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There are various forms of the Anthropic Principle, and the Weak Anthropic Principle in the version stated by Barrow and Tipler roughly says that the observed values of the physical and cosmological constants are not all equally probable, and this is partially due to only a certain range of these various constants being compatible with carbon-based life.

The assumption here seems to be that sentient life must necessarily be carbon-based. This could well be true, but how could one ever know that for sure? If it is possible that sentient life does not neecssarily have to be carbon-based, then how can we ever be sure what the real probabilities are for the observed values of the physical and cosmological constants?

Thanks.

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The assumption here seems to be that sentient life must necessarily be carbon-based. This could well be true, but how could one ever know that for sure?

We can't. This is a common misconception. The position comes from a weak understanding of biochemistry, forming the basis of carbon chauvinism. From the Wikipedia article:

In a 1999 Reason magazine article discussing the theory of a fine-tuned universe, Kenneth Silber quotes astrophysicist Victor J. Stenger using the term:

There is no good reason, says Stenger, to "assume that there's only one kind of life possible" - we know far too little about life in our own universe, let alone "other" universes, to reach such a conclusion. Stenger denounces as "carbon chauvinism" the assumption that life requires carbon; other chemical elements, such as silicon, can also form molecules of considerable complexity. Indeed, Stenger ventures, it is "molecular chauvinism" to assume that molecules are required at all; in a universe with different properties, atomic nuclei or other structures might assemble in totally unfamiliar ways.


If it is possible that sentient life does not necessarily have to be carbon-based, then how can we ever be sure what the real probabilities are for the observed values of the physical and cosmological constants?

We may never really know, since there may exist forms of "life" that is not even based on chemical bonds, but based on what we do know about chemistry and atomic bonds we can still make educated guesses. So while it is possible that non-carbon based life exists, scientists suggest that in order for life to come about using another element it must possess sufficient reorganizational complexity. This narrows it down to a lot fewer elements, and we know the quantities of these elements in various parts of the solar system, so from there were can make educated guesses about the probability of life occurring.

But yes, what you point out is exactly why the "Fine-tuned Universe" argument has never been very compelling...

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  1. The fact that life as we know is carbon-based seems to be due to some characteristics that carbon has that allows the necessary processes to take place ("That's Why Carbon Is A Tramp: Biology #1"). How can we be sure that life must be necessarily carbon-based? Well, it is hard to be sure for sure, but as a hypothesis it is consistent with our observations, and it is falsifiable. In 2010 it was thought to be discovered a form of life to use arsenic instead of phosphorus, but it is already being questioned. If we found some real evidence that there can be an alternative to carbon, our hypothesis could be falsified. There are some theoretical anternatives, but without concrete evidence and none as good as carbon, so it is our best shot so far.
  2. To talk about probabilities of cosmological constants we would have to be able to assess in some way the universe of possibilities (I'm using a frequentist probability approach here). Since we don't have (as far as we can tell) access to universes with different values for those constants, we have to rely on our mathematical models. But even using those doesn't solve the problem. Although we can think about the consequences of having different values, just knowing what the values are doesn't tell us what are their probabilities of existing. String Theory for example indicates that the values we see for the cosmological constants could be just a consequence of the characteristics of additional dimensions. But of course, that would just transpose the problem to somewhere else. Ultimately, even if there were some different values for the constants that would also allow sentient life, the anthropic principle can still be applied. It may be that it just happened we got this one.
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  • Welcome to Philosophy.SE! +1 Good answer, and thanks for the link to the TED talk. :) – stoicfury Aug 11 '12 at 15:04

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