How can we possibly explain the fine-tuning of the universe around values of physical quantities that permit biology, when a seemingly slight deviation from those values would render the universe sterile? At a conference in 1973 to celebrate the 500th anniversary of the birth of Nicklaus Copernicus, astrophysicist Brandon Carter coined the term "anthropic principle" to refer to the anti-Copernican stance that we do live in a privileged situation in space and time. He framed a weak and a strong form of the principle. The idea connects to fine-tuning but is even more controversial, in part because it gives intelligent observers a central role in the universe, and in part because there have been so many definitions in the research literature.
The weak anthropic principle says that we can only observe a universe that has properties such that intelligent observers can exist. People are inclined to say nothing and blink when they hear this—it seems like a truism or a tautology. The strong form says that the universe (and the physical parameters on which it depends) must have values that admit the creation and existence of intelligent observers. Playwright and novelist Michael Frayne put it this way in his book The Human Touch, "It's this simple paradox. The universe is very old and very large. Humankind, by comparison, is only a tiny disturbance in one small corner of it — and a very recent one. Yet the universe is only very large and very old because we are here to say it is... And yet, of course, we all know perfectly well that it is what it is whether we are here or not."
There are even more extreme or audacious versions. A "participatory" form of the principle is motivated by physicist John Wheeler’s idea that quantum effects create a deep connection between observers and the larger space container. In effect, observers create the universe. John Barrow and Frank Tipler’s book The Cosmological Anthropic Principle ends in a flight of fancy regarding the Omega Point, when complexity and intelligence evolve to their limit: "Life will have gained control of all matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread to all spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including all bits of knowledge which it is logically possible to know. And this is the end."
When the balloon of an idea gets so big and stretches so thin, there are people happy to prick the bubble. The strong form of the principle is the most tempting to smack down, since it smacks of teleology, the idea that the universe has a purpose. The strong anthropic principle has been co-opted by supporters of creationism and intelligent design in arguments not much different from those penned by Leibnitz three hundred years ago: "Now, as there is an infinity of possible universes in the Ideas of God, and as only one of them can exist, there must be a sufficient reason for God's choice, which determines him toward one rather than another."
Another critique comes from logic. Just as we shouldn’t be surprised that the universe has conditions consistent with our existence, we shouldn’t be surprised that it doesn’t have conditions that would be inconsistent with our existence, even though those conditions might not violate certain physical theories. The anthropic principle seems to invert cause and effect. Stephen Jay Gould compared the claim that the universe is fine-tuned for the benefit of our kind of life to saying that sausages were made long and narrow so that they could fit into hotdog buns, or that ships had been invented to house barnacles. Then there’s the central role given to intelligent observers. What’s so special about intelligent observers and why should we be the template for them? There’s no generalized theory of biology, and certainly not a theory of the pathways to intelligence, that requires an essential role for carbon or long-lived stars. If life exists in a wider range of physical parameters than we imagine from our single example, fine-tuning may not be as severe as we think.