The Anthropic Principle

Johnjoe McFadden

The Hindu

Wednesday, Sep 14, 2005

In The Hitchhikers Guide to the Galaxy, the Deep Thought computer announced that the answer to the ultimate question of life, the universe and everything was 42. Contemporary physicists are finding themselves just as frustrated as the hyperintelligent race of beings who built Deep Thought after discovering that the ultimate answer to their questions turns out to be a set of meaningless numbers. Many scientists conclude that the answers are contingent on biology and the facts of life. Imagine if the values of one or two of nature's fundamental constants were slightly different, say the strengths of forces that hold atoms together. One consequence might be that the Earth's oceans would regularly freeze. Water — essential for life — is unique in being lighter as a solid than as a liquid. So ice sheets float and form an insulating layer that stops the deeper waters freezing. If water was more conventional then the primordial oceans would never have stayed liquid for long enough for life to evolve. But then of course we would not be here to ponder our good fortune.

This is the point of the anthropic principle, which starts from the fact of our existence and then argues backwards to claim that the precise properties of the universe that emerged from the big bang had to be those that made the eventual emergence of humans inevitable. The unique properties of water depend on an exquisite level of fine-tuning of the fundamental constants. So why are these constants just right? Because if they weren't we wouldn't be here. There are lots of other fine tunings. Carbon, also essential for life, is made in stars by the fusion of three helium atoms. It is only due to an extraordinary "coincidence" in the resonant energies of helium, beryllium, and carbon that stars make lots of carbon. Change the resonant energy by just 0.0001 per cent and no carbon. Proponents of the anthropic principle claim it is pointless looking for theoretical explanations for the precise values of the fundamental constants; they are what they are because if they weren't we wouldn't be here. Opponents claim that the principle betrays a lack of imagination for assuming that other forms of life wouldn't be possible. But this is harder to sustain when considering some of the more cosmic consequences of tweaking the constants. If the weak force that binds atomic nuclei had been just a bit weaker, all hydrogen would have turned to helium without making any of the heavier elements. If the strong force had been a bit stronger, the universe would not even have had any atoms.

New research is making even the sceptics grudgingly accept the anthropic principle. A paper by Mario Livio and Martin Rees, the astronomer royal, explores the value of the cosmological constant, a measure of how much energy is contained in empty space. Without this value being tweaked to an extraordinary level of precision, the universe would be filled only with huge black holes or entirely empty of stars. But who's doing the tweaking? Another reason scientists are wary is that it seems to reverse the Copernican revolution and place humankind at the centre of the universe. Even worse, it could allow creationists to bring the G word back into science: a God to tweak all those knobs to make life possible. But if God is needed to tweak the universe's knobs then who was there to tweak God's knobs? Physicists such as Martin Rees and Stephen Hawking prefer another scenario whereby an infinite number of universes exist, each with different values of the fundamental constants. In just a few of them the constants have taken on the right values for the creation of stars, life and evolution.

For a biologist like me the anthropic principle has a persuasive charm. Physicists have long claimed that biology reduces to chemistry and chemistry reduces to physics. But now physics reduces to biology! To explain the values of the fundamental constants physicists have to look not at their equations but at the structure of the eye or the brain or the building blocks of life. Who knows which fundamental constants take their value from the fact that fish had to propel themselves through water before evolving into intelligent tetrapods like us. Biology explains everything.

© Guardian Newspapers Limited 2004
(The writer is a professor of molecular genetics at the University of Surrey and the author of Quantum Evolution.)