The contention that life exists elsewhere holds a mystery. If ten million planets in our galaxy alone are likely to support life, why have we not heard from any of it by now? It has been estimated that about a thousand of these planets would be supporting life that has evolved to the state of communicating by radio transmissions. Why has the SETI Institute (Search for Extraterrestrial Intelligence) not yet detected intelligence-bearing signals?
Moreover, why have we not found unquestionable evidence that we have been visited by aliens, or by one of their devices? We have already sent probes far beyond our own solar system, and within a decade or two will likely send similar instruments to exoplanets orbiting neighbouring stars. Indeed, in less than a couple of generations we may be visiting these planets ourselves. (Current technology necessitates journeys limited to a dozen years or so, but ion-accelerated drives already in use promise an ability to travel great distances in that amount of time.) This being so, we will inevitably attempt to colonize any habitable exoplanets we find, and, from those bases, we will certainly move onwards and outwards. All intelligent life, needing to replenish the resources it consumes throughout its industrial development on its home planet, will want to do the same. A few plausible assumptions and a couple of calculations suggest that any such pioneering life form will have colonized the whole of its own galaxy within a period of five to fifty million years.
This may seem a long time relative to our life span, but it is minuscule in evolutionary terms (we have to go back farther than that to find living dinosaurs). And it is an almost infinitesimal period on the galactic time scale. Life on other planets could well have begun a billion or more years before it arose on ours. Why, then, have we not been colonized by any of the intelligent civilizations that should have been able to do so in the past hundreds of million years? Or, why have we not at least been visited by some form of von Neumann probe (see The Expanding Universe, footnote 7 for a little more information about von Neumann probes) during that time?
Crawford suggests that the answer to this puzzle (called the “Fermi Paradox”) could be that, although the formation and evolution of life may indeed be universal, its subsequent development into intelligent life forms may be rare. An alternative explanation could be that von Neumann probes have already visited, or even now be present. If they were small enough—developed by aliens using nanotechnology, perhaps—we would not have noticed them.
However, the explanation may be even simpler. Perhaps life forms intelligent enough to use von Neumann probes, or to travel and colonize hospitable planets, avoid planets whose “intelligent” inhabitants are constantly at war among themselves. Landing would certainly lead to a transfer of technology, and would eventually equalize abilities of the two life forms. Far easier for them to wait to see if we mature to a stage where we no longer make war; seeking contact before then only invites trouble and is simply not what an intelligent being would do.
- The pyramids, Nazca lines, Stonehenge, crop circles and other occurrences have all been suggested as being possible evidence of alien visitation. However, all can be more credibly explained as being due to human effort.
- Ian Crawford, “Where are they?” Scientific American, July 2000, 38-43.
See also Peter D. Ward and Donald Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe (New York: Copernicus, 2000).
For the opposite view, read Amir D. Aczel, Probability 1: Why there must be Intelligent Life in the Universe (New York: Harcourt Brace & Company, 1998).