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The Universe seems to be 'fine-tuned' (for life), because any small changes in the 20 or so physical constants would make it very different, and presumably not hospitable to life. For example, stars would not be able to fuse hydrogen and helium if the electric charge of the electron had been only slightly different, or if the strength of the strong nuclear force had been only 2% greater diprotons would be stable and hydrogen would fuse too easily.
Another related point is that the early universe had a very low entropy, a very unlikely situation: statistical mechanics says that the most probable state of matter has a very high entropy, a state where life is not possible. The universe is now going towards that state, according to the second law of thermodynamics, resulting eventually in heat death. This paradox can be treated in the same way as fine-tuning.
Does "fine-tuning" need an explanation?
Some, like Stephen Jay Gould, believe that fine-tuning does not need any more explanation than a particular roll of dice that would result in a double six. Our universe had to have physical constants, and they just happen to be the ones that permit our existence. Moreover, any other combinations of physical constants may have resulted in a very different universe, one in which a human-like civilization may not emerge, but who is to say that it would not be hospitable to other, very different, forms of intelligent life?
What are the possible explanations?
There are actually three broad types of explanations: the universe is not finely tuned, the multiverse, and the intelligent designer. While the latter two are not incompatible with each other, accepting one of them makes the other one less necessary.
- The universe is not finely tuned explanation questions whether the universe is in fact finely tuned. The fact that a universe with different physical constants might be inhospitable to life as we know it does not necessarily mean that it is inhospitable to any form of life, and there is no known way of actually experimentally verifying whether a universe allows for life or not. Further, the overwhelming majority of this universe, especially the interstellar vacuum, appears to be devoid of life; other physical constants may exist that allow a much greater density of life than in this universe. The apparent rarity of life in our universe is, however, evidence that life does indeed require 'finely tuned' conditions for existence.
- The multiverse explanation assumes the existence of a mechanism that has created many universes with different physical constants, some of which are hospitable to intelligent life. And because we are intelligent beings, we would happen to be in an hospitable one. This approach has led to a lot of research into the anthropic principle. This explanation has been of particular interest to particle physicists because theories of everything do apparently generate large numbers of universes in which the physical constants are random.
- The intelligent design explanation assumes the existence of a being, principle or mechanism with the purpose to create a universe with intelligent life. It would thus somehow set the physical constants as we know them. This agent could be a God-like being.
A probabilistic discussion by mathematician Michael Ikeda and astronomer Bill Jefferys argues that the traditional reasoning about intelligent design from the presence of fine-tuning does not properly condition on the existence of life, and is also based on a incorrect reversal of conditional probabilities (in an example of the prosecutor's fallacy), which in this form erroneously claims that if fine-tuning is rare in naturalistic universes, then a fine-tuned universe is unlikely to be naturalistic. (In this context, "naturalistic" is taken to be synonymous with "not intelligently designed".)
They offer a proof that indicates one should in fact draw the opposite conclusion: the presence of fine-tuning actually argues against intelligent design. Their main theorem — under the assumptions that
- our universe exists and contains life (L),
- our universe is "life friendly" (F), and
- life can exist in a "naturalistic" (N) universe only if that universe is "life-friendly" (N&L ⇒ F), while life might exist in a "designed" universe even if it is not "life friendly"
states that, given our universe contains life (L), the probability that our universe is naturalistic, P(N|L), is less than the probability that our universe is naturalistic given that it is also fine-tuned, P(N|L&F) = P((N|L) | F). (That is, adding the assumption of fine tuning increases the probability that our universe is naturalistic, given that we know our universe contains life.) Thus, they argue ironically, supporters of intelligent design should try to prove that our universe is not fine-tuned. The philosopher of science Elliott Sober makes a similar argument.
- John Barrow (2003) The Constants of Nature, Pantheon Books, ISBN 0375422218
- Nick Bostrom (2002) Anthropic Bias: Observation Selection Effects in Science and Philosophy, Routledge, New York, ISBN 0415938589
- Design and the Anthropic Principle by Hugh Ross
- The Anthropic Principle Does Not Support Supernaturalism by Michael Ikeda and Bill Jefferys
- The Design Argument by Elliott Sober (Adobe pdf format)
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