SCIENTISTS are often impressed by how finely tuned the world is for the existence of life. If any of the world's many fundamental physical properties were slightly different, biological life would be impossible. The observation that we inhabit a world uniquely suited to us has led to the recent development of a scientific principle called the anthropic principle.
This asserts that we see the universe the way it is because we exist. There are two forms of the anthropic principle: a weak form and a strong form. Most scientists find the former a useful methodological principle. Others claim the anthropic principle is an irritating tautology and not a scientific principle at all.
The principle is lucidly described in Theories of Evervthing by J.D. Barrow (Clarendon Press, 1991).
The weak formulation recognises that human existence: requires certain conditions to be met regarding the past and present structures of the universe.
In other words, the conditions we observe in the universe must have allowed carbon dioxide based observers like ourselves to arise and evolve within a certain timeframe.
This timeframe must occur in a window of opportunity bounded at one end by the sun's death and at the other by the required presence of the elements that compose our bodies. Therefore, when intelligent observers find that the universe's conditions are just right for their existence, this is no reason for surprise or wonder.
One application of the weak anthropic principle is to "explain" why the universe is about 15 billion years old. The Earth's living systems are made from carbon based molecules in which the carbon is combined with hydrogen, nitrogen, phosphorus and oxygen.
Cosmologists tell us that in the beginning there was only hydrogen. The heavier elements in nature had to be bred in the interiors of the stars by nuclear fusion processes. At the ends of their fives, these stars explode and spew the heavier elements into space, where they become incorporated into molecules, planets and, eventually, people.
This process whereby the biological building blocks of, life are produced is long and slow. It took about 10 billion years from the big bang origin of the universe before our solar system was formed. It took over another billion years - before life began on Earth. Life subsequently evolved for about four billion years. And so this is why the universe is about 15 billion years old and about 15 billion light years in size.
The weak anthropic principle concerns itself with large scale observed properties of the universe. Strong versions of the principle attempt to account for the fundamental constants of the universe. There are many such constants, including the force of gravity, the charge on the electron and the ratio of photons (light particles) to nuclear particles (protons and neutrons).
IT HAS often been observed that the force of gravity, for example, is set at an ideal level for the existence of life. If the force of gravity were slightly higher the stars would burn more rapidly.
Consequently our sun would burn itself out in one billion years, instead of its present 10 billion year life span, and this would not allow enough time for the human species to evolve. On the other hand, if the force of gravity were slightly less than it is, the sun would burn more slowly and the temperature would be cooler.
As a result, the temperature on Earth would be too low to sustain life as we know it.
The ratio of photons to nuclear particles is also most convenient. This ratio plays a vital role in determining the range at which the cosmos expands. If the ratio were significantly higher than it is, the universe would expand so quickly that the stars and galaxies probably would not have formed, and so there would be no place for life to inhabit.
The strong form of the anthropic principle leads to the conclusion that there are many alternative universes, each with its own configuration and possibly its own laws of science. In most of these universes conditions will not be right for the development of life and so these places contain no scientists or philosophers to contemplate them and to ask questions such as: "Why is the universe the way we see it?"
Before the work of Nicolas Copernicus, mankind was assigned a position of pre eminent importance in the natural world. After Copernicus everything changed mankind was denied any special cosmological status. But now, if you accept the anthropic principle, limited status is again conferred on mankind.
Before Copernicus, the universe was thought to be centred on man. The anthropic principle would not restore that status but does accord some privilege to mankind.
It could be argued that the anthropic principle is as anthropocentric as cosmological thinking before the time of Copernicus. In its strong form, the principle seems to rule out the possibility of the existence of life that is not broadly similar to life as we know it.
This may not be sensible. After all, there are several examples of scientists credibly speculating on forms of life, radically different from life on Earth, in works of science fiction.
One could imagine such intelligent alien life forms contemplating their surroundings in other universes and devising their own anthropic principle to rule out a possibility of intelligent life in a universe such as ours because the constants of nature in our universe would not support a life form such as theirs!
It is important to point out that the anthropic principle (at least in the weak form) is not a causal explanation - it does not claim that the existence of life causes the constants of nature to have their observed values.
The principle acts more to constrain the features of the universe than to explain them. In other words, if the nature of the universe differed markedly from what we see, we would not be here to comment on it.
ALBERT EINSTEIN had the happy knack of making memorable, hence frequently quoted, comments. At one stage he said the reason he studied physics was to discover "if God had any choice in the way He made the universe".
Physicists really do not know yet if there was any choice in this matter. However, some believe that the theory of quantum mechanics leaves room for choice.
The origin of the universe involved quantum processes and such processes determine probability of events but not the events themselves.
It could therefore be argued that choices did exist and many were acted on and there may be many universes created in accordance with laws that allow for randomness in the fundamental constants. These fundamental constants may differ from universe to universe, leaving many universes unfit for life. In such a scenario the anthropic principle makes sense.
On the other hand, if the physical laws reigning at the birth of the universe allowed for only one set of values for the physical constants, then the anthropic principle would seem to be unnecessary. In this scenario the universe is absolutely determined from the start and the evolution of life is guaranteed.
Of course, there is another way of thinking that is a rival for the anthropic principle. This God principle can be stated sensibly along the lines that the reason we find the universe tailor made for our existence is because it was deliberately designed for our existence.
God created the universe the way it is as a home for intelligent life. Science can only deal with natural explanation and therefore cannot subscribe to the God principle. Religion responds to the unique fitness of the world for life with the God principle; science responds with the anthropic principle.
A cynic might remark that the anthropic principle is the closest some atheists can get to God.