The New York Times
It was at lunch in Los Alamos, N.M., in 1950 that Enrico Fermi, best known for building the first atomic reactor, asked the question that has haunted those who like to wonder about other life in the universe ever since.
Where is everybody?
In the half-century since Fermi posed the question, spacecraft have visited or inspected every planet but Pluto and all the major moons in the solar system, but have yet to find a sign of life. At the other end of the evolutionary scale, astronomers have examined thousands of stars for radio signals from extraterrestrials. So far, the silence is deafening.
Admittedly, those few thousand stars and one solar system represent only the barest trace of the potential riches of the galaxy, which holds some 200 billion stars. Depending on how plentiful Earthlike planets are and how likely the evolution of life and intelligence is, astronomers have argued that there could be millions of civilizations in the galaxy, or only one.
But if alien civilizations are plentiful and interstellar space travel is even vaguely attractive to them, Fermi's question has to be answered.
The argument behind it goes like this: Aliens could spread throughout the Milky Way in far less than the 10 billion years that the galaxy has been around, even if they travel at far less than the speed of light, Einstein's cosmic speed limit.
Suppose, for example, that the first alien starship that sets out for another star takes a million years to arrive there and become established. Then the new colony and the original civilization each send a spaceship on a similar voyage, and so forth, so that a wave of new colonies is formed, doubling the total number every million years.
After 10 million years, there will be 1,023 alien settlements, plus the original. After 20 million years, there will be a million. After 40 million years, if they keep it up, there would be a trillion -- more than there are stars in the galaxy.
By then, after 10 billion years, if there were more than one spacefaring civilization in the galaxy, they would be tripping over one another or one another's artifacts. But in fact there seems to be nothing.
If the logic of Fermi's argument is right, it will spell doom and frustration for astronomers' efforts to detect radio transmissions from distant societies, the project known as SETI, or the Search for Extraterrestrial Intelligence, because the presumed fact that they are not here means they are not there.
SETI enthusiasts concede that the paradox is not easy to get around. Perhaps interstellar travel is just too daunting and expensive, some say. But as Dr. Frank J. Tipler, a physicist at Tulane University, has pointed out, the aliens do not have to send themselves; they could send self-reproducing robots.
Moreover, California, and the islands of the Pacific before it, were settled by people who were willing to pack up their families, turn their backs on the known world and never look back. The scattering of humans over the whole earth from our origins in Africa is testimony to the ability of our own race at least, to keep on moving for a long time.
So where are they?
One answer is the imaginative and humbling-sounding "zoo hypothesis." It says that aliens are in fact here, or at least watching us, but are pledged to not interfere. It would mean that we are being treated more ethically than we have traditionally treated animals and indigenous cultures here on Earth.
Of course, "they" could be here, and we might not recognize them, any more than ants would recognize the Mona Lisa or, say, the Internet as artifacts of superior beings.
The discovery of even a lowly fungus in the shade of some foreign sun would rank as a landmark in the history not only of science, but also of humanity. The National Aeronautics and Space Administration has lately made the search for signs of extraterrestrial life, or of likely abodes for it, a centerpiece mission. But that mission, as astrobiologists concede, is hostage to our concepts of life and intelligence. We are good at looking for things like ourselves.
Still another answer is that even robot proxies are more expensive than electromagnetic waves and that we simply have not yet tapped into the galactic library, the cosmic Internet.
"Absence of evidence," as SETI astronomers like to say, "is not evidence of absence." The search has barely begun, and the scientific thing to do, they say, is to keep searching, expecting answers, trusting that we know the questions.