The Anthropic Principle


We have not yet detected life beyond Earth; still we are confronted with the mystery of our own existence. Where did we come from?Are we alone?Why havenít we found anyone else?Is life ubiquitous or unique?Modern cosmology tells us that we live in a universe that is enormously large and old, a universe in which we are insignificant players. Can there be any meaning to life in such a cosmic setting?Some scientists have posed a response to this question by presenting an unusual idea called the anthropic principle. The anthropic principle states that the very existence of life is connected with certain fundamental attributes of the universe.


The natural world exhibits some remarkable coincidences. Miss Marples, the detective hero of Agatha Christieís novels, once said, "A coincidence is always worth noticing. You can always discard it later if it is just a coincidence." Consider three fundamental forces of nature; the strong force, the weak force, and the electromagnetic force. If the strong force that binds atomic nuclei together were only a few percent stronger, there would be no stable atoms and stars would fizz through their lives in only a few years. If only a few percent weaker and stellar fusion would be impossible. Additionally, if the weak force in atomic nuclei were much stronger, the Big Bang would have converted all the hydrogen in the universe into helium, eliminating the existence of long-lived stars and water, the solvent for life. And if this force were much weaker, the universe would not even contain hydrogen. Finally, if the electromagnetic force were stronger, stars would be reduced to a feeble level of brightness, and no elements heavier than iron could be formed. If the electromagnetic force were weaker, all stars would be very hot and short-lived, decreasing the chances of life.


The forces of nature need only slightly different properties for the universe to be drastically different.If the forces were changed, the universe would still be a sensible place governed by physical laws. But it would be a universe without biological life!Variations in virtually any of the physical constants would lead to conditions in which life would be impossible ó either there would be no long-lived stars or there would be no heavy elements. If we take a step back and look at our situation on a cosmological scale, we find ourselves in a universe with a density parameter close to one. Changes in this parameter could cause a vastly different destiny for our universe.A universe with a density parameter much greater than one would collapse into heat death on a time scale much shorter than the current age of our universe. A universe with a density parameter much less than one would expand so rapidly that stars could not form out of the thinning gas. Each of these variations does represent plausible models for a universe, but they are all examples where biological life could not form. Consequently, we should not be surprised that we live in a universe so vast and old ó if it were much different, we could not exist!


Many scientists are uncomfortable with the anthropic principle. Humans observe the universe. One way of stating the anthropic principle is that we can only observe a universe that is capable of allowing observers to exist. This is a tautology, an obvious truth. As a scientific idea, the anthropic principle is flawed because it has no predictive power. Also, it bases all of its predictions about the possibility for life on a single data point, life on Earth. We donít know for sure that all life needs heavy elements and long-lived stars.Perhaps there is only one absolute requirement for life.It is almost certain that life cannot arise from a uniform and featureless conglomeration of molecules.Therefore, a departure from thermal equilibrium may be the only requirement for life.Science is not able to explain why we exist, nor if extraterrestrial life exists. Yet we continue to search, a search that involves how we know, the physical nature of the universe, and our place within it.




The Nature of Life

Are We Alone?

Where Are They?

The Best Way to Communicate

The History of SETI