The Third Xenological Revolution began sometime in the mid-seventies, when several prominent scientists challenged the conventional wisdom that intelligent life arises upon isolated islands, forever separated by the wide gulfs of interstellar space. Sanger, Bracewell, Forward, Bussard, and others demonstrated that it's possible to build to cross the emptiness between the stars. No "magic" is needed. It isn't necessary to repudiate Einstein. Whether by lightsail or by antimatter rocket, humanity may be launching starships within a few centuries.
These "Starships" would be nothing like the good old Enterprise. Limited to possibly a tenth of the speed of light, they could not travel terribly fast by interstellar standards. But clearly they could carry people, possibly living several generations in transit. The "slow-boat" generation ship of science fiction fame has been mathematically vindicated.
This is bad news?
Of course not. But the possibility of starships places a new and awesome burden on xenology. It presents us with a paradox that is very difficult to overcome.
What would we do if we had starships? If both history and literature tell us anything, we would look around for nice real estate and start colonizing. In fact, we wouldn't even need to find nice planets; stable stars with asteroid belts would do. Our own "belters" might by then prefer such virgin territory to "dirty planets" anyway.
Once the new colonies reached a high level of industry, say in a few hundred years or so, what would they do? Why, they'd send out more colony ships, of course. It seems obvious to almost anyone holding a book like this one.
Imagine a sphere of human settlement slowly expanding through space. How long would it take for colonies to be planted three hundred light-years from Earth? Even limiting ship speed to a tenth of the speed of light, and allowing each colony plenty of time to industrialize? Ten thousand years? Thirty thousand years?
Mankind has hardly changed at all, physically, in the last thirty thousand years. If we make a few social advances and avoid self-destruction, we should be able to fulfil the above scenario.
And why shouldn't anyone else? If this sort of expansion can occur once, why not for each of the million sophont races we calculated earlier? In well under 100,000 years the 200 light-year "average spacing" between races would be filled up!
Recent calculations by Eric Jones of Los Alamos Laboratories indicate that the scenario we have just described, of a slowly expanding sphere of settled solar systems, could fill the entire galaxy within sixty million years. It's not unreasonable to imagine at least one out of a million civilized races living that long. So why do we see no signs that the Earth has been colonized in the last sixty million years?
Why have we picked up no radio signals, when the stars should be humming with information and commerce?
Where are they?
This question marks the first traumatic awakening of the new science of xenology. It marks the end of a very short period of innocence. Starting around 1975 and building toward the present, the Third Xenological Revolution commenced. The dust has not yet settled, but one thing is clear. Some of our assumptions are wrong. The universe might turn out to be considerably more complicated than the scientist optimists of the late sixties had at first thought.
Of course, science fiction writers and readers could have told them that all along.
The Third Revolution in Xenology came with the realization that space should be filled with intelligent life. There appears to be no excuse any longer for the failure of SETI.
Indeed, why hasn't the Earth itself been colonized! The question, Where are they? might better be put, Why aren't they here? The quandary can be called the Mystery of the Great Silence.
We see no evidence for ancient alien cities in the Earth's crust. Venus and Mars apparently never were terraformed, though many now think we could tackle the job in a few centuries. The asteroids of the solar system appear to be untouched.
Most significantly, the Earth, until less than a billion years ago, was populated for two billion years by only primitive prokaryotic organisms. A visiting starship need not have landed colonists. All they'd have had to do was be careless with their garbage or latrine and the history of the Earth would be totally different.
It certainly looks as though we've been alone a very long time.
There have been several imaginative suggestions to explain the Great Silence. (At the end of this article we'll compile a partial list.)
Dr. Eric Jones, Dr. Frank Tipler, and Dr. Michael Hart all think it means that the early calculations of the probabilities of intelligent life were greatly overoptimistic. They suggest that the apparent absence of ETIs simply means that this part of the galaxy is uninhabited... that not race has got out there ahead of us to make an impact by colonization. Their Uniqueness Hypothesis implies that some or all of the factors ÿ(l,i,c) in the Drake Equation are really very small. For instance, some contend that intelligence such as ours is an evolutionary fluke.
Dr. Thomas Kuiper of JPL has presented strong arguments in refutation, showing that convergent evolution has happened frequently on Earth and might well occur elsewhere.
Dr. John Ball has dredged up the science fictional idea that the Earth is a "zoo" or wildlife preserve, and that extraterrestrials are already here, observing us. There are many variants to this concept, including "quarantine" (ETIs awaiting humanity's social maturity), a noninterference "Prime Directive," and many others. All imply we should add to the Drake Equation a factor to account for ETIs purposely avoiding contact.
Contact optimists, such as William Newman of Princeton and Carl Sagan of Cornell, have tried to make excuses for extraterrestrials. In a recent paper Newman and Sagan suggested that truly advanced cultures would practice zero population growth and thus feel less pressure to expand into virgin territory. The rate of "galaxy-filling" calculated under their extremely conservative assumptions is slow enough to make it barely possible that the nearest expanding space-faring race simply has not reached us yet.
Sagan and Newman further propose that techniques of life extension — immortality — would make individuals of a race very conservative. If a passion for risk-avoidance took hold, a species' rate of expansion, V, could drop to nil.
Might a race naturally graduate to other interests after a certain amount of time? Science fiction is filled with possibilities, from extra dimensions to realms of the mind far more attractive than drifting through space and clearing land on some new world. Such "maturity stages" would affect L in the Drake Equation, as well as the velocity of expansion.
Our assumptions for factors ÿ(l) might be too high. Although the precursors of life — sugar, amino acids, nucleic acids — seem likely to be about as common as stardust, it's possible that the next steps to life might be much, much harder to reach, requiring some rare catalyst to set the process off.
From physics and SF comes the dreadful idea of "deadly probes." Saberhagen's "Berserkers" might make life rare if some technological civilization accidentally let loose something so monstrous. Gregory Benford's variant on the idea is hardly more optimistic. A particularly paranoid advanced species might not want any potential competition to rise up elsewhere. Self replicating autonomous probes might be sent out to reproduce and fill the galaxy. Whenever new radio traffic indicates that new sentients are loose, these preprogrammed probes would home in on the signals with powerful bombs and stop the infection before it spreads.
It's already too late to call back the spherical wave of I Love Lucy, etc., that's already spreading through nearby space.
All of the hypotheses given above have their problems. Some seem to contradict the best knowledge we have in the field. Others, like the "zoo" theory, are almost innately untestable.
What we hope to do is to compile a list of these possibilities. I will start things off by talking about a few hypotheses that the xenologist speculators have mostly passed up. Some are a bit frightening.
In the Drake Equation the combined factor ÿ(i,c) — the fraction of life-planets on which intelligence and technology eventually evolve — is generally assigned a value of about 1 in 100. The xenologists who put forward the one percent argument support it by citing the apparent fact that it took four billion years for the Earth to give rise to merely one technological race. This is almost half the viable life span of the planet. Intelligent life would seem to be a rare and wonderful thing.
But is this assumption tenable? It appears to be the weakest link in the chain of logic.
Let's consider the life cycle of a "Nursery World," a planet with a stable biosphere in which the slow evolution to intelligence can take place.
Evolution appears to have proceeded gradually at first and then at an accelerating pace for over three billion years. Except for (maybe) the introduction of sex, and later of flowering planets, there is no evidence in the fossil record to support the idea that the Earth was ever suddenly invaded by extraterrestrials who, "with kith and kine," introduced advanced flora and fauna. The Great Silence seems, at first glance, to have stretched through the entire Paleozoic.
If we assume the Earth lay untampered with until at least the time of the Jurassic, we can guess that it takes about three billion years for life on a Nursery World to evolve to a level of complexity that makes intelligence feasible.
What if humanity suddenly vanished? Would it take another three billion years for intelligence once again to arise on Earth? If so, it's reasonable to accept the guess that the number of technological species to erupt per habitable planet is of order less than one.
But Homo sapiens is not the only species to have benefited from three billion years of evolution. Today's German cockroach may look a lot like his distant ancestors, but he has accumulated many little tricks his cousins in the Triassic never heard of. The size of genome of the raccoon and wolf is hardly smaller than that of man.
Consider what's happened since the Cretaceous-Tertiary Catastrophe approximately sixty five million years ago — the disaster that wiped out, over a period of a few hundred thousand years, almost every species of land animal whose adults massed more than forty kilos.
The creatures whose descendants went on to dominate the planet were small mammals: the early equivalents of mice, lemurs, and tree shrews. These humble animals expanded and diversified to all of ecological niches left vacant by the demise of the large reptiles. We are among their descendants.
In spite of the present arms race, man still lacks the ability to exterminate mice, although he will probably soon be able to do an efficient job on himself. The sudden demise of this star system's current technological race would not finish off the Earth as a nursery. If "mice" did it once, they could probably do it again.
We are led to suggest that suitable worlds must pass through long initial "fallow" periods before attaining a level of biological sophistication for intelligence. Afterward such planets should be able to produce sophont species at fairly short intervals, depending upon the time needed to recover from the damage done by the previous sentient race.
The interval between the Cretaceous Catastrophe and the present is a reasonable estimate for the time it takes to build a civilized race, once small and sturdy creatures have reached a high level of sophistication.
Let's go back to that expanding spacefaring species we were talking about earlier. Remember, calculations show that it might take as little as sixty million years for such a race to fill the galaxy. A question seldom asked by science fiction authors who write about colonization is, What happens to the colonized planet?
Unless the settlers leave large parts of their worlds fallow in wilderness preserves, or engage in "Uplift" bioengineering of local higher animals, their mere presence is likely to prevent the appearance of local sentient species. The cycle of production of intelligent species on a planet is probably delayed indefinitely by an active technological settlement. A world is not likely to serve as a useful nursery of intelligence so long as it is occupied by a spacefaring race.
When settlers finally do step aside — by attrition, disaster, exodus, or whatever — ecological recycling can resume, but recovery and regeneration of intelligence will take much more time, the longer a technological race occupied the planet.
It is generally assumed that a spacefaring race will expand into the galaxy either because of raw curiosity or population pressure. Either way, it's clear that the expansion soon becomes spherelike, with only the most recently settled worlds having much opportunity to seek new planets. For a race limited to slowboat technology, colonization will take place only in a thin shell surrounding an older, settled region within.
If population pressure is the primary motive for expansion, we have to wonder at the fate of the long-occupied worlds in the interior of the settled sphere, especially those near the Home planet. The words population pressure themselves suggest the likely fate of these worlds.
Consider the settlement of Polynesia from roughly 1500 B.C. to about A.D. 800. The island-hopping analogy with interstellar exploration is apt up to a point. Jones borrowed growth and emigration rates for his model of interstellar settlement from Polynesian history. The intrepid Polynesian example is used as testimony to the likely success and viability of "star-hopping" colonization ventures.
Polynesia may, indeed, be representative of interstellar settlement, but not in a pleasant sense. The Hollywood image of island life is paradisiacal, but Polynesian cultures were subject to regular cycles of extreme overpopulation controlled by blood culling of the adult male population, in war or ritual. There are many stories of islands whose men were almost wiped out, sometimes by internal strife, sometimes by invading males from other islands far away.
Meanwhile, introduction of domestic animals disrupted island ecosystems. Many native species were wiped out.
The most severe example is the island of Rapa Nui, also called Isla de Pasqua, or Easter Island. Isolated thousands of miles from its nearest neighbors, it was as much like an interstellar colony as any place in human history, when it was settled around A.D. 800. Mankind may devoutly hope to do better when finally embarked to the stars.
The Pasquans utterly destroyed the virgin ecosystem of Rapa Nui in a few generations, ravaging the forest until only banana trees were left. When no wood remained for houses or boats, they had to abandon the sea and its resources, along with all possibility of escape or trade. What remained was native rock — which they carved into hauntingly desolate images — and warfare.
When Europeans arrived, the natives of Rapa Nui had just about destroyed themselves.
Assume a settled sphere of expansion by an extraterrestrial intelligent species. What of the inner systems, within the sphere? The Polynesian example suggests a dismal image of increasing competition for dwindling resources with no escape valve for excess population, since all surrounding systems are in similar straits.
What happens to these inner worlds? They probably don't go looking to conquer their neighbors. Interstellar warfare seems to be a frightfully expensive proposition. conflict arising from population pressure is far more likely to be local, consisting of struggles for resources within each planetary system.
In an old settled system all available asteroids would long have been turned into habitats. Safe inner orbits with unhindered access to solar power would be at a premium.
Even the most efficient space structures will require frequent replenishment of volatile substances — gases such as oxygen, hydrogen, and nitrogen. Comets might supply part of this need, but terrestroid planets would be closer and rich in the desired light elements.
One might expect to see a profound cultural split between those living on planetary surfaces and those in space. Competition and misunderstandings might tempt the space-dwellers to take advantage of their superior position to dominate their planet-bound cousins. It would be simple to bombard the cities on a planet's surface with redirected asteroids until civilization there was obliterated. Factor L clearly falls in such a case.
(The space-born, long divorced from any attachment to planetary life, might even see a terrestroid planet as a likely source of building materials! It wouldn't be beyond their ability to pulverize a world such as the Earth by arranging planetary collisions. This would certainly affect not only L but also n(e), the number of planets on which life can evolve!)
In any event, the innocent higher animals suffer in the crossfire.
Let's return briefly to the episode about sixty-five million years ago known as the Cretaceous Tertiary Catastrophe. There were, at that time, many advanced species of reptiles. The best candidate among those for a species possibly ripe for development toward tool-using might have been Saurornithoides, a mid-sized bipedal carnivore with the highest brain-to-body mass of any reptile, approximately matching that of modern baboons. While there is no reason to think that this creature was particularly intelligent, he filled an ecological niche that might have been rigorous enough to encourage his glimmering abilities.
But Saurornithoides died out, along with virtually all of the other great reptiles during a relatively brief period by geological standards.
If the demise of the dinosaurs puzzles paleontologists, the problem has been even worse for the marine biologists. The dinosaurs, at least, took as long as a few million years to die out. The tiny sea microorganisms experienced a greater catastrophe. Over half of the species of phytoplankton went extinct within about one year.
The latter mystery, at least, now appears solved. Recent deep-core drillings have uncovered thin layers of clay rich in exotic elements, including iridium (up to 25 times normal abundance of some isotopes), at sedimentary levels associated with the end of the Cretaceous. Discoveries in locations as diverse as Italy and New Mexico all seem to correlate a sudden invasion of strange dust with the equally sudden disappearance of many classes of oceanic microorganisms. Scientists now conclude that a major meteorite impact kicked up a great pall of dust that severely altered weather, resulting in mass extinction by starvation when photosynthesis was interrupted.
For the marine creatures this seems sufficient, but don't forget that the dinosaurs were already dying out before this bombardment, starting with the greatest behemoths and so on down to the smaller herd animals. Their die-back was a lot like what we see happening to the wild animals of Africa at the hands of white and black "intelligent" beings. The meteorite seems to have been only one of the last straws for the great reptiles.
Might the demise of the dinosaurs, then, be part of a hidden pattern? Is it possible that an alien colony began a process of extinction that was by the meteorite (or meteorites) only finished?
A natural planetfall can't be distinguished from one targeted against ground settlements of a technological species. Is it possible that the dinosaurs were innocent bystanders in a genocidal war among alien settlers in the solar system?
The bombardment might only have been the last act in a more gradual ecological catastrophe that began half a million years before, when settlement of the planet resulted in extinction of species after species.
The introduction, about this time, of flowering plants, is another environmental perturbation that had profound ecological effects. It's not absurd to imagine this fitting into an overall pattern of outside intervention.
The settlement of Earth by a spacefaring race about seventy million years ago, then, offers one more (admittedly tenuous) explanation for the destruction of the higher terrestrial life-forms over a brief period.
If we make this hypothesis, however, where are the traces of this earlier technological occupancy? Over sixty million years of oxidation will destroy many artifacts, but certainly some might survive.
Who can say? The cities we look for may lie beneath astroblemes. A look at a geological map of the Earth shows that continental plate boundaries have proved to be choice living sites. These plate-edge regions have suffered pronounced geological changes that could have erased most traces of alien settlement.
The final test of this hypothesis would be found among the planetoids of the solar system. The asteroids might hold remnants of visits to our star by extraterrestrials... perhaps whole cities, the leftovers of great populations killed off, perhaps, by biological warfare in desperate retaliation by the Earthbound cousins they had annihilated.
This hypothetical explanation for the Cretaceous Mystery merely should take its place in a catalog of possibilities, perhaps near the bottom. Still, it's interesting to note that the period since that catastrophe — an interval that culminated in the development of Homo sapiens — is the same sixty million years suggested by Jones and others for an optimum minimal galaxy-filling by space traveling species.
If the ecological holocaust of the Cretaceous was a local manifestation of the death spasm of a prior spacefaring race, whose overpopulated sphere of settlement spoiled and self-destructed as the shell of colonization passed outward, then we humans may have come into being almost too late. Any longer, and the next wave — the expanding shell of still another spreading technological race — might have washed over Earth before we had the ability to assert property rights... assuming we have that ability now.
We may wonder if the Earth is the first Nursery World to have recovered sufficiently, since the last wave of "civilization" passed this way, to develop a species with intelligence. Whether or not the end of the Cretaceous corresponded to the agony of dying starfarers, it may well be that colonizing cultures inevitably leave behind them wastelands empty of intelligence and living voices.
If we humans initiate an era of interstellar travel of our own, we may find all around us the blasted remains of an earlier epoch. Would we then learn a lesson? Perhaps. But with the ever present opportunities for expansion, those humans who exercise self-restraint and environmental sensitivity toward their adopted worlds will not be able to force this tradition upon those who travel far away to establish newer colonies. A nucleus of selfishness is likely to expand more rapidly than a center of more rational colonization. While there may be zones where settlers preserve and protect the local ecospheres, cognizant of their long-range potential, others may be rapacious.
Certainly our environmental record here on Earth is a test. The list of extinct species, some of which might one day have become starfarers, is long and growing longer.
The Great Silence may be the sound of sands drifting up against monuments. It may be quiet testament to the fate of species which allow "population pressure" to be their motivation for the stars.
We'll begin a "morphological" analysis of the Great Silence by presenting the following list of possibilities.
FIRST: 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.
SECOND: "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.
THIRD: 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.
FOURTH: 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.
FIFTH: "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.
SIXTH: "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.
SEVENTH: 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.
Copyright © 1983 by David Brin. All rights reserved.
"Xenology: The Science of Asking Who's Out There" (published in full here) was 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: The Controversy Concerning Extraterrestrial Intelligent Life" — which appeared in the Quarterly Journal of Royal Astronomical Society, Fall 1983, v.24, pp 283-309.
John Billingham, ed., Life In The Universe (book)
Giuseppe Cocconi and Philip Morrison, "Searching for Interstellar Communications" (pdf)
Stephen H. Dole, "Habitable Planets for Man" (pdf)
Cyril Ponnamperuma and A. G. W. Cameron, Interstellar Communication: Scientific Perspectives (book)
I.S. Shklovskii and Carl Sagan, Intelligent Life In the Universe (book)
on interstellar travel technology:
R.N. Bracewell, Nature (London) 186 (1960): 670
Robert Forward, "Interstellar Flight Systems" AIAA Paper No. 80-0823 (1980)
A.R. Martin, ed., "Project Daedalus: Final Report of the British Interplanetary Society Starship Study" BIS (1978)
Gerard K. O'Neill, Physics Today 27 (1976): 32
on possible dispersal of intelligent life:
John A. Ball, Icarus 19 (1973) 347
John Billingham, ed., Life in the Universe. Cambridge, MA: MIT Press, 1981
A.G.W. Cameron, ed., Interstellar Communication, New York: W. A. Benjamin Inc., 1963, 1970
Michael Hart, Q.J.R.A.S. 16 (1975): 128
Eric Jones, Icarus 46 1981: 328
T.B.H. Kuiper and M. Morris, Science 196 (1977): 616
William I. Newman and Carl Sagan, Icarus 46 (1981): 293
I.S. Shklovskii and C. Sagan, Intelligent Life in the Universe. San Francisco: Holden Day, 1966
on the cretaceous catastrophe:
L.W. Alvarez, W. Alvarez, F. Asaro, and H. V. Michel. Science 208 (1980): 1095
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