Copyright © 1983, by David Brin. All rights reserved. No duplication or resale without permission.

The following article was first published in the 1980s in Analog Magazine as a popular adaptation of the much deeper and more scholarly 'classic' review of the field -- The Great Silence -- which appeared in the Quarterly Journal of Royal Astronomical Society, fall 1983, v.24, pp 283-309, which is also available on this site.

We'll begin a "morphological" analysis of the Great Silence by presenting the following list of possibilities.

1. Solitude -- We are unique in evolving technological intelligence.

   This hypothesis implies something is very wrong with current use of the Drake Equation. Habitable planets may be rare, or some "spark" may be needed to initiate life out of a prebiotic soup.

   The final step to intelligence may require some software miracle that makes it far more improbable than currently thought.

   Alternatively, the last term in the Drake Equation -- the average life span of technological species may be on the order of decades. This might be due to some "inevitability" of self destruction, or due to the "Deadly Probes" of Saberhagen and Benford.

2. "Magical" Technology -- It may be that technological species soon discover techniques that make radio and even colonization irrelevant. We may be on the verge of such discoveries right now, though it's hard to imagine any race totally abandoning the electromagnetic spectrum, whatever its other options.

3. Quarantine -- The hypothesis of the purposeful avoidance of contact.

   This is an idea long popular in science fiction. It explains the Great Silence by suggesting that the solar system is kept as a "zoo." Or benevolent species might want to let Nursery Worlds lie fallow for long periods, to nurture new sentience.

   Related ideas are that observers are awaiting mankind's social maturity or have quarantined us as dangerous, perhaps infected.

   Kuiper and Morris have also suggested that neighbors of a galactic radio club would not contact "beginners" because this would wreck our usefulness as members of the network. Making us information consumers too early would spoil is as information providers, whose unique experience would add richness to galactic culture.

   ETIs may visit the solar system for reasons having nothing to do with us.

   A problem with "Quarantine" is the galaxy's differential rotation. Our neighbors don't remain our neighbors. If during one epoch we live near environmentalists, ten million years later our sun could enter the domain of a less scrupulous race. The quarantine hypothesis appears to call for some degree of cultural uniformity in the galaxy... hard to accomplish in a relativistic universe.

4. Macrolife -- The abandonment of planet-dwelling as a lifestyle.

   Expansion will generally come from those colony worlds most recently settled. There might be a great selective process favoring those individuals suited to living in starships. One can imagine the pioneers eventually deciding that planet-bound existence is filthy and degrading. This might result in either of two different behaviors, each compatible with the Great Silence. Truly space-borne sophonts might greedily fragment terrestroid planets for building material and volatiles, leading to disastrous versions of "solitude" and "low rent" (see below), or they might cherish Nursery Worlds for what they are and protect them as in option "quarantine," without any conflict of interest or desire to use high-gravity real estate.

5. "Seniors Only" -- More alternate lifestyles.

   It's often suggested that spacefaring sophonts might "graduate" to other interests after a reasonable time. This would set a limit to the period of expansion, though not, perhaps, to exploration.

   Discovery of immortality could tend to promote conservatism, and an aversion to the dangers of spaceflight.

6. "Low Rent" -- Earth is inaccessible or undesirable.

   Spacefaring sophonts that otherwise had the means might choose to bypass Earth. A few possibilities to consider are the following:

   The one technique for travel faster than light (FTL) which has drawn some support from the physics community has been "geometro-dynamic" -- via controlled entry into the zone of influence of a black hole and traversing space-time through hyperdimensional shortcuts. If such a version of FTL travel were possible, convenient, and efficient, one might expect galactic civilization to cluster around entry and exit points, Long-range slowboat technology would languish.

   The fact, then, that astronomers have observed no nearby black holes may be a manifestation of the so-called Anthropic Principle. If a "usable" black hole were closer, the Earth would already have been settled, an ecological holocaust would have ensued, and we would not exist to observe the black hole. This the fact that we are here is consistent with a failure to observe nearby black holes.

   Another systematic effect that might make for periods of inaccessibility is the migration of the Sun around the center of the galaxy. We are currently on our way out of a gas-and-dust-rich spiral arm. In a few million years the Sun will be in an "open" area, where there are few bright, younger stars. Spiral arms are home to the dense interstellar hydrogen clouds. These are thought required to run Bussard ramscoops, but today that particular type of vehicle is falling into some disrepute. Besides, the clouds might also be hazards to other forms of travel.

   Earth life-forms rely almost totally on the left-handed isomers of complex organic proteins and amino acids. This might not be the case elsewhere. Should "dextro-" life dominate everywhere else, we might find Earth systematically avoided because there would be nothing here for prospective settlers to eat!

   These are just a few examples of an endless supply.

7. Migration Holocaust -- This category has received the most attention in this article. Transient occupation of a Nursery World by a techno-culture might cause extinction of local higher life-forms, delaying the local upsurge of intelligence and resulting in a neighborhood so depleted that we are the first to recover in the nearby area.

The quandary of the Great Silence gives the infant study of xenology its first traumatic struggle: between those who seek optimistic excuses for the apparent absence of sentient neighbors and those who enthusiastically accept the silence as evidence for humanity's isolation in an open frontier.

As humanity grows up, we're finding out just how complicated the universe can be. We've seen that "Galactic Empires" have implications far beyond anything considered even by the science fiction of the past. The universe has many more ways of being nasty, if it so chooses, than we had thought.

Opportunities do not, however, have to be taken up. While the author doesn't accept that elder species will necessarily be wiser and more restrained than contemporary humanity, he does suggest, and hope, that such noble races do crop up from time to time. If such a culture lived long, and retained much of the strength and vigor of youth, it might have taught a tradition of respect for the hidden potential of Life to all subsequent spacefaring species.

It might turn out that the Great Silence we're experiencing is like that of a child's nursery, wherein adults speak softly, lest they disturb the infant's extravagant and colorful time of dreaming.

On Interstellar Travel Technology:

Bracewell, R. N., Nature (London) 186 (1960): 670.

Forward, Robert. "Interstellar Flight Systems." AIAA Paper No. 80-0823 (1980).

Martin, A. R. "Project Daedalus -- Final Report of the British Interplanetary Society Starship Study." BIS. A. R. Martin, ed. (1978).

O'Neill, Gerard K. Physics Today 27 (1976): 32.

Ball, John A. Icarus 19 (1973); 347.

Billingham, John, ed. Life in the Universe. Cambridge, MA: MIT Press, 1981.

Cameron, A. G. W., ed. Interstellar Communications. New York: W. A. Benjamin Inc., 1963, 1970.

Hart, Michael, Q.J.R.A.S. 16 (1975): 128.

Jones, Eric. Icarus 46 1981: 328.

Kuiper, T.B.H. and M. Morris. Science 196 (1977): 616.

Newman, William I., and Carl Sagan. Icarus 46 (1981): 293.

Shklovskii, I.S. and C. Sagan. Intelligent Life in the Universe. San Francisco: Holden Day, 1966.

Alvarez, L. W., W. Alvarez, F. Asaro, and H. V. Michel. Science 208 (1980): 1095.