Are we alone? Humans have been endeavoring to find an answer to this question for generations. Many debates have ensued about the true answer. The optimists have generally held sway in the debate. In the 1930s, this optimism captured the physicist Enrico Fermi. 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?" If there is intelligent life out there, why we have not been contacted is a mystery. Radio astronomers have listened for radio messages and heard none. Our skies are not filled with alien visitors observing us or trying to contact us. There is no concrete evidence of "ancient astronauts" or alien visitations in earlier Earth history.
In a further demonstration of optimism, theorists have even argued that any one civilization could colonize all the habitable planets in the galaxy. Not only could it colonize the Milky Way, but it could do so on a time scale short compared to the age of the galaxy. Such a civilization could feasibly explore the galaxy without colonization by sending spacecraft to other planetary systems. These probes could then use local materials to replicate themselves and they could rapidly propagate through the galaxy, sending information back at light speed to the originators. The machines are called von Neumann probes, after a mathematics and computer science pioneer of the mid-20th century. 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.
If this were truly the case, we might expect that we have been contacted by an extraterrestrial civilization. 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 X-Files TV show 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. The belief in aliens can be equated with a modern religious metaphor. Perhaps a more interesting question than "Are we alone?" is "Why are we so lonely?&qquot;
There are other logical conclusions other than that arrived at by Fermi. One conclusion is that there are no other aliens; 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. Dolphins are probably intelligent and may be sentient, but they will never point telescopes at the stars and wonder if they are alone. 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.
Optimists in the search for extraterrestrial intelligence have an answer for the bleak scenario where 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.
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