False uses of the Anthropic Principle

There are other successful arguments which have been called 'anthropic', although they have nothing to do with selection effects or the existence of life. An illustrative example is Hoyle's prediction of a certain resonance in the nuclei of carbon [61]. Hoyle argued that for life to exist there must be carbon. Carbon is indeed plentiful in our universe and must have been made either during the big bang or in stars, as these are the only ways to synthesize copious amounts of chemical elements. Detailed studies show that it could not have been made in the big bang, so it must have been made in stars. Hoyle argued that carbon could only be formed in stars if there were a certain resonant state in carbon nuclei. He communicated this prediction to a group of experimentalists who went on to find this resonant state.

The success of Hoyle's prediction is sometimes used as support for the effectiveness of the AP. However, it has nothing whatsoever to do with the existence of life because the first step of his argument is unnecessary. The fact that we - or other living things - are made of carbon is totally unnecessary to the argument. Indeed, were there intelligent life-forms which evolved without carbon chemistry, they could just as easily make Hoyle's argument.

To be clear why Hoyle's argument does not employ the AP, let us examine its logical schema and then ask which step we would have to question were the prediction falsified. The key steps in the argument are as follows.

(i) X is necessary for life to exist.

(ii) X is true about our universe.

(iii) Using the laws of physics, as presently understood, together with other observed facts Y, we deduce that if X is true of our universe, then so is Z.

(iv) We therefore predict that Z is true.

In Hoyle's case, X is the statement that our universe is full of carbon, Y is the claim that this could only be made in stars, and Z is the existence of a certain resonance in carbon.

It is clear that the prediction of Z at step (iii) in no way depends on step (i). To see this, ask how we would react if Z were found not to be true. Our only option would be to question either Y or the deduction from the presently known laws of physics of Z. We might conclude that the deduction was wrong, for example, if we made a mistake in a calculation. If no such option worked, we might have to conclude that the laws of physics have to be modified. But we would never question (i), because - while true - it plays no role in the logic of the argument leading to the prediction for Z.

There are other examples of this kind of mistaken reasoning, in which an argument promoted as 'anthropic' actually has nothing to do with the existence of life, but is instead a straightforward deduction from observed facts.

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