The anthropic principle simply states that there are several universal constants and that these constants take on their values according the requirement that carbon based life can evolve at some point during the universe's history. The universe must be old enough that this has already occurred.
Brandon Carter, the British Cosmologist who proposed this principle in 1976, has gone further by stating that "the Universe must have those properties which allow life to develop within it at some stage in its history."
Some apologists have interpreted this to mean that the universe has been "fine tuned" for human life by an outside intelligence.
The argument can be summed up as follows:
The probability that each of these constants has lined up in a "life friendly" way, without the intervention of an outside intelligence, is astronomically small. Astronomer and Minister Hugh Ross counts more the 100 constants at a probability of about one chance in 10 to the power of 138 against their lining up as they have in our universe. With such a low probability of a "life friendly" universe, the only reasonable explanation for our existence is that God has "fine tuned" these attributes specifically to accommodate human life.
There are two major variations on the anthropic principle: "strong" and "weak". The strong anthropic principle (SAP) can also be divided into two other variations, "participatory" and "final".
- The weak anthropic principle (WAP): "The weak anthropic principle states that in a universe that is large or infinite in space and/or in time, the conditions necessary for the development of intelligent life will be met only in certain regions that are limited in space and time. The intelligent beings in these regions should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are necessary for their existence." (Steven Hawking. A Brief History of Time)
- The strong anthropic principle (SAP): This form states that a universe "must have those properties which allow life to develop within it at some stage of its history." (John Barrow and Frank Tipler, The Anthropic Cosmological Principle)
- The participatory anthropic principle (PAP): This form states that "Observers are necessary to bring the universe into being." (John Barrow and Frank Tipler, The Anthropic Cosmological Principle)
- The final anthropic principle (FAP): This form states that intelligences must evolve within a universe and that once evolved will not die out.
The FAP has also been dubbed "the Completely Ridiculous Anthropic Principle (CRAP)" by author and skeptic Martin Gardner.
- Because we don't know of other universes with different constants, attempting to list which constants can somehow vary is little more than speculation. There is no reason to assume that any "constant" can be changed. Furthermore, assuming it is somehow a knob that can be turned by a god effectively makes the anthropic principle assume its conclusion.
- Many of the features of humanity have developed as a result of our environment, rather than our environment being tailored to suit us. Arguments from the anthropic principle tend to beg the question, assuming the existence of a creator to "explain" the already understood reasons why we match our environment so well. As Douglas Adams, the humorist and atheist, once said: "A puddle wakes up one morning and thinks: 'This is a very interesting world I find myself in. It fits me very neatly. In fact it fits me so neatly... I mean really precise isn’t it?... It must have been made to have me in it.'"
- While the odds of a universe's fundamental constants having a specific set of values may be very low, the odds of them having some value is 1.0 (100%). It may be that life exists in our universe because it happened to form, by chance, with a set of universal constants that support life. In other words, humanity exists because of a lucky roll of the dice, so to speak.
- It is possible that terrestrial life is not the only form of life possible in our universe. For example, creatures on another planet might pass on their genes via a mechanism other than the DNA double helix. In other words, it is possible that our universe's constants are friendly to a broad variety of life forms.
- Similarly, it is possible that the universe's constants could have varied quite a bit, and still allow earth-style life to form. In other words, a broad variety of universes might be friendly to life as we know it.
- The strong, participatory, and final anthropic principles presuppose that life had to exist in our universe. This is an unwarranted assumption. If our universe could not support life, it would not contain life. There is no reason to suppose that our universe was "intended" or "supposed to" contain life.
- The underlying principles of the universe are not known. Without knowing these principles, applying odds to the settings of the Universal constants is disingenuous. Since we do not know how many 'settings' are possible for each constant we cannot assign valid odds for different 'settings'.
- It can be shown that the chances of a universe having "life friendly" universal constants, high though they may be, are lower than the chances of the existence of a supernatural creator. As Michael Ikeda and Bill Jefferys point out in their paper "The Anthropic Principle Does Not Support Supernaturalism" a self referential loop occurs when a supernatural entity is assumed as a creator. Each iteration of the loop decreases the chances of a supernatural entity's involvement in the settings of the universal constant.
- It is unknown whether this is the only iteration of "The Universe". If other universes exist or if this universe has oscillated through a series of Big Bangs and Big Crunches, the universal constants may have been reset many times. Given enough universes and/or Bangs, our "life friendly" settings would inevitably occur.
- The SAP and its variants assume that human observers are required for the existence of the universe. This is a common misrepresentation of the "Copenhagen interpretation" of quantum mechanics. It is taken from the mental experiment called Schrödinger's cat. A cat is placed in a sealed box into which poison will be pumped when the nucleus of a certain atom decays. According to the Copenhagen interpretation, the atom exists as both decayed and undecayed (superpositioned) until a measurement is made. Since the atom must exist in this superpositioned state, the cat must exist in the same state until the box is opened. Note that the cat does not cease to exist, nor does the atom's nucleus. They simply exist in an unobserved state. The 'wave forms' that represent the experiment's possibilities have not collapsed into a single 'choice'. If we accept the most mystical interpretation of quantum mechanics, the universe would still exist without human intelligence. It would simply exist in an unobserved state.
- Most physicists do not accept the most mystical interpretation of quantum mechanics. Instead they view 'wave form collapse' and 'superpositioning' as an extremely useful and accurate description of poorly understood processes.
- With the SAP, apologists are positing a God of the gaps. The SAP and its variants take as fact what most cosmologists take as speculation. Cosmologists are making highly educated guesses about how the universe works. No-one currently knows how the Universe started or what underpins it. This gap in our knowledge may provide a place for a god to exist, but humankind has examined other holes into which God was supposed to have climbed. In each case we have found nothing there but nature. It is a good bet that this gap houses fascinating things, but no God.
- The Anthropic Cosmological Principle: Apologists and Homilists Beware!, by Edwin A. Karlow, PhD
- Anthropic Principle: The Design Is In The Details
- The anthropic principle and the big bang: natural or supernatural?
- The Anthropic Principle Does Not Support Supernaturalism