One of the most intriguing questions in astronomy is whether or not the Earth alone harbors intelligent life. Either extraterrestrial life exists or it does not. Both possibilities have striking consequences. As American architect and designer Buck minster Fuller (1895-1983) said, "Sometimes I think we're alone. Sometimes I think we're not. In either case, the thought is staggering." The universe is a large and bountiful place. There are an enormous number of potential sites for life, and the chemical constituents of life are distributed widely across space. At first sight it appears unlikely that we should be alone. If intelligent life exists beyond the solar system, our world might well be influenced by it, for better or worse. At the very least, as anthropologist D.K. Stern has pointed out, the discovery of alien life "would irreversibly destroy our self-image as the pinnacle of creation." The alternative is equally profound. If we are the only intelligent creatures in the universe, it would imply a universe of incredible grandeur with us as the only, and quite temporary, spectators.
In the 1940s, the physicist Enrico Fermi took a back-of-the-envelope approach to this problem. He was heavily persuaded by the argument that the vast number of stars in the universe must imply that some of them must host planets with intelligent life. This line of reasoning led Fermi to ask the question "Where are they?" He estimated an advanced society could populate the entire Milky Way in just 5 to 50 million years. If there is intelligent life out there, why have we not been contacted? He considered how radio astronomers have listened for radio messages and heard none and how our skies are not filled with alien visitors observing us or trying to contact us. Looking around, all he saw was a lack of evidence for aliens in a galaxy that potentially could be as fully colonized as the land masses of our own planet Earth. Even looking at history, there is no concrete evidence of "ancient astronauts" or alien visitations in earlier Earth history.
Fermi and others have postulated that an advanced race could colonize the Milky Way on a timescale that is short compared to the age of the galaxy. Such a civilization could feasibly even explore the galaxy without biological colonization by sending unmanned (unaliened?) spacecraft to other planetary systems. These probes could then use local materials to replicate themselves and they could rapidly propagate through the galaxy, carrying out a technological colonization while sending information back at light speed to the originators. If this sounds far-fetched, remember that such a scenario requires only a modest extrapolation of our current technology; we are probably less than 100 years from robots that can construct other robots and propulsion systems that can reach a tenth the speed of light in space. Already, the Japanese are looking to build a fully robotic base on the Moon.
There are several logical answers to Fermi's "Where are they?" question. One possibility is that there simply isn't intelligent life out there; intelligent life is a unique consequence of random events in the universe. Alternatively, the universe could be heavily populated, but the nearest civilizations are in distant galaxies. Their radio messages would be million of years old by the time they reached Earth. Spaceships would be unlikely to reach Earth if limited to speeds less than that of light, as current physics demands. It is entirely consistent with our current knowledge to propose that simple microbial life is quite common in the universe, but that intelligence and technology are extremely rare.
It is also possible that space exploration and the desire for communication are uniquely human characteristics. After all, it can be argued that there is more than one intelligent species on our planet but that the technology and desire to explore and communicate are unique to humans. Technology is not even universal among human societies. Are humans fated to be explorers, bridge-builders, and scientists rather than artists, athletes, or daydreamers? Is the stereotyped aggressive Westerner more representative of the essence of humanity than the stereotyped contemplative Easterner? Our technocracy may be just one type of cultural activity rather than a natural consequence of biological evolution. Historically, patterns we once assumed to be results of our biology have turned out to be consequences of cultural influences (confusion between these two has led to racist and sexist biases that we are still trying to overcome). We have all been influenced by the appealing image from science fiction of space exploration and communication as a universal cultural activity. In fact, it is absurdly anthropocentric to suppose that beings on other planets would resemble us physically, psychologically, or socially.
Possibly the most significant answer to the question "where are they?" is that we may be separated from aliens more by evolutionary time than by actual physical space. Let us conduct a hypothetical experiment. Imagine a planetary twin of Earth that started evolving at exactly the same time. Even if life on both planets chose similar biochemical pathways, organisms on the two planets are likely to be out of evolutionary phase with one another. Just a slight difference in temperature between the planets could cause the evolutionary "clocks" on the two planets to be out of synch. If even 1 percent out of synch, life on our twin would be 40 million years behind or ahead of us. This is as far from us evolutionarily as we are from early mammals. If the clocks differ by 10% to 15%, then we are talking about the enormous evolutionary difference between single-celled microorganisms and ourselves! The timing argument carries one important consequence. We have had the capability for interstellar communication for only 50 years, which is a mere instant in evolutionary and cosmic time. Due to the time it takes signals to travel through interstellar space, it is likely that any civilization with which we could make contact will be far more advanced than us, by thousands or perhaps millions of years. Even if we were at the same evolutionary stage as our twin, the large distances coupled with anticipated lifetime of civilizations (the oldest civilizations on Earth have lasted only thousands of years) makes the idea of communication futile.
These aren't answers that satisfy everyone. Polls in the United States show that the public has an enormous susceptibility to the idea of alien visitation. Even without convincing evidence, over 50 percent of the population believes that we have already made contact. This isn't so surprising given that aliens have been in the popular culture for over 100 years, beginning with the influential H.G. Wells book War of the Worlds, and progressing to more recent phenomena like the TV shows X-Files V, and the entire Stargate franchise and movies like Men in Black. Long a staple of science fiction, alien life forms took a big step into the public consciousness through the powerful media of TV and film. As examples, consider the rise in popularity of the TV show Star Trek and the Star Wars movie series from small cults to the level of widespread cultural phenomena.
Popular culture paints two opposing versions of the alien myth. The first is the optimistic view, expressed by movies such as E.T., Close Encounters of the Third Kind, and more recently by Contact. This view demonstrates our desire for knowledge, camaraderie in a vast universe, and even salvation. Second is the pessimistic view, seen in movies such as Alien and Independence Day, which places us at jeopardy in a universe of superior life forms. That the later is more likely the truth is a concern of scientists as eminent as Steven Hawkings.
Despite our fears, we do continue to look. The search for alien life is a new scientific adventure in the making. Astronomers continue to study the possibilities of life beyond the Earth as a direct way of questioning our role in the universe. Admittedly, discussions of the existence, intelligence, psychology, or appearance of higher alien life forms are almost highly speculative. But an interdisciplinary group of researchers has begun to take these issues very seriously. Astronomers have detected planets in orbit around nearby stars, and modern observatories like Spitzer can actually measure the chemical composition of the some of these planets atmospheres. While we haven't yet discovered any habitable worlds, they have also found complex, carbon-based molecules in interstellar space and in meteorites. This confirms that complex chemistry is active beyond our own solar system. Chemists have studied the pathways by which replicating molecules and simple life forms can be synthesized from simple constituents. Evolutionary biologists have considered how life might evolve in complexity starting with single-celled organisms. Physicists have even calculated the rates at which interstellar cultures might populate the galaxy. Together, all these scientists have worked to predict what atmospheric clues life might create. It just may be that intelligent life is found first by how it pollutes chemically rather then how it transmits radio signals!
All this work feels like progress, but it still doesn't answer "Where are they?"
Optimists in the search for extraterrestrial intelligence have one additional, hopeful, answer for the bleak scenario that intelligent life is rare in the galaxy. Rather than use signals for communication from point to point, alien civilizations might use signals to store information in a vast, cosmic internet. If one civilization succeeded in sending probes across the galaxy, they could use these probes to set up a communication network across the galaxy. These artificial stations could beam information at planets, collect information from planets with intelligent life forms, and store the information in a growing database. Each newly intelligent civilization would not have to wait thousands of years of light travel time to communicate across the galaxy; they would only have to wait the much shorter time to tap into the nearest node of the network. The information in this cosmic internet would survive the death of any civilization and act as a kind of galactic consciousness.
All of these are hypothetical ideas, but we should recall the words of physicist Freeman Dyson: "Nature always has more imagination than us." We are a young intelligent species, peering into the vastness of our cosmic environment with the naivety of a newborn. We may be surprised at what we find.