on Earth if e.g., lambda or omega were changed (or if the surface tension of water differed and other anthropic “tunings” at the chemical/molecular level), but surely there are loads of other possible universes different than our own in which we would nonetheless find intelligent life, right? It seems the answer is no. The probability of finding ourselves in the particular universe we do is astronomically small if we were to consider just the number of possible ones (not unexpected, as the probability of picking one thing out of an infinite number is almost always going to be small or zero), but more importantly it’s the only one that allows complex life. The fine-tuning problem, in this sense, is that the universe shouldn’t be “tuned” without a tuner. It’s almost as if someone set things up very, very carefully so that we could be here. There are then three general classes of approach: 1) God did it 2) There’s something wrong with the evidence or the interpretation of it 3) Take the fact that we are here as fundamental. This last one is the anthropic choice: the universe has the properties it does because we are here, and if it didn’t have the properties it does we wouldn’t be. We’re the “first principle” from which we should reason.
