WHERE ARE THEY? / THE INTERSTELLAR INTERNET
by Greg Goebel
SETI experiments performed so far have not found anything that resembles an interstellar communications signal. Says Frank Drake of the SETI Institute: "All we know for
sure is that the sky is not
littered with powerful
microwave transmitters."
The great Italian physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. While faster-than-light, or "superluminal", flight is ruled out by contemporary physics, no law of physics
absolutely rules out interstellar flight at "subluminal" speeds,
though the physical requirements are formidable.
Assuming that stars are on the average about ten light-years apart; that an interstellar mission can be conducted at a speed of 10% of the speed of light; and that it takes four centuries for an interstellar colony to grow to the point where it can launch a pair of interstellar missions, then the "doubling time" of the interstellar colonies created by this advanced civilization would be 500 years. This would allow colonization of the entire Galaxy in five million years.
Even limiting an interstellar mission to 1% of the speed of light and assuming it takes a millennium for a society to get to the point where it can mount two interstellar missions, this still means the Galaxy would be completely populated in 20 million years. That is a very short interval on a cosmic time scale.
Given the lack of observable signals, as well as the lack of any persuasive evidence that extra-terrestials have ever visited this planet, Fermi's argument suggests that there is no such interstellar civilization. This depressing argument is called the "Fermi paradox".
*The fact that SETI searches have not come up with anything very interesting so far is not cause for despair. As the previous sections of this document show, trying to find another civilization in space is a difficult proposition, and we have only searched a small fraction of the entire "parameter space" of targets, frequencies, power levels, and so on.
The negative results do place limits on the proximity of certain "classes" of alien civilizations, as specified in a scheme proposed by Soviet SETI researcher Nikolai S. Kardashev in the early 1960s and later extended by Carl Sagan. In this scheme, a "Type I" civilization is one capable of using all the sunlight falling on the surface of an Earthlike planet for an interstellar signal; a "Type II" civilization is capable of harnessing the power of an entire star; and a "Type III" civilization is capable of making use of an entire Galaxy. Intermediate civilizations can be numerically defined on a logarithmic scale.
Assuming that an alien civilization is actually transmitting a signal that we could pick up, the searches so far rule out a Type I civilization within a spherical radius of 1,000 light-years, though there may be many civilizations comparable to our own within a few hundred light years that have remained undetected.
A similar analysis using the same assumption shows that there is no detectable Type II civilization in our Galaxy. In the early days of SETI, researchers assumed that such advanced civilizations were very common in our Galaxy. It is discouraging that this does not seem to be so.
However, it is important to emphasize that our SETI hunts have been based on assumptions on communications frequencies and technologies that may be laughable to alien societies, if they have the concept of humor. The lack of results do not say that alien civilizations don't exist. They only say that if they do, our most optimistic assumptions for getting in touch with them have proven unrealistic.
*There is another issue that hints as to why we don't see evidence of a large number of alien societies. That issue is time.
Our Sun is not a first-generation star. All first-generation stars are either very small and dim, or have exploded, or have burned out. This first generation synthesized the heavy elements needed to create planets and lifeforms. Later generations of stars, including our Sun, have been born and have died or will die in their turn.
Our Galaxy is more than 10 billion years old. Intelligent life and technological societies may have arisen and died out many times during this ten billion years. Assuming that an intelligent species survives for ten million years, that means that only 0.1% of all societies that have arisen during the history of our Galaxy are in existence now.
Science writer Timothy Ferris has suggested that since galactic societies are probably transitory, then if there is in fact an interstellar communications network, it consists mostly of automated systems that store the cumulative knowledge of vanished civilizations and communicate that knowledge through the Galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers".
Ferris suspects that if such an Interstellar Internet exists, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network probably still maintains some broadcast nodes in hopes of making contact with new civilizations. The Interstellar Internet may be out there, waiting for us to figure out how to link up with it.
