Anthropic principle

The anthropic principle is a controversial cosmological principle which states that the fact that the observable universe must be compatible with our powers of observation constrains the physical laws that are possible within that universe.

Proponents of this principle suggest that the universe appears to be "fine tuned" to allow the existence of life as we know it and that if any of the basic physical constants were different that life would not be possible. Papers have been written that argue that the anthropic principle would explain the physical constants such as the fine structure constant, the number of dimensions in the universe, and the cosmological constant. Proponents of the anthropic principle point out that these constants are not at "obvious" values and suggest that there may be many alternate universes in which these constants are set at a wide range of other values and that only within a small subset of these universes is intelligence possible. Naturally, the only universe that we would have the opportunity to observe would be one of those that lie within this subset.

Although there are several versions of the principle, they generally fall into two categories:

• the "Weak Anthropic Principle," which simply states that the existence of human life places limits on the laws of physics and physical constants that we will be able to observe. In this interpretation, the only restriction placed on the observable universe is that it be capable of producing observers; many variations may be possible.

• the "Strong Anthropic Principle," which states that our observable universe must be the only type of universe capable of evolving observers.

The anthropic principle has been criticized on a number of grounds. It has been argued that the anthropic principle is not scientifically testable or falsifible. Furthermore, it has also been argued that arguments that an alternate set of physical constants would not lead to intelligent life are at best premature and at worst completely untestable. Finally, the assumption that there may be an infinite number of alternate universes which have no observers strikes some scientists as a violation of Occam's Razor.

The Anthropic Cosmological Principle

In 1986, the controversial book The Anthropic Cosmological Principle by John D. Barrow[?] and Frank J. Tipler[?] (Oxford University Press) was published. In this book Barrow, a cosmological scientist, pioneered what he called the anthropic principle in order to deal with the seemingly incredible coincidences that allow for our presence in a universe that appears to be perfectly set up for our existence. Everything from the particular energy state of the electron to the exact level of the weak nuclear force seems to be tailored for us to exist. We live in a universe of numerous variables where only a slight change would cause all reality to literally fall apart. And yet, here we are. The anthropic principle states that the reason we are here to ponder this question at all, is due to the fact that all the correct variables are in place.

Some might say that this is a circular argument or even a bit contrived, while others would contend that it is merely a statement of logic. In any case, the thesis fails to take into account the possibility that life evolved to fit the pre-existing conditions.

See also: Inverse gambler's fallacy