"If God exists, there is a high probability that he would create sentient beings such as ourselves. You know, because we humans are so cool, and any old god would want us to exist. If God does not exist, then there is an extremely low probability that sentient beings like ourselves would come to exist. We know this because even if the laws of physics are necessarily configured in the way they are, there are still a bunch of fundamental physical constants whose values are arbitrary. A small change in any of these physical constants is unlikely to produce humans.Let me explain first why I like this argument, and then, later, why I disagree with it.
"So, given that humans exist, the probability of God is high."
I like this argument, because it involves physics, and gets people interested in physics. Now everyone is asking what a "fundamental physical constant" is. Well, I'll tell you! Most people think of things like "the speed of light" or "the planck constant" as fundamental physical constants, but in fact they're not. Those constants depend on our choice of units; they depend on our definition of a "meter", or a "second", or a "kilogram". So they are not really fundamental. All fundamental physical constants are essentially a ratio between two things. For example, the ratio of the mass of an up quark to the mass of a down quark. Or another example, the ratio of an electron's classical orbit speed to the speed of light (this is known as the fine structure constant). By John Baez's count, there are 26 such fundamental physical constants. Most of them are related to mass in some way. Incidentally, gravity is the least-understood physical force in quantum field theory--perhaps some of the physical constants will be eliminated when we figure quantum gravity out?
Many of the counterarguments, too, discuss physics. For instance, it can be argued that many of the fundamental constants are absolutely irrelevant to life. For instance, the mass of the top quark has absolutely no effect on us. The top quark has a lifetime of 5 x 10-25 seconds, and is not a component of any known object. Creating and detecting a top quark was a task that required some giant particle accelerators and many very clever physicists. Other arguably "useless" constants include the masses of neutrinos, muons, tauons, bottom quarks, charm and strange quarks, the CKM and MNS matrices, and the cosmological constant. But that's not all of them. In any case, even if all the fundamental constants were completely irrelevant, there's still the matter of the equations which relate the constants to physical laws.
Another physics counterargument is, how can we know that there would be no life without these particular fundamental constants? If you think we can just take a new set of fundamental constants and predict the prospects of life, then you have sorely overestimated the power of physicists. A prediction so complex must be informed by experiments and empirical observations. But we don't have any of those; we can only observe and experiment on our own universe. What we can do, however, is look at a specific process which led to life. For instance, the triple-alpha process allows helium nuclei to fuse, forming carbon, which is, as far as we know, essential to life. The triple alpha process seemingly depends on a lucky coincidence: carbon-12 has a quantum energy level which happens to be at 7.65 MeV. If we changed the fundamental constants, would this lucky coincidence disappear? Would there appear any new lucky coincidences to circumvent the process? I wouldn't claim to know.
A third physics counterargument is the idea of a multiverse. If we live in a multiverse, then our universe is simply one of many universes with different fundamental physical constant. We can restate the Fine-Tuning argument as follows:
"If there is a multiverse, there is a high probability that at least one of the universes would allow for our existence. If there is only one universe, then there is an extremely low probability that sentient beings like ourselves would come to exist. We know this because even if the laws of physics are necessarily configured in the way they are, there are still a bunch of fundamental physical constants whose values are arbitrary. A small change in any of these physical constants is unlikely to produce humans.We can use the same argument for the existence of a god, or for the existence of a multiverse. So why should we prefer god over the multiverse?
"So, given that humans exist, the probability of a multiverse is high."
I must say, my physics intuition tells me that this is a terrible argument for the multiverse. The reason physicists are entertaining the possibility isn't because they just thought up the multiverse and said, "Wouldn't that be philosophically satisfying?" At least, I hope that's not how it happened. I would hope that physicists thought it up because serious cosmological theories required or suggested it. And if we wish to support the idea, we must support it with real scientific experiments or deductions, not just some rhetorical argument. A lot of this cosmological theory is still up in the air, so I wouldn't jump the gun at this time.
If I find the Fine-Tuning argument for the multiverse so unconvincing, one wonders if the Fine-Tuning argument for a god is really any better.
However, none of these are my primary disagreement with the Fine-Tuning argument. I'm a physicist in training, so I would prefer not to commit to the idea that most constants are irrelevant, or that life could exist given different constants, or that the multiverse does or does not exist. Therefore, my primary response to the Fine-Tuning argument has nearly nothing to do with physics, and instead relates to logic.
...To be continued!