feature while maintaining the others, it was straightforward to compare their effects. The results were undeniable, setting aside years of controversy. The team estimated the typical ratio of intergalactic light with expansion to be only about one-half of that without it. Given the flood of energy produced by all the galaxies, the expansion of the universe thereby made little difference. Rather, the age factor produced by far the greatest impact, robbing the heavens of the vast majority of its potential brightness. Hence, even in a static universe, the sky would be plenty dark.

This experiment resolved Olbers’ paradox once and for all: The night sky is dark because the universe is still young, not because it is expanding. So the next time you stub your toe in the dark of night, you can justifiably blame it on the Big Bang.


Not only is space black, it is also silent. Surrounded by endless stellar reaches, Earth seems a lonely outpost devoid of communications from any other world. Given the likelihood of planetary systems orbiting many (if not most) of the stars we see, why hasn’t a single one of them sent us a simple hello? Could space be as empty of life as it is of light, or might there be another explanation?

The great Italian physicist Enrico Fermi pondered this dilemma in 1950 while taking a break from the rigors of Los Alamos Laboratory. It was the era of the UFO craze, and newspapers were brimming with speculations about the prospects for alien visitors. A clever cartoon about the subject caught his eye and led him to estimate the probability of extraterrestrial contact. During a casual lunch, he raised the topic with three of his colleagues—Edward Teller, Herbert York, and Emil Konopinski. While discussing sundry matters, Fermi suddenly asked, “Where is everybody?”

Fermi’s lunchtime companions knew him as a man of deep thought and did not take his question lightly. As an expert in seat-of-the-pants calculations, he was adept at ruling in or out various

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement