anthropic principle

Okay, I admit that discussion of the anthropic principle does not count as research (see rules at right). But really, the people who use the anthropic principle are confused! I agree that you can't observe a universe that cannot contain observers. No duh! But a universe that contains observers arbitrarily different from us is observable. Recall: The anthropic principle is invoked to reduce the range of possible predictions for the properties of the Universe from the vast range allowed by, eg, string theory. You can't use observations of our Universe (such as that it contains galaxies and carbon-based life) to trim down the possible fundamental predictions, because then they aren't predictions. You must only use the fact that the Universe is observed, which requires only observers, not carbon-based observers, not observers in galaxies, not observers around stars, etc. Interestingly, many people who have worked on this problem for many years, including Susskind and Vilenkin (who spoke today at NYU) have got this wrong, and they call the requirement that there be galaxies the anthropic principle.

The fact that the Universe contains galaxies is an observation; the requirement that it is in principle observable is properly the anthropic principle. Of course we don't know how to compute observability, given a stated theory (indeed, we cannot even predict the proton from the standard model, let alone observers!). Until observers are predictable, the anthropic principle is not useable for science, in practice.


  1. But really, the people who use the anthropic principle are confused!

    Yes, but no less than you are... ;)

    The evolutionary physics that derives the anthropic coincidences, (the "Goldilocks Enigma"), that have unfolded over the entire history of the univesre, is very specifically oriented toward the development of "sites" that where carbon-based life can arise and evolve.

    This most certainly does apply to galaxies, planets and even our local ecobalance.

  2. I'd agree with the posting above. If you don't have galaxy formation then you can't have life. A mass of ever-expanding hydrogen would not allow life to evolve. Neither would a universe that just rapidly collapsed in on itself. There does appear to be situations where no life could ever exist.

    The Anthropic Principle has moved on a bit from your impression. It now generally refers to the "multiverse" principle - many universes with the physical constants set differently in each universe. The fact that they are amenable for life in our universe is then just a selection effect.

    For more about this, see the Anthropic Principle page on my blog: http://www.ipod.org.uk/reality/