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. Physicists are finding themselves just as frustrated as the super-intelligent beings who built Deep Thought after discovering that the ultimate answer to their questions is a set of meaningless numbers. Many have concluded the answers are contingent on biology and the facts of life.
Imagine if one or two of nature’s fundamental constants were slightly different, say, the strength 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, the primordial oceans would never have stayed liquid 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 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% and no carbon.
Proponents of the anthropic principle claim it is pointless trying to explain 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 unimaginatively assumes other forms of life wouldn’t be possible. But this is harder to sustain when considering some of the cosmic consequences of tweaking the constants. If the weak force binding atomic nuclei had been just a bit weaker, all hydrogen would have turned to helium without making heavier elements. If the strong force had been a bit stronger, the universe would not have had any atoms.
New research is making even the sceptics grudgingly accept the anthro-pic principle. A paper by Mario Livio and Martin Rees, the astronomer royal, explores the value of the cosmological constant, which measures how much energy empty space contains. Without this value being tweaked to extraordinary precision, the universe would be filled only with huge black holes, or be entirely empty of stars.
But who’s doing the tweaking? Another reason scientists are wary is that the principle seems to reverse the Copernican revolution and place humankind at the centre of the universe. 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, who tweaked God’s knobs? Physicists likes Rees and Stephen Hawking prefer another scenario whereby an infinite number of universes exist, each with different fundamental constants. In a few, the constants have the right values for the creation of stars, life and evolution.
For a biologist such as myself, the anthropic principle has a persuasive charm. Physicists have long claimed that biology reduces to chemistry and chemistry to physics. But now physics reduces to biology! To explain the values of the fundamental constants, physicists have to look at the structure of the eye or 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. — Â