Thursday, October 8, 2009

Why are we here?

What is The Anthropic Principle?
Wikipedia gives a very nice and detailed discussion.
Although proposed in 1973 by Brandon Carter, I think it obtained wider prominence (and respectability) after a 1979 Nature paper by Martin Rees (now The Astronomer Royal) and B. Carr.

The basic idea is that the fundamental physical constants of nature are "fine tuned" so that life can exist. What do we need to get lots of carbon?

Changing the ground state energies of helium, berylium, carbon, and oxygen by as little as four per cent would produce a universe with insufficient carbon and oxygen.

Carbon is produced by the simultaneous encounter of three high speed helium nuclei. This collision is very unlikely and will only produce carbon if there is a "resonance" at a particular energy. Fred Hoyle predicted this in 1953 and it was discovered later experimentally.
Although Hoyle was an atheist, he said "a superintellect has monkeyed with physics, as well as with chemistry and biology."

If the gravitational constant was slightly larger stars would burn too rapidly. If it was slightly smaller stars would not be massive enough to produce heavy elements (iron, etc.).
If the strong nuclear force constant was slightly smaller hydrogen would be the only element in the universe. If it was slightly stronger nuclear fission would occur less frequently and there would be fewer heavy elements.

If the charge on the electron was smaller atoms consisting of electrons bound to nuclei would not exist. If it was larger it would be too hard for atoms to bond together to form molecules. Without molecules there would be no biochemistry!

Is all this "fine tuning" just a coincidence?
Should we be surprised since if it were not true we would not be here to talk about it?

Suppose you face a firing squad of one hundred expert marksman and they all miss! What is your response?

This is amazing. How could such an improbable event happen?
This isn't amazing because if they did not miss I would be dead and so would not be here to think about it.


  1. So I took it the 'anthropic principle' as used by Stephen Hawking was as per your last example. ie, not that things were designed so that we could be here, but that we can only talk about it BECAUSE we are here. Is "that" the anthropic principle - ie that we are anthopocentric in our observations?

  2. This is subtle.

    My understanding is that the anthropic principle (at least in its weaker forms) is simply the observation that everything appears to be "fine tuned" to parameters values that are necessary for our existence.

    Two possible interpretations are:

    * things are designed so we can be here

    * why is this so? is not a legitimate question since if it were not so we could not talk about it.

    In the Goldilocks dilemna, Paul Davies lists other options, which are summarised on Wikipedia.