The anthropic principle is astronomer Brandon Carter's 1974 response to the various anthropic coincidences in the universe. These coincidences are seemingly chance cosmic conditions that allow human life to exist in the universe. For example, the universe's electromagnetic force is 39 times stronger than gravity [source: Stenger]. If the two forces were more on par with one another, stars wouldn't burn long enough for life to develop on an orbiting planet. There are numerous anthropic coincidences involving various aspects of our universe, but they all break down to this: If X did not occur just so, then we wouldn't be here.
Carter hypothesized that anthropic coincidences are built into the universe's structure and that luck has nothing to do with it. He proposed two versions of the anthropic principle. The weak anthropic principle states that if the universe weren't compatible with human life, we wouldn't be here. The strong anthropic principle points out that because we live in a universe that can support life, only life-supporting universes are in the cards.
Some say that the anthropic principle is a sort of circular argument and even that it is not scientific because it has no predictions that can be proven false. Others have suggested that Carter was proposing the universe was sort of "custom designed" for humans and other Earth life, when the principle might have more to do with the fact that the very presence of carbon-based life forms imposed an effect on the universe that led to selection [source: San Francisco State University]. Other authors, however, have required carbon-based life as necessary to the strong anthropic principle [source: Stenger].
Until the cosmos reveals more of its secrets, scientists likely will debate principles and theories because they have yet to produce evidence for or against some of them. These advances occur often, especially with high-powered telescopes and quantum computing. In November 2011, for example, a team of astronomers made a discovery that could support part of the anthropic principle -- the possibility of changing physical constants.
If "changing constants" sounds like an oxymoron, that's part of the fun (and complexity) of quantum concepts. This discovery had to do with the fine-structure constant, which dictates exact electromagnetic force and resulting light wavelength absorption in atoms. Astrophysicists have been studying light spectrums reaching Earth through powerful telescopes for years and have noted that the fine-structure constant changes slowly through space. This could mean the place of Earth and its inhabitants in the vast universe has to do with constants that happen to be just right for our existence, in keeping with the anthropic principle [source: McAlpine]. Other independent studies will have to verify the data.
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