I wouldn't think it necessary to explain why we know that the universal constants must have been fined tuned for life to exist. After all, even Richard Dawkins acknowledges that much to be true. But if you dispute the point, I've already mentioned two books that explain the reasoning at length:
The Mind of God by Paul Davies and
Just Six Numbers by Martin Reese. A few excerpts from the first one:
[FONT="]We can write down the equations of physics and then tinker with them a bit to see what difference it makes. In this way theorists can construct artificial-model universes to test mathematically whether they can support life. Considerable effort has gone into studying this question. Most investigators conclude that the existence of complex systems, especially biological systems, is remarkably sensitive to the form of the laws of physics, and that in some cases the most minute changes to the laws are sufficient to wreck the chances of life arising.
[/FONT][FONT="]Most scientists have tacitly assumed that an approximately non-quantum (or "classical", to use the jargon) world would have emerged automatically from the big bang, even from a big bang in which quantum effects dominated. Recently, however, Hartle and Gellmann have challenged this assumption. They argue that the existence of an approximately classical world, in which well-defined material objects exist at distinct locations in space, and in which there is a well-defined concept of time, requires special cosmic initial conditions. Their calculations indicate that, for the majority of initial states, a generally classical world would not emerge.[/FONT]
[FONT="]A careful study suggests that the laws of the universe are remarkably felicitous for the emergence of richness and variety. In the case of living organisms, their existence seems to depend on a number of fortuitous coincidences that some scientists and philosophers have hailed as nothing short of astonishing.[/FONT]
Yes, yes, yes. We can imagine a bunch of universes incapable of supporting life as we know it. Any time you try to quantify the odds of those universes versus the odds of a universe capable of sustaining life as we know it, though, you're just making things up. It really not very different from saying "Why is there something instead of nothing?"
The fine tuning argument isn't new or scientific. It's just wearing a fancy hat.
That ignores a problem that I've already pointed out. Even if vast numbers of universes were springing out of nowhere, there's no reason to believe that each one would have a slightly different set of physical parameters: slightly different total mass, slightly different total energy, slightly different gravitational constant, etc...
Well where does that leave us? If the parameters are constrained (by what?) they all have to be just like this one, then. That seems bizarre, but not otherwise meaningful.
Even if we knew of some mechanism that causes universes to spring out of nowhere--which, needless to say, we do not--
Well that's not exactly true. We don't really know the universe or universes come from, and we're very much in disagreement about what the cause might be, but pretty much everyone agrees there exists or existed
something capable of spawning at least one universe.
I'm actually not entirely convinced, but I'm willing to grant the point for now.
Anyway, if we agree there exists or existed something that can create universes I find the idea it did one and then just stopped really weird.
a mechanism which produced many universes with the same properties seems much more probable than one in which the properties all vary a tiny little bit and thus allow one to escape the fine-tuning argument via multiple universes.
Then there are the two other objections that neither Dawkins nor any of his fans can answer. First, they're constantly telling me not to believe in things when there's no evidence for those things. There's certainly no evidence for any universe other than this one. No one's ever seen, heard, smelled, tasted or touched such a universe, nor any evidence originating from or otherwise suggesting the existence of such a universe. So, to be consistent with how atheists tell me to think, I have to reject any claims about multiple universes.
If we grant that at some point in the past the universe did not exist then the fact that it does now is very compelling evidence for some the existence of some mechanism capable of creating universes.
We've got no evidence it didn't peter-out after one or two, but it's got more evidence in its favor than any god. Some mechanism exists and is capable of one or more.
Really "God" in the context of the fine tuning argument is just the mechanism in question, plus a bunch of unsupported additional qualities.
Second, a vast multiverse is much more complex (by any definition) than a God capable of creating one universe. So by Dawkins' own reasoning the later is preferable to the former as an explanation for the existence of a universe capable of supporting life.
"Complex" is a tricky term, but if we stick with what it would mean when we talk about the the world around us seeming too "complex" to lack a designer a multiverse isn't more complex than a creator-god, it's just higher entropy. When people say the world is too complex to have just appeared they're not talking about the raw amount of information required to describe it, they're talking about how orderly, specific, organized, etc it is.
If instead of a bunch of discrete planets and stars our universe just consisted of a screaming, boiling mass of hydrogen a few trillion light years across that would be more "complex" than the universe we live in, in a certain sense. There'd be a lot more going on, more particles with more interactions. The universe would contain more information even if no one existed to appreciate it.
A multiverse haphazardly covering all possible configurations isn't ordered, specific, organized, or anything of the sort. It's just high-entropy. It's not more "complex" than something like a creator-god.
Like many people I find the anthropic principle utterly unsatisfactory; in fact, so does Dawkins. The fact of the matter is that we have a universe capable of supporting life; here we are, as proof of that. So it's natural for as to ask the reason for it. The anthropic principle doesn't give a reason.
We don't just have a universe with life in it. We have a universe you claim is specifically tailored to support life. The universe we see seems so perfect for the life it harbors... but of course any life-bearing universe is going to contain life that seems a pretty close match for it, and no one is ever going to observe anything different no matter how many alien configurations achieve a similar result.