The Possibilities of a SignalThese and similar arguments have been extensively discussed in the SETI literature. However, rather than dwell on evolution’s likely products, allow me to argue the case for detecting intelligence by pointing out that the universe offers many possibilities for providing us with a signal - more possibilities than at first may be apparent. First off, I would like to note that the usual view of aliens is highly conservative. When we consider the cut of ET’s jib, we generally incline to the models offered to us by Hollywood. Cinema aliens are highly anthropomorphic (as director Frank Capra once observed, “people are most interested in people”) and not very diverse. Movie aliens are like movie cowboys: their hats come in only two colors. There are good aliens, such as those in “Close Encounters of the Third Kind” and “E.T.” These are more than anthropomorphic. They look like children, and are clearly harmless. Black-hatted aliens, on the other hand, such as those in “Independence Day,” are modeled after our natural enemies: carnivores, snakes, insects, etc. These fictional aliens, both good and bad, populate our collective unconscious. They are what we expect to be keying the transmitter at the other end of a SETI signal. Their general properties are as follows:
That may be what we expect. But what is it that our search technology can actually find? What we are looking for are narrow-band, microwave signals - a type of emission that (as far as we know) is not produced naturally. So the requirements for SETI to succeed are (a) the existence of technology, i.e. a transmitter at the other end, and (b) most probably a deliberately broadcast signal designed to get our attention. This latter requirement derives from the fact that advanced civilizations may be relatively “radio quiet,” relying on low power (think of cellular phones) or highly directed (optical) communications. Consequently, they will not be leaking high-powered emissions into space. Furthermore, in order to inexpensively deliver an easily detected signal at interstellar distances, they will need to use reasonably large-sized transmitting antennas. Such antennas are highly directional (typical gains are 105 to 107 for terrestrial equipment). The picture we generally have of aliens, and one whose probability Professor DeVore has addressed, is hardly either described or proscribed by these simple technical requirements. There are other possibilities. So I hope you will let me (at least momentarily and in a spirit of general amity) to grant DeVore’s premise. Let us assume that high biological IQ is rare. I will continue to maintain that SETI could still find the universe rife with intelligence - with sentient entities. The argument requires letting your imagination run free. Think in terms of deep space, but also deep time. It is worth keeping in mind that the number of stars in the visible universe is ~1022, or comparable to the number of grains of sand on all the beaches of Earth. Approximately 10% of these resemble our sun. In addition, evidence for planets around sun-like stars is growing. At least 3-5% of the stars examined for extrasolar planets are found to have them. This implies the existence of billions of planetary systems in the Milky Way alone. (Within the visible universe, the number is approximately 100 billion times larger.) Finally, note that the universe is two to three times the age of Earth. We are the new kids on the block. It may be that the evolution of intelligence is unlikely. In an infinite universe, that wouldn’t matter. In a finite, but breathtakingly large universe - an old universe of 1022 stars - it probably doesn’t matter.
Contributed by: Dr. Seth Shostak |
