The study which is called SETI (Search for Extraterrestrial Intelligence) is plagued by two disturbing puzzles. One is this: If the galaxy is an inhabited place, then why have increasingly-sophisticated attempts to discover the radio transmissions of other races been, thus far, unsuccessful? Why the Great Silence? The other has been called the Fermi Paradox (most simply, "Why aren't they here?").
It is often helpful to look for different perspectives when trying to figure out a mystery. This paper is an attempt to approach this riddle from a seldom-used perspective.
In 1969, physicist Gerard K. O'Neill at Princeton University's Institute for Advanced Study began to work out a scenario for the future expansion of the human race into space. He devised what has come to be known as the Space Settlement proposal, which is the idea that the best way for humanity to establish colonies off the Earth may be to use space resources to build enormous, artificial habitats in orbit. It turned out that such habitats could be designed from the beginning to be surprisingly Earth-like.
Up until then, most thinking had centered around the idea of colonizing the
surfaces of other planets, sometimes after modifying their environments to
make them more Earth-like (called terraforming). But orbital settlements
seemed to offer many advantages over any planetary home:
Gerard O'Neill championed this idea in several scientific papers, starting with one in Physics Today 1 , and wrote a book for the general public entitled The High Frontier. 2 Although his concepts were somewhat influential on space-related thinking, it can be argued that they are not yet as influential as the planetary paradigm. Interest in the goal of terraforming Mars, for example, remains high. What does this less-widely-appreciated perspective offer us when considering the question of our uniqueness in the galaxy?
Here, I think, is where the divergence of opinion begins. I have noted that those who tend to believe in a planetary future for humanity also tend to hold out the hope for advanced civilizations somewhere nearby. Those who foresee a space-dwelling future for the human race tend to suspect that we are alone in this galaxy. The reason being that space-dwelling civilizations can be expected to so change the physical characteristics of the galaxy that the signs of their existence would be both obvious and unmistakable.
In 1959, Freeman Dyson (also with the Institute of Advanced Study) wrote a paper 3 on a concept that has since come to be known as the Dyson Sphere. His argument was that an advanced extraterrestrial civilization, regardless of the nature of its technology, is going to experience a constant increase in the amount of energy needed by that civilization. In time, the energy requirements would exceed the amount of energy conceivably available on the civilization's planet. At that point, there would be no recourse but to begin tapping the solar energy available in space. Dyson saw this process continuing until the home-star of the civilization was so completely surrounded by solar energy collectors and habitats that, from outside the system, the solar output would be severely dimmed, or perhaps totally obliterated.
It should be noted at this point that Dyson never proposed what the science-fiction writer has come to call a Dyson Sphere: a solid, continuous, hollow sphere. Such a structure would collapse under the gravitational pull of the sun at its center. A sphere could be spun to support the "equator", but there would be nothing to prevent the collapse of the "poles". Dyson instead hypothesized innumerable individual solar collectors, each in its own independent orbit. The term "Dyson Swarm" might be more accurate.
Dyson suggested that rather than searching for indicators of extraterrestrial civilizations in the radio spectrum, perhaps we should instead be looking in the infrared. A point source of energy equivalent in wattage to the output of a main-sequence sun, but entirely in the infrared spectrum, is precisely what we should expect a solar system to look like, provided that some industrial civilization was making 100% use of the star's solar output.
We have thus far found no such indicators. The coolest infrared sources located to date are consistent with dim, red dwarf stars; cool for a sun, but far too hot for a Dyson Swarm.
So we are left with the conclusion that whatever else may be out there in the galaxy, there are no nearby civilizations which harness most or all of the output of their local sun. But is it an inevitability that all technological civilizations must ascend to Dyson Swarm status?
Another type of extraterrestrial construct visible from cosmic distances can be found in Carl Sagan's science fiction novel Contact. 4 In it, a radio astronomer is informed that some of the extremely energetic phenomena we have detected, like Cygnus A, are the "galactic urban-renewal programs" of astonishingly-advanced civilizations. But this should probably be taken as an ingenious and provocative idea for a science fiction story, rather than as a serious proposal. Certainly any universe in which black holes exist can have extremely energetic phenomena. Occam's razor (also mentioned in the book) suggests that any phenomena which is explainable in terms of natural cosmic processes cannot be used as evidence for extraterrestrials.
So radio astronomers continue to scan the radio spectrum, looking for signals which are indisputably artificial. Gerard O'Neill was at least open to the idea that other technological cultures may exist in this galaxy. But one gets the impression he was never confident that a radio search for extraterrestrial intelligence would ever yield positive results. In The High Frontier he confessed (somewhat reluctantly) that one product his high-orbital manufacturing facilities would be excellent for would be the construction of an orbital version of Cyclops 5 , the SETI proposal for a vast array of intelligence-seeking radio telescopes.
Again, this was the High Frontier perspective in action. The "planetary mind-set" was that even if you wanted to build Cyclops off the Earth, the best location would be a crater on the far side of the moon, where the mass of the moon itself would shield the array from the radio noise of Earth. But O'Neill's proposal was a single radio dish in High Earth Orbit, five kilometers across, with a simple, disk-shaped aluminum baffle for a noise shield. Using thousands of miles of lunar rock for shielding was a bit of overkill; a millimeter of aluminum could do the job just as well.
Assuming both arrays were built from lunar resources, the only remaining issue was would the cost of lifting those resources from the moon plus the cost of the disk-shaped baffle be greater than the additional cost of building the array in a gravity environment where there was only access to solar energy half the time. Given the application of mass-driver technology to lunar ore-launching, the advantages of zero-G construction, and the simplicity of the shield, O'Neill felt the answer was no.
It is tempting to look at lunar craters and envision outfitting them as giant radio dishes, such as the one in Arecibo, Puerto Rico. But craters are the wrong shape. They are hemispheres. Radio dishes require parabolic shapes. To make a parabolic dish in a lunar crater would require extensive excavation in the middle, or very high lifts toward the edges (probably both). It will never be as simple as laying aluminum grills all over the ground.
But the major disadvantage of the Lunar Cyclops system versus the orbital one would be its fixed, immobile state. It would only be able to look in the direction the moon was pointing it in. The orbital Cyclops could point in any direction desired. If it was in a sufficiently-high orbit, no part of the sky would be blocked to it. O'Neill painted a humorous scene of a scientist in the uncomfortable position of trying to explain these facts to the very congressman who had approved the billions of dollars for his Lunar Cyclops proposal. This after it was discovered that the Encyclopedia Galactica was being beamed our way, but we were getting less than half of it.
1981 saw the publication of 2081: A Hopeful View of the Human Future, in which Gerard O'Neill went on to hypothesize replicators: machines which, given a supply of asteroidal ore and solar energy, can build a duplicate of themselves. 6 He then argues that any technological civilization in the galaxy can have easily placed a replicating space probe in orbit around every star of the Milky Way with only a modest investment; each probe reporting back to home base at the speed of light. The question then follows: If there is as many as one other civilization in the galaxy, where is the probe for our solar system? Either it is undetectable, or we are the first technological culture in the galaxy; the one which will eventually fill it with self-replicating space probes.
About ten years later, Marshall Savage wrote a book entitled The Millennial Project, How to Colonize the Galaxy In Eight Easy Steps. 7 His basic thesis was the following. Do you remember that "Genesis Effect" in Star Trek II: The Wrath of Kahn? That spherical shock wave which spread outward, and, everywhere it touched, converted dead, inanimate matter into living things and a clement environment? Well, if you want to know what the real Genesis Effect looks like, go find a mirror. We are destined to expand outward from Earth in a spherical wave traveling at just under the speed of light, and in every sterile, lifeless place the wave touches, raw matter will be converted into lakes and flowers and children.
His orbital habitat designs are different from those of O'Neill's, chiefly because they asked different questions. O'Neill asked, "How Earth-like an environment can we create in space?" Savage asked, "How can we create a life-sustaining environment in space?" But one issue upon which they were both agreed was that space is where it's at. Although Savage spends some time discussing domed habitats on the Moon and Mars, his "Solaria" civilization 8 is indisputably over 99% orbital habitats. This is a simple consequence of the fact that such settlements can reproduce themselves endlessly. Planetary surfaces will always be limiting to growth.
Savage foresaw his orbital habitats eventually numbering in the trillions. Since their life-support system involved the growing of algae, he wondered if their massed numbers wouldn't filter the sunlight green. 9 And given that we are witnessing no such "greening of the galaxy", he concludes that we do not share this galaxy with other advanced beings.
Even if extraterrestrials used a different type of life-support system, the argument remains the same. A star surrounded by trillions of artificial structures of any kind would show some distinct change in its spectral signature, if only the infrared shift earlier hypothesized by Dyson.
According to Marshall Savage, our SETI astronomers are cupping a hand to their ear, struggling to catch a whisper, when what they are looking for, if it indeed exists, would be a deafening roar of radio noise. 10 Imagine ten trillion cell phones going off at once, and you begin to get the idea. The counter argument to this is that extraterrestrials might use some communications technology as yet beyond our ability to detect. Perhaps when we turn on the world's first tachyon receiver, it will hum with ten trillion conversations. But it seems odd that ET's would totally ignore the electromagnetic spectrum. Surely there are a few applications left around which radio is still good for.
So we have two different views of the future which lead to vastly different-looking universes. On the one hand, if we live in a reality where intelligent life is typically found on planetary surfaces, we might expect one inhabited planet in a few widely-scattered solar systems. Some systems might have two inhabited planets: the planet of origin plus another, nearby world which has been terraformed, and is marginally inhabitable. Probably no single planet could support a population much greater than around 10 billion individuals. So figure maybe 10 or 15 billion per solar system total. If only one out of a hundred systems have a planet which is either inhabitable or amenable to terraforming, then that is not very many beings out there. Perhaps we could look at the night sky, and be unable to distinguish such a universe from a cosmic wilderness.
But on the other hand, if we live in a Gerard O'Neill / Freeman Dyson / Marshall Savage-type universe, the picture is very different indeed. O'Neill calculated that the resources of the asteroid belt alone would be sufficient to build, in the form of orbital space settlements, more than three thousand times the habitable surface area of the Earth. 11 And the asteroid belt only represents the most convenient source of raw materials. We would still have Mercury, our Moon, and the moons of the outer planets for industrial feedstock. In a universe where technological cultures go the orbital habitat route, we can expect solar systems with perhaps hundreds of thousands of times the Earth's living space. Trillions of intelligent beings residing in hundreds of billions of space settlements. Moreover, we can expect every solar system within reach of their starships to be so populated, not just the ones with certain planets with certain conditions orbiting certain-type suns. With orbital habitat technology in hand, even suns vastly different from our sun's spectral type can be colonized. All it takes is the proper color filtration. 12
If there is any truth to the Space Settlement concept, then we should expect a universe inhabited by intelligent beings to be lit up like a Christmas tree (at least where the infrared spectrum is concerned) with their cosmic-scale artifacts and construction projects. As Marshal Savage said, we could no more mistake that type of universe for a wilderness than we could confuse New York City with the Antarctic ice cap.
One objection raised against this logic is that perhaps ET's wouldn't concern themselves with such gross matters as reproducing themselves, or harnessing energy. Maybe they would devote themselves to some loftier goals of a spiritual (or at least invisible) nature. These assertions remain unconvincing. All of life, from slime molds to the reader, is driven by the same evolutionary mandate: proliferate and spread. It is difficult to imagine how life could develop in the first place other than via natural selection mechanisms. Although Extraterrestrials would doubtless move beyond Darwinian pressures when they entered their technological stage (as indeed we have begun to), by that point both their bodies and their minds would have already been shaped by their natural selection origins. The basic motivational imperatives would already be set, and couldn't be changed except by intentional self-reengineering.
But even this concept does not rescue the argument. If only one species out of a thousand retained its reproductive urges, that one race would be lords of the galaxy in only a few hundred thousand years, rapidly overwhelming the other, less fecund races. Perhaps many races might reengineer themselves to have no procreative desire. But the only explanation for either the Fermi Paradox or the Great Silence is that all races invariably do this without exception. Why this should be so is difficult to see.
Those with the traditional "conquer the planets" viewpoint believe in an inhabited galaxy because they can. Those who view the future from the "High Frontier" perspective tend to believe that we do not share the galaxy with other technological cultures, reasoning that the alternative is a galaxy which is so thoroughly-inhabited that the signs would be highly distinctive, and could not be overlooked.
1 O'Neill, G. K., "The Colonization of Space", Physics Today, Sept 1974. (Back)